A heterogeneous bio-inspired peroxide shunt for catalytic oxidation of organic molecules

Article abstract of DOI:10.1039/d0cc03468a

Heme enzymes are capable of catalytically oxidising organic substrates using peroxide via the formation of a high-valent intermediate. Iron porphyrins with three different axial ligands are created on self-assembled monolayer-modified gold electrodes, which can oxidize C-H bonds and epoxidize alkenes efficiently. The kinetic isotope effects suggest that the hydrogen atom transfer reaction by a highly reactive oxidant is likely to be the rate-determining step. Effect of different axial ligands and different secondary structures of the iron porphyrin confirms that the thiolate axial ligand with a hydrophobic distal pocket is the most efficient for this oxidation chemistry.

Full text of DOI:10.1039/d0cc03468a

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S. J. Connon, M. O. Senge et al.
Conformational control of nonplanar free base porphyrins:
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A Heterogeneous Bio-Inspired Peroxide Shunt for Catalytic
Oxidation of Organic Molecules
Manjistha Mukherjee, Abhishek Dey*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
have been made by changing the nature of the solvent and electronic
structure of the porphyrin to improve both the reactivity and
Heme enzymes are capable of catalytically oxidising organic
substrates using peroxide via the formation of high valent
intermediate. Iron porphyrins with three different axial ligands are selectivity during the oxidation in homogeneous medium18.
created on self assembled monolayer modified gold electrodes
which can oxidize C-H bonds and epoxidize alkenes efficiently. The
kinetic isotope effects suggest that Hydrogen atom transfer
reaction by a highly reactive oxidant is likely to be rate determining
step. Effect of different axial ligands and different secondary
structure of the iron porphyrin confirms that thiolate axial ligand
with a hydrophobic distal pocket is most efficient for this oxidation
chemistry.
However, sluggish reactivity towards these demanding substrates
rather allow the reactive intermediates to indulge in undesirable side
reactions which includes catalyst degradation. Moreover, an axial
thiolate ligand which is important for the reactivity of the native
enzyme could not be investigated vividly due to the synthetic
complexities and vulnerability of axial thiolate ligands19, 20 towards
oxidation in the presence of H2O2.
Emulating the rich chemistry of cytochrome P-450 (Cyt P450)
enzymes, which can catalyse a wide range of oxidation reactions,
including the hydroxylation of un-activated primary C-H bonds of
alkanes, in artificial systems has been a major goal for the catalysis
community for decades. Cytochrome P450 is known to form a high
valent FeIV=O bound to a porphyrin radical cation [(FeIV=O)P+. or
Compound I] species as the main oxidizing agent during the oxidation
of the small organic molecules.1, 2 This highly oxidizing intermediate
compound I can be derived from the reaction of the resting ferric
(FeIII) state of the enzyme with H2O2, known as ‘peroxide shunt’, as
well as via activation of dioxygen where the later pathway requires
electron transfer from a reductase component (Figure 1). A group of
Cyt P450 enzymes, known as peroxygenase, use peroxides as the
surrogate of O2. Though the use of peroxides can lead to inactivation
of enzymes, there are several examples where peroxides have been
used effectively in α-β hydroxylation, decarboxylation, oxidation of
the cyano groups etc.3-5
A
B
Previous investigations using synthetic ferric porphyrin complexes
for catalyzing alkane hydroxylation reactions have been conducted
extensively with oxidants such as PhIO, KHSO5, NaOCl, ROOH and
ozone 6-11. Since H2O2 is a biologically important and environmentally
benign oxidant, selective oxidation of hydrocarbons with H2O2 using
ferric porphyrin has always been a sought-after goal. However
radical-free (enzyme mimetic) hydroxylation of alkanes and selective
epoxidation with H2O2 is still difficult to achieve under physiological
condition due to the undesired side reactions like catalyst
decomposition and uncontrolled oxidation.12-17 Several attempts
Figure 1: A) Primary reaction taking place in the active site of
peroxidase and Cytochrome P450, evidence of similar intermediate
B) Schematic representation of the peroxide shunt.
