Manganese(III) complexes of novel chiral unsymmetrical BINOL-Salen ligands: Synthesis, characterization, and application in asymmetric epoxidation of olefins

Article abstract of DOI:10.1016/j.apcata.2011.12.001

Several chiral unsymmetrical and C₂-symmetric Mn(III)-BINOL-Salen complexes have been designed, synthesized and applied to the asymmetric epoxidation of non-functionalized alkenes. Experimental results show these complexes are effective in the

Full text of DOI:10.1016/j.apcata.2011.12.001

Applied Catalysis A: General 415–416 (2012) 40–46
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Applied Catalysis A: General
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Manganese(III) complexes of novel chiral unsymmetrical BINOL-Salen ligands:
Synthesis, characterization, and application in asymmetric epoxidation of olefins
Longhai Chena, Feixiang Chengc, Lei Jiab, Lei Wanga, Jian Weia, Jinfeng Zhanga, Lihui Yaoa,
Ning Tanga, Jincai Wua,∗
a Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical
Engineering, Lanzhou University, Lanzhou 730000, People’s Republic of China
b Department of Physics and Chemistry, Henan Polytechnic University, Jiaozuo 454000, People’s Republic of China
c College of Chemistry and Chemical Engineering, Qujing Normal University, Qujing 655011, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Several chiral unsymmetrical and C2-symmetric Mn(III)-BINOL-Salen complexes have been designed,
synthesized and applied to the asymmetric epoxidation of non-functionalized alkenes. Experimental
results show these complexes are effective in the catalytic asymmetric epoxidation of alkenes. The cat-
alyst 4c exhibited better enantioselectivity and reactivity than the catalysts 4b and 4a due to the steric
effect of the ligands. To understand the synergistic effect of the two different chiral centers in the cata-
lyst, the catalyst 6a has been investigated. By comparison of the enantioselectivity obtained by using 4c
and 6a, respectively, the positive experimental results have proved that the chiral stereogenic centers
in the diaminocyclohexane-derived catalysts played an important role in the current enantioselective
epoxidation. Besides, the comparison of enantioselectivity displayed by 4c and 7a further demonstrates
the significant influence through the cooperation of steric factors and chiral centers in catalyst.
© 2011 Elsevier B.V. All rights reserved.
Received 28 February 2011
Received in revised form
14 November 2011
Accepted 1 December 2011
Available online 9 December 2011
Keywords:
Chiral manganese(III) complex
Unsymmetrical BINOL-Salen
Asymmetric catalysis
Epoxidation
Olefins
1. Introduction
(e.g., catalyst immobilization on inorganic materials [15–17] and
polymers [18], catalyst functionalization by ionic liquid [19,20],
Chiral epoxid is one of the most important synthetic interme-
diates for its wide applications in chiral pharmaceuticals and fine
chemicals [1–5], and over the past three decades much attention
has been devoted to the development of new catalytic systems
for the synthesis of chiral epoxides. Among several catalysis sys-
tems reported previously, catalytic enantioselective epoxidation
of alkenes using chiral metal catalysts has proved to be the most
effective method for the synthesis of chiral epoxides [6–9]. Stim-
ulated by an important contribution from Jacobsen and Katsuki,
many other chemists have significantly advanced the catalytic
asymmetric epoxidation of unfunctionalized alkenes by introduc-
ing chiral MnIII-Salen catalysts due to their availability and wide
usefulness [10–12]. Although these chiral MnIII-Salen catalysts are
excellent catalysts for asymmetric epoxidation of alkenes with
high enantiomeric excesses, their utilization to some extent is lim-
ited by difficulties in separation of the catalyst from the resultant
epoxides as well as the recycling of the often expensive chiral
catalyst [13,14]. To address this issue, many attempts have been
made to discover the recoverable chiral MnIII-Salen complexes
and catalyst oligomers [21,22]) and highly active MnIII-Salen cat-
alysts [23] for asymmetric epoxidation of alkenes. Due to the fact
that the cooperative activation by two or more catalytic centers
with proper proximity could greatly increase the reactivity and
enantioselectivity of homogeneous chiral catalysts, the catalytic
centers is preferentially selected as one component of MnIII-Salen
complexes during our investigation on the development of novel
effective catalysts for asymmetric epoxidation of alkenes [24–26].
