HERARY Michigan State Universsty PLACE ll RETURN BOXtoromovombchockoumom your mood. To AVOID FINES Mum on or More data duo. DATE DUE DATE DUE DATE DUE Jm usu IsAnAMnnum MONEqual Opportunity Institution * Wilma-9.1 CHARACTERIZATION OF SEVERAL BORON-HETEROATOM COMPOUNDS AND THEIR REACTIONS WITH CARBON YL ACIDS By Jacqueline A. Iurchenko A THESIS Submitted to Michigan State University in partial fulfillment of the requirement for the degree of MASTER OF SCIENCE Department of Chemistry 1996 ABSTRACT CHARACTERIZATION OF SEVERAL BORON-HETERATOM COMPOUNDS AND THEIR REACTIONS WITH CARBON YL ACIDS By Jacqueline A. Iurchenko The purpose of this research was to study the reactivity of a series of boron - heteroatom compounds towards carbonyl acids. These compounds would serve as boron bases, in order to form boron enolates. The boron compounds synthesized include silylboranes, boron amides and boron phosphides. The ketones used in this investigation were pinacolone and diethyl ketone. The primary method of detection used to monitor the reactions was NMR spectroscopy. The enolization reaction for boron phosphides 19-21 is in competition with an addition reaction. The addition is a result of a nucleophilic attack of phosphorus on the carbonyl carbon. The extent of enolization for the bases was determined using 31P NMR spectroscopy. The most significant results were observed with compound 22, which showed nearly quantitative enolate formation and no indication of addition products. To Mom and Dad iii ACKNOWLEDGEMENTS I wish to thank Dr. Michael W. Rathke for his guidance and patience during the course of this research. I also wish to extend my thanks to Robert Elghanian, for putting up with me and assisting me in every possible way. To my family, Mom and Dad I wish to say THANK YOU. There are no words that can truly express my gratitude for the things you have done over the past several years, and throughout my entire life. To my brothers, Andy and Chris, a sincere thank you goes to you both. Growing up would not have been half as fun - or challenging! - without you. Finally, I wish to thank my Almighty Father for giving me my family to guide me and help me grow into the person I was made to be. ”All I ever have to be is what you’ve made me” Amy Grant vi TABLE OF CONTENTS List of Tables List of Symbols and Abbreviations I. Introduction II. Silyl Boranes Background Experimental III. Boron Amides Background Results Experimental IV. Boron Phosphides Background Results Experimental V. References vi vii 10 14 18 27 32 38 47 52 LIST OF TABLES Table 1: Comparison of Enolate Stereoselectivity with Benzaldehyde Table 2: Synthesis of Boron Enolates using dialkyl boron triflates Table 3: Synthesis of Boron Enolates using dialkyl boron halides Table 4: Synthesis of Silylboranes Table 5: Reactions of Silylboranes with ketones Table 6: Synthesis of Boron Amides Table 7: Reactions of Boron Amides with ketones Table 8: The first Boron Phosphides tested with carbonyl acids Table 9: Synthesis of Boron Phosphides Table 10: Reactions of Boron Phosphides with ketones vi Page 2 12 13 28 29 41 43 LIST OF SYMBOLS I - dimethylaminobis(trifluoromethyl)borane - (CF3)2B=NMe2 II - bis(trimethylsily1)aminodifluoroborane - FzB-N(SiMe3 )2 III - bis(trimethylsilyl)aminodichloroborane - C12B-N(SiMe3)2 LIST OF ABBREVIATIONS 9-BBN - borabicyclo[3.3.1]nonane But-Li - Butyl Lithium Bz - Benzyl group Dabco - 1,4-diazabicyclo[2.2.2]octane DBU - 1,8-Diazabicyclo[5.4.0]undec-7-ene DIP - diisopinocampheyl DPEA - Diisopropyl ethyl amine Et - Ethyl group (-CH2CHa) i-Pr - iso-Propyl group Me - Methyl group (-CHs) Mes - Mesityl group NMR - Nuclear Magnetic Resonance OTf - Triflate group Ph - Phenyl group s-Bu - secondary butyl group t-Bu - tertiary butyl group THF - Tetrahydrofuran TMS - Trimethylsilyl (SiMe3) vii INTRODUCTION Regiochemistry and stereochemistry both play an important role in the design of synthetic strategies. One well-known carbon - carbon bond forming reaction is the Aldol condensation. The aldol condensation uses an enolate, which is generated by removing an alpha hydrogen from a carbonyl compound with a strong base. The enolate anion then acts as a nucleophile towards a second carbonyl component, as shown in Eq. 11. O O o )k R Ru Base OH R' Aldol condensation product (1) Enolate chemistry is very important in organic synthesis because it offers chemists the ability to control both the regiochemistry and stereochemistry of the newly formed carbon - carbon bond. The goal of this research was to synthesize a boron heteroatom base that would form boron enolates. At present, boron enolates show a greater degree of stereoselectivity, in the aldol condensation, when compared to other metal enolates”. Enolate Metal (M) ErythrozThreo OM Li 8O :20 /K/ 3(04H9)2 >97 :3 CM Li 48 :52 6 B(C5H9)C5H13 4 :96 AI(QH5)2 50 :50 The general formula for the boron heteroatom bases we examined is LzB-ZRz, where Z is the heteroatom (which will be the proton acceptor), R represents various hydrocarbon ligands, and L is either a hydrocarbon or a heteroatom. To be useful bases in the generation of boron enolates three characteristics must be considered. First, the base should be strong enough to generate the enolate. Second, the base should deprotonate the carbonyl fast enough so that enolate equilibration does not compromise stereoselectivity. The third criteria should be a high degree of stereoselectivity in forming the enolate, this can be achieved by a six atom ring transition state, as shown in Eq. 7. L L \B/ O 0" ~‘?R2 Lea-2R2 + CH3 __. 1‘ ,A H H H CH3 0/ 0/ In this project we investigated three general classes of boron heteroatom bases - silyl boranes (Z = Si), boron amides (Z = N), and boron phosphides (Z = P). The rationale for choosing these three classes is as follows: for the silyl boranes, a driving force for deprotonation would be the replacement of a weak boron - silicon bond with a stronger silicon - hydrogen bond. Boron amides are boron analogs of the widely used lithium amide bases. Boron phosphides have a slightly longer boron heteroatom bond, which indicates less double bond character, when compared with boron amides. The longer bond may enable boron phosphides to be stronger bases than boron amides. This research involved the synthesis and characterization (NMR) of the boron heteroatom bases. Following synthesis was the characterization of the reaction of each boron base with representative carbonyl compounds. Egtures of Enolates Enolates are anions, and can made with various counterions such as Li, Mg, Zn, A1, and B. The convention for assigning enolate geometry, Z or E, depends on the relationship between the a-R group of the enolate and the position of the counterion, Eq. 3 illustrates“. M 0/” 0/ R1 \ R2 R1 \ H H Hz 2 E (3) As mentioned earlier, among the different metal enolates the boron enolate shows the greatest stereoselectivity5I6. The enhancement in stereoselectivity is derived from several different structural features. The boron enolate is unique because it does not exist as an oligomer in solution, like other metal enolates7. In addition, the boron aldolates (salt of the aldol) exist in a six-membered ring, Eq. 4 and 5. This transition state is in the chair 4 conformation and has no aggregation or chelation6. Lithium aldolates, which are also cyclic, may not involve covalent bonds between the lithium and oxygen atoms3. The short oxygen - boron bond, present in boron aldolates, allows them to be more compact, which enhances the steric interactions that control stereochemistry6. The stereochemistry of the aldol products obtained from boron enolates is consistent with the assumed transition state, Eqs. 4 and 5. The E isomer yields the anti, or threo, aldol condensation product while the Z isomer gives the syn, or erythro, product4. L R. I 0 9H H-o’§B\L E R" I.)=d ——» R' R R R” H u E- en 0' ate Ant: or threo product (4) L R' I O OH -O/?