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It has been recently demonstrated that a terminal thiolate group at organic compounds such as lipids, styrenes and sulfides in presence
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23-25
DOI: 10.1039/D0CC03468A
the end of a thiol functionalized gold electrode can bind iron of H2O2 (metMb) via the formation of compound I.
In this
picketfence (FePf) porphyrin (Figure2B) and can generate compound manuscript, the oxidation of a range of organic substrates by H2O2 in
I like intermediate from molecular oxygen using the electrode as the aqueous medium catalyzed by iron porphyrin immobilized on self-
reductase component and this can oxidize alkanes to alcohols and assembled monolayer on Au electrodes with varying axial ligands
alkenes to epoxide selectively with high turnover numbers (TON)21. (thiolate, phenolate and imidazole) are reported. The role of
The selectivity is imparted by the sterically congested “picket fence” different axial ligands and Kinetic Isotope effects (KIE) for those
architecture which restricts access to the reactive ferryl centre and constructs are also evaluated to understand the reactivity properly.
prefers the access of the secondary C-H bonds over tertiary C-H
The iron porphyrin complex, Fe-“picket-fence” (FePf) is attached to
undecane-1,11-dithiol (SHC11SH) modified gold electrode thinly
dispersed (surface coverage=5.23x10-13moles/cm2) between
octanethiol (C8SH) self-assembled monolayer (SAM) to mimic the
active site of Cytochrome P450 (Cyt P450). In the presence of
dissolved organic substrates this Cytochrome P450 analogue could
oxidize a number of organic substrates using H2O2. The products
were then extracted in organic solvent (Chloroform) and analysed
using GC-MS. Both hydroxylation of C-H bonds, having BDE as high as
100 kcal/mol, and epoxidation can be achieved with H2O2 with TON
of ~103 (Table 1). The TON is ~104 for the oxidation of weaker C-H
bonds of toluene (89.8 kcal/mol) and Cinnamaldehyde to yield
benzaldehyde and Cinnamic acid, respectively.
bonds. Thus, cyclohexane was only oxidized to cyclohexanol (no
cyclohexanone) and the 1o C-H of ethyl benzene could be oxidized in
the presence of the benzylic C-H bond. Compound I has been
detected using in-situ surface enhanced Resonance Raman
spectroscopy (SERRS) during the H2O2 disproportionation (HPD)
reaction by phenolate ligated iron porphyrin attached to the gold
electrodes where Fe-O vibration of the compound I species were
detected at 796 cm-1 and 803 cm-1 for porphyrins containing
hydrophilic (Fe-tetra ester porphyrin, FeEs4) and hydrophobic ( Fe-
‘picket fence’ porphyrin, FePf) distal structures respectively (Figure
2A, Supporting Information Figure S.8).22 The, compound I, so
generated, can be used to oxidize organic substrates.
Table1: Turnover numbers (TON) for the oxidation of substrates
by thiolate ligated FePf complex.
A
Bond
Dissociation
Energy
Product
Turnover Number for three
different axial ligands
RS-
OPh-
Imd
(kcal/mole)
3.4x103
2.1x103
1.6x103 5.8x102
7.4x102 9.2x102
(100)
1.9x103
3.3x102
1.3x102
5.3x104
5.2x102
4.6x104 2.2x104
1.7x102 74.5
C
B
(89.8)
1.5x104
1.2x104
ND
ND
(2o C-H = 85)
Figure 2: A) Schematic representation of generation of compound I
on top of the SAM modified gold electrode using H2O2 as the oxidant.
The structure of B) FePf and C) FeEs4 are shown.
8.2x104
4.1x104
7.9x103
(N.A)
These high TONs with respect to the amount of catalyst on the
surface are an less than those reported in a recent work where
substrate oxidation was achieved using this same construct by
electrochemical reductive activation of oxygen, i.e. when molecular
oxygen is used as the oxidant instead of H2O2 which replicates exactly
what happens during the peroxygenation by the enzymatic system.21
The lower reactivity with H2O2 is due to competing HPD reaction (Fig.
