Chiral Manganese Aminopyridine Complexes: the Versatile Catalysts of Chemo- and Stereoselective Oxidations with H2O2

Article abstract of DOI:10.1002/tcr.201700032

In the last decade, manganese(II) complexes with N-donor tetradentate aminopyridine ligands emerged as efficient catalysts of enantioselective epoxidation of olefins and direct selective oxidation of C?H groups in complex organic molecules, with environmentally benign oxidant hydrogen peroxide. In this personal account, we summarize the progress of these catalysts with regard to ligands design, structure-reactivity correlations, evaluation of the substrate scope, as well as mechanistic studies, shedding light on the nature of active sites and the mechanisms of selective oxygenations. Several practically promising catalytic syntheses with the aid of Mn aminopyridine catalysts are exemplified.

Full text of DOI:10.1002/tcr.201700032

Personal Account
DOI: 10.1002/tcr.201700032
Chiral Manganese Aminopyridine
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Complexes: the Versatile Catalysts of
Chemo- and Stereoselective Oxidations
with H2O2
T H E
C H E M I C A L
R E C O R D
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Roman V. Ottenbacher,*[a, b] Evgenii P. Talsi,[a, b] and Konstantin P. Bryliakov*[a, b]
Chem. Rec. 2017, 17, 1–14
© 2017 The Chemical Society of Japan & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Wiley Online Library
1

P e r s o n a l A c c o u n t
T H E C H E M I C A L R E C O R D
Abstract: In the last decade, manganese(II) complexes with N-donor tetradentate aminopyridine
ligands emerged as efficient catalysts of enantioselective epoxidation of olefins and direct selective
oxidation of CÀH groups in complex organic molecules, with environmentally benign oxidant
hydrogen peroxide. In this personal account, we summarize the progress of these catalysts with
regard to ligands design, structure-reactivity correlations, evaluation of the substrate scope, as
well as mechanistic studies, shedding light on the nature of active sites and the mechanisms of
selective oxygenations. Several practically promising catalytic syntheses with the aid of Mn
aminopyridine catalysts are exemplified.
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Keywords: asymmetric epoxidation, CÀH oxidation, hydrogen peroxide, manganese, mecha-
nism
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1. Introduction
101-102 times higher efficiencies than Mn-salen catalysts.[1c,l,m]
14 Selective oxygenation of organic substrates is a challenging
15 problem of modern chemistry.[1a] Such reactions could give
16 rich opportunities for functionality and complexity upbuild-
17 ing within a molecular structure in few synthesis steps. One
18 of these transformations is enantioselective epoxidation of
19 olefins, providing straightforward route to structurally diverse
20 optically active epoxides, including precursors to chiral
21 natural products or their bioactive synthetic analogues, as well
22 as approved drugs.[1b] Transition metal catalysis is regarded as
23 the most efficient and powerful method for the asymmetric
24 epoxidation reactions.[1c–e] Manganese salen based catalyst
25 systems, proposed in early 1990s by the groups of Katsuki[1f]
26 and of Jacobsen,[1g] have so far been the brightest example of
27 catalyst systems for the asymmetric epoxidation of unfunc-
28 tionalized olefins. The major drawbacks of Mn salen catalysts
29 have been low efficiency (typically 20–50 turnovers, TN),
30 and environmentally unfriendly oxidants (hypochlorites, or
31 m-chloroperoxybenzoic acid). Shul’pin with co-workers con-
32 tributed a family of catalysts, based on dinuclear manganese
33 complexes with 1,4,7-triazacyclononane derived ligands,
34 capable of conducting the oxidation of olefins, alkanes, and
35 alcohols with H2O2,[1h–k] sometimes with small ee.[1k]The last
36 decade has given birth to a new generation of manganese
37 based catalysts, relying on aminopyridine ligands, that have
38 exhibited excellent chemo- and stereoselectivity in the
39 asymmetric epoxidation of olefins with “green” H2O2, and
40
Moreover, it has been found that Mn aminopyridine
complexes are capable of catalyzing the selective oxidation of
non-activated aliphatic[1n] and benzylic[1o] C-H groups with
H2O2; very recently, the possibility to conduct these trans-
formations in enantioselective fashion has been demonstra-
ted.[1p,q] Reactions of this type hold a potential for the
paradigm shift in the syntheses of complex organic molecules,
taking into consideration the introduction of oxidized
functionality at late steps of multistep syntheses.[1r]
The present account is focused on the progress of
manganese aminopyridine and related complexes as catalysts
for enantioselective epoxidations and chemo- and stereo-
selective CÀH oxidations with hydrogen peroxide. Basically,
the contributions of groups of Costas, of Sun, and others will
be overviewed; howbeit, in accordance with the Personal
Account format, our own data on both the catalytic perform-
ance and mechanisms will be given the major attention.