\ . H I. -—d —»L R R R R" R" Z -enol at e syn or erythro product (5) Another consequence of the covalent nature of the bonding present in boron enolates is that boron aldolate complexes are stable. The stability of the complexes is reflected in the lack of equilibrium between the threo and erythro diastereomers, in refluxing ether for several hours, Eq. 6 35“. However, equilibrium between the threo and erythro aldolates is observed when the counterions are lithium, magnesium, potassium, or zinc3. R R R \ /R \ / /B\ Refluxing Ether /B\ 0 0 (several hours) 2 0 __5 E \ Ph Equilibration between the threo and erythro diastereomers lowers the stereoselectivity of subsequent reaction steps. The marked enhancement in the stereoselectivity exhibited by boron enolates makes them valuable reagents to chemists. Review of Boron Enolate Synthesis Tables 2 and 3, below, show the two most common methods used to synthesize enolates. Both methods use a tertiary amine base, a ketone, and a dialkyl borane with either a halogen or a triflate leaving group. IIIES II [E II . I’llll I'lll O Amine Base L28X + \)l\ ——’ E/Z Boron Enolates -78°C L X 3.3533 n-C4 H9 O-SOzC F3 2,6-lutidine C-C5H9 O-SOzMe DPEAa C2H5 O-COCMea NEIa L1-C5H9 pyridine L2-C5H13 Dabco” DBUc 1 ,1 ,3,3-tetramethylguanidine a) Diisopropyl ethyl amine b) 1,4-Diazabicyclo[2.2.2]octane c)1 ,8-Diazabicyclo[5.4.0]undec-7-e ne In table 2, where X = OTf, the ammonium triflate will precipitate out of ethereal and hydrocarbon solvents. The affects of ketone structure, the tertiary amine base, and the boryl triflate were studied and each individually contributes to the selectivity of the reaction3. IIIE'S || . [E || . l'llll Ill 0 Amine Base [-sz + —-> E/Z Boron Enolates ~78°C L x 9-BBN Cl (ficyclooctyl Br exonomomyl I disiamyl 2,5-dimethylcyclohexane In table 3, where X = halogen, the precipitation of an amine hydrochloride salt is common, in organic solvents. The boryl triflate reagents were reacted with many carbonyl systems including carboxylic acids, anhydrides, acid chlorides, esters, amides, thioesters, and ketoesters9. With the exception of acid chlorides, esters, and amides all the above functionality undergoes nearly quantitative enolate formation, the carboxylic acid requires two equivalents of boron reagent9. From the reactions described in the tables above, formation of the E enolate is favored in dilute solutions using non-polar solvents, while the formation of the Z enolate is favored in concentrated solutions using more polar solvents“). In addition, bulky bases favor the formation of the Z enolate while B enolate formation is favored by less bulky bases“). An advantage to the above method is the lack of solvent dependence. A small, but consistent solvent effect was noted, for a given boron ligand. In general, non-polar solvents results in a more compact transition state that enhances the stereoselectivity? A disadvantage for these systems is when a less hindered bases are used there is an irreversible complexation (Eq. 7) between the base and the boron reagent3. Because the complexation occurs before enolization the overall enolate yield is lows. 3sz + R3N :- ngl)—fiaa X (7) The 1,4 hydroboration of an a,B unsaturated acyclic ketone, shown in Eq. 8, is another method used to synthesize boron enolates“. BR2 0 H 0/ RZBH + /\)J\ —> M R = disiamyl dicyclohexyl diisOpinocampheyl di-2—isocaranyl (8) The 1,4 hydroboration reaction allows selective formation of the Z enolate from a variety of E ketones. The transition state is thought to be boat- like“. The subsequent aldol condensations are very stereoselective, selectivly giving the expected syn isomer“. For the reaction shown in Eq. 8, it is also possible to generate boron enolates from amides, imides, and esters using catalytic amounts of Rh(PPH3)C111. Several indirect methods of generating boron enolates were developed. These methods involve the use of a-diazocarbonyls, halogen-substituted enolates, and sulfur ylides with trialkylboranes, as shown in scheme 14. From these indirect methods is the enolate has incorporated into its own structure one of the ligands from the trialkylborane, this may limit the synthetic utility for these methods depending on what product structure is desired4. (Raga \ R1 CHN2 / O > I + R1/kgHBl' Labs. 0 (Rz)33 R1*§H-§Me2 J R2 Both BCl3 and PhBC12 can also be used to synthesize boron enolates. With BC13, to improve the yield of enolate, the ketone and BC13 are mixed together before the addition of base, this is done to overcome the boron- amine complexation (Eq 9)”. [SCI 3 .1 BCla \/|(|)\/ M \i/ + [ HEIDEtPr'zfi \ (9) The erythro, or syn, isomer, is selectively generated with PhBClz, Eq. 10. This reagent is also useful because it is easier to handle than dialkylboron triflates“. I- e I- — «- BPhC epncu o a / I2 0/ /u\ PhBClz i i-PerEt A ———> Et Et _ Et Et d El CHCH3 Ketone quantitative eyrthro The synthesis of a fast acting boron heteroatom base that can quantitatively deprotonate a carbonyl, through a cyclic transition state, that results in a boron enolate was the ultimate goal of this project. There are currently no examples of this kind of base reported in the literature. A boron heteroatom base offers the direct formation of a boron enolate, without the need for any additional reagents, such as excess base or ketone. The reactions of boron heteroatom bases are mild, 1:1 reactions between the base and the carbonyl acid. In addition, the boron enolates generated will offer superior selectivity in the aldol condensation. CHAPTER 1: SILYLBORANES Smthesis and Properties of Silylboranes The synthesis of silylboranes was accomplished, in the literature, using two methods. The first method, shown in Eq. 11, is a salt elimination reaction between a silicon anion and a dialkyl boron chloride specieslSaJSb. RzB-Cl + LiSiR'g —> FlzB'SiR'a + LiCl R = Et. NMe2 R' = Ph, Me, Et (11) The second method, shown in Eq. 12, is the combination of a dialkylboron chloride, a chlorotrialkylsilicon and two equivalents of an alkali meta115c. 2K RZB-CI + cuss»:3 —> RzB-SiR’a + KCI R = Et, NMez R' = Ph. Me. Et (12) So far this field of chemistry has focused primarily on the synthesis and characterization of various silylboranes that include -- tris(trimethylsilyl)- boranesle, (trimethylsilyl)boranes15C, (trimethylsilyl)boratesl5, and alkoxysilyl(amino)boranes17. Characterization of these compounds involved nuclear magnetic resonance (NMR) and mass spectrometry. A limited amount of information is available about the reactions of silylboranes of the type RzB-SiR’3. However, the chemistry of trimethylsilylbis- (dimethylamino)borane, (MezN)2B-SiMe3 is well documented. These reactions allowed us to look at the types of reactions that silylboranes are capable of, and from this information we could determine if silylboranes would be reasonable candidates for our study. In two separate papers, Noth synthesized trimethylsilylbis- (dimethylamino)borane and then studied the reaction, shown in Eqs. 13 and 14, with hydrochloric acid15a. 10 ll (NMeth-SiMeg ifl. NMezBl-SiMeg + lHfiMezlgl Cl (13) 3 HCI ———-> a e (NMeth-SiMeg NHMez-ClzB-SiMea + [HzNM°2lC' (14 ) The reactions shown in Eq. 13 and 14 illustrate the behavior of the silyl boranes in the presence of strong acids. Eq. 15 illustrates the reaction of alcohols with trimethylsilylbis(dimethylamino)borane1SC. . ROH (NM92)23'S|M63 4’ = B(OR)3 + Me3$iH (15) Noth also reported on the reactions of trimethylsilylbis(dimethyl- amino)borane with a diol and a diamine, as shown in Eq. 1615C. (NMesz-SIM63 + I I ——> - - + 2 HNMe ZR ZR R Z\B/Z R 2 |. When 2 .. N, R . Me S'Mea WhenZ=O,R=H (16) The above reactions, Eq. 15 and Eq. 16, are analogous to the reaction we want to accomplish. The silylborane dissociates, the boron bonds to an oxygen while the silicon becomes protonated. Our reactions would employ a less acidic carbonyl. However, evidence like the above reactions illustrate the potential silyl boranes have to be boron bases. Results We surmised that a silylborane, RzB-SiR’g, might function as a boron base towards carbonyl acids. A possible driving force may be the replacement of a weak boron - silicon bond (289 kj/mol) with a strong silicon - hydrogen 12 bond (531 kj/mol). The compounds shown in Table 4 were synthesized through the reaction of a silicon anion with a dialkylboron chloride. Ill I'S || . lS'llB RzB-Cl + LiSiFl'3 —. 923-3iR-3 + LiCl Compound 9 n' R(x) Temp. (°C) 1 o- CsH402 Me 25' 2 CI Me 25b 3 06H“) Me 25° 4 Cl SiMe3 25b 5 O-C5H402 SiMea 25a 6 06H“, SiMe3 25° a) solvent : 1:1 Hexane:Benzene b) solvent : Hexane c) solvent : Benzene Compounds 1-6 were characterized by both 29Si and 1 H NMR. Their behavior with two representative ketones was monitored with 1H NMR. The results of these studies are summarized in Table 5. 13 RzB-SiR'g + Ketone _. Compound pinacolone diethyl ketone Time'II 1" no reaction no reaction 24 hrs. no reaction no reaction 24hrs. no reaction no reaction 24hrs. 4“ no reaction no reaction 24hrs. 5" no reaction no reaction 24hrs. 6(1 no reaction no reaction 24hrs. a) At room temperature b) in THF 0) in Hexane d) in Benzene As Table 5 shows silyl boranes 1-6 are inert when combined with either a methyl or an ethyl ketone. The 1H NMR remained unchanged in all cases after a twenty-four hour period. As a result of the inactivity of the silyl boranes we have shown that these compounds do not function as bases to generate boron enolates from ketones. 