Enantioselective epoxidation of olefins with H2O2 catalyzed by bioinspired aminopyridine manganese complexes

Article abstract of DOI:10.1021/acs.orglett.5b03309

A novel family of bioinspired manganese(II) complexes bearing chiral aminopyridine ligands that possessed additional aromatic groups and strong donating dimethylamino groups were synthesized and characterized. These manganese complexes exhibited efficient

Full text of DOI:10.1021/acs.orglett.5b03309

Letter
pubs.acs.org/OrgLett
Enantioselective Epoxidation of Olefins with H O Catalyzed by
2
2
Bioinspired Aminopyridine Manganese Complexes
Duyi Shen, Bin Qiu, Daqian Xu, Chengxia Miao, Chungu Xia, and Wei Sun*
†
,‡
†,‡
†
†
†
,†
†State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of
Sciences, Lanzhou 730000, P. R. China
‡
University of Chinese Academy of Sciences, Beijing 100049, P. R. China
*
S Supporting Information
ABSTRACT: A novel family of bioinspired manganese(II) complexes
bearing chiral aminopyridine ligands that possessed additional aromatic
groups and strong donating dimethylamino groups were synthesized and
characterized. These manganese complexes exhibited efficient and
improved activities in the asymmetric epoxidation of various olefins,
such as styrene derivatives (up to 93% ee) with H O as the oxidant, even
2
2
with a catalytic amount of carboxylic acid as the additive.
symmetric epoxidation (AE) is undoubtedly one of the
most significant reactions since one or even two
selectivity of this manganese catalyst for AE, one strategy was
the ingenious modifications of the ligand MCP. Hence, after
the pioneering work of Stack, versatile manganese and iron
complexes containing aminopyridine N4 ligands were reported
and proved to be active in the catalytic AE with H O as the
A
stereogenic centers are constructed, and the resulting epoxides
1
can be further transformed in the synthesis of chiral molecules.
Over the past decades, numerous efforts have been dedicated to
the catalytic AE to achieve epoxides with high enantioselectiv-
ities and yields. For example, the Ti-catalyzed epoxidation by
2
2
12
oxidant. Gratifyingly, excellent yields and ee values (up to
9% ee) had been achieved for several types of olefins, such as
chalcones, cinamates, and chromenes by combining modified
9
2
3
Sharpless, the salen−Mn(III) complexes by Jacobsen and
4
5
13
Katsuki independently, and the chiral ketones by Shi are
among the greatest breakthroughs in AE. Despite the
tremendous progress of these recognized protocols, currently,
the pursuit of efficient catalytic systems is still highly desirable
from both the atom economic and ecological viewpoints.
Notably, natural nonheme enzymes can activate oxygen to
carry out oxidation reactions under mild conditions, which may6
offer an alternative method to the sustainable goal of AE.
Inspired by these structure motifs and powerful oxidative
properties of the active sites of such metalloenzymes, many
biomimetic catalysts have been synthesized and applied to the
manganese catalysts and appropriate carboxylic acid additives.
However, there is still substantial room for improvement in
terms of simple aromatic or aliphatic olefins. For example, in
the cases of styrene and stilbene derivatives, either the ee values
14
were generally low or their derivatives were less scrutinized.
In this context, the discovery of more stereoselective
manganese catalysts is highly desirable for this class of
challenging substrates.
It was noteworthy that we previously found the ee value of
styrene oxide was increased by about 20% when introducing
aromatic rings into both of the 2-pyridylmethyl positions of
7
AE. Indeed, the bioinspired AE is an appealing method
because low catalyst loadings (as low as 0.01 mol %) and
environmentally benign oxidants such as hydrogen peroxide
11,12c
[
Mn(R,R-MCP)(OTf) ] (Scheme 1).
In addition, Costas
2
and co-workers described that the amount of carboxylic acid
could be dramatically decreased to substoichiometric levels
(
H O ) are often employed in this catalytic system. More
2 2
(
from 14 equiv to 0.35 equiv with respect to olefin substrate)
importantly, probing into the concomitant metal−oxygen
by including dimethylamino groups to the 4-position of the
intermediates in bioinspired models will also benefit the
NMe2
8
−10
PDP ligand (PDP
, PDP = 2-[[2-(1-(pyridin-2-ylmethyl)-
illumination of mechanisms in nonheme enzymes in return.
