Highly Enantioselective C?H Oxidation of Arylalkanes with H2O2 in the Presence of Chiral Mn-Aminopyridine Complexes

Article abstract of DOI:10.1002/cctc.201701169

Bioinspired chiral Mn-aminopyridine complexes [(S,S)-LMn^II(OTf)₂] and [(R,R)-LMn^II(OTf)₂] (where (S,S)-L=(2S,2S)-1,1-bis((3-methyl-4-(2,2,2-trifluoroethoxy)pyridin-2-yl)methyl)-2,2-bipyrrolidine, and (R,R)-L=(2R,2R)-1,1-bis((3-methyl-4-(2,2,2-trifluoroethoxy)pyridin-2-yl)methyl)-2,2-bipyrrolidine) have been shown to efficiently catalyze the benzylic C?H oxidation of arylalkanes with hydrogen peroxide in the presence of carboxylic acid additives, affording enantiomerically enriched 1-arylalkanols and the corresponding ketones. Optically pure additive N-Boc-(L)-proline, in combination with [(R,R)-LMn^II(OTf)₂] complex, affords 1-arylalkanols in up to 86 % ee, which is the highest reported enantioselectivity for direct benzylic hydroxylations with H₂O₂ in the presence of transition-metal catalysts. Oxidative kinetic resolution only slightly contributes to the increase of the observed enantiomeric excess over the reaction course. The observed k_H/k_D values (3.5–3.6 for the oxidation of ethylbenzene/d₁₀-ethylbenzene) and competitive oxidation data are consistent with either a hydrogen-atom transfer/oxygen rebound or hydride transfer/oxygen rebound asymmetric hydroxylation mechanism.

Full text of DOI:10.1002/cctc.201701169

Accepted Article
Title: Highly Enantioselective C-H Oxidation of Arylalkanes with H2O2
in the Presence of Chiral Mn Aminopyridine Complexes
Authors: Evgenii Talsi, Denis Samsonenko, Roman Ottenbacher, and
Konstantin Petrovich Bryliakov
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Highly Enantioselective C-H Oxidation of Arylalkanes with H O in the
2
2
Presence of Chiral Mn Aminopyridine Complexes
a,b
a,c
a,b
Evgenii P. Talsi, Denis G. Samsonenko, Roman V. Ottenbacher, and Konstantin P. Bryliakov
a,b
*
a
Prof. Dr. E. P. Talsi, Dr. D. G. Samsonenko, Dr. R. V. Ottenbacher, Prof. Dr. K. P. Bryliakov,
Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russian Federation, E-mail:
bryliako@catalysis.ru
b
Prof. Dr. E. P. Talsi, Dr. R. V. Ottenbacher, Prof. Dr. K. P. Bryliakov, Boreskov Institute of
Catalysis, Pr. Lavrentieva 5, Novosibirsk 630090, Russian Federation
c
Dr. D. G. Samsonenko, Nikolaev Institute of Inorganic Chemistry, Pr. Lavrentieva 3, Novosibirsk
630090, Russian Federation
Abstract
Bioinspired chiral Mn-aminopyridine complexes [((S,S)-LMn (OTf) ] and [((R,R)-LMn (OTf) ]
II
II
2
2
(
where
(S,S)-L = (2S,2'S)-1,1'-bis((3-methyl-4-(2,2,2-trifluoroethoxy)pyridin-2-yl)methyl)-2,2'-
and (R,R)-L = (2R,2'R)-1,1'-bis((3-methyl-4-(2,2,2-trifluoroethoxy)pyridin-2-
bipyrrolidine,
yl)methyl)-2,2'-bipyrrolidine) have been shown to efficiently catalyze the benzylic C-H oxidation of
arylalkanes with hydrogen peroxide in the presence of carboxylic acid additives, affording
enantiomerically enriched 1-arylalkanols and the corresponding ketones. Optically pure additive N-
II
Boc-(L)-proline, in combination with [((R,R)-LMn (OTf) ] complex, affords 1-arylalkanols having
2
up to 86 % ee, which is the highest reported enantioselectivity for direct benzylic hydroxylations
with H O in the presence of transition metal catalysts. Oxidative kinetic resolution only slightly
2
2
contributes to the increase of the observed enantiomeric excess over the reaction course. The
observed k /k values (3.5…3.6 for the oxidation of ethylbenzene/d -ethylbenzene) and
H
D
10
competitive oxidation data are consistent with either hydrogen-atom transfer/oxygen rebound, or
hydride transfer/oxygen rebound asymmetric hydroxylation mechanism.
