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THE SYNTHESIS AND RADICAL FORMATION STUDIES
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presented by

Deena M. Conrad-Vlasak

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r. .___

THE SYNTHESIS AND RADICAL FORMATION STUDIES OF
PI-{ENOLIC PORPHYRINS

By
Deena M. Conrad-Vlasak

A THESIS
Submitted to
Michigan State University
in partial fulﬁllment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Chemistry
1993

ABSTRACT

THE SYNTHESIS AND RADICAL FORMATION OF PHENOLIC
PORPHYRINS

By

Deena M. Conrad-Vlasak

The role of meso-hydroxyporphyrin derivatives in heme degradation has
generated much interest. Many researchers have attempted to elucidate the
mechanism involved in this degradation process. Several steps have been
veriﬁed, yet the complete mechanism still has not been clariﬁed.

This work attempts to model the it radical nature of the meso-
hydroxyporphyrin which is believed to be an important feature in heme
degradation. Due to the instability of the meso-hydroxyporphyrin
framework under oxidative conditions, three phenolic porphyrin compounds
were studied: 5-(3,S-di-t-butyl-4-hydroxyphenyi)-13,17-diethyl-
2,3,7,8,12,18-hexamethylporphyrin, 5-(3,5-di-t-butyl-4-hydroxyphenyl)-13-
17,-diethyI-12,18-dimethylporphyrin and a diphenolic 5,15-bis-(3,5—di-t-
butyI-4—hydroxyphenyl)-2,8,14,18-tetraethyl-3 ,7 ,13,l7-
tetramethylporphyrin. Using EPR and UV-visible spectroscopy we were
able to detect radical formation and changes in the macrocyclic 1r systems.
Radicals were obtained by either alkaline oxidation or by irradiation with
white light. The macrocyclic framework proved stable under these
conditions. However, our results indicated that the unpaired electron density
resided mainly on the phenol ring rather than on the porphyrin suggesting
that the phenolic porphyrin compounds may not be a viable model for the
study of the biological heme degradation mechanism.

ACKNOWLEDGMENTS

I have passed through many pitfalls on my way to this degree and special
thanks needs to be given to those who helped me pick myself up and
continue on. First, I would like to extend a special thanks to Professor
C.K. Chang for working with me to try and solve the many problems I
encountered. Also, a great thanks is owed to the past and present members
of the Chang group. Dr. Wu for offering his guidance and sharing his
synthetic secrets, Ms Liang and Mr. Lee for sharing their starting material
and offering valuable information and Ms Geurin and Ms Morrison for
their friendship and encouragement. Lastly, Mr. Einhard Schmidt who
graciously extended his help whenever needed and provided great moral
support. My family, especially my husband Paul deserves the most thanks,
because without their unending love , support and Paul's computer know-
how I could not have made it this far.

This thesis is dedicated in loving memory of my youngest brother, Darin
Albert Conrad, who never had the chance to view learning as a wonderful
thing.

iii

TABLE OF CONTENTS

LIST OF TABLES ....................................................................................... vi
LIST OF FIGURES .................................................................................... vii
ABBREVIATIONS ...................................................................................... ix
Chapter 1: Introduction .............................................................................. 1
Degradation of Heme .......................................................................... 3
Studies Undertaken ............................................................................ 6
Goal of this work ................................................................................. 9
Chapter 2: Results and Discussion ........................................................... 12
Synthesis of the Oxophlorins (meso-Hydroxyporphyrins) ............ 12
Synthesis of the Phenolic Porphyrins .............................................. 12
Molecular Modeling .......................................................................... 20
Electronic Absorption Spectroscopy ............................................... 26
Oxidation of Porphyrin Derivatives ................................................ 26
Chemical Oxidation .......................................................................... 28
Electrochemical Oxidation ............................................................... 32
EPR Spectroscopy ............................................................................. 37
Chapter 3: Experimental ........................................................................... 47
3,4,5-TrimethyI-2-carhoethoxypyrrole ........................................... 47
3,4-Dimethyl-2-carboethoxy-5—pyrrole carboxylic acid ................ 48
3,4-Dimethyl-2,5-pyrrole dicarhoxylic acid .................................... 48
3,4-Dimethylpyrrole .......................................................................... 49
l,l9-Dideoxy-8,l2-diethyI-2,3,7,13,17,18-hexamethyl-
a,c-hiladiene-dihydrobromide .......................................................... 49
l,19-Dideoxy-8,12-diethyl-7,l3—dimethyl-aqc' hiladiene-
dihydrohromide ................................................................................ 49
13,17-Diethyl-l2,18-dimethyl-5-hydroxyporphyrin (2) ................. 50

iv

5-(3,5«di-t-hutyl-4-hydroxyphenyl)-13,l7-diethyl-

2,3,7,8,12,18-hexamethylporphyrin (4) ........................................... 50

5-(3,5-di-t-butyl-4-hydroxyphenyl)-13,l7-diethyl-

12,18-dimethyl porphyrin (5) ........................................................... 51

5,15-Bis(3,5-di-t-hutyl-4-hydroxyphenyl)-2,8,l4,18-

tetraethyI-3,7,13,l7-tetramethylporphyrin (6) ............................... 52

EPR Measurement ............................................................................ 52
LISI‘ OF REFERENCES ........................................................................... 53

LIST OF TABLES

Table 1: EPR results using base and oxygen as radical initiators ........... 40
Table 2: EPR results on degassed systems using light as the
radical initiator .......................................................................... 44

vi

Figure 1:
Figure 2:
Figure 3:
Figure 4:

Figure 5:
Figure 6:
Figure 7:
Figure 8:
Figure 9:

Figure 10:

Figure 11:
Figure 12:

Figure 13:
Figure 14:
Figure 15:
Figure 16:
Figure 17:

Figure 18:

LIST OF FIGURES

Porphyrin parent structure .......................................................... 1
Tautomeric structures of meso—hydroxyporphyrin ..................... 3
Proposed mechanism for biological heme degradation .............. 4
Autooxidation of mesohydroxyporphyrin systems showing
radical delocalization patterns. ................................................... 5
Model oxophlorin compounds .................................................... 6
Phenolic porphyrin model compounds ...................................... 8
TWO step oxidation of TBHPP-HZ ........................................... 10
Concerted mechanism for the oxidation of TBHPP-H2 ,,,,,,,,,,, 10
Synthesis of the unsubstituded meso-hydroxyporphyrin .......... 12
Synthesis of 5—0 ,5-di-t-butyl—4-hydroxyphenyl)-
l3,17-diethyl-2,3,7,8,12,18-hexamethylporphyrin ................... 14
Knorr synthesis of 3,4-dimethylpyrrole .................................... 15
NMR of 5-(3 ,5-di-t-butyl-4-hydroxyphenyl)-
I3,17—diethyl-2,3,7,8,12,l8-hexamethylporphyrin ................... 16
NMR of 5-(3 ,5-di-t-butyl-4—hydroxyphenyl)-
13,17-diethyl-12,18—dimethylporphyrin ................................... 18
IR of 5-(3,5-di-t-butyl-4-hydroxyphenyl)-13,17-
diethyl-2,3,7,8,12,18-hexamethylporphyrin ............................. 19
Synthesis of 5, 15-bis-(3 ,5-di-t-butyl-4—hydroxyphenyl)-
2,8,13,17-tetraethyI-3,7,12,l8-tetramethylporphyrin ............... 21
NMR of 5,15-bis-(3 ,5-di-t-butyl-4—hydroxyphenyl}
2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrin ............... 22
Molecular modeling of the free base monophenolic

porphyrin .................................................................................. 23
Molecular modeling of free base diphenolic compounds a) as
the phenol; b) as the quinone .................................................... 24

vii

Figure 19:

