Western Kentucky University
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Masters Theses & Specialist Projects
Graduate School
8-2014
Selective Oxidations by Iron(III) Porphyrins and
Iron(III) Corroles
Aaron Dalnamath Carver
Western Kentucky University, [email protected]
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Carver, Aaron Dalnamath, "Selective Oxidations by Iron(III) Porphyrins and Iron(III) Corroles" (2014). Masters Theses & Specialist
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SELECTIVE OXIDATIONS BY IRON(III) PORPHYRINS AND
IRON(III) CORROLES
A Thesis
Presented to
The Faculty of the Department of Chemistry
Western Kentucky University
Bowling Green, Kentucky
In Partial Fulfillment
Of the Requirements for the Degree
Master of Science
By
Aaron Dalnamath Carver
August 2014
ACKNOWLEDGEMENTS
First and foremost, I would like to thank GOD for this opportunity to be at this great
school and making this thesis possible. Words cannot express how grateful I am for Dr. Rui
Zhang who pushed me, guided me and helped me with research and with everyday life struggles.
Sometimes it’s when you’re about to drown and sink to a lonely deaths of despair someone
encourage you to swim harder, for that I am truly thankful for his mentorship. Without him I
would not be where I am today, and for that thank you.
During my time studying here I meet a lot of wonderful people, two of these people I
have the privilege of them sitting on my thesis committee. Dr. Donald Slocum who started me on
my research in organic chemistry, he took me in and gave me my first chance. I am humbled by
his compassion, and grateful for his knowledge that he shared with me. I am also grateful to have
meet Professors Larry Byrd who has helped me prepare for my teaching career. His insight in
education is truly astonishing, and the experiences he shared with me will never be forgotten. I
would also wish to give my sincere thanks to the Department Head Dr. Cathleen Webb. Her
kindness and devotion to the department is unmatched. I would like to thank her for all she has
done for me and my family.
I would like to thank all my research group members: Wei Long Luo, Zhibo Yuan,
Nowelle Altman, Nawras Asiri, Tse-Hong Chen, Hieu Vo, and Ka Wai Kwong. The Bonds that I
have formed with each one of you will never break thank you. This work is supported by a NSF
grant (CHE 1213971), and an internal grant from WKU Office of Research (RCAP 2012).
iii
Last and very dear to my heart I like to think my Family. My brother and sisters who
grew up with me we will always be the fab five, who has a great set of parents. Marion and Rev.
Fred R. Carver I like to think you both for providing me with love and compassion, and the
resolve to keep pushing on. My kids Aaron Jr. and Allanah it’s because of them I push to become
the man I should be. I realize you two give up the most why I have been writing this summer,
and I think you for your understanding. To my beautiful wife Dara Who I will always love, thank
you for keeping the house going while I have been writing this thesis. She is my heart, always.
iv
TABLE OF CONTENTS
LIST OF FIGURES ............................................................................................................................ vi
LIST OF SCHEMES........................................................................................................................... vii
LIST OF TABLES ............................................................................................................................. xii
ABSTRACT ........................................................................................................................................ ix
1. INTRODUCTION ............................................................................................................................ 1
1.1 Cytochrome P-450 Enzymes .......................................................................................................... 1
1.2 Biomimetic models with iron porphyrins ....................................................................................... 5
1.3 Biomimetic models with iron corroles ………..........................................................................…..6
1.4 Sulfoxidation reactions ............................................................................................................,,,,,,. 8
2. EXPERIMENTAL SECTION ................................................................................................. 10
2.1 Materials ................................................................................................................................ 10
2.2 Methods ................................................................................................................................. 10
2.2.1 Physical measurements ...................................................................................................... 10
2.2.2 Reagent purification ........................................................................................................... 12
2.2.3 Catalytic sulfoxidations by metalloporphyrin or metallocorrole……................................ 12
2.3.0 Synthesis and spectroscopic characterization………………............................................. 13
2.3.1 Meso-Tetramesitylporphyrin [H2TMP].............................................................................. 13
2.3.2 Iron(III) tetramesitylporphyrin chloride [FeIII(TMP)Cl)] .................................................. 16
2.3.3 Iron(III) tetramesitylporphyrin perchlorate [FeIII(TMP)ClO4)].......................................... 18
2.3.4 5,10,15-Tri(pentafluorophenyl)corrole [H3TPFC].............................................................. 19
2.3.5 Iron(III) corrole [FeIII(TPFC)·(OEt2)2]…………………................................................... 22
v
3. SELECTIVE OXIDATION OF SULFIDES TO SULFOXIDES CATALYZED BY IRON
PORPHYRINS WITH IODOBENZENE DIACETATE............................................................. 25
3.1 Introduction ........................................................................................................................... 25
3.2 General procedure for catalytic sulfoxidations...................................................................... 26
3.3 Results and discussions.......................................................................................................... 27
3.3.1 Solvent effect …………………………………………………………………..……...….27
3.3.2 Axial ligand effect of the catalyst on the catalytic oxidation of thioanisole....................... 28
3.3.4 Water effect on the catalytic oxidation of thioanisole........................................................ 29
3.3.4 Comparison of various amounts of PhI(OAc)2 in the catalytic oxidation of thioanisole
………………………………………………………………………………………………..… 31
3.3.5 Catalytic oxidation of substituted thioanisoles by PhI(OAc)2............................................ 32
4. CATALYTIC OXIDATIONS BY IRON CORROLES….......................................................35
4.1 Introduction............................................................................................................................ 35
4.2 Results and discussions.......................................................................................................... 35
4.2.1 Water effect on the catalytic oxidation of thioanisole…………….................................... 36
4.2.2 Solvent effect on the catalytic oxidation of thioanisole by FeIII(TPFC)............................. 38
4.2.3 Catalytic oxidation of substituted thioanisoles by PhI(OAc)2……………………….…………………. 39
4.3.0 Competition studies of sulfoxidation reactions…………………………...….................... 40
4.4 Mechanistic studies ............................................................................................................... 42
5. CONCLUSIONS...................................................................................................................... 46
REFERENCE .............................................................................................................................. 47
CURRICULUM VITAE .................................................................................................................... 52
ABBREVIATIONS AND SYMBOLS .............................................................................................. 53
vi
LIST OF FIGURES
Figure 1. Structure of iron protoporphyrin IX………………….................................................... 3
Figure 2. X-ray structure of cytochrome P-450cam....................................................................... 4
Figure 3. Structures of porphyrin, corrole and corrin..................................................................... 8
Figure 4. Agilent 8453 UV-vis diode array spectrophotometer....................................................11
Figure 5. JEOL ECA-500 MHz Nuclear Magnetic Resonance (NMR) spectrometer…………. 11
Figure 6. Mass Spectrometer-GC MS System ……………………………………..................... 12
Figure 7. The UV-vis spectrum of H2TMP in CH2Cl2..................................................................15
Figure 8. The 1H-NMR spectrum of H2TMP in CDCl3................................................................ 16
Figure 9. The UV-vis spectrum of FeIII(TMP)Cl in CH2Cl2......................................................... 17
Figure 10. The 1H-NMR spectrum of FeIII(TMP)Cl in CDCl3………......................................... 18
Figure 11. UV-vis spectra of the generated of FeIII(TMP)(ClO4)……………………………..…19
Figure 12. The UV-vis spectrum of H3TPFC in CH2Cl2……………………….......................... 21
Figure 13. The 1H-NMR spectrum of H3TPFC in CDCl3…………………………………….... 21
Figure 14. The UV-vis spectrum of FeIII(TPFC)·(OEt2)2 in diethyl ether................................... 23
Figure 15. The 1H-NMR spectrum of FeIII(TPFC)·(OEt2)2 in C6D6..............................................24
Figure 16. Time-resolved spectra of FeIII(TPFC) ……………………………..……………….. 43
Figure 17. Observed rate constants for reaction of FeIII(TPFC)………………..………………. 44
vii
LIST OF SCHEMES
Scheme 1. Monooxygenase reaction .............................................................................................. 2
Scheme 2. Stereospecific hydroxylation of the exo C-H bond at position 5 of camphor by
cytochrome P450cam….................................................................................................................. 4
Scheme 3. Two-step and one-pot synthesis of H2TMP…………… ............................................. 4
Scheme 4. Synthesis of FeIII(TMP)Cl............................................................................................14
Scheme 5. Synthesis of FeIII(TMP)X............................................................................................ 19
Scheme 6. Synthesis of H3TPFC……………………………...................................................... 20
Scheme 5. Synthesis of FeIII(TPFC)……..................................................................................... 23
Scheme 8. A proposed catalytic cycle for the catalytic oxidations by FeIII(TPFC).......................45
viii
LIST OF TABLES
Table 1. Solvent effect on the catalytic oxidation of thioanisole by FeIII(TMP)Cl……………...28
Table 2. Axial ligand effect on catalytic sulfoxidation reaction with PhI(OAc)2.........................29
Table 3. Water effect on the catalytic oxidation of thioanisole by FeIII(TMP)Cla....................... 30
Table 4. The effect of PhI(OAc)2 amount on catalytic sulfoxidation reaction…………............. 31
Table 5. Catalytic oxidation of thioanisole and substituted thioanisoles by
FeIII(TMP)Cl……………………………………………………………………………..…….. 33
Table 6. Water effect on the catalytic oxidation of thioanisole by FeIII(TPFC)…………........... 37
Table 7. Solvent effect on the catalytic oxidation of thioanisole by FeIII(TPFC)......................... 38
Table
8.