As well known, BINOL-Salen ligand as well as some of its metal
complexes have been approved as excellent catalysts for a series
of important asymmetric organic transformations [27–31]. Since
Katsuki first reported BINOL-Salen-based manganese complexes
for the asymmetric epoxidation of alkenes, much attention has
been devoted to expand this system [32–36]. To our knowledge,
however, there are only a few literatures concerning systematical
investigations on the effecting factors of the BINOL-Salen ligand
for the asymmetric reactions. Based on these facts, four novel
chiral unsymmetrical and one C2-symmetric BINOL-Salen ligands
derived from BINOL as well as the corresponding manganese(III)
complexes have been designed and further synthesized. The steric
effect and the synergetic effect of multi-chirality of these man-
ganese(III) catalysts have been investigated in the epoxidation
reaction. The catalytic activities of corresponding manganese(III)
∗
Corresponding author. Tel.: +86 931 8911218; fax: +86 931 8912582.
E-mail address: wujc@lzu.edu.cn (J. Wu).
0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.12.001

L. Chen et al. / Applied Catalysis A: General 415–416 (2012) 40–46
41
complexes towards asymmetric epoxidation of alkenes have been
2.2.1. Compound 3a
examined systematically with NaClO as the oxidant and PyNO as
the axial base, and the positive experimental results have proved
that five designed catalysts, especially catalyst 4c, are effective in
the current asymmetric epoxidation of alkenes. To the best of our
knowledge, this is the first report of unsymmetrical BINOL-Salen-
based manganese complexes with two different chiral centers as
catalysts for the asymmetric epoxidation of non-functionalized
alkenes.
Pale yellow solid (0.42 g, 79% yield) which was purified by flash
column chromatography using petroleum ether: ethyl acetate (4:1)
as eluent. 1H NMR (400 MHz, CDCl3): ı 8.52 (1H, s, CHN); 8.23
(1H, s, CHN); 7.88 (1H, d, J = 8.4 Hz, ArH); 7.81 (1H, d, J = 11.6 Hz,
ArH); 7.77 (1H, s, ArH); 7.73 (1H, d, J = 3.8 Hz, ArH); 7.45 (2H, d,
J = 9.2 Hz, ArH); 7.33–7.03 (7H, m, ArH); 6.89 (1H, d, J = 8.2 Hz, ArH);
6.73 (1H, t, ArH); 3.79 (3H, s, OCH3); 3.38 (1H, m, CH); 3.26 (1H,
m, CH); 2.09–1.65 (8H, m, CH2). 13C NMR (100 MHz, CDCl3): ı
169.8; 164.9; 154.5; 155.4; 154.1; 138.2; 131.4; 130.9; 129.6; 129.3;
129.1; 128.9; 128.8; 128.1; 126.4; 125.4; 124.8; 123.8; 123.6; 120.3;
118.2.; 117.7; 114.7; 73.3; 72.5; 71.8; 34.8; 31.9; 22.7; 19.2. LC-MS:
m/z 529.4 [M + H]+. Anal. calcd for C35H32N2O3: C, 79.52; H, 6.10;
N, 5.30. Found: C, 79.56; H, 6.11; N, 5.33. FT-IR (KBr): 3425, 2929,
2. Experimental
General remarks: All starting materials were obtained from com-
mercial supplier used without further purification unless otherwise
stated. 2,2-Dimethylchromene and its derivatives were synthe-
sized as described in Ref. [37]. The synthesis of 2a, 2c and 5
(Scheme 1) were carried out according to a modified procedure
[38–40]. (S)-(+)-3-Formyl-2-hydroxy-2ꢀ-methoxy-1,1ꢀ-binaphthyl
was prepared according to the literature method [41]. 1H NMR
and 13C NMR spectra were recorded on a Mercury Plus 400 spec-
trometer with TMS as internal standard. IR spectra were obtained
on a Nicolet 170SX FT-IR spectrophotometer as KBr discs. LC-MS
were performed on a Bruker Daltonics Esquire 6000 mass spec-
trometer. Elemental analyses were taken using a PerkinElmer 240C
analytical instrument. All reactions were monitored by TLC. TLC
was performed on glass plates coated with silica gel 60 F254. The
crude products were purified by flash chromatography. The enan-
tiomeric excesses of the chiral epoxides were determined by chiral
High-Performance Liquid Chromatography analysis (Daicel Chiral-
cel OJ-H and OB-H chiral column, n-hexane:i-PrOH = 90:10 (v/v),
1.0 mL/min, 254 nm) using a Waters 600 controller with 2996
pholodiode Array detector.
2842, 1731, 1633, 1455, 1376, 1257, 1191, 913, 811, 744, 654 cm−1
.