2. Asymmetric Epoxidation of Olefins on Mn(II)
Aminopyridine Complexes
In 2003, Stack and co-workers reported that manganese
complexes (R,R)-Mn-bpmcn (1a) and Mn-bpmen (2) (Fig-
ure 1) can efficiently catalyze the epoxidation of terminal
alkenes with peracetic acid.[2a] The C2-symmetrical N-donor
tetradentate ligands had good potential of structural mod-
ifications at both the diamine backbone and the pyridyl
fragments. Stack did not consider asymmetric epoxidations in
the presence of this class of complexes; the first step in this
direction was done by Costas with co-workers who demon-
strated that complexes 3a and 3b with pinene appended
ligands (Figure 1) catalyzed the epoxidation of several olefins
by peracetic acid and iodosylbenzene with up to 46% ee,
performing up to 200 TN.[2b] Sun with co-workers reported
that complexes 4a-4c (Figure 1) (1 mol. % loadings) medi-
ated the epoxidation of a,b-unsaturated ketones (with up to
89% ee, Table 1) and unfunctionalized alkenes (with up to
41
[a] Dr. R. V. Ottenbacher, Prof. Dr. E. P. Talsi,
Prof. Dr. K. P. Bryliakov
Chemistry Department
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Novosibirsk State University
Pirogova 2, Novosibirsk 630090 (Russia)
E-mail: ottenbacher@catalysis.ru
bryliako@catalysis.ru
[b] Dr. R. V. Ottenbacher, Prof. Dr. E. P. Talsi,
Prof. Dr. K. P. Bryliakov
Boreskov Institute of Catalysis
Pr. Lavrentieva 5, Novosibirsk 630090 (Russia)
Supporting information for this article is available on the WWW
under https://doi.org/10.1002/tcr.201700032
Chem. Rec. 2017, 17, 1–14
© 2017 The Chemical Society of Japan & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Wiley Online Library
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P e r s o n a l A c c o u n t
T H E C H E M I C A L R E C O R D
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46% ee) with H2O2 (6 equiv. vs. substrate) in the presence
of acetic acid additive (5 equiv.). More recently, Abdi with co-
workers reported structurally related complex 4d (Figure 1),
which was examined in epoxidations of different olefins with
hydrogen peroxide and demonstrated high efficiencies (800-
1000 TN) and moderate to good enantioselectivies (43-88%
ee).[2d]
complexes 10a and 10b.[2j] These catalysts furnished epoxides
with moderate enantioselectivity (66-73% ee, Table 1) in
most cases, and performed up to 1000 turnovers, consuming
1.2 moles of H2O2 per mole of olefin.
It is important to note that all Mn-aminopyridine
catalyzed epoxidations with H2O2 described above used acetic
acid as additive. We discovered that the use of more sterically
demanding carboxylic acid instead of acetic acid resulted in
remarkable improvement of enantioselectivity of Mn-pdp
catalyzed epoxidations.[3a] The enantioselectivity increased
with rising steric demand of the carboxylic acid, branched at
the a-carbon; 2-ethylhexanoic acid (EHA) has demonstrated
the best performance. These observations demonstrated that
the active oxidizing species incorporated the carboxylic acid
moiety, providing the simple and efficient approach to tuning
the catalysts enantioselectivity.
An alternative approach has been developed by Costas
and co-workers who contributed a series of Mn complexes
(11a and 11b) bearing electron donating substituents at
pyridine units of the ligand.[3b] Catalyst 11b (at 0.5 mol. %
loading) mediated the epoxidation of olefins with high
enantioselectivity (up to 98% ee) with H2O2 (2 equiv.) and
EHA as the additive. Catalyst 11a was shown to be active
toward diastereoselective b-epoxidation of unsaturated ster-
oids.[3b] We reported a novel catalyst (S,S)-Mn-dpn (11c,
Figure 1), which efficiently catalyzed the enantioselective
epoxidation of various electron-deficient olefins with up to
99% ee, performing 200–9000 turnovers (Table 2).[3c] More-
Our group contributed a novel 2,2’-bipyrrolidine based
complex Mn-pdp (5) (Figure 1) as an efficient catalyst for
10 asymmetric epoxidations with peracetic acid and hydrogen
11 peroxide.[2e] Mn-pdp was capable of yielding epoxides,
12 performing typically 1000 TN, and demonstrating moderate
13 to good enantioselection (up to 89% ee). In reactions with
14 H2O2, the oxidant was used in very small excess – 1.3
15 equivalents vs. olefin, which is more fruitful from the
16 practical perspective; this protocol was adopted from the
17 paper of Costas and co-workers who used it in epoxidations
18 with achiral Mn complex[2f]
Sun with co-workers designed a family of Mn(II)
.
19
20 complexes (6[2g], 7[2h] and 8[2i], Figure 1) bearing tetradentate
21 N-donor ligands with benzimidazole moieties instead of
22 pyridine ones, with various chiral backbones. It was found
23 that 6 catalyzed the epoxidation of substituted chalcones with
24 high enantioselectivity (up to 95% ee, Table 1), performing
25 300–500 turnovers. Pyridine-containing complex with (L)-
26 proline derived backbone (9[2g]) was also tested but demon-
27 strated moderate results (68% ee in chalcone epoxidation).
28 Costas with co-workers designed pinene-appended Mn-pdp
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30
Roman Ottenbacher is a research scientist
at the Boreskov Institute of Catalysis
(Novosibirsk, Russia). He was born in
Ust-Kamenogorsk (USSR) in 1989. In
2011 he graduated from Novosibirsk State
University. Roman Ottenbacher obtained
a Cand. Chem. Sci. degree (Ph.D., 2014)
in chemical kinetics and catalysis from the
Boreskov Institute of Catalysis (Novosi-
birsk). His research interests comprise
asymmetric catalysis and selective oxida-
tion reactions catalyzed by transition metal
complexes.
birsk), where he currently leads the Labo-
ratory of the Mechanistic Studies of
Catalytic Reactions. His research interests
encompass NMR and EPR spectroscopic
characterization of key intermediates of
homogeneous transition metal catalyzed
oxidations and polymerizations.
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Konstantin Bryliakov was born in Yoshkar-
Ola (USSR) in 1977. He obtained Cand.
Chem. Sci. degree (Ph.D.) in chemical
physics from the Institute of Chemical
Kinetics and Combustion (Novosibirsk) in
2001, and Dr. Chem. Sci. degree in
catalysis from the Boreskov Institute of
Catalysis (Novosibirsk) in 2008. Now he
is a leading research scientist at the
Boreskov Institute of Catalysis. His re-
search interests include selective transition
metal catalyzed oxidations and single-site
olefin polymerizations, and mechanistic
aspects of those reactions.
Evgenii Talsi was born in Ufa (USSR) in
1954. In 1983, he obtained a Cand.
Chem. Sci. degree (Ph.D.) in chemical
physics from the Institute of Chemical
Kinetics and Combustion (Novosibirsk).
