Highly enantioselective epoxidation of multisubstituted enones catalyzed by non-heme iron catalysts

Article abstract of DOI:10.1002/chem.201200992

Iron(II) efficiency: The iron complexes of N₄ ligands, derived from proline and benzimidazole, exhibited an unprecedented activity and enantioselectivity for the epoxidation of a variety of di- and trisubstituted enones (see scheme). This syste

Full text of DOI:10.1002/chem.201200992

DOI: 10.1002/chem.201200992
Highly Enantioselective Epoxidation of Multisubstituted Enones Catalyzed
by Non-Heme Iron Catalysts
Bin Wang,[a, b] Shoufeng Wang,[a] Chungu Xia,[a] and Wei Sun*[a]
Asymmetric epoxidation of olefins is one of the most im-
portant transformations in organic synthesis because the re-
sulting enantiomerically enriched epoxides are highly useful
intermediates and building blocks.[1] Over the past decades,
numerous efforts have been devoted to the development of
efficient catalysts for this reaction. Many catalyst systems in-
cluding chiral metal complexes[2,3] and organocatalysts[4,5]
have been successfully established. In light of the increasing
demand for green and sustainable chemistry, the develop-
ment of iron-based catalysts with considerable enantioselec-
tivity and activity is desirable due to their low toxicity, lower
price, and the benign environmental character of iron com-
pounds.[6] In this regard, only a few successful examples
have been published on the iron-catalyzed asymmetric epox-
idation of olefins (generally >90% enantiomeric excess
(ee)).[7] In 1999, the first approach of the enantioselective
epoxidation of styrene derivatives with iron-porphyrin com-
plexes was reported.[8] Later, Beller and co-workers have de-
scribed an iron-catalyzed methodology for the asymmetric
epoxidation of stilbene derivatives with 10 mol% catalyst
loading.[9] Yamamoto et al. recently developed the iron-cata-
lyzed asymmetric epoxidation of b,b-disubstituted enones in
fins.[13] We recently reported the enantioselective epoxida-
tion of a,b-enones (up to 87% ee) using non-heme iron
complexes of mcp derivatives (mcp=N,N’-dimethyl-N,N’-
bis(2-pyridylmethyl)-cyclohexane-1,2-diamine).[14] Common
features of these tetradentate ligands are that they contain
two pyridine donors as well as two sp3 N donors.[15] On the
basis of our previous studies, we reasoned that the fine
tuning of the rigidity of the ligands and nitrogen donors
might enhance the chiral induction. Herein, we report the
elaborate design of non-heme iron catalysts for the highly
enantioselective epoxidation of a broad range of multisubsti-
tuted enones with up to 98% ee.
The C1-symmetric tetradentate N ligands (N4) consisted of
more rigid chiral diamine template derived from l-proline
and two benzimidazole donors. The ligands were readily
prepared by the direct alkylation of (S)-N-methyl-2-amino-
methylpyrrolidine with 2-chloromethyl-benzoimdazole de-
rivatives (Figure 1).[16] Iron complexes were synthesized by
the presence of 5 mol% of FeACHTNUTRGNENG(U OTf)2 and 2 equiv of chiral
phenanthroline ligand. X-ray crystallography revealed that
a pseudo-C2-symmetric iron–ligand complex was responsible
for epoxidation.[10]
Iron(II) complexes containing linear tetradentate ligands
such as mep (mep=N,N’-dimethyl-N,N’-bis(2-pyridylmethyl)
ethylene-1,2-diamine) are typical non-heme model com-
pounds.[11] These model complexes have proven to be prom-
ising catalysts for the selective oxidation of organic sub-
strates.[12] Pioneering studies by Jacobsen and co-workers
have demonstrated that iron(II)-mep complex could effi-
ciently promote epoxidation of a variety of aliphatic ole-
Figure 1. New non-heme iron(II) catalysts used in this study.
treating N4 ligands with FeCl2 in CH3CN, then the anion
could be changed to OTf (OTf=CF3SO3) by the addition of
AgOTf. Solid-state X-ray analysis of a single crystal of com-
plex C1, containing 1-ethyl-benzoimdazoles donors, revealed
that the ligand coordinated the iron center in a cis-a topolo-
À
gy (Figure 2). The Fe N bond distances between the iron
and benzimidazole donors (complex C1: 2.141(5) and
[a] B. Wang, S. Wang, Prof. Dr. C. Xia, Prof. Dr. W. Sun
State Key Laboratory for Oxo Synthesis and Selective Oxidation
Lanzhou Institute of Chemical Physics
Chinese Academy of Sciences
À
2.156(6) ꢀ, respectively) are both shorter than Fe N distan-
ces between the iron and pyridyl donors (complex C4:
2.155(7) and 2.174(8) ꢀ, respectively).[17]
Lanzhou, 730000 (P.R. China)
Fax : (+86)931-827-7088
E-mail: wsun@licp.cas.cn
The preliminary screening of the catalysts was carried out
with chalcone as a model substrate using the H2O2/AcOH
protocol.[14,18] The complex C1 containing 1-ethylbenzimida-
zole donors gave the epoxide product with 82% ee using
1.0 mol% of catalyst loading at room temperature (Table 1,
entry 1). Further lowering of the temperature to À208C did
[b] B. Wang
Graduate School of the Chinese Academy of Sciences
Beijing, 100039 (P.R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200992.
7332
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Chem. Eur. J. 2012, 18, 7332 – 7335