Figure 20:

Figure 21:
Figure 22:
Figure 23:
Figure 24:

Figure 24:

Figure 25:

Figure 26:
Figure 27:

Figure 28:

Figure 29:

Figure 30:

Figure 31:

Molecular modeling of the Zn a) diquinone porphyrin and b)
the Zn monophenolic porphyrin .............................................. 25
UV-Vis absorption spectra of the free base porphyrins

(4,5,and 6), respectively in CH 202, under ambient

conditions .................................................................................. 27
UV-Vis spectrum from the oxidation of monophenolic
porphyrin (4), with MCPBA. .................................................... 29
UV-Vis spectrum of the ﬁnal oxidation product of the
diphenolic porphyrin (6) with MCPBA .................................... 3O
UV-Vis spectrum of Zn diphenolic porphyrin (6), after
oxidation with MCPBA ............................................................ 31
a)Proposed mechanism for the electrochemical oxidation of the
free base diphenolic porphyrin (6) ............................................ 34

b) UV-Vis spectra of electro-oxidation of (6) containing

0.1M tetra-n-butylammonium perchlorate as supporting
electrolyte ................................................................................. 35
Absorption spectra of the electrochemical oxidation of the Zn
diphenolic (6) in CH2C12 with 0.1M tetra-n—butylammonium
perchlorate as supporting electrolyte ........................................ 36
EPR spectrum compound (4) ................................................... 38
EPR spectrum 0f TBHPP-H2 under ambient conditions in
(11-1202, 10’3 M, under air with tetra-n-butylammonium

hydroxide added ....................................................................... 39
EPR spectra of compounds (4,5,and 6) in alkaline CH2C12,
under ambient conditions .......................................................... 41

EPR spectra of compounds (4 and 6) in neutral, oxygen free
CH2C12, at ambient temperature upon exposure to

white light ................................................................................ 42
EPR spectra from the electrochemical oxidation of the
diphenolic porphyrin compound (6) and its zinc analogue,
respectively ............................................................................... 43
EPR degassing system .............................................................. 45

viii

ABBREVIATIONS

EPR: Electron Paramagnetic Resonance
FAB: Fast Atom Bombardment
MCPBA: Meta-chloro-per—benzoic Acid

NMR: Nuclear Magnetic Resonance

0E0: Octaethyloxophlorin
TBHPP-HZ: Tetrakis-(3,5-ditert-butyl-4-hydroxyphenyl)porphyrin
TTFA: Thallic Triﬂuoroacetate

UV-vis: Ultra violet, Visible spectroscopy

ix

Chapter 1

Introduction

Porphyrins are tetrapyrrolic macrocycles joined in the (1 positions of the
pyrrole by a methene bridge. These conjugated 18 1t electron systems
represent a class of pigments with important biological functions.1 The
parent compound, porphine (Figure 1) does not occur naturally. Porphyrins
which contain two hydrogens in the central position are called free bases.
The addition of one proton to the free base porphyrin forms the monocation.
When all the inner nitrogens are protonated, the dication results. The
metallo-porphyrins are produced by the displacement of the two central
hydrogens of the free base by a metal ion. Synthetic porphyrins may bear
substituents at the methene bridges. These compounds shall be referred to as
meso substituted porphyrins.2 The numbering system used in this document
will follow that designated by the Commission of Nomenclature for
Biological Chemistry.3

 

Figure l: Porphyrin parent structure

2
The redox properties of porphyrins have become a major area of study.

Electron transfers are involved in the life processes of all biological
systems.4 Typical examples of electron transfer employing porphyrin
macrocyclic systems include photosynthesis, respiration, and redox mediated
enzyme catalysis.5 Therefore the electrochemistry of the porphyrin ring
itself is of considerable interest and may ultimately lead to a better
understanding of the role of this moiety in biological systems.6

The extensive 1r framework of the porphyrin facilitates the formation of it
radicals. The porphyrin 1t radicals are involved, as reaction intermediates, in
many important biochemical processes such as photosynthesis and
peroxidase reactions. In addition, one of the most interesting phenomena is
the proposed radical intermediate that is believed to be involved in the
degradation of heme.

In humans, the oxygen carrier hemoglobin has a life span of about 120 days,
after which degradation eventually occurs. Through studies done on the
ﬁnal degradation product, which is excreted from the body in fecal material,
an average person was found to excrete on the order of three to ﬁve hundred
milligrams of the open chain tetrapyrrolic waste per day. It has also been
shown that heme oxygenase, the enzyme that breaks down heme and
provides an endogenous source of CO, is found in large amounts in the
brain.7 It is proposed that the enzyme found in the brain may function to
generate CO which acts as a messenger. Thus, an important physiological
question arises concerning the molecular mechanism for the degradation of
heme. This process has been extensively studied}10 but a definite
mechanism has not been obtained. However, the formation of a meso-
hydroxyporphyrin structure has been shown to be crucial for heme
degradation.

Porphyrins which are oxidized at the meso position represent a novel class of
tetrapyrrole derivatives. The oxidation interrupts the conjugated 1! system
giving a ligand system with conformational properties distinct from their
parent porphyrin compounds.ll These so called oxoporphyrins can have one
(oxophlorin), two (dioxoporophodimethene), or four (xanthoporphyrinogen)
oxygens at the mesa positions.

Mum

The original ground work for the proposed heme degradation mechanism
was laid down by Fuhrhop12 and others 13’” in the study of oxophlorins. It
was found that the oxophlorins exist as tautomers of the
mesohydroxyporphyrins (Figure 2). An iron complex of the hydroxy
tautomer is believed to be an intermediate in heme catabolism.15 In the
following discussion, the backgroung literature of heme degradation will be
reviewed. This will provide a basis for the approach with which we are
taking to study the heme degradation meclmnism.

 

Figure 2: Meso-hydroxyporphyrin / oxophlorin tautomeric structures

The generally accepted mechanism for heme catabolism proceeds through an
oxidative ring cleavage of the porphyrin macrocycle (Figure 3). This causes
the formation of an unstable iron complex. As the cleavage occurs, the ring
undergoes a rapid decarbonylation to release CO, yielding an
oxaprotoporphyrin. The metal ion is released during this reaction and is
quickly scavenged and recycled. The complete mechanism is not yet
veriﬁed, but it is believed to involve a radical intermediate which prompts
the addition of oxygen. The oxaprotoporphyrin further reacts with another
molecule of oxygen to furnish the open chain biliverdin complex.16 This

 

 

co,rr co,n

MESO-HYDROXYPORPHYRIN Intramolecular

e" transfer

 

co,rr cop!
OXAPORPHYRIN

Figure 3: Proposed mechanism for biological heme degradation

5
complex then undergoes an enzymatic reduction to form the bilirubin

complex, which ultimately is excreted from the body.

In support of this mechanism it has previously been shown17 that, in vitro,
heme is ﬁrst hydroxylated at the meso position. This compound then
undergoes a deprotonation and intramolecular electron transfer, reducing the
iron center from the ferric to the ferrous state, forming a neutral 1! radical
which then reacts with molecular oxygen to give the oxaporphyrin and CO.

It has been shown that free base oxophlorins in the presence of air can be
reversibly oxidized to form stable, neutral 1r radicalslz'l4 (Figure 4). Even
in very pure preparations at room temperature, a small amount of radical can
always be detected by EPR.

 

Figure 4: Autoxidation of the mesohydroxyporphyrin system showing
radical delocalization patterns.

