Asymmetric epoxidation of chromenes using manganese(III) complexes with novel chiral salen-like schiff base ligands

Article abstract of DOI:10.1007/s10562-012-0787-3

Three novel chiral salen-like schiff base ligands and their Mn(III) complexes containing different amino acid unit have been synthesized and characterized. Asymmetric epoxidation reactions show these complexes are effective catalysts for the chromenes wit

Full text of DOI:10.1007/s10562-012-0787-3

Catal Lett (2012) 142:486–491
DOI 10.1007/s10562-012-0787-3
Asymmetric Epoxidation of Chromenes Using Manganese(III)
Complexes with Novel Chiral Salen-like Schiff Base Ligands
•
•
•
Longhai Chen Jian Wei Ning Tang
Feixiang Cheng
Received: 20 December 2011 / Accepted: 20 February 2012 / Published online: 6 March 2012
Ó Springer Science+Business Media, LLC 2012
Abstract Three novel chiral salen-like schiff base ligands
and their Mn(III) complexes containing different amino acid
unit have been synthesized and characterized. Asymmetric
epoxidation reactions show these complexes are effective
catalysts for the chromenes with buffer NaOCl as terminal
oxidant and pyridine N-oxide as co-catalyst in the presence of
ionic liquid. Good-to -excellent enantioselectivity and
acceptable yields can be obtained under optimum reaction
conditions. Catalyst 4c gives the highest ee (95%) for
6-chloro-2,2-dimethylchromene among these catalytic per-
formances. Furthermore, compared the enantioselectivity of
catalyst 4c with the other two catalysts 4a and 4b, the positive
experimental results suggest that the steric effect of the
ligands plays an important role in the asymmetric catalysis.
containing salen derivatives have been synthesized and
extensively used as catalysts for a range of asymmetric
reactions, such as aziridination [7], cyclopropanation [8],
Diels–Alder cycloaddition [9], lactide polymerization [10],
Michael addition [11], CO2 fixation [12], Sulfide oxidation
[13], hydrolytic kinetic resolution [14] and so on. Chiral
Mn(III) salen complexes have received much interest due to
their scope of applications as homogeneous catalysts in
asymmetric epoxidation of un-functionalised alkenes, which
have been studied extensively by Jacobsen, Katsuki and so on
[15, 16]. Althoughthesemetallosalen complexesare excellent
catalysts for asymmetric reactions, the separation and recy-
cling of homogeneous catalysts is problematic, making the
entire catalytic process economically nonviable for industrial
processes. Therefore, many attempts have been made to dis-
cover the recoverable metallosalen complexes, such as
immobilization ofcatalyst ontoinorganic materials[17, 18] or
polymer [19], ionic liquid-functionalized of catalyst [20, 21],
oligomer [22, 23] and highly active salen catalysts [24, 25].
Among these methods, ionic liquids have generated much
excitement in the field of organic synthesis, particularly for
metal complex catalysis, because ionic liquids are green
reaction media and can improve the separation, activity and
recycling of the catalysts [26–29].
Keywords Manganese(III) complex ꢀ Schiff base ꢀ
Asymmetric catalysis ꢀ Epoxidation ꢀ Ionic liquid ꢀ
Amino acid
1 Introduction
Chiral epoxides are extremely useful building blocks in the
synthesis of chiral compounds for the pharmaceuticals as well
as for fine chemicals [1–4]. The use of metal complexes of
chiral salen ligands in asymmetric synthesis has been wide-
spread in recent years [5, 6]. Numerous metal complexes
The structural and electronic properties of salen ligands
play the important roles in the catalytic activities. A great
deal of chiral schiff base ligands have been synthesized for
enantioselective catalytic reactions over the past decade,
but the vast majority of such studies have been focused on
the Jacobson–Katsuki type chiral catalysts, the study of
novel salen-like chiral schiff base ligands has attracted less
attention [30, 31]. Amino acids are one of the most
important classes of the building blocks of life: they are the
structural subunits of proteins, peptides, and many sec-
ondary metabolites. They are also the interesting ligands
L. Chen ꢀ J. Wei ꢀ N. Tang
College of Chemistry and Chemical Engineering, Lanzhou
University, Lanzhou 730000, People’s Republic of China
F. Cheng (&)
College of Chemistry and Chemical Engineering, Qujing Normal
University, Qujing 655011, People’s Republic of China
e-mail: chengfx2010@163.com
123

Asymmetric Epoxidation of Chromenes
487
with cheap chiral center and some of their metal complexes
have been approved as excellent catalysts for many
important asymmetric organic transformations [32]. Amino
acids have received considerable attention to get cheaper
and highly active chiral salen catalyst. To the best of our
knowledge, few people plough into creating amide-based
salen-like schiff ligands. Herein, we first report the syn-
thesis and characterization of three novel chiral salen-like
schiff base ligands and their Mn(III) complexes derived
from different amino acids. Their activities in asymmetric
epoxidation of chromenes using NaClO as the oxidant,
PyNO as the axial base, and ionic liquid as the reaction
solvent have been studied and discussed systematically.
ethanol solution (40 mL) of compound (2a–2c, 1.0 mmol).