2.2.2. Compound 3b
Pale yellow solid (0.48 g, 82% yield) which was purified by flash
column chromatography using petroleum ether: ethyl acetate (6:1)
as eluent. 1H NMR (400 MHz, CDCl3): ı 13.91 (1H, s, OH); 13.01 (1H,
s, OH); 8.56 (1H, s, CHN); 8.23 (1H, s, CHN); 7.97 (1H, d, J = 8.6 Hz,
ArH); 7.86 (2H, d, J = 8.2 Hz, ArH); 7.81 (1H, s, ArH); 7.77 (1H, d,
J = 3.2 Hz, ArH); 7.45 (2H, d, J = 8.6 Hz, ArH); 7.33–6.95 (7H, m, ArH);
6.93 (2H, t, ArH); 3.68 (3H, s, OCH3); 3.41 (1H, m, CH); 3.25 (1H,
m, CH); 2.01–1.66 (8H, m, CH2); 1.43 (9H, s, C(CH3)3). 13C NMR
(100 MHz, CDCl3): ı 165.7; 165.1; 160.3; 155.2; 154.5; 137.1; 133.8;
133.2; 129.7; 129.6; 129.3; 128.9; 128.7; 128.0; 127.9; 127.3; 126.4;
125.1; 124.8; 123.6; 123.1; 120.7; 118.6.; 117.7; 114.7; 72.9; 72.2;
71.8; 34.8; 33.1; 31.9; 29.4; 22.7; 19.2. LC-MS: m/z 585.5 [M + H]+.
Anal. calcd for C39H40N2O3: C, 80.11; H, 6.89; N, 4.79. Found: C,
80.13; H, 6.92; N, 4.81. FT-IR (KBr): 3433, 2924, 2866, 1624, 1507,
1376, 1258, 1195, 1080, 1017, 913, 800, 745 cm−1
.
2.2.3. Compound 3c
2.1. Preparation of half-unit ligands (2b)
Yellow solid (0.56 g, 88% yield) which was purified by flash col-
umn chromatography using petroleum ether: ethyl acetate (5:1) as
eluent. 1H NMR (400 MHz, CDCl3): ı 13.71 (1H, s, OH); 12.99 (1H, s,
OH); 8.55 (1H, s, CHN); 8.22 (1H, s, CHN); 7.96 (1H, d, J = 9.2 Hz, ArH);
7.85 (1H, d, J = 8.4 Hz, ArH); 7.79 (1H, s, ArH); 7.75 (1H, d, J = 8 Hz,
ArH); 7.44 (1H, d, J = 8.6 Hz, ArH); 7.32–7.14 (6H, m, ArH); 7.03 (1H,
d, J = 7.2 Hz, ArH); 6.91 (1H, s, ArH); 3.66 (3H, s, OCH3); 3.43 (1H, m,
CH); 3.23 (1H, m, CH); 1.99–1.53 (8H, m, CH2); 1.44 (9H, s, C(CH3)3);
1.19 (9H, s, C(CH3)3). 13C NMR (100 MHz, CDCl3): ı 165.9; 165.1;
157.9; 154.5; 139.9; 136.4; 133.3; 130.9; 129.6; 129.5; 128.9; 128.7;
128.0; 127.9; 127.3; 126.9; 126.4; 125.9; 125.2; 124.7; 123.6; 123.1;
120.8.; 119.1; 117.8; 114.6; 73.0; 72.2; 71.7; 34.9; 33.3; 32.9; 31.9;
29.5; 29.4; 22.7; 19.2. LC-MS: m/z 641.5 [M + H]+. Anal. calcd for
C43H48N2O3: C, 80.59; H, 7.55; N, 4.37. Found: C, 80.62; H, 7.58; N,
4.41. FT-IR (KBr): 3386, 2972, 2925, 1629, 1451, 1380, 1266, 1087,
2.1.1. Synthesis of N-(2-hydroxyl-3-tert-butylbenzaldehyde)-1-
amino-2-cyclohexaneimine
(2b)
3-Tert-butylsalicylaldehyde (0.18 g, 1.0 mmol) in chloroform
(50 mL) was added dropwise to the vigorously stirred solution of
(1R,2R)-(−)-diaminocyclohexane (0.12 g, 1.0 mmol) in chloroform
◦
˚
(150 mL) containing 4 A molecular sieves at 0 C. The reaction mix-
ture was stirred for 48 h, and then the solvent was removed under
reduced pressure to give the crude product. The crude product
was then purified by flash chromatography on silica gel (petroleum
ether/EtOAc = 4:1) affording a yellow oil (0.25 g, 91% yield). 1H NMR
(400 MHz, CDCl3): ı 13.85 (1H, s, OH); 8.28 (1H, s, CHN); 7.24 (1H,
d–d, J = 6.4 Hz, J = 1.6 Hz, ArH); 6.98 (1H, d–d, J = 6 Hz, J = 1.6 Hz, ArH);
6.71(1H, t, J = 7.6 Hz, ArH); 3.32 (1H, m, CH); 1.99-1.73 (9H, m,
CH CH2); 1.39 (9H, s, C(CH3)3). 13C NMR (100 MHz, CDCl3): ı 165.5;
160.3; 137.1; 129.8; 129.2; 118.6; 117.7; 72.4; 34.8; 33.1; 29.7;
29.3; 24.3. LC-MS: m/z 275.3 [M + H]+. Anal. calcd for C17H26N2O:
C, 74.41; H, 9.55; N, 10.21. Found: C, 74.44; H, 9.60; N, 10.24.