1
3a,15,16
pyrrolidin-2-yl)pyrrolidin-1-yl]methyl] pyridine).
As
The landmark studies of AE catalyzed by bioinspired
such, it was envisioned that the development of manganese
complexes bearing chiral N4 ligands that possessed aromatic
rings in 2-pyridylmethyl positions and dimethylamino groups
on pyridine rings was reasonable and feasible (Scheme 1).
Meanwhile, such new manganese complexes not only would
aminopyridine manganese complexes began at 2003 by Stack
1
1
and co-workers. They described that a manganese complex
[
Mn(R,R-MCP)(OTf) ] (MCP = N,N′-bis(2-pyridylmethyl)-
2
−
N,N′-dimethyl-trans-1,2-diaminocyclohexane, OTf = CF SO )
3
3
could rapidly catalyze the epoxidation of electron-deficient
olefins using peracetic acid (AcOOH) as the oxidant with high
turnover numbers (TONs), while providing a 10% ee for the
AE of vinyl cyclohexane. In order to enhance the stereo-
Received: November 17, 2015
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b03309
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 1. Manganese(II) Complexes with Newly Designed
Aminopyridine Ligands
Table 1. Reaction Development with Various Aminopyridine
Manganese Catalysts
a
b
entry
cat. (x)
y
GC yield (%)
ee (%)
1
2
3
4
5
6
7
8
C2a (0.1)
C2a (0.1)
C2b (0.1)
C1a (0.1)
C1a (0.05)
C1a (0.1)
C1b (0.1)
C1c (0.1)
25
500
25
25
25
20
25
25
trace
70
−
72
63
77
77
77
73
67
63
84
28
82
50
37
a
Reaction conditions: H O (30% aqueous solution, 1.3 equiv) diluted
2
2
with 0.5 mL of MeCN was delivered through a syringe pump over 1 h
to a stirred solution of catalyst (0.05−0.1 mol %), EHA (20−500 mol
retain the high efficiency of aminopyridine manganese catalysts
but also exhibit potential in enhancing enantioselectivities in
the AE of simple olefins with less carboxylic acid additives
owing to the incorporation of the two beneficial substituents.
Herein, we report the preparation, reactivity, and substrate
scope of bioinspired manganese complexes supported by a new
family of aminopyridine ligands where the electronic properties
can be tuned by the introduction of both aromatic rings and
dimethylamino groups on the ligands. The present manganese
catalysts (Figure 1) show improved catalytic activities and
enantioselectivities in the AE of styrenes, trans-stilbenes, and
aliphatic olefins, even in the presence of 25 mol % of carboxylic
acid.
%
), internal standard (decane), and substrate (0.5 mmol) in 1.0 mL of
b
MeCN at −30 °C. The ee values were determined by GC.
substituents. Both manganese complexes exhibited lower
NMe2
reactivity than that of PMCP
Mn, C1a.
In the case of nonheme iron or manganese catalysts,
carboxylic acid as a coligand would lead to an efficient
III
heterolytic O−O bond cleavage of M (OOH) (M = Fe, Mn)
9a,c,12d,e
species to form high-valent metal-oxo species.
Despite
the fact that mechanisms of epoxidation catalyzed by nonheme
iron or manganese catalysts require further elucidation, the
involvement of the carboxylate high-valent metal-oxo inter-
mediates in the catalytic epoxidation reactions have been
9
e
9c
supported experimentally and theoretically, which are
thought to be reactive epoxidizing intermediates. Bryliakov et
12e,h
15,16
al.
and Costas et al.
had also demonstrated that the
stereoselectivity of AE could be modulated by the use of
carboxylic acid. In order to achieve high enantioselectivity, a
variety of carboxylic acids were also screened. First, acetic acid
(
AA) was investigated in the AE of styrene, providing low
conversion and a moderate ee value (Figure 2). With the
Figure 1. Manganese(II) complexes used in this study.
In exploratory experiments, we selected the AE of styrene as
1
2h
the model reaction and 2-ethylhexanic acid (EHA) as the
additive to study the catalytic activity and stereoselectivity of
several well-designed manganese catalysts. As can be seen in
Table 1, almost no reaction occurred with the PMCPMn (C2a)
catalyst in the presence of 25 mol % EHA (Table 1, entry 1).