14 EXPERIMENTAL The solvents used - THF, benzene, and hexane - were dried over calcium hydride, distilled and stored in an inert N2 atmosphere prior to use. A1131P, 11B, 295i, 13C, 1H NMR data was obtained using the either the Varian 300 MHz or Gemini 300 MHz NMR. The external references for 31 P, 11B, 29Si spectra were HaPO4, BF3 032, and TMS respectively. Unless otherwise stated the internal reference for the 13C and 1H data was CDC13 with TMS. All reagents and reaction products were handled in an inert atmosphere of N2. NMR samples were sealed in 5mm NMR tubes, capped with a rubber septum and secured with Teflon tape. Synthesis of tetrakis(trimethylsilyl)sila£l§; 0.914 moles (50% excess) of lithium flatted lithium rods were put into a 500 mL round bottomed flask (equipped with a magnetic stirrer and N2 inlet/ outlet) and washed with dry hexane. The cleaned lithium was then suspened in 100 mL of dry THF and 0.358 moles of TMSCl was added using a syringe. A 250 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, was charged with 75 mL of dry THF and 75.3 mmoles of SiCl4. Using a syrings 20mL of the SiCl4 solution was added to the lithium metal/TMSCl soution. The mixture warmed slightly and turned dark brown in color, after 4 hours of stirring the remainder of the SiCl4 mixture is added and the mixture is stirred at room temperature overnight. The solution was filtered into a bed of Celite, in a buchner funnel attached to an aspirator, to remove unreacted lithium metal. The filtrate was then refiltered over a fresh bed of Celite. The filtrate was drowned in 50 mL HCl/ 150 mL H20, at 00C. The resulting 2 layers were extracted twice with 100 mL of ether and the ether layer was evaporated under reduced pressure. The yellow solid product was recrystillized using acetone to yield 9.36 g (40%) tetrakis(trimethylsilyl)silane, which is a white solid. The 29Si NMR has resonances at -9.7 ppm (Megsi) and -135.5 ppm (Megsi)4Si ppm. Synthesis of tris(trimethylsilyl)silyl lithiuml§; 6.5 moles of tetrakis- (trimethylsilyl)silane was placed in a 50 mL round bottom flask and pumped 15 dry overnight with a vacuum. The 50 mL round bottom flask was then equipped with a magnetic stirrer and N2 inlet/ outlet line, and charged with 25 mL THF and 6.6 moles of methyl lithium. The solution turned bright yellow. After 24 hours the solvents were removed under reduced pressure and the solid residue was stirred in pentane for 2.5 hours. The solution was canula filtered into a new 50 mL flask, that was flushed with N2 and equipped with a magnetic stirrer and N2 inlet/ outlet line. At -780C, crystals formed in the flask and the solvents were removed with a syringe. The crystals were then recrystalized with pentane : THF solution. The product (>95% yield) is a pale yellow solid and the 29Si NMR has a single resonance at -5.3 ppm. Synthesis of BMmethylsilylgmemlborme (1): A 10 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, was charged with 3.0 mmol of a 1:1 hexane : benzene solution of cathecol boron chloride. At room temperature, 3.0 mmol of the lithium salt of hexamethyldisilane was added, via syringe. The mixture became slightly warm and a solid precipitate was formed upon completion of addition. The solution was allowed to stir at room temperature for 30 minutes, the solvents were evaporated and NMR analysis was performed. 11B : (C5D6) - 19.66, 23.13 ppm Synthesis of B-trimethylsilyldichloroborane (2): In a 10 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 0.5 mmol of a hexane solution of boron trichloride was added. At room temperature, 0.5 mmol of LiSiMe3 was added using a syringe. The solution was allowed to stir at room temperature for 30 minutes, the solvents were evaporated and NMR analysis was performed. 11B : (Hexane: D2O,external) — 28.75 ppm thesis of B-trimeth lsil 1dic clohex lborane 3 : In a 5 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 1 mmol of dicyclohexyl boron chloride was dissolved in 0.5 mL of hexane. At room temperature, 1 mmol of LiSiMe3 in benzene, was added with a syringe. The 16 solution was allowed to stir at room temperature for 30 minutes, the solvents were evaporated and NMR analysis was performed. NB : (c606) - 73.45 ppm Synthesis of B-tris(trimethylsilyl)silyldichloroborane (4): A 25 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, was charged with 0.6 mmol of a hexane solution of boron trichloride, an additional 0.5 mL of benzene was also introduced. At room temperature, 0.6 mmol of a benzene solution of tris(trimethylsilyl)silyl lithium was added. A salt precipitate was formed upon completion of the addition. The solution was allowed to stir at room temperature for 60 minutes, the solvents were evaporated and NMR analysis was performed. 11B : (Cal-16, D2O external) - 10.49 ppm 1H: (CsDe) - 0.232 ppm Smthesis of B—tris(trimethylsilyl)silylcathecolborane (5): In a 25 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 0.6 mmol of a 1:1 benzene : hexane solution of cathecol boron chloride was dissolved in an additional 0.5 mL of benzene. At room temperature, 0.6 mmol of a benzene solution of tris(trimethylsilyl)silyl lithium was added. A salt precipitate was formed upon the completion of addition. The solution was allowed to stir at room temperature for 60 minutes, the solvents were evaporated and NMR analysis was performed. 113 : (céHé, D20, external) - 39.67, 21.58, 7.75 ppm ngthesis of B-tris(trimethylsilyl)silyldicyclohexylborane (6): In a 25 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 0.6 mmol of dicyclohexyl boron chloride was dissolved in 0.5 mL of benzene. At room temperature, 0.6 mmol of a benzene solution of tris(trimethylsilyl)- silyl lithium was added. A salt precipitate was formed as the addition proceeded. The solution was allowed to stir at room temperature for 60 minutes, the solvents were evaporated and NMR analysis was performed. 17 11B : (Cd-I5, D20, external) - 51.692, 52.02 ppm (doublet) 1H : (C6145, D20, external) - 0.24 ppm CHAPTER 2: BORON AMIDES Literature survey of Boron Amides Boron amides can be viewed as boron analogs of lithium amides, which are very powerful bases. However, electron diffraction data of several boron amides exhibit short boron - nitrogen bonds. The average bond length of a boron amide is 1.41 angstroms while a typical single bond is around 1.58 angstroms“). In addition, many unsymmetrical boron amides exhibit restricted boron - nitrogen bond rotation and can exist in cis/ trans isomers in solution”. The shortened boron - nitrogen bond and the cis/ trans isomerization present both imply a strong pi bond moment between boron and nitrogen”. Both of these factors may limit how effective these compounds are as boron bases. Boron amides are well reviewed in the literature. Dimethylamino- bis(trifluoromethyl)borane, (CF3)2B=N(CH3)2, (I), was studied in great detail by Ansorge and co-workers. Dimethylamino-bis(trifluoromethyl)borane is thought to be planar through comparison, by electron diffraction, with other boron amides and from data obtained from theoretical calculations”. The data indicate the planarity comes from a boron - nitrogen pi bondzo. Reaction of (I) with 1,3 dienes and B-unsaturated ketones gave [2+4] cycloaddition and B-alkylation products, shown in Eq. 1720. 18 19 0 CF M M9 0 (CFalz 3‘9 W 9 H \9/ = + Me ——> I l / \ 59 M9 Me Me Me [2+4] cycloaddition O CHzB(CF3)2 Me Me B-alkylation (17) The reaction of (I) with terminal alkynes and terminal alkenes yields aminoboration products (Scheme 2). For the reactions shown in scheme 2, the methyl group on nitrogen is a hydride source”. Scheme 2: Reaction of (I) with various functional groups F30\- ,IMe HCECR B=N ——>RHC=CHB(CF3)2 . CH3N=CH2 F3C’ | Me \KC:CR RHC -CHB(CF3)2 . CH3N=CH2 0 ll C(CF3)2 (F30)2C(H)O-B(CF3)2 . Cl~|3N=CHz F3C(H)C=NB(CF3)2 . CH3N=CH2 The reaction of (I) with epoxides, (Eq. 18), gave ring expansion. When the epoxide is symmetrical, polymerization results”. 20 HC\—’CHZ ' + HZCL\CHR e /NM°2 0‘?CF3)2 Internal SNZ/ \SN1H2 CH2 \CHR «$53-$92 “30%;”... R - Me, Et, Bz, CF3,CH2F R = Ph (18) Eq. 19 shows (1) underwent an ene—type reaction with thiocarbonyl compounds23. R” R‘ R' S _ Rearrangement _>=S R"6-CH2R' + I —> :___R.. E?— ?? 9.2.x” F3 0““ C‘ "’IMe F.2‘C‘“ ‘ (”Me L F3C Me _ 3F30 M8 Reaction will stop here if R' = Me and R' = SB (19) The type of reaction shown in Eq. 19 is similar to the reaction we plan to explore between boron amides and carbonyls. The enethiol intermediate shown in the above reaction is a sulfur analog of a boron enolate. Ansorge also reported addition of (I) to isocyanates and isothiocyanates, (Eq. 20)“. 21 10°C Mari/x RN=C=X + l —> (CF3)23| —u;2 X - 8.0 R . Me,t—Bu,Et Ph 1 6° C . lNMez lsomerizes at 60 C. If R: t-Bu or Ph RN —C / then no isomeriztion. mm X = S and R = Ph will isomerize. (CFalzB x - I NMGZ / “l-i’ (CF3)23'—x (20) In 1994, the first boron-nitrogen analogs of cyclopropane were synthesized by the adding (I) to carbenes, shown in Eq. 212525. R, .R I + ch=fl=fi --—> ,C\ (F30)2B—NMe2 R = H, SiMeg, CHzPh Stable up to 90°C (21) Brauer, Eq. 22, studied the addition of HX across the boron - nitrogen bond of (1)27. (F30)2@=@HRZ + HX —-> (FaC)2(X)B-NHR2 R = Et, i-Pr X=Cl,Br,F,OH (22) The reaction shown in Eq. 22 shows the behavior of boron amides with strong acids. We want to get similar results using a less acidic proton on carbonyl compounds. Burger studied, Eq. 23, the alkylation of (I) by 1- alkeneszs. When a bulky group is placed on the alkene there is a hydride transfer from one of the methyl groups on nitrogen to the olefinic carbon, Eq. 2423. 