The reaction mixture was stirred under refluxing for 8 h
before stirring was discontinued. After cooling down to
room temperature, the solvent was evaporated under
reduced pressure. The residue was purified by flash chro-
matography on silica gel, eluted with petroleum ether-
EtOAc (5:1, v/v) to afford the desired product as a yellow
solid.
2.3.1.1 Compound 3a Yield: 0.33 g (83%). 1H NMR
(400 MHz, CDCl3): d 12.55 (1H, s), 8.71 (1H, s), 8.49 (1H,
s), 8.21 (1H, s), 7.49 (1H, d, J = 2.4 Hz), 7.17 (1H, d,
J = 2 Hz), 7.13 (1H, t, J = 7.6 Hz), 7.02 (2H, d,
J = 8 Hz), 6.87 (1H, t, J = 7.6 Hz), 4.21 (1H, d,
J = 6.8 Hz) 1.68 (2H, d, J = 6.8 Hz), 1.48 (9H, s), 1.32
(9H, s). 13C NMR (100 MHz, CDCl3): d 172.3, 169.1,
157.6, 148.6, 141.3, 137.1, 128.6, 127.4, 126.9, 124.9,
122.2, 120.5, 119.7, 117.5, 68.6, 35.1, 34.2, 31.4, 29.4,
21.3. LC–MS: m/z 397.4 [M?H]?. Anal. Calcd for
C24H32N2O3: C, 72.70; H, 8.13; N, 7.06. Found: C, 72.80;
H, 8.18; N, 7.14. FT-IR (KBr): 3385, 2959, 2870, 1658,
1622, 1595, 1539, 1455, 1386, 1248, 1174, 1103, 1038,
2 Experimental Section
2.1 Materials
All starting materials were purchased from the Tianjin
Chemical Reagent Factory or Aldrich Chemical Company.
All solvents and raw materials were of analytical grade and
used without further purification unless otherwise stated.
2,2-Dimethylchromene and its derivatives [33], a-amino
amide and its derivatives (2a–2c) [34], and ionic liquid L-1-
ethyl-3-(10-hydroxy-20-propanyl)imidazolium bromide [35]
were synthesized according to the literature procedures.
750 cm-1
.
2.3.1.2 Compound 3b Yield: 0.38 g (89%). 1H NMR
(400 MHz, CDCl3): d 12.63 (1H, s), 8.74 (1H, s), 8.41 (1H,
s), 8.13 (1H, s), 7.50 (1H, d, J = 2.4 Hz), 7.19 (1H, d,
J = 2.4 Hz), 7.11 (1H, t, J = 7.2 Hz), 6.99 (2H, dd,
J = 7.2, 1.2 Hz), 6.87 (1H, t, J = 7.6 Hz), 3.89 (1H, d,
J = 4 Hz), 2.61–2.57 (1H, m), 1.48 (9H, s), 1.33 (9H, s),
1.08 (3H, d, J = 6.8 Hz), 1.09 (3H, d, J = 6.8 Hz). 13C
NMR (100 MHz, CDCl3): d 171.1, 169.7, 157.6, 148.7,
141.1, 137.0, 136.3, 129.7, 128.6, 127.4, 127.0, 126.9, 124.8,
122.3, 120.5, 119.7, 117.4, 41.3, 35.1, 34.2, 31.4, 29.4. LC–
MS: m/z 425.4 [M?H]?. Anal. Calcd for C26H36N2O3: C,
73.55; H, 8.55; N, 6.60. Found: C, 73.67; H, 8.62; N, 6.69.
FT-IR (KBr): 3380, 2960, 2871, 1657, 1626, 1532, 1457,
2.2 Physical Methods and Analysis
1H NMR and 13C NMR spectra were recorded on a Mercury
Plus 400 spectrometer with TMS as internal standard. IR
spectra were obtained on a Nicolet 170SX FT-IR spectro-
photometer as KBr discs. LC–MS spectra were performed on
a Bruker Daltonics Esquire 6000 mass spectrometer. Ele-
mental analyses were taken using a Perkin-Elmer 240C ana-
lytical instrument. All reactionsweremonitoredbyTLC. TLC
was performed on glass plates coated with silica gel 60F254.