Catalytic
oxidation
of
thioanisole
and
substituted
thioanisoles
by
FeIII(TPFC)………………………………………………………………………………………40
Table 9. Relative rate constants ratios for iron(III) corrole catalyzed sulfoxidation ……..….... 42
ix
SELECTIVE OXIDATIONS BY IRON(III) PORPHYRINS AND
IRON(III) CORROLES
Aaron Carver
August 2014
51 Pages
Directed by: Rui Zhang. Donald Slocum, Larry Byrd
Department of Chemistry
Western Kentucky University
The selective oxidation of organic compounds represents a leading technology for
chemical industries. They are used in chemical synthesis in the pharmaceutical and
petrochemicals industries, and possible the decontamination of harmful substances. However,
oxidations reaction are among the most challenging processes to control. Many stoichiometric
oxidants with heavy metals are expensive, or toxic maybe both, and therefore unfeasible to be
utilized. The ideal processes for catalytic oxidation would use molecular oxygen or hydrogen
peroxide as the primary oxygen source, with transition metal catalysts to mimic the predominant
oxidation catalysts in Nature, the cytochrome P450 enzymes. This study focuses on the synthesis
of porphyrin and corrole macrocyclic ligands and the corresponding iron(III) complexes which
are fully characterized by UV-vis, GC/MS, and NMR spectroscopies.
In this work, the potential of catalytic oxidation reactions towards organic sulfides by
these metal complexes were studied. The iron(III) porphyrin and iron(III) corrole catalysts have
shown excellent activity and selectivity for sulfoxidation reactions. Various reaction conditions
and environmental effects were investigated including solvent, axial ligands, water, amounts of
oxygen source, and substrate scope. The optimal conditions were determined for iron(III)
porphyrin/ corrole-catalyzed sulfoxidations with PhI(OAc)2 as the mild oxygen source.
Competitive catalytic oxidation of substituted thioanisoles versus thioanisole by iron(III)
corrole with PhI(OAc)2 were studied. The spectral studies of iron(III) corrole with PhI(OAc)2 in
x
the presence of organic sulfide showed that a well-known diiron(IV)-µ-oxo biscorrole was
formed with a second-order rate constant of k2= (3.5 ± 0.3)×103 M-1·s-1. A catalytic cycle was
proposed on the basis of the mechanistic study, suggesting a highly reactive iron(V)-oxo corrole
as the active oxidizing intermediate.
xi
1. INTRODUCTION
1.1
Cytochrome P-450 Enzymes
Catalytic oxidation is of great importance in organic synthesis and the key to
understanding chemical reactions in Nature as well.1 Organic oxygenated compounds are
valuable synthetic reagents for the production of a variety of chemicals, pharmaceuticals in
industry and in the organic lab.2 Optically active oxygenated compounds are also useful
intermediates in medicinal and pharmaceutical chemistry to prepare the therapeutic agents.2
Many stoichiometric reagents are available, for oxidations unfortunately, most are harmful to the
environment or expensive, and a simple procedure is still elusive. The development of new
processes that employ transition metals as substrate-selective catalysts with an environmentally
friendly oxygen source such as molecular oxygen, or hydrogen peroxide for catalytic oxidation is
the most significant goals in oxidation chemistry.3,4,5
In biological systems, catalytic oxidations mediated by oxidative enzymes form the basis
of a great many biogenesis and biodegradative (aerobically or anaerobically) processes.5 Many of
these biological oxidatants are facilitated by heme-containing oxygenases, and the most notable
is cytochrome P450 monooxygenase (CYP450). The P450s family of enzymes have a variety of
critical roles in biochemical reactions in all domains of life. The cytochrome P450s enzymes can
transfer one oxygen atom from atmospheric molecular oxygen into a substrate then reduce the
second oxygen to a water molecule, with reducing agents such as NADH (nicotinamide adenine
dinucleotide) or NADPH (nicotinamide adenine dinucleotide phosphate) as electron donor via
1
electron transport protein systems.6 The cytochrome P450s have been known to catalyze a wide
variety of oxidation reactions in Nature, from mammalian drug metabolism, to synthesize
ergosterol and facilitate pathogenesis by detoxifying the chemical defenses of target host plants.7
Some of the more important reactions fall under oxidation reactions like epoxidation,
hydroxylation, dealkylation, dehydrogenation, and oxidation of amines, sulfides, alcohols and
aldehydes.5,6,8,9 Enormous efforts go into developing biomimetic oxidation catalysts each year. In
all these cysteinato-heme enzymes, the prolific group is composed of an iron(III)
protoporphyrin-IX covalently linked to the protein by the sulfur atom of a proximal cysteine
ligand (Figure 1).10 The highly potent enzymes catalyze the hydroxylation of saturated carbon
hydrogen bonds, the epoxidation of doubles, the oxidation of heteroatoms, oxidation, of
aromatics, and many more reactions. 5,6,8,9
Scheme 1. Monooxygenase reaction.
2
Figure 1. Structure of iron protoporphyrin IX
The name cytochrome P450 enzymes arises from the reduced protein which efficiently
binds carbon monoxide with a strong absorption at λmax= 450 nm.11 The majority of the
cytochrome P450s are membrane-bound, associated with the inner mitochondrial and the
endoplasmic reticulum of microsomal membranes.6 Some prokaryotes use hydrocarbons as both
carbon and energy sources containing cytochrome P450s, that is found in Pseudomonas putida.6
This soluble bacterial P450cam is very stable and easy to be purified in quantitative amounts. The
first three-dimensional structure of cytochrome P450cam was reported in 1986 by Poulos et al,12
in which showed the stereospecific 5-exo hydroxylation of the bicyclic terpene camphor to 5exo-hydroxycamphor (Scheme 2).11, 13, 14
3
Figure 2. X-ray structure of cytochrome P-450cam.
Scheme 2. Stereospecific hydroxylation of the exo C-H bond at position 5 of camphor by
cytochrome P450cam.
The cytochrome P450 enzymes have very unique spectral properties and can catalyze a
variety of bio-transformations. Therefore, a great amount of research has been done toward
creating synthetic version of these natural enzymes.3,8 In this work, iron porphyrins and iron
corrole complexes are synthesized and characterized.
This thesis focuses on the synthetic
organic chemistry, spectroscopic characterization and mechanistic studies of catalytic sulfide
4
oxidation with iron porphyrin and corrole complexes in the presence of a mild sacrificial oxygen
source, i.e. iodobenzene diacetate.
1.2 Biomimetic models with iron porphyrins
Nature mediates intrinsically difficult oxidations with the use of iron porphyrins with
dioxygen at the heme center of the cytochrome P450 enzymes, and scientists have focused on
this highly effective catalytic system. Groves and co-workers first reported the first oxidation
system with a simple iron porphyrin in 1979.14 Different models of metalloporphyrin have been
synthesized and used to develop useful oxidation catalysts and to understand the reaction
mechanisms.15 The transition high-valent metal-oxo porphyrins additionally serve as a center
within the reactions that involve oxidation and whose mediation is enabled by various synthetic
types of metalloporphyrins.16
Studies using synthetic metalloporphyrins as models for cytochrome P450 have afforded
important clues into the biomimic nature of the enzymatic processes.16 High-valent metal-oxo
porphyrins have been known as reactive intermediates in the catalytic oxidation cycles and
observed in the reactions of synthetic metalloporphyrins with various oxygen sources such as
PhIO, m-CPBA and tBuOOH.
