Mechanistic elucidation of C-H oxidation by electron rich non-heme iron(IV)-oxo at room temperature

Article abstract of DOI:10.1039/c5cc04803f

Non-heme iron(iv)-oxo species form iron(iii) intermediates during hydrogen atom abstraction (HAA) from the C-H bond. While synthesizing a room temperature stable, electron rich, non-heme iron(iv)-oxo compound, we obtained iron(iii)-hydroxide, iron(iii)-alkoxide and hydroxylated-substrate-bound iron(ii) as the detectable intermediates. The present study revealed that a radical rebound pathway was operative for benzylic C-H oxidation of ethylbenzene and cumene. A dissociative pathway for cyclohexane oxidation was established based on UV-vis and radical trap experiments. Interestingly, experimental evidence including O-18 labeling and mechanistic study suggested an electron transfer mechanism to be operative during C-H oxidation of alcohols (e.g. benzyl alcohol and cyclobutanol). The present report, therefore, unveils non-heme iron(iv)-oxo promoted substrate-dependent C-H oxidation pathways which are of synthetic as well as biological significance.

Full text of DOI:10.1039/c5cc04803f

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Mechanistic Elucidation of C–H Oxidation by Electron Rich
Non-heme Iron(IV)-oxo at Room Temperature
Sujoy Rana, Aniruddha Dey and Debabrata Maiti*
Cite this: DOI: 10.1039/x0xx00000x
Received 00th June 2015,
Accepted 00th June 2015
DOI: 10.1039/x0xx00000x
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Nonꢀheme iron(IV)ꢀoxo species form iron(III) intermediates during reactions of iron(III)ꢀhydroxides. Such pathway is well accepted for
[
5ꢀ6]
hydrogen atom abstraction (HAA) from C–H bond. By synthesizing iron(IV/V)ꢀoxo and manganese (IV)ꢀoxo complexes.
a room temperature stable, electron rich, nonꢀheme iron(IV)ꢀoxo
Although radical rebound pathway has been established for
ruthenium(IV)ꢀoxo, gathering concrete evidences for the same in
compound, we obtained iron(III)ꢀhydroxide, iron(III)ꢀalkoxide and
hydroxylatedꢀsubstrateꢀbound iron(II) as the detectable
[7]
case of iron(IV)ꢀoxo requires further study. We thought to
synthesize a modified N4Py ligand scaffold (L) with electron rich
substituents at picolyl moiety. We were particularly intrigued by the
DFT data of Feꢀ(N4Py) complex which showed greater HOMO
contribution by two picolyl moieties that resulted in shorter Feꢀ
N(picolyl) distance in Feꢀ(N4Py) complex. We rationalized that
introduction of electron donating group (such as 4ꢀOMe) in picolyl
unit will further shorten the Feꢀ(N4Py) distance and will increase
intermediates. Present study revealed that a radical rebound pathway
was operative for benzylic C–H oxidation of ethylbenzene and
cumene. A dissociative pathway for cyclohexane oxidation was
established based on UVꢀvis and radical trap experiments.
Interestingly, experimental evidences including Oꢀ18 labeling and
mechanistic study suggested an electron transfer mechanism to be
operative during C–H oxidation of alcohols (e.g. benzyl alcohol and
cyclobutanol). The present report, therefore, unveils nonꢀheme
iron(IV)ꢀoxo promoted substrateꢀdependent C–H oxidation pathways
of synthetic as well as biological significance.
[8]
[8]
HOMO contribution (Figure 1).
Consequently it will produce
more reactive reaction intermediates, which may be verified by
[3dꢀf, 4a, 4b]
detailed mechanistic studies.
Highꢀvalent ironꢀoxo species are responsible for C–H oxidations
in numerous biological and chemical transformations for both heme
[
1]
and nonꢀheme enzymes.
Heme enzymes like cytochrome P450
carry out alkane hydroxylation, olefin epoxidation and sulfoxidation
[
2]
involving iron(IV)ꢀoxo porphyrin πꢀcation radical.
