Highly Efficient Aromatic C?H Oxidation with H2O2 in the Presence of Iron Complexes of the PDP Family

Article abstract of DOI:10.1002/cctc.201800832

The catalytic activity of a series of iron complexes of the PDP family (PDP=N,N′-bis(2-pyridylmethyl)-2,2′-bipyrrolidine) in the oxidation of aromatic substrates with H₂O₂ has been studied. In the presence of acetic acid, these complexes efficiently catalyze the oxidation of benzene and alkylbenzenes with high selectivity for oxygen incorporation into the aromatic ring (up to 93 %), performing up to 84 catalytic turnovers. The parent complex, [(PDP)(OTf)₂], has demonstrated the highest catalytic efficiency and aromatic oxidation selectivity. The yield of products of oxidation of different substrates increases in line with increasing number of electron-donating alkyl groups of the substrates: halogenbenzenesH/k_D (0.90–0.92), agrees with the S_EAr oxidation mechanism. Low-temperature EPR studies have witnessed the presence of low-spin (g₁=2.071, g₂=2.008, g₃=1.960) perferryl intermediates, demonstrating direct reactivity toward benzene. The oxidation of m-xylene in the presence of H₂¹⁸O has shown the same probability of ¹⁸O incorporation into the aromatic ring and the aliphatic moieties, which is indicative that both the aromatic and aliphatic oxidations are conducted by the same active species.

Full text of DOI:10.1002/cctc.201800832

Accepted Article
Title: Highly Efficient Aromatic C-H Oxidation with H2O2 in the
Presence of Iron Complexes of the PDP Family
Authors: Nikolay Tkachenko, Roman Ottenbacher, Oleg Lyakin,
Alexandra Zima, Denis Samsonenko, Evgenii Talsi, and
Konstantin Petrovich Bryliakov
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Highly Efficient Aromatic C-H Oxidation with H2O2 in the
Presence of Iron Complexes of the PDP Family
Nikolay V. Tkachenko,[a, b] Roman V. Ottenbacher,[a, b] Oleg Y. Lyakin,[a, b] Alexandra M. Zima,[a, b]
Denis G. Samsonenko,[b, c] Evgenii P. Talsi,[a, b] Konstantin P. Bryliakov*[a, b]
[a] N. V. Tkachenko, Dr. R. V. Ottenbacher, Dr. O. Y. Lyakin, A. M. Zima, Prof. Dr. E. P. Talsi,
Prof. Dr. K. P. Bryliakov
Boreskov Institute of Catalysis, Pr. Lavrentieva 5, Novosibirsk 630090 (Russia), E-mail:
bryliako@catalysis.ru
[b] N. V. Tkachenko, Dr. R. V. Ottenbacher, Dr. O. Y. Lyakin, A. M. Zima, Dr. D. G. Samsonenko,
Prof. Dr. E. P. Talsi, Prof. Dr. K. P. Bryliakov
Novosibirsk State University, Pirogova 2, Novosibirsk 630090 (Russia)
[c] Dr. D. G. Samsonenko
Nikolaev Institute of Inorganic Chemistry, Pr. Lavrentieva 3, Novosibirsk 630090 (Russia)
Abstract
The catalytic activity of a series of iron complexes of the PDP family (PDP = N,N′-bis(2-
pyridylmethyl)-2,2′-bipyrrolidine) in the oxidation of aromatic substrates with H2O2 has been
studied. In the presence of acetic acid, these complexes efficiently catalyze the oxidation of benzene
and alkylbenzenes with high selectivity for oxygen incorporation into the aromatic ring (up to
93%), performing up to 84 catalytic turnovers. The parent complex, [(PDP)Fe(OTf)2], has
demonstrated the highest catalytic efficiency and aromatic oxidation selectivity. The yield of
products of oxidation of different substrates increases in line with increasing number of electron-
donating alkyl groups of the substrates: halogenbenzenes < benzene < monoalkylbenzenes <
dialkylbenzenes, which, together with the results of competitive oxidation experiments and the
observation of inverse primary kH/kD (0.90–0.92), agrees with the SEAr oxidation mechanism. Low-
temperature EPR studies have witnessed the presence of low-spin (g1 = 2.071, g2 = 2.008, g3 =
1.960) perferryl intermediates, demonstrating direct reactivity toward benzene. The oxidation of m-
18
xylene in the presence of H2 O has shown the same probability of 18O incorporation into the
1
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aromatic ring and the aliphatic moieties, which is indicative that both the aromatic and aliphatic
oxidations are conducted by the same active species.