The crude products were purified by flash chromatography.
The enantiomeric excesses of the chiral epoxides were
determined by chiral high-performance liquid chromatograph
analysis (Daicel Chiralcel OD-H chiral column, n-hexane:
i-PrOH = 99:1 (v/v), 1.0 mL/min, 254 nm) using a water
600 controller with 2996 photodiode array detector.
1390, 1273, 1248, 1173, 1103, 826, 750 cm-1
.
2.3.1.3 Compound 3c Yield: 0.38 g (81%). 1H NMR
(400 MHz, CDCl3): d 12.45 (1H, s), 8.63 (1H, s), 8.12 (1H,
s), 8.05 (1H, s), 7.46 (1H, d, J = 2.4 Hz), 7.26–7.14 (5H,
m), 7.13 (1H, t, J = 6.4 Hz), 7.09–6.93 (3H, m), 6.85 (1H,
t, J = 6.4 Hz), 4.23 (1H, dd, J = 4.8, 3.6 Hz), 3.46 (1H,
dd, J = 10, 3.6 Hz), 3.23 (1H, dd, J = 8.4, 5.2 Hz), 1.47
(9H, s), 1.28 (9H, s). 13C NMR (100 MHz, CDCl3): d
171.6, 169.9, 157.8, 148.9, 141.3, 137.2, 128.7, 127.5,
126.9, 124.9, 122.3, 120.5, 119.9, 117.5, 79.1, 35.2, 34.2,
32.6, 31.4, 29.4, 19.7, 17.2. LC–MS: m/z 473.4 [M?H]?.
Anal. Calcd for C30H36N2O3: C, 76.24; H, 7.68; N, 5.93.
Found: C, 76.33; H, 7.76; N, 6.01. FT-IR (KBr): 3357,
2957, 2864, 1656, 1627, 1534, 1454, 1388, 1238, 1174,
2.3 Preparation
2.3.1 General Procedure for the Preparation of the Novel
Chiral Schiff Base Ligands (3a–3c)
The ethanol solution (40 mL) of 3,5-di-tert-butylsalicylal-
dehyde (0.24 g, 1.0 mmol) was added dropwise to the
1087, 1033, 847, 751 cm-1
.
123

488
L. Chen et al.
2.3.2 General Procedure for the Preparation
of the Mn(III) Complexes (4a–4c)
64.23; H, 6.11; N, 4.99. Found: C, 64.16; H, 6.03; N, 4.90.
FT-IR (KBr): 3420, 2957, 2868, 1609, 1558, 1473, 1385,
1243, 1171, 1083, 1027, 844, 749 cm-1
.
A mixture of Mn(OAc)2ꢀ4H2O (3.0 mmol) and new ligand
(3a–3c) (1.0 mmol) in ethanol (40 mL) was heated to 80 °C
for 6 h under nitrogen atmosphere. After cooling down 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 9 10 mL). The extract was washed
with water (2 9 10 mL), brine and dried over anhydrous
Na2SO4, and then concentrated to give the crude product.
The crude product was recrystallized from petroleum ether
yielding the desired complex as a dark brown powder.
2.3.3 General Procedure for the Asymmetric
Epoxidation of Chromenes
The substrate (2.0 mmol) and PyNO (0.3 mmol) were added
to the CH2Cl2 (4 mL) solution of Mn(III) complex
(0.10 mmol) in the presence of ionic liquid L-1-ethyl-3-(10-
hydroxy-20-propanyl)imidazolium bromide. After the addi-
tion of buffered NaOCl (4.0 mmol) (pH 11.3) 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 9 10 mL). The organic layer
was separated, washed with water and brine, and then dried
with MgSO4. The solvent was removed under reduced
pressure to afford the crude product, which was purified by
flash chromatography on silica gel (petroleum ether/
CH2Cl2 = 2:1) to afford the corresponding epoxide. The
enantiomeric excesses of the chiral epoxide were determined
by chiral high-performance liquid chromatograph analysis.
2.3.2.1 Complex 4a Yield: 0.36 g (74%). LC–MS:
m/z 449.2 [M-Cl]?. Anal. Calcd for C24H30ClMnN2O3: C,
59.45; H, 6.24; N, 5.78. Found: C, 59.36; H, 6.17; N, 5.71.
