Chiral copper-Schiff base catalyst for asymmetric henry reaction

Article abstract of DOI:10.1134/S1070428007120020

Five copper-Schiff base complexes were synthesized conveniently from copper(II) acetate monohydrate, salicylaldehydes, and amino alcohols. The complexes were shown to be effective as catalysts in the asymmetric Henry reaction affording nitro alkanols in u

Full text of DOI:10.1134/S1070428007120020

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2007, Vol. 43, No. 12, pp. 1754–1756. © Pleiades Publishing, Ltd., 2007.
Published in Russian in Zhurnal Organicheskoi Khimii, 2007, Vol. 43, No. 12, pp. 1754–1755.
Chiral Copper–Schiff Base Catalyst for Asymmetric
Henry Reaction*
Yong-guang Gao, Na Chen, Hua-jiang Wu, and Xin-sheng Li
Department of Chemistry, Zhejiang Normal University, Jinhua, Zhejiang, 321004 China
e-mail: sky33@zjnu.cn
Received November 19, 2006
Abstract—Five copper–Schiff base complexes were synthesized conveniently from copper(II) acetate mono-
hydrate, salicylaldehydes, and amino alcohols. The complexes were shown to be effective as catalysts in the
asymmetric Henry reaction affording nitro alkanols in up to 98% yield with moderate and good enantiomeric
excess (up to ee 38.6%).
DOI: 10.1134/S1070428007120020
Syntheses of nitro alcohols via the Henry reaction
have been extensively studied during the last decade
complexes with Schiff bases have seldom been
reported for asymmetric Henry reaction. In the present
communication we report on the use of copper–Schiff
base complexes in asymmetric Henry reaction.
[
1–3]. Recent contributions to this area have been
highlighted by Shibasaki and Trost [4–11]. Among ef-
ficient catalysts for the Henry reaction, bimetallic zinc
complexes [6, 7], rare-earth lithium BINOL complexes
Five copper–Schiff base complexes were synthe-
sized according to the procedures described in [17, 18].
Their structures are shown below. Initially, we ex-
amined the Henry reaction of p-nitrobenzaldehyde
with nitromethane, catalyzed by complex Ia in various
solvents (Scheme 1), and some representative results
[
[
2, 4, 5, 12–15], chiral quaternary ammonium salts
10, 11], and complexes derived from bis(dihydrooxa-
zole) and bis(dihydrothiazole) ligands [1, 3, 8, 9, 16]
were shown to be potent. On the other hand, copper
t-Bu
t-Bu
Me
Me
OBu-t
OBu-t
OC H
8
17
17
OC H
N
N
8
Cu/2
O
Cu/2 O
O
O
t-Bu
t-Bu
Ia
Ib
Ar
Ar
O
O
O
O
Ph
O
Ph
H
Cu
Cu
H
N
N
N
N
Cu/2
O
Cu/2
O
O
Me
Ic
Id
Ie
*
The text was submitted by the authors in English.
1754

CHIRAL COPPER–SCHIFF BASE CATALYST FOR ASYMMETRIC HENRY REACTION
1755
Scheme 1.
Table 1. Henry reaction of p-nitrobenzaldehyde with nitro-
methane in the presence of complex Ia (5 mol %) in dif-
CHO
ferent solvents (reaction time 24 h, room temperature)
+
MeNO2
a
b
O N
2
Solvent
Yield, %
ee, % (R)
OH
Tetrahydrofuran
Nitromethane
Acetonitrile
Toluene
20
90
90
30
40
93
85
87
10.9
01.4
19.0
01.1
10.6
15.0
17.2
25.4
Ia (5 mol %)
*
NO2
room temperature
O N
2
Diethyl ether
Ethyl acetate
Methylene chloride
Ethanol
are summarized in Table 1. The reactions were carried
out in different solvents at room temperature using
mol % of Ia. The results showed that ethanol was the
best solvent: the corresponding nitro alcohols were ob-
tained in high yield with moderate enantiomeric excess
5
a
Hereinafter, the yield of the isolated product is given.
(
ee 25.4% for the R enantiomer, Table 1). Compounds
b
Hereinafter, determined by HPLC; the absolute configuration
was assigned by comparison of the retention time with published
data [1].
Ia–Ie were then tested in the same reaction using
ethanol as solvent (Table 2). The reactions were com-
plete within 24 h at room temperature. It is seen that
complexes Ia and Ib ensure high yields of 2-nitro-1-
Table 2. Henry reaction of p-nitrobenzaldehyde with nitro-
methane in the presence of complexes Ia–Ie (5 mol %) in
ethanol (reaction time 24 h, room temperature)
(
4-nitrophenyl)ethanol and moderate ee values (25.4
and 19%, respectively). We also performed the Henry
reaction of nitromethane with different aromatic alde-
hydes [12] with a view to estimate its scope. Complex
Ia worked well with the examined benzaldehydes
Catalyst no.
Yield, %
ee, % (R)
25.4
Ia
Ib
Ic
Id
Ie
87
98
40
45
50
(
Table 3), but better enantioselectivity was observed
19.0
with substituted benzaldehydes compared to unsubsti-
tuted. The product obtained from p-chlorobenzalde-
hyde was characterized by higher optical purity than
that formed from p-nitrobenzaldehyde, On the other
hand, the chemical yield in the reaction with p-nitro-
benzaldehyde was greater than in the reaction with
p-chlorobenzaldehyde.
07.4
09.0
03.1
Table 3. Henry reaction of nitromethane with substituted
benzaldehydes in the presence of complex Ia (5 mol %) in
ethanol (reaction time 24 h, room temperature)
EXPERIMENTAL
Aldehyde
Yield, %
ee, % (R)
The ee values were determined by HPLC using
a Chiralcel™ OD-H column (eluent propan-2-ol–
hexane). The H NMR spectra were recorded on
Benzaldehyde
65
50
70
87
06.6
30.2
38.6
25.4
1
o-Chlorobenzaldehyde
p-Chlorobenzaldehyde
p-Nitrobenzaldehyde
a Bruker AV-400 spectrometer at a frequency of
4
00 MHz using chloroform-d as solvent and tetra-
methylsilane as internal reference.
General procedure for the Henry reaction.
A mixture of 5 mol % of catalyst Ia–Ie and 2.5 mmol
of nitromethane in 5 ml of the corresponding solvent
was stirred for an hour at room temperature, 0.5 mmol
of aromatic aldehyde was added, the mixture was
stirred for 24 h at room temperature, the solvent was
evaporated under reduced pressure, and the crude
product was purified by column chromatography (15%
of ethyl acetate in petroleum ether).
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