22 3 F F Fac \Qla‘ HMe F3C >393 C/3CF3 H C=CMe 8 2 M92”? 1 I ‘fiHMe l 2—i "'2C H2C 2 \C=CH2 \c’ Me/ “ CH2 (23) 01:3 | Me CF = I, —" — I H20 4., Hzc‘ sz H2C/ \\C R = s-Bu, Ph, Mes H/‘C\- ‘H \C 2 SiMea, SiEta 3 H2“ (24) Burger also showed that (1) underwent an ene-type reaction with nitriles and carbonyl compounds. Eq. 25 shows the ene-type reaction with a nitrile, which forms in a boron - carbon bond29. R 622.5“ N" \H (F3C)2 )é—NMez l I + NECCH2R —> N i 3' Rearrangement ,CH HC’R H 4: //C F C \€ I N\ |ll ‘ 3 ’2 We. (Face-€113 92 (25) The reactivity of (I) with alkenes, nitriles, and carbonyl groups shown in the above equations show the electrophilic nature of the boron as it adds across the multiple bond. Two of the boron amides studied in this project were also synthesized and studied in the literature, bis(trimethylsilyl)aminodifluoroborane, F2B-N(SiMe3)2 - (II), and bis(trimethylsilyl)aminodichloroborane, 23 Cl2B-N(SiMe3 )2 - (III). Geymayer published a procedure, Eq. 26, for the synthesis of (II)3°a. -7e°c . BF3.OEt2 + NaN(SiMe3)2 ——> FzB'N(SIM63)2 (26) Russ synthesized (11), Eq. 27, by heating BF3 and tris(trimethylsilyl)- amine31. 13 ° , 131:3 + N(SiMe3)3 ——> FzB-N(SIMea)2 + TMSF (27) In a third publication, Gerrard synthesized (11), Eq. 28, through the loss of HCI from a boron-amine complex using a secondary amine base32. -HCI by R2NH ——> (Measi)2NH.BC|3 (MeaSibNBC'z + [Him2 18' (23) Although many reactions of compounds (II) and (III) are reported in the literature, there are no current reports of any reactions with ketones or aldehydes. However, following reactions of (II) and (III) do indicate the potential these boron amides have to behave as boron bases towards carbonyls. This information encouraged us to study the reactions of these compounds in more detail. Klingebiel and co-workers investigated the reaction of (II) with the lithium salt of hexamethylcyclotrisilazanes, (Eq. 29)”. '1' H -/N~ . (11) .,N\ . Mezsl' SIMez ., Me2SII SlMe2 HN\ ,.N Li HN\ ,,N-BFN(SiMe3)2 S'l SI Mez Mez 1) But-Li \mm RFN(SiMe3)2 1) B tL' jFMSiMeah U‘ I . \ . Mezsi'fl‘SiMez ‘— M9281| SIMez (MeasnzNFe-m i.N-BFN(SiMe3)2 2) l") HN\Si.N-BFN(SIMea)2 s . M92 M62 (29) 24 Eq. 30 shows the reaction that Meller studied that involved (II) and 3,4 lutidine34. _ Hexane F _ - Na/K ——-> N--BN(SiM6 ) / an \. / 3 - NaF/KF ‘ ‘ 2 (30) In a series of reports Elter documented the reactions of (II) with various silylamines, Eqs. 31-36 35:36. 3(Me38i)2N 31:2 _. C[(Me3$i)2N- -BF + TMSF 3 (31) IF (ll) Leash!“ (M6351)2N'B’+ TMSF N(SiMe3)2 (32) (II) (W (MeasihN-B-MSiMeah .1. TMSF 200°C F (33) (ll) Measi—NRZ (MeeSi)2N -3_ -NR2 + TMSF R=Meet F n-CgHg, n-butyl (34) (II) + (MeaSiiz-NR —-> (Megsi)2N-B'—i\|l-SiMe3 R=Me,Et F n (35) 25 Measi Me3Si\ ,F (II) + :N—H —> N-B I \ R R F a = i-Pr, Me (36) Elter also disclosed the reaction of (II) with organolithium compounds, shown in Eq. 3737. F (II) + RLi ——>(Me3s1)2N-BjR + LiF n = 2.4.6-tri-t-butyl phenyl 1 Heat Mafia-55R + TMSF (37) Klingebiel, in two separate publications, investigated the reaction of (II) with both tris(trimethylsilyl)methane and tris(trimethylsilyl)silane, Eq. 38 38,39. , F (H) + (MeasilsMLi (Measi)2N-ef M(SiMe3)3 M =c, Si 400-5oo°c M25113; M(SiMe3)3 (38) The chemistry of the dichloro derivative, C12B-N(SiMe3)2 - (III) was also studied. Two separate reports showed that when three equivalents of (III) were heated to 140°C the products obtained were the corresponding borazine‘wv41 or diazadiboretidine40, Eq. 39. CI ' Cl SiMe 140°C . B\ . \ / 3 3 (M8331)2N-BC|2 Me381N / NSIMe3 + BI_NI CIA SCI N—B \N/ / \ M8381 Cl Sims (39) 26 Neilson studied the reaction of (III) with several nucleophiles, shown in Eq. 40-4342. (m) t-butyl Lithium pi —.> ' Ether 0°C (MGSS'kN-B‘t-butyi (40) M ' H l Cl 0") MC (Megsi)2N-e' . Ether 0°C ‘CstiMee (41) i-PngCl ,CI (Ill) ——> (MegsihN-BV Ether 0°C I—Pr (42) MeasiNMez _ , (Ill) ————> (MeasllzN-B. - TMSCI ””92 (43) Wells and co-workers studied the action of lithium aluminum hydride, Eq. 44, on (111)“. LiAlH4 , (III) ——> (Me3Si)2N-BHZ Ether 0°C (44) Wells also studied the reaction of (III) with 1,1,1,3,3,3-hexamethyl- disilazane, Eq. 4544. (Megsi)2NH , (13' . (Ill) _, (Me381)2N-B-NHSiMe3 (45) Currently, there are two papers that report a reaction of (I) with ketones. For these special cases (I) acted like a boron base, Eqs. 46 and 47, and a boron enol derivative is formed, in small yield23129. 27 B(CF3 )2 . NHMGZ F3C / Me \ / ° 0 B=N 1' ——> / \ i . F36 Me F30 CH3 F3C CH2 (46) /B(CF3)2 . NHMez F3C M6 0 O \B_N/ + \Jk/ —* W / \ \ F30 Me (47) These reports showed that through modification of the ligands on boron it is possible to synthesize a boron enolate with a boron amide. For the above two examples, it is the action of the CF3 group on boron that enhances the electron deficiency of the boron, despite the pi bonding from the nitrogen”. With this in mind we set out to find other boron amides that could enolize carbonyl acids. Results The three boron amides pictured in Eq. 48 were synthesized in our laboratory, through the 1:1 reaction of dicyclohexylboron chloride with the lithium salt of the secondary amine. None of these compounds give boron enolates as products when reacted with ketones. The assumption for the lack of reactivity is the presence of the boron - nitrogen pi bond. Chx /R B—N R - Phenyl, isopropyl, and SiMe3 Chx \R (43) Because of the reactivity of (CF3)2B-NMe2 we hoped that by further modification of the ligands on both boron and the nitrogen we may be able to synthesize other suitable boron amides that would function as boron bases. Boron amides 7-17, shown in table 6, were synthesized through the action of a lithium dialkylamide on a boron halide. 28 I II 5.5 I . [E E 'I RzB‘X + LiNR'2 _. RzB-NR'z + LiX Compound R X R' R(x) Temp. (°C) 7 F F SiMeaa 0° 8 Cl CI SiMea 0° 9 F F Ph“ od 10 CI Cl Ph -78° 11 F F i-Prza 0° 12 F F H,COCH3" 0b 13 CI 01 H,COCH3 0b 14 F F H,COPha 0° 15 cu Cl H,COPh 0b 16 ”'0‘; CI H, t-butyl 0' _N.’ Me 17 NMe2 Cl H, t-butyl 0a a) Ratio EN = 2:1 b) solvent : Hexane c) solvent : Ether d) solvent : Benzene e) solvent : THF f) solvent : Toulene After characterization of compounds 7-17 with both 11B and 1H NMR each was then individually reacted with pinacolone and diethyl ketone and 29 monitored using 1H NMR. The results of those reactions are summarized in Table 7. III IE I' [I 'l '|||| Rza-NR'Z + Ketone R'T' Compound pinacolone diethyl ketone Time 7° no reaction no reaction 9 hrs‘I 8° no reaction no reaction 10hrs° 9° sell-condensation overnight" 10° sell-condensation 1Ohrsb 1 1° self-condensation overnightb 12° no reaction no reaction 18hrs‘I 13° no reaction no reaction overnight“ 14" no reaction no reaction overnighta 15° no reaction no reaction overnight‘3 16" sell-condensation ovemight' 17° no reaction no reaction overnighta a) At room temperature b) In a 50°C water bath c) in Benzene d) in THF 0) in Toluene Evidence of any reaction was determined by a change in the proton NMR spectra. Both the disappearances of the a - hydrogen peaks (1.76 ppm for pinacolone and 2.0 ppm for diethyl ketone) and the appearance of enolate peaks (approx. 4.0-5.0 ppm) indicated that enolization had occurred. No evidence of any such change in the 1H NMR spectra for compounds 7, 8, 12- 15, and 17 was observed. 30 However, compounds 9—11 underwent self-condensation. Self- condensation was also observed by Sugasawa, who used C12 B-NEtz - diethylaminodichloroborane45. In his report, Sugasawa explained that the vinyloxyaminochloroborane assisted in the self-condensation reaction of the ketones. The carbonyl compounds used in Sugasawas study included cyclohexanone, propanal, and cyclopentanone. A typical reaction involved the combination of the ketone, diethylaminodichloroborane, and triethylamine in a 1:1:2 ratio, respectively“. The mixture was stirred in dichloromethane at 4-8 0C, for 20 hours. The product obtained was the boron enolate, a vinyloxyaminochloroborane, Eq. 49. If the reaction was allowed to occur at a higher temperature, the self- condensation product would result45. This would explain why boron amides 9-11 and 16 all gave the self-condensation products, the reaction temperature for these reactions was at, or above, room temperature. 0 0 CI -3’ \ (+3 + EtzNBClzfla—> or NEt2 + [HNEtaa (49) Sugasawa also reported that the use of triethylamine is necessary, Eq. 49, to help prevent self-condensation. The triethylamine also plays a role in the diastereoselection of the subsequent aldol condensation reaction“. H R M 9'13 '53" v ”as o 43’ 2 / H ' R=¢NO Ph /T5H H30 1 H30 0 l HF; 'i' g 3 H . 