Then high conversion was observed only when the amount of
EHA was increased to 5.0 equiv (Table 1, entry 2), which was
Figure 2. Screening the carboxylic acid for the AE of styrene. H O
(30% aqueous solution, 1.3 equiv) diluted with 0.5 mL of MeCN was
2
2
12c
consistent with our previous conditions.
In the case of
delivered through syringe pump over 1 h to a stirred solution of
NMe2
NMe2
PMCP
Mn, C1a (0.1 mol %), acid additive (25 mol %), internal
MCP
Mn (C2b) catalyst, a considerable yield was observed
NMe2
standard (decane), and substrate (0.5 mmol) in 1.0 mL of MeCN at
30 °C.
with only 25 mol % EHA (Table 1, entry 3). The PMCP
Mn (C1a) catalyst, which could be considered as an integrated
one of C2a and C2b, was then tested in the AE of styrene. To
our delight, full conversion and a further improved ee value
−
increase in the steric bulk at the α-carbon of the carboxylic acid,
the ee values could be enhanced clearly (Figure 2). The use of
dimethylbutanic acid (DMBA) provided a comparable outcome
with that of EHA (Figure 2, 78% ee), and this figure could be
ascribed to the highest one in the AE of styrene in comparison
(77%) were achieved with only 25 mol % of EHA and 0.1 mol
%
of catalyst at −30 °C (Table 1, entry 4). Lowering the
catalyst loading and amount of EHA led to a decrease in
conversion while the ee values remained unchanged (Table 1,
entries 5 and 6). The series of manganese complexes with
different aromatic rings, such as 4-tBu-phenyl (BPMCP
Mn, C1b) and 1-naphthyl (1-NMCP
12,13
with other nonheme iron or manganese catalysts so far.
In
NMe2
addition, racemic ibuprofen (Ibu) showed identical ee
compared to S-Ibu. A chiral diacid, D-camphoric acid (D-
CPA), was also involved to provide a good result compared
NMe2
Mn, C1c), were also
evaluated to study the stereo- and electro-properties of the
B
DOI: 10.1021/acs.orglett.5b03309
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
with that of DMBA. The AE of styrene was efficiently
promoted by the use of Boc-protected L-proline except for
the minimally lower ee value (Figure 2; for details see Table S1
Table 2. Asymmetric Epoxidation of Various Olefins by
a
NMe2
PMCP
Mn (C1a) and H O
2 2
1
6
in SI).
After the optimized conditions were identified, the
asymmetric epoxidation of simple aromatic olefins including
styrenes and trans-stilbenes were explored using this catalytic
system. In general, the ee values of the epoxides of styrene
derivatives were enhanced with regard to previous nonheme
manganese systems. The halo atoms on the para-position of
styrene nearly had no effect on the enantioselectivity, while the
methyl group led to the decrease of the ee value (Table 2,
entries 2−4). In the case of styrene bearing a methyl group on
the ortho-position, 80% ee was observed (Table 2, entry 6). To
our delight, an excellent ee value of 93% was obtained for the
AE of cis-β-methylstyrene (Table 2, entry 7). However, the ee
value dramatically declined to 52% when there were two methyl
groups in the β-position (Table 2, entry 9). In the case of trans-
β-methylstyrene, nearly racemic epoxide was produced (Table
2, entry 8). Presently, the application of the nonheme iron or
manganese catalyst was limited by the lack of selectivity during
the epoxidation of trans-stilbenes. It was noteworthy that AE of
trans-stilbene with C1a under the same conditions provided
quite good stereoselectivity (Table 2, entry 10, 88% ee). The
reaction of ortho-Me-substituted trans-stilbene provided the
epoxide in 90% ee (Table 2, entry 13). Furthermore, the trans-
stilbenes with bulkier substituents resulted in diminished
enantioselectivity (Table 2, entry 14). cis-Stilbene was often
used as a probe substrate to simply study the reaction pathway
1
7
for epoxidation. In this work, the AE of cis-stilbene led to the
formation of two stereoisomeric epoxides and the ee value of
trans-epoxides was very low (Table 2, entry 15). It would point
to the formation of a radical intermediate that would allow C−
18
C bond rotation to give both stereoisomers. In the cases of
trisubstituted olefins, moderate to good yields as well as
improved ee values were observed (Table 2, entry 16; Table S2
in SI, entry 1). In addition, this manganese catalyst C1a also
exhibited good chemo- and enantioselectivities in the AE of
other types of olefins. For example, simple aliphatic alkenes,
which were always thought of as challenging substrates in
bioinspired catalysis, could be converted to their epoxides with
1
1a,12c
improved enantioselectivities (Table 2, entries 17−19).
