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I“.; 31 .. 2‘2? .X - u: 53%: .; ii.- ' L. if; 52 r}?- - “1:31!" 3g . *9, S! if THEBEB A F:/'-\ ,I“.'\ 2’3 9 L1, #77 5(6/ This is to certify that the dissertation entitled New Approaches to Highly Sterically Encumbered Porphyrins as Heme Active Sites presented by Chen-Yu Yeh has been accepted towards fulfillment of the requirements for Ph.D. degree in Chemistry 613164.; '~ -\:/ Major professor f \ <3 /~"U Date ”if / —NH—CO2' + H”). When the hemoglobin is reoxgenated at the lungs, it releases protons. These protons shift the equilibrium to the left, converting bicarbonate ions to the less soluble CO2 which is exhaled. In addition, when carbon dioxide dissolves in water, it produces bicarbonates and protons. The protons are (picked up by deoxyhemoglobin and bicarbonate ions are carried back to the lungs as the counterions. The microenvironment of the heme which controls the ligand binding is a fascinating research topic. Dioxygen affinities (Pl/2 of dioxygen binding) of various hemoproteins at room temperature may range from 0.002 torr for Ascaris hemoglobin to 2.5 torr for Aplysia hemoglobin. 12,13 Furthermore, the presence of carbon monoxide, an intrinsicly toxic ligand produced when one molecule of heme is catabolized by heme oxygenase, complicates dioxygen transport and storage. 14 The heme proteins are capable of discriminating between dioxygen and carbon monoxide. The relative CO/O2 affinity is described by “M” (partition coefficient). The M values in hemoproteins range from 6000 for glycera Hb, to 20—40 for Mb, to 0.02 for Ascaris Hb, and are thought to be related to the nature of the Fe-CO geometry regulated by protein residues.15’l6 The assessment of the factors is frequently difficult in the protein system. The studies of model compounds which are structurally and functionally similar to the active sites of the metallproteins can provide a detailed understanding of the structure and function of the active site which can not be obtained by studying the protein itself. Thus a model compound similar to the active sites in hemoglobin can be assembled from an iron porphyrin and a ligand. In the past three decades numerous model compounds have been synthesized to mimic the functions of hemoproteins and elucidate the factors which influence the dioxygen affinities. One of requirements for the synthetic model compounds is their ability to bind oxygen reversibly. However, unhindered ferrous hemes are oxidized rapidly and irreversibly in the presence of oxygen. Two general autoxidation pathways have been proposed: (1) p—oxo dimer formation and (2) H" (H2O) catalyzed autoxidation. Fe"(O2)(P)(B) + Fe"(P)(B) —9 —> (B)(P)Fe"'OFem(P)(B) (1-7) Fe"(O2)(P)(B) + H+ —+ Fem(P)(B) + HO2’ (1-8) Where P is porphyrin dianion and B the axial ligand. In order to thwart p-oxo dimer formation that leads to irreversible oxidation, the approaches which have been employed successfully are low-temperature stabilization,17 immobilization of porphyrins into polymers,18’19 and construction of heme models with sterically hindered substituents.20' 22 It has been known that the iron-dioxygen complexes are relatively stable at low temperature. Chang and Traylor17 reported the reversible oxygenation of a chelated Fe(II) porphyrin in which an imidazole is covalently attached to a pyrrolic position. The Pm was estimated to be 0.2 torr in dichloromethane solution at —45 °C. A similar system with a covalently linked pyridine showed rapid oxidation at -45 °C. After the report by Chang and Traylor, many papers described the reactions of dioxygen with flat hemes and chelated hemes.23'26 In the presence of axial ligands such as alkyl imidazoles, pyridine, simple amine-type ligands, and DMF, these systems can reversibly bind dioxygen at temperatures below —45 °C. In summary, imidazole appears to be the best ligand and dioxygen affinities are higher in solvents of high dielectric constant. In hemoproteins, globins provide hydrophobic environments to prevent the active sites from irreversible oxidation and suppress proton-driven oxidation in the presence of dioxygen. Synthetic polymers can fill the roles of proteins. Wang was the first to report a polymer-encapsulated heme system in which dioxygen binds to the heme reversibly.27 The results suggested that the hydrophobic environments provided by polystyrene matrix excluded water molecules and the isolation of the hemes from each other prevented the oxidation by dimerization. Chang and Traylor reported a similar system in which a porphyrin with a built-in imidazole was embedded in a polystyrene film.28 A reversible oxygenation was observed by spectrophotometry. Oxygen-binding to the iron(II) porphyrin attached to a modified silica gel was also reported.29 In this system, an imidazole group was covalently linked on the surface of silica gel and Fe(H)TPP was coordinated to the imidazole. The active sites are capable of binding molecular oxygen reversibly. The spectrum of the oxygen adduct of a water soluble complex of poly(1-vinyl-2-methylimidazole) (PMI) and protoporphinatoiron IX agreed with that of oxyheme when its aqueous solution was exposed to oxygen at -10 °C to —30 °C. The oxyheme returned to the deoxyheme upon flushing the solution with nitrogen, and the oxy-deoxy cycle was repeated several times at low temperature. The results suggest that the water soluble but hydrophobic polymer created a hydrophobic environments for oxygen-binding to the five-coordinate protoheme complex in cold aqueous media and the proton-driven irreversible oxidation was suppressed. Another class of stable heme complexes is when the heme itself is attached to a polymer. Fuhrhop was the first to employ this approach.30 He incorporated the vinyl side chains of iron protoporphyrin IX dimethyl ester into the back-bone of a polymer. The solid polymers with <10% imidazole reacted with dioxygen reversibly. In solution the heme was oxidized by oxygen rapidly. A similar approach was taken by Tsuchida. A picket-fence iron porphyrin was covalently bound to the central hydrophobic block of a triblock copolymer: poly(ethylene oxide)/polystyrene/poly(ethylene oxide). This system 10 1.. a". l“ showed reversible oxygenation at room temperature and the half-life of the oxygen adduct was estimated to be half a day, although the oxygen association and dissociation occurred slowly. This is the first report that dioxygen binds to the heme reversibly in aqueous solution at room temperature. The use of meso-tetraphenylporphyrin (TPP) derivatives, however, resulted in somewhat different spectral features from that of protoporphyrin D(. However, the use of biological porphyrin (e. g. protoporphyrin IX) often leads to irreversible oxidation due to a mu stacked aggregate which induces an unfavorable electron transfer. Tsuchida has recently reported the reversible oxygenation of a lipid/protoporphyrin having three long alkyl chains and an axial imidazole embeded into the bilayer membrane of the phospholipid vesicle.31 He also found that the oxygen- binding affinity was affected by the phase transition of the membrane. 19 32 The oxygen- binding affinity is higher below the gel-(liguid crystal) phase transition temperature (T c) of the bilayer membrane and lower above the Tc. That is, the T- and R-states of hemoglobin were mimicked by the phase transition behavior of the phospholipid vesicle. In order to inhibit the irreversible oxidation of iron(II) porphyrins via dimerization, many steric hindered porphyrins have been constructed. Modified porphyrins for modeling hemoproteins can be divided into two categories: systems in which one face of the porphyrin is protected by sterically hindered groups (Figure 1-3), and systems in which both faces of the porphyrin are protected by sterically hindered groups (Figure 1- 4). The first attempted synthesis of a single-face-hindered porphyrin, in which one face of the porphyrin was protected by a cyclophane strap, was reported by Traylor.33 However, this porphyrin was prepared in very poor yield and no chemical studies were reported. The earliest successful iron porphyrin model for Hb and Mb active sites was reported by Collman.34’35 He described the reversible dioxygen-binding of a “picket- fence” model compound having four pivalarnido groups on one side of porphyrin plane and leaving the other side unencumbered for axial ligand coordination. The four bulky groups provide a hydrophobic pocket for complexation of dioxygen. In the presence of 11 excess axial ligand, the oxygenation-deoxygenation was repeated several times at room temperature without appreciable decomposition. In order to mimic the T state of hemoglobin, the sterically hindered axial ligand, 1,2—dimethylimidazole was employed.22 In the presence of 1,2-Me2Irn, the 5-coordinate complex was formed and the iron(II) was pulled out of porphyrin plane due to the steric interactions of methyl group and the it electrons on porphyrin ring (Figure 1-2). It is interesting that the use of this hindered imidazole can effectively reduce the oxygen affinity to the level found in T-state Hb. After the picket fence porphyrin, a series of tailed picket fence porphyrins were prepared.36'38 In this system, the axial ligand, either imidazole or pyridine was built into the porphyrin. In addition to the picket fence model compounds, many types of sterically protected model systems have been synthesized and their binding properties have also been reported. These include strapped,39,40 capped“,42 pocket,16a43 picnic basket,36’44 crowned,45 and cofacial porphyrins21 as well as hybrids46 of these classifications. Some examples are shown in Figure 1-3. Most of the model compounds can bind dioxygen reversibly in aprotic organic solvents at room temperature in the presence of a high concentration of the axial ligand. However, in the absence or at low concentrations of the axial ligand, the iron center oxidizes irreversibly via the formation of p-oxo dimer from the unprotected side. Another attempt to prevent p-oxo dimer formation is to introduce sterically encumbered groups to both sides of the heme. Figure 1-4 shows some of double-sided porphyrins with or without built-in nitrogenous base. The “bis-pocket”,47 “bis- fenced”,48 “basket-handle”,49,50 and “doubly-bridged”51 porphyrins are suitable model compounds for dioxygen binding. The stability of the oxyhemes to oxidation is related to the degree of steric hindrance. By studying the O2 binding to hemoproteins and iron(II) porphyrin synthetic model compounds, it has been shown that the heme reactivity is governed by the microenvironment near the active sites. As noted, the Pm of dioxygen binding and the 12 .p' A): ,...\,, ‘0'-“ .1 I ’Fu. ‘1'.’ “A i am 00251 Figure 1-3. Some examples of single-sided heme model compounds. (a) cyclophane; (b) picket fence; (c) hybrid; (d) cofacial; (e)crowned; (f) capped porphyrins. 13 @- arr 1°4— w Figure 1-4. Some examples of double-sided heme model compounds. (a) bis-pocket; (b) basket-handle; (c) bis-fenced; (d) integrated porphyrins. l4 relative CO/O2 affinity to the hemes vary from system to system. The factors that affect the small molecule binding to the heme include the nature of axial ligand, solvent effects, distal steric interactions, dipole—dipole interactions, and hydrogen bonding. Fe(II) porphyrins can bind to dioxygen in the presence of a variety of axial ligands such as imidazoles, pyridine, simple amines, and some donor solvents. The effects of the axial ligand on the gasous ligand binding to ferrous porphyrins have been an interest. There are major electronic changes at the iron center upon 02 binding. Therefore, electronic effects in axial ligand coordination influence the small molecule affinities to iron(II) porphyrins. Chang and Traylor reported that the oxygen affinity of an imidazole- chelated iron(II) porphyrin is about 20-fold higher than the pyridine-chelated analogue.52 Kinetic studies show that the effects of axial ligands reflect on the dissociation (off) rates. This is consistent with the fact that imidazole can donate more electron density to the iron for It back-bonding than pyridine. Similar results from O2 and CO binding to three “hanging base” ferrous porphyrins were also reported by Momenteau.53 A reduction of the dissociation rate for oxygen was observed when imidazole was replaced by pyridine. This appears to be consistent with the greater basicity of the imidazole. In addition to electronic effects of axial ligands, steric effects are appreciable as well. The studies on O2 and CO binding to pocket Fe(II) porphyrin show that the O2 affinity is about 35 times lower using 1,2-dimethylimidazole as the axial ligand than using l-methylimidazole.36 The reduction in 02 affinity can be ascribed to the severe steric interaction between the 2- methyl group of imidazole and the porphyrin plane. Traylor and coworker studied 02 affinities of a series of appended-base porphyrins. In this system, increasing the rigidity, or shortening the length of the covalent linkage results in a decrease in 02 binding affinities.54 The heme protein crystal structures suggest that hydrogen bonding to the N-H of the proximal imidazole, by releasing electron density to the heme iron, should increase dioxygen affinity. From this point of view, Traylor et al. studied the O2 and CO binding 15 affinities of caped iron(H) porphyrins in the presence of internally hydrogen-bonded imidazole.55 However, they found that proximal hydrogen bonding has little effect upon 02 or CO binding affinity. The picket-fence iron porphyrin forms a very stable oxyheme in toluene at room temperature and has a long lifetime compared to those of the others. The high stability of the oxygen adduct was ascribed to the amides in the pickets. The roles of the amide groups for stabilizing the oxyheme have been debated for two decades. The hydrogen bonding between the terminal oxygen and the hydrogens of the amide residues, or the dipole-dipole interactions are known to contribute to the high dioxygen binding affinity.56 In order to quantify the effects of dipole-dipole interctions on heme ligand binding, Chang and coworkers reported the kinetics of O2 and CO binding to heme model compounds equipped with a range of groups of varying dipole moments positioned near the acitve site.57 They have found that the dipolar forces can play a significant role in regulating oxygen affinities of the hemes. Tsuchida et al. synthesized the iron complexes of double-sided porphyrins having four ester groups on each side of the porphyrin plane, and reported the kinetics of O2 binding (Figure 14c and l—4d).52 This model compound exhibits good stability to oxidation when exposed to oxygen at room temperature. The Pm of this bis-fenced iron(H) porphyrin with ester bulky groups was estimated to be 866 torr at room temperature using 1,2-dimethylimidazole as the axial ligand. Under the same conditions, the oxygen affinity of this model compound is lower than that of the picket-fence iron porphyrin having a more polar environment. Another factor, which causes the reduced dioxygen affinity, is a decrease in the basicity of the axial ligand. It has been reported that decreasing the basicity of the axial ligand results in a reduction in oxygen affinity. In this case, it appears that the unfavorable steric interaction between the axial base and the ester fence, as evidenced from small formation constants of the base and the heme, must play an important role in the reduced oxygen affinity. This is further confirmed by the oxygen-binding affinity of a later version of the double-sided iron 16 porphyrin in which a covalently linked imidazole was built into the porphyrin as the axial ligand.58 In this system, a Pm of 13 torr was reported, which is slightly higher than that of the tailed picket-fence heme of Collman. The O2 binding behavior of double-sided iron(II) porphyrin complexes modified by amide residues was also reported by Tsuchida.59 They demonstrated that increasing the local polarity of the binding site results in an increase in the O2 binding affinity, as reflected by the reduced dissociation rate. The hydrogen bonding between bound oxygen and the distal histidine residue in hemoproteins has been a focal point of interest. The first successful synthetic model compound which showed hydrogen bonding between the terminal oxygen and amide groups was reported by Momenteau and Lavalette (Figure 1-5a).53’6O The amide-linked basket handle porphyrin has a lO—fold higher oxygen binding affinity than its ether-linked analogue. The interaction was further confirmed by 1H and 170 NMR of the Fe-O2 moiety. Chang designed a series of Co(II) l-naphthyl porphyrins substituted with arnido, carboxy, and hydroxymethyl groups at the 8-naphthyl position and demonstrated their 02 binding behavior (Figure 1-5b).61 Thermodynamic results show that the presence of a protic group near the dioxygen binding site significantly increases the O2 adduct formation constant. The large gain in enthalpy, -22 kcal/mol and -13 kcal/mol for carboxylate and hydroxyl groups, respectively, indicates that intramolecular hydrogen bonding occurred upon the coordination of O2 to the active center. The large negative entropy is also consistent with the loss of rotational degree of freedom of the bound O2. Chang et al. also reported the O2 binding affinities of series tailed hemes equipped with appended polar groups near the coordination site.57 For models with protic groups, the 02 off rate is substantially reduced due to the hydrogen bonding with the bound 02. The stabilization energy of the hydrogen bonding was estimated to be 0.7 and 1.6 kcal/mol for an alcohol and a secondary amide, respectively. Another study of H-bonded oxyheme models was provided by Reed and coworkers (Figure 1-5c).62 The synthesis of a 17 Figure 1-5. Some examples of heme model compounds. 18 variety of picket fence porphyrins having one of the four pickets replaced by passive and protic groups and the thermodynamic and kinetic results of their 02 affinities were reported. A 9—fold increase, corresponding to a free energy difference of 1.3 kcal/mol, in 02 affinity of the phenylurea analogue compared to picket-fence porphyrin was observed. The studies on the synthetic model compounds having intramolecular hydrogen bonding with bound oxygen have provided us informative data. However, the geometry and orientation of the bound oxygen and the protic group near the active site are well understood. The X-ray crystal structures and neutron diffraction data of oxyhemoproteins showed that the N-H bond of the distal histidine is in fact restrained from optimal alignment for strong hydrogen bonding and is not coplanar with the Fe-O-O moiety. Rather, it is located off to the side so that the hydrogen bonding between the distal histidine and Fe-O2 is an oblique interaction.62 This indicates that the histidine residue can interact with both the iron-bound oxygen and the terminal oxygen. Recently, Chang et al. have prepared anthracene and naphthalene Kemp’s acid porphyrins and reported the oxygen binding behavior of the cobalt(II) complexes (Figure l-Sd and 1- 5e).63 In the naphthalene case, a 104-fold enhancement of the O2 affinity from the ester to the acid was observed. Furthermore, AH and AS are relatively small in naphthalene Kemp’s acid model compared to those in the naphthoic acid model, in which it has coplanar and inflexible Fe-O-O...H resulting in the highest gain in AH and the highest loss in AS. The high 02 affinity and small AH and AS demonstrate that a high 02 affinity does not necessarily come from a maximum Fe-O-O...H interaction since the smaller loss in entropy is obviously more than enough to compensate for the loss in enthalpy. As noted, one molecule of carbon monoxide is produced when one molecule of heme is catabolized by heme oxygenase. The heme proteins are capable of discriminating between dioxygen and carbon monoxide. In contrast, simple heme models are unable to mimic the O2/CO discrimination of hemoproteins and bind to CO with a much higher affinity than hemoproteins. Structural determinations and neutron 19 Ft. diffraction studies of the CO adducts of hemoproteins showed that the Fe-C-O unit was either tilted or bent by as much as 120-140°,64'66 whereas CO preferentially binds to the Fe(II) center of simple heme models in a linear fashion.67~68 However, recent high resolution x-ray crystal structures for the CO adducts of some sterically constrained porphyrins and hemoproteins showed that there are only small degrees of tilting and bending for the Fe-C-O unit.69'70 Recent spectroscopic studies also support this conclusion.71 The Fe-C-O bond in Mb was found to be oriented < 7° from the heme normal. It is widely accepted that the distorted binding of CO resulted from close contacts with distal residues in the proteins. These interactions are referred to as distal steric effects. The FeCO unit is forced to adopt a nonlinear geometry by steric interactions whereas the bound oxygen, being naturally bent, does not suffer these interactions, resulting in a discrimination between the binding of CO and O2 in hemoproteins. To mimic the steric discrimination of hemoproteins against the binding of CO many types of sterically hindered porphyrins have been synthesized. These include “cofacial” diporphyrin,21 and “strapped”,39’72»73 “picnic basket”,44’56 “pocket”,16v74 and “capped”75'77 porphyrins. The cofacial diporphyrin was first synthesized by Chang (Figure 1-3d).21 The O2 and CO binding to the diporphyrinatocopper-iron was studied in benzene solution containing excess N-alkylimidazoles. It was considered that the inert porphinato copper, covalently attached to the porphinato iron, protects the iron—oxygen adduct. Indeed, the oxyheme complex formed in the Cu-Fe dimers was so stable that the Pm values can be measured directly by gas titration. The most striking result is that distal steric hindrance can affect ligand binding. The CO-binding affinity of the Cu-Fe dimer was suppressed and was similar to that of Mb. The Fe-Fe dimer showed the same reduced CO-binding affinity, but it bound two CO molecules due to two binding sites. It also showed a strong cooperativity in the binding reaction. The cooperative parameter, n, was estimated for CO-binding to the Fe-Fe dimer to be 3.4. The studies on the CO and O2 binding of picket fence and pocket iron(H) porphyrins reported by Collman showed 20 fimfi‘l um _..