In 1991, Evgenii Talsi obtained a Dr.
Chem. Sci. degree in catalysis from the
Boreskov Institute of Catalysis (Novosi-
Chem. Rec. 2017, 17, 1–14
© 2017 The Chemical Society of Japan & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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P e r s o n a l A c c o u n t
T H E C H E M I C A L R E C O R D
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Table 1. Asymmetric epoxidation of (E)-chalcone with H2O2 in
the presence of Mn aminopyridine complexes.
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4
5
6
Catalyst (load-
ing, mol. %)
T, Additive
8C
Epoxide
yield, %
ee, %
(config.)
Ref.
7
8
9
4c (1.0)
4d (0.1)
5 (0.1)
5 (0.1)
10b (0.1)
6 (0.2)
r.t. AcOH
91
93
98
97
93
93
93
92
95
100
100
78
86
78
93
72
92
91
90
96
95
98
93
97
32
2c
2d
2e
3a
2j
2 g
2 h
2i
3b
3c
3c
3d
3e
3 g
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
0
AcOH
-30 AcOH
-30 EHA a
-40 AcOH
-20 AcOH
-20 AcOH
-30 EHA a
-30 EHA a
-30 EHA a
-30 EHA a
7 (0.5)
8 (0.6)
11b (0.5)
11a (0.1)
11c (0.2)
12a (1.0)
12d (1.0)
13a (1.0)
-30 DMBA b 95
-20 H2SO4 94
0
0
0
0
Boc-(L)- 26
proline
Boc -(L)- 100
proline
13b (1.0)
13c (0.1)
13d (0.1)
15a (0.2)
50
63
62
96
3 g
AcOH
51
27
85
this
work
this
work
3 h
AcOH
-40 AcOH
[a] 2-Ethylhexanoic acid. [b] 2,2-Dimethylbutanoic acid.
over, a series of conjugated cis-olefins was successfully
epoxidized in the presence of Mn-dpn, with good to excellent
enantioselectivities (74-97% ee, Table 2).
The NH2- and NMe2-substituted catalysts 11b–c may be
considered as the “theoretical limit” of the approach relying
on the introduction of the electron-donating substituents,
since the synthesis of complexes with ligands bearing stronger
electron-donors may be problematic. At the same time, the
broad choice of available acid additives, as well as possibilities
for the modification of the chiral backbone, or tuning the
ligands’ steric properties leave enough room for further
developments.
For example, Sun with co-workers examined chiral 1,2-
cyclohexanediamine-derived Mn complexes with NMe2 sub-
stituents at pyridine rings (12a-12c, Figure 1) as catalysts of
asymmetric epoxidation with H2O2.[3d] In the presence of 2,2-
dimethylbutanoic acid additive, the epoxidation of aliphatic
and trisubstituted alkenes proceeded with moderate enantio-
selectivities (31-64% ee), whereas the epoxidation of trans-
stilbenes, cis-alkenes and substituted styrenes gave products
with good to excellent optical yields (80-98% ee). Nam and
Sun with co-workers reported asymmetric epoxidation with
Figure 1. Mn aminopyridine complexes discussed in this work.
Chem. Rec. 2017, 17, 1–14
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P e r s o n a l A c c o u n t
T H E C H E M I C A L R E C O R D
Table 2. Enantioselective epoxidation of olefins with H2O2 in the presence of Mn–dpn catalyst.[a]
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2
3
4
5
6
7
8
9
Olefin
Catalyst loading [mol. %]
Conversion [%]
Yield [%]
ee [%]
91
Ref.
0.1
0.1
0.1
100
100
100
98
98
98
this work
this work
this work
78
74
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
0.1
0.2
100
100
100
100
99
98
3c
3c
0.5
0.5
0.2
0.5
0.5
0.5
0.1
89
91
90
91
80
88
100
87
88
90
91
80
88
98
96
96
93
89
94
96
94
3c
3c
3c
3c
3c
3c
this work
0.1
0.1
100
100
99
99
97
97
this work
this work
[a] At 238 K; [H2O2]/[substrate]/[EHA]=130:100:100 mmol, H2O2 was added during 30 min and mixture stirred for 3 h.
45 H2O2, catalyzed by complex 12d, using sulfuric acid as co-
complex 12d and graphene oxide (GO). GO was used as
additive (in reduced amounts: 14 mg of GO per 1 mmol of
substrate).[3f] The authors explained its co-catalytic activity by
the presence of oxidized (carboxylic) groups in the GO
structure, presumably playing the same role as “traditional”
carboxylic acids. The catalyst was used at 0.5% loadings,
mediating the asymmetric epoxidation of substituted chal-
46 catalytic additive.[3e] The molecule of sulfuric acid was shown
2À
47 to be important for the catalytic reaction, the SO4 anions
48 and H+ both being necessary. The catalyst showed 100–500
49 TN in the epoxidation of various olefins (unfunctionalized
50 alkenes, a,b-unsaturated ketones and esters) with moderate to
51 excellent enantioselectivities (60-98% ee). Very recently, Sun
52 and co-workers described the catalyst system based on
Chem. Rec. 2017, 17, 1–14
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P e r s o n a l A c c o u n t
T H E C H E M I C A L R E C O R D
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cones (82-99% ee), chromenes (90-92% ee), cinnamic esters
(84-93% ee), and styrenes (41-49% ee).[3f]
The atomistic-level understanding of the mechanism of
the catalytic asymmetric oxygen transfer, and of the nature of
catalytically active species is of high fundamental significance,
paving the way towards rational modifications of the ligand
structure, aimed at acquiring more efficient and selective
catalytic systems. As yet, manganese-aminopyridine based
catalyst systems lack direct spectroscopic evidence for the
active oxidizing species. At the same time, several indirect
methods have been successfully applied, providing insight
into the mechanistic landscape of Mn aminopyridine catalyst
systems.