'H C H 31 When triethylamine was present the ratio of threo to erythro products was 2.4:1, without the triethylamine, the diastereoselection increased to 5:1 (threo to erythro)”. The two proposed transition states, Eq. 50, show that in the presence of triethylamine, the protonation adjacent to the carbonyl that would lead to the erythro product is enhanced by the NEtz group, which helps block the upper face that results in the threo isomer. 32 EXPERIMENTAL The solvents used - hexane and diethyl ether - were dried over calcium hydride, distilled and stored in an inert N2 atmosphere prior to use. All 31F, 11B, 13C, 1H NMR data was obtained using the either the Varian 300 MHz or Gemini 300 MHz NMR The external references for 31F, 11B, spectra were HaPO4 and BF3OEt2, respectively. Unless otherwise noted the internal reference for the 13C and 1H data was CDC13 with TMS. All reagents and reaction products were handled in an inert atmosphere of N2. N MR samples were sealed in 5mm NMR tubes, capped with a rubber septum and secured with Teflon tape. The butyl lithium used below was purchased from Aldrich as a 1.6 M solution in hexane. Smthesis of disilylalted ethanolamine systemsfl; In a 250 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 0.047 moles of N-alkylethanolamine was dissolved in THF. At -78° C, 0.09 mmoles of butyl lithium was added via syringe. The pale yellow solution was allowed to stir for 10 minutes then was warmed to room temperature and then stirred for an additional 10 minutes. Then solution was cooled to -30° C and 0.09 moles of TMSCI was slowly added via syringe. After stirring for 15 minutes at room temperature the colorless solution was vacuum distilled using a vigreux column. The solution is stored under inert atmosphere in a round bottom flask with a small piece of calcium hydride until needed. Smthesis of the 1-Aza-2-Bora-3-oxacyclopentane systemsfl; A 100 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, was charged with 2.5 mmol of a BC13 solution. At -78° C, 2.5 mmol of the disilylated ethanolamine derivative was added using a syringe. An immediate precipitate was formed. After stirring for 15 minutes at room temperature the solvents were removed under reduced pressure to give a pale yellow soild product - a 1,3,2-oxazaborolidine. Smthesis of difluoro-bis(trimethylsilyl)amino borane (”$3 In a 10 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 1 33 mmol of 1,1,1,3,3,3—hexamethyldisilazane was dissolved in 4 mL of hexane. At 0°C, 1 mmol butyl lithium was added. A salt precipitate was formed as the addition proceeded. The solution was allowed to stir at room temperature for 15 minutes. In a separate 25 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 1 mmol of BF3 OEt2 was dissolved in 2 mL of ether. At 0°C, the lithium salt of the amine was added to this flask. After several minutes the solution became cloudy. The mixture was allowed to stir for 1 hour at room temperature. After filtration, the solvents were evaporated under reduced pressure, and NMR analysis was performed. 11B : (Cd-16, D20, external) - 17.0 ppm 1H : (CgHé, D20, external) - 0.003 ppm Synthesis of bis(trimethylsilyl)gminodichloroborane (8210-; In a 10 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 1 mmol of 1,1,1,3,3,3-hexamethyldisilazane was dissolved in 4 mL of hexane. At 0°C, 1 mmol of butyl lithium was added. A salt precipitate was formed as the addition proceeded. The solution was allowed to stir at room temperature for 15 minutes. In a separate 25 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 1 mmol hexane solution of BCl3 was dissolved in 2 mL of additional hexane. Next, at 0°C, the lithium salt of the amine was added to this flask. After several minutes the solution became cloudy. The mixture was allowed to stir for 1 hour at room temperature. After filtration, the solvents were evaporated under reduced pressure, and NMR analysis was performed. 11B : (C6H6, D20, external) - 36.56 ppm 1H : (CéHé, D20, external) - 0.015 ppm Synthesis of diphenylaminodichloroboranefi (9): In a 10 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 0.162 g (1 mmol) of diphenyl amine was dissolved in 4 mL of hexane. At 0°C, 1 mmol of butyl lithium was added. A salt precipitate was formed as the addition 34 proceeded. The solution was allowed to stir at room temperature for 15 minutes. In a separate 25 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 1 mmol of BF3 OEt2 was dissolved in 2 mL of ether. Then at 0°C the lithium salt of the amine was added to this flask. After several minutes the solution became cloudy. The mixture was allowed to stir for 1 hour at room temperature. After filtration, the solvents were evaporated under reduced pressure, and NMR analysis was performed. 11B : (Cd-16, D20, external) - 1.015, 23.780 ppm Synthesis of diphenylaminodichlorobora_n_ei§_(m)_: In a 10 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 0.162 g (1 mmol) of diphenylamine was dissolved in 4 mL of hexane. At 0°C, 1 mmol of butyl lithium was added. A salt precipitate was formed as the addition proceeded. The solution was allowed to stir at room temperature for 15 minutes. In a separate 25 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 1 mmol of a hexane solution of BC13 was dissolved in 2 mL of additional hexane. At -78°C, the lithium salt of the amine was added to this flask. After several minutes the solution became cloudy. The mixture was allowed to stir for 1 hour at room temperature. After filtration, the solvents were evaporated under reduced pressure, and NMR was analysis performed. 11l3 : (Cd-16, D20, external) - 32.25 ppm Smthesis of diisopropylaminodifluoroboranefi (11): In a 10 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 1 mmol of diisopropyl amine was dissolved in 4 mL of hexane. At 00C, 1 mmol of butyl lithium was added. A salt precipitate was formed as the addition proceeded. The solution was allowed to stir at room temperature for 15 minutes. In a separate 25 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 1 mmol of BFgOEt2 was dissolved in 2 mL of 35 ether. At 0°C, the lithium salt of the amine was added to this flask, after several minutes the solution became cloudy. The mixture was allowed to stir for 1 hour at room temperature. After filtration, the solvents were evaporated under reduced pressure, and NMR analysis was performed. 11B : (C5136, D20, external) - 0.444, 17.394, 24.403 ppm Smthesis of N-(difluoroboro)acetamide (12L In a 10 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 0.05967 g (1 mmol) of acetamide was dissolved in 4 mL of hexane. At 0°C, 1 mmol of butyl lithium was added. A salt precipitate was formed as the addition proceeded. The solution was allowed to stir at room temperature for 15 minutes. In a separate 25 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 1 mmol of BF3OEt2 was dissolved in 2 mL of ether. At 0°C, the lithium salt of the amine was added to this flask. After several minutes the solution became cloudy. The mixture was allowed to stir for 1 hour at room temperature. After filtration, the solvents were evaporated under reduced pressure, and N MR analysis was performed. 11B : (C6116, D20, external) - 16.97, 0.412, 0.88 ppm Synthesis of N-(dichloroboro)acteamide (13): In a 10 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 0.5967 g (1 mmol) of acetamide was dissolved in 4 mL of hexane. At 0°C, 1 mmol of butyl lithium was added. A salt precipitate was formed as the addition proceeded. The solution was allowed to stir at room temperature for 15 minutes. In a separate 25 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 1 mmol of a hexane solution of BCl3 was dissolved in 2 mL of additional hexane. At 0°C, the lithium salt of the amine was added to this flask. After several minutes the solution became cloudy. The mixture was allowed to stir for 1 hour at room temperature. After filtration, the solvents were evaporated under reduced pressure, and N MR analysis was performed. 11B : (CeHs, D20, external) - 18.51 ppm 36 Synthesis of N-(difluoroboro)benzamide (14): In a 10 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 0.1211 g (Immol) of benzamide was dissolved in 4 mL of hexane. At 0°C, 1 mmol of butyl lithium was added. A salt precipitate was formed as the addition proceeded. The solution was allowed to stir at room temperature for 15 minutes. In a separate 25 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 1 mmol of BF3 OEt2 was dissolved in 2 mL of ether. At 0°C the lithium salt of the amine was added to this flask. After several minutes the solution became cloudy. The mixture was allowed to stir for 1 hour at room temperature. After filtration, the solvents were evaporated under reduced pressure, and N MR analysis was performed. 