‘ that decreasing the cavity size of the iron(II) pocket porphyrin series suppressed the CO affinities without substantially affecting o2 affinities.36 In the R state, the most encumbered pocket porphyrin has 60-fold lower CO affinity than that of picket fence complex. The kinetic data suggested that the reduced CO affinities were almost entirely reflected in the decreased association rates. In the case of O2 binding, both association and dissociation rates are reduced in similar extent resulting unchanged O2 affinities. This allows the model iron(H) porphyrins to discriminate between CO and O2 binding. Momenteau designed a series of strapped porphyrins in which the amount of central steric hindrance is modulated by means of an aliphatic chain of various length attached to the pyrrole carbons.39 The CO affinities in this system are reduced by several orders of magnitude. Kinetic studies showed that the reduction of CO affinties could be ascribed to the central steric interactions. Ibers and coworkers reported the binding behavior of l,2,4,5-linked capped model hemes having a cavity very near the limit to hold a linear Fe-C-O linkage (Figure l-3f).41 They found that the Pm values of O2 and CO binding at room temperature are 100 and 200 torr, respectively. The resultant M value is among the lowest obtained in model hemes and indicates that steric interactions inhibit CO binding. The C=O stretching frequency of 2014 cm-1, greater than those in other model compounds, is a clear indication of significantly reduced Fe back-bonding and hence of a stronger C=O bond. Recently Collman et al. synthesized a series of aza-crown-capped heme models and studied their steric interactions of gas binding (Figure 1-5f).75’78 The most striking result is that the Fe(II) complex of the “cyclam” capped porphyrin exhibits a normal 02 affinity and does not bind CO at all (at 1 atrn); the M value is estimated to be less than 0.007! The x-ray structure of the Zn complex showed that each amide nitrogen is coplanar with the three atoms covalently attached to it as well as the hydrogen-bonded cyclam nitrogen. The hydrogen bonding between amide hydrogen atoms and cyclam nitrogen atoms results in a less flexible cap, and the distance which the cap can span is reduced. Furthermore, the methylene groups just over the metal core have hydrogen 21 atoms positioned right at the axis perpendicular to the porphyrin and passing through the metal center. The naturally bent O2 ligand does not suffer the unfavored steric interactions, whereas the CO ligand is strongly distorted and hence destabilized. Catalytic Oxidation Dioxygen is both a terminal electron acceptor and a source of biosynthesis of various molecules in metabolic pathways. The four-electron reduction of dioxygen to give two molecules of water per dioxygen molecule represents the major source of energy in aerobic organisms. The use of dioxygen in biosynthesis involves the enzyme- catalyzed incorporation of one or both of the oxygen atoms of dioxygen into substrate. The enzymes involved in the activation of dioxygen are either monooxygenase or dioxygenase enzymes, depending on whether one or both oxygen atoms from dioxygen are incorporated into the substrate. The reactions of dioxygen with organic substrates are thermodynamically favorable, i.e., exothermic. However, direct reactions of dioxygen with organic substrates at ambient temperature are intrinsically slow, unless the substrate is a good reducing agent. If this were not the case, dioxygen would spontaneously react with organic substrates and would be harmful or fatal rather than useful for living organisms. To understand the sluggishness of dioxygen reactions with organic compounds, we must consider the kinetic barrier to these reactions. The low kinetic reactivity of dioxygen to organic compounds arises from its triplet ground state, i.e., it contains two unpaired electrons. Typical organic substrates have singlet ground states (no unpaired electrons) and the resulting products from their oxidation reactions also have singlet ground states. The reaction of triplet dioxygen with singlet organic compounds to give singlet products is a spin-forbidden process.79’80 One way of circumventing this barrier is via the spin allowed, but energy-demanding formation of an unstable triplet intermediate followed by a spin conversion to a singlet product. However, this reaction is highly endothermic for most organic compounds and occurs 22 only with easily oxidizable singlet organic molecules, such as reduced flavins. x112+o2 ——> [XH' +HO2I] —) XH-O2H (1-9) The reaction involves the formation of a caged radical pair, followed by spin inversion to singlet products. Another way to overcome the kinetic barrier for the reactions of triplet dioxygen is to include a transition metal ion such as iron or copper. Transition metals in appropriate oxidation states can react with dioxygen to form the corresponding dioxygen adducts which can participate in reaction pathways that result in the oxidation of organic substrates. In biological systems, the activation of dioxygen is achieved by utilizing metalloenzymes, many of which are heme-containing enzymes.81 Understanding how these metalloenzymes work and the design of synthetic model compounds has been a major research area in bioinorganic chemistry. . In this respect monooxygenases, in particular, cytochrome P450 monooxygenases have attracted much attention in recent years. In late 19505, it was shown that liver microsomes contained a CO-binding pigment which exhibited an unusual absorption band at 450 nm.82 A particularly important breakthrough for understanding the role of the CO-binding pigment was made by Estabrook and coworkers.83 They described the catalytic role for the pigment in microsomal drug and steroid hydroxylation and showed that the photochemical spectrum has a maximum absorption at 450 nm. In 1964, Omura and Sato demonstrated that the CO-binding pigment was a heme protein containing protoheme D( and assigned it the name “cytochrome P450”.84 The term cytochrome P450 was then widely adopted for this class of monooxygenases having a maximum absorption near 450 nm in the presence of carbon monoxide. Cytochrome P450 monooxygenases are now known to exist ubiquitously in nature. A great number of P450 monooxygenases have been isolated from a variety of mammalian tissues and organs as well as in plants, insects, yeasts, bacteria, and so on. These monooxygenases have been shown to play the central role in 23 the catalysis of a variety of important biosynthetic pathways, such as steroid hormone and prostaglandin biosynthesis, and detoxification of a wide range of drugs and xenobiotics.85 Examples of such reactions include hydroxylation of aliphatic and aromatic compounds, epoxidation of alkenes and arenes, amine oxidation, sulfide oxidation, and oxidative dealkylation of heteroatoms. Many of the P450 enzymes have been difficult to characterize because they are tightly bound to membranes in mammalian systems and consequently are relatively insoluble in aqueous solutions. Initial efforts to release the membrane—bound P450s from the membrane by detergent solubilization resulted in deactivation of these monooxygenases. However, a soluble bacterial P450 monooxygenase (P450cam) has been isolated from Pseudomonas putida.86 P450cam has been the model system from which many mechanistic, catalytic, and spectroscopic studies have been carried out including an X-ray structure determination. Much of our knowledge and current concepts of the mechanism of P450 catalysis are derived from P450cam. The first X-ray crystal structure of P450cam was reported by Poulos and coworkers in 1985.87 This enzyme consists of a single polypeptide chain having a triangular shape and a Fe-protoporphyrin IX nearly parallel to the plane of the triangle. The heme prosthetic group is deeply embedded in the hydrophobic environments with no covalent attachments between the porphyrin ring and the protein. One axial ligand bound to the iron is a cysteinyl thiolate, and there are no close contacts between the heme and amino acids in the distal site. In the resting state without bound substrate, the iron is predominently low-spin Fe(III), and a hydrogen-bonded network of six water molecules occupies the active site probably having a water molecule as the sixth coordination ligand.88 The starting point in the catalytic cycle of cytochrome P450 involves the binding of camphor to ferric P450, which results in the displacement of all of the water molecules from the active site as shown in Figure 1-6. The heme becomes five-coordinate which 24 H‘O’H N— N R-OH //\'|:e$// N"--—"'I N RH S \ %0 Cys/ o N——N N |——N \ / / 90%) and vinyltrimethylsilane (82%). The ee’s obtained for these two olefins exceed the highest values from any previously reported catalytic systems, 34 Figure 1-7. Some examples of chiral metalloporphyrins. 35 including the remarkable Mn(salen) derivatives. Results and Presentation Numerous sterically hindered iron(II) porphyrins have been synthesized. Each of these sterically hindered porphyrins is encumbered on at least one side of the heme. By studying hemoproteins and various model compounds, it has been shown that dipole- dipole interaction and hydrogen bonding of the bound dioxygen and the residues around the active sites strongly affect the oxygen affinity of the hemes. In view of this point, it is interesting to design a new type of model compound having nonpolar protected pockets on both sides of the porphyrin plane. Recently, Barton and Zard developed a convenient method for the synthesis of a-free pyrroles by the reaction of nitroalkenes and isocyanoacetates. In chapter 2, we describe the synthesis of a series of sterically hindered ,B-substituted bis-pocket porphyrins by using the methods of Barton-lard and of Ono- Maruyarna, and also the oxygen binding of the iron complexes. In spite of extensive studies on the oxidations of organic substrates catalyzed by metalloporphyrins, their use in shape-selective catalysis has been less explored. In chapter 3, the shape selectvities of epoxidation reactions catalyzed by the manganese complexes including a rigid porphyrin and the selective-ligation of the zinc complexes are described. In catalytic epoxidation and ligation, the substrate can approach the active site from the top or the side of the porphyrin plane. Only a limited number of ,6- substituted water-soluble porphyrins have been synthesized to date. Chapter 4 describes the synthesis of a water-soluble ,B-substituted porphyrin, and the electrochemical and ligation properites of its iron complex. Most chiral porphyrins prepared for assymmetric oxidations of alkanes and alkenes are planar. In Chapter 5, we describe the synthesis of nonplanar chiral porphyrins, and the correlation of the absolute conformation of the nickel complexes and their circular dichroism patterns. The manganese complexes of these chiral nonplanar porphyrins can be used as chiral catalysts. 36 Chapter 2 SYNTHESIS OF fl—SUBSTITUTED BIS-POCKET PORPHYRIN AND OXYGEN BINDING OF THEIR IRON (H) COMPLEXES Introduction The oxygen transport and storage functions of hemoproteins continue to be a topic of considerable interest. In order to elucidate the factors that influence the oxygen affinity, model compounds have been used extensively.51,63,72,147’148 One essential requirement for the synthetic models is their ability to bind oxygen reversibly. However, unhindered ferrous hemes are irreversibly oxidized rapidly in the presence of oxygen. Two general autoxidation pathways have been proposed: (1) p-oxo dimer formation and (2) H+ (H2O) catalyzed autoxidation as discussed in Chapter 1.23 In order to thwart ,u-oxo dimer formation leading to irreversible oxidation, Numerous sterically hindered iron(II) porphyrins have been synthesized, with either one face or both faces of the heme protected. As the single-face hindered hemes can still be oxidized when low concentration of axial base is used,72 attempts have been made to prepare doubly-shielded porphyrin complexes such as bis-pocket porphyrins,47 bis- fenced porphyrins,59a149 and basket-handle porphyrins.150 These model compounds have played an important role to increase our knowledge on hemoglobin. However, most such model complexes are based on mesa-substituted TPP. Only limited cases involve )6- substituted cyclophane porphyrins because of the synthetic difficulties and poor yield. 151 We have now developed a general method to synthesize a series of sterically hindered fl- substituted bis-pocket hemes that bind oxygen reversibly. Our approach gave a much higher yield than a previously reported mesa-substituted bis-pocket TTPPP.47 37 Results and Discussion Synthesis The structures of a series of fl-substituted bis-pocket porphyrins are shown in Figure 2-1. The synthesis of intermediate pyrrole is shown in Scheme 2-1. Aniline 1 was converted to the corresponding diazonium salt by reaction with sodium nitrite under acidic conditions. The diazonium solution was then treated with potassium cyanide in the presence of CuCN as the catalyst under alkaline conditions to give nitrile 2 in 71% yield. Nitrile 2 was reduced to aldehyde 3 in 90% yield by reacting with DIBAL-H. Aldehyde 3 was then condensed with nitroethane in the presence of NI140Ac to afford nitropropene 4 in an excellent yield. lH NMR of the singlet vinylogous proton at 6 8.08 ppm showed that the only product obtained had the nitro group trans to the aryl group. Empolying the method developed by Barton and Zard for the condensation of nitroalkene and isocyanoacetates,152’153 nitropropene 4 was treated with ethyl isocyanoacetate in the presence of excess DBU in THF at room temperature to give pyrrole 5 in 93% yield after addition of HClm). Pyrrole 5 contains two bromine substituents, which can be replaced by aryl groups using Suzuki cross-coupling with boronic acids.154 The aryl boronic acids used for Suzuki cross coupling reactions were prepared by the typical Grignard method (Scheme 2-2).155 The aryl bromides were first converted to Grignard reagents, and the reaction with trimethyl borate afforded arylboronic acids in excellent yields after hydrolyzing with 10% HClmq). The Suzuki coupling reactions were carried out in gently~ refluxing DMF with K2CO3 as base and Pd(PPh3)4 as catalyst under argon for 2 days to give the doubly coupled pyrrole as the major product in good yields (Scheme 2-3). A mono-coupled pyrrole was also obtained as the minor product, in which one bromine was substituted by an aryl group and the other bromine was replaced by hydrogen. The use of 4 instead of 2.5 equivalents of the boronic acid had little effect on the yield of the expected di-coupled product and did not suppress the formation of mono-coupled product. The TLC Rf’s of the di- and mono-coupled products are close to each 38 Ar = Q HMO ' H30 H3C Figure 2-1. Structures of sterically hindered fl-substituted bis-pockets porphyrins. Br Br 1. NaN02/H” DIBAL—H F NH2 F CHO 2. KCN CHZC'2 Br Br 1 2 3 Br F NH4OAc H DBU Br F \ + CNCHQCOZEI CH30H2N02 N02 THF / \ Br Br CH3 N C02Et 4 5 Scheme 2-1 Mg 1. B(OMe)3/THF Ar_Br Ar_MgBr : Ar-B(OH)3 THF 2. H+ 6 - 8 H3O s Ar = Hac+©— H30 Scheme 2-2 40 Br /\ N Br 002Et Ar Ar-B(OH)2 / \ Ar Pd(PPh3)4, Na2003/DMF, H20 N 002Et 9-12 . O 10 Ar = H3CO—Q H30 12 Ar = Hac+©> H30 Scheme 2—3 41 other. After column chromatography, recrystallization was necessary to separate the desired di-coupled product. As shown in Scheme 24, an alternative route for the synthesis of pyrroles with bulky groups was also tested. The bromine substituents of nitropropene 4 was converted to aryl groups using Suzuki cross coupling. Pyrrole 14, however, could not be obtained by the same procedure of condensation using DBU as base. We found no evidence for pyrrole formation even after 2 days of reaction in reluxing THF, and only recovered mostly unreacted starting material 13. The problem was partially solved by using a much stronger and expensive non-nucleophilic base than DBU to promote the condensation. Thus, in the presence of stoichiometric amount of phosphazene base P4-t-Bu, pyrrole 14 was obtained in about 35% yield. 156 For the synthesis of porphyrin from pyrroles such as 9, we started with the reduction of the pyrrole ester to a very reactive 2-hydroxymethylpyrrole followed by cyclization and oxidation. Initially, the pyrrole ester was reduced by LiAlH4 in THF;157 after quenching with water and extracting with ether, the mixture was evaporated to dryness. We found that some of pyrrole ester was overreduced to 2-methylpyrrole with LiA1H4 even when the reduction was carried out at low temperature. This resulted in a low yield on the cyclization. Therefore, we employed milder reductants such as DIBAL- H and Red-Al for the reduction of pyrrole ester. The use of 10 equivalents of Red-Al converted the pyrrole ester to the corresponding pyrrole alcohol as shown in Scheme 2-5. The conversion was tested under various conditions, and the best yield was obtained if the reaction was performed in ether for an hour at room temperature. Longer reaction times led to overreduction of the ester. The work-up step involved the slow addition of minimum amount of water at 0 °C to quench the excess Red-Al and then extraction of the product with CH2C12 or ether. The use of excess water made it difficult to separate the organic and water layers during the extraction step. In general, the reduction of the pyrrole ester to the pyrrole alcohol by Red—Al afforded a quantitative yield. Without 42 Ph H Ph CNCH2002E1 \ NC > Ph CH 2 base / \ Ph 3 N COgEt ‘3 14 Scheme 2-4 F Ar Red- Al / \ Ar 2:; (5 ‘,F N COzst CH:OH 9-12 1. CH2(OCH3)2, H” 2. 000 15 Ar _©_ 16 Al’ =H300‘Q" 17 Ar :0 O 18 Ar H=30$-< >— H3C Scheme 2-5 further purification, acetic acid was added to the pyrrole alcohol and the mixture was heated on steam bath in open air for 12 h. Under the reaction conditions, the porphyrinogen intermediate was converted to porphyrin without the use of oxidant such as p-chloranil. This approach can only be employed satisfactorily to give about 20% yields for less hindered porphyrins 15 and 16. However, the conversions of pyrrole 11 and 12 to porphyrin 17 and 18 respectively, were not successful under the same conditions. Recently, Ono et al. have reported the preparation of B-substituted porphyrins by the reduction of pyrrole esters with LiA1H4 followed by treatment with an acid catalyst and oxidation with chloranil or oxygen. 157 They found that the hydroxymethyl group at the pyrrole a-position was eliminated as formaldehyde by an acid catalyst. Additional formaldehyde is required to obtain a geod yield. Alternatively, they used dimethoxymethane as an equivalent of formaldehyde. A procedure similar to Ono’s method was employed for the synthesis of our highly hindered porphyrins (Scheme 2-5). The general procedure involved the reduction of the pyrrole ester to the pyrrole alcohol with Red-A1 instead of LiAlH4 in ether for 1 h at room temperature followed by extraction. Without further purification, the reactive pyrrole alcohol was immediately cyclized in CH2C12 in the presence of 3 equivalent of dimethoxymethane for 12 h using p-TsOH as catalyst and then oxidized by DDQ. The use of p-chloranil as the oxidant resulted in partial oxidation even after 12h at elevated temperatures. By employing this procedure, the yields for porphyrins 15 and 16 were 48% and 45%, respectively. Suslick et al. reported the synthesis of a mesa-substituted “bis-pocket” porphyrin, TTPPP, by the condensation of 2,4,6-triphenylbenzaldehyde with pyrrole in refluxing propionic acid. Under the best conditions, they obtained only 1% of the product.47 Our synthesis, obviously, is a much more practical approach to highly hindered porphyrin molecules. Encouraged by this success using Ono’s method, the same strategy was employed to synthesize porphyrins l7 and 18, which could not be obtained under acetic acid conditions. The yields for porphyrins l7 and 18 were 21% and 11%, respectively. In our system, the yield of porphyrin is related to the steric hindrance of the substituents on the terphenyl groups. For example, porphyrin 18 has the most crowded substituents and gives the lowest yield. The good solubility of these sterically hindered porphyrins in organic solvents, except for porphyrin 17, allowed us to easily purify and characterize them. The purification of these porphyrins involved chromatography and then recrystallization from CH2C12 and methanol. In general, porphyrins insoluble in most organic solvents became soluble upon treatment with CF3CO2H to convert the free bases to their protonated forms. However, this was not the case for porphyrin 17. As long as it precipitates it does not dissolve in organic solvents even in 30% CF3CO2H in CH2C12. Fortunately, porphyrin 17 is soluble in a solution of CH2C12 previously washed with concentrated Ham). This allowed us to purify porphyrin 17 by recrystallizing it several times from CH2C12 containing HCl, and methanol. In general, the chemical cyclization of monopyrroles substituted with the 2- hydroxymethyl group or other active methylene groups results in a mixture of the four possible regioisomeric porphyrins (Type I-IV) due to the scarnbling of dipyrrylmethane intermediates during the cyclization step. 158 Indeed, on the basis of proton NMR spectra of the porphyrins whose mesa-protons appeared as a multiplet, scrambling produced a trace amount of a mixture of isomers. However, the undesired isomers can be removed by recrystallizing the products from CH2C12 and methanol as evidenced by a sharp singlet for mesa-protons. The amount of undesired isomers were somewhat dependent on the bulkiness of the pyrrole. This suggested that the steric bulk of the aryl group has a major influence in directing the pyrrole cyclization as less hindered aryl substitutents invariably result in a mixture of substitution patterns. 