We found that even achiral catalysts 13a and 13b
(Figure 1) can conduct olefin epoxidation enantioselectively
in the presence of optically active additive (N-Boc-(L)-
proline), affording chalcone epoxide with 32% and 50% ee,
respectively (Table 1), which represents a rare case of chiral
environment amplification.[3g] The modification of chiral
diamine backbone either with (R)-1-phenyl-1,2-diamino-
10 ethane (13c, SI) or with (R,R)-1,2-diphenyl-1,2-diamino-
11 ethane (13d, SI) resulted in inferior chalcone epoxidation
12 yields and ee’s (Table 1), compared to bipyrrolidine-based
13 Mn-pdp. Curiously, C1-symmetrical complex 13c provided
14 virtually the same level of enantioinduction as it’s C2-
15 symmetrical counterpart 13d.
Previously, isotopic 18O-labeling studies proved to be a
powerful technique in mechanistic studies of non-heme iron-
based catalyst systems for hydrocarbon oxidations.[4a,b] How-
ever, straight application of this method to Mn amino-
pyridine systems (addition of H218O to the Mn complex/
H2O2/carboxylic acid mixtures) displayed zero 18O content in
epoxide product, owing to the lost competition between
H218O and abundant carboxylic acid for the Mn coordination
site.[3c] To our delight, Mn-dpo catalyst appeared to be
capable of operating in the absence of any carboxylic acid; in
effect, styrene oxidation with H216O2 in the presence of excess
H218O yielded styrene epoxide with 35% 18O enrichment.[3c]
The oxidation of more electron deficient (and thus less
reactive) p-CF3-styrene resulted in 44% 18O content in the
epoxide. These data clearly indicate that the oxygen transfer
from the active species to the olefin competes with the
isotopic scrambling within the active species. Moreover, for
both substrates, the reaction mixture contained significant
amount of mono-18O-labeled diols (87% and 86% for
styrene and p-CF3-styrene, respectively). Doubly-labeled diols
were absent, indicating that the diols did not originate from
the hydrolytic cleavage of the initially formed epoxides. The
observed data were evidence in favor of a “water-assisted”
16
Very recently, Huang with co-workers have reported a
17 new family of Mn complexes with chiral ligands composed of
18 modified bipyrrolidine backbone with fused benzene rings
19 (namely, 1,1’-biisoindoline).[3h] Complexes 14, 15a and 15b
20 (Figure 1) have demonstrated high to excellent enantioselec-
21 tivities (94.9-99.6% ee) in the epoxidation of a number of
22 trans-cinnamic acid amide derivatives and of a,b-unsaturated
23 ketoamides with H2O2. Chalcone-type and related substrates
24 can also be epoxidized with good to high stereselectivity (82-
25 99% ee). The catalysts performed 400–500 TN, consuming
26 nearly stoichiometric amount of H2O2 (1.2 equiv.).[3h]
27
Gao with co-workers designed a family of non-amino-
28 pyridine, “porphyrin-inspired” ligands (16a–i, Figure 2), and
29 reported that their complexes with Mn(OTf)2, prepared in-
30 situ, catalyzed the enantioselective epoxidation of olefins with
31 H2O2 in up to 99% optical yield.[3i,j] The catalyst system
32 demonstrated superior results in the epoxidations of 2,2-
33 dimethylchromene derivatives (up to 99% ee), cis-olefins
34 (93% ee for indene and 96% ee for 1,2-dihydronaphthalene),
35 trans-stilbene (97% ee).[3j] At the same time, other trans- and
mechanism of olefins oxidation, analogous to the one
[4a]
´
36 tri-substituted olefins were converted to corresponding
described by Mas-Balleste and Que for similar iron-based
37 epoxides with much poorer enantioselection (51-67% ee).[3j]
systems (Scheme 1).
38 The catalyst loadings were as high as 0.5-1.0 mol. %; the
39 system required carboxylic acid additive (acetic, 1-adamanta-
40 noic or cyclohexanoic), and 2–4 equivalents of H2O2 with
41 respect to substrate.
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43
44
45
46
47
48
49
According to this mechanism, the initial Mn(II) pre-
catalyst is oxidized to Mn(III)-OOH species, which then
converts to the MnV =O(OH) complex through peroxide
bond heterolysis, assisted with a molecule of 18O-labeled
water. In effect, the resulting MnV =O(OH) species contains
one labeled oxygen atom (hydroxo group) and one non-
labeled oxygen atom (oxo group). The hydrogen atom can
exchange between them by virtue of the oxo-hydroxo
tautomerism. At the final step, the terminal oxygen atom is
transferred to the olefinic double bond to form partially 18O-
labeled (<50% 18O) epoxide. Alternatively, both oxygen
atoms can be transferred to the olefin, affording singly-labeled
diol.
50
51
52
Figure 2. “Porphyrin-inspired” ligands for in-situ prepared Mn complexes.
In the presence of carboxylic acid additive, the reaction
mechanism is shifted towards the so-called “carboxylic acid
Chem. Rec. 2017, 17, 1–14
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P e r s o n a l A c c o u n t
T H E C H E M I C A L R E C O R D
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2
3
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level of asymmetric induction. In the presence of carboxylic
acid additives, formation of diols has not been observed.