COMMUNICATION
is still highly desirable; in particular, the development of an
iron-catalyzed methodology would be highly advanta-
AHCTUNGTRENNUNG
geous.[5a,23] Under the above established reaction conditions,
the conformationally rigid cyclic enones were employed as
substrates to test the possibility. To our delight, the desired
trisubstituted a,b-epoxyketones with a quaternary carbon
center were obtained in good yields with excellent enantio-
selectivities (Table 2). In particular, cyclic enones derived
Table 2. Substrate scope of the enantioselective epoxidation of trisubsti-
tuted enones.[a]
Figure 2. Crystal structure of the iron(II) complex C1.
Table 1. Screening the reaction conditions.[a]
Entry
n
R
Yield
[%][b]
ee
[%][c]
1
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
Ph
Ph
90
92
92
98
89
96
82
72
86
82
99
0
91
94
91
97
98
94
87
89
95
94
97
–
2[d]
3
Entry
Cat
T
HOAc
[equiv]
H2O2
[equiv]
Yield
[%][b]
ee
o-Me-C6H4
m-Me-C6H4
p-Me-C6H4
p-Me-C6H4
o-Cl-C6H4
m-Cl-C6H4
m-Cl-C6H4
p-Cl-C6H4
p-F-C6H4
p-CF3-C6H4
Ph
[oC]
G
N
[%][c]
4
5
1[d]
2[d]
3
4
5
6
7
8
9
C1
C1
C1
C1
C1
C1
C1
C1
C2
C3
C4
RT
À20
À20
À40
À20
À20
À20
À20
À20
À20
À20
5
5
5
5
3
2
3
3
3
3
3
2
2
2
2
2
2
1.5
1.2
1.2
1.2
1.2
80
92
85
80
86
73
90
89
90
83
68
82
83
92
91
92
87
92
92
88
86
56
6[d]
7
8
9[d]
10
11
12
13
14[d]
15[e]
16[e]
78
80
80
86
85
83
91
93
10
11
Ph
p-Me-C6H4
p-F-C6H4
[a] Unless otherwise mentioned, all reactions were performed in CH3CN
(1.5 mL) with substrates (0.25 mmol) and iron(II) complex (2 mol%).
50% H2O2 was added dropwise through a syringe pump for over an hour
and then an hour of stirring was allowed. [b] Isolated product yield.
[c] Determined by HPLC analysis (Daicel chiral OD-H column).
[d] 1 mol% of iron(II) complex C1.
17
Ph
67
89
[a] Unless otherwise mentioned, reaction conditions: 1.2 equiv of 50%
H2O2 diluted with CH3CN (0.5 mL) was delivered by syringe pump over
an hour to a stirred solution of catalyst C1 (5ꢂ10À3 mmol, 2 mol%),
HOAc (0.75 mmol) and substrate (0.25 mmol) in CH3CN (1.0 mL) at
À208C under Ar atmosphere, and the mixture was stirred for an addi-
tional hour. [b] Isolated product yield. [c] Determined by HPLC analysis,
see the Supporting Information for details. [d] 2 mol% of complex C2.
[e] CH3CN (1.5 mL) and CH2Cl2 (1.0 mL) as solvent.
not lead to an increase in selectivity (Table 1, entry 2). The
enantioselectivity of product reached as high as 92% when
the catalyst loading was raised to 2.0 mol% (Table 1, entry
3). After testing the loading of acetic acid, we identified that
best results were achieved upon addition of acetic acid
(3 equiv) with respect to substrate (Table 1, entries 3, 5, and
6).[19,20] Furthermore, the loading of H2O2 was successfully
lowered to 1.2 equiv with no decrease of yield and enantio-
selectivity (Table 1, entries 7 and 8). Finally, we evaluated
the abilities of the four iron complexes of N4 ligands for the
asymmetric epoxidation under the optimized conditions. Im-
portantly, benzimidazole donors of the newly designed N4 li-
gands are essential to obtain high yield and ee values
(Table 1, entries 8–11).[21] The use of complex C4 as catalyst
(bearing pyridine donors), only resulted in a moderate yield
and ee value (Table 1, entry 11).
from a-tetralone are good candidates toward forming the
quaternary carbon center through the iron-catalyzed asym-
metric epoxidation (Table 2, entries 1–11, up to 98% ee). On
the other hand, a cyclic enone bearing a p-CF3 group was
found to be inactive for this catalyst system (Table 2,
entry 12). In addition, the cyclic enones bearing a seven-
membered ring were well tolerated, thereby providing the
a,b-epoxyketones with up to 93% ee (Table 2, entries 13–
16). Notably, the epoxidation of trisubstituted enones with
an O-heterocycle proceeded smoothly with comparable
enantioselectivity (Table 2, entry 17). The comparison ex-
periments between complexes C1 and C2 were also per-
formed. When the R group is a phenyl group or a m-Cl-
phenyl, the complex C2 leads to better enantioselectivity
Trisubstituted cyclic a,b-epoxyketones are versatile inter-
mediates in organic synthesis, due to possessing a chiral qua-
ternary carbon center.[22] The enantioselective construction
of such quaternary carbon centers bearing an oxygen atom
Chem. Eur. J. 2012, 18, 7332 – 7335
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7333