45 The conversion of a pyrrole ester into the corresponding porphyrin was also tested by other means. For example, the pyrrole ester was hydrolyzed and decarboxylated under alkaline condition, and then treated with formaldehyde in refluxing benzene in the presence of acid followed by oxidation with DDQ. However, the expected porphyrins could not be obtained in good yields by this procedure. As shown in Sheme 2-6, iron was inserted into porphyrins 15, 16, and 18 by treatment with FeBr2 in refluxing DMF overnight followed by column chromatography and washing with 5-10% HCl(aq) until the axial ligand was substituted by chloride as evidenced by UV-vis. The preparation of the iron complex of porphyrin 17 can not be accomplished by this method, but can be achieved with Fe(CO)5 and 12 in refluxing toluene followed by chromatography, washing with 10% HCl(aq) gave 78% of the iron porphyrin chloride. Using the synthetic strategy described, we successfully introduced various substituted terphenyl groups into porphyrins. These substituents fine-tune the porphyrin properties and influence the oxygen binding affinity. The preparation of ferrous porphyrins could not be achieved by the usual method with Na2S2O4 in water-toluene solution under anaerobic atmosphere}59 but was successful with Red-Al (scheme 2-7). However, when a large excess Red-A1 was employed, ring reduction took place as evidenced by UV-vis spectra. In order to prevent ring reduction, benzophenone was chosen to quench excess Red-A1. The UV—vis spectra of the 1-MeIm-iron(11) porphyrin complexes obtained by treatment with Red-A1 were identical to that obtained by spectroelectrochemical method. The equilibria between iron(II) prophyrins and axial ligands are as follows: KB FeP(B) (2-1) B FeP+B FeP(B) + B FeP(B)2 (2-2) where P represents the porphyrinato ligand, and B is an axial ligand. In this study, 1- 46 FeBr2/DMF 20 Ar $3000— H30 ' H3C Scheme 2-6 47 Fe(CO)5/Toluene methylimidazole (l-MeIm) and l,2-dimethylimidazole (1,2-Me2Im) were chosen as axial ligands. Previous work has shown that for unhindered imidazoles iron(II) porphyrin complexes bind the second axial ligand more strongly than the first one (KBB > K3) for unhindered imidazoles.37116ovl61 In the case of the sterically hindered base 1,2-Me21m, iron(H) porphyrins form five-coordinate adducts cleanly (KBB << Kg) due to the repulsive interactions between 2-methyl group on the axial ligand and the electrons of the porphyrin ring. Therefore, 1,2-dimethylimidazole has been used to mimic T-state hemoglobin.162 Suslick et al. showed that the equilibrium constant, KBKBB, for binding two l-methylimidazoles is about 5 x 109 M'2 for FenTI‘PPP.47 Attempts to obtain the equilibrium constants by spectrophotomeric titration with l-MeIm for our iron(II) porphyrins were not successful. Therefore, we switched to electrochemistry for the investigation of 1-MeIm binding affinity in this system. Electrochemistry of Iron Porphyrin 20 In order to investigate the binding ability of the iron(II) porphyrins to 1-MeIm, cyclic voltammetry was employed. Figure 2-2 shows the cyclic voltammograms of iron porphyrin 20 in CH2C12 in the presence of various concentrations of l-MeIm. In the "V" at -0.55 V was observed. absence of l-MeIm an irreversible reductive peak of Fe When the concentration of 1-MeIm increased, a new redox couple at -0.29 V formed and the reductive current at -0.55 V decreased. When the ratio of porphyrin 20 and l-MeIm was more than 2, the irreversible reductive current for Fen"ll disappears. The CV’s of iron porphyrin 20 remain unchanged as the concentration of 1-MeIm reaches 0.1 M. The results are consistent with KBB > K3, in that we did not observe two separate steps of 1- MeIm ligation to the iron(II) porphyrin. In the presence of oxygen the oxidative peak of Fe(II) at 028 V separated to two peaks as shown in Figure 2-2. The first, with E22,, at - 0.23 V involved the oxidation of iron(H) center, the other one with E3p at -0.08 V is assigned to the oxidation of the oxygen iron(II) porphyrin adduct. Compared to the first 48 F H3C -_-H co—Q— .: moéO— 20 Ar 3 22 Ar H C 3 Red-AI / N828204 / F H3C ... = EEC->0 24 Ar _H3CO-©— 26 Ar H C Scheme 2-7 49 a, E. Wig/‘9 (we/>7 .1? £49 I l ' l 1 l l I +0.2 0.0 -0.2 -0.4 -0.6 E (V vs. Ag/AgCl) I Figure 2-2. Cyclic voltammograms of iron porphyrin 20 in 0.1 M solution of TBAP in CH2C12. [20]:[1-Me1m] = (a) 1:0; under N2; (b) 1:0.5; under N2; (c) 1:1; under N2; ((1) 1:1.5; under N2; (e) 1:2; under N2; (1) 1:2; under 02; (g) 1:2; after holding the potential at —0.5 V for 2 min under 02. 50 oxidative current, the second one is more pronounced when the potential is held at -0.55 V for 2 minutes. Based on CV’s of iron porphyrin 20 in O2-saturated and N2- saturated CH2C12 solutions, the electrochemical reactions of iron porphyrin 20 can be expressed by the following equations: FemP(l-Melm)2 + e‘ Fe"P(1-Melm)2 13" = -O.28 v (23) K0 l=e"i>(1-Meltn)2 + 02 ___2_ Fe"p(1-MeIm)(02) (24) FenP(1-Me1m)(02) + l-MeIm =FemP(I-Mehn)2 + 02 + e' E“, = -0.08 v (25) where P is the dianion of porphyrin 16. The results also indicate that a stable oxygen iron(II) porphyrin adduct can be obtained. 02 Binding All of the iron(II) porphyrins used for O2 binding studies were prepared in situ by reduction of the corresponding iron(III) porphyrins with a minimum amount of Red-Al. When a large excess Red-Al was present, porphyrin ring reduction sOmetimes was evidenced by UV-vis spectra. In order to prevent ring reduction benzophenone was chosen to quench excess Red-A1. In the presence of 1-MeIm or 1,2-Me2Irn the UV—vis spectra of the iron(II) porphyrins obtained by this method were identical with that obtained in electrochemical cells. Oxygenation at 25 °C were achieved by all the iron(II) porphyrins with l-MeIm or 1,2-Me21m as the axial ligand. The stability of the oxygen adducts and oxygen affinities of the iron porphyrins were dependent on the concentration of the axial ligands. When 2.5 equivalents of l-MeIm were used, no clean isosbestic points were observed. An increase in the concentration of l-MeIm resulted in a decrease in oxygen affinity. Therefore, 5 equivalents of 1-MeIm were chosen as the optimum ligand-to-heme ratio to achieve reproducible Im-Fe-O2 formation and to maximize oxygen affinity. With 1,2-Me2Im as the axial ligand, high concentrations of the base must be employed. When the concentration of 1,2-Me2Im is less than 0.1 M, no well-defined 51 isosbestic points were observed. Figure 2-3 shows the spectral change in the Q band of iron(II) porphyrin 25 in toluene solution in the presence of 5 equivalents of 1-MeIm upon exposure to dioxygen. When the O2 pressure increases, the peaks at 534 and 562 nm shift to 548 and 580 nm with isosbestic points at 512, 540, 554 and 570 nm. With 1,2-Me21m as the axial ligand, the Soret band at 444 nm shifts to 426 nm upon addition of 02 as shown in Figure 2-4. The oxygen adducts readily changed to the corresponding CO adducts upon bubbling carbon monoxide gas in the solution. Removing 02 with freeze- pump-thaw cycles restored about 90 % of the deoxy adducts. The stability of the O2 adducts of these highly shielded hemes is quite remarkable with the half-life of heme 23 more than 2 h at the half-saturation O2 pressure, and much longer for the more bulky 24, 25 and 26. Table 2-1 shows the Pu202 for these iron(H) complexes under various titration conditions. The oxygen affinities of these hemes are necessarily low in comparison with myoglobin and its model compounds because of the hydrophobic nature of the heme center created by the nonpolar shielding wings. The oxygen affinity of heme 23 using 1,2-Me2Im as base (Pl/2O2 = 467 torr) is close to that of the mesa-substituted bis-pocket porphyrin, Fe(H)TI‘PPP, reported by Suslick, in which P1002 is 508 torr.47 Previously, it was established that Fe-O2 binding is enhanced by H-bonding or dipolar interactions present near the heme center and that increased solvent polarity results in higher 02 affinities of model compounds due to the stabilization of the expected charge separation in such complexes.57v163 Indeed, solvent effects were also observed in our heavily shielded iron porphyrins: as solvent polarity increases, 02 affinity increases. The fact that solvent may play a role in governing the O2 affinity can be seen in another way. Among our hemes, the order of oxygen affinity is 26 > 25 > 24 > 23, suggesting that the size, or the degree of shielding, has an effect on the O2 binding. Since the polarity at the heme micro-environment cannot differ too greatly, the higher oxygen affinity associated with the bulkier wings is potentially due to two factors: the planarity effect and the solvation effect. The planarity effect was suggested for certain ortho- 52 1‘2 562 1.1 70° a 600i "9 b 500- n0.9 c 0.3400. :1 . 11:6.8- ”L :3: 33-?“ d 100- 59-6- e 580 o . . QB 5' 0 500 1000 1500 3 - P02/AA ”0.4' .D 18.3“ 8.2“ 0.1' 0.3 v . , . f, 1 580 558 880 650 738 Hevolengthtnm) Figure 2-3. Spectral changes of iron(II) porphyrin 25 at 25 °C in toluene in the presence of 1- MeIm upon addition of O2. [25]:[1-Melm] = 1:5, P02 = (a) 0; (b) 20.3; (c) 50.7; (d) 152; (e) 304; (f) 608 torr. Inset: Plot of P02 versus PO2/AA at 562 nm. 53 (to Hammad!» Hbeorbencetfll) 9 9 9 «b C! O O 0 N 400 458 500 556 600 “i Have I ength (he) Figure 2-4. Spectral changes of iron(II) porphyrin 25 at 25 °C in toluene in the presence of 0.3 M 1,2-Me2Im upon addition of 02. P02 = (a) 0; (b) 10.1; (c) 25.3; (d) 50.7; (e) 101.3; (1) 202.7; (g) 354.7; (h) 608 torr. Inset: Plot of P02 versus P02/AA at 426 nm. 54 Table 2-1. P1/2 of dioxygen-binding for iron(II) porphyrin complexes Fe(II) porphyrin Solvent Ligand“ P 1,202 (torr) P 1,2C°(torr) 23 Toluene l-MeIm 583 Toluene 1 ,2-Me2Im 467 0.050 Chlorobenzene l-MeIm 200 24 Toluene l-MeIm 214 Toluene l ,2-Me2Im 304 0.045 Chlorobenzene l-MeIm 56 25 Toluene l-MeIm 98 Toluene 1,2-Me2Im 144 0.026 Chlorobenzene l-MeIm 20 26 Toluene l,2-Me2Im 10.3 0.008 a [FeP]:[l-MeIm] = 1:5; [l,2-Me2Im] = 0.3 M. 55 substituted tetraphenylporphyrin (TPP) in which the ortho groups, by virtue of steric interactions with the porphyrin plane, hinder the porphyrin ring doming or deformation.164 The result is an increase in ligand binding at the sixth site. Since porphyrin 26 shows steric interactions among the shielding wings and produces a tighter enclosure above and below the plane, the porphyrin ring therefore could be more resistant to dorning as compared with the other three. Collman et al., on the other hand, proposed that with “flat” iron(II) porphyrins (e.g., FeTPP or “chelated mesoheme”), the unligated five—coordinate species might be subject to a stronger solvation stabilization than the protected “picket-fence” complexes (Figure 2-5), thus extra solvent reorganization energy is required for flat hemes changing from five-coordination to six-coordination, resulting in lower oxygen affinities.43 The higher oxygen affinities for 25 and 26, compared to 23 and 24 could be similarly due to the more effective disruption of the solvent shell by the bulky wings surrounding the heme micro-enviroment. Judging from the X-ray structures as described below, it seems that these ,B-substituted shielding wings still have some degree of flexibility to rotate, the planarity effect therefore cannot play a major role to influence the ligand binding affinity. Previous work has shown that more than a 10-fold decrease in oxygen affinity was observed when the axial ligand changed from l-MeIm to 1,2-Me21m.36152 However, the oxygen affinities using l—MeIm and l,2-Me2Irn as bases in our case are in the same order since the binding of dioxygen molecules must compete with that of 1-MeIm when using l-Me-Im as the base. Therefore, the use of a low l-MeIm concentration would minimize the competition with dioxygen ligand and increase the dioxygen affinities. C0 binding The conditions used for C0 binding affinities of these iron(H) porphyrins were similar to those for 02 binding studies. The values of Pmco binding were measured spectrophotometrically by the addition of various quantities of 1% C0 in N2 using 1.2- 56 \ _____ / /—\\\ / _ —————— / \\ _____ / \ __—'—__ L o L fi 3 B /—\\ /// /—\\\\\ /—\ \ / \ L 1lull B \ \\ \_/// \ \\—/// \\ \_:/ Figure 2-5. Schematic representation of solvation effects of iron(II) porphyrins. Upper scheme, simple porphyrins; lower scheme, protected porphyrins. 57 Me21m as base. Figure 2-6 shows the Soret band change of 25 in toluene solution in the presence of 0.3 M l,2-Me2Im upon exposure to C0. When the C0 pressure increases, the peak at 442 nm shifts to 426 nm with isosbestic points at 432 and 464 nm. Table 2—1 contains the values of P1/2Co of the iron(ID porphyrins. As noted, the nonpolar nature of the binding site of our bis-pocket porphyrins resulted in lower oxygen affinities compared to the porphyrins having a polar active site. In contrast, the C0 affinities are not decreased related to other iron(II) porphyrins indicating little charge separation for Fe-CO binding. For example, the Plum of Fe'ITPPP(1,2-Me21m) with a nonpolar binding site is 9.1 x 10'3 torr and the PmCO of FeTpivPP(1,2-—Me2Im) with a polar binding site is 8.9 x 10'3 torr.37,47 The data in table 2-1 show that increasing the steric hindrance in the bis- pocket porphyrin series increases C0 affinities. This can also be explained by planarity and solvation effects encountered in the case of 02 binding affinity studies. X-ray Crystal Structures The crystal structure of porphyrin 16 is shown in Figure 2-7. Table 2-2 shows the crystal data and refinement parameters. Table 2-3 lists the average out-of-plane displacements, bond lengths, and bond angles. As Figure 2-7 shows, the macrocycle of pophyrin 16 is basically planar. The average displacement from the 24-atom mean plane is only 0.021 A for nitrogen, 0.051 A for cm... 0.043 A for c... and 0.047 A for c2 atoms. The core size (defined as half of the average distance between opposite nitrogens) is 2.028 A, which is slightly shorter than those of H2TPP (2.099 A) and H20EP (2.098 A).165 In 1995, Chang and coworkers reported the synthesis and the structure of a bis-pocket porphyrin without mesa-substituents.”6 The 2,6- isopropylphenyl groups at ,B-positions are nearly perpendicular to the porphyrin plane. For porphyrin 16, the phenyl groups at ,B-positions rotate toward porphyrin plane. The average dihedral angle of the porphyrin plane and the phenyl groups is 663°. The dihedral angles between adjacent phenyl groups range from 49 to 54° and the two ortho- 58 426 , T is - 1 ‘ A 0.5- is- 3.; .. Zr: 0 _ - 440 i3 J. 8’ 05 .. ..1 8 r: ' -1 - “I fi 10— g -1.5 . ‘ -3 -2 -1 o < _ L09 Pc i I I l 400 450 500 550 600 650 Wavelength (nm) Figure 2-6. Spectral changes of iron(II) porphyrin 25 in toluene in the presence of 0.3 M 1,2—Me2Im upon addition of C0. The following pressures of CO were used: 0, 0.001, 0.0025, 0.005, 0.01, 0.02, 0.04, 0.10, 0.25, 2.5 torr. Inset: Plot of Log [(Ao-A)/(A—A...)] versus Log Pco at 440 nm. 59 I- E 1' 1‘ ‘ ll/ 13w $1") in in ... ‘3‘“; 0(2) 0(3) 1. .1, 014A) :_ ‘0 011A) 0(1) (9 1- 013A) F111 1. I. = . Em “i“ Figure 2-7. X-ray crystal structure of porphyrin 16. Hydrogen atoms have been omitted for Clarity. 6O Table 2-2. Crystal data, Intensity Measurements, and Refinement Parameters for Porphyrins 16 and 21. Porphyrin 16 Porphyrin 21 Formula C110H83N403F4C112 FeC144H96N404F4C1 Formula Weight 2095034 2113.66 Crystal System Monoclinic Monoclinic Space Group P21/n Cc Temperature, K 298 298 a, A l4.352(3) 25.800(5) b, A 18.684(3) 21.447(4) c, A 20.259(5) 21.936(4) [3, deg 97.333(23) 103.33(1) v, A3 5,388.1(19) 11,811(3) Z 2 4 Crystal Dimensions, mm 0.4 x 0.5 x 0.5 0.05 x 0.1 x 0.2 0,3,3, g cm" 1.292 1.189 p, crn‘l 0.784 17.37 F(000) 2,164 4,396 26 (max), deg 45 120.2 Scan Type 6726 a) Scan Width, deg (0.65 + 0.35 tan 6) (1.21 + 0.3 tan 6) No. of Unique Reflections 7,026 9,059 No. of Reflections Observed 2,770 5,114 No. of Parameters 641 1,247 R; Rwal 0.100; 0.079 0.108;0.069 S 2.34 5.16 aR=izlll=ol-|1=c| ilZIFo|,Rw=[Ew(iiFo|-1Fci If/prozim. 61 Table 2-3. Average Out-of-Plane Displacements, Bond Lengths, and Bond Angles for porphyrins l6 and 21. Porphyrin 16 Porphyrin 21 Displacement (A)a Fe 0.5272 N 0.021 0.038 Ca 0.043 0.075 Cp 0.047 0.126 Cm 0.051 0.059 Bond Length (A) Fe-Cl 2.181 Fe-N 2.125 Cd-Cfi 1.444 1.410 CarCm 1.389 1.387 N-Ca 1.362 1.358 Cp-Cp 1.363 1.356 Bond Angle (deg) N-Fe-N 86.5 N-CaGC 110.4 108.9 Cd-N—Ca 106.3 107.3 Cm-Ca-Cp 124.9 126.9 N-Ca-Cm 124.6 123.8 Car-ny -Cp 106.4 107.0 Car-Cm -Ca 126.9 129.1 a From the least-square plane of the 24 atoms. 62 groups are approximately perpendicular to each other. The terphenyl group is about 6.4 A tall (from the porphyrin plane), which is sufficient to prevent the formation of p-oxo iron(III) dimer. This has been confmned by the dioxygen binding experiments described above. Figure 2-8 shows the X-ray crystal structure of porphyrin 21. The crystallographic data and structural summary are given in Tables 2—2 and 2-3, respectively. Surprisingly, in the presence of highly hindered substituents at ,B-positions the porphyrin macrocycle is only slightly distorted. The average deviation from the 24-atom mean plane is 0.038 A for nitrogen, 0.059 A for Cmm, 0.075 A for Ca, and 0.126 A for C5 atoms. The core size is 2.058 A. The out—of-plane distance of iron is 0.527 A and the distance of Fe-Cl is 2.181 A. As expected, the phenyl groups directly attached to porphyrin macrocycle rotate toward the ring to prevent the steric interactions between the highly hindered substituents. The average dihedral angle of the phenyl and the macrocycle planes is 624°, which is smaller than that for porphyrin 16. The dihedral angles between the adjacent phenyl groups range from 25 to 51° with an average of 405°. The hindered substituents at ,B-positions are about 7.9 A tall and the pockets are better protected than those in the case of porphyrin 16. Therefore, the iron(II) complex porphyrin 21 is more stable than that of porphyrin 16 upon dioxygenation. Conclusions We have described the synthesis of highly hindered flsubstituted bis-pocket porphyrins in which both sides of the porphyrin plane are protected to provide a nonpolar environment. The iron(II) complexes of these porphyrins are capable of binding dioxygen irreversibly at room temperature, indicating that the hindered substituents can prevent the irreversible oxidation of the coordination site via the formation of the p-oxo dimer. The decreased 02 affinities of these iron complexes are ascribed to the nonpolar nature of the binding site. This can be confirmed by the observation that increasing the 63 Figure 2-8. X-ray crystal structure of porphyrin 21. Hydrogen atoms have been omitted for clarity. polarity of solvents increases the 02 affinity. The results described here are consistent with the fact that in hemoproteins the polar environments near the active site can stabilize the oxygenated adduct and result in a higher 02 affinity. Experimental Materials All reagents and solvents were obtained from commercial sources and were used without further purification unless otherwise noted. Dry dichloromethane and Chlorobenzene were obtained by refluxing and distilling over CaHz. Dry toluene and THF were obtained by refluxing and distilling over sodium/benzophenone. 1,2- Dimethylimidazole and l-methylimidazole were distilled over sodium under reduced pressure. Silica gel for flash chromatography was 60-200 mesh, manufactured by Fisher Scientific. Analytical TLC was performed by using Eastman Kodak 13181 silica gel sheets. Compositions of solvent mixtures are quoted as ratios of volume. Instrumentation 1H NMR (300 MHz) spectra were recorded on a Varian Gemini spectrometer. Chemical shifts were reported in ppm relative to the residual proton in deuterated chloroform (7.24 ppm). Absorption spectra were recorded on a Shimadzu UV—160, Varian Carry 219, or HP 8452A spectrometer. Mass spetra were obtained on a benchtop VG Trio-1 mass spectrometer. FAB-MS mass spectra were obtained on a JEOL HX-110 HF double focusing spectrometer operating in the positive ion detection mode. Oxygen Affinity Measurements A solution of porphyrin Fe(III)Cl in solvent containing the appropriate concentration of 1-methylimidazole or l,2-dimethylimidazole was introduced to a three- armed tonometer previously flushed with argon. A small amount of Red-Al in toluene 65 was introduced to one arm by a syringe and benzophenone was placed into the other arm of the tonometer. After deaeration by freeze-pump—thaw cycle, iron(III) was reduced to iron(II) by mixing with Red-Al. Excess Red-Al was then quenched by benzophenone when the reduction of iron(IH) was complete. Spectral changes were recorded, with isosbestic points, by the addition of aliquots of oxygen from a gas manifold via a gas- tight valve to the solution. After equilibrium measurements were made, reversibility was checked by vacuum removal of oxygen; in all cases >90% reversibility was achieved. Oxygen partial pressures were determined from injections of known volumes of gas into the tonometer of known volume. The data from the spectrophotometric titrations were fitted to the equation described as follows. Oxygen binding of iron(II) porphyrins can be expressed by the following equationzzz’52 KO FeP(B) + 02 2 FeP(B)(02) (2-6) The data from the spectrophotometric titrations can be determined in two ways. (1) When the absorbance of the completely oxygenated adduct can be obtained, the data are fitted to the Hill equation: log[y/(1-y)] = log P02 - 108 Pl/Z (2‘7) where y equals the fraction of the total oxygenated adduct. Values for Pm were determined from the x intercept of the regression line for a plot of log [y/(l-y)] vs. log P02. (2) When only partial dioxygenation occurred at the highest 0; pressure that we used, the data were fitted by equation (2-7) employed by Collman et al.22 P02 = [Fe(P)]rbAfi(P02/AA) ' P1/2 (2'8) where Pm is equal to 1/Koz and is defined as the pressure of dioxygen at which one-half of the available bonding sites are oxygenated. Fe(P) is the total concentration of iron porphyrin, b is the cell path length, As is the difference in molar extinction coefficient between the oxy and the deoxy complexes, AA is the difference between the absorbance 66 at the particular P02 and the absorbance of deoxy complex at the same wavelength. Because [Fe(P)]TbAa is a constant, a plot of P02 vs. POz/AA gives a straight line with slope [Fe(P)]TbAe and intercept Pm. Pm values were determined at various wavelengths with large spectral changes. The deviation within each titration is less than 10 %. In all cases of oxygen titrations, well—defined isosbestic points were observed. Carbon Monoxide Affinity Measurements The experimental procedures used for these studies were similar to those described above for oxygen affinity measurements. Partial pressures of CO were continuously varied from 1 x 10'3 to 7.6 torr by the injections of 1% of CO in N2. The data from the spectrophotometric titrations were treated in one of two ways described above. Crystallography Crystals of porphyrins l6 and 21 were grown from the CH2C12/MeOH mixture and benzene, respectively, by slow evaporation. To prevent solvent evaporation, the chosen crystals were coated with hydrocarbon oil and mounted on glass fibers. The diffraction data of porphyrins 16 were collected on a Nonius CAD4 diffractometer with graphite- monochromated Mo Koc radiation. The diffraction data of porphyrins 21 were collected on a Rigaku AFCSR diffractometer with graphite-monochromated Mo Kat radiation. The crystal structures were solved by direct methods and refined by full matrix least-squares using NRCVAX program. All non-hydrogen atoms were refined anisotropically, while hydrogen atoms were refined isotropically. Crystallographic details for the structures are given in Table 2-2. 2,6-Dibromo-4-fluorobenzonitrile (2) Sodium nitrite (15.18 g, 0.22 mmol) was added portionwise to a magnetically stirred concentrated sulfuric acid (100 ml) at 0 °C. The resulting mixture was allowed to warm 67 to 55 °C and then held at room temperature. This nitrosyl sulfuric acid was then added dropwise to an acetic acid (110 ml) solution of 2,6-dibromo-4-fluoroaniline (53.6 g, 0.2 mol) at 20 °C. After stirring for l h the diazonium solution was added dropwise to a mechanically stirred solution containing KCN (65.12 g, 1 mol), CuCN (21.49, 240 mmol), and Na2C03 (340 g, 3.2 mol) in 1 l of water at 0 °C. When the addition was complete, the mixture was stirred at room temperature for l h. The mixture was then filtered, washed with water and dried. The resulting crude product was dissolved in benzene and insoluble solid was removed by filtration. Benzene was removed under reduced pressure and the solid was chromatographed on silica gel eluting with CH2C12 and hexanes (1:1) to afford 35.9 g (71%) of nitrile 2. mp. 103-105 °C; 1H NMR (300 MHz, CDC13): 6 7.41 (2H, (1); MS found m/e 278.8, cacld. 