In order to draw comparisons of the radical species found in the oxophlorin
compounds, we chose to study the free base octaethyloxophlorin (1) (Figure
5) and its paramagnetic zinc species prepared previously by Clezy.13 This
compound undergoes oxidative decomposition to a biliverdin-like complex
under basic conditions in the presence of visible light and air.14 The
intermediate in this process is a radical, detectable by EPR. Also
synthesized and studied was a ”topless" or unsubstituted version (2) of the
mesohydroxyporphyrin. By removal of the side chains in the 2, 3, 7, and 8
positions of the porphyrin ring. We hope to observe the inﬂuence of the
pyrrole protons on the EPR signal so that information of electron density
distributions of the radical species can be obtained.

 

(1) (2)

Figure 5: Model oxophlorin compounds

As an alternative approach to studying the intermediate reaction sequence
of heme degradation, a model was needed that would be stable under the
conditions of oxidation, reduction, protonation, deprotonation, and radical
formation. The oxophlorins are unstable under some of these conditions,

7
and rapidly open to form biliverdin-like complexs. Traylor, et 01.13

previously reported the facile two electron oxidation of meso-tetrakis-(3,5-
di-tert-butyl-4-hydroxyphenyl)porphyrin, (TBHPP-H 2) (3) 18,19 that takes
place through a radical intermediate. In this compound, all the meso
positions are substituted with sterically hindered phenols. These groups aid
in stabilization of the phenoxy radical and the macrocyclic framework.

Three meso-arylhydroxy porphyrins (Figure 6) have been synthesized.
These compounds have an extended, conjugated system with both donor I
acceptor abilities. . The porphyrin, with its large conjugated system, can
provide good radical delocalization. The phenol group provides the site for
independent radical formation. On all of the synthesized compounds tertiary
butyl groups were placed on the phenols in the meta positions to prevent
dimerization. The electrochemistry of aromatic diols and quinones such as
these is well characterized20 and generally follows the sequence:

.. . ,
.45: ___..- ‘
v—— <— ,

OH OH OH

HYDROQUINONE OX0 ANION SEMIQUINONE QUINONE

-H+

.6

ﬂ

   

Conformational changes in the phenolic porphyrins should have a large
effect on the distribution of unpaired electron density. Due to the steric
repulsion in TBHPP-H2 (3) between the phenols and the porphyrin ring,
coplanarity was not observed. Instead, the molecule has a saddle like shape.
However, with compounds bearing only one or two phenol groups, there is
more possibility of coplanarity. This would improve electron transfer
through better 1r overlap.

 

(4)

 

 

(5)

 

Figure 6: Phenolic porphyrin model compounds

9
In all of the compounds tested, a radical could be generated by air oxidation

in alkaline solution as described by Milgrom.21 These long-lived radicals
further reacted and became EPR silent. Two mechanisms have been
proposed for the oxidation of the TBHPP-H2 compound. The ﬁrst is a two
step process21 where a base is used to form the 0x0 anion and then one
electron is removed from the anion, probably by molecular oxygen, to form
a phenoxy radical. This radical then reacts with a second equivalent of
oxygen to yield the diquinone porphodimethene (Figure 7). The other, most
recently proposed mechanism22 is a concerted two electron oxidation
directly to the quinone which may then revert to the radical by
conproportionation of 1 and 3, allowing for slow oxidation back to 3

(Figure 8).

In all cases, EPR was employed to monitor the formation of the radical. The
EPR signals provided information about the electron delocalization pattern
in the molecule. An EPR signal could also be induced by irradiation of the
porphyrin phenols with white light in the absence of oxygen and base. The
resulting EPR signal had a better, more deﬁned structural pattern. The
lifetime of these radicals was much shorter, lasting only a few seconds after
removal of the radiation source.

The last technique used for radical induction was electrochemistry. By
applying a known voltage and following changes observed in the electronic
absorption of the molecule, we could test intermediates for radical activity.

W

Meso-hydroxyporphyrins have a tendency to form radicals and are believed
to play a major role in the heme degradation reactions. However, they
readily cleave under oxidative conditions the intermediate steps of the heme
degradation mechanism are difﬁcult to study. The TBHPP-H2 compound
provides much greater stability, but shows less correlation to biological
redox reactions. The goal of this work is to combine the positive attributes
of the TBI-IPP-H2 molecule and those of the oxophlorin system to produce a

new set of model compounds for the study of redox reactions in biological

1 1
systems, especially those involved in the degradation of heme. This led to

the synthesis of three mesa-phenolic porphyrin compounds. The first
compound (4) has the structure 5-(3, 5-di-t-butyl-4—hydroxyphenyl)—13, 17-
diethyl 2, 3 ,7, 8, 12, 18-hexamethyl porphyrin. The second compound 5-
(3,5—di-t—butyl-4-hydroxyphenyl)-13,17-diethyl-12, 18-dimethylporphyrin
(5) has greater freedom of rotation between the aryl meso substituent and the
porphyrin ring due to the absence of the 3, 7 methyl groups. The last
compound 5,15-bis-(3,5-di-t-butyl-4-hydroxyphenyl)-2,8,14,18-tetraethyl-
3,7,13,17-tetramethylporphyrin (6) bears two meso aryl substituents directly
across from one another. This compound resembles the well documented
TBHPP—Hz compound. The structures of these compounds were veriﬁed by
FAB mass spectrometry and NMR analysis. UV-visible spectroscopy and
electrochemisz were employed to determine oxidation, metal insertion,
and other reactions of the macrocycles. Extensive EPR studies using various
radical-inducing techniques were also performed to test for the radical
intermediate. It is postulated that through compiling the results of this study
and future work, we may develop a better understanding of the true
mechanism involved in heme degradation.

Chapter 2

Results and Discussion

 

Meso-hydroxyoctaethylporphyrin (1) was prepared from Zn(II)OEP with
Th(III) trifluoroacetate according to a literature procedure.23 Meso-
hydroxyporphyrin (2) was made by condensation of 2,2'-dipyrrylketone24
and 3 ,3'-diethyl-5 ,5 '.diformyl-4,4'-dimethyl-2 ,2'-dipyrrylmethane 25 in acetic
acid in the presence of HBr (Figure 9).

N
H Ii _AsorI_, 30H. ,
”We“ ..

(2)

Figure 9: Synthesis of unsubstituted meso-hydroxyporphyrin (2)

 

A goal of this work was to create an intermediate system that would provide
a model stable enough to maintain the macrocycle frame-work yet be similar

12

13
to the oxophlorin compounds found in biological systems. For these reasons

the mono and diphenol porphyrin, systems were synthesized. These
compounds offered both protection for the radical and the possibility of
some electron delocalization onto the macrocycle as is seen with the
oxophlorins. For comparison, the tetra-(3,5-di-t-butyl-4-hydroxy)-
phenylporphyrin was also prepared by the standard benzaldehyde / pyrrole
condensation reaction in propionic acid. The monophenolic compounds (4)
and (5) were synthesized in two major steps from pyrrole and dipyrrole
units. A tetrapyrrolic salt (a,c-biladiene) was formed by the condensation of
5,5'—diformyldipyrromethanes with pyrroles containing an a free position.
These powdery, dark colored salts were cyclized into the phenolic
porphyrins by condensing with 3,5—di-t-butyl-4-hydroxy benzaldehyde
(Figure 10).

The ﬁrst step involved in the preparation of the tetra pyrrolic salt was the
formation of the single pyrrole unit (7) (Figure 11). It is formed following
the basic Knorr synthesis.26 Once pyrrole (7) is formed, it was converted to
the di-a-free pyrrole which is able to couple with the diformyl-
dipyrromethane, to form the tetrapyrrolic salt.