11B : (C6146, D20, external) - -1.221 ppm Synthesis of N-(dichloroboromenzamide (15h In a 10 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 0.1211 g (1 mmol) of benzamide was dissolved in 4 mL of hexane. At 0°C, 1 mmol of butyl lithium was added. A salt precipitate was formed as the addition proceeded. The solution was allowed to stir at room temperature for 15 minutes. In a separate 25 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 1 mmol of a hexane solution of BCl3 was dissolved in 2 mL of additional hexane. At 0°C the lithium salt of the amine was added to this flask. After several minutes the solution became cloudy. The mixture was allowed to stir for 1 hour at room temperature. After filtration, the solvents were evaporated under reduced pressure, and NMR was analysis performed. 11B : (Cd-I6, D20, external) - 15.17 ppm Synthesis of 1-methyl-3-Q-butylamino)-1,3,2 oxazaborolidine Q6): In a 10 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 0.262 mL (1 mmol) of t-butyl amine was dissolved in 4 mL of hexane. At 0°C, 1.56 mL (1 mmol) of butyl lithium was added. A salt precipitate was formed 37 as the addition proceeded. The solution was allowed to stir at room temperature for 15 minutes. In a separate 25 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 1mmol of N-methyl-1,3,2 oxazaborolidine was dissolved in 5 mL of toluene and the lithium salt of the amide was added to this flask. After several minutes the solution became cloudy. The mixture was allowed to stir for 1 hour at room temperature. The resulting mixture was filtered, the solvents were evaporated under reduced pressure, and NMR analysis was performed. 11B : (TI-IF, D20, external) - 27.923, 24.641, 8.705 ppm Synthesis of bis(dimethylamino)-t-butylgalminoborayne (17): In a 10 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 0.262 mL (1 mmol) of t-butyl amine was dissolved in 4 mL of hexane. At 0°C, 1 mmol of butyl lithium was added. A salt precipitate was formed as the addition proceeded. The solution was allowed to stir at room temperature for 15 minutes. In a separate 25 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 5 mmol of BCL3 was dissloved in 2 mL of hexane. At -78°C, the litium salt of deiethyl amine was added. After several minutes the solution became cloudy. The mixture was allowed to stir for 15 minutes at -78°C and for then for 1 hour at room temperature. The resulting mixture was filterd and the solvents were evaporated under reduced pressure. The solid was then suspended in hexane and the lithium salt of t- butyl amine was introduced at -78°C. This mixture stirred for 45 minutes at -78°C then at room temperature for 1.5 hours. After filtration, the solvents were evaporated under reduced pressure, and NMR analysis was performed. 11B : (Toluene, D20, external) - 45.413, 18.235, -17.513 ppm CHAPTER 3: BORON PHOSPHIDES Background The lack of any significant success with boron amides led us to next examine the reactions of boron phosphides with carbonyl acids. We postulated that a longer boron - phosphorus bond may enable the boron phosphides to act as better bases than boron amides, toward carbonyls. Unlike many boron amides, boron phosphides of the type R2 B-PR'2, have little or no double bond character between the boron and phosphorus, if R is a heteroatom. In fact, compounds with the formula, (R2N)2B-PR’2, are monomeric, which indicates double bond character, but the double bond is between the nitrogen and the boron", In these compounds competition exisists between nitrogen and phosphorus for the electron donation into the empty p orbital on boron. Nitrogen is more effective in the donation of its lone pair so, pi bonding exists, but it is between nitrogen and boron, and not between boron and phosphorus. A shortened boron - nitrogen bond has been measured in several boron phosphides". The pi bonding between boron and nitrogen would allow phosphorus to be both basic and nucleophilic. Teteraalkyl boron phoshpides (R and R’ = hydrocarbons) exhibit considerable double bond character between boron and phosphorus”. In fact, tetraalkyl boron phosphides form trimers, tetramers and in some cases are polymers“. Tetraalkyl boron phosphides can be monomeric if the R group is sterically demanding, and many examples of monomeric, tetraalkyl boron phosphides can be found in the literature49'53. Work done by Coates and Livingstone showed the boron phosphide Ph2 B-PPh2 is a monomer50. This compound, shown in Eq. 51, is a monomer because of conjugation between the boron and the aryl groups, not because of a double bond with phosphorus. 38 39 (51) Control over the extent of the double bond character between boron and phosphorus, in boron phosphides, has been accomplished by careful selection of ligands for boron and phosphorus. Electron withdrawing substituents on boron enhance the double bond character”. Also, placement of either electropositive substituents or bulky groups on phosphorus, which would force phosphorus to become more planar, increase the extent of pi bonding54. Measurement of the length of the boron - phosphorus bond, the degree of pyramidicity at phosphorus, and phosphorus barrier to inversion all indicate the degree of pi bonding between boron and phosphorus“. In summary, a short boron - phosphorus bond, a low degree of pyramidicity at phosphorus and, a low barrier to inversion of phosphorus imply conjugation between boron and phosphorus47r54. Boron phosphides are most often synthesized through a salt elimination reaction between a phosphorus anion and the appropriate boron chloride, shown in Eq. 5247. R ’R" -78 °C R\ /R" \ - _ LiCl B—Cl + LIP\ —-> ’8 P\ + R. / Rm R0 Rm (52) The chemistry of boron phosphides is not well documented. The major focus in this area of chemistry is synthesis and characterization, including the type of bond between boron and phosphorus and the existance as monomers or aggregates. 40 In 1994, Noth showed that a monomeric boron phosphide reacted with an acyl chloride, (Eq. 53), to give several different reaction products”. SiMe3 u R B—%LSiMe nae-HSiMeaiz + R'CCI ——> 2 kc: 3 meme: R'=Ph, Me eO R. 1 -msm R O (I? R B P 3M >/ 0 YR R'CCI 2 )\ 3 \ P 3R2 <=J 923_p .>=o x . .. O R 21:15 ——————» P>—O\Bae >=o’ >=°’ R R (53) The products obtained for this reaction depend on borons substituents for example, when bulky tertiary butyl groups are put on boron, as in Eq. 54, the reaction will stop at 1:1 stoichiometry49. X—HSiMefiz + ROCI —» o X X >=P(SiM93)2 R (54) This reaction gave some insight to the reactivity of boron phosphides and with this information we began our studies with boron phosphides. Tetraalkyl boron phosphides, R2 B-PR'2, (when R = Me or Et and R' = t- butyl or SiMeg) are commercially used as wide band gap semiconductors and as thermal coatings after pyrolytic polymerization55. 41 Results Our work with boron phosphides and their 1:1 reactions with carbonyl compounds yielded several observations. First, the boron phosphides we synthesized exist as monomer and dimers in solution, Eq. 55, which indicates little double bond character between the boron and phosphorus. Second, phosphorus exhibits a strong tendancy to add to the carbonyl carbon, as illustrated in Eq. 55. The addition to the carbonyl represented the most challenging aspect of this research. In order to quantitatively enolize carbonyl compounds the addition reaction had to be stopped. Monomer/dimer Addition by phosphorus “2 038 2 ‘?_‘.’R2 O Rza'PRz : : ——> RQP — B Ra "‘2 (55) Compounds A-E in Table 8, below, show the boron phosphides that were synthesized in our laboratory, before this work. R28-X + LiPR'2 _. R23.pn'2 + LiX Compound Fl X R' A cyclohexane CI t—butyl B cyclohexane Cl Ph (3 r—0~§. Cl Ph _N’ ’ Tos D (i-Per)2 Cl Ph E O- CsH 402 CI Ph 42 Of the above boron phosphides, A was able to enolize ketones however, the addition product is also present. Boron phosphide B gave the boron enolate only for the bulky diisopropyl ketone, and the reaction of E to give enolates is very slow. The remaining boron phosphides, in table 8, gave no evidence of enolization. In order to form enolates, the ligands on both boron and phosphorus needed modification. We want to make boron more electron deficient and, at the same time, make the phosphorus more basic. To this end, the boron phosphides in Table 9 were synthesized by a salt elimination reaction between a lithium dialkylphosphide salt and the corresponding boron chloride species. 43 Rae-x + LiPR'z —~ R28 -PR'2 + UX Compound R x R' 900 Temp. (°C) 18 DIP Cl Ph -78° ‘—".