Nonꢀheme
enzymes such as Rieske oxygenase, αꢀketoglutarate dependent
dioxygenases, TauDꢀJ, routinely perform biochemical oxidative
transformations involving iron(IV)ꢀoxo intermediate. Intense
experimental work has been devoted for mimicking the chemistry of
[
3]
heme/nonꢀheme enzymes.
Nonꢀheme iron(IV/V)ꢀoxo complexes abstract hydrogen atom
from C–H bonds in the rate determining step to form iron (III)
•
[4]
hydroxide and radical species (R ).
These active species,
depending on their properties, can pursue a radical rebound, radical
nonꢀrebound or an electron transfer mechanism to form the
[
4l, 5]
respective C–H oxidation products (Scheme 1).
Following
radical rebound pathway, the in situ formed iron(II)ꢀspecies and
Scheme 1. C–H oxidations by nonꢀheme iron (IV)ꢀoxo
alcohol can undergo comproportionation reaction in presence of
OMe,Me II
The nonꢀheme iron complex [(N4Py)
Fe (CH CN)] (OTf) (1)
3 2
[
3b]
another equivalent of iron(IV)ꢀoxo (Scheme 1).
dissociative pathway, iron(III)ꢀhydroxide and substrate radical
generated upon HAA) becomes separated from solvent cage
In case of
was synthesized by reacting Fe(OTf) .2CH CN with an
2
3
OMe,Me
[5]
electronically enriched and substituted N4Py
Complex 1 was also characterized by Xꢀray (Figure 1), ESIꢀMS
m/z, 688.150), UVꢀvis spectroscopy (maximum at 459 nm due to
ligand.
(
resulting in subsequent radical trapped products and other side
(
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DOI: 10.1039/C C04803F
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[
9]
1
13
LMCT).
(
NMR (0ꢀ10 ppm, Hꢀ and 0ꢀ200 ppm, Cꢀ) and EPR produced cyclohexanol (~15% yield, 52% Oꢀ18 enriched) whereas
[
10],[11]
silent) studies indicated the diamagnetic character of 1.
Electrochemical study of complex 1 showed lower Fe /Fe
reduction potential (E1/2 ~ 0.84 V vs SCE) compared to that of
unsubstituted N4Py iron(II) complex (E1/2 ~ 1.01 V vs SCE).
This further suggested that iron(III) species for 1 is more stable
compared to unsubstituted N4Py iron(III) complex. The
ethyl benzene gave 1ꢀphenyl ethanol (yield, 22%; 60% Oꢀ18
labeled).
III II
18O
18OH
Radical
CH(Me)Ph
[
8],[12]
2+
4+ PhCH Me
3+
rebound
2
LFe 18OH
LFe
2
LFe PhCHMe
4
HAA
3
18O
OMe,Me IV
2+
Me
Comproportionation
4+
corresponding iron (IV)ꢀoxo species, [(N4Py)
Fe (O)] (2)
2
LFe
N
N
OMe
was synthesized by reacting 1 with iodosyl benzene in acetonitrile at
room temperature. Characteristic UVꢀvis maximum at 692 nm (ϵ ~
Me
Me
Ph
N
18O
N
N
18OH
3+
LFe
ꢀ
1
ꢀ1
4
32 M cm ) due to ligand field transitions (dꢀd transition) was also
3+ Me
OMe
LFe
+
[
4a, 13]
IV
III
Me
observed.