FT-IR (KBr): 3379, 2957, 2866, 1615, 1568, 1530, 1476,
1385, 1242, 1171, 1103, 1027, 749 cm-1
.
2.3.2.2 Complex 4b Yield: 0.38 g (74%). LC–MS:
m/z 477.3 [M-Cl]?. Anal. Calcd for C26H34ClMnN2O3: C,
60.88; H, 6.68; N, 5.46. Found: C, 60.81; H, 6.59; N, 5.39.
FT-IR (KBr): 3382, 2959, 2870, 1609, 1556, 1474, 1385,
3 Result and Discussion
1271, 1242, 1172, 1104, 840,748 cm-1
.
3.1 Synthesis and Characterizations
2.3.2.3 Complex 4c Yield: 0.45 g (80%). LC–MS:
m/z 525.2 [M-Cl]?. Anal. Calcd for C30H34ClMnN2O3: C,
The outline of the synthesis of three novel chiral salen-like
schiff base ligands 3a–3c and their Mn(III) complexes
R
O
O
N
HN
R
O
EtOH
HN
OH
HO
H2N
OH
+
HO
3
1
2
R
N
O
O
NH
4
Mn(OAC)2.4H2O / EtOH
LiCl.H2O
Mn
Cl
O
2a R = CH3
3a R = CH3
4a R = CH3
4b R = i-Pr
3b R = i-Pr
2b R = i-Pr
2c R = CH2-Ph
3c R = CH2-Ph
4c R = CH2-Ph
Scheme 1 Synthesis of ligands 3a–3c and corresponding Mn(III) complexes 4a–4c
123

Asymmetric Epoxidation of Chromenes
489
4a–4c is presented in Scheme 1. These ligands have been
prepared by two steps, the a-amino amide and its derivative
(2a–2c) bearing a cheap chiral center have been first pre-
pared according to the reported procedure [34], then
compound (2a–2c) reacted with 3,5-di-tert-butylsalicylal-
dehyde, respectively, in a 1:1 molar ratio affording salen-
like schiff base ligand (3a–3c). To elucidate electronic and
steric effects of chiral center in the catalytic reaction sys-
tematically, L-alanine, L-valine, and L-phenylalanine were
used as the starting materials, which bear a methyl, iso-
propyl or benzyl group as the donor substituent on the
amino acid moiety, respectively. The compounds 3a–3c in
EtOH solution were heated with excess Mn(OAc)2ꢀ4H2O
for insertion of the Mn(II) center. Then, LiCl was added
and the mixture was heated to reflux for an additional 2 h
to obtain the Mn(III) complexes. These compounds were
well characterized by NMR, LC–MS, FT-IR, and Ele-
mental analyses.
3.2 Catalytic Activity Studies
It is known that some 2,2-disubstituted-3,4-epoxychromene
derivatives show unique physiological activity and have
wide range of applications [36]. The asymmetric catalytic
activity of the Mn(III) complexes 4a–4c are systematically
investigated with the substrates of 2,2-dimethylchromene
(A), 2,2,6-trimethylchromene (B), 6-tert-butyl-2,2-dim-
ethylchromene (C), 6-chloro-2,2-dimethylchromene (D),
6-nitro-2,2-dimethylchromene (E), and 6-methoxy-2,2-
dimethylchromene (F). Ionic liquid L-1-ethyl-3-(10-
hydroxy-20-propanyl)imidazolium bromide is used as the
reaction solvent to improve the separation of chiral Mn(III)
Table 1 Asymmetric epoxidation of chromenes using complexes 4a, 4b and 4c as catalystsa in the presence of ionic liquidb
R2
R4
O
O
O
R1
R3
R2
R4
R1
*
O
O
F
catalyst / ionic liquid
CH2Cl2 / H2O
O
O
+ NaClO
R3
MeO
*
H3C
O2N
t-Bu
Cl
A
C
E
B
D
Entry
Alkene
Catalyst
Time (h)c
Yield (%)d
ee (%)e
Configurationf
1
2
A
B
C
D
E
F
4a
4a
4a
4a
4a
4a
4b
4b
4b
4b
4b
4b
4c
4c
4c
4c
4c
4c
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
86
77
79
89
83
81
89
86
83
92
88
83
90
87
88
98
92
88
81
83
83
88
81
85
84
91
89
91
87
90
91
94
92
95
93
94
3R,4R-(?)
3R,4R-(?)
3R,4R-(?)
3R,4R-(?)
3R,4R-(?)
3R,4R-(?)