‘ Me 20 03. Cl Ph 38" ét 21 E0~§. Cl Ph -78b i—Pr 0‘ 22 j g: Cl SiMe3 ~78” Me 23 Cl Cl Ph -78" 24 F F Ph" 78‘ 25 Cl Cl SiMe3 -78° 26 F F SiMeg“ -78° 27 O- 05H 4% Cl SiMe3 '786 a) solvent : Hexane b) solvent : Toluene c) solvent : 1:1 Hexane : Benzene d) Ratio BZP = 2:1 Each boron phosphide was characterized by 31F, 11B and 1 H NMR. The reactions with diethyl ketone and pinacolone were monitored using both 31P anle NMR. Table 10, below, outlines the results of those reactions. RzB-PR'Z + Ketone Compound pinacolone diethyl ketone Time 18" no reaction no reaction 24 hrs.a 19° 80% addition >90°/. addition approx. 15 min.b 20° 75% addition 31 °/. addition approx. 15 min.b 21° 8.0% addition 35% addition iohrs.b 22° enolization enolization 1.5 hrs.° 23' no reaction no reaction 24hrs.° 24' no reaction no reaction 24hrs.‘3| 25' no reaction no reaction 24hrs.a 26' no reaction no reaction 24hrs.‘ 27° no reaction no reaction 24hrs.a a) At room temperature. b) Ketone added at -78°C and slowly warmed to room temperature while in spectrometer. c) Ketone added at -20°C and slowly warmed to room temperature while in spectrometer. e) in Hexane d) in Toluene t) in Benzene Discussion For compounds 18-21, 23, and 24 the boron phosphide peak (31F) is between -250 and -255 ppm, while the 31F peak for diphenyl phosphine (HPPh2) is located at -40 ppm. A successful enolization would involve a decrease in the (31F) boron phosphide peak and a corresponding increase in the secondary phosphine peak. The computer integrator was used to detect fluctuations in those peaks. 45 Compounds 19-21 showed a decrease in the boron phosphide peak and a corresponding increase in the diphenyl phosphine peak, which indicated enolization. However, the enolization reaction was in competition with addition of phosphorus to the ketone. The appearance of a new phosphorus resonance at around 12 ppm was the addition product. The data in table 10 show for compounds 19-21, the addition reaction is fast and the products are in significant yield. The boron phosphide (31F) resonance for compounds 22, and 25-27 is found between -255 and -265 ppm, while the resonance for bis(trimethyl- silyl)phosphine (HP(SiMe3 )2) is located at ~240 ppm. The 31P NMR was monitored during both reactions of compound 22 with pinacolone and diethylketone. These reactions showed a disappearance of the boron phosphide peak along with a corresponding increase in the secondary phosphine resonance, again indications of successful enolization. In contrast to compounds 19—21, there was no new phosphorus resonance. Upon completion of the reaction (boron phosphide peak less than 10%, based on integration values) the 1H NMR was observed to confirm the presence of enolate peaks (between 4.0 - 5.0 ppm). This is the first known example of a boron phosphide that will enolize carbonyls, in a nearly quantitative fashion, like pinacolone and diethyl ketone. The major drawback to this base is the time required to form the enolate, longer reaction times decrease the overall stereoselectivity. The geometry of the enolate obtained with the reaction of compound 22 and diethyl ketone was determined. The geometry assignment of the enolate derived from diethyl ketone was taken from Brown’s procedure9. This involved a 1:1 reaction between the enolate, of diethyl ketone, and benzaldehyde. The 1H NMR of the aldol condensation product gave resonances for the syn or erythro isomer which indicated that the enolate geometry was Z, Eq. 56. 46 Me “Q Q 3" + HP(SiMea)2 o ’B—P(SiMea)2 + W —" o’ the \/l\/ PhCHO Aggie 0” (56) The results of this research showed that ligand modification from phenyl to trimethylsilyl suppressed the addition reaction. However, when the addition reaction is suppressed, the enolization also dramatically slows down. The phenyl group on phosphorus did not lower the nucleophilicity of phosphorus enough to prevent addition, which would explain the predominance of the addition product seen with the boron phophides that had the phenyl ligand. However, the trimethylsilyl ligand did reduce the nucleophilicity at phosphorus and suppressed the addition. Unfortunately, the trimethylsilyl ligand also slowed down enolization. Slower reaction time allows for equilibration of the ketone, and increases the chance for self- condensation of the unenolized ketone. 47 EXPERIMENTAL The solvents used - hexane, toulene, THF, and benzene - were dried over calcium hydride, distilled and stored in an inert N2 atmosphere prior to use. All 31F, 11B, 13C, 1H NMR data was obtained using the either the Varian 300 MHz or Gemini 300 MHz NMR. The external references for 31F, 11B, spectra were 1-13P04 and BF3 OEt2, respectively. Unless otherwise noted the internal reference for the 13C and 1 H data was CDC13 with TMS. All reagents and reaction products were handled in an inert atmosphere of N2. N MR samples were sealed in 5mm NMR tubes, capped with a rubber septum and secured with Teflon tape. The butyl lithium used below was purchased from Aldrich as a 1.6M solution in hexane. Smthesis of disilylalted ethanolamine systemsfin In a 250 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 0.047 mmoles of N—alkylethanolamine was dissolved in THF. At -780 C, 0.09 mmoles of butyl lithium was added via syringe. The pale yellow solution was allowed to stir for 10 minutes then was warmed to room temperature and then stirred for an additional 10 minutes. Then solution was cooled to -30° C and 0.09 moles of TMSCI was slowly added via syringe. After stirring for 15 minutes at room temperature the colorless solution was vacuum distilled using a vigreux column. The solution is stored under inert atmosphere in a round bottom flask with a small piece of calcium hydride until needed. Smthesis of the 1-Aza-2-Bor§-3-oxngclopentane systemséé; A 100 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, was charged with 2.5 mmol of a BC13 solution. At -780 C, 2.5 mmol of the disilylated ethanolamine derivative was added using a syringe. An immediate precipitate was formed. After stirring for 15 minutes at room temperature the solvents were removed under reduced pressure to give a pale yellow soild product - a 1,3,2-oxazaborolidine. Synthesis of Lithium bis(trimethylsiyl)phosphide-bisgtetrahydrofuran25—6—3 In a 100 mL round bottom flask equipped with a magnetic stirrer and N2 48 inlet/ outlet line, a THF solution Tris(trimethylsilyl)phosphine55 (30 moles), at 0° C, an ether solution of methyl lithium (11 ml of a 2.69M solution) was added slowly over twenty minutes, while stirring. The resulting pale yellow solution was stirred for twenty minutes at 0° C, then at room temperature for 8 hours. The solvents were removed under reduced pressure and the yellow residue was suspended in 30ml of pentane. THF was added to the solution until dissolution was achieved (aprrox. 10 ml). At -78° C, yellow crystals precipitated out and the solvents were removed with a syringe. 31F data gave a single peak at -298ppm. The product 15 stored' in a round bottom flask, in a freezer. Synthesis of B-diphenylphosphinodiisopinocampheylborane (18): In a 10 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 0.20 mL of a 0.5 M solution in hexane of DIP chloride was dissolved in an additional 1.0 mL of hexane. At 0°C, 1.5 mmoles lithium diphenyl phosphine (in a THF solution) was added with a syringe. A salt precipitate was formed upon completion of addition. The solution was allowed to stir at 0°C for 30 minutes, then at room temperature for 1 hour. The solvents were removed under reduced pressure and NMR analysis was performed. 31F : (Hexane, D20, external) - 5.372 ppm 11B : (Hexane, D20, external) - 88.00, 89.017 ppm (doublet) Smthesis of 1-methyl-3-diphenylphosphino-13,2-oxazaborolidine (19): In a 10 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 1 mmol of N -methyl-1 ,3,2-oxazaborolidine is dissolved in 5 mL of toluene and 2.5 mmol of lithium diphenyl phosphine (in a THF solution) was added with a syringe at -78°C. The solution was stirred at -78°C for 30 minutes then at room temperature for 1 hour. After filtration, the solvents were evaporated under reduced pressure and N MR analysis was performed. 31P:(C61-16, D20, external) - 65.0 ppm 49 Smthesis of 1-ethyl-3-diphenylphosphino-1,3, -oxazaborolidine (20): In a 10 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 1mmol of N-ethyl-1,3,2-oxazaborolidine was dissolved in 5 mL of toluene and then 1 mmol of a THF solution of lithium diphenyl phosphine was added via syringe, at -78°C. The solution was stirred at -78°C for 30 minutes then at room temperature for 1 hour. After filtration, the solvents were evaporated under reduced pressure and NMR analysis was performed. 31P : (C6136, D20, external) - -63.69 ppm 11B : (Cal-16, D20, external) - 33.65 ppm Smthesis of 1-isopronyl-3-diphenylphosphino~1,3l2-oxazaborolidine (21): In a 10 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, Immol of N-isopropyl-I,3,2-oxazaborolidine was dissolved in 5 mL of toluene and then 1 mmol of a THF solution of lithium diphenyl phosphine was added via syringe, at -78°C. The solution was stirred at -78°C for 30 minutes then at room temperature for 1 hour. After filtration, the solvents were evaporated under reduced pressure and NMR analysis was performed. 