Competitive epoxidations of para-substituted chalcones,
catalyzed by Mn-pdp, confirmed the electrophilic nature of
the active species. Substrates with electron-withdrawing
groups reacted slower but with higher enantioselectivity.[3c]
The linear Brown-Okamoto correlation between logarithm of
epoxide formation rates and the substituents constants s+
p
9
was documented, indicating formation of electron-deficient
transition state. The observed slope 1+ of -1.51 is typical for
olefin epoxidations with oxo-metal species proceeding via the
electron-transfer mechanism of the rate-limiting stage,
followed by the formation of acyclic, presumably carboca-
tionic intermediate. Supplementary proof for the formation
of acyclic intermediates was gained in the epoxidation of
epimerization-prone cis-stilbene: catalysts Mn-pdp and Mn-
dpo afforded 5% and 7% of trans-epoxide, respectively.[3c]
The most likely reason for this is the existence of the relatively
short-lived carbocationic intermediate, which can either
directly collapse to afford cis-epoxide or could undergo
rotation around the single CÀC bond, subsequent ring
closure yielding trans-epoxide.[3c]
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11
12
13
14
15
16
17
18
19
20
21
22
23
24
Scheme 1. Proposed “water-assisted” epoxidation mechanism in the presence
of Mn aminopyridine catalysts and H218O water.[3c] 18O atoms are in red.
25 assisted” pathway[3c] (Scheme 2), previously invoked in
3. Direct Selective Oxidation of C-H Groups on
Mn(II) Aminopyridine Complexes
26 analogous non-heme iron catalyst systems.[4a] This mechanism
27 assumes the formation of manganese(V)-oxo species through
28 OÀO bond heterolysis, facilitated by the carboxylic acid
The first report on Mn-aminopyridine catalyzed CÀH
oxidation, using H2O2 as the oxygen source, came from
Costas and co-workers who found that achiral complex 17
(Figure 3) performed 8 turnovers in the oxidation of cis-1,2-
dimethylcyclohexane (cis-DMCH) with H2O2.[5a] We ob-
served much higher catalytic efficiencies (up to 970 TN) of
complexes 1b, 5 and 9 (Figure 1) in the oxidations of various
aliphatic substrates containing secondary or tertiary CÀH
groups, in the presence of acetic acid additive.[1n] Cis-DMCH
was oxidized to the corresponding tertiary alcohol with high
selectivity (up to 96%) and excellent stereospecificity (>
99% retention of configuration, RC). Oxidation of excess of
cyclohexane showed alcohol/ketone ratios of 4.9-5.1, while
the oxidation of adamantane proceeded with high selectivity
29 molecule. In effect, the carboxylate moiety remains incorpo-
30 rated into the structure of active species and thus affects the
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
Scheme 2. Proposed “carboxylic acid-assisted” epoxidation mechanism in the
presence of Mn aminopyridine catalysts.[3c]
.
Figure 3. Some Mn aminopyridine complexes used in CÀH oxidations.
Chem. Rec. 2017, 17, 1–14
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for tertiary CÀH groups (38/28=40-49). These data clearly
pointed to a metal-based oxidation pathway rather than to a
radical-driven one. Sun with co-workers reported catalyst 18
(Figure 3) to promote benzylic and aliphatic CÀH oxidations
with high selectivity to tertiary CÀH groups (38/28=34)
rather similar to that of 1b, 5 and 9 (see above).[1o] Catalyst
18 conducted up to 164 TN; in benzylic oxidations,
formation of benzyl acetate by-products was detected.
We studied the catalytic activity of complexes 11a and
10 11e in benzylic oxidations by H2O2 in the presence of acetic
11 acid.[5b] The oxidation of cumenes proceeded preferentially to
12 the corresponding alcohols (with cumyl acetate and acetophe-
13 none as minor by-products), the catalysts performing up to
14 760 TN. The oxidation mechanism was explored.[5b] Normal
15 primary KIE (kH/kD =3.5-3.9) for the oxidation of cumene/
16 a-D-cumene was indication that the hydrogen atom lays on
17 the reaction coordinate, participating in C–H bond breaking
18 step. Linear Brown-Okamoto correlation for the oxidation of
19 a series of para-substituted cumenes was observed, with 1+ of
20 -1.0 providing evidence for electrophilic active species, and
21 electron-deficient transition state. Linear correlation between
22 the C–H bond dissociation energies (BDEs) of the benzylic
23 substrates and the logarithms of their oxidation rates implied
24 a common (e.g. hydrogen atom transfer, HAT) oxidation
25 mechanism. Oxidations of cis-DMCH led to partial loss of
26 stereochemistry with the rise of temperature (RC>99% at
27 0 8C and RC=94.3% at +30 8C), reflecting the formation
28 of short-lived (possibly radical) intermediate. In the absence
29 of carboxylic acid additive, the oxidation of cis-DMCH and
30 adamantane with H216O2 in the presence of excess of H218O
31 resulted in the formation of partially labeled (up to 27% 18O)
32 alcohols, witnessing the viability of the “water-assisted”
33 mechanism for the formation of active, presumably oxo-
34 manganese(V) species. In the presence of carboxylic acid
35 additive, the “carboxylic acid-assisted” pathway (similar to
36 that in Scheme 2) was the case. The data obtained were
37 consistent with the rebound mechanism (Scheme 3, left),
38 earlier postulated for the catalytic cycle of cytochrome
39 P450.[5b]
Scheme 3. Proposed “bifurcated rebound mechanism” mechanism of CÀH
oxidations with H218O2 in the presence of Mn aminopyridine catalysts.[5b,c]
18O atoms are in red.