3,5
The first high-valent iron-oxo porphyrin complex was reported
by Groves et al. by oxidation of FeIII(TMP)Cl with m-chloroperbenzoic acid (m-CPBA) at low
temperature.11 The iron(IV)-oxo porphyrin radical cations, namely Compound I analogues, are
observed intermediates in peroxidase and catalase enzymes.8 The similar species are not detected
under turnover conditions, but are also thought to be active oxidants in the cytochrome P450
enzymes.1 In the last few decades, ruthenium porphyrins have been extensively studied as
5
oxidation catalysts, and the goal has been to explore biological mechanisms as well as
formulating catalysts that mimic enzymes owing to the highly comparable periodic linkages to
iron coordination chemistry.17,18 The nature of this research marked the significance as it expands
the appreciation of catalyzed oxidations by iron(III) porphyrin complexes, and a understanding
of various biochemical P450s’ reactions. In the petroleum industries selective hydrocarbon
oxygenation is very important technology for converting derivatives of petroleum into valuable
commodity chemicals.16 With new laws, the invention of new alternatives with environment
friendly oxygen source such as O2 and H2O2 are of great significance.2, Transitional metals with
viable oxidation states might be useful in this regard. Therefore, a lot of consideration has been
dealt to the generation of biomimetric transition catalysts for regioselective and stereoselective
oxidations of organic.19
1.3 Biomimetic models with iron corroles
Corroles, are tetrapyrrolic macrocycles, which are structurally close to porphyrin, and
related to the cobalt-chelating corrin in vitamin B12 (Figure 3). Corrole tetrapyrrolic macrocycle
has a reduced meso-carbon that is replaced by a direct pyrrole to pyrrole link. Its inner cavity has
three inner core amine protons that help form the trianionic ligand. A transitional metal can bond
with the amine transitioning to a higher oxidation state that’s closely related to that of
porphyrins. Some of these macrocycles form metal complexes with metal ions, that show
different properties compared with metalloporphyrins.20 Corrole is more acidic than porphyrin,
and easier to form anionic species in basic conditions.21,
22
The main difference between
porphyrins and corroles is π-system of corroles is much more electron-rich than porphyrins.10
6
Therefore, trianionic correlate ligands supports unusually higher metal oxidation states than
dianionic porphyrinato ligand. Its reduced symmetry, small cavity, and ability to stabilize highvalent transition metal-oxo species has result in the increasing interests on corroles and their
metal complexes.23,24 Here recently, the coordination chemistry of corrole-metal complexes have
been revealed with respect to high metal oxidation states.25 In 1999, Gross and co-coworkers
explored a new and efficient synthetic pathway to prepare corroles, i.e. the solvent-free
condensation of pyrrole and aldehydes under unusual reaction conditions.26 Gross and coworkers were also first to report utilization of metallocorrole complexes for the catalytic
epoxidation, hydroxylation, and cyclopropanation in the presence of iodosylbenzene and
carbenoids.20
Figure 3. Structures of porphyrin, corrole and corrin.
The most common metals used in corrole catalysts is iron mainly due to its high
oxidation state in +4. The +5 oxidation state in catalytic macrocycle-iron-oxo chemistry exists
mainly as an iron(IV)-oxo complex with the ligand oxidized to a radical cation.27 The highly
oxidized iron O atoms are rare and a reported iron(V)-oxo complex supported by a tetraanionic
ligand showed truly unprecedented reactivity.28 We have shown that a fluorinated diiron(IV)-µoxo biscorrole catalyzed aerobic oxidation of alkenes and activated hydrocarbons under visible
7
light irradiation. The observed photocatalytic oxidation is ascribed to a photo-disproportionation
mechanism to afford a highly reactive corrole-iron(V)-oxo species that can be directly observed
by laser flash photolysis methods.29
1.4 Sulfoxidation reactions
Catalytic oxidation processes are key technologies for chemical synthesis in the
pharmaceutical and petrochemicals industries, but oxidations reaction are among the most
challenging processes to control. Many stoichiometric oxidants with heavy metals are expensive
or toxic maybe both, and therefore unfeasible. The central ideal of a green catalytic oxidation
process would use molecular oxygen or hydrogen peroxide as the primary oxygen source. The
selective oxidation of sulfides to sulfoxides is of great importance in organic synthesis. In
addition, organic sulfoxides are commonly utilized as precursors for biologically active and
chemically important compounds, and of great significance for synthetic intermediates for
production of a wide range of chemically and biologically active molecules including therapeutic
agents.30
The oxidation of sulfides to sulfoxides is a straightforward method for the synthesis of
sulfoxides.31 The reaction conditions including time, temperature and the amount of oxidants are
hard to control, these requirements are often hard to meet to avoid sulfone formation as over
oxidation product. Considerable attention in the development of selective oxidations methods is
still challenge, and undoubtedly appealing.32 A variety of reagents are available in sulfoxidation
reactions, most of them are unsatisfactory either because they are harmful or too expensive.
8
In spite of that the use of hydrogen peroxide as an oxidant has been studied as an ideal
oxidant sulfoxidation catalyzed by metalloporphyrins complexes with sacrificial oxidants has
received less attention. The studies on the epoxidation and hydroxylation of hydrocarbons by
trans-dioxoruthenium(VI) porphyrins are well established in the literature.5 So our studies is to
identify the true oxidants formed during turnover conditions when sacrificial oxidants are
employed with transition metal catalysts.33 So a way to evaluate if the same species was active is
to compare the ratios of product formed under catalytic turnover conditions to the ratios of rate
constants measured. In this study, our major objective is to fully explore the catalytic potential of
iron porphyrin and corrole complexes for the selective oxidation of sulfides to sulfoxides under
mild reaction conditions.
9
2. EXPERIMENTAL SECTION
2.1 Materials
All commercial reagents were of the best available purity and used as supplied unless
otherwise specified. Iodobenzene diacetate or (diacetoxyiodo)benzene, [PhI(OAc)2], was
purchased from Aldrich Chemical Co. and used as such. The organic solvents used for synthesis
and column purification, including acetone, ethanol, ethyl acetate, hexane, methanol, acetonitrile,
chloroform, dichloromethane, and N,N-dimethylformamide (DMF), were of analytical grade and
used as received without having to perform further purification. All substrates and reagents for
kinetic and catalytic studies were of the best available purity from Aldrich Chemical Co.,
including thioanisole, 4-methoxythioanisole, 4-methylthioanisole, 4-fluorothioanisole, 4chlorothioanisole, 4-bromothioanisole, silver perchlorate, silver chlorate, silver nitrate, silver
bromate, and iron chloride. The pyrrole was obtained from Aldrich Chemical Co. and freshly
distilled before use. Mesitaldehyde, 2,3,4,5,6-pentafluorobenzaldehyde, boron trifluoride diethyl
etherate
(BF3·OEt2),
2,3-dichloro-5,6-dicyano-p-benzequinone
(DDQ),
triethylamine,
chloroform-d, acetonitrile-d3, methanol-d4, and benzene-d6, were purchased from Aldrich
Chemical Co. and also used as received.
2.2 Methods
2.2.1 Physical measurements
UV-vis spectra were recorded on an Agilent 8453 diode array spectrophotometer (Figure
1). IR spectra were obtained on a Bio-Rad FT-IR spectrometer. 1H-NMR was performed on a
JEOL ECA-500 MHz spectrometer at 298K with tetramethylsilane (TMS) as internal standard
(Figure 2). Gas Chromatograph analyses were conducted on an Agilent GC6890/MS5973
10
equipped with a flame ionization detector (FID) using a DB-5 capillary column (Figure 3). The
above GC/MS system is also coupled with an auto simple injector. Reactions of
FeIII(TPFC)·(OEt2)2 with excess of PhI(OAc)2 were conducted in a chloroform and methanol
solutions, respectively, at 23 ± 2 ºC.