276.85 for CszBrzFN. 2,6-Dibromo-4-fluorobenzaldehyde (3) To a solution of aniline l (2.78 g, 10 mmol) in 20 ml CH2C12 was added DIBAL-H (1 M solution in hexane, 12 ml, 12 mol) at 0 °C under argon. The solution was stirred at room temperature for 4 h and then poured into 30 ml 6 N HCl(aq) in an ice bath. After stirring for 1h, the mixture was extracted with CH2C12. The dichloromethane solution was dried over anhydrous sodium sulfate and then the solvent was removed under reduced pressure. The crude product was purified by flash chromatography on silica gel eluting with CH2C12 and hexanes (1:1) to give 2.54 g (90%) of aldehyde 3. mp. 85-86 °C; 1H NMR (300 MHz, CDC13): 6 10.20 (1H, s, CH0), 7.43 (2H, s, phenyl); MS found m/e 282, cacld. 279.85 for C7H3Br2FO. Ethyl 4-methyl-3-(2,6-dibromo-4-fluorophenyl)-2-pyrrolecarboxylate (S) The mixture of aldehyde 3 (2.8 g, 10 mmol) and NH40Ac (1.1 g, 14 mmol) in 12 ml nitroethane was refluxed under argon for 12 h. The solution was cooled to 20 °C and then the salt was filtered. Excess nitroethane was removed in vacuo. The crude product 68 was purified by flash chromatography on silica gel, eluting with CH2C12 and hexanes (3:7) to give orange-yellow oil of nitropropene 4 (2.6 g, 76%). Ethyl isocyanoacetate (0.95 g, 8.4 mmol) was added to a THF solution (10 ml) of nitropropene 4 (2.6 g, 7.6 mmol) cooled in an ice bath under argon. To the solution was added dropwise DBU (2.1g, 15.2 mmol) in 10 ml THF. The mixture was stirred at room temperature for 24 h, and then poured into 100 ml of 6 N HCl(aq). The solution was stirred for 5 min, filtered, washed with water, and then dried under reduced pressure to give 2.86 g (93%) of pyrrole 5. mp. 161-162 °C; 1H NMR (300 MHz, CDClg): 6 9.05 (1H, br 5, NH), 7.37 (2H, d, phenyl), 6.82 (1H, d, pyrryl), 4.09 (2H, q, CH2), 1.85 (3H, s, CH3), 1.04 (3H, t, CH3); MS found m/e 405, cacld. 402.92 for C 14H12Br2FN02. 4-Methoxyphenylboronic acid (6) Mg (2.67 g, 0.11 mmol) was placed in a 500 ml three-necked round-bottomed flask equipped with a reflux condenser. The system was flushed with argon for 20 min while the Mg and flask were heated with a heating mantle. The apparatus was cooled and a solution of 4-bromoanisole (18.7 g, 0.1 mol) in THF (200 ml) was added from an additional funnel. The mixture was heated to initiate the reaction. The remaining solution of 4-bromoanisole was added rapidly enough to maintain a gentle reflux. The solution was refluxed for 4 h and then transfer to an additional funnel. To a solution of trimethyl borate (10.39 g, 0.1 mol) in THF (50 ml) cooled at —78 °C was slowly added the Grignard reagent under argon. The solution was stirred overnight while warming up to room temperature slowly. After acidified with 100 ml of 10% HCl(aq), the product was extracted with 200 ml of ether three times, and dried over anhydrous Na2SO4. The solvent was removed under reduced pressure. The solid was then precipitated from actone and water to give 8.05 g (53%) of boronic acid 6. 1H NMR (300 MHz, CDC13)I 6 8.18 (2H, d, phenyl), 7.02 (2H, d, phenyl), 3.90 (3H, s, CH3). 69 Biphenylboronic acid (7) A procedure similar to that used for the synthesis of boronic acid 6 was followed to give the product in 70% yield. mp. 255-257 °C; 1H NMR (300 MHz, CDC13): 6 8.33 (2H, d), 7.75 (2H, d), 7.82-7.58 (4H, m), 7.52-7.34 (3H, m). 4-(t-butyl)phenylboronic acid (8) A procedure similar to that used for the synthesis of boronic acid 6 was followed to give the product in 58% yield. mp. 202-203 °C ; 1H NMR (300 MHz, CDC13): 6 8.19 (2H, d, phenyl), 7.55 (2H, d, phenyl), 1.40 (3H, s, CH3): Ethyl 4-methyl-3-(2,6-diphenyl-4-fluorophenyl)-2-pyrrolecarboxylate (9) The mixture of pyrrole 5 (1g, 2.47 mmol), phenylboronic acid (750 mg, 6.18 mmol), and Pd(PPh3)4 (200 mg, 0.17 mmol) in 25 ml of DMF containing 5 ml of 2 M Na2CO3(aq) was purged with argon for 10 min. The solution was gently refluxed under argon for 2 d. The reaction mixture was then cooled to room temperature and inorganic solids were removed by filtration. The filtrate was concentrated in vacuo, and then extrated with CH2C12. The solvent was dried over anhydrous Na2SO4 and removed under reduced pressure. The product was purified by column chromatography on silica gel eluting with CH2C12 and hexanes (4:6). The crude product was recrystallized from methanol to give 620 mg (63%) of pyrrole 9. mp. 180-181 °C; 1H NMR (300 MHz, CDCl3): 6 8.53 (1H, br 8, NH), 7.18-7.04 (12H, m, phenyl), 6.41 (1H, d, pyrryl), 4.01 (2H, q, CH2), 1.52 (3H, t, CH3); MS found We 399.3, cacld. 399.16 for C26H22FNO2. Ethyl 4-methyl-3-(2,6-bis-(4-methoxylphenyl)-4-fluorophenyl)-2-pyrrolecarboxylate (10) A procedure similar to that used for the synthesis of pyrrole 9 was followed to give the product in 68% yield. mp. 158-160 °C; 1H NMR (300 MHz, CDC13): 6 8.90 (1H, br s, 70 NH), 7.03 (2H, d, phenyl), 6.98 (4H, d, phenyl), 6.68 (4H, d, phenyl), 6.44 (1H, d, pyrryl), 4.01 (2H, q, CH3), 3.74 (6H, s, CH3), 1.52 (3H, s, CH3), 1.05 (3H, t, CH3); MS found m/e 459.3, cacld. 459.18 for C23H26FNO4. Ethyl 4-methyl-3-(2,6-bis-(4-biphenyl)-4-fluorophenyl)-2-pyrrolecarboxylate (11) A procedure similar to that used for the synthesis of pyrrole 9 was followed to give the product in 92% yield. mp. 230-232 °C; 1H NMR (300 MHz, CDC13): 6 8.59 (1H, br s, NH), 7.56 (4H, d, phenyl), 7.46-7.35 (8H, m, phenyl), 7.34-7.26 (2H, m, phenyl), 7.16 (4H, d, phenyl), 7.15 (2H, d, phenyl), 6.45 (1H, d, pyrryl), 4.05 (2H, q, CH2), 1.58 (3H, s, CH3), 1.09 (3H, t, CH3); MS found m/e 551.4, cacld. 551.23 for C33H30FNO2. Ethyl 4-methyl-3-(2,6-bis-(4-t-butylphenyl)-4-fluorophenyl)-2-pyrrolecarboxylate (12) A procedure similar to that used for the synthesis of pyrrole 9 was followed to give the product in 74% yield. mp. 190-192 °C; 1H NMR (300 MHz, CDC13): 68.53 (1H,br 8, NH), 7.13 (4H, d, phenyl), 7.06 (2H, d, phenyl), 6.99 (4H, d, phenyl), 6.42 (1H, d, pyrryl), 3.99 (2H, q, CH2), 1.51 (3H, s, CH3), 1.24 (18H, s, CH3), 1.09 (3H, t, CH3); MS found We 511.5, cacld. 511.29 for C34H33FNO2. 2,7,12,17-Tetramethyl-3,8,13,lS-tetrakis(2,6-diphenyl-4-fluorophenyl)porphyrin (15) Method a: To a 30 ml ether solution of pyrrole (399 mg, 1 mmol) was added Red-Al (3 ml, 65% in toluene, 10 mmol) dropwise at 0 °C under argon. When the addition of Red-Al was complete, the mixture was returned to room temperature and stirred for 1 h. The excess Red-Al was destroyed by the addition of ice at 0 °C, follow by 5 ml water. The resulting mixture was extracted with 30 ml CH2C12 twice. The combined organic portions were washed with water, dried over Na2SO4, and concentrated under reduced pressure to give 71 yellow an oily residue. The crude product, without any purification, was dissolved in 20 ml acetic acid and heated over a steam bath in open air overnight. Solvent was removed in vacuo and then the residue was taken into dichloromethane, washed with water and purified by silica gel column chromatography eluting with CH2C12 to give the product as a purple solid. The solid was recrystalized from CH2C12 and methanol to afford 71 mg (21%) of porphyrin 15. 1H NMR (300 MHz, CDC13): 6 9.23 (4H, s, mesa), 7.44 (8H, d, phenyl), 7.01 (16H, d, phenyl), 6.65 (8H, t, phenyl), 6.55 (16H, t, phenyl), 2.85 (12H, 5, CH3), -4.24 (2H, br 3, NH); UV-vis (toluene, 714m nm (8)): 636 (6,400), 580 (9,000), 546 (14,800), 509 (19,200), 418 (162,200) ; MS (FAB): found We 1351.8, cacld. 1350.52 for C96H66F4N4- Method b: The same reduction procedure in method a was followed to obtain the alcohol which was then dissolved in 40 ml CH2C12. To the solution was added dimethoxymethane (0.28 ml, 3 mmol) and p-toluenesulfonic acid monohydrate (38 mg, 0.2 mmol). After stirring the mixture overnight under argon, DDQ (227 mg, 1 mmol) was added and the solution was stirred for a further hour. The solvent was removed in vacuo to give a dark residue. The residue was chromatoghraphed on silica gel eluting with CH2C12. The purple fraction was collected and recrystallized from CH2C12 and methanol to give 162 mg (48%) of porphyrin 15. 2,7,12,17-Tetramethyl-3,8,l3,18-tetra(2,6-bis-(4-methoxyphenyl)-4-fluor0phenyl) porphyrin (16) Method b was employed to give the product in 45% yield. 1H NMR (300 MHz, CDC13): 6 9.32 (4H, s, mesa), 7.38 (8H, d, phenyl), 6.92 (16H, (1, phenyl), 6.15 (16H, d, phenyl). 3.31 (24H, 3, CH3), 2.87 (12H, 5, CH3), -4.21 (2H, br 5, NH); UV-vis (toluene, Am, nm 72 (rel intens)): 638 (0.07), 582 (0.08), 549 (0.12), 512 (0.14), 421 (1.00); MS (FAB): found m/e 1591.3, cacld. 1590.61 for C104H32F4N403. 2,7,12,17-Tetramethyl-3,8,13,l8-tetra(2,6-bis-(4-biphenyl)-4-fluorophenyl)porphyrin (17) Method b was employed. After oxidation with DDQ, the mixture was passed through a short silica gel column eluting with CH2C12. The purple fraction was collected and evaporated in vacuo. The crude product was consecutively recrystallized from CH2C12, which had been washed with concentrated HCl(aq), and methanol to give the product in 21% yield. 1H NMR (300 MHz, CDC13): 69.33 (4H, s, mesa), 7.47 (8H, d, phenyl), 7.05 (16H, d, phenyl), 7.0-6.8 (40H, m, phenyl), 6.69 (16H, d, phenyl), 2.92 (12H, 3, CH3), - 4.03 (2H, br 8, NH); UV-vis (toluene, 24m nm (8)): 637 (7,300), 582 (8,700), 549 (14,300), 513 (17,200), 422 (135,400); MS (FAB): found m/e 1962.4, cacld. 1958.77 for C144H98F4N4- 2,7,12,17-Tetramethyl-3,8,13,l8-tetra(2,6-bis-(4-t-hutylphenyl)-4-fluorophenyl) porphyrin (18) Method b was employed to give the product in 11% yield. 1H NMR (300 MHz, CDC13): 6 9.30 (4H, s, mesa), 7.40 (8H, d, phenyl), 6.92 (16H, d, phenyl), 6.59 (16H, (1, phenyl), 2.87 (12H, 3, CH3), 0.78 (72H, 5, CH3), -4.18 (2H, br 5, NH); UV-vis (Toluene, km, nm (8)): 637 (5,800), 582 (7,200), 547 (12,600), 512 (16,000), 421 (152,000); MS (FAB): found m/e 1800.6, cacld. 1799.02 for C123H130F4N4. Fe(III) 2,7,12,17-tetramethyl-3,8,l3,18-tetrakis(2,6-diphenyl-4-fluorophenyl) porphyrin chloride (19) Free base 15 (27 mg, 0.02 mmol), anhydrous FeBr2 (86 mg, 0.4 mmol), and 0.5 ml of pyridine were added to 20 ml of DMF and refluxed under argon. After 12 h, the reaction 73 was completed as monitored by the UV-vis spectrum. The solvent was evaporated to a minimum amount in vacua and the iron porphyrin was purified over silica gel column eluting with CH2C12 and methanol (97:3). The collected iron porphyrin was washed twice with 5-10% HCl(aq) and dried with Na2SO4, and the solvent was evaporated under reduced pressure, yielding 20 mg (83%) of 19. UV-vis (toluene, km” nm (rel intens)): 641 (0.08), 546 (0.14), 513 (0.13), 418 (1.00), 393 (0.89); MS (FAB): found m/e 1405.5, cacld. 1404.44 for C96H64F4N4Fe. Fe(III) 2,7,12,17-tetramethyI-3,8,13,l8-tetrakis(2,6-bis(4-methoxyphenyl)-4-fluoro- phenyl)porphyrin chloride (20) A procedure similar to that of 19 was employed to give 20 in 78% yield. UV-vis(toluene, 24m nm (8)): 643 (5,700), 546 (11,700), 512 (11,800), 416 (75,000); MS (FAB): found We 1645.7, cacld.1644.53 for C104H30F4N403Fe. Fe(III) 2,7,12,l7-tetramethyl-3,8,13,l8-tetrakis(2,6-bis(4-t-butylphenyI)-4-fluoro- phenyl)porphyrin chloride (22) A procedure similar to that used for the synthesis of 19 was employed to give 22 in 76% yield. UV-vis(toluene, Mm nm (8)): 644 (4,600), 511 (11,400), 415 (86,400); MS (FAB): found m/e 1854.4, cacld. 1852.94 for C123H123F4N4Fe. Fe(IH) 2,7,12,17-tetramethyl-3,8,13,l8-tetrakis(2,6-bis(4-biphenyl)-4-fluorophenyl) porphyrin chloride (21) To a solution of the free base 17 (20 mg, 0.01 mmol) and iodine (5 mg, 0.02 mmol) in 20 ml of toluene was added iron pentacarbonyl (118 mg, 0.6 mmol) and the mixture was refluxed under argon. After 2 h, the reaction was completed as monitored by the visible spectrum. The solvent was evaporated in vacua and the iron porphyrin was purified over silica gel column eluting with CH2C12 and methanol (95:5). The collected iron porphyrin 74 was washed twice with 5-10% HCl(aq) and dried over anhydrous Na2SO4, and the solvent was evaporated under reduced pressure, yielding 16 mg (78%) of iron porphyrin. UV-vis (toluene, km, nm (8)): 643 (5,700), 546 (11,600), 514 (11,700), 421 (80,200); MS (FAB): found m/e 2014.4, cacld. 2012.69 for C144H96F4N4Fe. 75 Chapter 3 SHAPE-SELECT IVE EPOXIDATION AND LIGATION OF B-SUBSTITUTED PORPHYRINS Introduction Many sterically hindered metalloporphyrins, such as bis-pocket,47 basket handle,132’139 dendritic,137 steroidal,166 and othersl42’1453167’168 have been used to mimic the selective epoxidation of cytochrome P450 enzymes. The synthesis of such sterically encumbered metal centers for shape- or regio-selective catalysis continues to be of great interest. However, synthetic difficulty limits the use of sterically hindered mesa- substituted metalloporphyrins as shape-selective catalysts, although the presence of bulky substituents at the artha-positions of mesa-phenyl groups exhibits high stability during the catalytic reaction.116 In Chapter 2, we have described the synthesis of sterically hindered fl-substituted porphyrins. This new class of porphyrins that we employed for dioxygen binding is also used for shape-selective epoxidation catalysis. Figure 3-1 gives examples of selective oxidations. For example, under thermodynamic control, the internal C=C bond is more reactive than the external C=C bond when reacted with simple oxidants such as peracids. In contract, under the influence of catalysts, the selectivity of the catalysts may be controlled by the steric interactions and van der Waals contact between the substrate and the substituents at the periphery of the porphyrin. Therefore, these catalysts can selectively epoxidize the external C=C bond (Figure 3-2). In the catalytic reaction, the substrate might approach the active site from the top or the sides of the porphyrin plane. The selectivity is basically controlled by the accessibility to the active site or the cavity size of the steric superstructure which can be varied by synthesis. 76 HYDROXYLATION: allylic, benzylic > 3° > 2° > 1° less preferred 0 *' I”. I] ll preferred EPOXIDATION: more substituted > less substituted W ‘8 “ preferred less preferred Figure 3-1. The preferred oxidation sites of hydrocarbons under thermodynamic control. 77 lodosylbenzene major product Figure 3-2. Intra-molecular shape-selective epoxidation. 78 The introduction of different substituents should allow us to modify the structures of the sterically hindered ,B-substituted porphyrins, thus regulating their properties. However, the structures of the porphyrins are flexible due to the rotation or vibration of the substituents. To better control the micro-environment of the active site of the metalloporphyrin, it is necessary to reduce the flexibility of the porphyrin structure. In view of this point, it would be interesting to examine the activities of these catalysts as a function of the rigidity of the superstructure. We therefore designed and synthesized a rigid porphyrin in which the shielding wings are locked by intramolecular hydrogen bonding, thereby blocking the top entrance of the channel as well as limiting access via the sideway wings. In addition to the regioselective epoxidation reactions that can be demonstrated by using these hindered porphyrins as catalysts, their unique structure also proves to be useful as an example of molecular recognition. Molecular recognition of neutral molecules that control or initiate specific interactions is the essence of biological chemistry. In addition to their biological importance, porphyrins provide a potentially useful framework for artificial acceptors since they have a well-defined binding pocket, can be functionalized at the mesa- and fi-positions, and can be inserted with a number of metals. Thus, metalloporphyrins have been constructed as the acceptors for the recognition of small molecules such as carbohydrates,169 amino acid derivatives,170' 172 and quinone derivatives.173’174 However, shape-selective ligation has been less examined. 175,176 In this chapter, we describe the synthesis, characterization, and utilization of this new class of sterically hindered metalloporphyrins as catalysts for shape-selective epoxidation reactions. We also report the shape-selective ligation of the zinc porphyrins to substituted pyridines and alkyl amines. 79 Results and Discussion Synthesis Unlike mesa-phenyl bis-pocket porphyrin synthesized by Suslick et al., our 6- substituted porphyrins can be prepared in good yields as described in Chapter 2 and substituents with different hindrance can be easily introduced to the periphery of the porphyrins to control the cavity size. In this chapter, the synthetic efforts are focused on a system in which intramolecular hydrogen bonding can take place among the shielding terphenyl groups. Initially, we tried to synthesize porphyrins 27 and 28, having carboxylic acids and amides on the terphenyl groups, respectively, as shown in Figure 3- 3. Porphyrin 27 could be converted to porphyrin 28 by treatment with thionyl chloride followed by NH3 gas. The key step for the formation of pyrrole 31 involves Suzuki cross-coupling of pyrrole 5 and commercially available boronic acid 30 (Scheme 3-1). However, the Suzuki coupling gave a low yield and a mixture of mono- and di- substituted pyrroles, which could not be separated with chromatography. The use of ester 32 instead of acid 30 did not improve the yield. Therefore, an alternative preparation of pyrrole 31 is necessary. Miyaura177 and Giroux178 have reported a modified Suzuki coupling which involved the use of PdCl2(dppf), pinacol ester of diboron, and arylhalides. The modified coupling reaction employing the method as shown in Scheme 3-2 also gave a mixture of mono-, di-substituted, and debrominated pyrroles in low yields for which the separation was tedious. We then switched to the preparation of porphyrin 29 (Figure 3-3). The key feature for the preparation of porphyrin 29 is the synthesis of a pyrrole having an amine substituted terphenyl group at ,B-position, which can be easily converted to the desired amide group. Boronic acid 37, in which the amine group was protected by two benzyl groups, was prepared by the route shown in Scheme 3-3. Dibenzylaniline 34 was brominated with Br2 in CHCl3 at 0 °C to give bromide 35, which was then reacted with 10 equivalent of Mg to give Grignard reagent 36. The use of excess Mg was necessary 80 Figure 3-3. Structures of porphyrins with intramolecular hydrogen bonding. H Br Ar—B(OH)2 / \ Br Pd(PPh3)4r N32003/DMF, H20 / \ Ar N (302Et N CO2Et 5 30Ar=~©—002H + F Ar 32 Ar =-.-<;>»cozcn3 / \ Ar N 0023 31 Ar = Ocow 33 Ar =OCO2CH3 Scheme 3-1 0‘ ’0 OOIB-B\ BF‘OCOZH PdCl2(dppf), KOAc/DMF= Br iii: 0% /\ Br N COZEI 5 PdCl2(dppf), Na2CC)3/DMF= Ar Ar C0251 CO2 Et COH E1 31 Ar = HOcozH Scheme 3-2 82 for the preparation of Grignard reagent 36 since the reaction proceeded much slower than most of Grignard reactions and the use of 1.1 equivalent of Mg resulted in a low yield of the boronic acid. The reaction of Grignard reagent 36 and trimethyl borate gave the desired boronic acid 37 after hydrolysis with 10% HCl(aq). The work-up step for boronic acid 37 must be carried out carefully since the amphoteric product can dissolve in both acidic and alkaline solutions. We obtained crude boronic acid 37 after hydrolysis with 10% HCl(aq) and separation of organic layer from the reaction mixture followed by the removal of THF. The crude product, containing the protonated form of boronic acid 37, was washed with saturated NaHCO3(aq) using ether as the solvent. After the removal of solvent, CH2C12 was added to the oily residue and the pure boronic acid was obtained in 44% yield upon heating on steam bath followed by filtration. Several routes have been examined for the preparation of porphyrin 29. In Scheme 34, pyrrole 38 was first prepared in 70% by Suzuki cross-coupling of pyrrole 5 and 2.5 equivalent of boronic acid 37 in the presence of Pd(PPh3)4 as catalyst. Porphyrin 40 was obtained in about 5 % yield from pyrrole 38, after being reduced by Red-Al to give pyrrole 39. Unfortunately, attempts to convert porphyrin 40 to porphyrin 41 with hydrogenation over Pd/C were unsuccessful. Thus, an alternative method was tested as shown in Scheme 3-5. Hydrogenation of pyrrole 38 in the presence of catalytic amount of Pd/C gave pyrrole 42. Reduction of pyrrole 42 with Red-Al followed by cyclization gave only a small amount of porphyrin 41, presumably due to the unprotected amine group. It was hOped that the protection of amine group by the reaction with acetic anhydride would result in an increase in the yield of the desired porphyrin. To test this idea, pyrrole 42 was converted to pyrrole 44 quantitatively by reacting with acetic anhydride (Scheme 3-6). Pyrrole ester 44 was then reduced to the corresponding pyrrole alcohol with Red-A1 in THF. Unfortunately, under the reduction condition, the acetamide groups were readily reduced to the amine groups. After reacting with acetic anhydride and followed by cyclization, pyrrole 4S afforded the secondary acetamido 83 Ph Ph ©~~J s-CW 1“ r j CHCI3 W Ph Ph 34 35 Mg THF Ph Ph k 3(OCH3)3 ) N—Q—Bmz - Bng-Q—N ( THF \ Ph Ph Scheme 3-3 84 Ph k F rN-Q—B(OH)2 F Br Ph Ar 37 /\ Ar /N\ (>032; Pd(PPh3)4. Na2C03/DMF, H20 N (30252 Ph 33 Ar =—©—N: Ph Red-Alfl' HF F Ar Ar 1. HIGH Cl / \ 2.000 2 2 N CHZOH Scheme 3-4 85 Ar Ar / \ Ar H2/Pt,Cfl'HF / \ Ar N ooze: N 00213 42 Al‘ =O-NH2 Red-AI/T HF F Ar : + / \ Ar ézéloé’HZC'z N CH20“ 43 Ar =-©—NH2 41 Ar =-©-NH2 Scheme 3-5 86 Ar Ar / \ Ar H2/Pt,C/THF 7 / \ Ar N COzEt N 002a Ph } 33 Ar: /_\ N1 42 Ar =0an Ph 0(C0CH3)2 CH2CI2 F F Ar Ar Red-Alfl' HF / \ AI” 7 / \ Ar N CH2OH N CO2Et 45 Ar =0ij E1 0(cocr-13)2 CH2CI2 \ F 1. H+/CH2CI2 2.DDQ H 44 Ar = N' 00,2 Scheme 3-6 87 porphyrin 46 in 8% yield. Evidence supporting the formation of porphyrin 46 was given by mass and 1H NMR spectra. To counteract the unwanted reduction on amide groups, we therefore reduced pyrrole ester 42 to pyrrole alcohol 43 with Red-Al without the protection of amine groups (Scheme 3-7). As mentioned above, the presence of unprotected amine groups during the cyclization reaction resulted in an unsatisfactory yield of porphyrin. The protection of amines with acetyl groups was necessary for pyrrole alcohol 43 before cyclization. Pyrrole alcohol 47 was produced in situ by the treatment of pyrrole alcohol 43 with acetic anhydride in CH2C12 followed by cyclization in the presence of 3 equivalent of trimethoxymethane and a catalytic amount of trifluoroacetic acid gave about 6% of porphyrin 29 after oxidation with DDQ. Because the rather low yield of porphyrin 29 could be due to the low solubility of the intermediates formed during the cyclization reaction, we decided to change the solvent system to a 5:1 mixture of CH2C12 and acetic acid instead of CH2C12. With the new solvent system, the cyclization of pyrrole alcohol 47 employing Ono’s method followed by oxidation with DDQ, column chromatography, and recrystallization afforded porphyrin 29 in 37% yield. The higher yield for porphyrin 29 compared to the most hindered porphyrins described in Chapter 2 could be ascribed to the presence of intramolecular hydrogen bonding among the amide groups. The solubility of porphyrin 29 is not good in organic solvents. However, in the presence of trifluoroacetic acid porphyrin 29 can be protonated as evidenced by UV-vis spectra, and thus dissolved in polar solvents such as CH2C12, CHCl3, DMF, and methanol. Porphyrin 29 was characterized by mass, 1H NMR, and IR spectroscopies. The 1H NMR spectrum of 29 in CDCl3 containing 1% TFA showed that the signal for the methyl groups of amides shifted upfield due to shielding of the porphyrin ring current. In the mass spectrum, consecutive cleavages of CH3 and HNCOCH3 was observed. The IR spectrum exhibited N-H and C=O stretching. 