4A) by the compound I formed during the reaction of ferric porphyrin
with H2O2. Notably, the oxidation of cyclohexane with H2O2 offer
both Cyclohexanol and cyclohexanone with a ratio of 17:10 whereas
Generation of compound I with phenolate bound Fe-porphyrin,
mentioned earlier, raises the possibility of the formation of
compound I during the same HPD reaction by thiolate and imidazole
ligated iron porphyrins in this heterogeneous construct. Such
hypothesis gets support from the fact that cytP450 is known to
generate compound I from H2O2 and Metmyoglobin (metMb) with
Histidine as axial ligand assists in oxidative reactions of a variety of
a. School of Chemical Science, Indian Association for the Cultivation of Science,
Kolkata, india, E-mail: icad@iacs.res.in
†Electronic
Supplementary
Information
(ESI)
available:
See
DOI: 10.1039/x0xx00000x
2 | J. Name., 2012, 00, 1-3
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when previously, using molecular O2, only cyclohexanol was (Figure 5) along with the thiolate (RS-) ligated analVoiegwueArt(icalelrOenaldinye
-
DOI: 10.1039/D0CC03468A
obtained as the exclusive product. The primary oxidant formed when discussed). The results show that both Imd and OPh bound FePf can
oxygen was used as an oxidant was compound I and the selectivity catalyze the oxidation of very strong C-H bonds of substrates like
was imparted by the steric hindrance of the four tertiary butyl group Cyclohexane (with BDE˜100kcal/mole) and epoxidize alkenes with
limiting access of the substrate to the high-valent oxo species at the high TONs using H2O2 as the oxidant. This directly implicates
centre. Accordingly, when the steric hindrance was minimized by compound I as the key oxidizing species as most theoretical and
decreasing the number of bulky pivolyl group in the distal structure experimental data suggest that other oxidants like compound 0 and
of the porphyrin geometry (iron half-picket, FehPf) (Supporting compound II are incapable to achieve both these oxidations.26-28The
information, Figure S.7) the selectivity decreased and cyclohexanol decrease in the TON for both the hydroxylation and epoxidation
and cyclohexanone were found with 26:10 ratio under the same (Table 1 and figure 5) with axial imidazole and phenolate ligands
conditions the same catalytic cycle (Figure 3).21 With H2O2 as an suggest that the thiolate axial ligand works best for the
oxidant, the ratio of cyclohexanol:cyclohexanone increased to 3:7 for monooxygenation reactions. This is consistent with site directed
the same porphyrin i.e. FehPf. The over oxidation increases as the mutants of cytochrome P450cam where replacement of the axial
concentration of H2O2 is increased (Figure 4B) in the reaction cysteine with histidine reduced the oxidation activity by an order of
medium and in control experiments H2O2 in buffered solution could magnitude.29
oxidize cyclohexanol to cyclohexanone without any catalyst
(Supporting information, Figure S.5) indicating that the selectivity is
lost due to the excess H2O2 present in the medium. Note that H2O2
cannot oxidize C-H bond of cyclohexane without a catalyst
(Supporting information Figure S.6).
B
A
Figure 5: Comparison between the Log(TON) of cyclohexane, styrene,
Figure 3: Access of cyclohexanol for over oxidation in case of A) FePf
toluene and cinnamaldehyde to form cyclohexanol, Styrene oxide,
and B) FehPf
benzaldehyde and cinnamic acid by thiolate (orange), phenolate
(red) and imidazole (blue) ligated FePf.
Kinetic Isotope effect (KIE) on the oxidation is determined using d8-
Toluene. The H/D isotope effect is determined to be 3.6 for a thiolate
bound FePf and this value falls in the classical range for Hydrogen
atom transfer (HAT) reaction and indicates that HAT of the C-H bond
is the rate determining step (rds). Moreover, the imidazole bound
FePf shows a KIE of 13.02 for Toluene-d8 oxidation and the Phenolate
bound FePf shows KIE of 22.8 (Supporting information, Figure S.5).