Figure 1. Agilent 8453 UV-vis diode array spectrophotometer.
Figure 2. JEOL ECA-500 MHz Nuclear Magnetic Resonance (NMR) spectrometer.
11
Figure 3. Mass Spectrometer-GC MS System
2.2.2 Reagent purification
The pyrrole was commercially available and purified by distillation. Pyrrole (ca. 25 mL)
was added into a 50 mL round-bottom flask which contained a magnetic spin vane and waterjacketed condenser. The solution was heated to the boiling point of the pyrrole at 130 oC, and all
the distillate was collected above this temperature. The freshly distilled colorless pyrrole was
directly used for the synthesis of the free porphyrin and corrole ligands.
2.2.3 Catalytic sulfoxidations by metalloporphyrin or metallocorrole
The catalytic reactions generally consisted of approximate 0.5-1 μmol of iron(III) catalyst
FeIII(TMP)Cl or FeIII(TPFC), in 2 mL of methanol solution containing 0.5 mmol of organic
substrates, 1.5 equivalent of PhI(OAc)2 as oxygen source (0.75 mmol). The reaction solution was
stirred at 23 ± 2 ºC until the reaction was complete. Aliquots of the reaction solution were
12
analyzed by 1H-NMR or GC/MS spectrometer to determine the product yields with an internal
standard (diphenylmethane). All reactions were run at least twice, and the data reported in
sections 2 and 3 represent the average of these reactions.
2.3 Synthesis and spectroscopic characterization of iron(III) porphyrin and iron(III) corrole
catalysts.
2.3.1 meso-Tetramesitylporphyrin [H2TMP]
The sterically encumbered free porphyrin ligand was prepared based on the well-known
method described by Lindsey and co-workers,34 as shown in Scheme 1. A l-L round-bottomed
flask fitted with a reflux condenser was charged with freshly distilled pyrrole (347 μL, 5 mmol),
mesitaldhyde (736 μL, 5 mmol) and chloroform (500 mL). Ethyl alcohol (3.47 mL, 0.5% v/v)
was added as co-catalyst. The solution was purged with argon for 5 min. Boron trifluoride
diethyl etherate (BF3·OEt2) (660 μL, 1.65 mmol) was added as strong Lewis acid catalyst in a
dropwise manner, and the reaction mixture was stirred for 1 h at room temperature. This Lewis
acid was known to catalyze the pyrrole-aldehyde condensation to form porphyrinogen as
intermediate product in the first step of reactions (Scheme 1).
13
Scheme 1. Two-step and one-pot synthesis of H2TMP.
The reaction was monitored by UV-vis spectroscopy to confirm that porphyrinogen was
formed during the room temperature reaction with a signature absorption at λmax = 478 nm. The
one-electron oxidant of 2,3-dichloro-5,6-dicyano-p-benzequinone (DDQ) (957 mg) was then
added to the reaction mixture and the solution was gently refluxed for 1 h. After the solution was
cooled to room temperature, triethylamine (920 μL, 6.6 mmol) was added to neutralize the
mixture to give the desired product absorption at λmax = 418 nm. The solution was evaporated to
dryness. The solid crude product was washed by large excess of methanol under vacuum until
the filtrate was colorless. The typical impurities such as poly-pyrromethanes and quinine
components are highly soluble in the methanol and can be easily removed. The further
purification was performed on column chromatography with silica gel. After placing the crude
product into the wet column, dichloromethane was used to elute the desired product. The mesotetramesitylporphyrin (H2TMP) was obtained as purple solid after removing the solvent by rotary
evaporation, and further characterized by UV-vis (Figure 4) and 1H–NMR (Figure 5), matching
literature reported values.34
14
[H2TMP] Yield = 25.1%. 1H-NMR (500 MHz, CDCl3): δ, ppm: -2.50 (s, 2H, NH), 1.81 (s, 24H,
o-CH3), 2.62 (s, 12H, p-CH3), 7.25 (s, 8H, m-ArH), 8.61 (s, 8H, β-pyrrole). UV-vis (CH2Cl2)
λmax/nm: 418 (Soret), 443, 513, 546, 594, 645.
2.5
418
Absorbance (AU)
2.0
1.5
1.0
0.5
443
513
0.0
300
400
546
500
594
645
600
700
Wavelength
Figure 4. The UV-vis spectrum of H2TMP in CH2Cl2.
6
methyl(24H,ortho)
5
methyl(12H,para)
Intensity
4
phenyl(8H)
3
pyrrolic(8H)
TMS
2
1
Inner(2H)
0
10
8
6
4
2
0
-2
Chemical Shift (ppm)
Figure 5. The 1H-NMR spectrum of H2TMP in CDCl3.
15
2.3.2 Iron(III) tetramesitylporphyrin chloride [FeIII(TMP)Cl)]
The FeIII(TMP)Cl were synthesized by literature known methods similar to the work with
ruthenium(II) carbonyl porphyrins (Scheme 2).35 Free porphyrin ligand (100 mg) was dissolved
in 30 mL of N,N-dimethyl formamide (DMF) in a round bottom-flask. Excess amounts of
iron(II) chloride salt (FeCl2, 500 mg) was added and the solution was then refluxed for 1 h. UVvis spectroscopy was used to monitor the reaction based on the known Soret band of the desired
iron(III) complex. The solution was cooled to room temperature, and evaporated to dryness
under vacuum. The resulting solid was dissolved in CH2Cl2 in the presences of excess HCl (3N)
and stirred for 30 min. The final product was purified by column chromatography (basic Al2O3).
A large excess amount of CH2Cl2 was used to elute the desired product. The final product was
obtained as a dark green solid and after the solvent was removed via rotary evaporation.
The
iron(III) tetramesityporphyrin chloride was characterized by UV-vis (Figure 6) and 1H-NMR
(Figure 10) with the typical pattern expected for the paramagnetic complexes.
Scheme 2. Synthesis of FeIII(TMP)Cl.
16
[FeIII(TMP)Cl] Yield = 95%. 1H-NMR (500MHz, CDCl3): δ, ppm: 82 ppm, 17 ppm, 18 ppm.
UV-vis (CH2Cl2) λmax/nm: 418 (Soret), 508, 375.
2.5
418
Absorbance (AU)
2.0
1.5
375
1.0
508
0.5
0.0
300
400
500
600
700
Wavelength (nm)
Figure 6. The UV-vis spectrum of FeIII(TMP)Cl in CH2Cl2.
6
5
Intensity
4
3
2
1
0
100
80
60
40
20
0
Chemical Shift (ppm)
Figure 7. The 1H-NMR spectrum of FeIII(TMP)Cl in CDCl3.
17
2.3.2 Iron(III) tetramesitylporphyrin perchlorate [FeIII(TMP)ClO4 )]
The FeIII(TMP)(ClO4) was readily prepared by a axial ligand exchange with
corresponding silver salts. Excess amount of AgClO4 (ca. 1 mg) was dissolved in a small amount
of CH3CN in a vial. In another vial, 2 mg of FeIII(TMP)Cl was added to CHCl3 (0.5 ml) and
dissolved. Immediately a couple of drops of the AgClO4 solution was added and a color change
was noticed. The entire transformation was monitored by UV-vis spectroscopy (Figure 8). The
same synthetic process was also used to syntheize FeIII(TMP)(ClO3), FeIII(TMP)(NO2), and
FeIII(TMP)(NO3), as described in Scheme 3.
Scheme 3. Synthesis of FeIII(TMP)X.
18
2.5
Absorbance
2.0
1.5
FeIII(TMP)Cl
FeIII(TMP)ClO4
1.0
0.5
0.0
300
400
500
600
700
Wavelength (nm)
Figure 8. UV-vis spectra of the generated of FeIII(TMP)(ClO4) (solid line) from the reaction of
FeIII(TMP)Cl (dash line) with AgClO4.