88 F F Ar Ar N COzEt N COZEI S’h 38 Ar =‘O—N] 42 Ar =-< >—NH2 Ph Red-Al/T HF F F Ar Ar O(COCH3)2 / \ Ar a N CH2OH CH2CI2 / \ Ar ,H 47 Ar = 031% 1. H*/CH2CI2, Acetic acid 2.000 43 Ar =-©-NH2 Scheme 3-7 89 The metallation of porphyrin 29 carried out with the typical method in DMF was unsuccessful since the rigid pockets created by the intramolecular hydrogen bonding did not allow the metal ions to approach the porphyrin center. Therefore, the removal of the amine protecting groups was necessary and it could be achieved by hydrolysis in a refluxing mixture of methanol and water in the presence of H2SO4 for 3 days (Scheme 3- 8). After hydrolysis, the metal was inserted into the porphyrin in refluxing dry DMF in the presence of excess metal salt and pyridine. Without work-up, excess acetic anhydride was added to the reaction mixture and stirred for 2 hours at room temperature under a nitrogen atmosphere. The metalloporphyrin was purified by flash chromatography of A1203 eluting with a mixture of CH2C12 and methanol. The zinc and iron complexes of porphyrin 29 were recrystalized from CH2C12 and methanol and the manganese complex was recrystalized from CH2C12, methanol, and toluene. These metal complexes showed low solubility in organic solvent. However, the solubility increased when mixtures of solvents such as CH2C12 and methanol were used. The mass spectra of the metal complexes of porphyrin 29 also showed consecutive cleavages of CH3 and HNCOCH3 as observed for free base. The insertion of Mn(IIDCl to the other fl-substituted porphyrins for shape selective epoxidation was achieved by the reaction with MnCl2-4H2O in refluxing DMF. These manganese complexes showed considerable solubility in polar and nonpolar solvents and exhibited less solubility in alkanes. X-ray Crystal Structure of Porphyrin 49 The crystal structure of porphyrin 49 consists of an independent porphyrin molecule, two solvated CH3OH, and two solvated H2O. Figure 3-4 shows the crystal structure of porphyrin 49. Some of the solvated molecules are omitted for clarity. Table 3-1 shows the crystal data and refinement parameters. As shown in Figure 3-4, the macrocycle of pophyrin 16 is basically planar. Table 3-2 lists the average out-of-plane 90 41 Ar =-O-NH2 sz Py, DMF Py, DMF 48 M = Zn 49 M = Fe(lll), L = CH3O' so M = Mn(lll), L = CH30° Scheme 3-8 91 UCl26Al -, C(48) C‘SBA’O C(2SAl C(47) C(SSA) a: ) g ‘) /-, r) ,.) 0(4) MGM/‘7' . \’ - ‘ . 0(2Al u , f) v v - ‘ -, C(33) \r J .I . 3, 3 5' ) C(57) " 0(3) ) " I" , Fill ’) V H7752)” n f” 3, ,, T) UFi2l Fl2Al , , I ‘ , , ' ’) FHA, ) \’ 013A) ’0 . a 3, A, - 3 'r ’I - a 7) ’ ) ’)N(4Al I r, 3013 A) ’) 0‘2) M3) (SA C(34Al ’) ’) ( 47A (25) (2)645) or . -- C W é) ‘" @017) Figure 3-4. X-ray crystal structure of porphyrin 49. Hydrogen atoms and some of the solvated molecules have been omitted for clarity. 92 Table 3-1. Crystal data, Intensity Measurements, and Refinement parameters for Porphyrin 49. Formula FeC; 15H103F4N12013 Formula Weight 1,992.94 Crystal System Triclinic Space Group P21 Temperature, K 120 a, A 112.7762(5) b, A l3.7811(6) c, A 14.4907(5) a 90.994(2) 13. deg 99.156(l) y 90.280(2) v, A3 2,518.42(12) Z 1 Crystal Dimensions, mm 0.2 x 0.07 x 0.06 Dcalcd, g cm'1 1.314 F(000) 1,043 Orange, deg 1.42 — 22.50 Transm Range 0.6891 — 0.9280 Reflection collected 16,490 No. of Unique Reflections 6,473 No. of Parameters 618 R; Rwa 0.1284; 0.3143 8 1.062 aR=>:l|1=ol-l1=cl IIZIFol,Rw=[2w(HFo|-|Fc||)2/ZwFoz]m. 93 Table 3-2. Average Out-of-Plane Displacements, Bond Lengths, and Bond Angles for Porphyrin 49. Displacement (A)a Fe 0.557 Cm 0.037 N 0.005 C5 0.030 Ca 0.031 Bond Length (A) Fe-Ol 1.845 Car-Cm 1.375 Fe-N 2.086 N-Ca 1.389 Car-Cfi 1.457 C5-C5 1.375 Bond Angle (deg) Fe-Ol-C57 129.2 Cm-CaGC 125.5 N-Fe-N 85.9 N -Ca)-Cm 124.0 N-Cd-Cp 110.5 Cd-Cp-Cp 106.7 Ca-N-Ca 105.6 Cd-Cm-Ca 127.6 a From the least-square plane of the 24 atoms. 94 displacements, bond lengths, and bond angles. The X-ray structure shows the occurrence of the intramolecular hydrogen bonding among the anilides, but they fall short of forming a circular lock. On each side of the porphyrin plane, one of the four amide groups rotates out to break the interlock, with the dihedral angle between the amide plane and the attached phenyl plane being about 43°. The reason for the tilting of the amide may be ascribed to the interference of the solvated molecules and/or the crystal packing forces. The X-ray structure shows that there is intermolecular hydrogen bonding between an amide and a solvated water, in which the distance of O7(H2O)-N6(amide) is 2.794 A. Even though the expected interlocking amide circle is interrupted, intramolecular hydrogen bonding interactions among the other amides are present. For example, the distances for O2-N5A and O4A-N4A are 2.894 and 2.957 A, respectively. The intramolecular hydrogen bonding interactions of these amides still partially block the top of the pocket and partially constrain the rotation of the phenyl groups. The crystals for crystallography were grown from a mixture of CH2C12 and methanol. The use of the protic solvent could be the cause of the imperfection of the intramolecular hydrogen bonding. We believe that in nonprotic solvents better intramolecular hydrogen bonding interactions can be formed. This hypothesis seems to be borne out by the exceptionally high selectivity observed in the catalytic epoxidation reactions as discussed below. Shape-Selective Epoxidation Reactions The structures of the manganese porphyrins used for selective epoxidation are shown in Figure 3-5. The epoxidation reactions with the manganese porphyrins were performed in CH2C12 using iodosylbenzene as oxygen donor. To study the steric effects of our manganese porphyrins on the shape selective epoxidation of alkenes, both intra- and intermolecular selectivity tests were performed. In the first case, a series of non- conjugated dienes containing both internal and external C=C bonds were used to investigate the relative selectivity. In the second case, a series of 1:1 mixtures of cis- 95 so Ar=<©—N'H (Mn(TMTAP)(OM6)) O)— 51 Ar = (Mn(TMTBPP)(CI)) 52 ms“ (Mn(TMTNP)(CI)) 53 Ar=—©§ (Mn(TMTt-BP)(CI)) 54 Ar = Q (Mn(TMTPP)(Cl)) Figure 3-5. Structures of the manganese complexes of ,B-substituted bis-pocket porphyrins. 96 cyclooctene and various alkenes with different sizes and shapes were employed for intermolecular selectivity. Control epoxidation reactions were carried out with Mn(TPP)(C1). Figure 3-6 shows the data for the intramolecular epoxidation selectivity. The ,B-substituted bis-pocket porphyrins are more selective than Mn(TPP)(Cl) and Mn(T(2,4,6-OMeP)P)(OAc). Suslick et al. have reported the synthesis and shape selectivity of dendrimer porphyrins.136 Our system exhibited selectivity similar to those dendrimer porphyrins. Among the fl-substituted porphyrins, 50 (MnTMTAP) with intramolecular hydrogen bonding, shows the highest selectivity. Figure 3-7 shows the intermolecular selectivity for the various alkene mixtures with the fl-substituted bis- pocket porphyrins. For unhindered catalysts, more preferential attack should occur at air-cyclooctene than l-alkenes. For hindered catalysts, one can expect that the relative reactivity of l-alkenes increases. The intermolecular selectivity of flsubstituted bis— pocket porphyrins is much higher than that of meta-substituted dendrimer porphyrins reported by Suslick.136 Our system can be as much as thirty times more selective than Mn(TPP)(Cl), whereas mesa-phenyl substituted dendrimer porphyrins, are only two to three times better than Mn(TPP)(C1). Among these bis-pocket porphyrins, 50 shows the highest selectivity, indicating the intramolecular hydrogen bonds are doing their job in tying up the pockets above and below. Without intramolecular hydrogen bonding, 51 shows higher selectivity than the other porphyrins, indicating that the biphenyl groups hinder the pocket more effectively than p-tert-butylphenyl and 2-naphthy1 groups. Metalloporphyrins have a marked proclivity for self-destruction in oxidizing media. Generally, destructive oxidation occurs at the mesa-carbon.116 The oxidative stability of the metalloporphyrin can be increased by introducing steric hindered substituents at mesa-positions and electron withdrawing groups at the periphery of the porphyrin. The system we studied here is mesa-unsubstituted. One would expect that our system would show lower oxidative stability compared to mesa-substituted porphyrins. The stability of 97 5......5......vi!!..i!!.!!!.ilhhh....... m), D. e w ) ) eflwmwm flawemam PQWWMWW an and nnnnnnn MMMMMMM DEE—unfl- [- . a _ _ < a E _ 9. 8. 7 6 5. 4 3 1. 0 0 0 0 0 o 0 0 0 3:83 Ectosxonm .233. Figure 3-6. A plot of external epoxide to the total epoxide for intramolecular shape- selectivity of various Mn(III) porphyrin catalysts. 98 n Mn(TPP) Mn(TMTt-BP) 2.) Mn(TMTNP) )2) Mn(TMTBPP) I Mn(TMTAP) 60 Estes: o. REESE: when”. fififififlfifififififififififibbtbbbbbbbb w///////////////////////////////////////////////////. I V//////////////////////////////////////////////////A V\\\\\\\\\\\\\\\\\\\\\\\N 99 § \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\§ the epoxides were normalized to corresponding [Mn(TPP)(Cl)] values. Errors are estimated at 5% relative. 10- Figure 3-7. Epoxidation results for the intermolecular mixture of alkenes. The ratios of these fl-substituted porphyrins was investigated in oxidizing media. The reactions were followed under pseudo-unimolecular conditions in dilute porphyrin solutions in benzene under argon. The stability of the porphyrin was monitored by the rate of bleaching of the Soret peak. Porphyrin concentrations were maintained around ~10 pM and ~10 mM in m-chloroperbenzoic acid. Mn(TPP)(Cl) shows an half-life of 510.5 min, while the porphyrins, 54, 53 and 52 have half lives of 11 h, 13 h, and 1510.5 h, respectively. Surprisingly, our system shows high oxidative stability comparable to that of the extremely hindered Mn(T(2,4,6-PhP)P)(OAc) (half-life, 25 h). We also carried out the reaction of the nickel complex of porphyrin 16 and Fe(F2oTPP)(C1) in the presence of iodosybenzene in benzene under an argon atmosphere. Catabolic oxidation of the nickel porphyrin at mesa-carbons was not observed. Instead, one of the methyl groups at ,6- positions was converted to aldehyde or carboxylic acid under the oxidative media. The results indicate that the steric protection of the metal center prevents self-destruction of the catalyst, and thus increases the oxidative stability of these ,B-substituted porphyrins. The reactivity of these ,B-substituted porphyrins are similar to that of unhindered catalyst, Mn(TPP)(C1). T umover numbers (i.e., mol product/mol metalloporphyrins/s) observed for the bis-pocket porphyrins studied here are in the range of 1 to 3 sec'1 comparable to 3 to 4 sec" of Mn(TPP)(C1). Figures 3-8, 3-9, and 3-10 show the computer-generated molecular models of the free base of bis-pocket porphyrins, 54, 53, and 50. Porphyrin 54 has a ~0.5 nm pocket on both faces of the porphyrin while it is limited in the case of 50 and 53. All of the bis- pocket porphyrins show negligible cavities along their sides. Furthermore, porphyrin 50 shows perfect intramolecular hydrogen bonding between the para-carboxamido groups, even though the X-ray crystal structure of the iron complex shows that one pair of the amide groups did not align perfectly. The intramolecular hydrogen bonding obviously can hold the substituents in position completely blocking top access and limiting sideway approach to the metal center. In contrast to the dendrimer porphyrins, the more crowded 100 Top View Side View Figure 3-8. Computer-generated molecular models of the free base of 54. The top view shows a cavity of around 5 A on both faces of the porphyrin. The side view shows a negligible cavity. 101 Top View Side View Figure 3-9. Computer-generated molecular models of the free base of 53. The top and side views show the pockets are fully blocked 102 Top View Side View Figure 3-10. Computer-generated molecular models of the free base of 50. The top view shows perfect intramolecular hydrogen bonding between amide groups. Both top and side views show the pockets are fully blocked. 103 porphyrins, 50, 51, 52, and 53, show reduced openings on both top and side ways. However, due to the higher flexibility in the fl—substituents for these porphyrins, there are many conformations possible that would open up the pocket for the incoming alkene. The lower selectivity of 51, 52, and 53 relative to 50 indicates that free rotation of the substituents without hydrogen bonding would open up a bigger cavity than that of acetalinide groups with hydrogen bonding. These molecular modeling studies are consistent with the high shape-selectivity of these ,B-substituted porphyrins in the catalytic epoxidation reactions. Shape-Selective Ligation In the shape-selective ligation studies, we employed the zinc complexes of our ,8- substituted porphyrins as hosts since they are 5-coordinated in the presence of bases. The insertion of zinc was accomplished by the reaction of free bases with zinc salt in refluxing DMF. The structures of zinc porphyrins used for the shape-selective ligation are shown in Figure 3-11. Various substituted pyridines and primary amines were chosen to probe the shape-selectivity on ligation. As an example, Figure 3-12 shows the spectral changes of zinc porphyrin 58 at the Soret band at various 4-phenylpyridine concentrations in toluene. Upon addition of 4-phenylpyridine, the Soret band at 419 nm shifts to 431 nm with clean isosbestic points. The equilibrium constant (Keg) can be obtained from the plot of [base]/AA versus [base] as shown in the inset of Figure 3- 12.175 In each case, the plot gave a straight line with an intercept equal to l/Keq. Figure 3-13 shows the binding constants of ,B-substituted zinc porphyrins and Zn(TPP) with various nitrogenous ligands. All zinc porphyrins bind para-substituted amines (e.g., 4-phenylpyridine) better than meta-substituted ones (e. g., 3-phenylpyridine) due to the steric interactions between the meta-substituent and the fl-substitutents on the porphyrin. The decreased Keq’s of the zinc porphyrins binding to 3-chloropyridine and 3-bromopyrindine can be ascribed to both steric interaction and electron defficiency on 104 55 Ar: Q (Zn(TMTPP)) 56 Ar=\. (Zn(TMTNP» 57 Ar = (Zn(TMTBPP» 53 Ar: 04 (Zn(TMTt-BP)) Figure 3-11. Structures of the zinc complexes of ,B-substituted porphyrins for shape- selective ligation. 105 ‘ 0.06 5. 1.0 — g °'°“‘ 0. 0 i a _ fl 0 ‘ . V) 0.05 0.1 Q - [4-PhPy]/AA 0.5 - T I ' fi— - y 375 400 425 450 475 500 Wavelength (nm) Figure 3-12. Spectral changes of 58 in the Soret band region in toluene upon titration with 4-phenylpyridine. Inset: Plot of 4-phenylpyridine concentration versus 4-phenylpyridine divided by absorbance changes at 431 nm. 106 El ZnTPP B ZnTMTPP El ZnTMTNP fl ZnTMTBPP I ZnTMTt-BP 4 an !—-u_-——~——_——————-——-——————————————————_———-—————____—___——————_—__———__-_-———-—-— H1111 -.————__—_.—_—__———_.—___—_—_—~_—___—-_—__——__——————————.—______——______—_F~_PFPH IIIIIIllIll-III-Ill-IIIIIIIIIIIIIIIIIIII-I-Ill-III...-III-IIIIIIIIIIIIIII GS‘UG“KCCUGGEGg§K§S“SCCCUUQfiNUCUUUUUNxCCQUS““““§‘K“‘C€C¢“§V§ llIIIII-IIllIll-IIll-Ill-lIII-lIII-llIll-III-I-lllIll-IllllllllllIll-IIIIIIIIIIII cCfiEGNSfi‘g§§h333HR‘RR3\‘Ufi¥Cg§‘§SgKGSQeNU§SKS‘3‘“‘C‘S“C§C§¥“““W‘K NH: Ligand binding constants for ,B-substituted zinc porphyrins relative to Figure 3-13. Zn(TPP). Errors in Keq values are less than :t10%. 107 the pyridine ring. As expected, compared to Zn(TPP), zinc porphyrins 55, 56, and 57 exhibit higher affinities to both aromatic and alkyl amines due to the hydrophobic pockets, whereas the more closed-pocketed zinc porphyrin 58, as the molecular models shown in Figures 3-9, exhibits a lower affinity to the bases. The computer-generated molecular models show the steric interactions between the phenyl group of the ligand and the tert—butyl groups of the porphyrin upon binding with 4-phenylpyridine (Figure 3-14). When a bigger ligand, cinchonidine, was used the ,B-substituted zinc porphyrins showed lower Keq’s compared to Zn(TPP). Moreover, zinc porphyrin 58 does not bind cinchonidine at all as evidenced by the UV-vis spectrum, which did not change upon addition of cinchonidine. The discrimination between small and big ligands for these bis- pocket porphyrins is consistent with the difference in the pocket size and shape around the binding center. These results are also consistent with those of dendrimer-zinc porphyrins reported by Suslick. 175 It is noteworthy that zinc porphyrin 57 shows the highest affinities to aromatic amines and about the same binding constants to alkyl amines as compared to zinc porphyrins 55 and 56, even though the computer-generated molecular model of the free base shows much more restrictions on both top and side approaches (Figure 3-15). The high binding constants of zinc porphyrin 57 with substituted pyridine can be ascribed to TE-‘lt interactions. As an example, Figure 3-16 shows 1t-1t interactions between the aromatic ring of the ligand and the aromatic substituents of the porphyrin. The presence of 1T-1'r interactions is further confirmed by the similar binding ability of 55, 56, and 57 to alkyl amines, which do not have aromatic rings for 1t-1t interactions with the aromatic rings on porphyrins. Thus, the size of the pocket is not the only factor that influences the binding of amines to the porphyrins. 108 Top View Side View Figure 3-14. Computer-generated molecular models of the 58—4-phenylpyridine complex. The models show the steric interactions between the phenyl group of the ligand and the ten-butyl groups of the porphyrin. 109 Top View Side View Figure 3-15. Computer-generated molecular models of 57. The top and side views show the pockets are fully blocked. 110 Top View Side View Figure 3-16. Computer-generated molecular models of the 57-4-phenylpyridine complex. The model of the complex shows the 1H: interactions between the phenyl group of the ligand and the phenyl groups of the porphyrin. 111 Conclusions Shape-selective epoxidation of alkenes has been carried out by using sterically hindered manganese porphyrins as catalysts. The size of the shielding superstructures is responsible for the shape selectivity. The moderately hindered porphyrin, 54, shows a cavity size of ~ 0.5 nm on the top, which allows the substrate to approach the reaction center. The more crowded porphyrins, 50, 51, 52, and 53, have more restricted access on both the top and sides. However, larger sideway openings created by conformational changes of the shielding wings may allow easier access to the reaction center. Higher selectivity is observed with the increase in steric crowding on both the face and sides of the porphyrins. Moreover, porphyrin 50 shows the highest shape-selectivity due to intrarnolecualr hydrogen bonding. In our system, the steric protection of the active site by substituents at ,B-positions also increases the stability of catalysts during the oxidation reacitons. The fl-substituted zinc porphyrins show shape-selectivity on ligation with various amines having different sizes and shapes. The hydrophobic pockets of the porphyrins can stabilize the coordinating ligand, thus showing higher binding constants than those of Zn(TPP). While the size of the pocket of the porphyrin reflects on the selectivity for ligation to small and bulky ligands, other interactions such as n—rl: stacking also play an important role in the shape selectivity of ligation. Experimental Materials All reagents and solvents were obtained from commercial sources and were used without further purification unless otherwise noted. Dichloromethane was distilled over CaH2 under N2 atmosphere. THF was distilled over Na/benzophenone. All the l-alkenes and dienes were purchased from Aldrich and were used as received. Silica gel was of 60- 200 mesh, manufactured by Fisher Scientific. Analytical TLC was performed on 112 Eastman Kodak 13181 silica gel sheets. Compositions of solvent mixtures are quoted as ratios of volume. Instrumentation 1H NMR (300 MHz) spectra were recorded on a Varian Gemini spectrometer. Chemical shifts were reported in ppm relative to the residual proton in deuterated chloroform (7.24 ppm). Absorption spectra were recorded on a Shimadzu UV-l60, or Varian Carry 219 spectrometer. Mass spetra were obtained on a benchtop VG Trio-1 mass spectrometer. FAB-MS mass spectra were obtained on a J EOL HX-110 HF double focusing spectrometer operating in the positive ion detection mode. The epoxidation products were analyzed using a Varian GL 3700 series capillary gas chromatograph and a Hewlett-Packard GCMS. Energy minimized structures of ,B-pyrrole substituted porphyrins were performed on an Indigo Silicon Graphics System using Spartan software packages. Crystallography Crystals of porphyrin 49 were grown from a mixture of CH2C12 and methanol by slow evaporation. To prevent solvent evaporation, the chosen crystal was coated with hydrocarbon oil and mounted on a Nonius CAD4 diffractometer with graphite- monochromated Mo Kor radiation. The crystal structure was solved by direct methods and refined by full matrix least-squares using the NRCVAX program. All non-hydrogen atoms were refined anisotropically, while hydrogen atoms were refined isotropically. Crystallographic details for the structure are given in Table 3-1. Epoxidation Reactions All epoxidation reactions were performed in CH2C12 under an argon atmosphere. Iodosylbenzene was employed as the oxygen donor. Standard epoxides were obtained 113 from Aldrich or synthesized from reported procedures. Epoxidation reactions were carried out in CH2C12 (1 ml) solution containing 1 M of catalyst, 0.5 mM of alkene and 10-20 ,uM of iodosylbenzene. The reaction mixture was vigorously stirred at room temperature for 30 min under argon. To this solution an internal standard, n-decane or n- octane was added and the products were analyzed by GC and GCMS. Control epoxidation reactions were performed with Mn(TPP)(Cl) under similar conditions. In all cases, the yields of the epoxides were greater than 70% based on the amount of iodosylbenzene employed. 