Formation of the tetrapyrrolic salt was carried out using two equivalents of
compound (10) and one equivalent of compound (11) in methanol with a
small amount of hydrogen bromide. The product was compound (12), a
burnt-red colored tetrapyrrolic salt. No puriﬁcation was needed. The ﬁnal
condensation to the porphyrin was achieved by cyclization of compound
(12) with aldehyde (13) in a heated methanol I_ HBr solution. The product
was separated by column chromatography. The porphyrin (4) was bright red
and was obtained in 33% yield. Veriﬁcation was carried out by NMR
(Figure 12) and FAB mass spectrometry. The NMR showed two peaks
above 10 ppm, indicative of the two different meso protons in compound (4).
A single peak at 7.8 represents the two aromatic hydrogens of the phenol.
At 2.5, 3.5, and 3.6 singlets appear. These singlets are due to the three
different methyl groups found on compound (4). The triplet and quartet
correspond to the ethyl groups in positions 13 and 17 on the porphyrin ring.
Two very large, sharp singlets appear around 1.5 and 1.6. These are most
likely due to steric repulsion of the phenol unit and the porphyrin ring

 

332
5:2
\
z
/
9?
m-z\ |
‘* Y
.
gﬁ
/\

 

 

(4)

Figure 10: Synthesis of 5-(3,5-di-t-butyl-4—hydroxyphenyl)-l3,l7-diethyl-
2,3,7,8,12,18-hexamethylp0rphyrin

15

 

MOB +NaN02 ACOH PM

 

 

 

OEt
NOI-I

/ \ 1 S0202 Zn,AcOH
H02 H (D215! 2. H20 \‘—_(X)ZB
(8) KOH, Econ-H20

reﬂux (8:1)
It
NaOAc-3 H20, KOAc
JCS ~>

H02 H COzH reﬂux

(9) (10)

Figure 11: Knorr's pyrrole synthesis of 3,4—dimethylpyrrole

l6

aesaeaseoaagééimam
-3505:.m_522.323.?.eanseaavn e :22 é 95mm

at...~.a....a.~.s......pt .

;

 

 

 

 

 

 

17
causing the two t-butyl groups of the phenols to have slightly different

chemical environments. From this spectrum, we can deduce that the phenol
ring does not lie planar with the porphyrin ring. Instead, it most likely
resides at some tilted angle to the porphyrin plane.

The unsubstituted monophenolic porphyrin was synthesized in the exact
same manner. The only exception was the starting pyrrole. Commercially
available pyrrole was used instead of 3,4—dimethylpyrrole. The yield of the
tetrapyrrole salt was only 62%, considerably less than that for the methylated
salt. Also, there are three possible products due to the possible attack at the
less active B positions of the pyrrole. Final cyclization gave a 3-7% yield.
After puriﬁcation, a bright red solid (5) was isolated and veriﬁed by NMR
(Figure 13) and FAB mass spectrometry. The NMR of the unsubstituted
monophenolic porphyrin (5) was very difﬁcult to obtain and appears very
noisy. This was due to the paramagnetic character which could be observed
by EPR using the NMR solution. The two expected meso proton peaks
occur above 10 ppm. The unsubstituted pynole hydrogens appear as a pair
of doublets at 9.1 and 9.35 ppm. They appear at a very high ﬁeld due to the
fact that they are outside the ring current and therefore are deshielded. The
aromatic hydrogens of the phenol appear as a singlet at 8.1 ppm. The
broadened triplet (1.9 ppm) and quartet (4.15 ppm) represent the ethyl
substituents. Two large singlets represent the two t-butyl groups and again
lead us to the conclusion that the ring resides at an angle to the porphyrin
macrocycle.

Zinc was inserted into these monophenolic porphyrins by heating with zinc
acetate in CHCI3 / MeOH. A small amount of radical was present under
ambient conditions without initiation, much like that reported 12 for the
oxophlorin systems. Oxidation of the system using silver perchlorate
showed a single absorption band (425 nm) that was red shifted from the
starting material. The IR spectrum showed both ketone and hydroxy
resonance's for the non-oxidized system (Figure 14), while the oxidized
system showed no hydroxy absorption.

l8

cesarean—.3388
a:£73505:.m_Assesses—.3423???n.«I e «22 "2 page

tDDDIDDDDDDbDDEDbbFDDDDDbbwbbbPIDDbDDPFDDDDDDD FDDDDDDDDwbﬁbebbbb~bbbleIDb~i —DDD'DDI

.F’bDDFDDDEDDDPP

 

 

 

 

19

anaeassoagiwessasieoe
-2.maé?2§§2=ib£-ve-nea e E n: 2:5

 

 

 

 

 

 

 

 

 

20
Synthesis of the diphenolic porphyrin (6) was done under argon by a

condensation reaction involving an a free dipyrrole (14) and compound (13)
(Figure 15). An NMR (Figure 16) and a FAB mass spectrum were taken for
compound veriﬁcation. The NMR data suggested some macrocyclic
distortion of the diphenolic compound. The spectrum revealed one type of
meso proton (10.2 ppm). However, two types of aromatic protons (7.85,
7.35 ppm) as well as two types of t-butyl groups (1.6, 1.7 ppm) were seen.
This indicates different chemical environments of the groups. The methyl
groups on the macrocyclic framework are represented by a singlet at 2.5
ppm. The ethyl group was seen as the quartet at 4.05 ppm and a triplet at
1.8 ppm. The bulky methyl groups on the D carbons of the pyrrole introduce
steric interactions and inhibit the phenol from residing in the same plane.
Therefore, the different environments seen by the aromatic and t—butyl
groups are likely due to geometric changes created by the phenol rings being
out of the porphyrin plane.

Zinc could be inserted after free base porphyrin puriﬁcation or before the
work-up. If zinc acetate was added to the reaction mixture, the separation of
zinc porphyrin was simpliﬁed. This causes no problems in acquiring the free
base porphyrins because the metal can be easily removed by washing with
HCl.

MM:

Molecular modeling of the model compounds (4, 5, and 6) was carried out
on an Indigo system using the software package Biographies. The modeling
results supported the NMR results. Modeling showed that the porphyrin
rings were not planar (Figures 17 and 18). Both the hydroxy and quinone
forms of the free base compounds are seveme twisted about the porphyrin
core. The zinc hydroxy analogs have a saddle shape, pseudo—C2 geometry
(Figure 19). In the zinc hydroxy porphyrins, the metal has a pyramidal
geometry. It resides slightly above the porphyrin plane, alleviating some of
the molecule‘s strain. When the mono-oxidized compound was modeled for
both compounds (4) and (5), the phenol ligand tried to gain coplanarity with
the porphyrin ring, but due to steric interactions in (4) complete coplanarity

21

 

(14) (13)

l. MEOH l Argon
2. p—toluene sulfonic acid
3. NaOAc

 

 

t-butyl "’t—butyl
OH

(6)

Figure 15: Synthesis of 5,15-bis-(3,5-di-t-butyl-4—
hydroxyphenyl)-2,8,13,17-tetraethyl-3,7,12,18-
tetramethylporphyrin

22

aesaeoegoseaae_.Nwhoseoﬁaééaa
-222323-1asses.seen_.n e «22 ”2 use".

EPLPDDDDbDDFPIDDIFDFDDFDDP”

vv-b

 

"’DDI’DDmeDDbFD:

DD-DDFDDDFPD

m

’PDDbPPb’PDDPFDPPDD

L

 

a.