2.3.4 5,10,15-Tris(pentafluorophenyl)corrole [H3TPFC]
The procedure consisted of a solvent-free condensation reaction of pyrrole (15 mmol) and
pentafluorobenzaldehyde(15 mmol) as the efficient synthetic pathway for the preparation of
triphenyl corroles as described by Gross and co-workers.26 The reactants was dissolved in
CH2Cl2 (10 mL) and the mixture was then added to a 20 mL beaker containing basic alumina
oxide (3 g) as a solid support. The mixture was heated with stirring in an open vessel above 60
ºC (inner temperature). After the solvent was evaporated, the reaction was maintained above 60
ºC for 4 hours, and the mixture became dark orange and black. CH2Cl2 (50 mL) was then added
to dissolve the product and the solution was filtered under vacuum. The filtrate was stirred with
DDQ (1.7g, 7.5 mmol) for 1 h. The reaction progress was checked by TLC
19
(dichloromethane/hexanes = 1:1), and a purple band with strong fluorescence under long-wave
UV light should be seen. The isolation and purification of final product was performed on a long
column (silica gel) with hexane/CH2Cl2 (3:1, v/v) as eluent. The second column for further
purification might be needed, if any. The desired product H3TPFC was characterized by UV-vis
(Figure 9) and 1H-NMR (Figure 10), matching the literature reported values.36
Scheme 4. Synthesis of H3TPFC
[H3TPFC] Yield = 7%. 1H-NMR (500 MHz, CDCl3): δ, ppm: -2.25 (s, 3H), 8.57 (d, 4H), 8.75 (d,
2H), 9.10 (d, 2H). UV-vis (CH2Cl2) λmax/nm: 408 (Soret), 560, 602.
2.5
Absorance (AU)
2.0
408
1.5
1.0
556
0.5
0.0
300
602
400
500
600
700
Wavelength (nm)
Figure 9. The UV-vis spectrum of H3TPFC in CH2Cl2.
20
3.0
TMS
2.5
Absorption
2.0
X
1.5
Solvent
1.0
0.5
Pyrrolic-H
8.57, 8.75,
9.10
0.0
10
8
6
4
2
0
-2
-4
Chemical Shift (ppm)
Figure 10. The 1H-NMR spectrum of H3TPFC in CDCl3
2.3.5 Iron(III) corrole [FeIII(TPFC)·(OEt2)2]
FeIII(TPFC) complex was obtained by metalation of free corrole ligand (H3TPFC) in hot
DMF with FeCl2 as the metal source (Scheme 5). Following to the procedure described by
Simkhovich.37 a solution of free corrole ligand H3TPFC (50 mg, 68 mmol) was dissolved in
DMF (30 mL) in a two-neck 100-mL round-bottom flask fitted with reflux condenser. The
solution was purged with argon and heated gently. Large excess amount of iron(II) chloride
(FeIICl2) (127 mg, 680 mmol) was added. The mixture was heated with stirring for 1 hr. UV-vis
spectroscopy was used to monitor the reaction process. The reaction mixture was cooled to room
temperature. High boiling point solvent DMF was removed by vacuum distillation. The crude
product was collected and dissolved in diethyl ether, then followed by a column chromatography
on silica gel (diethyl ether). Isolation of the desired Fe III(TPFC)·(OEt2)2 complex was confirmed
21
by UV-vis (Figure 11) and 1H-NMR (Figure 12), consistent with the pattern of paramagnetic
compound.38
[FeIII(TPFC)·(OEt2)2] Yield = 85%. 1H-NMR (500 MHz, C6D6): δ, ppm: 19.71 (s, 2H), 13.37 (s,
2H), -60.03 (s, 2H), -126.05 (s, 2H). UV-vis (CH2Cl2) λmax/nm: 402 (Soret), 542.
Scheme 5. Synthesis of FeIII(TPFC)
1.4
Absorbance (AU)
1.2
402
1.0
0.8
0.6
0.4
542
0.2
0.0
300
400
500
600
700
Wavelength (nm)
Figure 11. The UV-vis spectrum of FeIII(TPFC)·(OEt2)2 in diethyl ether.
22
5
Solvent
4
Intensity
TMS
3
Paramagnetic complex
X
2
1
0
50
0
-50
-100
Chemical Shift (ppm)
Figure 12. The 1H-NMR spectrum of FeIII(TPFC)·(OEt2)2 in C6D6.
23
3. SELECTIVE OXIDATION OF SULFIDES TO SULFOXIDES
CATALYZED BY IRON PORPHYRINS WITH IODOBENZENE
DIACETATE
3.1 Introduction
The selective oxidation of sulfides to sulfoxides (sulfoxidation) is of great importance in
organic synthesis.39 Organic sulfoxides are important synthetic reagents for the production of a
diverse class of chemically and biologically momentous molecules. Optically active sulfoxides
form useful intermediates for medical and pharmaceutical drugs such as antiulcer (proton pump
inhibitors), antibacterial, antifungal, antiatherosclerotic, antihypertensive and cardiotonic agents,
parapsychology and vasodilatation.39 A large family of electrophilic reagents such as peracids,
and highly toxic oxo metal oxidants have been utilized for the oxidation of conventional sulfides
with the objective of obtaining a high selectivity for sulfoxide without over oxidation to give
sulfone.39,30 Nevertheless, the procedures that are currently reported seldom provide the ideal
combination of high selectivity, fast reaction kinetics, and excellent product yields.
In the past decades, many D–block metals complexes40 have been synthesized to mimic
the major oxidation catalysts in Nature, namely the cytochrome P450 enzymes.2,40 In biomimetic
catalytic oxidations, a transition metal catalyst is oxidized to a high–valent metal–oxo by a
sacrificial oxidant oxygen source, and then the reactive transition metal–oxo intermediate
oxidizes the substrate.11 In this regard, the iron porphyrin complexes are among the most
extensively studied biomimetic oxidation catalysts in view of their rich coordination and redox
chemistry.41,14 Iron porphyrins are shown to catalyze the oxidation of a wide variety of organic
24
substrates, particularly for alkenes, bezylic hydrocarbons and arenes.42,43,44,45 The catalysts can be
recycled several times to reach product turnovers of over 10,000 or by performing the oxidation
at low catalyst loadings, as demonstrated in a number of selected cases.46,47
In this section, our goal was to fully explore the potential of iron porphyrins for catalytic
sulfoxidation reactions with iodobenzene diacetate [PhI(OAc)2], which is commercially
available, easy to handle, and safe to use. Different from the sacrificial oxidants in conventional
use for metalloporhyrin catalyzed reactions, PhI(OAc)2 does not show appreciable reactivity
towards organic substrates and does not damage the porphyrin catalyst under general catalytic
conditions. Due to its mild oxidizing ability, it has been rarely employed in the
matalloporphyrin-catalyzed oxidations. To our knowledge, using PhI(OAc)2 for the oxidation of
organic sulfides catalyzed by iron porphyrin complexes is unprecedented. In this study, we have
discovered that PhI(OAc)2 is an efficient oxygen source associated with iron porphyrins to
facilitate the selective oxidation of sulfides to sulfoxides. In most cases, quantitative conversions
of sulfides to sulfoxides have been achieved with good selectivity.
3.2
General procedure for catalytic sulfoxidations
In general, a typical catalytic reaction was carried out in a vial at room temperature (23 ±
2ºC) in 2 mL of deuterated solvent of CDCl3 solution, and the mild oxygen source PhI(OAc)2
(0.75 mmol) was added and dissolved in the solution. Sulfide substrate was added to this
mixture, and to this mixture iron(III) porphyrin (1 µmol) was added as a catalyst. Aliquots of the
reaction solution at constant time interval were analyzed by 1H-NMR or GC/MS to determine the
25
formed products and yields with an internal standard. All reactions were run at least in duplicate,
and the data reported in this section represents the average of these reactions.
3.3
Results and discussions
With the mild oxygen source of PhI(OAc)2, we have evaluated iron(III) porphyrins in
catalyitic oxidation of thioanisole as a standard substrate under different conditions. In general,
the iron(III) porphyrins can facilitate the selective oxidation of sulfide to the corresponding
sulfoxide with good to excellent selectivity (>95%). The control experiment shows that in the
absence of the iron(III) porphyrin catalyst, no oxidized product was observed.