1049, 881, 804, 736 cm−1
.
2.2.4. Compound 6
Yellow solid (0.51 g, 80% yield) which was purified by flash col-
umn chromatography using petroleum ether: ethyl acetate (6:1) as
eluent. 1H NMR (400 MHz, CDCl3): ı 13.38 (1H, s, OH); 12.55 (1H, s,
OH); 8.95 (1H, s, CHN); 8.63 (1H, s, CHN); 8.07 (1H, s, ArH); 7.96 (1H,
d, J = 8.8 Hz, ArH); 7.85 (2H, d, J = 8.4 Hz, ArH); 7.45 (1H, d, J = 8.8 Hz,
ArH); 7.38 (1H, d, J = 2 Hz, ArH); 7.33–7.09 (11H, m, ArH); 3.76 (3H,
s, OCH3); 1.29 (9H, s, C(CH3)3); 1.26 (9H, s, C(CH3)3). 13C NMR
(100 MHz, CDCl3): ı 165.3; 164.8; 164.7; 158.6; 154.5; 142.0; 134.8;
130.9; 130.2; 129.7; 129.6; 128.8; 128.4; 128.2; 127.9; 127.5; 127.3;
126.8; 126.7; 126.5; 125.3; 125.2; 124.9; 124.2; 123.7; 123.5; 123.2;
121.9; 121.3; 119.9; 114.5; 35.1; 31.5; 31.4; 29.4; 29.2. LC-MS: m/z
635.5 [M + H]+. Anal. calcd for C43H42N2O3: C, 81.36; H, 6.67; N,
4.41. Found: C, 81.40; H, 6.68; N, 4.45. FT-IR (KBr): 3433, 2924, 2854,
2.2. General procedure for the preparation of new unsymmetrical
BINOL-Salen ligands (3a–3c and 6)
To a solution of half-unit ligands (2a–2c and 5) (1.0 mmol)
in ethanol (40 mL) was added dropwise a solution of (S)-(+)-
3-formyl-2-hydroxy-2ꢀ-methoxy-1,1ꢀ-binaphthyl (1.0 mmol) in
ethanol (40 mL). The reaction mixture was stirred under refluxing
for 12 h, and then cooled to room temperature. The solvent of the
resulting mixture was removed under reduced pressure, and the
crude product was then purified by flash chromatography on silica
gel (petroleum ether/EtOAc) affording a yellow solid.
1613, 1566, 1459, 1368, 1269, 1179, 1080, 1021, 800, 752 cm−1
.

42
L. Chen et al. / Applied Catalysis A: General 415–416 (2012) 40–46
Scheme 1. Preparation of ligands and corresponding manganese(III) complexes.
