Mayani et al.
JOCArticle
Schiff bases,13a,14 thiols,15 thiophene,16 bipiperidine,1a amino-
pyridine,17 oxabispidines,18 with diverse metals, viz., Zn,2d,19
Sm,20 Cu,1a,3b,17,21 Mg,22a Cr,14a La,4a,d,21b Li,4c,e,6 Na,4h
Ag,13b Pd,4b Zr,4a and Yb,22b have been used as catalyst for
the Henry reaction. Among these, the Cu-catalyzed nitroal-
dol reaction performed at room temperature is intriguing
due to its inexpensive and less toxic nature together with high
catalytic activity under homogeneous reaction conditions.1a,13,21
Further, due to mechanistic reasons both acidic and basic
sites within a single catalytic system are preferred. For this
reason a variety of bases, viz., potassium hydroxide,23a,b
cetyltrimethyl ammonium hydroxide,24a sodium carbonate,21b
triethyl amine,21b 2,6-lutidine,11 pyridine,11 and aromatic
imines,8b have found application as additives to improve
the efficiency of typically acidic metal complex based cata-
lysts in nitroaldol reactions.
FIGURE 1. Schematic presentation of chiral copper complex sup-
ported on silica.
Heterogeneous asymmetric catalysis has attracted much
attention due to its potential advantages, such as easy
product separation, recycling of the expensive chiral cata-
lysts, and the possibility of making a fix-bed reactor for
conducting reactions on continuous mode.25 Although fer-
vent activity was seen in the asymmetric nitroaldol reaction
under homogeneous condition, only a handful of recyclable
homogeneous and heterogeneous catalysts have been repor-
ted.2d,12d,16,22a,26a,26b The synthesis of chiral metal complexes
supported on various silicas for their use as asymmetric
heterogeneous catalysts in various organic transformations
including chiral lanthanum-lithium-binaphthol complex
on silica for the nitroaldol reaction has been of continued
interest to us.6 Recently we have reported the synthesis of
copper complexes of (S)-amino alcohol-supported silica as
chiral stationary phase and chiral ligand exchange stationary
phase for the chromatographic separation of racemic com-
pounds.27 Herein, we have investigated the application of
these materials as recyclable heterogeneous catalysts for
asymmetric nitroaldol reaction to afford chiral nitroalcohols
in good to excellent yields and enantioselectivity. These
materials, however, required a chiral imine as a promoter
in order to show high catalytic performance.
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Siliceous hexagonal SBA-15 and mesostructured cellular
foams (MCF) have substantially large pores (7-35 nm),
which make them potential materials as catalyst support.
Chiral complex supported on SBA-15, henceforth desig-
nated as catalyst A was synthesized and characterized by
various physicochemical and spectral studies according to
our previous report.27 To see the effect of pore size on the
catalytic performance of the copper complex we have also
prepared MCF (a bigger pore size silica) supported catalyst
B.28 Both the supported complexes (A and B; Figure 1) were
used as catalysts in nitroaldol reaction of various aldehydes
at room temperature.
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Chem.—Eur. J. 2009, 15, 12764.
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(b) Trost, B. M.; Yeh, V. S. C.; Ito, H.; Bremeyer, N. Org. Lett. 2002, 4, 2621.
To start with, we have carried out the nitroaldol reaction
of benzaldehyde with nitromethane as model substrates in
the presence of catalyst A and B in ethanol at rt. Both of
catalysts A and B provided the nitroaldol product in ∼90%
yield with ee values of 75% and 80%, respectively (Table 1,
entries 1 and 2). Both SBA-15 and MCF have similar BET
surface area but ultralarge pore size and a hydrothermally
robust framework of MCF might have contributed toward
better enantioselectivity of nitroaldol products. Next, we
evaluated the role of various organic bases 4-6a-e as
additives with complex A as a representative heterogeneous
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(28) See the Supporting Information for details.
6192 J. Org. Chem. Vol. 75, No. 18, 2010