As in the case of other state-of-the-art biomimetic procotol-
13a,b
s,
as expected, excellent yields and stereoselectivities for the
derivatives of chromene and chalcone were also achieved by
using this manganese complex in a low loading of DMBA
(
Table 2, entries 20; Table S2, entries 2−4).
To demonstrate the utility of this reaction, a gram-scale AE
reaction was performed. Treatment of 1.08 g of trans-stilbene
produced epoxide in a 68% isolated yield and 88% ee under
standard conditions (Scheme 2). The ee value of the epoxide
reached 93% after recrystallization. Even though the reaction
scale was magnified up to 20 times, a comparable ee value and
synthetically valuable yield could be still obtained.
a
Reaction conditions: For styrene derivatives, H O (30% aqueous
2
2
solution, 1.3 equiv) diluted with 0.5 mL of MeCN was delivered
through a syringe pump over 1 h to a stirred solution of C1a (0.1 mol
%
mmol) in 1.0 mL of MeCN in the air at −30 °C. For stilbene
derivatives, the scale of substrates was 0.3 mmol, while the solvent was
a mixture of MeCN and CH Cl (v/v = 1:1). Isolated yields were
listed in the brackets. The ee values were measured by GC and HPLC
with chiral columns. cis-Epoxides/trans-Epoxides = 7:3. Ee value of
trans-epoxides. C1a (0.4 mol %), H O (2.0 equiv), DMBA (100 mol
%
), DMBA (25 mol %), internal standard (decane), and substrate (0.5
b
In summary, we have prepared and characterized several
aminopyridine ligands and their manganese complexes that
possessed additional aromatic groups and strong electron-
donating dimethylamino groups. As expected, these manganese
complexes exhibited excellent stereoselectivities in the asym-
metric epoxidation of various olefins (e.g., styrenes, up to 93%
ee), with environmental H O as the oxidant in the catalytic
2
2
c
d
e
f
2
2
).
the reaction mechanism and development of more robust
catalytic systems are underway.
2
2
amount of the carboxylic acid partner. Further elucidation of
C
DOI: 10.1021/acs.orglett.5b03309
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
J. S.; Gu
̈
ell, M.; Ribas, X.; Luis, J. M.; Cronin, L.; Costas, M. Nat.
Scheme 2. Gram-Scale AE of trans-Stilbene Catalyzed by
NMe2
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PMCP
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5
(
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ASSOCIATED CONTENT
* Supporting Information
■
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S
2
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b03309.
2004, 6, 3119. (c) Murphy, A.; Stack, T. D. P. J. Mol. Catal. A: Chem.
2006, 251, 78.
(
12) For selected examples of AE catalyzed by bioinspired
manganese catalysts with MCP-derived ligands, see: (a) Guillemot,
Synthetic procedures for the newly designed ligands and
manganese(II) complexes, catalytic procedures, NMR,
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G.; Neuburger, M.; Pfaltz, A. Chem.−Eur. J. 2007, 13, 8960.
(b) Gomez, L.; Garcia-Bosch, I.; Company, A.; Sala, X.; Fontrodona,
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AUTHOR INFORMATION
Corresponding Author
■
*
E-mail: wsun@licp.cas.cn.
Notes
̈
(g) Garcia-Bosch, I.; Gomez, L.; Polo, A.; Ribas, X.; Costas, M. Adv.
Synth. Catal. 2012, 354, 65. (h) Lyakin, O. Y.; Ottenbacher, R. V.;
Bryliakov, K. P.; Talsi, E. P. ACS Catal. 2012, 2, 1196. (i) Maity, N. C.;
Bera, P. K.; Ghosh, D.; Abdi, S. H. R.; Kureshy, R. I.; Khan, N. H.;
Bajaj, H. C.; Suresh, E. Catal. Sci. Technol. 2014, 4, 208.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We acknowledge financial support of this work from the
National Natural Science Foundation of China (21473226 and
1133011).
■
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