Introduction
Selective C-H oxidation, driven by bioinspired iron complexes, mimicking the catalytic
properties of natural Fe containing oxygenases, has attracted great attention in the last years.[1-3] So
far, the major focus of research related to the catalytic properties of bioinspired iron catalysts has
been the oxidation of aliphatic C-H groups,[3, 4] whereas aromatic C-H oxidations with H2O2 have
remained challenging. Some examples of C-H hydroxylations of arene substrates have been
reported since 2005,[5-8] however, they either demonstrated low efficiency (1-2 turnovers at best), or
operated under model, high-substrate-excess conditions. The nature of the active iron-oxygen
intermediates remained unclear.
In the last few years, several bioinspired iron-catalyzed C-H oxidations of aromatic substrates
with H2O2, with the catalysts performing >10 turnovers, have been reported,[9-12] and attempts to
probe the oxidation mechanism on the basis of indirect (kinetic) data have been made.[9, 12] Very
recently, we have studied the catalyst system based on iron complex [(PDP*)FeIII(µ-
OH)2FeIII(PDP*)](OTf)4, 1-OTf (PDP* = N,N′-bis(3,5-dimethyl-4-methoxypyridyl-2-methyl)-(S,S)-
2,2′-bipyrrolidine), using EPR spectroscopy, and found that the kinetics of decay of the low-spin
iron(V)-oxo intermediates was affected by the presence of aromatic substrates.[13] Moreover, it has
been shown that 1-OTf (Figure 1) is capable of mediating the hydroxylation of substituted benzenes
with good efficiency (up to 36 TN) and high selectivity for aromatic oxidation (up to 91%).
Herewith, we present the systematic investigation of iron aminopyridine complexes, bearing
different substituents at the pyridine moieties, and different counteranions, in the aromatic oxidation
of substituted benzenes. The nature of the ligand and the counteranion has been found to affect their
catalytic properties. Novel EPR spectroscopic data, supporting the key role of the perferryl active
2
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species, are presented, that, in combination with kinetic and isotopic labeling data, bear evidence in
favor of the electrophilic aromatic substitution mechanism, operated by iron(V)-oxo intermediates.
Figure 1. Structures of iron complexes considered in this study.
Results and discussion
Very recently, we have demonstrated that complex 1-OTf can efficiently catalyze the C-H
oxidation of aromatic substrates with H2O2 in the presence of acetic acid (AA).[13] In this work, we
first studied the effect of counteranion on the catalytic reactivity of complexes 1-OTf, 1-ClO4, 1-
NO3, 1-Cl and 1-BPh4 featuring the same organic ligand framework, toward o-xylene. Preliminary
experiments (Table S1, Supporting Information, SI) revealed the optimized reaction conditions, that
were further used for screening various complexes. The results are presented in Table 1 (entries 1-
5).[14] Triflate complex 1-OTf demonstrated the highest efficiency and selectivity in the series.
Perchlorate complex 1-ClO4 showed slightly lower catalytic activity; complexes with other
counteranions appeared to be much less efficient and selective. A similar counteranion effect has
been observed for complexes 3-OTf and 3-ClO4 (Table 1, entries 7-8).
Here we notice that under the reaction conditions, in addition to the mono-oxygenation
products, namely “aromatic” (ring oxygenation products) and “aliphatic” (side chain oxygenation
products), noticeable amounts of mixed aromatic/aliphatic double oxygenation products were
detected (for details see Tables S2-S5 of the SI). Nevertheless, in compliance with our aim to
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develop efficient catalysts of direct aromatic oxygenation, in Tables 1 and 2 we have calculated the
selectivity of oxygen incorporation into the aromatic ring, as {yield of aromatic oxidation products
+ yield of aromatic/aliphatic oxidation products}/overall amount of oxidized products.