4-Bromo-N, N dibenzylaniline (35) To the solution of dibenzylaniline 34 (50.00 g, 0.183 mol) in 125 ml of CHC13 at 0 °C was added a solution of Br2 (29.38 g, 0.183 mol) in 125 ml of CHC13 through an addition funnel. After the addition of Br2 was completed, the solution was stirred at room temperature for 10 min. The reaction mixture was then washed with Na2CO3m). The organic layer was separated and dried over anhydrous N a2SO4 and then the solvent was removed in vacuo. The crude product was recrystallized from CH2C12 and methanol to give 60.6 g (94%) of 35. mp. 125-126 °C; 1H NMR (300 MHz, CDC13): 6 7.40-7.15 (12H, m, phenyl), 6.59 (2H, d, phenyl), 4.64 (8H, s, CH2); MS found We 351.1, cacld. 351.06 for C20H13BrN; Anal. found: C, 68.96; H, 5.03; N, 3.59. cacld: C, 68.18; H, 5.15; N, 3.98. for C20H13BrN. 4-(N, N -Dibenzylaminophenyl)boronic acid (37) The arylboronic acid was prepared by standard Grignard techniques. Mg (39.50 g, 1.72 mol) was placed in a 500 ml three-necked round-bottomed flask equipped with a reflux condenser and mechanical stirrer. The system was flushed with nitrogen for 20 min while the Mg and flask were heated with a heating mantle. The apparatus was cooled and 4-bromo-N,N-dibenzylaniline 35 (60.60 g, 0.172 mol) was added to THF (250 ml) in the 114 reaction flask. The resulting mixture was refluxed for 4 hr and then transferred to an addition funnel. Under argon the Grignard reagent was slowly added to a solution of trimethyl borate (21.5 ml, 0.189 mol) in THF (250 ml) cooled at -78 °C. The solution was stirred overnight at room temperature. After acidification with 50 ml of 10% HCl(aq), the organic layer was separated from the mixture. THF was then removed in vacuo. The oily residue was dissolved in ether and washed with saturated NaHCO3m). The organic layer was collected and dried over anhydrous Na2SO4. After removal of the solvent, 100 ml of CH2C12 was added to the crude product and heated on steam bath. The precipitates were filtered to give 23.8 g (43.6%) of boronic acid 37. 1H NMR (300 MHz, CDC13): a 8.00 (2H, d, phenyl), 7.40-7.30 (10H, m, phenyl), 6.82 (2H, d, phenyl), 4.73 (8H, s, CH2); Anal. found: C, 78.69; H, 6.01; N, 4.18. cacld: C, 75.67; H, 6.36; N, 4.42 for ConzoBNoz. Ethyl 4-methyl-3-(2,6-bis(4-(N,N-dibenzylaminophenyl))-4-fluorophenyl)-2-pyrrole- carboxylate (38) A mixture of pyrrole 5 (1.00 g, 2.47 mmol), boronic acid 37 (1.97 g, 6.2 mmol), and Pd(PPh3)4 (200 mg, 0.17 mmol) in 25 ml DMF containing 5 ml 2M Na2CO3m) was purged with N2 for 10 min. The solution was gently refluxed under N2 for 2 d. The reaction mixture was then cooled to room temperature and the inorganic solids were removed by filtration. The filtrate was concentrated in vacuo, and then extracted with CH2C12. The solvent was dried over anhydrous Na2SO4 and removed under reduced pressure. The product was separated by column chromatography on silica gel eluting with CH2C12 and hexanes (4:6) to give 4.1 g (70%) of pyrrole 38. The product for analysis was recrystalized from CH2C12 and methanol. mp. 197-198 °C; IH NMR (300 MHz, DMSO-da): 6 11.05 (1H,br (1, NH), 7.35-7.10 (20H, m, phenyl), 6.93 (2H, d, phenyl), 6.75 (4H, d, phenyl), 6.51(1H, d, pyrryl), 6.45 (2H, d, phenyl), 4.58 (8H, s, 115 CH2), 3.79 (2H, q, CH2 of ethyl), 1.35 (3H, s, CH3), 0.86 (3H, 1, CH3 of ethyl); MS found We 789.1, cacld. 790. for C54H43FN3O2. Ethyl 4-methyl-3-(2,6-his(4-aminophenyl)-4-fluorophenyl)-2-pyrrolecarboxylate 42. To a solution of pyrrole 38 (790 mg, 1 mmol) in dried THF (30 ml) was added 10% palladium on charcoal (50 mg). The mixture was deaerated first and hydrogenated at room temperature under 1 atm of pressure. After stirring for 24 h, the reaction mixture was filtered, the solvent was removed in vacua and the residue was chromatographed on silica gel eluting with CH2C12 and methanol (50:1). The crude product was recrystallized from CH2C12 and toluene to give 352 mg (82%) of 38. mp. 145-148 °C; 1H NMR (300 MHz, CDC13): 6 8.65 (1H,br s, NH), 6.99 (2H, d, phenyl), 6.85 (4H, d, phenyl), 6.45 (5H, m, pyrryl and phenyl), 3.99 (2H, q, CH2 of ethyl), 3.55 (4H, br s, NH2), 1.03 (3H, t, CH3 of ethyl); MS: found m/e 429.2, cacld. 429.49 for C26H24FN 302. Ethyl 4-methyl-3-(2,6-bis(4-acetanilino)-4-fluorophenyl)-2-pyrrolecarboxylate (44) Excess acetic anhydride was added to a solution of pyrrole 42 (430 mg, 1 mmol) in 5 ml of CH2C12. The resulting mixture was stirred at room temperature under nitrogen. After stirring for 20 min, the reaction mixture was filtered to give the product quantitaitvely. m.p. >260 °C; 1H NMR (300 MHz, DMSO-dé): a 11.14 (1H,br 8, NH of pyrrole), 9.86 (2H, br 3, NH of amide), 7.37 (4H, d, phenyl), 7.10 (2H, d, phenyl), 6.95 (4H, d, phenyl), 6.52 (1H, d, pyrryl), 3.91 (2H, q, CH2 of ethyl), 2.00 (6H, s, CH3 of amide), 1.43 (3H, s, CH3), 0.97 (3H, t, CH3 of ethyl); MS: found m/e 513.1, cacld. 513.57 for C30H23FN3O4. 2,7,12,17 -Tetramethyl-3,8,13,18-tetrakis(2,6-bis(4-acetanilino)-4-fluorophenyl)- porphyrin (29) To a 30 ml THF solution of pyrrole 42 (430 mg, 1 mmol) was dropwise added Red-Al (6 ml, 65% in toluene, 20 mol) at 0 °C under argon. When the addition of Red-Al was 116 complete, the mixture was returned to room temperature and stirred for 6 hr. To the resulting solution was added 40 ml of CH2C12. The excess Red-Al was destroyed by the addition of ice at 0 °C. The liquid layer was washed with water twice. The organic layer was separated, dried over N a2SO4, and concentrated under reduced pressure to give white solid. Without further purification, the white solid was added to 50 ml of CH2C12. To the solution was added acetic anhydride (0.51 g, 5 mmol) and the resulting solution was stirred at room temperature under nitrogen. After stirring for 10 min, acetic acid (10 ml) was added to the solution followed by the addition of dimethoxymethane (0.28 ml, 3 mmol) and trifluoroacetic acid (23 mg, 0.2 mmol). After stirring overnight under nitrogen, DDQ (227 mg, 1 mmol) was added and the solution was stirred for a further hour. The solvent was removed in vacua to give a dark residue. The residue was chromatographed on silica gel eluting with a mixture of CH2C12, methanol, and trifluoroacetic acid (20022021). The purple fraction was collected and recrystallized from CH2C12, methanol and trifluroacetic acid to give 167 mg (37%) of porphyrin. IR: vmax 3314 (N-H), 1674 (C=O) cm-l; UV-vis (0.2% TFA in CH2C12, 24w nm (8)): 605 (12,400), 564 (20,600), 412 (160,300); lH NMR (300 MHz, 1% TFA in CDC13): 6 10.02 (4H, s, mesa), 8.04 (8H, 8, NH of amide), 7.49 (8H, d, phenyl), 6.90 (16H, (1, phenyl), 6.70 (16H, d, phenyl), 3.12 (12H, 5, CH3), 1.92 (24H, s, CH3 of amide), -3.70 - —4.2 (3H, 2 br s, N-H); MS (FAB): found We 1710.6, 1725.8, 1737.0, 1750.5, 1766.5, 1779.0, 1792.5, 1809.7 (M++1), 1825.0, 1831.9, cacld. 1808.02. for C112H90F4N1203. 2,7,12,17-Tetramethyl-3,8,13,18-tetrakis(2,6-bis(4-aminophenyl)-4-fiuorophenyl) porphyrin (41) Porphyrin 29 (180 mg, 0.1 mmol) was added to a mixture of methanol, H2804, and water (90:12:1). After the solution was refluxed for 24 h under nitrogen, 2 ml of water was added and the resulting mixture was refluxed for 48 h. The solution was cooled in an ice bath and neutralized with NaOH(aq). The mixture was filtered and the solid was washed 117 with water to give the hydrolyzed porphyrin quantitatively. UV-vis (5% MeOH in CH2C12, 2mm nm (rel intens)): 636 (0.05), 581 (0.06), 551 (0.10), 515 (0.11), 421 (1.00); lH NMR (300 MHz, DMSO-dg): 69.21 (4H, s, mesa), 7.39 (8H, d, phenyl), 6.65 (16H, (1, phenyl), 5.81 (16H, d, phenyl), 4.50 (16H, s, NH2), 2.78 (12H, s, CH3); MS (FAB): found We 1473.3, cacld. 1471.72 for C96H74F4N12. Zn(II) 2,7,12,17-Tetramethyl-3,8,l3,18-tetrakis(2,6-bis(4-acetanilino)-4-fluoro- phenyl)porphyrin (48) Porphyrin 41 (30 mg, 0.02 mmol), Zn(OAc)2-4H2O (440 mg, 2.0 mmol), and 5 ml of pyridine were added to 10 ml of DMF and refluxed under nitrogen. After 2 h, the reaction was completed as monitored by the visible spectrum. The solution mixture was cooled to room temperature and then 2 ml of acetic anhydride was added to the reaction mixture. After stirring for 2 h at room temperaturen, the solution was concentrated in vacuo. Water was added to the residue, the mixture filtered, and the solid air-dried. The solid was chromatographed on A1203 eluting with CH2C12 and methanol (20:1). The product was recrystallized from CH2C12 and methanol to give 32 mg (86%) of Zinc porphyrin 48. IR: vmx 3314 (N-H), 1675 (C=O) cm"; UV-vis (5% MeOH in CH2C12, 2m, nm (8)): 586 (16,900), 551 (15,800), 426 (177,600); 1H NMR (300 MHz, 20% CD3OD in CDC13): 6 9.06 (4H, s, mesa), 7.22 (8H, d, phenyl), 6.82 (16H, (1, phenyl), 6.57 (16H, (1, phenyl), 2.70 (12H, 5, CH3), 1.61 (24H, 5, CH3 of amide); MS (FAB): found rn/e 1812.8, 1826.9, 1841.8, 1870.4 (M++2), 1884.4, cacld. 1868.61 for C) 12H88F4N 1208211- Fe(III) 2,7,12,l7-tetramethyl-3,8,l3,18-tetrakis(2,6-bis(4-acetanilino)-4-fluoro- phenyl)porphyrin methoxide (49) The free base 41 was treated with excess FeBr2 employing a procedure similar to that used for the synthesis of 48 except that a longer reaction time was needed. The crude 118 product after chromatography was recrystallized from CH2C12 and methanol to give 85% of iron porphyrin 49. IR: vmax 3314 (N-H), 1676 (C=O) cm"; UV-vis (5% MeOH in CH2C12, 24,.“ nm (rel intens)): 599 (0.12), 412 (1.00); MS (FAB): found m/e 1805.6, 1819.9, 1833.9, 1847.6, 1862.5 (M++2), 1877.4, cacld. 1860.61 for C112H33F4N1203Fe. Mn(III) 2,7,12,17-tetramethyl-3,8,l3,l8-tetrakis(2,6-his(4-acetanilino)-4-fluoro- phenyl)porphyrin methoxide (50) The free base 41 was treated with excess MnC12-4H2O employing a procedure similar to that used for the synthesis of 48 except that a longer reaction time was needed. The product after chromatography was recrystallized from CH2C12, methanol, and toluene to give 82% of maganese porphyrin 50. IR: v“... 3314 (N-H), 1673 ((2:0) em“; UV-vis (5% MeOH in CH2C12, Km” nrn (rel intens)): 704 (0.02), 590 (0.14), 557 (0.20), 469 (1.00), 377 (0.93); MS (FAB): found m/e 1804.6, 1819.9, 1832.9, 1847.7, 1861.5 (MW-2), 1877.32, cacld. 1859.62 for C132H33F4N1203Mn. 2-Naphthylboronic acid The boronic acid was prepared by the standard Grignard method as described in Chapter 2. mp. >260 °C; 1H NMR (300 MHz, CDC13): 6 8.88 (1H, s), 8.32 ( 1H, dd), 8.08 (1H, dd), 7.99 (1H, d), 7.92 ( 1H, dd), 7.64-7.42 (2H, m). Ethyl 4-methyl-3-(2,6-bis-(2-naphthyl)-4-fluorophenyl)-2-pyrrolecarhoxylate A procedure similar to that used for the synthesis of pyrrole 9 in Chater 2 was employed to give the product in 63% yield. mp. 167-168 °C; 1H NMR (300 MHz, CDC13): 6 8.48 (1H,br 5, NH), 7.78-7.67 (4H, m, arryl), 7.66 (2H, d, arryl), 7.58 (2H, d, arryl), 7.46-7.36 (4H, m, arryl), 7.22 -7.15 (4H, m, arryl), 6.32 (1H, d, pyrryl), 4.04 (2H, q, CH2), 1.56 (3H, s, CH3), 1.08 (3H, t, CH3); MS found m/e 499.4, cacld. 499.19 for C34H26FNO2. 119 2,7,12,17-Tetramethyl-3,8,13,18-tetra(2,6-bis-(2-naphthyl)-4-fluorophenyl) porphyrin A procedure similar to that used for the synthesis of porphyrin 15 in Chapter 2 was employed to give the product in 32% yield. 1H NMR (300 MHz, CDC13): 6 9.21 (4H, s, mesa), 7.90 (8H, s, arryl), 7.51 (8H, d, arryl), 7.49 (8H, d, arryl), 7.22-7.10 (24H, m, arryl), 6.46 (8H, d, arryl), 6.30 (8H, d, arryl), 2.86 (12H, s, CH3), -4.46 (2H,br s, NH); UV-vis (Toluene, km, nm (8)): 637 (6,100), 582 (7,700), 548 (13,100), 512 (16,500), 423 (158,200); MS (FAB): found m/e 1752.9, cacld. 1750.65 for C123H32N4F4. Mn(III) 2,7,12,17-tetramethyl-3,8,l3,18-tetrakis(2,6-diphenyl-4-fluorophenyl) porphyrin chloride (54) Free base 15 (27 mg, 0.02 mmol), MnCl2-4H2O (198 mg, 1.0 mmol), and 0.5 ml of pyridine were added to 20 ml of DMF and refluxed under argon overnight. The solvent was evaporated in vacua and the manganese porphyrin was purified on silica gel column eluting with CH2C12 and methanol (95:5). The collected product was washed with saturated NaClm) and dried with Na2SO4, and the solvent was evaporated under reduced pressure, yielding 24 mg (87%) of manganese porphyrin 54. UV-vis(toluene, 34m nm (rel intens)): 780 (0.02), 577 (0.17), 483 (0.91), 373 (1.00); MS (FAB): found m/e 1404.6, cacld. 1403.44 for C96H64F4N4Mn. Mn(III) 2,7,12,17-tetramethyl-3,8,13,18-tetrakis(2,6-his(4-t-hutylphenyI)-4-fluoro- phenyl)porphyrin chloride (53) A procedure similar to that used for the synthesis of 54 was employed to give the product in 76% yield. UV-vis(toluene, 711m nm (rel intens)): 783 (0.02), 576 (0.17), 482 (1.00), 375 (0.91); MS (FAB): found We 1853.2, cacld. 1851.95 for C123H123F4N4Mn. 120 Mn(III) 2,7,12,17-tetramethyl-3,8,13,l8-tetrakis(2,6-bis(2-naphthyl)-4-fluorophenyl) porphyrin chloride (52) A procedure similar to that used for the synthesis of 54 was employed to give the product in 88% yield. UV-vis (toluene, 714,.” nm (rel intens)): 785 (0.02), 577 (0.18), 483 ( 1.00), 378 (0.81); MS (FAB): found m/e 1805.0, cacld. 1803.57 for C123H30N4F4MII. Mn(III) 2,7,12,l7-Tetramethyl-3,8,l3,l8-tetra(2,6-bis-(4-biphenyl)-4-fluorophenyl) porphyrin chloride (51) A procedure similar to that used for the synthesis of 54 was employed to give the product in 83% yield. UV-vis (toluene, 2m, nm (8)): 786 (1,500), 604 (5,500), 576 (12,000), 483 (70,500), 375 (58,100); MS (FAB): found m/e 2012.8, cacld. 2011.70 for C144H96F4N4Mn Zn(II) 2,7,12,17-Tetramethyl-3,8,l3,18-tetrakis(2,6-diphenyl-4-fluorophenyl) porphyrin (55) To a solution of free base 15 (27 mg, 0.02 mmol) in 10 ml of CHC13 and 10 ml of DMF was added a saturated solution of zinc acetate (0.5 ml). The solution was refluxed for 1 hr, and then concentrated in vacuo. Water was added to the residue and the solid was filtered. The solid was chromatographed on basic alumina eluting with CH2C12 to give 26 mg (94%) of zinc porphyrin 32. 'H NMR (300 MHz, CDC13): 68.86 (4H, s, mesa), 7.12 (8H, d, phenyl), 6.74 (16H, (1, phenyl), 6.40-6.20 (24H, m, phenyl), 2.59 (12H, 8, CH3); UV-vis (toluene, km, nm (8)): 582 (26,000), 546 ( 16,000), 422 (253,500); MS (FAB): found m/e1414.1, cacld. 1412.44 for C96H64F4N4Zn. Zn(II) 2,7,12,17-Tetramethyl-3,8,13,18-tetrakis(2,6-bis(2-naphthyl)-4-fluorophenyl) porphyrin (56) A procedure similar to that used for the synthesis of 55 was employed to give the product in 90% yield. 1H NMR (300 MHz, CDC13): 69.20 (4H, s, mesa), 7.85 (8H, s, naphthyl), 121 7.49 (8H, d, phenyl), 7.42 (8H, d, naphthyl), 7.20-7.00-6.20 (24H, m, naphthyl), 6.51 (8H, d, naphthyl), 6.29 (8H, d, naphthyl), 2.86 (12H, 8, CH3); UV-vis (toluene, Mm nm (8)): 588 (20,200), 552 (17,500), 432 (211,500); MS (FAB): found m/e 1814.5, cacld. 1812.56 for C123H30N4F4Zn. Zn(II) 2,7,12,17-Tetramethyl-3,8,13,18-tetra(2,6-bis-(4-hiphenyl)-4-fluorophenyl) porphyrin (57) A procedure similar to that used for the synthesis of 55 was employed to give the product in 90% yield. 1H NMR (300 MHz, CDC13): 6 9.29 (4H, s, mesa), 7.47 (8H, d, phenyl), 7.03 (16H, (1, phenyl), 6.97-6.73 (40H, m, phenyl), 6.67 (16H, d, phenyl), 2.90 (12H, 5, CH3); UV-vis (toluene, lam nm (rel intens)): 585 (0.16), 548 (0.14), 432 (1.00); MS (FAB): found m/e 2022.3, cacld. 2020.69 for C144H96F4N4Zn. Zn(II) 2,7,12,17-tetramethyl-3,8,13,18-tetrakis(2,6-bis(4-t-butylphenyl)-4- fiuorophenyl) porphyrin (58) A procedure similar to that used for the synthesis of 55 was employed to give the product in 93% yield. 1H NMR (300 MHz, CDC13): 6 9.25 (4H, s, mesa), 7.39 (8H, d, phenyl), 6.89 (16H, (1, phenyl), 6.55 (16H, s, phenyl), 2.83 (12H, 5, CH3), 0.72 (72H, 8, CH3); UV- vis (toluene, Mm nrn (rel intens)): 582 (0.11), 546 (0.06), 423 (1.00); MS (FAB): found m/e 1862.8, cacld. 1860.97 for C128H123F4N4Zn 122 Chapter 4 A STERICALLY HINDERED PORPHYRIN MADE WATER SOLUBLE Introduction Metalloporphyrins serve many functions in biological system. These functions include dioxygen transportation and storage,1693179 electron transfer,l733180 and biocatalysis.129,1813182 These diverse biological functions of metalloporphyrins have spawned numerous studies involving model complexes. Most porphyrins are only sparingly soluble in organic solvents. Since water is a major component of many biological systems, it would be of great interest to study porphyrins in an aqueous environment. Furthermore, the advantages of an aqueous system are numerous. First, water is a bountiful source of active oxygen in electrocatalytic oxidation of water mediated by metalloporphyrins. Secondly, H20 and OH‘ as the axial ligands influence the reactivity of redox centers. Thirdly, redox reactions may be controlled by adjusting the pH of the solution. Electrochemical and chemical redox reactions of water-soluble metalloporphyrins have been extensively studied to elucidate the catalytic properties and to understand the ligand effects and substituent effects.183'185 However, compared to the voluminous studies carried out in organic solvents, water-soluble systems are less well studied. One of the reasons is that highly modified water-soluble porphyrins are much more difficult to synthesize. The other reason is that many simple water-soluble porphyrins can aggregate in solution, either through rt-rt interactions or through formation of ,u-oxo dimers in alkaline aqueous solution.186'188 These aggregates complicate the studies of these porphyrins because their properties are different from those of monomers. It is important to prevent the formation of dimer in the studies of iron porphyrins in aqueous solution. Therefore, some sterically hindered water-soluble porphyrins have been 123 synthesized.186,189il90 Almost all synthetic water-soluble porphyrins available to date are mesa-substituted. Only limited results have been reported for fl-substituted water- soluble porphyrins.191 We have now synthesized a highly hindered Water-soluble porphyrin based on the fl-substituted terphenyl wings as described in previous chapters. This porphyrin is highly water-soluble over the whole range of pH values and has no propensity to form p-oxo dimer due to the steric hindrance of terphenyl groups. The catalysis of dioxygen reduction by metalloporphyrins is a current topic.192‘ 194 Recent studies of 02 reduction have shown that water soluble iron porphyrins catalyze the reduction of both dioxygen and hydrogen peroxide via an EC mechanism. 195. 196 (I) E step FemP + e' FeuP (4-1) (11) C step 2 Fe"? + 02 + 2 H+ —— 2 FemP + H202 (4-2) 2 Fe"P + H202 + 2 H+ —— 2 FemP + 2 H20 (4-3) Iron(IH) porphyrin is reduced electrochemically to iron(H) porphyrin, which reduces 02 to H202 and then H20 stepwise in aqueous solutions. We investigated 02 reduction catalyzed by a sterically hindered fl-substituted water-soluble iron porphyrin at various pH values. The biological functions of metalloporphyrins are dependent on the protein residues surrounding them and the axial ligands. For example, the number and the nature of axial ligands,197 the binding geometry of the axial ligands,1983199 and the hydrogen bonding64 between the protein and axial ligands appear to be of importance in regulating the biological functions of the active sites. It is known that the coordination properties of water-soluble porphyrins are influenced by the peripheral structure. Miskelly and coworker have shown that the perfluorinated water-soluble porphyrin they synthesized 124 has a hydrophobic environment about the metal center.1893200 This behavior was examined by the binding of organic molecules. In water solutions of the porphyrin, addition of small amounts of organic solvents or water soluble organic molecules resulted in the displacement of axial water molecules from six-coordinate nickel porphyrin to form the square planar four-coordinate nickel species. Similar behavior for the iron(III) complex of methylated (nicotinarnidophenyl) porphyrin was also observed. 187 The axial water molecules of the iron complex can not be replaced by anions such as Cl', Br', and N031 Thus, it is of interest to learn what chemical and structural features of the porphyrin determine its coordination chemistry. Our water-soluble porphyrin having four substituted terphenyl groups at ,B-positions shows hydrophobic character about the binding site. The coordination chemistry of the iron complex has been investigated. Results and Discussion Synthesis The synthesis of H2TSPP and H2TSDCPP has been reported.353201 The reactions were performed in hot sulfuric solution and hot fuming sulfuric acid solution, respectively. The structures of the fl-substituted porphyrins used for sulfonation are shown in Scheme 1. The synthesis of the parent porphyrins has been described in Chapter 2. We started the sulfonation of the terphenylporphyrin 59 at various temperatures in sulfuric acid solutions. However, a mixture of sulfonated porphyrins was produced. The mixture could not be separated by typical methods such as chromatography. Attempts to solve this problem by substitution of hydrogen by fluorine to deactivate the phenyl group directly attached to the porphyrin ring (15) also did not prevent the formation of a mixture of sulfonated porphyrins. Our approach was then switched to the activation of the terminal phenyl groups by the introduction of methoxy groups (16), and this permitted us to obtain a single sulfonated porphyrin 60 under a mild reaction condition. It has been shown that unreacted starting material was detected in the 125 H so H230, st04 2 4 Mixture Mixture 2,7, 12, 17 -Tetramethyl-3,8, 13, 18-tetra(2,6-bis-(4-methoxy-3-sulfonatophenyl)-4- fluorophenyl)porphyrin (H2TMTSPP) Scheme 4-1 126 sulfonation reaction of TPP under the mild reaction condition.186 A similar result was also observed in our system using the typical procedure of stirring the porphyrin in H2804 at room temperature. The porphyrin did not completely dissolve in H2S04 during the course of the reaction. This problem can be solved by dissolving the porphyrin in CH2C12 followed by the addition of H2S04. The porphyrin was protonated immediately and converted to its diacid form, thus dissolving in H2S04. The sulfonation was carried out for 1 h at room temperature. The work-up of the reaction involved the addition of water and neutralization with alkaline solution. The crude product was precipitated from methanol and acetone several times to give 85% of sulfonated porphyrin 61 (H2TMTSPP). The solubility of this porphyrin is good in the whole range (0-14) of pH of aqueous solution. It is also soluble in highly polar organic solvents such as MeOH, DMSO, and DMF. The identification of the sulfonated porphyrin was based on the 1H NMR spectra in D20 and deuterated DMSO. The 1H NMR of the sulfonated porphyrin in D20 shows three peaks for the aromatic hydrogens of the sulfonated methoxyphenyl groups at 8.07, 6.27, and 5.84 ppm. The integrated area of the three peaks gave the expected 1:1:1 ratio. There is a singlet at 9.23 ppm due to the hydrogens at meso-positions, a doublet at 7.51 ppm due to the hydrogens on the phenyl groups attached to the fl-positions, a singlet at 2.96 ppm due to methoxy groups, and a singlet at 2.71 ppm which arises from the methyl groups at fl-positions. The integration of these peaks is consistent with the structure. There is no evidence for the demethylation of the methoxy groups under the sulfonation condition. The NMR spectrum gives definite evidence for sulfonation occurring at all the eight carbons artha to the methoxyl group. In the mass spectrum, the molecular ion peak (M=2406) was absent. However, the largest peak at m/z 2369 was observed. This could be assigned to the species after loss of one CH3 and one Na, and gain of one H (M - CH3 - Na + H) from the molecular ion. Along with the largest peaks some other intense peaks observed are consistent with the consecutive loss of sodium ions. The insertion of iron 127 was accomplished by the typical method of refluxing an aqueous solution containing the sulfonated free base and excess FeS04 under argon for l h. The excess iron salt was precipated out as Fe(0H)3 by adusting the pH of the solution mixture to the range of 12- 13 by NaOH(aq). The iron porphyrin 61 (FeTMTSPP) was then precipitated from methanol and acetone several times to give 84% of iron porphyrin. pKa of iron porphyrin 61 (FeTMTSPP) The pKa of the water ligated FeTMTSPPP was determined by spectrophotometric titration. Figure 4-1 shows the Q band spectral change of FeTMTSPP as a function of pH. The absorption peaks at 504 and 624 nm in pH 6.83 solution shift to 486 and 606 nm in pH 9.90 solution with isosbestic points at 508, 518 and 636 nm. A pKa of 8.11 is calculated from spectrophotometric titration. The reaction thus involves a one-proton transfer between the acid and base forms. (H20)FemTMTSPP (H0)FemTMTSPP + H+ pKa = 8.11 (44) The observed pKa is the higher than other sulfonated iron porphyrins. For example, iron(III) mesa-tetrakis(3-sulfonatomesityl)porphyrin (FemTSMP) and iron(III) mesa- tetrakis(4-sulfonatophenyl) porphyrin (FemTSPP) have pKa’s at 6.6 and 7.0, respectively.202 The higher pKa value for FeTMTSPP is probably due to the electron- donating groups at ,B-positions, which increase the electron density of porphyrin ring and iron center. In the pH range 1.0 - 6.0, the absorption spectra do not change significantly. No significant change in the absorption spectra was observed in the pH range 10.0 - 13.0 as well. As expected, there was no evidence for dimerization of FeTMTSPP. Titration of FemTMTSPP with anions No significant spectral change was observed when the acid and base forms of FeTMTSPP were titrated with sulfate, perchlorate, and phosphate. The same was 128 486 1.8 1.2- _”‘ ‘ Ear 3; 31.8: 303‘ E . 