W. Sun et al.
than that of complex C1 (Table 2, entry 1 vs. 2 and 8 vs. 9).
In the case of substitution by p-Me-phenyl, a higher ee value
was observed with complex C1 as the catalyst (Table 2,
entry 5 vs. 6).
To further demonstrate the substrate scope of the non-
heme iron-catalyzed epoxidation, a series of chalcone deriv-
atives were also evaluated under optimized conditions. It
should be emphasized that the level of enantiocontrol for all
substrates was impressive (Table 3). The chalcone deriva-
mains obscure. When the asymmetric epoxidation of chal-
cone was conducted under standard conditions in the pres-
ence of a large excess of H218O (20 equiv), the 18O labeling
product was observed (see the Supporting Information).
The results suggest that high-valent Fe=O species are in-
volved as the active oxidant in the catalytic process. In addi-
tion, the role of acetic acid was also studied. Several acids
such as acetic acid, propionic acid, and HClO4, were used in
the present reaction, but only carboxylic acids promoted the
epoxidation reaction. With KOAc as an additive, no reaction
occurred. These observations may support the carboxylic
acid-assisted pathway proposed by Mas-Ballestꢃ and
Que.[19a]
Table 3. Enantioselective epoxidation of chalcone derivatives.[a]
In conclusion, we have developed a novel series of tetra-
dentate N4 ligands derived from proline and benzimidazole.
The resulting iron complexes of N4 ligands exhibited an un-
precedented activity and enantioselectivity for the epoxida-
tion of a variety of di- and trisubstituted enones. This
system, which is based on synthetic non-heme iron catalysts,
provides ready access to a wide range of epoxyketones of
high enantiomeric purity. Further studies to extend the sub-
strate scope of this non-heme system, the detailed mecha-
nism, as well as the development of even more efficient iron
catalysts are underway.
Entry
R1
R2
Yield
[%][b]
ee
[%][c,d]
1
2
3
4
5
6
7
8
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
89
90
99
96
98
93
99
97
78
96
90
78
80
90
93
93
92
91
95
94
93
92
97
94
91
87
88
80
74
93
92
90
p-Me-C6H4
m-Me-C6H4
o-Br-C6H4
m-Br-C6H4
o-Cl-C6H4
m-Cl-C6H4
p-Cl-C6H4
Ph
Ph
Ph
Ph
Ph
9
o-Cl-C6H4
p-Cl-C6H4
p-F-C6H4
p-MeO-C6H4
p-CF3-C6H4
p-Me-C6H4
p-Me-C6H4
p-Cl-C6H4
10
11
12
13
14
15
16
Acknowledgements
p-Cl-C6H4
p-F-C6H4
p-Cl-C6H4
This work was supported by the “Hundred Talents Program” of the Chi-
nese Academy of Sciences and the National Natural Science Foundation
of China (20873166, 21133011 and 21073210).
[a] Reaction conditions: 1.2 equiv of 50% H2O2 diluted with CH3CN
(0.5 mL) was delivered by syringe pump over an hour to a stirred solu-
tion of catalyst (5ꢂ10À3 mmol, 2 mol%), HOAc (0.75 mmol) and sub-
strate (0.25 mmol) in CH3CN (1.0 mL) at À208C under an Ar atmos-
phere, then the mixture was stirred for an additional hour. [b] Isolated
product yield. [c] Determined by HPLC analysis, see the Supporting In-
formation for details. [d] Absolute configurations were determined to be
(2R,3S) by comparison of the HPLC retention times with known data.
Keywords: benzimidazole · enones · epoxidation · iron ·
N4 ligands
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Highly Enantioselective Epoxidation of Multisubstituted Enones
COMMUNICATION
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Received: March 23, 2012
Published online: May 13, 2012
Chem. Eur. J. 2012, 18, 7332 – 7335
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7335