ester is formed as by-product of the catalytic reaction, the
ratio of the alcohol and acetate formation being dependent
on the relative rates of the OH and OC(O)R rebound, that
follow the hydrogen-atom or hydride abstraction step (in
Scheme 3, the radical, hydrogen-atom transfer mechanism is
shown).[5c] To identify this new effect, the term “bifurcated
rebound mechanism” was proposed.[5c] We notice that the
formation of the ester byproduct can be suppressed by using
carboxylic acids bulkier than acetic acid, that are less prone to
bifurcated rebound.[5c]
Malik with co-workers carried out a research of 6 different
catalyst systems for CÀH oxidations.[5d] Mn-pdp demon-
strated high efficiency and good chemoselectivity in the
oxidation of a series of p-substituted cumenes bearing
electron-withdrawing groups to the corresponding cumyl
alcohols.[5d]
40
The origin of the acetate by-products in benzylic
41 oxidations has been a matter of separate study.[5c] Tentative
42 assumption that the ester stems from the alcohol and the
43 abundant carboxylic acid[5b] was ruled out, since in a separate
44 experiment, cumyl alcohol did not react with acetic acid
45 under the standard catalytic conditions. Besides, the oxidation
46 of cumene with 70% 18O-enriched hydrogen peroxide in the
47 presence of 16O-acetic acid, catalyzed by 11c, 11e or 13b,
48 resulted in>60% 18O enrichment of the resulting alcohol,
49 while the acetate contained exclusively non-labeled oxygen
50 atoms. This indicates that the common Fischer esterification
51 mechanism does not operate the formation of acetate in Mn
52 aminopyridine catalyzed CÀH oxidations. Most plausibly, the
The enantioselective oxidation of prochiral CÀH groups
is a particular challenge for bioinspired oxidation catalysis,
holding
a potential of revolutionizing fine synthetic
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chemistry. Until 2017, the water-soluble porphyrin, reported
by Simonneaux with co-workers, was the only Mn-based
catalyst, capable of conducting the enantioselective CÀH
hydroxylations (of alkyl arenes) with H2O2.[5e] In water-
methanol solutions, ethylbenzene and its derivatives con-
verted to 1-Ph-ethanols in 32–57% ee, with up to 40 TN
performed by the catalyst.
Manganese aminobenzimidazole complexes were also
examined as catalysts for the oxidation of secondary alcohols
10 to ketones. Sun with co-workers reported complex 18 to
11 catalyze the oxidation of a series of benzylic and aliphatic
12 secondary alcohols with H2O2 in the presence of acetic
13 acid.[5f] The catalysts exhibited 500–4700 TN with moderate
14 to good chemoselectivity.[5f]
Scheme 5. Enantioselective CÀH hydroxylation of ethylbenzenes and oxida-
tive kinetic resolution of 1-arylalkanols in the presence of Mn aminopyridine
catalyst.[1q,s]
.
15
Gao with co-workers applied in-situ prepared “porphyrin-
16 inspired” complex (ligand 16d/Mn(OTf)2) in the catalytic
17 oxidation of a variety of benzylic, allylic, propargylic, and
18 aliphatic alcohols to the corresponding aldehydes or keto-
19 nes.[5g]
by stereoconvergent oxidative kinetic resolution of the
resulting benzylic alcohols.
20
Bietti and Costas proposed a desymmetriazation-based
The kinetic resolution reaction (Scheme 5) was studied in
detail.[1q] In the presence of EHA, there was no resolution in
1-Ph-ethanol oxidation, while in the absence of any acid, the
chiral recognition was the best, the krel approaching 7.2 and
8.8 for the resolution of 1-phenylethanol and 1-(p-methyl-
phenyl)ethanol, respectively (Scheme 5).
21 approach to circumvent the problem of direct enantioselective
22 hydroxylation of prochiral CÀH groups. In the substrates
23 they considered (monosubstituted cyclohexane derivatives,
24 Scheme 4), the prochiral center did not coincide with the
25 reaction center, the latter being oxidized to the ketone
26 without stopping at the first, hydroxylation, step. In effect,
27 Mn aminopyridine complexes 19 and 20 (Figure 3), showed
The mechanistic study provided evidence for an electro-
philic active oxidant (Hammett 1 value of À1.2); the
observed primary KIE (kH/kD =2.2) for the oxidation of 1-D-
phenylethanol-1 indicating rate-limiting CÀH abstraction
step. The oxidation of ethers of 1-aryl ethanols in the
presence of catalyst 21 afforded the acetophenones as well,
thereby witnessing the CÀH abstraction / oxygen-rebound /
gem-diol dehydration mechanism of ketone formation. In the
oxidations of the relatively small alcohols (1-phenylethanol
and p-methylphenyl-1-ethanol), the observed krel substantially
(from 4 to 7.2 and from 5.5 to 8.8, respectively) enhanced in
the course of the reaction, in parallel with increasing (S)-/(R)-
substrate ratio, pointing to participation of the benzylic
alcohol in the kinetic resolution as a chiral auxiliary ligand. In
effect, the formation of two diastereomeric active sites,
exhibiting different alcohol stereoselectivity, resulted in
monotonous increase of the observed krel. This dynamic non-
linear effect (NLE) is related to the previously described
asymmetric autocatalysis and asymmetric autoinduction phe-
nomena, yet not identical with either of those.[1q] To identify
this effect, the term asymmetric autoamplification (chiral
autoamplification) has been proposed.
28 51–53% yields and 43–44% ee in t-butylcyclohexane
29 ketonization at the 3rd position.[1p] The best substrates for
30 these catalysts were found to be amide-substituted cyclo-
31 hexanes (Scheme 4), affording ketones with good yields (up
32 to 85%) and good to high ee’s (62-96%).[1p]
33
34
35
36
37
38
39
40
Scheme 4. Enantioselective ketonization of cyclohexane derivatives in the
presence of Mn aminopyridine catalysts.[5f]
.