302) 39.8% -o.7 . C a) 5.5 7 7.5 8 5:5 9 9.5 99.5“ p" L '1 o . 71 DB. 4" (I: . B. 2" [4 l Q 8 sea séa" ' 760' Wavelength(nm) Figure 4). Spectral changes of FemTMTSPP in 0.1M NaC104 at different pH. pH = 6.83 - 9.90. Inset: Plot of log ((Ao-A)/(A-A.,.,)) vs. pH. 129 observed when titrated with nitrite at pH 10.0. Similar results were also observed in the systems in which the mesa-phenyl groups have some bulky groups attached at the ortho- positions.187 This is probably because the highly hindered substituents, terphenyl groups at the ,B-positions provide hydrophobic environment which impedes the ligation of anions to the metal. However, at pH 6.1, the spectra of 61 changed significantly when titrated with nitrite. Figure 4-2 shows the spectral changes of 61 at pH 6.1 in the presence of various nitrite concentrations. The bands at 414, 506 and 628 nm decrease while those at 430, 540, and 578 nm increase with isosbestic points at 422, 526, and 592 nm. The resulting spectrum is identical with that of Fe"TMTSPP(N0) obtained by directly bubbling N0 into FemTMTSPP. In solutions, N02’ disproportionates to N03' and N0 according to the following equation.203 2 N0 + N03" + 2 OH (4-5) 3 N02' + H20 K: 1.1x 1020 M‘2 Scheidt et al. have reported that the reactions between iron(III) porphyrin complexes and nitrite salts resulted only in the isolation of nitrosyl complexes rather than the expected nitrite complexes.204 The proposed mechanism involves the formation of nitrite complexes that react with excess nitrite ions to produce the nitrosyl complexes. Based on the redox potentials of N0 and (N0)Fem"°T PP, Su and coworkers have reported that NO is thermodynamically capable of reducing (N0)Fem(TMPyP).203 Based on the observation that nitrite does not ligate to FemTMTSPP at pH 10.0, we assume that the formation of Fe"TMTSPP(N0) involves N0 ligation to iron(IH) followed by reduction of FemTMTSPP(N0). The inset of Figure 4-2 demonstrates that the formation constant Kf is 2.1 x 106 M, and one N0 coordinates to the iron center. Titration of FemTMTSPP with imidazole Figure 4-3 shows the Q band spectral changes of FemTMTSPP titrated with imidazole having concentrations ranged from 0 to 3 x 10“1 M in a pH 6.5 phosphate 130 )l 5', d.) . 5? 0.5 O . 5 _ 11 ) g 01 ..o )1 8': '°‘5 ‘.1 ‘6 10— ) 4‘ VJ \‘ -1.5 . . .13 r/ -7 -6 -5 -4 < - / 1.09 (cone. of N0) 0-5‘ 578 506 540 T 3 T 628 1 A J, 0.0......"A.. , 400 500 600 700 Wavelength (nm) Figure 4-2. Spectral changes of FemTMTSPP in pH 6.1 of 0.1 M phosphate solution in the presence of various concentrations of NaN02. [NaNO2] = 0 - 0.67 M. Inset: Plot of log ((Ao-A)/(A—A..,)) versus log (conc. of N0). 131 588 682 788 Navelength(nm) Figure 4-3. Spectral changes of FemTMTSPP in pH 6.5 of 0.1 M phosphate solution in the presence of various concentration of imidazole. [ImH] = 0 — 3 x 10‘4 M. 132 buffer solution. As the concentration of imidazole increases, the absorbances at 504 and 626 nm decrease while that at 538 nm increases with isosbestic points at 552 and 594 nm. The absorption spectra do not change as the imidazole amount is greater than two equivalents of FemTMTSPP and remain unchanged as the imidazole concentration reaches 0.1 M. Thus, the total number of imidazole ligated to the iron porphyrin is two. (H20)FemTMTSPP + 2 11111-1 —— (ImH)2FemTMTSPP + H20 (445) Ashley and coworker reported the imidazole ligation of Cr(III)TSPP with distinct stages for the first and second ligations. The values of K. and K2 were estimated to be 1.0 x 104 and 2.9 x 102 for the acid form, and 1.9 x 103 and 3.3 for the base form.205 Su et al. reported the coordination properties of iron(III) mesa-tetrakis(3-sulfonatomesityl)- porphyrin with imidazole.203 They found that in the spectrophotometric titration of FemTSMP wrth imidazole, distinct stages for the first and second imidazole ligation were not observed. Two imidazoles were coordinated to the iron center. Titration of FenTMTSPP with imidazole The determination of the equilibrium constants for imidazole ligation was achieved by spectrophotometric titration at pH 6.5 using phosphate buffer. FemTMTSPP was reduced to FenTMTSPP by sodium dithionite and the measurements were made under nitrogen. Fig. 5-4 shows how the Q band spectra change of FenTMTSPP in the presence of various imidazole concentrations. It is noteworthy that in the titration of FenTMTSPP with imidazole, distinct stages for the first and second base ligations were observed. As the concentration of imidazole increases in the range of 0 - 2 x 10“4 M, a broad band at 500 - 600 nm shifts to 528 and 558 nm with isosbestic points at 506, 540, 550 and 566 nm. Upon further titration with imidazole, the absorption bands at 528 and 558 nm shift to 532 and 562 nm. The isosbestic points are at 530, 542, 554 and 568 nm. No further 133 .— A 1.2 31.2) a: . 0 08.8 C G .D L21.15 O "a 4 .n . a: 1 0.8 < 0.6 - 0.4 0.2 o . -o.2 - -0.4 < -0.6 - -0.a . -6.5 6.5 4.5 Log (cone. of Im) “MMWNNM (.— LN 7GB 3 1 . ‘5 32. C I! 98. L o 4 .3 .2 .38 . Log (cone. of Im) 12 a. a u U I ‘ I V I V j see 553 see 552 7212 Have length (run) Figure 4-4. Spectral changes of FeuTMTSPP reduced by dithionite in pH 6.5 of 0.1 M phosphate solution in the presence of various concentrations of imidazole under N2. [ImH] = (a) 0 — 2 x 10“ M; (b) 2 x 10“ M — 5 x 10'2 M. Inset: Plot of log ((Ao-A)/(A-A..)) versus log (cone. of imidazole). 134 change in absorbance was seen after reaching 0.05 M in imidazole concentration. The equilibria between iron(H) porphyrin and axial bases are expressed as follows: FeHTMTSPP + ImH v—FeuTMTSPPamH) K, = 7.1 x 106 (4-7) Fe‘ITMTSPParnH) + ImH Fe“TMTSPF(IrnH)2 K2 = 1.0 x 103 (4-3) The concentration of free imidazole ([IrnH]) was calculated from the total imidazole concentration ([Ime] + [IrnH]), the pKa of imidazole (6.95), and the pH of the solution. The binding constants were calculated from plots of log [(A - Ao)/(A... - A)] vs. log [IrnH], where A0 is the absorbance of iron(III) porphyrin at a particular wavelength in the absence of imidazole, A... the absorbance in the presence of a large excess imidazole, and A is the absorbance at a particular imidazole concentration. Both plots of log [(A - Ao)/(A... - A)] vs. log [IrnH] shows a slope of 1 :1: 0.1 indicating one imidazole ligation for each stage. The equilibrium constants, K. = 7.1 :l: 0.1 x 106 and 1.0 :1: 0.1 x 103, were calculated from the intercepts of the plots. The binding behavior of FenTMTSPP with imidazole is different from those of most other iron(II) porphyrins that bind the second axial ligand more strongly than the first for unhindered imidazole“)52 It should be noted, that the binding behavior is also different from those of the fl-substituted porphyrins we used for 02 binding studies. The dioxygen binding to FenTMTSPP in aqueous solution in the presence of imidazole was our goal. However, in the presence of 02, iron(II) oxidized to iron(III) immediately. This is probably because the hydrophobic pockets can not impede the approach of water molecules, which act as proton sources to accelerate the oxidation of the iron(II) porphyrin-02 adduct. Electrochemistry of FeTBMSPFPP Figure 4-5 shows the cyclic voltammograms of FeTMTSPP in various pH solutions under N2. An irreversible reduction wave of Fem/"TMTSPP at about -0.40 V was 135 Current 1 L 1 1 1 1 1 J l l 0.00 —0.20 —0.40 —0.60 E (V vs. Ag/AgCl) Figure 4-5. Cyclic voltammograms of FemTMTSPP in 0.1 M Na2SO4 solution at scan rate of 100 mV/s. (a) pH = 1.0; (b) pH = 6.4; (c) pH = 9.1. 136 observed, indicating an overpotential for the reduction or a chemical reaction following the reduction. Figure 4-6 shows thin layer spectra of FeTMTSPP in various pH solutions. When the pH is lower than 2, the bands at 414, 504 and 628 nm shift to 418, 574 and 616 nm upon reduction. The new spectrum is consistent with that of the diacid form, H4TMTSPP2+, in aqueous pH 1 solution. The results indicate that demetallation followed the reduction at pH < 2. In pH 4.0 solution, the absorption peaks at 414, 504 and 628 nm shift to 424 and 558 nm upon reduction. The OTI'LE method has been used to determine the formal potentials. However, attempts to obtain the formal potential of Fem/"TMTSPP could not be achieved. The spectrum of FemTMTSPP did not change at Eappl. = -0.40 V at pH 4.0 but changed completely to FeuTMTSPP when the applied potential was stepped to -0.50 V for 30 min. The new spectrum upon electroreduction is identical with that of FenTMTSPP obtained from chemical reduction of FemTMTSPP by sodium dithionite. ' The spectrum of FemTMTSPP could not be regenerated by stepping the potential to -0.40 V, but could be regenerated by stepping the potential to -0.30 V. These results suggest that there is an overpotential for the conversion between iron(III) and iron(II) with glassy carbon.202 The overpotential and slow heterogeneous electron-transfer rate are probably due to the increased distance between iron center and the surface of the electrode caused by the negatively charged sulfonate groups and bulky terphenyl groups. As the pH increases, a more negative potential is needed to reduce the base form of FemTMTSPP. The bands at 412, 486, and 606 nm shift to 424, and 558 nm upon reduction as shown in Figure 4-6. At pH 10.0, the spectrum of Fe'"TMTSPP does not change at E3”), = -070 V, but changes completely to FeuTMTSPP at -0.80 V. The spectrum of FenTMTSPP could not be regenerated even at -0.50 V, but could be regenerated by stepping the potential to -0.40 V. In alkaline solutions this overpotential is more pronounced since the surface of the glassy carbon electrode is covered by carboxylate groups which would repulse FeTMTSPP containing eight negatively charged sulfonate groups. 137 wanna-u a; 13ch ..-..:’E m 414 Absorbance 200 300 400 500 600 700 ' 800 Wavelength Figure 4-6. Time-resolved spectral changes of FemTMTSPP reduction in various pH solutions under N2. (a) pH = 1.0; (b) pH = 4.0; (c) pH = 10.0. 138 02 Reduction Catalyzed by FeTMTSPP Numerous of studies on the 02 reduction catalyzed by metalloporphyrins have been reported.206’207 Kuwana et al. have shown that CoTMPyP catalyzed two-electron reduction of 02 to H202 and FeTMPyP catalyzes four-electron reduction of 02 to H20 via H202.196 Figure 4-7 shows the cyclic voltammograms of electrocatalytic 02 reduction by FeTMTSPP in various pH solutions under 02. As the pH increases, Epmg moves toward more negative potentials. This is due to the higher overpotential at higher pH as mentioned above, or the rate-determining step of the 02 reduction involving protons. The electrocatalytic reduction of 02 was studied with varying amounts of FeTMTSPP in 02- saturated pH 2 solution. As the iron porphyrin concentration is increased from 0.2 to 1.0 mM. Emu and catalytic current do not change. These results indicate that the kinetic rate of C step is very fast. The electrochemistry of adsorbed FeTMTSPP in the absence and presence of 02 was also investigated. The reduction is not significantly different from that of bulk FeTMTSPP. Adsorbed FeTMTSPP catalyzes the reduction of 02 as well. As the scan number increases, Epm, shifts to nagative direction and ipm, decreases. This phenomenon was also observed in bulk FeTMTSPP solution. The possibility is that A demetallation and/or decomposition of FeTMTSPP deactivates the electrode. Dual catalysts: CoTPyP and FeTMTSPP. Figure 4-8 shows the cyclic voltammograms of 02 reduction by adsorbed CoTPyP and dissolved FeTMTSPP in 0.05 M H2304 solution. In the presence of adsorbed CoTPyP, a double wave was observed. The first wave corresponds to the reduction of 02 to hydrogen peroxide catalyzed by CoTPyP. The hydrogen peroxide produced was then reduced to water by FenTMTSPP. If FeuTMTSPP does not reduce H202 to H20, then the second catalytic wave would be absent. Thus, the results are consistent with the iron porphyrin reducing 02 to H20 via H202. 139 Current 1 1 l _1 1 1 1 3 1 1 +0.20 0.00 -0.20 -0.40 -0.60 E (V vs. Ag/AgCl) Figure 4-7. Cyclic voltammograms for 02 reduction in 02-saturated solutions containing 1.0 x 10'3 M FemT MT SPP in various pH solutions at a scan rate of 100 mV/s. (a) pH = 1.0; (b) pH = 6.4; (c) pH = 9.1. 140 IluA b Current 1 J 1 l 1 l 1 1 1 J -0-20 0.00 +0.20 +0.40 E (V vs. Ag/AgCl) Figure 4-8. Cyclic voltammograms for 02 reduction catalyzed by adsorbed CoTPyP and dissolved FeTMTSPP in pH 1 solution at a scan rate of 100 mV/s. (a) N2- saturated, catalyst = adsorbed CoTPyP and solution FeTMTSPP; (b) 02- saturated, catalyst = solution FeTMTSPP; (c) 02-saturated, catalyst = CoTPyP and solution FeTMTSPP. 141 Oxidation of FeTMTSPP Figure 4-9 shows the oxidation of FeTMTSPP in various pH solutions. At pH 1.0, there is a redox couple at E” = +0.73 V. The peak-to-peak separation at the scan rate of 100 mV/sec is 60 mV. As pH increases, the Em does not change but waves become quasi-reversible. To further probe the number of electrons transfered and the reaction center, thin-layer spectroelectrochemistry was performed in different pH solutions. Figure 4-10 shows the thin-layer spectra of FeTMTSPP at pH 1.0. The band at 416 nm decreases dramatically upon oxidation. The broad band in the region of 500-800 nm is the typical pattern for a porphine ring radical cation. The plot of log[0]/[R] as a function of 5pr shows that the E°' of FemTMTSPP/FemTMTSPP” is +0.73 v and the oxidation involves one electron transfer (slope = 72 mV). Based on the above evidence, the redox reaction is then assigned as (H20)Fc‘"TMTSPP‘ + c‘ —— (H20)FemTMTSPP 13‘”: +0.73 v (49) The iron porphyrin radical cation is stable at pH < 2 on the OTTLE time scale and starts to decompose at pH > 3 upon oxidation. Numerous iron(IV) porphyrins and iron(IV) porphyrin radical cations have been used to catalyze the oxidation reactions of organic substrates.20132083209 It is well known that the redox potential of the metal center for water-soluble metalloporphyrins is dependent on pH values. The electrocatalytic properties of Fen/TMTSPP and FeWTMTSPP" would be of particular interest. Our appoach was to perform the spectroelectrochemistry of FeTMTSPP in alkaline solutions, but unfortunately, we could not obtain FeWTMTSPP or FeNTMTSPP" at a potential negative of the formal potential of FemTMTSPP/FemTMTSPP‘” even at pH 13.0. As mentioned, there is an overpotential for the conversion between Fem/"TMTSPP. Therefore, an overpotential for the conversion of Fen"IV is expected. When the potential was increased until oxidation occurred, decomposition was observed from the decrease in absorbances. 142 1111A ‘ _ 1M ' filiflfi‘ Current 0 111- ' l 1.), ' I 1 1 l 1 +0.80 +0.40 0.06 E (V vs. Ag/AgCl) Figure 4-9. Cyclic voltammograms of FemTMTSPP oxidation at various pH values. (a) pH = 1.0; (b) pH = 4.6; (c) pH = 6.4; ((1) pH = 13.0. 143 1.4- 416 1.2- a 2 _ b C 8 1.0: d g 0.8— e .0 . f lI-t o 0.6- g ,8 - 1.1 <1 04— i 0.2— k 0.0 . . . . , . , . r . 300 400 500 600 700 Wavelength (nm) Figure 4—10. Thin-layer spectra of FemTMTSPP at different applied potentials (vs. Ag/AgCl) in pH 1.0 solution. Eappl. = (a) +0.60; (b) +0.65; (c) +0.67; ((1) +0.70; (e) +0.72; (1) +0.74; (g) +0.76; (h) +0.79; (i) +0.82; (1) +0.85; (k) +0.90 V. Inset: Plot of log [0]/[R] vs. Em... Conclusions We have successfully synthesized a new highly sterically crowded B—substituted water-soluble porphyrin (H2TMTSPP) by the introdution of activating and deactivating groups into the terphenyl groups. The spectrophotometric data of the iron(III) complex (FemTMTSPP) support the absence of ,u-oxo dimeric species over the whole pH range due to the added sterically hindered substituents at ,B-positions. Similar to anionic water- soluble porphyrins, only a single pKa was observed whereas most of cationic water- soluble porphyrins show two pKa’s. The pka of FemTMTSPP(0H2) was estimated to be 8.11 which is the highest among sulfonated iron porphyrins due to the electron-donating nature of the substituents at ,B-positions. It is unlikely that the ligated water molecules in FeTMTSPP(0H2) can be replaced by anions such as 3042', N02', C104) and P043” since the spectra did not change when titrated with these anions, whereas significant spectral changes did occur when titrated with imidazole. When titrated with N02‘ in acidic solutions, the N0 produced ligated to FeTMTSPP and converted iron(III) to iron(II). These coordination studies demonstrated that the hydrophobic pockets prohibited the coordination of anionic ligands. The E°’of Fem/"TMTSPP can not be obtained with cyclic voltammetry and electrospectrophotometry due to the overpotential. When pH < 2, demetallation of FeTMTSPP occurred upon reduction. The mechanism of catalytic 02 reduction by FeTMTSPP is consistent with the proposed ‘2+2’ mechanism. The oxidation of FeTMTSPP was also investigated by cyclic voltammetry and electrospectrophotometry. The 13°: of Fe'I‘TMTSPP/FemTMTSPP" is +0.73 v obtained by cyclic voltammetry and electrophotospectrometry. Attempts to oxidize iron(III) to iron(IV) by electrochemical and chemical methods were unsuccessful. 145 I, H fl.‘ Experimental Materials All pyrroles and porphyrins used were synthesized by the same procedure described in Chapter 2. All reagents and solvents were obtained from commercial sources and were used without further purification unless otherwise noted. Silica gel for chromatography was 60-200 mesh, manufactured by Fisher Scientific. Analytical TLC was performed on Eastman Kodak 13181 silica gel sheets. Compositions of solvent mixtures are quoted as ratios of volume. Eletrochemical Measurements All aqueous solutions used for electrochemistry were prepared with doubly distilled deionized water. Solutions were deoxygenated by purging with nitrogen gas. Buffer solutions ranging from pH 1 to pH 14 were prepared from H2S04, potassium hydrogen phthalate (KHP), borate, carbonate, and NaOH. Dilute solutions of H2804 and NaOH were used for the adjustment of pH. The pH values of the solutions were measured before and after electrochemical experiments. The measured pH values were within :l:0.05 pH units. All experiments were performed at room temperature. Metalloporphyrin concentrations ranged from 0.5 to 1.0 mM and contained 0.2 N Na2S04 as electrolyte. Instrumentation 1H NMR (300 MHz) spectra were recorded on a Varian Gemini spectrometer. Chemical shifts were reported in ppm relative to the residual proton in deuterated chloroform (7.24 ppm), DMSO-d6 (2.49 ppm), or D20 (4.63 ppm). Absorption spectra were recorded on a Shimadzu UV-160, Varian Carry 219, or HP 8452A spectrometer. Mass spetra were obtained on a benchtop VG Trio-1 mass spectrometer. FAB-MS mass spectra were obtained on a JEOL HX-110 HF double focusing spetrometer operating in the positive ion detection mode. Electrochemistry was accomplished with a three- 146 electrode potentiostat (Bioanalytical Systems, Model CV-27) and a BAS X-Y recorder. Cyclic voltammetry was conducted with the use of a home-made three-electrode cell in which a BAS glassy carbon electrode (0.07 cmz) was used as working electrode and a platinum wire as auxiliary electrode. All potentials taken were referenced to a home- made Ag/AgCl/KCl (sat.) electrode. The working electrode was polished with 0.03 pm aluminum on Buehler felt pads prior to each experiment. The reproducibility of individual potential values was within 21:5 mV. The spectroelectrochemical experiments were accomplished with the use of a 1 mm cuvette, 100 mesh platinum gauze as the working electrode, a platinum wire as the auxiliary electrode, and a Ag/AgCl reference electrode. The design of cuvettes for spectroelectrochemical measurements has been described.2 10 2,7,12,17-Tetramethyl-3,8,13,18-tetra(2,6-his-(4-methoxy-3-sulfonatophenyl)-4- fluorophenyl)porphyrin (60) Porphyrin 16 (0.5 g) was added to 20 ml of CH2C12 and stirred for 5 min. The mixture was then cooled to 0 °C and 20 ml of concentrated H2804 was added. After the porphyrin was protonated and completely dissolved, the CH2C12 was removed by a pipette. The solution was stirred at room temperature for 1 h and then was cautiously diluted with two volumes of water at 0 °C. The resulting solution was neutralized with NaOH(aq) to a pH of 7-8. Methanol was added to the solution to precipitate Na2S04,and the mixture was filtered through a sintered glass frit to remove the salt. The solvent was removed and the solid containing a small amount of Na2S04 was then dissolved in a minimum amount of methanol. The mixture was filtered to remove the Na2S04. The sulfonated porphyrin was precipitated from methanol and acetone three times, air dried, and yielded 85% of sulfonated porphyrin. 