Complex 2 showed slightly more negative Fe /Fe
L = N4PyOMe,Me
5
3
reduction potential (Ep,c ~ ꢀ0.19 V vs SCE) compared to that of
Scheme 3. Intermediates during reaction of 2 and ethylbenzene
IV
2+
unsubstituted [(N4Py)Fe (O)] (Ep,c ~ ꢀ0.15 V vs SCE). Notably, 2
was found to be stable at room temperature for few days (t ~50 h at
The ESIꢀMS data obtained upon addition of ethylbenzene to 2
suggested formation of iron(III)ꢀhydroxide (3, m/z, 705.150), 1ꢀ
1
/2
[
8]
3
0 °C in air). The ESIꢀMS characterization of complex 2 revealed
OMe,Me
II
OMe,Me IV
+
phenylethanol
bound
intermediate,
[(N4Py)
Fe
major isotopic peak at 704.145 due to [(N4Py)
Fe (O)](OTf)
+
18
(HO(Me)CHPh)](OTf) (4) (m/z, 810.22; Figure 2g) and iron(III)ꢀ
which was shifted to 706.150 upon Oꢀ18 labeling with H O (~95%
2
OMe,Me III
+
[
14]
1
alkoxide, [(N4Py)
Fe (O(Me) CHPh)](OTf) (5, m/z, 809.215;
Oꢀ18 incorporation, vide infra).
The H NMR spectrum (–20 to
Figures 2b and 2e) (Scheme 3). Most interestingly, 1ꢀphenylethanol
bound intermediate [(N4Py)
formed as a consequence of radical rebound step was rapidly
oxidized by 2 to produce 3 and 5.
5
0 ppm) along with EPR silent behaviour at 77 K suggested
OMe,Me II
+
[
10, 15]
Fe (HO(Me)CHPh)](OTf) (4)
paramagnetic character of 2 (likely in the S=1 spin state).
[
7]
Formation of 5 occurred via
comproportionation reaction of 1 and 2 in presence of 1ꢀphenyl
ethanol. This was further verified by adding 1ꢀphenyl ethanol to a
solution of 2 in acetonitrile where both 3 and 5 were simultaneously
detected.
Furthermore, the formation of iron(III) complex was confirmed
from rhombic signal at g = 1.94, g = 2.11, g = 2.10 and axial signal
1
2
3
Figure 1. ORTEP diagram of complex 1 (CCDC, 1051845) and at g = 4.17, g = 5.98 by EPR experiment of a solution of 2 and ethyl
1
2
[
5, 15, 18]
DFT optimized geometry of 2 using B3LYP/LANL2DZ with benzene (or cumene) (Figure 3a and 3b).
Replacing ethyl
OMe,Me
N4Py
ligand
benzene by cumene also showed formation of iron(III)ꢀhydroxide (3)
and iron(III)ꢀalkoxide species, [(N4Py)
(OTf) (5a, m/z, 823.21; Scheme 4), which upon O labeling shifted
by two mass unit (m/z, 825.21; Figures 4a and 4b) along with 70%
Oꢀ18 enriched cumyl alcohol.
OMe,Me III
Fe O(Me) CPh)]
2
18O
18OH
+
18
4
+
PhCH OH
3+
Electron transfer
2
LFe
LFe PhCHOH
PhCHO
2
LFe
1
+
HAA
% 18O labeling
5
2
3
UV-vis 692 nm,
EPR active
1
18O 707.150 (16O 705.150)
EPR silent, H NMR
L = N4PyOMe,Me
18O
706.140 (16O 704.145)
OH
O4
LFe
OH
3+
LFe
O
OH
+
Electron transfer
2+
HAA
LFe
Exclusive product
2
3
1
OMe,Me IV
2+
Scheme 2. Alcohol oxidations by [(N4Py)
Fe (O)] (2)
Oxidation of benzyl alcohol by 2 provided benzaldehyde as the
sole product (yield, 86%). Labeling study showed 5% Oꢀ18
incorporation in benzaldehyde. Furthermore, C–H oxidation of
PhCH OH and PhCD OH (~95%, D enriched) provided kinetic
2
2
isotope effect value, 11 which suggests that the initial hydrogen
[
8] [16] [3b]
atom abstraction is the rate determining step.
Cyclobutanol
ꢀ
as mechanistic probe provided cyclobutanone exclusively (2e
oxidation product) (ring open product 4ꢀhydroxybutanal was not
detected) without any Oꢀ18 labeling (Scheme 2). These observations
suggested that following HAA, an electron transfer mechanism is
operational during C–H oxidation of benzyl alcohol.