41
42
43
44
Very recently, the first Mn-aminopyridine catalyzed
45 asymmetric CÀH hydroxylation has been reported. In
46 particular, complex 21 has been found to conduct the CÀH
47 hydroxylation of ethylbenzene and p-ethyltoluene in 41 and
48 50% ee, respectively, in the presence of EHA,[1q] and in 76
49 and 86% ee, respectively, in the presence of N-Boc-(L)-
50 proline,[1s] performing up to>500 turnovers (Scheme 5).[1q]
51 In the presence of acetic acid, the oxidation was accompanied
52
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4. Intramolecular Competitive Epoxidation / C-H
Oxidation Reactions Catalyzed by Mn(II)
Aminopyridine Complexes
allylic alcohols were oxidized to corresponding aldehydes or
ketones with 34–99% yields.[5g]
In this work, we have studied the oxidation of allylic
alcohols. Cinnamic alcohol has been oxidized to the
corresponding epoxy alcohol in the presence of 11a with
96% GC yield (Scheme 6, see SI for details). The oxidation
of b-methyl-cinnamic alcohol, mediated by 11d, has also
proceeded towards predominant formation of epoxide (82%
yield (SI), Scheme 6). Contrariwise, the oxidation of a-
vinylbenzyl alcohol on 11d afforded enone as the major
product (69% yield (SI), Scheme 6). These data demonstrate
that Mn aminopyridine complexes can be tuned towards
either the preferential epoxidation or to CÀH oxidation of
substrates with olefinic and alcohol functionalities.
The oxidation chemoselectivity is very important character-
istic of catalyst systems, especially for substrates containing
more than one functional group prone to oxidation. The
readily available models for such compounds are allylic
alcohols, that can be oxidized either to the corresponding
10 epoxides or to a,b-unsaturated ketones (aldehydes). Some
11 examples of oxidation of these substrates have been reported
12 in the literature for Mn aminopyridine and related catalysts.
13 Sun with co-workers reported the oxidation of a-vinylbenzyl
14 alcohol on catalyst 18 (0.2 mol. %) with H2O2 (1.5 equiv. vs.
15 substrate) in the presence of acetic acid (Scheme 6).[5f] The
16 reaction yielded enone (43%) and over-oxidation product
17 epoxyketone (30%).[5f] Gao with co-workers examined the
18 oxidation of cinnamic alcohol over a series of in-situ prepared
5. Examples of Preparative Syntheses with the Aid
of Mn(II) Aminopyridine Complexes
19 “porphyrin-inspired” complexes (16a-h, Figure 2) with differ-
20 ent carboxylic acid additives.[5g] The substrate was selectively
So far, there have been several reports on syntheses of valuable
chemical products with the aid of Mn aminopyridine
catalysts. Some examples are listed below.
21 oxidized to cinnamaldehyde (in up to 86% yield, Scheme 6)
22 in the presence of 16e/Mn(OTf)2 (1 mol. %) and 1-
23 adamantanecarboxylic acid (50 mol. %). High excess of
24 oxidant (6 equiv. of H2O2) was required. A number of other
Sun with co-workers demonstrated that complex 6 can be
used to obtain epoxide 22 (Scheme 7) with rather high
selectivity (22:23=7:1);[2g] the latter may serve as the side
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
Scheme 6. Examples of synthetic applications of Mn catalysts for enantiose-
51
52
lective epoxidations. Isolated yields are given for each stage. ACA – 1-
adamantanecarboxylic acid.
Scheme 7. Examples of synthetic applications of Mn catalysts for enantiose-
lective epoxidations. Isolated yields are given for each stage.
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chain of some biologically active compounds: epoxomicin
(potent in vivo anti-tumor and anti-inflammatory agent)[2g]
and carfilzomib (irreversible proteasome inhibitor).[2g]
Generally, chiral epoxides are not the final products of the
multistep catalytic syntheses; instead, they are considered as
valuable chiral precursors in the syntheses of complex
optically active organic structures. Within this paradigm, we
have obtained 3 biologically active products starting from
readily available cis-olefins, subjected to enantioselective
10 epoxidation in the presence of Mn aminopyridine complexes
11 (Scheme 7). Cis-b-methyl-styrene was converted to corre-
12 sponding epoxide in 95% yield and 86% ee using 11e and in
13 97% yield and 91% ee using 11d as catalyst. Further ring
14 opening with methylamine, followed by workup and crystal-
15 lization, afforded both enantiomers of nasal decongestant
16 pseudoephedrine 24 with 89% and 98% ee, respectively (SI).
17 The epoxidation of 2,2-dimethylchromene-6-carbonitrile,
18 catalyzed by 11d, afforded the corresponding epoxide in
19 nearly quantitative yield and extremely high ee (99.7 % (SI),
20 Scheme 7). It was further reacted with 2-pyrrolidone in the
21 presence of NaH, to furnish (3S,4R)-25, known as anti-
22 hypertensive agent levcromakalim.[6a] The epoxidation of tert-
23 butyl cis-cinnamate was also highly enantioselective: 97% ee
24 (SI), which is apparently the highest reported epoxidation
25 enantioselectivity for this substrate. Subsequent transforma-
26 tions afforded optically pure Taxol side chain acid (2R,3S)-26
27 (component of anti-tumor drug) with 99% ee (Scheme 7).[6b]
Scheme 8. Examples of synthetic applications of Mn catalysts for selective
CÀH oxidation.
28
Several preparative CÀH oxidations in the presence of
29 Mn aminopyridine complexes have been reported
30 (Scheme 8). Natural (-)-menthyl acetate was oxidized to
31 product 27 with high selectivity on catalyst Mn-pdp.[1n]
32 (+)-Sclareolide was obtained from (-)-ambroxide via CÀH
33 oxidation with H2O2, catalyzed by Mn-dpo.[5b] The latter
34 catalyst was also capable of mediating the conversion of
35 simple tetrahydrofuran molecule to g-butyrolactone, the
36 direct precursor of sodium oxibate, used to treat excessive
37 daytime sleepiness.[6c]
and 2-fold excess of H2O2: in this case, benzaldehyde was
formed in 59% yield with 99.7% selectivity. Taking into
account the very small difference between the CÀH BDE of
benzyl alcohol (87.5 kcal/mol)[7] and benzaldehyde
(88.7 kcal/mol),[7] such benzaldehyde selectivity is remarkable
and may hold considerable practical promise.