2.3. Synthesis of (1R,2R)-(−)-N,Nꢀ-Bis((S)-1,1ꢀ-2-hydroxy-2ꢀ-
1ꢀ-binaphthyl (1.0 mmol) in ethanol (5 mL) and stirred under
reflux for 6 h. After the removal of the solvent under reduced
pressure, the crude product was purified by flash chromatography
on silica gel (petroleum ether/EtOAc = 4:1) affording a yellow solid
(0.64 g, 87% yield). 1H NMR (400 MHz, CDCl3): ı 8.48 (2H, s, CHN);
7.98 (2H, d, J = 8.8 Hz, ArH); 7.86 (2H, d, J = 8.4 Hz, ArH); 7.74 (2H,
s, ArH); 7.73 (2H, d, J = 2.4 Hz, ArH); 7.53 (2H, d, J = 8.2 Hz, ArH);
7.43 (2H, d, J = 8.8 Hz, ArH); 7.33–7.17 (8H, m, CH2); 7.04 (2H, d,
methoxy-3-naphthylidene)-1,2-cyclohexanediamine
(7)
The chiral 1,2-cyclohexanediammonium mono-L-tartrate
(0.50 mmol) and K2CO3 (1.0 mmol) were dissolved in 1.2 mL
50% ethanol. The obtained solution was then added dropwise
to the solution of (S)-(+)-3-formyl-2-hydroxy-2ꢀ-methoxy-1,

L. Chen et al. / Applied Catalysis A: General 415–416 (2012) 40–46
43
J = 3.6 Hz, ArH); 3.51 (6H, s, OCH3); 3.31 (2H, d, J = 10 Hz, CH2);
1.97–1.56 (8H, m, CH2). 13C NMR (100 MHz, CDCl3): ␦ 165.3; 155.2;
154.4; 135.4; 133.8; 133.2; 130.9; 129.6; 129.5; 128.8; 128.5;
127.9; 127.4; 126.3; 125.2; 124.7; 123.6; 120.6; 119.3; 117.11;
114.8; 73.03; 31.5; 24.1; 19.2. LC-MS: m/z 735.4 [M + H]+. Anal.
calcd for C50H42N2O4: C, 81.72; H, 5.76; N, 3.81. Found: C, 81.75;
H, 5.80; N, 3.84. FT-IR (KBr): 3468, 2921, 2842, 1625, 1503, 1455,
ether/CH2Cl2 = 2:1) affording the corresponding epoxide. The enan-
tiomeric excesses of the chiral epoxide were determined by chiral
High-Performance Liquid Chromatography analysis.
3. Results and discussion
3.1. Synthesis of the novel chiral BINOL-Salen ligands and
corresponding Mn(III) complexes
1376, 1262, 1179, 1088, 808, 744 cm−1
.
2.4. General procedure for the preparation of the corresponding
Mn(III)-BINOL-Salen complexes (4a–4c, 6a and 7a)
Taking into consideration the above-described previous
studies, we synthesized four novel chiral unsymmetrical
BINOL-Salen ligands (3a–3c, and 6), which bear a tert-butyl
group or phenyl group as the bulky substituent on the half-
unit ligand moiety (as show in Scheme 1). The BINOL-Salen
ligands were synthesized by two-step procedure, wherein
we first prepared the half-unit ligands bearing a Schiff base
framework according to the reported procedure, [37−39]
N-(2-hydroxylbenzaldehyde)-1-amino-2-cyclohexaneimine
The ethanol solution of new ligands (3a–3c, 6 and 7) (1.0 mmol)
was stirred with the ethanol solution of Mn(OAc)2·4H2O (3.0 mmol)
under nitrogen atmosphere with refluxing for 6 h. The reaction mix-
ture was cooled to room temperature. Lithium chloride (6.0 mmol)
was added and the resulting mixture was refluxed for additional
2 h while exposed to air. The solvent was removed under reduced
pressure and the residue was extracted with dichloromethane
(3 × 10 mL). The extract was washed with water (2 × 10 mL), brine
and dried over anhydrous Na2SO4, and then concentrated to give
the crude product. The crude product was recrystallized with
petroleum ether affording the desired complexes.
(2a)
2-cyclohexaneimine
butylbenzaldehyde)-1-amino-2-cyclohexaneimine
N-(2-hydroxyl-3,5-di-tert-butylbenzaldehyde)-1-amino-2-
N-(2-hydroxyl-3-tert-butylbenzaldehyde)-1-amino-
(2b), N-(2-hydroxyl-3,5-di-tert-
(2c) and
benzeneimine (5). Then the half-unit ligands reacted with
(S)-(+)-3-formyl-2-hydroxy-2ꢀ-methoxy-1,1ꢀ-binaphthyl, respec-
tively, in a 1:1 molar ratio affording unsymmetrical BINOL-Salen
ligands. To elucidate steric effect and chiral center effect of this
catalytic reaction systematically, one C2-symmetric BINOL-Salen
ligand (7) was also prepared (as show in Scheme 1). With these
ligands in hand, the corresponding Mn(III) complexes (4a–4c, 6a
and 7a) were synthesized, respectively (as show in Scheme 1). All
of the synthesized ligands and complexes were well characterized
by NMR, LC-MS, FT-IR, and Elemental analyses.
2.4.1. Compound 4a
Dark brown powder (0.55 g, 89% yield). LC-MS: m/z 581.4
[M–Cl]+. Anal. calcd for C35H30ClMnN2O3: C, 68.13; H, 4.90; N, 4.54.