Next, the effect of substituents at the aminopyridine ligand was tested. It was found that the
presence of amino group (3-OTf and 3-ClO4) instead of methoxy group (1-OTf and 1-ClO4)
caused virtually no effect on the efficiency and selectivity of the process (Table 1, entries 7-8).
Also, dimeric complex 2-ClO4 (Figure 2), bearing methyl and trifluoroethoxy substituents at the
common PDP core, showed results similar to those of 1-ClO4 (Table 1, entry 9), while complex 4-
OTf, featuring NMe2 substituents, appeared to be less efficient (Table 1, entries 10), as well as the
Fe-tpa complex 5-ClO4 (Table 1, entry 11). Eventually, the highest efficiency and selectivity for
aromatic oxidations in the series was demonstrated by complex 6-OTf. Control experiment,
conducted under the same reaction conditions using 1 mol.% of Fe(OTf)2 as catalyst indicated very
low conversion (<1 TN), with 2-methylbenzaldehyde as the main oxidation product.
Table 1. Catalytic oxidation of o-xylene with H2O2 in the presence of complexes 1-7.[a]
Selectivity for
TN[d]
aromatic
oxidation [%][c]
Aromatic/
aliphatic[b]
Aliphatic Aromatic
oxidation[b] oxidation[b]
Entry Catalyst
Conversi
on (%)
1
2
32
22.5
5.7
2.8
3.8
34.5
32.5
21
1.9
2.7
1.5
0.9
1.3
1.5
1.8
1.7
3.0
1.5
2.6
17.3
6.5
5.7
7.5
1.3
0.4
0.6
10.6
6.0
4.1
7.7
2.6
1.3
88
77
65
52
55
91
78
75
78
72
67
47
1-OTf
1-ClO4
1-NO3
1-Cl
29.2
7.7
3
1.7
4
0.8
3.6
5
1.1
4.9
1-BPh4
6-OTf
3-OTf
3-ClO4
2-ClO4
4-OTf
5-ClO4
6
14.7
14.4
8.8
50.2
39.4
25.6
30
7
8
9
22
6.1
10
11
13.5
10
5.5
16.8
14.7
4.0
4
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[a] Reaction conditions: 0 °C; o-xylene (100 µmol), H2O2 (400 µmol), CH3COOH (1000 µmol), catalyst
(1.24 µmol of Fe), CH3CN (400 µL), oxidant was added by a syringe pump over 60 min, and the mixture was
stirred for 1.5 h, followed by GC/MS analysis. [b] Yields, moles of products/mole of Fe. Detailed
compositions of reaction products are provided in Table S2, SI. [c] calculated as 100%·{aromatic oxidation
products + aromatic+aliphatic oxidation products}/overall amount of oxidized products. [d] TN, defined as
{moles of single oxidation products/mole of Fe}+2{moles of double oxidation products/mole of Fe}.
Ketones and aldehydes were considered as double oxidation products.
Figure 2. Molecular structure of the diferric unit of complex 2-ClO4. Thermal ellipsoids at 50%
probability level. Fe: orange, O: red, N: blue, C: grey, F: yellow. Hydrogen atoms and perchlorate
counteranions are omitted for clarity.
Further, we examined the activity of the most efficient catalyst, 6-OTf, toward different
aromatic substrates (Table 2). In several cases (Table 2, entries 7, 8, 10, 12), high catalytic
efficiency (>30 TN) was demonstrated. As a general trend, the conversions for differently
substituted benzenes varied in the following order: halogenbenzenes < benzene <
monoalkylbenzenes < dialkylbenzenes. This order agrees with the increasing electrophilicity of the
aromatic ring, governed by the number of donor alkyl substituents.