FDR-DDDFDbFD

23

 

Figure 17: Molecular modeling of the free base monophenolic
porphyrin

24

 

Figure 18: Molecular modeling of free base diphenolic compounds
a) as the phenol; b) as the quinone

 

Figure 19: Molecular modeling of the Zn a) diquinone porphyrin and
b) the Zn monophenolic porphyrin \

26
was not observed. Upon oxidation of compound (6) the two quinone ligands

twisted to a near perpendicular arrangement to the macrocyclic ring. Also,
severe rufﬂing of the porphyrin macrocycle core was observed. The
oxidized zinc diphenolic porphyrin analog showed severe strain. The
quinones were twisted with respect to the macrocycle which itself was very
distorted. This helps to explain why upon oxidation only the radical could
be detected. The use of molecular modeling has proven a valuable tool to
aid in determining possible structural geometry.

 

All UV-Vis spectra were taken using either dichloromethane or chloroform
as the solvent and utilizing one centimeter square cuvettes. In general,
porphyrin spectra consist of a very intense soret band at about 370 to 420 nm
and several visible (ﬁnger) peaks in the range of 500 to 650 nm. This can be
seen in the free base spectra of compounds (4), (5), and (6) (Figure 20).
These bands may be interpreted by the four-orbital model proposed by
Gouterman and coworkers.27 Gouterman's model shows the excited
electronic transitions extending ﬁom the nearly degenerate HOMO's (am
and a2u) into the degenerate LUMO pair ((23). The porphyrin ring's lowest
singlet excited state conﬁgurations, 1(a In. eg) and 1(a2u, e;) are of the same
Eu symmetry and almost equal in energy. These transitions have very strong
electronic interactions which are mixed with conﬁguration interactions and
yield the visible Qo,o hands when the transition dipoles almost cancel.
Additive conﬁgurations result in observance of the soret band. The closer
the energies of the lowest singlet excited state conﬁgurations, the weaker the
Qo,o band. Protonation, oxidation, or reduction bring about noticeable
changes in the absorption spectra making it a very diagnostic tool for
porphyrin research.

 

Both molecular modeling and N MR results have shown that the porphyrin
macrocycle is non-planar and most likely severely puckered. This puckering

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

00.40. TT
p 4)
(“NJ WM 0
D (I
one“ Qt L I
. 300.0 f.IIO «III/DIV. I . 000..
02.000
b I
. I
(‘/3,U.)
> Fk‘
00.000 r .
an... I... “III/OW. I I". U
01.". i r
i it
(:53’8.) ’
? (I
..e... g Mb”
300.. I... “ll/DIV.) 00.:

Figure 20: UV-Vis spectra of free base phenolic porphyrins (4, 5, and 6),
respectively in CH zClz under ambient conditions

28
minimizes steric interaction between substituents. The additive effects of

increased conjugation from the phenol substituents and the conformational
changes are reﬂected in the ease of oxidation of these porphyrin compounds
and in the red shift of their absorption spectra.

Ch . ”2.“.

Oxidations were carried out on the two oxophlorin compounds and on
compounds 4, 5, and 6 using MCPBA as an oxidizing agent. When followed
spectrophotometrically, addition of a few drops of MCPBA in chloroform
caused the soret band of the oxophlorins, and compounds 4 and 5 to decrease
in intensity (Figure 21). The four visible bands were also lost, and dramatic
color changes occurred. This is consistent with the radical behavior known
to exist in the oxophlorins”, suggesting radical formation in our synthetic
compounds. As we add more MCPBA the macrocyclic frame-work is
destroyed.

However, when the free base diphenol (6) was titrated with MCPBA it
underwent a red shift relative to the original soret, showing one large,
shifted band (453 nm). This signiﬁes oxidation to the quinone-like
product27 (Figure 22). As more MCPBA was added, the macrocycle was
destroyed. Accompanying this shift the color slowly changed from red to
orange-brown. During the oxidation the absorption bands broaden and
decrease in intensity. The ﬁnal oxidation product contains the
porphodimethene skeleton.

In contrast, the zinc diphenol complex underwent rapid color changes when
titrated with MCPBA, ending with a green compound having a broad, two
band absorption spectrum (658, 462 nm) (Figure 23). This compound has
not yet been identiﬁed but is EPR silent, showing that the radical has been
quenched.

29

 

 

 

I.
:e
O
r: t F i 4.
O
I A’
e
3
one
0
\
A I
2
V
O
o
0
e-
r '11)
M
3
ll - ,
it t \ 4.
ﬁ 0
O
N
C A C
O 0 O
O 03 O
o 0‘ e
N no 0
0 °\ 0
CC
V

 

Figure 21: UV-Vis spectrum from the oxidation of monophenolic
porphyrin (4) with MCPBA

30

 

 

O
O

 

 

 

-- Pear: -- -- UOLLEY --
1 at: 1 nos
. 366.3 0.203 _
ol.eee ‘ A A

(8)518.) -

 

 

90.”.

 

O . -

. , . I u
:0... tea ecuulplo.> 300.3
riser LgngL_ e.eegg

 

Figure 22: UV-Visible spectrum of the ﬁnal oxidation product of
diphenolic porphyrin (6) with MCPBA

31

-- PEAK -- -- URLLEY --
a 983 a 983
658 0 0.216 786.0 0.000
462.5 0.265 556.5 0.086

DATA PROCESSING Y/N ?

 

 

 

 

 

 

 

+0.409 . .
0.100
(exoru.>, .
+0.0en , , . 5
300.0 100.0(NM/DIU.) 00
14:25 2/22 '93 [4800.0NM 0.000

 

Figure 23: UV-Vis spectrum of Zn diphenolic porphyrin (6), after
cuhhmonwmmlWCPBA.

32
El h ° ° ion

To more fully understand the oxidative process of the model compounds,
electrochemical techniques were employed to monitor spectral changes
during oxidation. The monophenolic compounds behaved as anticipated.
Upon application of 0.65 V (vs Ag/AgCl electrode) the original soret at 405
nm decreases in intensity and an EPR signal was detected. This signiﬁes the
formation of a radical. Further oxidation took place when the voltage was
increased to 0.9 V to give a spectrum consisting of a large, broad band at
430 nm and a small band at 563 nm. Continued electrolysis at this voltage
eventually caused the peaks to merge to one broad band. This intermediate
was EPR silent and most likely represents the quinone. When the voltage
was increased to 1.5 V another species was formed which was red shifted
(445 nm) and EPR active. This showed that the porphyrin macrocyclic ring
can be oxidized past the formation of the monoquinone oxophlorin. Further
increase in voltage resulted in destruction of the macrocycle.

Electrochemical oxidation of the free base diphenolic porphyrin presented
some interesting results. Oxidation appears to proceed through 3 steps
(Figure 24), the ﬁrst being protonation, followed by formation of a radical
intermediate and ﬁnally the complete oxidation to the porphodimethane
compound. Upon application of 0.9 V the ﬁrst intermediate was formed. It
was red shifted (453 nm) from the original soret (409 nm) and EPR inactive.
This compound was thought to be the porphyrin diacid. This was veriﬁed by
acidifying the original starting material in CH2C12 with TFA, and comparing
the UV-visible spectra. When the voltage was increased to 1.05 V the diacid
was further oxidized to show a broad, two band intermediate purple in color
and EPR active (586.5, 459.5 nm). The EPR consists of a triplet spectrum,
which showed the radical was delocalized most extensively on the phenol
ligand. Further oxidation yielded an intermediate green in color and EPR
inactive (510, 590 nm). When the solution of the ﬁnal oxidized product was
exposed to air it reverted back to the radical species.

In the zinc diphenolic compound (6) the oxidation began at 0.45 V but did
not go to completion until 0.65 V. The oxidation was monitored by using

33
UV-visibie spectroscopy (Figure 25). This ﬁrst intermediate like that of the

free base was EPR silent. As the voltage is increased to 0.9 V a second
intermediate having an intense EPR signal was obtained. It was orange in
color and had a UV-visible band at 467 nm. This compound most likely
represents the diradical species. The oxidation potential could be increased
to 1.5 volts with no spectral changes. Thus, due to the conformational
constraints imposed on the macrocycle by the zinc atom, the molecule
cannot adopt the quinone form. As this compound was exposed to air it
quickly reverted to the unknown green EPR inactive species seen upon
oxidation with MCPBA.