Found: C, 68.17; H, 4.96; N, 4.60. FT-IR (KBr): 3433, 2927, 2855,
1608, 1442, 1342, 1263, 1147, 1018, 906, 810, 755, 689 cm−1
.
2.4.2. Compound 4b
Dark brown powder (0.51 g, 76% yield). LC-MS: m/z 637.4
[M–Cl]+. Anal. calcd for C39H38ClMnN2O3: C, 69.59; H, 5.69; N, 4.16.
Found: C, 69.64; H, 5.70; N, 4.23. FT-IR (KBr): 3417, 2944, 2850,
1613, 1545, 1328, 1263, 1195, 1083, 1022, 891, 808, 752 cm−1
.
3.2. The catalytic performance of unsymmetrical
Mn(III)-BINOL-Salen complexes 4a, 4b and 4c
2.4.3. Compound 4c
Dark brown powder (0.63 g, 86% yield). LC-MS: m/z 693.4
[M–Cl]+. Anal. calcd for C43H46ClMnN2O3: C, 70.82; H, 6.36; N, 3.84.
Found: C, 70.90; H, 6.39; N, 3.88. FT-IR (KBr): 3448, 2953, 2850,
To understand the steric effect of the unsymmetrical Mn(III)-
BINOL-Salen complexes for asymmetric epoxidation of alkenes, the
ligands were altered with only substituent group of the salicylalde-
hyde framework. With the unsymmetrical Mn(III)-BINOL-Salen
complexes (4a, 4b and 4c) in hand, the asymmetric epoxide
reactions were systematically investigated with the substrates of
styrene (A), trans-stilbene (B), 2,2-dimethylchromene (C), 2,2,6-
trimethylchromene (D), 6-tert-butyl-2,2-dimethylchromene (E), 6-
chloro-2,2-dimethylchromene (F), 6-nitro-2,2-dimethylchromene
(G), and 6-methoxy-2,2-dimethylchromene (H). As is known to
all, the organic co-catalyst plays an important role in the Jacob-
sen epoxidation. Based on this point, Pyridine N-oxide was used
as co-catalyst since it has remarkable effects on both the activity
and enantioselectivity of the enantioselcetive epoxidation. Table 1
summarizes the catalytic performance of the complexes 4a, 4b and
4c in the enantioselective epoxidation of alkenes with NaClO as
the oxidant and PyNO as the axial base. The positive experimental
results show these complexes are effective catalysts for the asym-
metric epoxidation of alkenes. As expected, the reaction with 4a
proceeded at 0 ◦C in CH2Cl2, giving high entantioselectivity and
chemical yield, especially for 2,2-dimethylchromene (9–91% ee,
64–82% yield) (Table 1, entries 1–8). Using catalyst 4b instead of 4a
led to remarkable enhanced entantioselectivity and chemical yield
(73–91% yield, 16–95% ee) (Table 1, entries 9–16). This feature can
be attributed to complex 4b bearing a tert-butyl group as the bulky
substituent on the salicylaldehyde moiety at the 3 position. This
result supports the assumption that steric bulk at the 3,3ꢀ-position
of the Salen ligand can enhance entantioselectivity of alkenes [4]. To
understand the steric effects and electronic effects of catalyst in the
1610, 1530, 1433, 1388, 1082, 1023, 839, 807, 749 cm−1
.
2.4.4. Compound 6a
Brown powder (0.61 g, 85% yield). LC-MS: m/z 687.4 [M–Cl]+.
Anal. calcd for C43H40ClMnN2O3: C, 71.42; H, 5.58; N, 3.87. Found:
C, 71.44; H, 5.64; N, 3.90. FT-IR (KBr): 3425, 2953, 2850, 1598, 1573,
1560, 1385, 1248, 1178, 1083, 1017, 807, 747 cm−1
.
2.4.5. Compound 7a
Brown powder (0.72 g, 87% yield). LC-MS: m/z 787.4 [M–Cl]+.
Anal. calcd for C50H40ClMnN2O4: C, 72.95; H, 4.90; N, 3.40. Found:
C, 72.98; H, 4.93; N, 3.49. FT-IR (KBr): 3472, 2921, 2842, 1609, 1577,
1423, 1342, 1245, 1188, 1081, 806, 748 cm−1
.