3.3.1 Solvent effect
The solvent effect on the catalytic oxidation of thioanisole by FeIII(TMP)Cl was first
investigated and the results are summarized in Table 1. Chloroform, methanol, and acetonitrile
were used as solvent in this study. As shown in Table 1 a significant solvent effect was observed,
indicating the solvent played an important role in the catalytic reactions. Instead of CDCl 3 (Table
1, entry 1), the use of CD3OD as the solvent resulted in an enhanced yield (Table 1,entry 2). The
reaction was also carried out in acetonitrile, and much slower reaction was observed with only
15% conversion (Table 1, entry 3). This might be due to its poor solubility of PhI(OAc)2.
Overall, methanol was much more favorable to the oxidation of sulfide to sulfoxide, presumably
because of its polar nature and good solubility of the PhI(OAc)2.
26
Table 1. Solvent effect on the catalytic oxidation of thioanisole by FeIII(TMP)Cla
a
Entry
Solvent
Conv(%)b
Selectivityb
1
CDCl3
18
95:5
2
CD3OD
46
95:5
3
CD3CN
15
95:5
All reactions were performed under the following conditions: thioanisole (0.5 mmol),
FeIII(TMP)Cl catalyst (1µmol), solvent (2 mL), iodobenzene diacetate [PhI(OAc)2] (0.75 mmol).
b
Conversions (conv.), and product selectivity (surfoxide:sulfone) were determined by 1H-NMR
and/or quantitative GC-MS analysis with an internal standard (1,2,4-trichlorobenzene) on the
crude reaction mixture over 40 min.
3.3.2 Axial ligand effect of the catalyst on the catalytic oxidation of thioanisole
We carried out the catalytic sulfoxidations by FeIII(TMP)X in CDCl3 with different axial
ligands (X = Cl, ClO4, ClO3, NO3, and NO2) and the results are summarized in Table 2. As
evident in Table 2, the axial ligand in the catalyst showed a significant effect on the sulfoxidation
reaction. The FeIII(TMP)(ClO4) (Table 2, entry 3) and the FeIII(TMP)(NO3)(Table 2, entry 5)
gave the best selectivity and highest conversion. Among the axial ligands that we studied, ClO4and NO3- have relativively weak coordinating ability to the iron, suggesting that a rapid reaction
with PhI(OAc)2 to generate the active oxidizing species. The FeIII(TMP)(ClO3) and
FeIII(TMP)(NO2) (Table 2, entries 2 and 4) gave a quantitative conversion, however, with
27
significant formation of the undesired sulfone. Cl- is known to be the strongest coordinating axial
ligand to the metal, gaving the slowest reaction under the identical conditions. Therefore, the
best selectivity for sulfoxide was observed with Cl- as the axial ligand (entry 1)
Table 2 Axial ligand effect on catalytic sulfoxidation reaction with PhI(OAc)2.
Entry
a
Catalyst
Conv(%)b
Selectivityb
1
FeIII(TMP)Cl
82
99:01
2
FeIII(TMP)(ClO3)
100
77:23
3
FeIII(TMP)(ClO4)
96
97:03
4
FeIII(TMP)(NO2)
100
88:12
5
FeIII(TMP)(NO3)
100
98:02
All reaction were performed under the following conditions: thioanisole (0.5 mmol),
FeIII(TMP)Cl catalyst (1µmol), CDCl3 (2 mL), iodobenzene diacetate [PhI(OAc)2] (0.75 mmol).
b
Conversions (Conv), and product selectivity (sulfoxide:sulfone) were determined by 1H-NMR
and/or quantitative GC-MS analysis with an internal standard (1,2,4-trichlorobenzene) on the
crude reaction mixture over 20 h.
3.3.4 Water effect on the catalytic oxidation of thioanisole
Similar to the literature known work reported by Nam and co-workers,16 we have
observed an significant water effect on the catalytic reactions. Table 3 shows that when the
reactions were carried out in the presence of a small amount of H2O, sulfoxidation reactions
proceeded much more rapidly and 100% conversion and high selectivity (> 95:5) can be obtained
28
within 40 min. On the other side, in the absence of water, the reaction preceded much slowly and
only 18% conversion was obtained (Table 3, entry 1). The enhanced reactivity by a small amount
of H2O is possibly due to the generaration of a more oxidizing oxidant PhIO from the reaction of
PhI(OAc)2 with H2O.
Table 3 Water effect on the catalytic oxidation of thioanisole by FeIII(TMP)Cla
a
Entry
H2O (µL)
Conv(%)b
Selectivityb
1
0
18
95:5
2
4.5
60
95:5
3
6.75
64
95:5
4
9
100
95:5
All reactions were performed under the following conditions: thioanisole (0.5 mmol),
FeIII(TMP)Cl catalyst (1µmol), CDCl3 (2 mL), iodobenzene diacetate [PhI(OAc)2] (0.75 mmol).
b
Conversions (conv), and product selectivity were determined by 1H-NMR and/or quantitative
GC-MS analysis with an internal standard (1,2,4-trichlorobenzene) on the crude reaction mixture
over 40 min.
29
3.3.4 Comparison of various amounts of PhI(OAc)2 in the catalytic oxidation of thioanisole
The use of PhI(OAc)2 as an efficient oxygen sources in ruthenium(II) poprhyrincatalyzed oxidation of thioanisole has been recently reported by our group.73Following similar
study, we also investigated the amount of the oxygen source in catalytic sulfoxidation reaction
by iron porphyrin catalyst. As expected, the use of more PhI(OAc)2 resulted in a faster reaction
(Table 4). For example, the addition of PhI(OAc)2 (1.0 equiv. with respect to thioanisole)
converted thioanisole to the corresponding sulfoxide with 52% conversion. Increase the amount
of PhI(OAc)2 to 1.5 equivalent (entry 3) gave 60% conversion within the same reaction time.
The use of 2 equivalent of PhI(OAc)2 can give a 100% conversion with a minor formation of
sulfone (entry 4).
Table 4.The effect of PhI(OAc)2 amount on catalytic sulfoxidation reactiona.
Entry
PhI(OAc)2(equiv.)
Conv(%)b
Selectivityb
1
0.5
43
99:1
2
1.0
52
99:1
3
1.5
60
99:1
4
2.0
100
96:4
30
a
All reactions were performed under the following conditions: thioanisole (0.5 mmol),
FeIII(TMP)Cl catalyst (1µmol), CDCl3 (2 mL), (0.5 – 2.0 equivalent of) iodobenzene diacetate
[PhI(OAc)2].
b
Conversions (conv), and product selectivity (sulfoxide:sulfone) were determined by quantitative
GC-MS analysis with an internal standard (1,2,4-trichlorobenzene) on the crude reaction mixture
over 20 h.
3.3.5 Catalytic oxidation of substituted thioanisoles by PhI(OAc)2
The substrate scope of the unprecedented catalytic sulfoxidations was explored in the
absence H2O and varing substituted thioanisoles were studied. Table 5 lists the oxidized products
and corresponding selectivities using the FeIII(TMP)Cl as the catalyst. The different substrates
that
we
examined
include
thioanisole,
4-methoxythioanisole,
4-fluorothioanisole,
4-
chlorothioanisole, and 4-bromothioanisole. In all cases, catalytic oxidation of substituted
thioanisoles proceeded with a good conversion of sulfides into the corresponding sulfoxides
and/or sulfones. Of note, the introduction of electron-donating groups’ such as methoxy in the
substrate resulted in increased reactivity compared to the other substituted thioanisole.
FeIII(TMP)Cl as catalyst gave excellent reactivity and selectivity in the presence of iodobenzene
diacetate for the oxidation of organic sulfides.
31
Table 5. Catalytic oxidation of thioanisole and substituted thioanisoles by FeIII(TMP)Cl
Conv (%)b
Selectivityb
1
71
95:5
2
46
95:5
3
52
95:5
4
14
95:5
5
63
95:5
Entry
a
Substrate
Product
All reactions were performed under the following conditions: thioanisole ( 0.5 mmol),
FeIII(TMP)Cl catalyst (1µmol), CDCl3 (2 mL), iodobenzene diacetate [PhI(OAc)2] (0.75 mmol).
b
Conversions (Conv), and product selectivity (sulfoxide:sulfone) were determined by 1H-NMR
and/or quantitative GC-MS analysis with an internal standard (1,2,4-trichlorobenzene) on the
crude reaction mixture over 17 h.