38
The authors have a feeling that the high efficiency and
6. Conclusions and Outlook
39 selectivity of currently available Mn aminopyridine catalysts,
40 in combination with the cheap oxidant, may well justify their
41 use in the “general-purpose” CÀH oxidations, rather than in
42 the syntheses of high-added-value complex molecules only. To
43 illustrate this, in this work we have tested the selective
44 oxidation of benzyl alcohol with H2O2 in the presence of a
45 series of Mn catalysts (5, 11a, 11d and 21, see Table S4 in
46 the SI). The best conversions and selectivities (Scheme 8)
47 have been achieved under inert atmosphere with catalyst 11d
48 (0.1 mol. % loading). When EHA was used as additive
49 (1 equiv. vs. substrate) with small excess of H2O2 (1.3 equiv.)
50 the yield of benzaldehyde was 57% (PhCHO/PhCOOH~
51 2.5). However, much better selectivity has been documented
52 when using 2,2-dimethylbutanoic acid (2,2-DMBA) additive
Manganese aminopyridine and structurally related catalysts of
enantioselective epoxidation and selective CÀH oxidation
reactions have been one of the brightest recent developments
of transition metal catalyzed oxidation chemistry. The unique
combination of the catalytic properties of the Mn amino-
pyridine complexes, i.e. (1) the versatility, (2) the high
chemo- and stereoselectivities they demonstrate (up 99% ee
for epoxidations, and up to >99% RC or up to 96% ee for
CÀH oxidations), (3) the high catalytic efficiencies (up to
1000–9000 TN), and (4) the high activity (the reactions are
complete within a few hours at 0.01-0.1 mol. % catalyst
loadings at À30…0 8C) has been unprecedented in the
asymmetric oxidation catalysis. In addition, the cheap and
Chem. Rec. 2017, 17, 1–14
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environmentally benign oxidant hydrogen peroxide should be
mentioned, which may be used in virtually substoichiometric
amounts. In recent years, the substrate scope of these catalyst
systems continues broadening, in parallel with the design of
novel ligand architectures and fine tuning catalysts for
different applications. Altogether, this brings Mn amino-
pyridine and related catalysts on the top of current catalytic
asymmetric oxidation catalysis.
Acknowledgements
The authors are grateful to the Russian Science Foundation
for the financial support (grant 17-13-01117), and thank Dr.
M. V. Shashkov for the GC-MS measurements. R.V.O.
acknowledges the Russian Foundation for Basic Research
(grant 16-33-00014), which support was used to obtain some
so far unpublished experimental results (SI), surveyed in the
present review.
In addition, Mn aminopyridine catalysts have appeared
10 extremely fruitful in theoretical respect, providing several
11 examples of novel or rare catalytic effects. The mechanistic
12 data available so far corroborate the involvement of electro-
13 philic, presumably manganese(V)-oxo species, capable of
14 oxidizing both olefinic groups in epoxides (via rate-limiting
15 electron-transfer) and CÀH groups in alkanes and alcohols
16 (via rate-limiting hydrogen-atom or hydride transfer). The
17 oxidation of racemic secondary benzylic alcohols on Mn
18 aminopyridine catalysts has been found occurring stereo-
19 selectively, and, remarkably, has revealed a novel non-linear
20 effect in asymmetric catalysis, exhibiting itself as progressive
21 increase of the stereoselectivity factor (krel) in line with
22 increasing ratio of the less reactive vs. more reactive alcohol
23 over the reaction course. To unambiguously identify this
24 effect, the term asymmetric autoamplification has been
25 proposed.[1q]
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26
A novel reactivity feature has been observed, leading to
27 the formation of a mixture of cumyl alcohol (major product)
28 and cumyl carboxylate (minor product) in the oxidation of
29 cumenes with catalyst systems Mn complex/H2O2/carboxylic
30 acid. This mixture of products is due to the competition
31 between the OH and OC(O)R rebound, following the rate-
32 limiting hydrogen-atom or hydride abstraction step. It is very
33 likely that such “bifurcated rebound” also accounts for the
34 formation of benzyl acetate by-product in the course of
35 ethylbenzene oxidation.[1o,5b]
36
In addition, the Mn aminopyridine based catalysts have
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38 effect which, in particular, enabled an achiral catalyst to
39 mediate olefin epoxidation in enantioselective fashion.[3g]
40
In spite of the tremendous progress of Mn aminopyridine
41 catalysts in the last years, challenging problems remain, e.g.
42 (1) the lack of direct spectroscopic evidence for the active
43 oxidizing species, (2) the lack of Mn catalysts, capable of
44 efficiently oxidizing strong CÀH bonds (with BDEs>
45 100 kcal/mol), (3) the absence of Mn catalyst systems for the
46 direct enantioselective hydroxylation of prochiral methylenic
47 sites with acceptable alcohol selectivity, (4) the lack of
48 preparative-scale synthetic procedures, etc. As it is, the
49 authors are pleased to predict that the future of Mn
50 aminopyridine catalysts may be even better than their past
51 and present, in both fundamental and practical aspects.
52
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Published online on && &&, 0000
Chem. Rec. 2017, 17, 1–14
© 2017 The Chemical Society of Japan & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Wiley Online Library
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PERSONAL ACCOUNT
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Manganese is back to the asymmetric
oxidation catalysis: in the last few years,
Mn aminopyridine and structurally
related complexes have emerged as
highly efficient and stereoselective
catalysts of asymmetric epoxidations,
CÀH oxidations, and oxidative kinetic
resolution of secondary alcohols, with
green H2O2 as the terminal oxidant.
Dr. R. V. Ottenbacher*, Prof. Dr. E. P.
Talsi, Prof. Dr. K. P. Bryliakov*
1 – 14
Chiral Manganese Aminopyridine
Complexes: the Versatile Catalysts of
Chemo- and Stereoselective Oxida-
tions with H2O2
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