1
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Introduction
The design of biomimetic transition metal based catalyst systems, capable of conducting the
1
,2
selective C-H oxidations, had been continuous attraction for years; this interest had been further
encouraged by the publications by White and co-workers who showed that bioinspired iron
aminopyridine complexes mediate the oxidation of non-activated aliphatic C-H groups in complex
3
organic molecules with environmentally benign oxidant H O in a predictably selective fashion.
2
2
From the practical perspective, the reactivity and site-selectivity are the key factors, determining the
possibility of introducing oxygen atoms at particular position of a desired substrate; ideally,
development of a variety of transition metal catalysts covering whole range of C-H groups of
potential substrates may be the gate to a new era of C-H oxofunctionalization. In recent years, there
has been appreciable progress in this direction. For example, the interplay between steric bulk and
electronic properties of the elaborate chiral aminopyridine iron catalysts enabled Costas and co-
workers to manipulate the oxidation selectivity of complex steroidal substrates, in particular
4
diverting it toward 2° C-H groups over 3° C-H groups. Others have documented the high
selectivity and efficiency of manganese based catalysts in catalyzed aliphatic and benzylic C-H
5
oxidations, including the oxidation of arylalkanes and benzylic alcohols.
Apparently, next step would be achieving stereoselectivity in “green” C-H oxidations with
H O mediated by highly efficient metal based catalysts. Until very recently, it was only the
2
2
manganese porphyrin complex 1 (Figure 1), reported by Simonneaux with co-workers, for which
6
a,b
the asymmetric oxidation of prochiral benzylic groups with H O had been documented, with the
2
2
oxidation of p-ethyltoluene yielding the corresponding 1-(p-tolyl)ethanol with up to 57 % ee. The
catalytic protocol required rather high (2.5 mol. %) catalyst loading and a 5-fold excess (relative to
alkane) of H O . Costas and Bietti developed a desymmetrization-based protocol for the oxidation
2
2
of monosubstituted cyclohexanes in the presence of manganese complexes 2a and 2b (Figure 1); the
rd
oxidation occurred preferentially at the 3 position, affording the corresponding chiral C3-
6
c
ketones. For the best substrates – bulky amide-substituted cyclohexanes – enantiomeric excess
2
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10.1002/cctc.201701169
values of 62-96 % ee were reported using 2 mol. % of catalyst 2b, 3.5 equiv. of H O , and
2
2
6
c
cyclopropanecarboxylic acid as the catalytic additive.
In a recent paper, mostly dedicated to the kinetic resolution of 1-arylalkanols, we announced
the oxidation of ethylbenzene and several p-substituted homologues with H O in the presence of
2
2
manganese complex 3 and 2-ethylhexanoic acid, affording chiral 1-arylalkanols with up to 50 % ee,
6
d
the catalyst performing several hundreds of catalytic turnovers. In this contribution, we are
presenting the detailed study of the asymmetric benzylic oxidation of arylalkanes, with substantial
improvements of the initial protocol. In particular, it has been found that the chiral environment
amplification (the use of chiral catalytic additive N-Boc-proline) drastically enhances the oxidation
stereoselectivity up to > 80 % ee; the asymmetric induction level slightly benefits from the
oxidative kinetic resolution of the 1-arylalkanol. The chiral match/mismatch effects and the
preliminary mechanistic data are discussed.
Figure 1. Manganese based catalysts for the asymmetric C-H oxidations with H O .
2
2
3
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Results and discussion
6
d
The oxidation of ethylbenzene in the presence of manganese complex (S,S)-3 and its enantiomer
R,R)-3 (Figure 2), using various carboxylic acids as catalytic additives (Figure 3) has been studied.
(
The results are collected in Table 1. The oxidation in the presence of acetic acid has afforded a
mixture of acetophenone and 1-phenylethanol, the latter having 34 % ee optical purity (Table 1,
entry 1). The use of n-butyric acid leads to poor oxidation yield, whereas branched carboxylic acids,
with substituents at the α-carbon, usually ensure higher yields and higher enantioselectivities
compared with acetic acid (Table 1, entries 3-6), 2-ethylhexanoic and cyclohexanecarboxylic acid
demonstrating the highest enantioselectivity and substrate conversion at the same time (entries 5 an
6). Complex (R,R)-3 has shown the same catalytic performance with EHA (Table 1, entries 6 and
7). We notice that for the case of EHA, the contribution of oxidative kinetic resolution of the
6d
initially formed 1-phenylethanol can be ruled out, the observed enantioselectivity being solely
generated at the asymmetric C-H oxidation step itself.