31P : (Toluene, D20, external) - -66.385 ppm 11B : (Toluene, D20, external) - 31.38 ppm Synthesis of N-methyl-3-bis(trimethylsilylphophino)-1,3,2-oxazaborolidine (223 In a 25 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 1 mmol of N-methyl-1,3,2-oxazaborolidine was dissolved in 5 mL of toluene . At -78°C, 1 mmol of a THF solution of lithium bis(dimethylsilyl)phosphide-bis-THF was added. A salt precipitate was formed upon completion of addition. The solution was allowed to stir at -78°C for 30 minutes, then at room temperature for 1 hour. After filtration, the solvents were evaporated under reduced pressure and NMR analysis was performed. 31P : (Toluene, THF, D20, external) - -269.28 ppm 11B : (Toluene, THF, D20, external) - 25.64, 6.549 ppm 50 Synthesis of dichloro-diphenylphosphinoborane (23): In a 10 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 2 mmol of diphenyl phosphine was dissolved in 1.0 mL of hexane. At 0°C, 1 mmol of butyl lithium was added. The solution stirred at 0°C for 15 minutes then at room temperature for 15 minutes. In a separate 25 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 2 mmoles of a hexane solution of BC13 was dissolved in 2 ml. of additional hexane at -78°C. The lithium phosphide was added at -78°C and the solution was stirred at -78°C for 30 minutes then at room temperature for 1 hour. After filtration, the solvents were evaporated under reduced pressure and NMR analysis was performed. 31F : (Cd-15, D20, external) - 37.462, 36.972 ppm 11B : (C5H6, D20, external) - 17.41, 7.46 ppm Smthesis of difluoro—dinhenylphosphinoborane (24): In a 10 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 2 mmoles diphenyl phosphine was dissolved in 2 mL of hexane. At 0°C, 2 mmoles butyl lithium was added. The solution stirred at 0°C for 15 minutes then at room temperature for 15 minutes. In a separate 25 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 4 moles of BF30Et2 was dissolved in 2 mL of ether at -78°C. The lithium phosphide was added at -78°C and the solution was stirred at -78°C for 30 minutes then at room temperature for 1 hour. After filtration, the solvents were evaporated under reduced pressure and NMR analysis was performed. 31P : (Cd-16, D20, external) - -12.680, 37.97 ppm 11B : (C6Ho, D20, external) - 0.127 ppm Smthesis of dichloro-bis(trimethylsilylphosphino)borane (25): In a 25 ml. round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 2 moles of a hexane solution of BC13 was dissolved in an additional 2 ml. of hexane at -78°C. To this solution was added 1 mmol of a THF solution of 51 lithium bis(dimethylsilyl)phosphide-bis-THF, at -780C. The solution stirred at -78°C for 30 minutes, then at room temperature for 1 hour. After filtration, the solvents were evaporated under reduced pressure and NMR analysis was performed. 31F : (C6116, D20, external) - -40.0 ppm 11B : (C6116, D20, external) - 17.63, -3.04 ppm Synthesis of difluoro—bis(trimethylsilyl)phosphinoborane (26): In a 25 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 2 moles of BF3OEt2 was dissolved in 2 mL of THF at -78°C. To this solution was added 1 mmol of a THF solution of lithium bis(dimethylsilyl)phosphide- bis-THF, at -78°C. The solution was stirred at -78°C for 30 minutes then at room temperature for 1 hour. After filtration, the solvents were evaporated under reduced pressure and NMR analysis was performed. 31F : (Cd-16, D20, external) - -234.0, -250.0 ppm 11B : (C6116, D20, external) — 17.71, 0.206, -1.11 ppm Synthesis of B-bis(trimethylsilylphophino)cathecolborane (27): To a 25 mL round bottom flask equipped with a magnetic stirrer and N2 inlet/ outlet line, 2 mmoles of a 1:1 benzene : hexane solution of cathechol boron chloride was added. At -78°C, 2 moles of a THF solution of lithium bis(dimethylsilyl)- phosphide-bis-TI-IF was added. The solution was stirred at -78°C for 30 minutes, then at room temperature for 1 hour. After filtration, the solvents were evaporated under reduced pressure and NMR analysis was performed. 31F : (Hexane, D20, external) - -253.81 ppm 11B : (Hexane, D20, external) - 38.68, 20.819, 18.95 ppm REFERENCES REFERENCES 1. C. H. Heathcock, in Asymmetric Synthesis, Vol 3,]. D. Morrison (ed.), Academic Press, New York, 1984, Chap 3. 2. D. A. Evans, J. V. Nelson, E. Vogle, T. R. Taber, I. Am. Chem. Soc., 103, 3099 (1981). 3. E. Buncel, T Durst (eds.), Comprehensive Carbanion Chemistry, Elsevier, New York, 1981, Vol. 2, 186-207. 4. D. A. Evans, I. V. Nelson, T. R. Nelson, Topics in Stereochemistry, 13, 1 (1982). 5. a) C. H. Heathcock, et. al., I. Org. Chem, 45, 1066 (1980). b) R. Hoffmann, et. al., Tett. Lett., 25, 1781 (1984). c) C. Gennari, et. al., Tetrahedron, 40, 4051 (1984). d) T. Mukayama, Bull. Chem. Soc. Jpn., 53, 174 (1988). 6. F. A. Carey, R. I. Sundberg, Advanced Organic Chemistny Chap 2.1 pg 66. 7. a) L. Iackman, B. C. Lange, Tetrahedron, 33, 2737 (1977). b) P. G. Williard, G. Carpenter, I. Am Chem. Soc., 107, 3345 (1985). 8. D. A. Evans, J. V. Nelson, E. Vogel, and T. R. Taber, ].Am Chem. Soc., 103, 3099 (1981). 9. H. C. Brown, K. Ganesan, R. K. Dhar, ]. Org. Chem, 57, 3767 (1992). 10. H. C. Brown, I. Org. Chem, 58, 7162 (1993). 11. C. Trombini, et. al., Tet. Lett., 32, 1229 (1991). 12. H. C. Brown, et. al, I. Org. Chem, 42, 1197 (1977). 13. D. Seebach, et. al., Helv. Chem. Acta., 69, 604 (1986). 52 14 15. 16. 17. 18. 19. 20. 21. 24. 26 27. 28 29 53 H. Hamana, Chem. Lett., 1729 (1984). a) H. Noth, G. Hollerer, Chem. Ber., 99, 2197 (1966). b) H. Noth, W. Biffar, Z. Nat, 36B, 1509 (1981). c) H. Noth, W. Biffar, R. Schwerthoffer, Liebigs Ann. Chem, 2067 (1981). W. Biffar, H. Noth, Chem. Ber., 115, 934 (1982). I. Pfeiffer, W. Maringgele, A. Meller, Z. Anorg. AIIg. Chem, 511, 185 (1984). A. Brook, G. Gutekunst, I. Organomet. Chem, 225, 1 (1982). M. F. Lappert, P. Power, A. R Sanger, R. C. Srivastava; Metal and Metalloid Amides; Ellis-Horwood: Chichester, UK., 1980. A. Ansorge, et al. I. Organomet. Chem, 396, 253 (1990). H. Burger, T. Hagen, G. Pawelke, I. Fluorine Chem, 55, 323 (1991). . H. Burger, T. Hagen, G. Pawelke, Z. Nat. B. Chem. Sal, 48B, 935 (1993). . H. Burger, G. Pawelke, I. Rothe, I. Organomet Chem, 474, 43 (1994). A. Ansorge, D. I. Brauer, H. Burger, F. Dorrenbach, T. Hagen, G. Pawelke, W. Weuter, I. Organomet. Chem, 407, 283 (1991). H. Burger, et. al., Angew. Chem. Int. Ed. Engl., 32, 384 (1993). A. Ansorge, DJ. Brauer, H. Burger, T. Hagen, G. Pawelke, I. Organomet. Chem, 467, 1 (1994). D. I. Brauer, H. Buger, F. Dorrenbach, G. Pawelke, W. Weuter, I. Organomet. Chem, 378, 125 (1989). H. Burger, T. Hagen, G. Pawelke, I. Organomet. Chem, 456, 19 (1993). A. Ansorge, D. I. Brauer, H. Burger, T. Hagen, and G. Pawelke, I . Organomet. Chem, 444, 5 (1993). 30. 31. 32. 33. 35. 37. 38. 39. 41. 42. 43 44 45 46 54 a) P. Geymayer, E. G. Rochow, U. Wannagat, Angew. Chem. Int. Ed. Engl., 3, 633 (1964). b) W. Haubold, U. Kraatz, Z. Anorg. Allg. Chem, 421, 105 (1976). R. Russ, A. G. MacDiarmad, Angew. Chem. Int. Ed., 3, 509 (1964). W. Gerrard, H. R. Hudson, E.F. Mooney, I. Chem. Soc., 1568 (1960). U. Klingebiel, Z. Nat., 4GB, 913 (1985). . A. Meller, et. al., Z. Nat., 483, 561 (1993). G. Elter, O. Glemser, W. Herzog, Inorg. Nucl. Chem. Lett., 8, 191 (1972). . G. Elter, O. Glemser, W. Herzog, Chem. Ber., 105, 115 (1972). G. Elter, I. Organomet. Chem, 381, 299 (1990). M. Hasse, U. Klingebiel, R Brese, M. Polk, Chem. Ber., 119, 1117 (1986). U. Klingbiel, M. Hasse, Angew. Chem. Int. Ed., 24, 324 (1985). . P. antzold, A. Richter, T. Thijssen, S. Wurtenberg, Chem. Ber., 112, 3811 (1979). P. Geymayer, E. G. Rochow, Montash. Chem, 97, 428 (1966). R Neilson, B. Li, M. Goodman, Inorg. Chem, 23, 1368 (1984). R. Wells, W. P. Nutt, Inorg. Chem, 21, 2469 (1982). R. Wells, A.L. Collins, Inorg. Chem, 5, 1327 (1966). T. Sugasawa, T. Toyoda, K. Saskura, Synth. Comm, 9, 583 (1979). H. Kohn, A. Meller, Z. Nat. B, 35b, 447 (1980). 55 47. P. Power, Angew. Chem. Int. Ed. Engl., 29, 499 (1990). 48. P. Power, M. Olmsted, X. Feng, Inorg. Chem, 25, 4616 (1986). 49. H. Noth, S. Staude, M. Thomann, I. Kroner, R. T. Paine, Chem. Ber., 127, 1923 (1994). 50. G. E. Coates, I.G. Livingstone, I. Chem. Soc., 1000 (1961). 51. T. I. Groshens, K.T. Higa, R Nissan, R]. Butcher, A.I. Freyer, Organometallics, 12, 2904 (1993). 52. H. Noth, 8. Stand, M. Thomann, R.T. Paine, Chem. Ber., 126, 611 (1993). 53. H. Karsch, et. al., I. Organomet. Chem, 361, C25 (1989). 54. P. Power, et. al., Pure and Appl. Chem, 63, 859 (1991). 55. T. I. Groshens, C. E. Johnson, I. Organomet Chem, 480, 11 (1994). 56. G. Becker, H. Schrnist, G. Uhl, Inorganic Synth., 29, 243 (1990). "1111111111111111ES