2.5. General procedure for the asymmetric epoxidation of olefins
The substrate (2.0 mmol) and PyNO (0.30 mmol) as the axial
ligand were added to the dichloromethane (4 mL) solution of
Mn(III)-Binol-Salen complex (0.10 mmol). After the addition of
buffered NaOCl solution (4.0 mmol, pH 11.3) as the oxidant at 0 ◦C,
the resulting mixture was stirred vigorously and monitored by
TLC. After the reaction was totally completed, the mixture was
diluted with CH2Cl2 (2 × 10 mL). The organic layer was separated,
washed with water and brine, and then dried with MgSO4. The
solvent was removed under reduced pressure, and the crude prod-
uct was purified by flash chromatography on silica gel (petroleum

44
L. Chen et al. / Applied Catalysis A: General 415–416 (2012) 40–46
Table 1
Asymmetric epoxidation of alkenes using complexes 4a, 4b and 4c as catalystsa
.
Entry
Alkeneb
Catalyst
Time (h)c
Yield (%)d
ee (%)e
Configurationf
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
A
B
C
D
E
4a
4a
4a
4a
4a
4a
4a
4a
4b
4b
4b
4b
4b
4b
4b
4b
4c
4c
4c
4c
4c
4c
4c
4c
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
81
77
81
68
82
64
82
68
88
83
91
73
83
82
83
74
91
83
95
88
89
93
87
79
27
9
R-(+)
R-(+)
91
89
82
65
52
86
36
26
95
90
91
93
89
92
40
26
97
91
93
95
94
97
3R,4R-(+)
3R,4R-(+)
3R,4R-(+)
3R,4R-(+)
3R,4R-(+)
3R,4R-(+)
R-(+)
F
G
H
A
B
C
D
E
R-(+)
3R,4R-(+)
3R,4R-(+)
3R,4R-(+)
3R,4R-(+)
3R,4R-(+)
3R,4R-(+)
R-(+)
F
G
H
A
B
C
D
E
F
G
H
R-(+)
3R,4R-(+)
3R,4R-(+)
3R,4R-(+)
3R,4R-(+)
3R,4R-(+)
3R,4R-(+)
a
Reaction conditions: substrate (2 mmol), catalyst (5 mol%), NaOCl (4 mmol), PyNO (15 mol%).
A: styrene; B: trans-stilbene; C: 2,2-dimethylchromene; D: 2,2,6-trimethylchromene; E: 6-tert-butyl-2,2-dimethylchromene; F: 6-chloro-2,2-dimethylchromene; G:
b
6-nitro-2,2-dimethylchromene; H: 6-methoxy-2,2-dimethylchromene.
c
Monitored by TLC every other 20 min.
Isolated yield.
Determined by HPLC on chiral OJ-H and OB-H column.
Absolute configuration was determined by comparison of the sign of [␣]D with the literature value.
d
e
f
catalytic reaction systematically, we also examined the asymmet-
ric epoxidation of alkenes with 4c as the catalyst under the same
condition. It was also noteworthy that complex 4c derived from
chiral unsymmetrical BINOL-Salen (3c) gave higher entantioselec-
tivity on alkenes epoxidation than its analogue catalysts (4a and 4b)
(79–95% yield, 26–97% ee) (Table 1, entries 17–24). The remarkable
improvement in the performance of 4c can be attributed to the two
tert-butyl groups as the bulky substituent on the salicylaldehyde
moiety at the 3,5-position. This result supports the assumption that
steric bulk at the 5,5ꢀ-position of the Salen ligand has an equally
profound effect of entantioselectivity [42]. With comparison of the
activities of 4a and 4b with that of 4c, we conclude that different
attachment positions of chiral unsymmetrical Mn(III)-BINOL-Salen
complex had crucial effects on the catalytic performance of the cor-
responding Mn(III) complex. It demonstrates that steric factors play
an important role in the related asymmetric catalysis.
3.3. The catalytic performance of unsymmetrical
Mn(III)-BINOL-Salen complex 6a
To further investigate the synergistic effect of two differ-
ent chiral centers in the unsymmetrical Mn(III)-BINOL-Salen
Table 2
Asymmetric epoxidation of alkenes using complex 6a as catalysta
.
Entry
Alkeneb
Catalyst
Time (h)c
Yield (%)d
ee (%)e
Configurationf
1
2
3
4
5
6
7
8
A
B
C
D
E
F
G
H
6a
6a
6a
6a
6a
6a
6a
6a
4
4
4
4
4
4
4
4
82
78
73
64
88
91
84
93
21
8
R-(+)
R-(+)
24
21
25
10
11
22
3R,4R-(+)
3R,4R-(+)
3R,4R-(+)
3R,4R-(+)
3R,4R-(+)
3R,4R-(+)
a
Reaction conditions: substrate (2 mmol), catalyst (5 mol%), NaOCl (4 mmol), PyNO (15 mol%).