6
d
Figure 2. Molecular structures of complex (S,S)-3 and its enantiomer (R,R)-3. Mn magenta, N
blue, S orange, F yellow, C grey, O red. Thermal ellipsoids drawn at 50 % probability level. Three
(out of four) complexes of the asymmetric unit, hydrogen atoms, and rotational disorder of triflates
have been omitted for clarity. Full asymmetric unit of (R,R)-3 can be found in Figure S1 (SI).
4
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10.1002/cctc.201701169
Figure 3. Structure of carboxylic acids used as additives in this work, and the abbreviations used.
Table 1. Oxidation of ethylbenzene and p-ethyltoluene with H O in the presence of complexes
2
2
a
(S,S)-3 and (R,R)-3.
HO
O
Mn cat.
+
+ byproducts
CH3CN
RCOOH
H2O2
X
X
X
X = H, CH3
c
No catalyst
X
additive (µmol)
substrate
conversion
yield of alcohol / A / K
ketone / other
ee of
config.
b
alcohol
d
(
µmol)
(µmol)
(%)
e
1
2
3
4
5
6
7
(S,S)-3
(S,S)-3
(S,S)-3
(S,S)-3
(S,S)-3
(S,S)-3
(R,R)-3
H
AA (600)
BA (100)
37
15 / 19 / 3
0.8
2
34
10
35
30
41
35
34
(S)
(S)
(S)
(S)
(S)
(S)
(R)
H
H
H
H
H
H
7
4 / 2 / 1
EBA (100)
DMBA (100)
CHA (100)
EHA (100)
EHA (100)
34
35
41
43
40
11 / 23 / < 1
14 / 21 / ‒
13 / 27 / 1
15 / 27 / 1
15 / 24 / 1
0.5
0.7
0.5
0.5
0.6
8
9
1
1
1
(S,S)-3
(S,S)-3
(S,S)-3
(S,S)-3
(S,S)-3
CH3
CH3
CH3
CH3
CH3
AA (100)
EBA (100)
DMBA (100)
CHA (100)
EHA (100)
31
31
31
27
44
20 / 8 / 3
21 / 8 / 2
19 / 10 / 2
17 / 8 / 2
2.6
2.6
1.9
2.1
1.6
41
40
44
42
50
(S)
(S)
(S)
(S)
(S)
0
1
2
e
25 / 16 / 3
1
1
1
1
3
4
5
6
7
(S,S)-3
(R,R)-3
(R,R)-3
(S,S)-3
(R,R)-3
CH3
CH3
CH3
H
Boc-L-Pro (100)
Boc-L-Pro (100)
Boc-L-Pro (30)
Boc-L-Pro (30)
Boc-L-Pro (30)
28
29
22
31
22
16 / 9 / 3
13 / 13 / 3
13 / 6 / 3
10 / 20 / 1
10 / 12 / ‒
1.8
1.0
2.2
0.5
0.8
50
86
80
49
76
(S)
(R)
(R)
(S)
(R)
1
H
a
At 0 °C; [substrate]:[H O ]: [Mn] = 200 µmol : 70 µmol : 0.1 µmol, oxidant added via syringe
2
2
pump over 30 min and the mixture stirred for an additional 3 h, and product yields and ees were
determined by analyzing the reaction mixture by chiral HPLC. Other products were: esters; traces
b
5
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c
d
of (1-hydroperoxyethyl)benzene, etc. (see SI for details). Alcohol/ketone ratio. Determined by
chiral HPLC. From ref. 6d.
e
The oxidation of p-ethyltoluene in the presence of a variety of carboxylic acids (Table 1,
entries 8-12) has shown somewhat different trend: in this case, CHA demonstrated inferior results
than DMBA and EHA; the use of EHA ensured the highest substrate conversion and the highest
alcohol ee at the same time (entries 10-12). In this case, the alcohol/ketone ratios (1.6…2.6, entries
8-12) were higher than for the oxidation of ethylbenzene in the presence of branched acids
(0.5…0.7, entries 3-7), apparently, due to somewhat higher reactivity of p-ethyltoluene, compared
with ethylbenzene (see below).