The difference between the free base porphyrin and the zinc complex lies in
the conformation and ﬂexibility of the macrocycle. In order to form the
diquinone, a high degree of coplanarity of the two meso phenyl groups to the
macrocycle is necessary. This is possible only if the porphyrin adopts a
severely warped, saddle-shaped conformation. Steric hindrance to
coplanarity about a double bond raises the ground state energy level but
leaves the excited state relatively unchanged”. This accounts for the red
shifts observed during oxidation. Insertion of a metal atom into the
porphyrin system causes the macrocycle to become more rigid. Therefore
the macrocycle may not adapt to the conformation necessary for quinone
formation. The restriction by the metal allowed the phenoxy groups to retain
their unpaired electron spin density. For the two mono phenols, (4 and 5),
chemical oxidation led only to the formation of radicals, while
electrochemical oxidation led to the quinone species. The absorption spectra
revealed no appreciable difference between the two radical species. We used
EPR spectroscopy to further examine the radical character.

34

 

 

 

Figure 24: a) Proposed mechanism for the electrochemical oxidation of
drphenolrc porphyrin (6)

35

.....

.i ‘ . '. ' . '.'. ;. ..
. .. . . "
_..:; .::..' ... .,_|.
._ . ,. ' ; IIT.

Lia/O \) l 9:0

     

Figure 24: b) UV-Vis absorption spectra from the electrochemical
oxidation of (6) in CH2C12, containing 0.1 M tetra-n-

butylammonium perchlorate as supporting electrolyte

36

ljiwf.y

 

 

 

 

 

 

   

 

 

 

 

 

 

';:f..::..,.., .i::!.':l .,I..

¢"1i‘ .1 V . .. 1"“ What-in” -

. l 4;,1 ;! ill“ "I ‘1 lilting. ~

4”. .,..” I“!

:‘1 l 11” A" l III-t
‘ I

Iii. In 1 ' Mi till

ago 440 590

 

 

 

 

no
..._.._

‘.‘.. 1- .‘-

- - r 4.1 ’

 

 

 

 

 

 

 

 

 

 

__

 

Figure 25: Absorption spectra of the electrochemical oxidation of Zn
diphenol (6) in CH2C12, containing 0.1M tetra-n-
butylammonium perchlorate as supporting electrolyte

37
W

For a typical organic species, a magnetic ﬁeld around 3400 Gauss is used.
The AE for this ﬁeld strength is on the order of 1024 J, representing a
frequency of electromagnetic radiation on the order of 10 GHz. The g value
for a free electron is 2.0023. In most organic free radicals, the electron is
highly delocalized and the angular momentum is nearly quenched.
Therefore the g values do not differ greatly from that of the free electron.
The calculation of the g values was derived from equations (1). The
following is a typical example calculation for one of our model compounds
(Figure 26). The g value for this spectrum is 2.0049, a typical number for an
organic free radical.

Example for spectrum A (Figure 26):
AB = gBH and AE=hu, therefore g = ho / BH = 2.(X)49 (I)
where

u = 9.56 x 10 9

h=6.626X10'27 ergsec

B = 9.274 x 10 '21

H= 3417.5 G

Further information can be obtained from the peak to trough distances.
These numbers show the extent of electron coupling in our molecules, ﬁom
which the spin density distribution on the skeleton can be determined. 29

Using the TBHPP-H2 as a reference compound (Figure 27) one can predict
what kind of spectrum should be seen. If a radical is formed on a phenol
oxygen as it is in TBHPP-H2 one should see a triplet spectrum. This is due
to the splitting caused by the two equivalent protons (I=ll2) in the ortho
position of the phenol. If however the radical is delocalized evenly over the
macrocycle one should observe a broad singlet spectrum. If the electron
density is more localized on certain areas of the macrocycle one should see a
decrease of the coupling constant as the protons are further removed from
the spin center.

38

gins e.

 

AB :- gBH M 21
g = hv / Bil Where was x 10 9 1:26.626 x lo -
3:9.274 x 10‘“. ii- 3411.5 0

g=2.0049

Figure 26: An EPR spectrum of compound (4)

39

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 27: EPR spectrum of TBHPP-H2 10-3 M in CH2C12, open to air
with tetra-n-butylammonium hydroxide added

40
The EPR data were obtained in three ways. The ﬁrst method, described by

Milgrom, et (11.,21 involved the use of approximately 10-3 M samples in
freshly distilled dichloromethane. This solution was saturated with oxygen.
Approximately 1.0 ml of this saturated solution was injected into a dry
argon-ﬁlled vial to which 0.2 ml of methanolic tetra-n-butylammonium
hydroxide was added. All of our compounds went through rapid color
changes, typical for radical formation. A broad singlet EPR spectrum was
observed for each. The g values and peak to trough distances are listed in
Table I. The use of oxygen has both advantages and disadvantages. It
increases the life of the radical by propagating new radical formation.
Unfortunately, however, it causes a broadening of the peaks leading to a
decrease in resolution. This can be seen by comparing the spectra for
compounds 4, 5, and 6 in alkali dichloromethane saturated with oxygen
(Figure 28) with those run in the base free, anaerobic environment
(Figure 29) or with the electrochemically oxidized spectra of the diphenolic
model compounds (Figure 30).

Table 1: EPRresultsusingbaseandoxygenasradicalinitiators

 

 

 

Compound Number of peaks L peak to trougL
1 singlet 2.0030 10 G
(Zn) 1 singlet 1.990 10 G
2 singlet 2.0060 4.4 G
(Zn) 2 singlet 2 .0016 9 G
3 triplet 2.0023 1.4 G
4 singlet 2.0049 2.5 G
(Zn) 4 singlet 2.0005 3.5 G
5 singlet 2 DUB 3.0 G
6 singlet 2 .(Xl33 3.5 G
) 6 singlet 2.0062 4.0 G

 

 

 

 

 

 

 

 

Figure 28: EPR spectra of compounds (4, 5, and 6) in alkaline CH2C12
under ambient conditions

42

“70-0-7 I O

 

JO

 

I. P

 

 

 

A

33.0 33'. 340. IOIO 3423 3430
FIOI‘ (GAUGE)

 

I.

 

 

 

 

 

 

 

 

 

 

Figure 29: EPR spectra of compounds (4 and 6) in neutral, oxygen-free,
CH2C12 at ambient temperature, upon exposure to white light

22
13
q

 

3400

 

 

:3 a
ﬁt 3%
'3 _ V
{L 'o
" '3
[gr rig:

 

 

 

and-udhh-ediﬂi
m an 30 a
Field use

 

 

 

 

 

 

Figure 30: EPR spectra from the electrochemical oxidation of the
diphenolic porphyrin compound (6) and its zinc analog,
respectively, showing the ﬁrst radical intermediate detected

44
The second method of radical sample preparation entailed the use of light

irradiation in a special EPR degassing system (Figure 31). The samples
were prepared in CH2C12, degassed by freeze-thaw cycles, and immediately
analyzed. The spectra obtained under continuing light irradiation were much
sharper, and some splitting could be observed. For g values, peak to trough
widths, and the number of peaks, see Table 2 .