In summary, our studies have discovered an efficient method for the highly selective
oxidation of sulfides to sulfoxides with iron porphyrin as catalyst in the presence of PhI(OAc)2.
All the factors that effected the catalytic sulfoxidation reactions were investigated. Various
thioanisoles could be successfully oxidized with good conversions (most quantitative) and good
selectivities. A significant solvent, axial ligand and water effects on the rate of catalytic
sulfoxidation were observed. Also all catalytic oxidation reactions of substituted thioanisoles
were finished within 17 hours with negligible amount of over-oxidation products of sulfones.
32
4. CATALYTIC OXIDATIONS BY IRON CORROLES
4.1 Introduction
High-valent metal-oxo transients are active intermediates in many catalytic oxidation
processes in nature and in the laboratory.2 In biological systems iron is one of the most common
metals in catalyst, where it is found in forms of iron porphyrins and other closely related
modified versions.1 One of these is the tetrapyrrolic macrocycle known as corrole. Although
corroles are not found in biological systems, it is similar to porphyrins. The corroles is an
example of a synthetic porphyrinoid macrocycle that is compared to the P450 enzyme synthetic
version of porphyrin. Corrole tetrapyrrolic macrocycle has a reduced meso-carbon that is
replaced by a direct pyrrole to pyrrole link. Its inner cavity has three inner core amine protons to
form the trianionic ligand. A transitional metal can bond with the amine transitioning to a higher
oxidation state that’s closely related to that of porphyrins. In view of the promising catalytic
properties, a number of metallocorroles have been prepared and explored in a wide variety of
catalytic reactions, including oxidations. In this section, we have explored the potential of
iron(III) corrole for the selective oxidation of sulfides with PhI(OAc)2 as the mild oxygen source.
4.2
Results and discussion
The catalytic potential of FeIII(TPFC) with PhI(OAc)2 was evaluated for the oxidation of
thioanisole as the standard substrate under different conditions. In genereral, the FeIII(TPFC) can
facilitate the selective oxidation of sulfide to the corresponding sulfoxide with good to excellent
selectivity (> 95%). The control experiment shows in the absence of the iron(III) corrole catalyst,
no sulfoxidation was observed.
33
4.2.1 Water effect on the catalytic oxidation of thioanisole
The effect of water on the catalytic oxidation of thioanisole by Fe III(TPFC) was first
investigated and the results were summarized in Table 6. In the presence of a small amount of
H2O, a general accelerating trend is evident, i.e. the conversion of sulfoxidation reactions
increase with the increased amounts of H2O. However, undesired sulfone also forms, as a result
of the overoxidation of sulfoxid (Table 6, entry 6, 7, and 8). One possible speculation is that the
more oxidizing PhIO that is formed by the reaction of PhI(OAc)2 with H2O could over oxidize
sulfoxide to produce sulfone. More studies will be needed to understand the water effect on the
catalytic sulfoxidation with iron(III) corrole complex.
34
Table 6 Water effect on the catalytic oxidation of thioanisole by FeIII(TPFC) in the presence of
PhI(OAc)2.a
a
Entry
H2O (µL)
Conv(%)b
Selectivityb
1
0
26
100:0
2
2
81
99:1
3
3
91
97:3
4
4
92
97:3
5
5
92
97:3
6
10
100
66:34
7
20
100
58:42
8
30
100
37:63
All reactions were performed under the following conditions: thioanisole (0.5 mmol),
FeIII(TPFC) catalyst (1µmol), CDCl3 (2 mL), iodobenzene diacetate [PhI(OAc)2] (0.75 mmol).
b
Conversions (conv), and product selectivity (sulfoxide:surfone) were determined by 1H-NMR
and/or quantitative GC-MS analysis with an internal standard (1,2,4-trichlorobenzene) on the
crude reaction mixture.
4.2.2 Solvent effect on the catalytic oxidation of thioanisole by FeIII(TPFC)
Immediately apparent from examination of Table 7 a solvent effect on the reaction was
observed and CH3OH as the solvent gave the fastest reacting time (Table 7, entry 4).
35
Chloroform, methanol, dichloromethane, and acetonitrile were used as solvent in this study. As
shown in Table 7 a significant solvent effect was observed, indicating the solvent played an
important role in the catalytic reactions. It took only 5 min to reach a complete conversion in
methanol as the solvent (Table 7, entry 4). The reaction was also carried out in acetonitrile, and
much slower reaction was observed with a selectivity of 84:16 (Table 7, entry 1). The less polar
CH2Cl2 gave the slowest reaction compared to other two polar solvents used in this study ( entry
2 and 3).
Table 7 Solvent effect on the catalytic oxidation of thioanisole by FeIII(TPFC) with PhI(OAc)2a
a
Entry
Solvent
Time (min)
Con(%)b
Selectivityb
1
CH3CN
9
100
84:16
2
CH3CN:CH2Cl2
30
100
84:16
3
CH2Cl2
80
100
82:18
4
CH3OH
5
100
83:17
All reactions were performed under the following conditions: thioanisole (0.5 mmol),
FeIII(TPFC) catalyst (1µmol), solvent (2 mL), iodobenzene diacetate [PhI(OAc)2] (0.75 mmol) in
the presence of 5µL of H2O.
b
Conversions (conv), and product selectivity (sulfoxide:sulfone) were determined by 1H-NMR
and/or quantitative GC-MS analysis with an internal standard (1,2,4-trichlorobenzene) on the
crude reaction mixture.
36
4.2.3 Catalytic oxidation of substituted thioanisoles by PhI(OAc)2
The substrate scope of the unprecedented catalytic sulfoxidations was explored under
optimized conditions. Table 8 lists the oxidized products and corresponding selectivities using
the FeIII(TPFC) as the catalyst, respectively. In analogy with what we observed for the oxidation
thioanisole, all catalytic oxidation of substituted thioanisoles proceeded with a quantitative
conversion into the corresponding sulfoxides with good selectivity, and only traces of sulfones
were detected in all cases. As is evident, the introduction of electron-donating groups like methyl
or methoxy in the organic substrates resulted in increased selectivity with only 2% sulfone
formation. An increased reactivity was also observed for p-methoxythioanisole (entry 5)
compared to the other substituted thioanisoles (Table 8, entries 2, 3, 4, and 6).
Table 8 Catalytic oxidation of thioanisole and substituted thioanisoles by FeIII(TPFC)a
Conv (%)b
Selectivityb
1
100
93:7
2
90
97:3
3
79
97:3
4
86
95:5
5
93
98:2
6
53
98:2
Entry
Substrate
Product
37
a
All reactions were performed under the following conditions: thioanisole (0.5 mmol),
FeIII(TPFC) catalyst (1µmol), CH3CN (0.5 mL), iodobenzene diacetate [PhI(OAc)2] (0.25 mmol)
in the presence of 5µL of H2O.
b
Conversions (Conv), and product selectivity were determined by 1H-NMR (JEOL 500M) and/or
quantitative GC-MS analysis with an internal standard (1,2,4-trichlorobenzene) on the crude
reaction mixture at 20 min.
4.3
Competition studies of sulfoxidation reactions
One objective of this study is to identify the oxidants during the catalytic conditions with
iron corrole catalyst. The highly reactive O=FeV(TPFC) a proposed oxidizing intermediate,
however it is not necessarily the active oxidant under catalytic turnover conditions. One method
to evaluate whether the same species of metal-oxo intermediate was active in different conditions
was by comparing the ratios of products formed under catalytic conditions to the ratios of the
rate constants measured in direct kinetic studies. Table 9 contains the competition results where
iodobenzene diacetate served as oxygen source with FeIII(TPFC) catalysts. In this case, limited
amount of sacrificial oxygen source PhI(OAc)2 was used to keep the conversion under 20% in
the purpose of reducing potential problems in product analysis due to secondary oxidations and
multiple oxidation products. Table 9 contains ratios of relative rate constants (krel) from a series
of completion kinetic studies using iron(III) corrole catalyst in the presence of PhI(OAc)2 as the
oxygen source. For cases where the ratios of krel > 1 (entries 2 and 3), substituted thioanisoles
including p-methoxythioanisole and p-methylthioansole are more reactive than the thioanisole
under the same catalytic conditions. The substituted thioanisoles with different halogen groups
(entries 1, 4 and 5) are less reactive than thioanisoles as indicated by the values of krel < 1.