Commercially available 2-ethylhexanoic acid is a racemic mixture of (R) and (S)
enantiomers, which may lead to two diastereomeric catalytically active sites, featuring different
stereoselectivities and thus deteriorating the ee. One could expect that the use of proper (optically
pure) chiral carboxylic acid as additive could positively affect the stereochemical outcome of the C-
7
H oxidation reaction, like it was previously reported for the asymmetric epoxidations of olefins and
6
d
oxidative kinetic resolution of 1-arylalkanols. Herewith, the possibility of such chiral additive
amplification has been tested using optically pure N-Boc-(L)-proline as the chiral additive (Table 2,
entries 13, 14). The oxidation of p-ethyltoluene by the system (S,S)-3/Boc-(L)-Pro/H O has
2
2
demonstrated the same enantioselectivity as that by the system with (S,S)-3/EHA/H O (entries 13
2
2
vs. 12). By contrast, the enantioselectivity of the system (R,R)-3/Boc-(L)-Pro/H O has been much
2
2
higher than that of (S,S)-3/Boc-(L)-Pro/H O (cf. entries 14 and 13) at similar substrate conversion
2
2
levels, thus indicating proper chiral matching between the chiral catalyst and chiral additive. When
the amount of added Boc-(L)-Pro was reduced from 100 to 30 µmol, the conversion dropped
slightly, but the alcohol/ketone ratio increased twice (the yield of alcohol was the same, the yield of
ketone being twice smaller) (entry 15 vs. 14). The enantioselectivity was several per cent lower,
probably due to smaller contribution of oxidative kinetic resolution (see below). The oxidation of
6
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ethylbenzene by the system (R,R)-3/Boc-(L)-Pro/H O2 has also been found much more
2
enantioselective than by (S,S)-3/Boc-(L)-Pro/H O (76 vs. 49 % ee, entries 16 and 17).
2
2
The oxidation of several substrates by the system (R,R)-3/Boc-(L)-Pro/H O has been
2
2
studied (Table 2). In some cases (p-Br-ethylbenzene, p-ethyltoluene, isobutylbenzene),
enantioselectivities approaching or exceeding 80 % ee have been observed. This is much higher
than for the asymmetric C-H oxidations with H O , catalyzed by manganese porphyrin complex 1
2
2
6
a,b
(which showed 57 % ee at best),
catalysts (S,S)-3 and (R,R)-3 also being much more efficient
(performing 200-300 of turnovers under the reaction conditions), and producing 50-160 molecules
of chiral alcohol per molecule of catalysts. The oxidant efficiency was moderate, 70 µmol of H O
2
2
affording 10-20 µmol of products (alcohol + ketone, Table 2). Nevertheless, this efficency is
comparable with that of the system of Costas and Bietti, which requires 3.5 equiv. of H O , and is
2
2
much better than for the Cu/Au nanoparticles-based system of Hua and co-workers, which requires
6
e
ca. 15 equivalents of H O .
2
2
a
Table 2. Oxidation of alkylarenes with H O in the presence of complexes (R,R)-3.
2
2
R
HO
R
O
R
(
R,R)-3
CH CN
+
+ byproducts
3
Boc-(L)-proline
H O
X
2
2
X
X
b
c
d
No
substrate
substrate
conversion
yield of alcohol / A / K
ketone / other
TON ee of alcohol config.
d
(%)
(
µmol)
(µmol)
1
2
3
4
22
10 / 12 / ‒
5 / 10 / < 0.5
13 / 6 / 3
0.8
0.5
2.2
1.4
340
250
25
76
78
80
79
(R)
(R)
(R)
15
22
e
9.5
5.5 / 4 / < 0.5
135
n/a
n/a
f
e
5
6
13.5
20
7 / 6 / 0.5
1.3
1.0
190
300
70
44
10 / 10 / ‒
17 / 5 / ‒
(R)
(R)
7
22
3.4
270
19
7
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a
At 0 °C; [substrate]:[H O ]:[additive]:[Mn] = 200 µmol : 70 µmol : 30 µmol : 0.1 µmol, oxidant
2
2
added via syringe pump over 30 min and the mixture stirred for an additional 3 h, and product
b
yields and ees were determined by analyzing the reaction mixture by chiral HPLC. Alcohol/ketone
c
d
ratio. For this table, turnover numbers, TON, have been calculated as ([A]+2[K])/[Mn]0.
e
f
Determined by chiral HPLC. Not assigned. Product yields were determined by GS-MS.