Table 2: EPR results on degassed systems using light as the radical
. ‘I' tor

 

 

 

 

 

 

Compound Number of peaks 3 peak to trougL_
1 quartet 2.0028 5.0 G
(Zn) 1 singlet 1.9940 10.0 G
2 triplet 2.0091 20.0 G
(Zn) 2 singlet 2.0016 9.0 G
3 triplet 2.0023 2.0 G
4 triplet 2.0034 1.5 G
(Zn) 4 singlet 1.9978 4.8 G
5

6 triplet 2.0064 1.5 G
(Zn) 6 triplet 1.9984 4.9 G

 

 

 

 

 

When the oxophlorin derivatives were analyzed in THF solution the
resolution was much better than that in CH2C12_ In the metalated
oxophlorins, radical was always present even without light. When the
metalated samples were irradiated, the EPR signal increased in height
yielding a broad ﬁrst order spectrum.

To probe for solvent effects on the radical, compound (4) was dissolved in a
nonredox active solvent (deuterated benzene) and degassed as previously

45

 

 

 

 

 

 

Reaction
Vessel

Tube

 

 

Figure 31: EPR degassing system

 

Vacuum
Pump

 

 

46
described. It was found that in 100% deuterated benzene, a light induced

EPR signal could be observed, although it was much less intense than those
previously seen. When the solvent was changed to less pure benzene (96%
deuterated), the spectrum regained its normal amplitude. This suggests that
impurities in the solvents may act as electron acceptors, aiding in the
formation of the radical and in extending radical lifetimes.

When ZnOEP, a porphyrin having no electron acceptor group, was
irradiated, no EPR signal was seen. It can therefore be concluded that the
presence of a phenol electron acceptor is necessary to induce the radical.

In the model compounds, based on the NMR and molecular modeling, there
are geometric constraints favoring a nonplanar geometry which impede
electron transfer into the macrocycle. The tertiary butyl groups in the ortho
positions of the phenols tend to stabilize the radical on the. oxygen. This
allows only a small amount of electron density to pass into the macrocyclic
ring. This was conﬁrmed by the EPR spectra in which some triplet
character, masked by singlet character, can be detected. However, in the
oxophlorins, where no such constraints exist, full delocalization resulted in
well resolved spectra.

These experiments help to explain the electron delocalizing pattern of the
mesa-conjugated porphyrins. We have shown the facile formation of radical
species in mesa-hydroxy, as well as in mesa-phenolic porphyrins. These
radicals are long-lived and can delocalize (depending on structural
constraints) at least partially on the macrocyclic ﬁame work. This could
support the possibility of attack by molecular oxygen to form the
oxoprotoporphyrin which could then degrade to the open chain biliverdin-
like complex seen in the degradation of heme.

Chapter 3

Ex mrimental

UV-visible spectra were taken with either a Varian Cary 219 spectrometer or
a Shimadzu UV-160, using either dichloromethane or chloroform as the
solvent in one centimeter square cuvettes. EPR spectra were taken with a
Varian E4 100 kHz EPR spectrometer. The ﬁeld was calibrated by a DPPH
standard. Proton NMR spectra were obtained on a Gemini 300 or VXR 300
spectrometer in either deuterated dichloromethane or deuterated chloroform.
FAB mass spectrometry was performed on a Jeol HX110 mass spectrometer.
Puriﬁcation of samples was carried out on preparative thin layer
chromatography plates (silica gel) of 1000 or 1500 mm thickness.
Electrochemical oxidations were conducted with an ECO model 550
potentiostat equipped with a model 731 digital integrator. A circulating cell
consisting of two platinum electrodes sealed through the cell wall and
connected with a cuvette was used for photometric monitoring of
electrolysis. All electrochemical measurements were carried out in 0.1 M
tetra-n-butylammonium perchlorate - dichloromethane solutions under

argon.

 

Ethyl acetoacetate (130 g, 1 mole) was dissolved in 200 ml of AcOH. To
this solution a saturated solution of NaN02 (69.5 g in 175 ml H20) was
added dropwise while stirring. The temperature of the reaction mixture was
kept below 20' C using an external ice bath. After completing all additions,
stirring was continued for 1 hour. In a separate ﬂask, (114 g , 1 mole) 3-
methyl-2,4 pentanedione was dissolved in 450 ml AcOH and 50 g of zinc
powder was added. To this mixture, the above oxime solution was slowly
added dropwise, under mechanical stirring, and another 200 g of zinc
powder was added in small quantities until the oxime addition was
completed. The reaction temperature was maintained below 90' C. After
47

48
completion of all additions, the mixture was reﬂuxed for 2 hours. The hot

solution was then poured into 5 liters of ice water. After 2 hours the crude
pyrrole precipitated. The crude product was collected by ﬁltration, washed
with water, and recrystallized from EtOH. Yield: 57%; mp 119-121° C

lit. mp 120-123° C

 

Under inert atmosphere, using dry solvents and glassware protected from
moisture, 3,4,5-trimethyl-2-carboethoxypyrrole (56 g, 0.3 mole) was
dissolved in 300 ml of CHzClz and 500 ml of ether. To this solution SOzC12
(77 ml, 0.77 mol) in 200 ml CH2C12 was added in a rapid dropwise manner.
After the ﬁnal addition, stining was continued for approximately 30
minutes. The solvent was then removed via rote-evaporation. The orange
oil residue was added to a hot acetone I water solution (75:25, 500 ml) and
heated to reﬂux for 30 minutes. The cnrde pyrrole acid crystallized out from
solution upon cooling. It was collected by ﬁltration and washed with water.
The solid was dissolved in 500 ml hot EtOH, 150 ml saturated NaHCO3 was
added, and the mixture warmed on a steam bath. A cotton plugged funnel
was used to ﬁlter the insoluble oil-like substances from the hot solution. The
clean solution was acidiﬁed with concentrated HCI. Pure acid pyrrole,
precipitating out as a white powder, was collected by ﬁlitration, washed with
water, and air dried. Yield: 95% mp l65-167° C lit. mp 168-170" C

W2

3,4-DimethyI-2-carboethoxy-5-pyrrole carboxylic acid was dissolved in 400
mlofEtOHand50m1H20. KOH (23 g) wasaddedandthesolutionwas
reﬂuxed for 6-8 hours. Most of the solvent was removed via roto-
evaporation. The solution was cooled in an ice bath and 200 ml of water
was added. Acidiﬁcation with acetic acid precipitated out the diacid. The
product was collected by ﬁltration, washed with water, and air dried. Yield:
57%; Decomposes 205° C

49
W31

3,4-Dimethyl-2,5-pyrrole dicarboxylic acid (10 g) was ground with 40 g of
NaOAc and 40 g of KOAc until a uniform mixture was achieved. This
mixture was transferred to a round bottom flask and heated until the mixture
melted. The heating was continued at 150° C for approximately 30 minutes.
The solution was allowed to cool and was partitioned with water. It was
then diluted and extracted using CH 2C12. It was dried over NazSO4 and
ﬁltered. The solvent was removed. Puriﬁcation was accomplished by
distillation with an air-cooled condenser, and the product stored in the
freezer. Yield: 15%; mp 31-33° C; bp 75° C using an aspirator to reduce
pressure and an oil bath at 105° C lit. mp 32-33° C bp750 162-164° C

1H NMR (CDCI3): d: 2.05 (6H, s), 6.5 (2H, broad s)

i ‘ . . - . ‘ . °. ° .
hum. 1‘ ' ‘UL'IJ . 7 ‘1‘L9:.t.1' '1.‘ ’_' 1.”. LI 1...

lel I I el

3,3'-Diethyl-5,5'-diformyl-4,4’—dimethyI-2 ,2-dipyrrylmethane34 (286 mg, 1
mmol) and 3,4—dimethylpyrrole (222 mg, 2.0 mmol) in methanol (20 ml)
were treated with hydrobromic acid (1.5 ml, 48%), and the solution was
heated on a steam bath for 5 minutes. The solution was allowed to cool and
remain at room temperatme for 2 hours. The product was ﬁltered, washed
with methanol containing a little hydrobromic acid, washed with ether, and
dried in the air to give red-brown prisms with a green luster. Yield: 82%
Decomposes 215° C

I " , . - .'. . . . . . ' ° '.
.4L'x’1';‘ - - .lk'l.' , AllllraJ In) -12 ”aim-rut ”-1 ‘- 1r! “LU. '1

3,3'-Diethyl-5,5'-diformyl-4,4'-dimethyl-2,2-dipyrrylmethane34 (100 mg,
0.35 mmol) and pyrrole (47 mg 0.70 mmol) in methanol (7 ml) were treated
with hydrobromic acid (0.5 mi, 48%). The solution was heated on a steam
bath for 5 minutes. The solution was allowed to cool and remain at room
temperature for 2 hours. The product was ﬁltered, washed with methanol

50
containing a little hydrobromic acid, washed with ether and dried in the air

to give red-brown prisms with a green luster. Yield: 62%; Decomposes
210° C.