A: styrene; B: trans-stilbene; C: 2,2-dimethylchromene; D: 2,2,6-trimethylchromene; E: 6-tert-butyl-2,2-dimethylchromene; F: 6-chloro-2,2-dimethylchromene; G:
b
6-nitro-2,2-dimethylchromene; H: 6-methoxy-2,2-dimethylchromene.
c
Monitored by TLC every other 20 min.
Isolated yield.
Determined by HPLC on chiral OJ-H and OB-H column.
Absolute configuration was determined by comparison of the sign of [␣]D with the literature value.
d
e
f

L. Chen et al. / Applied Catalysis A: General 415–416 (2012) 40–46
45
Table 3
Asymmetric epoxidation of alkenes using complex 7a as catalysta
.
Entry
Alkeneb
Catalyst
Time (h)c
Yield (%)d
ee (%)e
Configurationf
1
2
3
4
5
6
7
8
A
B
C
D
E
F
G
H
7a
7a
7a
7a
7a
7a
7a
7a
4
4
4
4
4
4
4
4
82
85
78
76
89
79
81
69
31
13
94
90
91
92
80
91
R-(+)
R-(+)
3R,4R-(+)
3R,4R-(+)
3R,4R-(+)
3R,4R-(+)
3R,4R-(+)
3R,4R-(+)
a
Reaction conditions: substrate (2 mmol), catalyst (5 mol%), NaOCl (4 mmol), PyNO (15 mol%).
A: styrene; B: trans-stilbene; C: 2,2-dimethylchromene; D: 2,2,6-trimethylchromene; E: 6-tert-butyl-2,2-dimethylchromene; F: 6-chloro-2,2-dimethylchromene; G:
b
6-nitro-2,2-dimethylchromene; H: 6-methoxy-2,2-dimethylchromene.
c
Monitored by TLC every other 20 min.
Isolated yield.
Determined by HPLC on chiral OJ-H and OB-H column.
Absolute configuration was determined by comparison of the sign of [␣]D with the literature value.
d
e
f
complex, the asymmetric epoxidation of alkenes with catalyst 6a
were examined. Table 2 summarizes the catalytic performance of
the unsymmetrical Mn(III)-BINOL-Salen complex 6a in the enan-
tioselective epoxidation of alkenes under the controlled condition,
displaying lower entantioselectivity (8–25% ee) (Table 2, entries
1–8). From the results in Tables 1 and 2, it could be seen that the
different asymmetric induction observed in the epoxidation with
complexes 4c and 6a might attribute to the different chiral centers
in 4c and 6a. These results, to some extent, support the assump-
tion that the catalytic activity and entantioselectivity in the present
epoxidation were related to the synergistic effect of chiral centers
of catalyst.
suggested that the cooperation of steric factors and chiral cen-
ters of Mn(III)-BINOL-Salen complex play an important role in the
catalytic performances. Notably, this work emphasizes a combina-
tional strategy to encourage more attempts to explore new highly
effective catalyst for the enantioselective epoxidation of nonfunc-
tional olefins, and the further optimizations and studies of the scope
and mechanism of the asymmetric olefin epoxidation catalyzed
by the designed chiral BINOL-Salen complexes are ongoing in our
laboratory.
Acknowledgments
We thank the financial supports from the National Natural Sci-
ence Foundation of China (no. 21071069, 20601011 and 20571035),
Science Foundation of Gansu Province of China (0803RJZA103),
Scientific Research Foundation for the Returned Overseas Chinese
Scholars and the Fundamental Research Funds of the Central Uni-
versities of China, the Ministry of Education of China.
3.4. The catalytic performance of C2-symmetric
Mn(III)-BINOL-Salen complex 7a
To further understand the cooperation of steric factors and chi-
ral centers in the Mn(III)-BINOL-Salen complex, the epoxidation of
alkenes with catalyst 7a was also tested. As summarized in Table 3,
the catalytic performance of the C2-symmetric Mn(III)-BINOL-
Salen complexe 7a in the enantioselective epoxidation of alkenes
gave the high chemical yield and entantioselectivity (69–89% yield,
13–94% ee) (Table 3, entries 1–8). Compared with the activities of
6a and 4c as catalyst, the catalytic results from 7a apparently show
that the cooperation of steric factors and chiral centers of Mn(III)-
BINOL-Salen complex has an important influence on the current
catalytic asymmetric epoxidation, and this fact is consistent with
that observed by others [9,43].
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