38
Table 4 Relative rate constants ratios for iron(III) corrole catalyzed sulfoxidation.a
Entry
a
Substrates
Oxygen Source
krelb
1
p-flurorthioanisole/thioanisole
PhI(OAc)2
0.978
2
p-methoxythioanisole/thioanisole
PhI(OAc)2
1.58
3
p-methylthioansole/thioanisole
PhI(OAc)2
1.11
4
p-bromothioanisole/thioanisole
PhI(OAc)2
0.296
5
p-chlorothioanisole/thioanisole
PhI(OAc)2
0.983
A reaction solution containing equal amounts of two substrates, e.g., thioanisole (0.25 mmol)
and substituted thioanisole (0.25 mmol), FeIII(TPFC) catalyst (0.5 µmol) was prepared in
methanol (2 mL). Iodobenzene diacetate (PhI(OAc)2) (0.5 mmol), and 8 µL of Di-water was
added.
b
The ratio of the oxidized products is one over thioanisole sulfoixde and measured by GC-MS.
4.4 Mechanistic studies
To probe the identity of the active oxidizing species under catalytic conditions, we
conducted the chemical oxidation reaction of FeIII(TPFC) catalyst by PhI(OAc)2 in CH3OH in
the absence of sulfides. The time-resolved spectra for the reaction of FeIII(TPFC) with PhI(OAc)2
are shown in Figure 15. A large excess of PhI(OAc)2 can oxidize the FeIII(TPFC) rapidly to a
product with absorption of λmax at 380 nm, which is known to be diiron (IV) µ-oxo dimer, i.e.
[FeIV(TPFC)]2O. A shift of the Soret band in the UV-vis spectrum in Figure 15 is noticed as the
39
dimer is generated. The diiron(IV) µ-oxo-bis precursor was also prepared in a different method
and characterized by UV-vis, and 1H NMR.38 The electron-withdrawing substituents such as F
on corrole ligand are necessary for the formation of µ-oxo dimer. Presumably, the electronwithdrawing subsitutents would stabilize the iron(IV) complex in a dimeric form by reducing the
electron density of metal atoms.
1.4
FeIV-O- FeIV
1.2
FeIII(TPFC)
380 nm
Absorbance (AU)
1.0
404 nm
0.8
540 nm
0.6
0.4
0.2
0.0
300
400
500
600
700
Wavelength (nm)
Figure 15. Time-resolved spectra of FeIII(TPFC) reacting with PhI(OAc)2 ( 2.5 mmol) in
methanol over 30 s.
When PhI(OAc)2 was present, the formation of the stable µ-oxo dimer accelerated
linearly as the function of PhI(OAc)2 concentration. This result indicates a second-order reaction
of FeIV(TPFC) and PhI(OAc)2, and therefore the plot slope gave a second-order rate constant of
(3.5 ± 0.3) ×103 M-1. s-1 (Figure 16).
40
0.35
0.30
kobs(s-1)
0.25
0.20
0.15
0.10
0.05
0.00
2
4
6
8
10
12
PhI(OAc)2 /mM
Figure 16. Observed rate constants for reaction of FeIII(TPFC) with varying concentrations of
PhI(OAc)2 in MeOH at 23 ± 2 °C.
On the basis of the present experimental facts, a catalytic cycle is proposed in Scheme 1.
First, oxidation of FeIII(TPFC) with PhI(OAc)2 generated a highly reactive iron(V)-oxo (i,e.
O=FeV(TPFC) complex as the major oxidizing intermediate which subsequently oxidize an
organic sulfide selectively to sulfoxide in view of its high reactivity. After the oxygen transfer
from the iron(V) µ-oxo species to the sulfide, the precursor iron(III) corrole complex (Scheme 1)
was reformed. In the absence of organic substrate, the highly reactive O=FeV(TPFC) rapidly
converted to a stable iron(IV) µ-oxo dimer as we observed in Figure 1.
41
PhI(OAc)2
PhI + HOAc
B
FeIII(TPFC)
O=FeV(TPFC)
(TPFC)FeIV-O-FeIV(TPFC)
A
O
R1 S
R2
R1 S
R2
Scheme 8. A proposed catalytic cycle for the catalytic oxidations by Fe III(TPFC) in the presence
of PhI(OAc)2 as an oxygen source. A: reaction pathway with substrate; B: reaction pathway
without substrate.
42
5. CONCLUSIONS
Following the literature known methods, the porphyrin and corrole free ligands and the
corresponding
iron
complexes
were
successfully
synthesized
and
spectroscopically
characterized. All the spectroscopical data including UV-vis and 1H-NMR are matching
literature reported.
The catalytic oxidation of organic sulfides by iron(III) complexes was studied in the
presence of iodobenzene diacetate [PhI(OAc)2] as a mild oxygen source. Different reaction
factors including solvent effect, catalyst amount, axial ligands of the iron(III) porphyrins and
substrate scope that affect the catalytic oxidation reactions have been evaluated to identify the
optimal conditions. It was also found that in the presence of small amounts of H2O, the
sulfoxidations proceeded at an increased rate. Nearly quantitative conversion and over > 95%
selectivity of sulfoxides were obtained for all catalytic sulfoxidations by iron(III) porphrin and
corrole complexes.
The competition study by iron(III) corrole complexes in the presence of iodobenzene
diacetate were studied for substited thioanisoles versus thioanisole. The mechanistic results have
implication for the sulfoxidation reactions catalyzed by iron(III) corrole complexes that the
active oxidants generated with iodobenzene diacetate are the highly reactive iron(V)-oxo corrole
that can convert to a stable iron(IV)-µ-oxo biscorrole in the absence of organic sulfides.
43
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49
CURRICULUM VITAE
Articles in peer-reviewed journals
1.
Chen, T.-H.; Yuan, Z.; Carver, A.; Zhang, R., Visible Light-Promoted Selective
Oxidation of Sulfides to Sulfoxides Catalyzed by Ruthenium Porphyrins with
Iodobenzene Diacetate. Appl. Catal. A 2014 478, 275-282.
2. Tse-Hong Chen, Kai Wai Kwong, Aaron Carver, Weilong Luo and Rui Zhang, An
Efficient Oxidation with Iodobenzene Diacetate Catalyzed by Iron(III) Corrole
Complexes: Synthetic and Mechanistic Aspects, Org. Lett., 2014, manuscript ready for
submission.
Presentations at academic conferences:
1.
Carver, A.; Chen, T.-H.; Zhang, R. “Synthesis and Catalytic Studies of an Iron(III)
Corrole for the Selective Oxidation of Organic Sulfides” Kentucky Academy of Science,
Morehead, Ky, November 2013.
2.
Carver, A.; Chen, T.-H.; Zhang, R. “Synthesis and Catalytic Studies of Iron(III) Corrole
towards Selective Oxidation of Organic Sulfides ”WKU Student Research Conference, Bowling
Green, Ky, March 2013.
50
ABBREVIATIONS AND SYMBOLS
BF3·OEt2
Boron trifluoride diethyl etherate
DDQ
2,3-Dichloro-5,6-dicyano-p-benzequinone
DMF
N,N-Dimethylformamide
FID
Flame ionization detector
FeIII(Cor) ·(OEt)2
Iron(III) corrole dietherate complexes
FeIII(TPFC) ·(OEt)2
Iron(III) 5,10,15-tri(pentafluorophenyl)corrole
[FeIV(TPFC)]2O
Diiron(IV) μ-oxo biscorrole
GC
Gas chromatograph
1
Proton nuclear magnetic resonance
H-NMR
H2TMP
meso-Tetramesitylporphyrin
H2TPFPP
meso-Tetrakis(5,10,15,20-pentafluorophenyl)porphyrin
H3TPFC
5,10,15-Tri(pentafluorophenyl)corrole
k2
Second-order rate constant
kobs
Observed pseudo-first-order rate constant
krel
Relative rate constant
m-CPBA
meta-Chloroperoxybenzoic acid
MS
Mass spectroscopy
PhIO
Iodosylbenzene
PhI(OAc)2
Iodobenzene diacetate
UV-vis
Ultraviolet-visible
51