Subsequently, we have studied C–H oxidation chemistry of 2 using
Figure 2. ESIꢀMS of the intermediates during reaction of 2 and
ethylbenzene (red line, experimental and black line, simulated,
spectra were recorded after 5 min of addition). ESIꢀMS of 2 (2a), 3
[
16ꢀ17]
(
2b), 18ꢀOꢀ2 (2c), 18ꢀOꢀ3 (2d), 5 (2e), 18ꢀOꢀ5 (2f), 4 and 18ꢀOꢀ4
[
8]
[
8]
(2g).
ethylbenzene (Scheme 3), cumene and cyclohexane. Cyclohexane
2
| J. Name., 2012, 00, 1-3
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C
0.
O
10
M
39
M
/C
U
5C
N
C
I
0
C
4
A
8
T
03IO
F
N
ꢀ
1
ꢀ1
ethylbenzene is similar to its unsubstituted analogue (0.0021 M s
ꢀ
1
ꢀ1
ꢀ1 ꢀ1 [4a, 5]
vs 0.0031 M s or 0.008 M s ).
Notably, during the C–H
oxidation reactions of 2 with ethylbenzene, cumene, triphenyl
methane, benzyl alcohol and cyclobutanol (500 equiv.) iron(II) was
regenerated. Initially after 1ꢀ2 hour of the reaction, 40ꢀ60% of
iron(II) species was regenerated. After 48 hours of the reaction,
iron(II) was obtained quantitatively (~95%). On the contrary,
II
2+
unsubstituted [Fe (N4Py)(O)]
complex generated ~95% of
iron(III) species via dissociative pathway after completion of the
[
5]
reaction with ethyl benzene, cumene and triphenyl methane.
Figure 3. EPR spectra (acetonitrile, 77 K) obtained from reaction
between 2 and (a) ethylbenzene (b) cumene
The formation of 5a was presumed to occur via
comproportionation reaction. This was verified when 2ꢀphenylꢀ2ꢀ
propanol was added to a solution of 2, trace amount of 3 and 5a were
observed after 30 min. Notably, a significant amount of these
compounds was formed after 16 h. Complex 2 decayed with time to
form 1, which in presence of 2ꢀphenylꢀ2ꢀpropanol and 2 underwent
comproportionation reaction to form 3 and 5a. Expectedly,
formation of 3 and 5a (Scheme 4) were observed within 10 minutes
via comproportionation reaction when 2ꢀphenylꢀ2ꢀpropanol was
added to a mixture of 1 and 2.
Figure 5. (a) UVꢀvis change at 692 nm for 2, in presence of cumene,
(b) kinetics plot for cumene and ethylbenzene
Cyclohexane oxidation by 2 produced major amount of iron(III)
18O
18OH
3+
LFe
Radical
18OH
LFe + PhCMe
2
(~90%) and minor amount of iron(II) species (10%). Moreover,
2+
4+
PhCHMe
2
rebound
LFe
2
PhCMe2
cyclohexyl radical was trapped in the form of cyclohexyl bromide on
HAA
[8],[11]
3
18O
adding CCl Br (or CBr ) during cyclohexane oxidation by 2.
3 4
Me
Comproportionation
4+
These experimental evidences suggested that cyclohexane oxidation
was likely following a dissociative pathway.
LFe
2
N
N
N
OMe
Me
Me
N
Ph
No radical trapped or brominated product was found during the
N
18O
18
OH
Me
[11],[20]
reaction of 2 with ethylbenzene or cumene.
Therefore, the
3
+
OMe
Me
+
3+
Me
LFe
LFe
substrate based organic radicals failed to escape from solvent cage
L = N4PyOMe,Me
[20c]
3
for ethylbenzene and cumene.