Table 2. Catalytic oxidation of aromatic substrates with H2O2 in the presence of complex 6-OTf.[a]
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Selectivity for
aromatic
oxidation [%][c]
Aliphatic Aromatic
oxidation[b] oxidation[b]
Conversion
[%]
No
Substrate
Aromatic/
aliphatic[b]
TN[d]
1
2
3
4
benzene
toluene
14.5
26
–
11.8
14.0
12.5
9.4
–
5.4
6.4
1.0
3.8
10.6
17.8
6.0
5.2
10.5
3.5
–
–
21.4
38.7
40.7
19.8
35.1
50.2
84.2
78.2
41.1
56.2
43.4
8.6
1.4
3.1
1.8
1.6
1.5
1.4
2.2
8.0
7.4
6.3
–
93
86
83
89
91
91
92
68
76
74
–
ethylbenzene
cumene
27
15.5
25.5
34.5
57.5
51.5
32
5
6
7
8
9
isobutylbenzene
o-xylene
14.4
14.7
24.7
32.3
12.2
14.5
16.4
6.3
m-xylene
p-xylene
2-ethyltoluene
10 3-ethyltoluene
11 4-ethyltoluene
12 chlorobenzene
13 bromobenzene
41
33
8
6
–
4.9
–
–
8.9
[a] Reaction conditions: 0 °C; substrate (100 µmol), H2O2 (400 µmol), CH3COOH (1000 µmol), catalyst
6-OTf (1.24 mol.%), CH3CN (400 µL), oxidant was added by a syringe pump over 60 min, and the mixture
was stirred for 1.5 h, followed by GC/MS analysis. [b] Yields, moles of products/mole of Fe. Detailed
compositions of the reaction mixtures are provided in Table S3, SI. [c] calculated as 100%·{aromatic
oxidation products + aromatic+aliphatic oxidation products}/overall amount of oxidized products. [d] TN,
defined as {moles of single oxidation products/mole of Fe}+2{moles of double oxidation products/mole of
Fe}. Ketones and aldehydes were considered as double oxidation products.
The mechanism of the aromatic oxidation is of obvious interest. In previous mechanistic studies
of aromatic oxidations, mediated by iron complexes with different polydentate donor ligands,
electrophilic aromatic substitution mechanism was proposed.[9, 12] Recently, we confirmed the direct
reactivity of the highly electrophilic iron(V)-oxo intermediate [(PDP*)FeV=O(OC(O)CH3)]2+, 1a
(PDP* = N,N′-bis(3,5-dimethyl-4-methoxypyridyl-2-methyl)-(S,S)-2,2′-bipyrrolidine), stemming
from catalyst 1-OTf, toward aromatic substrates.[13] The present study provides the following data
on the oxidation mechanism.
6
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First of all, in the reaction mixtures of Tables 1 and 2, no biaryls (products of radical arene
coupling) have been found. Moreover, the oxidations of o-xylene under an argon atmosphere
demonstrated virtually the same results as under ambient air (Table S4, entry 2), which also rules
out significant contribution of pathways involving trapping dioxygen with free radicals. Further, in
the course of the oxidation of o-xylene, the addition of CBrCl3, the radical trap, well-known to
suppress the incorporation of oxygen into C-H groups in radical-type oxygenations,[15-17] caused
virtually no effect in our experiments; no brominated products were detected (Table S4, entry 3).
Further, we evaluated the primary KIE values for the oxidation of benzene/benzene-d6 in the
presence of complexes 2-ClO4 and 6-OTf. In both cases, inverse kH/kD values were found (0.92 and
0.90 respectively; for the experimental details, see SI), similar to those previously observed for
12, 13, 18]
related oxidations in the presence of iron complexes,[9,
supposedly occurring via
electrophilic aromatic substitution mechanism. Such significant primary inverse KIE is consistent
with the rate-limiting sp2 to sp3 conversion during σ-complex formation at the deuterium-substituted
position.[18-21]
Experiments on competitive oxidations of monosubstituted arenes with different (electron-
donating and electron-withdrawing) substituents vs. benzene have shown that the yield of
oxygenated products increases in line with decreasing electron deficiency of the substrate. In
particular, more electron-rich toluene demonstrated higher yield of oxygenated products than
benzene, while chloro- and bromobenzene showed much lower conversions than benzene (Scheme
1 and Table S5, SI). Such picture is fully consisted with the previously invoked electrophilic
oxidation mechanism.[9,12,13]
The exact nature of the catalytically active sites is a question of primary significance.