1H NMR (300 MHz, D20): 6 9.32 (4H, s, mesa), 7.38 (8H, d, phenyl), 6.92 (16H, (1, phenyl), 6.15 (16H, d, phenyl), 3.31 (24H, t, CH3), 2.87 (12H, 8, CH3), -4.21 (2H,br s, NH); UV-vis(H20 (pH 8), Mm nm (rel intens)): 147 628 (0.04), 575 (0.06), 552 (0.09), 514 (0.08), 417 (1.00); MS: found m/e 2175.4, 2201.3, 2223.9, 2247.2, 2269.3, 2288.0, 2312.1, 2335.0, 2348.6, 2371.2, 2391.2, 2412.9, 2435.0 cacld. 2406.12. for C104H74F4N4032S3Na3. Anal. found: C, 43.73; H, 4.55; N, 1.70. calcd.: C, 43.43; H, 4.42; N, 1.95 for C104H74F4N403283Nag x 26 H20 Fe(III) 2,7,12,l7-Tetramethyl-3,8,l3,18-tetra(2,6-his-(4-methoxy-3-sulfonatophenyl)- 4-fluorophenyl)porphyrin (61) The free base was metallated by refluxing with an excess of FeS04 in water between pH 5 and 8 for 1 h, at which time there was no longer any spectrophotometric evidence for the unreacted free base. The solution was cooled and the pH was adjusted between 12 and 13 to precipitate excess iron(III) as Fe(OH)3. The mixture was filtered through a. sintered glass frit to remove the salt. The solvent was then removed and the solid was purified by precipitation from methanol and acetone three times, and air dried to yield 83% of the iron porphyrin. UV—vis(H20 (pH 4), Am nm (rel intens)): 627 (0.04), 506 (0.09), 413 (1.00); Anal. found: C, 40.95; H, 5.04; N, 1.60. calcd.: C, 40.16; H, 4.67; N, 1.80 for C104H72F4N4032S3NagFe x 32 H20 148 Chapter 5 CHIRAL NONPLANAR PORPHYRINS Introduction The distortion of tetrapyrrole macrocycles has been observed in biological systems such as the bacterial photosynthetic reaction centers,211 vitamin Bt2-dependent enzymes,212 cofactor F430 of methylreductase,213’214 and photosynthetic antenna complexes.215 The distortion is presumably caused by the protein environments surrounding the macrocycle.216 In particular, the axial ligands coordinated to the metal center, substituents covalently attached to the macrocycle, and the amino acid residues in the vicinity of the active site are undoubtedly important. The presumption of protein induced distortion is based on the fact that the isolated macrocycles are nearly planar in solution and nonplanar in proteins. It has been suggested that the nonplanar distortions of the macrocycles play an important role in their biological function. For examples, recent structural data for photosynthetic centers showed that the chromophores have multiple nonplanar conformations.217 The conformational variations have been believed to shift the energy of the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals of the chromophores, thus modulating their optical properties, and redox potentials, with consequent effects on the electron-transfer rates of the reaction centers. For cytochromes c, high resolution X-ray structures have shown that the iron porphyrin is distorted from planarity by a significant degree.218 The distortions are believed to be related to the modification of redox properties of the hemes. Additional evidence is the observation that the nonplanar distortions are conserved for proteins belonging to the same functional class. For example, the ruffling distortion is highly conserved for mitochondrial cytochromes c isolated from diverse species.219 These conserved distorted structures are most likely to influence enzymatic functions. 149 The investigation of nonplanar porphyrins has been an active area due to the significant relationship between macrocycle distortion and physiochemical properties in biological systems.220,221 In the past years, a number of nonplanar porphyrins have been synthesized and their physical and chemical properties have been demonstrated. Studies on nonplanar model compounds have contributed to the better understanding the origin of porphyrin nonplanar distortions. These model compounds also provided information about the effects of the distortions on the porphyrins. Numerous X-ray structural data of highly substituted nonplanar porphyrins have elucidated various conformations for the macrocycleszzz'224 In general, these conformations can be classified into four types, the saddled, ruffled, waved, and domed conformations as shown in Figure 51225 Among these four conformations, saddled and ruffled are the most common observed in nonplanar porphyrins. For the saddled conformation, the porphyrin mesa carbons are in the porphyrin mean plane whereas the pyrrole units are alternatively above and below the mean plane of the porphyrin. NiOETPP and ZnOETPP are specific examples for the porphyrins with the saddled conformation.223t226 The X- ray crystal structure shows that NiOETPP is severely nonplanar and adopts an 84 saddle conformation. The mesa carbons lie nearly in the plane of the macrocycle with the average displacement of the C5 atoms from the mean plane of the molecule is 1.23 A. The angles for the adjacent pyrrole planes are 34.5° and for opposite pyrroles 580°. The ruffled conformation is quite different from the saddled conformation. In the ruffled conformation the pyrrole nitrogens are in the mean plane whereas the mesa carbons are alternatively above and below the porphyrin mean plane. The [3 carbons in the same pyrrolic unit for the ruffled conformation are on the opposite sides of the porphyrin mean plane since the pyrrolic units are twisted whereas in the saddled conformation the ,8 carbons in the same pyrrolic unit are on the same side of the mean plane. The crystal structure of NiDPP, in which there are twelve phenyl groups at mesa- and ,B-positions, shows a well-defined ruffled conformation.227a228 The pyrrolic units 150 Fnll Figure 5-1. The saddled, ruffled, waved, and domed nonplanar porphyrin conformations. Filled circles represent atoms above the least-squares plane, and open circles correspond to atoms below the plane; atoms not circled are in the plane. 151 exhibit twist angles of 22.81° with respect to the porphyrin mean plane. A maximum displacement of 0.885 A for the mesa carbons and an average diplacement of 0.430 A of the core atoms from the porphyrin mean plane have been observed. Waved and domed conformations are less often reported than are ruffled and saddled. The waved conformations exhibit smaller deviations from the porphyrin mean plane than do saddled and ruffled conformations. The domed conformations are only observed when a porphyrin is ligated to a large metal ion, usually having one or more ligands coordinated. Because there is a significant relationship between nonplanarity and functions of tetrapyrrole complexes in biological systems, it is important to know the factors that control the nonplanar structure of the macrocycle. Numerous X-ray structures of synthetic nonplanr porphyrins point to four factors that relate to the conformation of the macrocycle have been found.229‘235 These factors include peripheral substituents, the central metal, the axial ligand, and the environment of the macrocycle. Peripheral substituent efftects include the number, size, orientation, and electronic properties. When the porphyrin has bulky groups at mesa- and fl-positions there are steric interactions between the mesa-substituents and the adjacent ,6 carbons. There are additional steric interactions between the mesa- and fl-substituents. X-ray crystal structures show that the bulkier the substituents are, the more distorted the porphyrin will be. The nature of the metal center in the macrocycle is integral in determining the degree of distortion. A shorter M-N bond results in a decrease in the porphyrin core size, thus distorting the macrocycle. In the case of NiOETPP and ZnOETPP, the Ni-N bond (1.906 A) is shorter than Zn-N (2.063 A), and therefore a more non-planar structure was observed in NiOETPP.22332263227 Structural and theoretical studies show that the axial ligand is also a factor that influences the conformation of the porphyrin.2363237 Numerous crystal structures of Fe(TMP) complexes with various axial ligands suggest that the bulky planar axial ligands are capable of interacting with the peripheral substituents and induce strong ruffling in the porphyrin core.2383239 The environment of the macrocycle can be 152 considered as solvents in synthetic model compounds and proteins in biological systems. Both chemical and physical properties of non-planar porphyrins, which are different from those of planar porphyrins, have been demonstrated. The most common spectroscopic features of non-planar porphyrins are the red shifts at the Soret and Q bands in the UV-vis spectrumzzot240 Shelnutt and coworkers have synthesized and studied the spectroscopic properties of a series of mesa-tetrasubstituted porphyrins.220 The porphyrin with simple linear alkane substituents have the Soret band near 430 nm and Q bands near 540 and 580 nm, whereas the porphyrin with bulky adarnantyl groups give greatly red-shifted spectra with the Soret band near 470 nm and Q bands near 600 and 650 nm. Resonance Raman spectroscopy has been proved to be a powerful technique for quantifying the conformational equilibrium between non-planar and planar conformers of porphyrins.2263241‘243 It has been shown that there is a relationship between the core size of the porphyrin and the resonance Raman frequencies between 1300 and 1500 cm’1 which are called core-size marker lines. The studies on synthetic model compounds have demonstrated a correlation between the frequencies of structure-sensitive lines and the degree of distortion of the macrocycle. The distortion of the porphyrin shifts the energy levels of the frontier orbitals, HOMO and LUMO. Fajer et al. have reported that non-planar porphyrins are easier to oxidize and harder to reduce compared to planar porphyrins.219 Based on the electrochemistry of nonplanar O-bonded iron(IH) porphyrins, Kadish and coworkers have shown how the nonplanarity of the porphyrin influences the electron transfer site and demonstrated that the facile oxidation of OETPP derivatives compared to their 0EP analogs can be explained by the distortion of the porphyrin macrocycle.24‘4»24S In a series of brominated tetraphenylporphyrins, the porphyrin is initially harder to oxidize due to the electron-withdrawing ability of the added bromine groups and subsequently becomes easier to oxidize since the addition of more bromine substituents result in the 153 nonplanarity of the macrocycle. Some other chemical and physical properties of nonplanar porphyrins have also been investigated. These studies include NMR and EPR spectra, axial ligand affinity, and basicity.2323246'249 The chemistry of chiral porphyrins has attracted much attention because of the interest in the development of new chiral ligands and receptors for asymmetric catalysis and the modeling of biologically important reactions.1"'532503251 The chiralities of hemes in biological systems are induced by the protein pocket, while in synthetic planar porphyrins, they are often derived from chiral auxiliaries. Nonplanar porphyrins, however, may have intrinsic chirality associated with the nonequivalent up-and-down pyrrole quadrons.2523253 Typically, nonplanar porphyrins in solution are capable of undergoing flip-flop of the two enantiomeric saddle forms giving rise to racemization. The racemization results in the difficulty to isolate the enantiomer from each other.254 Inoue and coworkers reported the photoinduced conformational ruffling of a “single-arrned” porphyrin, derived from etioporphyrin, with a single pivaloylamino group at one of mesa-positions.252 The mono-substituted porphyrin is achiral when planar since its mirror images can be superimposed by C2 rotation. However, the pivalarnide substituent at the mesa position is located on either side of the porphyrin plane due to the steric interactions with the adjacent substituents, thus making the porphyrin chiral. The enantiomers of the porphyrin were isolated by chiral chromatography. The 1H NMR spectra of the enantiomers were both the same, whereas the circular dichroism spectra were perfect mirror images of each other. The enantiomers racemized slowly at room temperature and no racemization was observed below 0 °C. The factors that influence the racemization process were also demonstrated. These factors include the size of the substituents, temperature, the nature of the central metals, bases, and photoirradiation. Recently, Aida and coworkers used a D2-symmetic fully substituted porphyrin that has a nonplanar structure as a conceptually new chirality sensor.253 The chirality sensor can recognize chiral carboxylic acids through self-assembly and memorizes the acquired 154 information within its skeleton even after the assembly is broken as evidenced by CD spectroscopy. In the presence of a chiral carboxylic acid one of the two conformers was preferred. The CD spectra of the (R)- and (S)-mandelate porphyrin complexes were perfect mirror images of each other. However, the absolute porphyrin conformations of the enantiomers have not been determined. We have synthesized a series of fully substituted chiral porphyrins by the introduction of either one or two chiral auxiliaries into the porphyrins as shown in Figure 6-2. Thus, a chiral environment around the metal center was created. The nonplanar chiral porphyrins we synthesized can be used for asymmetric epoxidation of alkenes and hydroxylation of alkanes. As noted above, the absolute conformation of the nonplanar chiral porphyrin has not been well established. In our system, one of the two conformations was preferred when a chiral auxiliary was attached to the nonplanar porphyrin. This has been observed by CD spectroscopy. The CD spectra of the (R) and (S) nonplanar porphyrins are mirror images of each other. The absolute conformations of single armed fully substituted porphyrins were determined by CD and X-ray Aspectroscopies, thus their correlation with CD profiles was established. Results and Discussion Synthesis The key feature in our preparation of chiral nonplanar porphyrins was to develop an efficient route for the introduction of various chiral auxiliaries into the porphyrins. To attach the chiral auxiliary to the porphyrin, we chose xanthene as the rigid spacer group to bridge the porphyrin macrocycle and the chiral auxiliary. The intermediate, diformyl xanthene 64, was prepared from xanthene 63 by treatment with butyl lithium in the presence of TMEDA and DMF, followed by hydrolysis as shown in Scheme 54.255 Xanthene 63 was prepared from commercially available xanthone 62 and trimethyl aluminum in toluene.256 155 Figure 5-2. Structures of nonplanar porphyrins with (a) one chiral auxiliary; (b) two chiral auxiliaries on the same side; (c) two chiral auxiliaries on the opposite side. 156 Q ff + o o N coza @7040 62 H O 65 ' TiCl4/CH2CI2 lAlMea / \ / O 53 EtO2C N N ‘C02Et O .. .. 66 1. BuLi ' NaOW(CH20H)2 2. DMF Q CHO / \ / \ O 64 N N O CHO H 67 H K J 1. BF OEt CH Cl T 2. 0030. 2, - 2 2 Ni(0Ac)2/DMF mixture 68 (inseparable) Scheme 5-1 157 We started the synthesis of the porphyrins with two xanthene bridges (Scheme 5-1). Initially, porphyrin 68 was chosen as the key intermediate, in which the formyl group on xanthene can be modified to create a chiral environment. Dipyrrylmethane 66 was obtained in 90% yield by the reaction of 2 equivalents of pyrrole 65 and benzaldehyde. Dipyrrylmethane 66 was decarboxylated in refluxing ethylene glycol containing NaOH to give dipyrrylmethane 67, which was then immediately coupled with diformylxanthene 64 in the presence of BF3'0Et2 followed by oxidation with DDQ to give a mixture of porphyrins.253 The use of p-chloranil resulted in incomplete oxidation even after refluxing the reaction overnight. Attempts to isolate the desired products, cis- and trans- porphyrin 68, were unsuccessful due to the scrambling of the porphyrin during the cyclization.257 It has been reported that the scarnbling can be suppressed by the use of solid catalysts such as silica gel258 or montmorillonite clay K-10,259 or of a dehydrating agent such as molecular sieves.253 However, attempts to minimize the scrambling during cyclization step using either solid catalysts or molecular sieves failed. Since it was impossible to isolate porphyrin 68 we employed diethylpyrrole 69 instead of ethylmethyl pyrrole 65 as the starting material. The use of diethylpyrrole 69 allowed us to reduce the number of porphyrin isomers even if severe scrambling of porphyrins occurred. A procedure similar to that of porphyrin 68 was empolyed as shown in Scheme 6-2. Dipyrrylmethane 70 can be prepared from pyrrole 69 and benzaldehyde either in CH2C12 catalyzed by T104260 or in ethanol in the presence of HC104(aq) in higher than 90% yield. Decarboxylation of dipyrrylmethane 70 gave dipyrrolemethane 71, which was then treated with diformylxanthene 64 followed by oxidation with DDQ to give a mixture of protonated porphyrins. Since the protonated porphyrins were inseparable with chromatography we inserted nickel into the porphyrins by treating with excess Ni(0Ac)2 in refluxing DMF. This allowed us to use chromatography to separate some of porphyrins from the mixture. The first band was identified as NiOETPP, which was formed due to scrambling during cyclization. The 158 H“ Atf /\ N H 69 0023 + cho \—/ CHO cm 64 T)C),/CH2C)2 / \ / \ Et02C N N C02Et H H 70 NaOH/(CHZOH)2 / \ / \ N N H H 71 1. BF oOEt CH Cl 2. 0030 2/ 2 2 Ni(0Ac)2/DMF mixture 73 (inseparable) 72 (separable) Scheme 5-2 159 aria-turn second band, which was the major product, was separated in 9% yield and identified as porphyrin 72. We expected that the third band was the trans form of porphyrin 73. Unfortunately, this band contains two porphyrins and they could not be separated. As noted, the use of silica gel, montmorillonite clay K-10, or molecular sieves could not minimize the scrambling. Our approach was then switched to the introduction of hindered substituents to the phenyl groups at mesa-positions. Dipyrrylmethane 74 was obtained in 72% yield from the reaction of pyrrole 69 and mesitaldehyde in CH2C12 in the presence of TiCl4 as shown in Scheme 5-3. Reactions using ethanol as solvent and HC104 as catalyst gave a lower yield and resulted in incomplete reaction. Decarboxylation of dipyrrylmethane 74 followed by reaction with diformylxanthene 64, oxidation with DDQ and nickel insertion afforded a mixture of nickel porphyrins. As expected, the introduction of hindered substituents into the phenyl groups at mesa- positions minimized the scrambling. The major product was the expected trans form of porphyrin 76 isolated with chromatography in 18% yield. It was hoped that the minor product was the cis form of porphyrin 74. However, the cis porphyrin could not be separated from the mixture, presumably due to scrambling. We also tried the preparation of porphyrin 79 using dichlorobenzaldehyde instead of mesitaldehyde as shown in Scheme 6—4. In this case, both the trans and cis forms of porphyrin 79 were separated with chromatography in 22% and 17% yields, respectively. We were able to anchor chiral auxiliaries to the xanthene bridge of porphyrin 76 and 79, which have two xanthyl groups at the opposite mesa-positions. The synthetic strategy for the synthesis of chiral porphyrins was shown in Scheme 54. The aldehyde on xanthyl group was converted to the nitrile by treating with hydroxylarnine hydrochloride in refluxing 98% formic acid for 24 hr under argon. We could not convert the nitrile to the carboxylate group using an alkaline solution of ethylene glycol. Thus, acidic conditions were employed. The nitrile was converted to the acid in a refluxing mixture of acetic acid, water, and concentrated sulfuric acid. The carboxylic acid was 160 rich/CH Cl Ar M9 W + Ar—CHO 2 2 / \ / \ 74 Ar=Me-Q- N 002E! Et02C N N COZEI Me H H H Cl 69 77 Ar = Cl NaOH/(CHZOH)2 O Me Ar ° + / \ / \ 75 “m“:q N N e H H o o) 54 78 Ar=Q— 1.BF oer 0 Cl 0' 2. 0013 2] H2 2 Ni(0Ac)2/DMF mixture cis-76 (inseparable) trans-76 (separable) trans- and cis-79 (separable) Scheme 5—3 161 NHonCI ——> trans-76 Ar = Met-Q— Me Cl cis- and trans-79 Ar = Q» CI Me trans-81 Ar = MeQ— Me Me Cl trans-82 Ar = Me-Q— cis- and trans-84 Ar = Me Cl CI cis- and trans-85 Ar = Q trans-82 (separable) cis-85 (separable) trans-85 (inseparable) Scheme 5-4 162 converted to the acid chloride by treating with thionyl chloride in refluxing CH2C12. After the solvent was removed in vacuo, chiral amines such as the (R) and (S) forms of 1- (1-naphthyl)ethylamine were coupled with the acid chloride of the porphyrin. However, a mixture of porphyrins was obtained for the preparation of trans-85 and isolation using chromatography was not successful. The yields for cis-85 and trans-82 were 20% and 13%, respectively. Obviously, the low yield for cis-85 was due to the steric interactions between the chiral auxiliaries on the same side of the porphyrin mean plane. The low yield for trans-82 could be explained by the increased steric interactions between the coming nucleophile and the porphyrin. In our system, one of the two conformations was preferred as the first chiral auxiliary was anchored to the xanthene. Once the conformation was fixed, the porphyrin lost flexibility and the steric hindrance between the incoming auxiliary and the pyrrole unit increases during the coupling, thus resulting in low yields. In Scheme 54, most of conditions used were acidic and dematallation occurred during the reaction. To facilitate the purification of the porphyrins, it is necessary to metallate the porphyrin with Ni(0Ac)2 in each step. To improve the yields of the chiral porphyrins, the effects of steric interactions during the reaction must be circumvented. Therefore, we switched to the synthesis of the chiral porphyrins with only one xanthene bridge. Two synthetic strategies, shown in scheme 5-5 and 5-6, were used for the preparation of the porphyrins anchored one xanthyl group. The first strategy shown in Scheme 5-5 was similar to that of TPP proposed by Lindsey.261 A 4:32] mixture of diethylpyrrole 86, benzaldehyde or pentafluoro-benzaldehyde, and diformylxanthene 64 was condensed in CH2C12 in the presence of BF3'0Et2 to give a mixture of porphyrins. After nickel insertion, the mixture was separated by chromatography to afford the porphyrins 72 and 87, in 11% and 10% yields, respectively. To improve the yield of the xanthene-anchored porphyrins, we also tried “2 + 2” condensation (Scheme 5-6).253 For the preparation of porphyrin 72, a 2: 1 :1 mixture of di-a-free dipyrrylmethane 71, bezaldehyde, and diformylxanthene 64 163 O O CHO O H WJ' ”CHO + o 1 BF OEt2/CH or; 0 Ar . C H CHO 2. 00%) 2 2 O % Ar 3. Ni(0Ac)2/DMF 86 Ar 64 MC}- F F 72 ”=0— 6 . . F F 87 Ar=F