Although radicals formed via
5
a
dissociative pathway have been trapped as per the prescient
knowledge from reported literature for nonꢀheme iron(IV)ꢀoxo, our
experimental observations suggested that following HAA, the
iron(III)ꢀhydroxide and the exogenous substrate based radical may
not undergo dissociation (e.g. in case of ethylbenzene and
Scheme 4. Radical rebound pathway for C–H oxidation of cumene
[
21]
cumene).
The reaction followed a radical rebound pathway and
produced an iron(II)ꢀalcohol coordinated product that was
[
20c]
subsequently oxidized by 2.
In summary we have synthesized an electron rich, room
temperature stable and reactive nonꢀheme iron(IV)ꢀoxo species
Figure 4. ESIꢀMS of the intermediates, 5a (4a) and 18ꢀO labeled 5a
4b) during reaction of 2 and cumene (red line, experimental and
black line, simulated)
(
OMe,Me IV
+
[
(N4Py)
Fe (O)](OTf) (2).
The iron(IV)ꢀoxo derived
intermediates like iron(III)ꢀhydroxide (3), iron(III)ꢀalkoxide (5) and
In presence of ethylbenzene (or cumene), the absorbance vs time substrateꢀbound iron(II) species (4) were detected from the reaction
plot for complex 2 (decay profile at 692 nm) was fitted with the mixture. The mechanistic switch during C–H oxidation by nonꢀ
[
3b, 4a, 5]
heme iron(IV)ꢀoxo complex 2 mainly depends on the stability of the
pseudoꢀfirst order reaction profile (rate constant, k ; Figure 5).
1
A straight line was obtained by plotting the different values of k1 radical generated after HAA. More stable radical preferred electron
against concentration of the substrate. The slope of this plot yielded transfer pathway (Scheme 2), whereas moderately stable radical
[
8]
the second order rate constant (k , Figure 5).
During C–H underwent radical rebound pathway (Schemes 3 and 4). Least stable
2
ꢀ
oxidation reaction by 2, cumene reacted slightly faster (k ~ 0.01 M radical of all (e.g. in case of cyclohexane) underwent dissociative
2
1
ꢀ1
ꢀ1 ꢀ1
s ) compared to ethylbenzene (k ~ 0.0021 M s ) due to higher pathway.
2
[
19]
benzylic C–H bond strength.
Reaction of cumene and complex 2
occurred with a slightly faster rate (~5 times, k ~ 0.01 M s vs k =
.002 M s for FeꢀN4Pyꢀoxo) compared to that for unsubstituted Mumbai 400076, India
Notes and references
ꢀ
1 ꢀ1
2
2
†
Department of Chemistry, Indian Institute of Technology Bombay, Powai,
ꢀ
1
ꢀ1
[4a]
0
FeꢀN4Pyꢀ(oxo) complex. However, reaction rate of 2 with
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DOI: 10.1039/C C04803F
Jou5Crnal Name
This activity is supported by SERB, India. Financial support received from 11 G. Roelfes, M. Lubben, K. Chen, R. Y. N. Ho, A. Meetsma, S.
CSIRꢀIndia (fellowships to S. R.) is gratefully acknowledged. DM sincerely Genseberger, R. M. Hermant, R. Hage, S. K. Mandal, V. G. Young, Y. Zang,
thanks Dr. Sayam Sen Gupta (NCL Pune), Dr. Tapan K. Paine (IACS H. Kooijman, A. L. Spek, L. Que Jr and B. L. Feringa, Inorg. Chem., 1999,
Kolkata) and their group members for stimulating scientific discussions and 38, 1929.
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Y.ꢀM. Lee, M. Clémancey, R. GarciaꢀSerres, T. Nomura, T. Ogura, J.ꢀM.
Latour, B. Hedman, K. O. Hodgson, W. Nam and E. I. Solomon, J. Am.
Chem. Soc., 2012, 134, 11791; (h) A. R. McDonald, Y. Guo, V. V. Vu, E. L.