The large KIE values suggest that HAT from the benzylic C-H bond of
Toluene by compound I is the rate-limiting step for both the catalytic
cycles and there is substantial tunnelling in the Transition state (TS).
These large isotope effects are also suggestive of tunnelling through
a TS having very different polarity relative to the thiolate axial
ligand.30 The higher KIE for phenolate bound FePf than the Imidazole
bound analogue supports the hypothesis of Shaik et. Al.30 The barrier
of hydrogen atom tunnelling depends on the electrostatic
B
δ
δ
δ
-
+
-
stabilization of the H as it moves between the C and O (Figure 6B)
moieties in the Transition state (TS)30. The neutral Imidazole axial
ligand results in a lesser electrostatic stabilisation in the TS than the
anionic phenolate axial ligand in the rate-limiting step. Thus, the
lesser electrostatically stabilised transition state for imidazole bound
compound I go through a wider tunnelling barrier and thus results in
lower KIE values than more electrostatically stabilised T.S of
phenolate bound compound I. The strong covalent interaction
Figure 4: Schematic representation of A) the competing HPD reaction
along with substrate oxidation by compound I and B) increase in the
overoxidation with increase in the H2O2 concentration
The effect of axial ligands on the substrate oxidation activity is
evaluated with Imidazole (Imd) and Phenolate (OPh-) ligated FePf
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J. Name., 2013, 00, 1-3 | 3
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between the thiolate and the iron is likely to results in lesser ionic 7.
character in the TS and less KIE than phenolate as observed
experimentally.
F. Montanari, S. Banfi, G. Pozzi and S. Quici, in
View Article Online
Metalloporphyrins Catalyzed OxidDatOioI:n1s0,.1e0d3s9./FD.0CC03468A
Montanari and L. Casella, Springer Netherlands,
Dordrecht, 1994, DOI: 10.1007/978-94-017-2247-6_5, pp.
149-173.
8.
J. T. Groves, R. C. Haushalter, M. Nakamura, T. E. Nemo
and B. J. Evans, J. Am. Chem. Soc., 1981, 103, 2884-2886.
J. T. Groves and T. E. Nemo, J. Am. Chem. Soc., 1983, 105,
6243-6248.
B
9.
10.
T. J. McMurry and J. T. Groves, in Cytochrome P-450:
Structure, Mechanism, and Biochemistry, ed. P. R. O. de
Montellano, Springer US, Boston, MA, 1986, DOI:
10.1007/978-1-4757-9939-2_1, pp. 1-28.
Z. Gross and L. Simkhovich, Journal of Molecular Catalysis
A: Chemical, 1997, 117, 243-248.
B. S. Lane and K. Burgess, Chem. Rev., 2003, 103, 2457-
2474.
T. G. Traylor, C. Kim, W.-P. Fann and C. L. Perrin,
Tetrahedron, 1998, 54, 7977-7986.
T. G. Traylor, C. Kim, J. L. Richards, F. Xu and C. L. Perrin, J.
Am. Chem. Soc., 1995, 117, 3468-3474.
T. G. Traylor, S. Tsuchiya, Y. S. Byun and C. Kim, J. Am.
Chem. Soc., 1993, 115, 2775-2781.
T. G. Traylor, W. P. Fann and D. Bandyopadhyay, J. Am.
Chem. Soc., 1989, 111, 8009-8010.
W. Nam, H. J. Han, S.-Y. Oh, Y. J. Lee, M.-H. Choi, S.-Y.
Han, C. Kim, S. K. Woo and W. Shin, J. Am. Chem. Soc.,
2000, 122, 8677-8684.
11.
12.
13.
14.
15.
16.
17.
Figure 6: A. Tunnelling lowers the observed barrier relative to the
semi classical TS. B. schematic representation of the TS where L
represents the different axial ligations.
In summary, the catalytic oxidation of strong C-H bonds and
epoxidation of alkene was demonstrated using H2O2 with high TON
at room temperatures and in aqueous medium by thiolate bound
FePf. The in situ generated compound I via a peroxide shunt abstracts
hydrogen atom from C-H bond as evident from the isotope effects.