Apparently, in the presence of Boc-(L)-Pro, kinetic resolution of the initially formed
6
d
alcohol could contribute to the observed enantioselectivity. The effect of kinetic resolution on the
asymmetric oxidation of ethylbenzene by the system (R,R)-3/Boc-(L)-Pro/H O has been tested by
2
2
extending the reaction to higher conversions (Figure 4). One can see that the enantiomeric excess
slowly increases from 71 % ee at ca. 200 catalyst turnovers to 83 % ee at ca. 750 turnovers, which
gain is not very significant; the major source of asymmetric induction remains enantioselective
oxidation itself. One can see that the yield of 1-phenylethanol approaches its maximum value of ca.
10 µmol and further slowly decreases over the reaction course, while the amount of ketone
monotonously grows up, thus becoming the major reaction product.
3
3
2
2
2
1
1
6
2
8
4
0
6
2
90
80
70
60
50
40
30
20
10
0
µ
8
4
0
0
100 200 300 400 500 600 700 800
catalyst TON
Figure 4. Plot of the amounts of alcohol (blue), ketone (black), and of alcohol ee (red) in the course
of ethylbenzene oxidation by the system (R,R)-3/Boc-(L)-Pro/H O ([substrate]:[Mn]:[Boc-(L)-Pro]
2
2
=
200 µmol : 0.1 µmol : 30 µmol) at 0 °C vs. catalyst turnover number, TON, calculated as
([alcohol]+2[ketone])/[(R,R)-3]0.
To get preliminary data on the oxidation mechanism, primary KIE for the oxidation of
ethylbenzene/d -ethylbenzene in the presence of catalysts 3 has been measured. Three experiments,
1
0
8
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10.1002/cctc.201701169
performed in the presence of different carboxylic acids, have returned very close k /k values of
H
D
3.5...3.6 (similar to those previously documented for the oxidations of cumene/α-D-cumene
5
c
derivatives in the presence of structurally similar Mn catalysts ), thus corroborating that the
benzylic hydrogen lies on the reaction coordinate of the rate-limiting step.
Next, competitive oxidations of p-substituted ethylbenzenes have been performed (Figure
8
+
5
), revealing good linear Brown-Okamoto correlation, with ρ value of ca. -1.4, which fits within
9
typical range for hydroxylations by electron-deficient transition-metal complexes (-1.0…-2.0), and
+
is close to the ρ value of -1.0, reported for the oxidation of p-substituted cumenes in the presence
5
c
of a structurally similar manganese catalyst. Previously, we proposed the benzylic oxidation (of
5
c
cumenes) to proceed via rate-limiting hydrogen abstraction, followed by oxygen rebound. At the
same time, keeping in mind (1) the apparently high electrophilicity of the putative
oxomanganese(V) species, and (2) the stability of benzylic cations, the Log(k /k ) correlation vs.
X
H
+
polar substituent parameter σp may as well reflect hydride abstraction mechanism (Figure 6).
10
Hydride transfer mechanism has been previously invoked in oxometal-mediated C-H oxidations.
At the moment there is not enough data to reasonably discriminate between the two scenarios;
however, this issue will be addressed in a subsequent separate study.
Table 3. KIE values for oxidation of ethylbenzene/d -ethylbenzene by catalyst systems
1
0
a
3
/acid/H O .
2 2
No
acid
AA
Catalyst
conversion,
k /k
H
D
µmol
1
2
(S,S)-3
(S,S)-3
(R,R)-3
146
137
127
3.6
3.6
3.5
PVA
EHA
3
a
Conditions: 0 °C, [ethylbenzene]:[d -ethylbenzene]:[(R,R)-3]:[H O ] = 150 µmol : 300 µmol :
10
2
2
0.2 µmol : 450 µmol), oxidant added via syringe pump over 2 h, the mixture stirred for an
additional 3 h; 10 mg of PPh was added to ensure the absence of traces of hydroperoxides, and the
3
mixture was analyzed by NMR and chiral HPLC to provide the yields of alcohols, ketones, and
residual alkylarenes. For details see Supporting Information.
9
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0
,6
,2
p-Ме
0
p-Н
p-Br
-0,2
-0,6
-1,0
-1,4
p-NO
2
y = -1.36x - 0.019
R = 0.999
2
-0,6 -0,4 -0,2
0,0
0,2
0,4
0,6
0,8
1,0
+
σp
8
Figure 5. Hammett plot for the competitive oxidation of p-substituted ethylbenzenes with H O .