Bominaar, E. Munck and L. Que, Jr., Chem. Sci., 2012, 3, 1680; (i) F. T. de
Oliveira, A. Chanda, D. Banerjee, X. Shan, S. Mondal, L. Que, Jr., E. L.
Bominaar, E. Münck and T. J. Collins, Science, 2007, 315, 835; (j) S. Kundu,
J. V. K. Thompson, A. D. Ryabov and T. J. Collins, J. Am. Chem. Soc., 2011,
OH
TOC Graphic
OH
O
or
O
or
Electron Transfer
Me
Me
1
8
18O
Me
OH
N
N
OMe
Fe 4+
1
33, 18546.
Me
4
(a) J. Kaizer, E. J. Klinker, N. Y. Oh, J.ꢀU. Rohde, W. J. Song, A. Stubna, J.
N
N
N
Me
Radical Rebound
Kim, E. Münck, W. Nam and L. Que, Jr., J. Am. Chem. Soc., 2003, 126, 472;
b) C. V. Sastri, S. M. Sook, P. M. Joo, K. K. Mook and W. Nam, Chem.
1
8
(
OMe
OH
Me
Commun., 2005, 1405; (c) H. Hirao, D. Kumar, L. Que, Jr. and S. Shaik, J.
Am. Chem. Soc., 2006, 128, 8590; (d) D. Kumar, S. P. de Visser and S. Shaik,
J. Am. Chem. Soc., 2003, 125, 13024; (e) D. Kumar, H. Hirao, L. Que, Jr. and
S. Shaik, J. Am. Chem. Soc., 2005, 127, 8026; (f) D. Kumar, B. Karamzadeh,
G. N. Sastry and S. P. de Visser, J. Am. Chem. Soc., 2010, 132, 7656; (g) W.
Nam, Acc. Chem. Res., 2007, 40, 522; (h) W. Nam, Y.ꢀM. Lee and S.
Fukuzumi, Acc. Chem. Res., 2014, 47, 1146; (i) S. Ye and F. Neese, Proc.
Natl. Acad. Sci., 2011, 108, 1228; (j) Y. J. Jeong, Y. Kang, A.ꢀR. Han, Y.ꢀM.
Lee, H. Kotani, S. Fukuzumi and W. Nam, Angew. Chem. Int. Ed., 2008, 47,
Dissociative
Pathway
7
5
5
1
5
321; k) W. Lai, C. Li, H. Chen and S. Shaik, Angew. Chem. Int. Ed., 2012,
1, 5556; (l) C. Geng, S. Ye and F. Neese, Angew. Chem. Int. Ed., 2010, 49,
717; (m) P. Comba, M. Maurer and P. Vadivelu, Inorg. Chem., 2009, 48,
0389.
K.ꢀB. Cho, X. Wu, Y.ꢀM. Lee, Y. H. Kwon, S. Shaik and W. Nam, J. Am.
Chem. Soc., 2012, 134, 20222.
(a) X. Wu, M. S. Seo, K. M. Davis, Y.ꢀM. Lee, J. Chen, K.ꢀB. Cho, Y. N.
6
Pushkar and W. Nam, J. Am. Chem. Soc., 2011, 133, 20088; (b) E. Kwon, K.ꢀ
B. Cho, S. Hong and W. Nam, Chem. Commun., 2014, 50, 5572.
7
T. Kojima, K. Nakayama, K. Ikemura, T. Ogura and S. Fukuzumi, J. Am.
Chem. Soc., 2011, 133, 11692.
8
9
See supporting information for detailed description.
M. Lubben, A. Meetsma, E. C. Wilkinson, B. L. Feringa and L. Que Jr,
Angew. Chem. Int. Ed., 1995, 34, 1512.
0 E. J. Klinker, J. Kaizer, W. W. Brennessel, N. L. Woodrum, C. J. Cramer
and L. Que Jr, Angew. Chem. Int. Ed., 2005, 44, 3690.
1
4
| J. Name., 2012, 00, 1-3
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