Previously, it was proposed that iron(V) oxo intermediates are the true active species conducting the
hydroxylation of aromatic substrates;[5, 7, 21, 22] moreover, very recently, direct EPR spectroscopic
observation of the reactivity of such intermediate 1a, stemming from complex 1-OTf, toward
arenes has been reported.[13] In this work, formation of similar active species
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[(PDPMeOCH2CF3)FeV=O(OC(O)CH3)]2+, 2a, having EPR pattern characteristic of perferryl species
(g1 = 2.071, g2 = 2.008, g3 = 1.960),[23-27] has been observed, too, in the systems 2-
ClO4/H2O2/AcOH and 2-ClO4/CH3CO3H/AcOH (Figure 3). The concentration of 2a in Figure 3 did
not exceed 6 % of the initial concentration of 2-ClO4. Furthermore, the observed pseudo-first-order
rate constant of the decay of intermediate 2a at −85 °C (Figure 3, bottom) dramatically increased
upon the addition of 0.02 M of benzene (Figure 4), thus providing the opportunity to evaluate the
second-order rate constant for the direct reaction of benzene with 2a (k2 ∼ 5 M−1s−1) following the
previously developed approach.[13]
Scheme 1. Competitive oxidation of benzene/substituted benzene es. Detailed composition of the
reaction mixtures are provided in Table S5, SI. . (Yields are given in mol products/mol Fe).
8
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2.071
2a
2.008
2a
*
*
0 min
*
3 min
6 min
9 min
15 min
3200
3280
3360
H / G
3440
3520
1.0
k1 = 2.2 × 10−3 s−1
0.8
0.6
0.4
0.2
(−70 °C)
c/c0
0
2
4
6
8
10 12 14 16
t / min
Figure 3. (Top) EPR spectra (−196 °C) of the sample 2-ClO4/CH3CO3H/AA
([Fe]:[CH3CO3H]:[AA] = 1:3:10, [Fe] = 0.04 M), frozen after mixing the reagents during 5 min at
−70 °C in a CH2Cl2/CH3CN mixture (v/v = 1.8:1) and storing the sample at −70 °C for various
times. (Bottom) Relative concentration of 2a (c/c0) vs. time calculated from the decay of its EPR
signal. Asterisks denote resonances of an unknown iron complex raised upon the decay of 2a. The
resonance of 2a at g3 = 1.960 overlaps with the resonances of unknown complex.
9
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2.071
2a
2a
2.008
A)
2a
1.960
B)
C)
*
*
3200
3280
3360
H / G
3440
3520
Figure 4. EPR spectra (−196 °C) of the (A) sample 2/CH3CO3H/AA ([Fe]:[CH3CO3H]:[AA] =
1:3:10, [Fe] = 0.04 M): frozen after mixing the reagents during 1 min at −70 °C; (B) sample in “A”
frozen 0.5 min after the addition of benzene (0.02 M) at −85 °C; (C) sample in “A” frozen 2 min
after the addition of benzene (0.02 M) at −85 °C. CH2Cl2/CH3CN mixture (v/v = 1.8:1) was used as
a solvent. Asterisks denote resonances of an unknown iron complex.