3R,4R-(?)
3R,4R-(?)
3R,4R-(?)
3R,4R-(?)
3R,4R-(?)
3R,4R-(?)
3R,4R-(?)
3R,4R-(?)
3R,4R-(?)
3R,4R-(?)
3R,4R-(?)
3R,4R-(?)
3
4
5
6
7
A
B
C
D
E
F
8
9
10
11
12
13
14
15
16
17
18
A
B
C
D
E
F
A 2,2-dimethylchromene, B 2,2,6-trimethylchromene, C 6-tert-butyl-2,2-dimethylchromene, D 6-chloro-2,2-dimethylchromene, E 6-nitro-2,2-
dimethylchromene, F 6-methoxy-2,2-dimethylchromene
a
Reaction conditions: substrate (2 mmol), catalyst (5 mol%), NaOCl (4 mmol), PyNO (15 mol%)
Ionic liquid: L-1-ethyl-3-(10-hydroxy-20-propanyl)imidazolium bromide
b
c
Monitored by TLC every other 20 min
d
Isolated yield
e
Determined by HPLC on chiral OD-H column
f
Absolute configuration is determined by comparison of the sign of [a] D with the literature value
123

490
L. Chen et al.
salen complexes [35]. Organic co-catalyst plays an
important role in the Jacobsen epoxidation, therefore,
pyridine N-oxide is used as co-catalyst due to its remark-
able effects on both the activity and enantioselectivity of
the enantioselcetive epoxidation [37, 38]. Asymmetric
epoxidation reactions are performed with 5 mol% of the
complexes 4a–4c with substrates in CH2Cl2 at 0 °C using
buffer NaOCl (pH 11.3) as terminal oxidant and pyridine
N-oxide as co-catalyst in the presence of ionic liquid.
Table 1 summarizes the catalytic performance of com-
plexes 4a–4c in the enantioselective epoxidation of
chromenes. As expected, reaction solvent plays a crucial
role in the asymmetric epoxidation of chromene. The
reaction time of ionic liquid system is very short, and the
product can be easily separated due to the solvent power of
the ionic liquid for many organic and inorganic substances
compared with the well-documented reaction time of
Jacobson type chiral catalysts [39, 40]. The experimental
results show these complexes are effective catalysts for the
asymmetric epoxidation of chromenes in the presence of
ionic liquid. It is noteworthy that the catalyst 4c shows high
enantioselectivity in excess of 91% and excellent chemical
yield (yield 87–98%, ee 91–95%) (Table 1, entries 13–18),
especially epoxidation of 6-chloro-2,2-dimethylchromene
(yield 98%, ee 95%). The use of catalyst 4a insteading of
4c leads to remarkable reduced entantioselectivity and
chemical yield (yield 77–89%, ee 81–88%) (Table 1,
entries 1–6). This feature can be attributed to the steric
factors of the chiral center of the catalyst, the steric hin-
drance of benzyl group is lager than the methyl group. To
elucidate the steric effect of catalyst in the catalytic reac-
tion systematically, the catalyst 4b with isopropyl group
has been prepared. The asymmetric epoxidation of
chromenes with catalyst 4b is examined under the same
condition. As expected, the catalyst 4b gives better
chemical yield and entantioselectivity than the catalyst 4a,
but poorer than that of the catalyst 4c (Table 1, entries
7–12). Furthermore, compared the epoxidation activities of
catalyst 4c with the other two catalysts 4a and 4b, we can
conclude that different substituent groups of chiral schiff
base complex have crucial effects on the catalytic perfor-
mances. This result supports the assumption that steric
factors and electronic effects play the important roles in the
asymmetric catalysis.
functionalized by different amino acids have been pre-
pared. These Mn(III) complexes are efficient catalysts for
the asymmetric epoxidation of chromenes with good-to-
excellent chemical yields and enantioselectivity using
NaClO as the oxidant in the presence of PyNO as the axial
base, especially epoxidation of 6-chloro-2,2-dimethyl-
chromene with catalyst 4c in the presence of ionic liquid.
The steric effect of chiral center in the catalyst has crucial
effect on the catalytic performance of the Mn(III) complex.
The complex 4c bearing a benzyl group exhibits the best
catalytic performance in the epoxidation reaction. These
observations suggest that the cooperation of steric factor
and chiral center of the complex plays an important role in
the catalytic performances.
Acknowledgments We are grateful to the National Natural Science
Foundation of China (21171078) and the Yunnan provincial science
and technology department (2010ZC148, 2009ZD008) for financial
support.
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