7-Di h l-12 ' l- h rin

2,2'-Dipyrryllretone32 (160 mg, 1 mmol) and 3,3'—diethyl-5,5'-diformyl-4,4'-
dimethyl—2,2'.dipyrrylmethane33 (372 mg, 1 mmol) were dissolved in HOAc
(30 mL) heated over a steam bath. To this warm solution, acetic anhydride
(10 mL) and HBr (0.5 mL of 45% hydrobromic acid in 10 mL of acetic acid)
were added, and the mixture was heated at 90° for 15 min. The mixture was
partitioned in CH 2Clz and water. The organic phase was separated and
washed with water and evaporated. The residue was chromatographed on
silica gel to yield 13,17-diethyl-12,18-dimethyl-5-acetoxyporphin (5 mg).
1H NMR(CDC13): d: 10.07 (2H, s), 9.95 (1H, s), 9.32 and 9.34 ( 4H, dd),
4.07 (4H, m), 3.57 (6H, s), 3.03 (3H, s), 1.88 (6H, t), -3.47 (2H, s).

UV-Vis (CH2C12): 619.5 nm, 568, 530.5, 497.5, 398.5, phyllo-type.

The acetoxy porphyrin was hydrolyzed using methanolic tetra-n-
butylammonium hydroxide and immediawa analyzed by EPR.

.,..“ '15 ‘ i, u tunic-r! . rig-.,.! - . 7.5 .-

W151

A suspension of 1,19-dideoxy-8,12-diethyl-2,3,7,13,17,18-hexamethyl-a,c-
biladiene-dihydrobromide (100 mg, 0.166 mmol) in methanol (25 ml)
containing a ten-fold excess of 3,5-t-butyl-4-hydroxybenzaldehyde (389 mg,
1.66 mmol) and four drops of acetic acid saturated with HBr (48%) were
heated under reﬂux for 24 hours. The mixture was cooled and treated with
an excess of solid sodium bicarbonate. The product was extracted with
CHzClz rota-evaporated to dryness and redissolved in CHzClz. This
solution was chromatographed on silica gel using dichloromethane as the
eluant. The product was further separated on silica gel plates using CH2C12
and less than 1% formic acid. The main band was collected to give a deep
red solid. Yield: 33%. Decomposes 178 °C

51
FAB mass spectrum: mlz (relative intensiIY); 654.4 (100); 444.1 (12); 308

(12); 154 (49)

1H NMR (CD2C12): d: 10.2 (2H, s), 10.0 (1H, s), 7.8 (2H, s), 5.6 (1H, s),
4.1 (4H, q), 3.7 (6H, s), 3.6 (6H, s), 2.5 (6H, s), 1.9 (6H, q), 1.6 (9H, s),,
1.3 (9H, s)

UV-Vis. (CH2C12): 403.5 nm, 501.5 nm, 533.0 nm, 569.5 nm, 622.5 nm
e = 1.687 x 105

1121.1 -4- 111.; 011': l 7 11er - (attire:

A suspension of 1,19-dideoxy-8,12-diethyl-7,13-dimethyl-a,c-biladiene-
dihydrobromide (100 mg, 0.167 mmol) in methanol (25 ml) containing an
excess of 3,5-t-butyl-4—hydroxybenzaldehyde (389 mg, 1.66 mmol) and four
drops of acetic acid saturated with HBr (48 %) were heated under reﬂux for
24 hours. The mixture was cooled and treated with an excess of solid
sodium bicarbonate. It was then extracted with CH2C12, rote-evaporated to
dryness, and redissolved in CH2C12. This solution was column
chromatographed on silica gel using dichloromethane / hexane (4:1) as the
eluant. The product was further separated on silica plates using CH2C12 and
less than 1% formic acid. The main band was collected and solvent removed
yielding a cherry red solid. Yield: 3—7% Decomposes 195 °C

E1 Mass Spectrum: mlz (relative intensity): 598 (38); 478 (39); 436 (100);
421 (42); 207 (15)

1H NMR(CD2C12): d: 10.2 (2H, s), 10.0 (1H, s), 9.35 (2H, d), 9.1 (2H, d),
8.1 (2H, s), 5.6 (1H, s), 4.1 (4H, q), 1.9 (6H, t), 3.6 (6H, s), 1.65 (9H, s),,
1.5 (9H, s)

UV-Vis. (CH2C12): 406.0 nm, 501.0 nm, 531.0 nm, 568.0 nm
e = 1.6673 x105

52
l " .‘ll-alhlllt st'l en 2.5141 - 1‘s 1- 71 17;

mam/1mm

4,4'-Diethyl-3,3'-dimethyl-2,2'-dipyrrylmethane31 (230 mg, Immol) and 3,5 -
di-t-butyl-4-hydroxybenzaldehyde (234 mg, 1 mmol) were dissolved in 11.5
ml of methanol. The solution was deaerated by bubbling with argon for 10
minutes, then p-toluenesulfonic acid (53.2 mg, 0.28 mmol) was added. The
mixture was stirred for 2 hours. The solution was then neutralized with
anhydrous NaOAc, extracted with CH2C12 and the solvent removed via roto-
evaporation. The product was dissolved in CH2C12 and puriﬁed on a silica
gel column using 80% CH2C12 and 20% C6H12. Yield : 35%.

Decomposes 217 °C

FAB mass spectrometry : 888 (100); 444.1 (12); 307.3 (27); 154 (100)

1H NMR (CD2C12): d: 10.2 (2H, s), 7.85 (2H, s), 7.4 (2H, s), 5.6 (2H, s),
4.0 (8H, q), 2.5 (12H, s), 1.8 (12H, t), 1.55 (18H, s), 1.6 (18H, s)

UV-Vis. (CH2C12)1409.5 nm, 506.5 nm, 539.5 11111, 573.5 nm
e = 1.168 x 105

BMW

Prior to each sample analysis a DPPH I CHzClz standard was run to calibrate
the frequency. All of the EPR samples were measured in dry CH2C12 at
room temperature at a concentration of approximately 10'3 M. A 4 mm
quartz tube was used. The only exception to this was one sample which was
run in deuterated benzene. Two different methods were used to prepare the
radicals for analysis. The ﬁrst method entailed the use of alkaline CH2C12
saturated with oxygen. The second used white light irradiation of deairated
solutions as discussed in Chapter 2. EPR spectra from electrochemical
reactions were run at ambient temperature in the presence of oxygen. The
ﬁeld was ﬁrst set at 3400 G and then scanned for detection of a signal. The
modulation, gain, and ﬁeld sweep was optimized for each compound to gain
the maximum resolution.

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