2
2
For details see Supporting Information.
Figure 6. Two alternatives for the proposed mechanism of benzylic C-H oxidation wirh H O in the
2
2
presence of Mn aminopyridine complexes.
Conclusions
Herewith, we have shown that bioinspired chiral Mn-aminopyridine complexes [((S,S)-
II
II
LMn (OTf) ] and [((R,R)-LMn (OTf) ] (where (S,S)-L = (2S,2'S)-1,1'-bis((3-methyl-4-(2,2,2-
2
2
trifluoroethoxy)pyridin-2-yl)methyl)-2,2'-bipyrrolidine, and (R,R)-L = (2S,2'S)-1,1'-bis((3-methyl-4-
2,2,2-trifluoroethoxy)pyridin-2-yl)methyl)-2,2'-bipyrrolidine) are capable of catalyzing the
benzylic oxidation of arylalkanes with hydrogen peroxide, performing several hundreds of catalytic
(
1
0
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turnovers, and affording enantiomerically enriched 1-arylalkanols and the corresponding ketones.
The choice of carboxylic acid additive is crucial for achieving high alcohol/ketone ratio and high
enantioselectivity. While the use of simple achiral or racemic carboxylic acids results in low to
moderate enantioselectivities (10-50 % ee), optically pure additive N-Boc-(S)-proline, in
II
combination with [((R,R)-LMn (OTf) ], affords 1-arylalkanols having up to 86 % ee. This is the
2
highest reported enantioselectivity level for direct catalyzed benzylic hydroxylations with H O , and
2
2
is among the highest enantioselectivities ever reported for transition metal catalyzed benzylic C-H
1
1
oxidations with other oxidants (2,6-dichloropyridine N-oxide and iodosylarenes). In addition, the
catalytic efficiency of our catalysts (in mol of chiral alcohol per mol of catalyst) is also among the
6
,11
highest reported values.
Oxidative kinetic resolution has relatively small contribution to the amplification of the
enantiomeric excess over the reaction course, the major source of asymmetric induction being
enantioselective oxidation itself. Currently available mechanistic data (k /k values of 3.5…3.6 and
H
D
+
linear Brown-Okamoto correlations with ρ value of -1.4) may reflect the hydrogen-atom
transfer/oxygen rebound or hydride-transfer/oxygen rebound oxidation mechanism. Detailed
mechanistic study of Mn-aminopyridine catalyzed asymmetric benzylic C-H oxidations will be
reported in a separate study.
At the moment, the major drawback is the pronounced overoxidation of the formed chiral
alcohol to the ketone, which makes one use high excess of substrate and stop the reaction at low
conversion levels, so that the yield of chiral alcohol is limited to 50-160 molecules per one catalyst
molecule, which restricts the immediate synthetic application of the proposed system. This
challenging problem will be addressed in further investigations.
Associated content
Supporting information, including description of materials and methods, synthetic procedures,
general procedures for C-H oxidations, HPLC separation details, details of KIE and competitive
oxidation experiments, and crystal data for complex (R,R)-3. CCDC 1555788 ((R,R)-3) contains the
1
1
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10.1002/cctc.201701169
supplementary crystallographic data for this paper. These data can be obtained free of charge from
the Cambridge Crystallographic Data Centre via ww.ccdc.cam.ac.uk/data_request/cif and from the
authors.
Acknowledgements
Financial support from the Russian Science Foundation (project 17-13-01117) is gratefully
acknowledged.
Keywords
Asymmetric catalysis; C-H hydroxylation; enzyme models; hydrogen peroxide; manganese
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Layout 1:
FULL PAPER
Towards green asymmetric C-H
oxidations: catalyst system based on
bioinspired Mn aminopyridine
Evgenii P. Talsi, Denis G. Samsonenko,
Roman V. Ottenbacher, and Konstantin
P. Bryliakov*
complex is presented, which is
capable of catalyzing the asymmetric
benzylic oxidation of arylalkanes with
hydrogen peroxide, affording 1-
arylalkanols with up to 86 % ee.
Page No. – Page No.
Highly Enantioselective C-H
Oxidation of Arylalkanes with H
the Presence of Chiral Mn
Aminopyridine Complexes
2 2
O in
1
8
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