Given the widely-accepted “water-assisted” mechanism of the formation of FeV=O
intermediates,[28-30] isotopic labeling experiment could be used to corroborate the involvement of
the perferryl species in the oxidation, by tracing the inclusion of 18O atom stemming from externally
added 18O-water to the oxidation products. The only technical problem could be the presence of
high excess of acetic acid additive, competing with water for the Fe coordination site. Fortunately,
catalyst 6-OTf has been found to catalyze the oxidation of m-xylene without any acid additives,
performing several TN, thus opening the gate for the isotopic labeling experiment as shown in
18
Scheme 2. In particular, the addition of 30 equiv of H2 O (vs. substrate) to the system
16
6-OTf/H2 O2 (90%) has resulted in the formation of aromatic and aromatic/aliphatic oxidation
products with 4% and 9% 18O incorporation respectively (Scheme 2). The monohydroxylation
18
products witness 4% O incorporation, which corroborates the water-assisted mechanism, leading
10
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to partial oxygen scrambling. The products of dihydroxylation (either aromatic/aromatic or
aromatic/aliphatic) predictably demonstrated doubled 18O percentage (7–9%, Scheme S1, SI),
which reflects the same probabilities of external 18O oxygen incorporation into the aromatic and
aliphatic moieties of the substrate. This is indicative that both the aromatic and aliphatic oxidations
are conducted by the same active perferryl species.
Scheme 2. Water-assisted mechanism of the formation of the FeV=O intermediate, together with the
isotopic labeling scheme and 18O label distribution (colored in red) in the major products of m-
xylene oxidation, assuming the proposed SEAr hydroxylation mechanism. Yields are given in mol
products/mol Fe.
Conclusions
A series of bioinspired iron complexes with bipyrrolidine-derived aminopyridine ligands have been
studied as catalysts of aromatic hydroxylation of substituted benzenes with hydrogen peroxide in
the presence of acetic acid. High efficiencies (up to 84 TN) and selectivities toward aromatic
oxidation (up to 93%) have been achieved under optimized reaction conditions. The substrate
conversions increase in the following order: halogenbenzenes < benzene < monoalkylbenzenes <
dialkylbenzenes, in line with increasing nucleophilicity of the arene. This, in combination with the
competitive oxidation data and the sizeable primary inverse KIE (0.90–0.92), points to the
electrophilic aromatic substitution mechanism. EPR spectroscopic measurements have revealed the
presence of highly active perferryl intermediate 2a [(PDPMeOCH2CF3)FeV=O(OC(O)CH3)]2+, directly
11
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18
reacting with benzene even at −85 °C. Isotopic labeling experiments with excess H2 O, conducted
in the absence of acetic acid, provide evidence in favor of the [LFeV=16O(18OH)] intermediate,
existing in equilibrium with its isomer [LFeV=18O(16OH)], which results in formation of partially
18O labeled products of aromatic oxidation: 4% 18O for monooxygenated products and 7–9% 18O for
dioxygenated products. Altogether, these data are consistent with the SEAr oxidation mechanism,
driven by the electrophilic LFeV=O intermediates.
Experimental section
Standard oxidation procedure
Substrate (100 µmol) and acetic acid (1000 µmol) were added to the solution of corresponding iron
complex (1.24 µmol of Fe) in CH3CN (400 µL), and the mixture was thermostated at 0 °C. Then,
the solution of H2O2 (400 µmol) in CH3CN (100 µL) was injected by a syringe pump over 60 min
upon stirring. The resulting mixture was stirred for 1.5 h and subjected to GC/MS analysis.
The detailed experimental procedures and data are provided in the Supporting Information.
Acknowledgements
This work was conducted with the financial support from the Russian Science Foundation (project
17-13-01117). AMZ thanks Russian Foundation of Basic research (project 18-33-00462 for young
scientists) for the support of the EPR detection and monitoring of the perferryl intermediates.
Keywords
Aromatic substitution, hydrogen peroxide, intermediates, iron, oxidation
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FULL PAPER
N. V. Tkachenko, R. V. Ottenbacher, O.
Y. Lyakin, A. M. Zima, D. G.
Samsonenko, E. P. Talsi, K. P.
Bryliakov*
Highly Efficient Aromatic C-H
Oxidation with H2O2 in the Presence
of Iron Complexes of the PDP Family
Bioinspired iron complexes catalyze the oxidation of benzene and alkylbenzenes
with H2O2, with high efficiency and selectivity for aromatic oxidation. Catalytic,
mechanistic and spectroscopic data bear evidence in favor of the electrophilic
aromatic substitution mechanism, driven by perferryl species.
15
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