Unlike with O2, the excess H2O2 in the medium led to overoxidation
of alcohols. Moreover, variation of the axial ligand from thiolate to
phenolate to imidazole does not only result in the decrease in the
yield but also results in significantly high isotope effect suggesting an
involvement of a transition state of very different polarity.30 The
effect of hydrophobic distal pocket was also carried out which shows
a show a significant decrease in both the TON and TOF of the
oxidation hydrophobic substrates like cyclohexanol and styrene
(Supporting information Figure S.9).These results open up new
reaction engineering approaches for harnessing a green oxidant like
H2O2 for useful chemical oxidations and also offer direct insight into
the role of axial ligands in tuning the reactivity of very reactive
intermediate compound I.
18.
19.
20.
W. Nam, S.-Y. Oh, Y. J. Sun, J. Kim, W.-K. Kim, S. K. Woo
and W. Shin, J. Org. Chem., 2003, 68, 7903-7906.
J. P. Collman, T. N. Sorrell and B. M. Hoffman, J. Am.
Chem. Soc., 1975, 97, 913-914.
F. Tani, M. Matsu-ura, S. Nakayama, M. Ichimura, N.
Nakamura and Y. Naruta, J. Am. Chem. Soc., 2001, 123,
1133-1142.
M. Mukherjee and A. Dey, ACS Cent. Sci., 2019, DOI:
10.1021/acscentsci.9b00046.
K. Sengupta, S. Chatterjee and A. Dey, ACS Catal., 2016, 6,
1382-1388.
T. Egawa, H. Shimada and Y. Ishimura, J. Biol. Chem.,
2000, 275, 34858-34866.
S.-i. Ozaki, T. Matsui and Y. Watanabe, J. Am. Chem. Soc.,
1997, 119, 6666-6667.
T. Matsui, S.-i. Ozaki and Y. Watanabe, J. Biol. Chem.,
1997, 272, 32735-32738.
E. Derat, D. Kumar, H. Hirao and S. Shaik, J. Am. Chem.
Soc., 2006, 128, 473-484.
R. Raag, S. A. Martinis, S. G. Sligar and T. L. Poulos,
Biochemistry, 1991, 30, 11420-11429.
I. G. Denisov, T. M. Makris, S. G. Sligar and I. Schlichting,
Chem. Rev., 2005, 105, 2253-2278.
K. Auclair, P. Moënne-Loccoz and P. R. Ortiz de
Montellano, J. Am. Chem. Soc., 2001, 123, 4877-4885.
J. E. M. N. Klein, D. Mandal, W.-M. Ching, D. Mallick, L.
Que and S. Shaik, J. Am. Chem. Soc., 2017, 139, 18705-
18713.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
Conflicts of interest
There are no conflicts to declare.
References
1.
2.
M. Sono, M. P. Roach, E. D. Coulter and J. H. Dawson,
Chem. Rev., 1996, 96, 2841-2888.
J. T. Groves, in Cytochrome P450: Structure, Mechanism,
and Biochemistry, ed. P. R. Ortiz de Montellano, Springer
US, Boston, MA, 2005, DOI: 10.1007/0-387-27447-2_1,
pp. 1-43.
3.
4.
5.
6.
T. Fujishiro, O. Shoji, S. Nagano, H. Sugimoto, Y. Shiro and
Y. Watanabe, J. Biol. Chem., 2011, 286, 29941-29950.
A. W. Munro, K. J. McLean, J. L. Grant and T. M. Makris,
Biochemical Society Transactions, 2018, 46, 183-196.
O. Shoji and Y. Watanabe, JBIC Journal of Biological
Inorganic Chemistry, 2014, 19, 529-539.
B. Meunier, in Metalloporphyrins Catalyzed Oxidations,
eds. F. Montanari and L. Casella, Springer Netherlands,
Dordrecht, 1994, DOI: 10.1007/978-94-017-2247-6_1, pp.
1-47.
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