pictorially the changes in the specific morphologies of
poly2A as such and on the LiAl-poly2A. In poly2A a well-
developed system of microspheres are seen (Figure 1, top
photograph) forming large fused aggregates of microgel
particles.18 The large pore sizes seen in these polymers may
facilitate the penetration of the reactants into the polymer
matrix. By SEM we also observed the changing morphology
of LiAl-poly2A where the microspheres get hidden indicative
of the agglomeration of microspheres with aluminum and
lithium metals (Figure 1, bottom photograph). On the basis
of our earlier studies on the structure of the heterobimetallic
catalyst13 LiAl-1, we suggest that the polymer chains entangle
to attain a similar structure (Scheme 1)
On using the amino diol 1 as ligand, we could obtain just
5.3% ee of the Michael adduct, whereas on using poly2A
as the ligand the ee of the product rose to 51%. But with
poly2b, where the chain length space was increased by higher
styrene incorporation, the ee of the product fell to 11.5%
(Table 1).
From this we may infer that the polymer chiral catalyst
LiAl-poly2a is more stereospecific compared to 1, and the
drop in the specificity is dramatic on increasing the chain
length with varying amount of styrene. We can explain the
significantly enhanced enantioselectivity for poly2a-pro-
moted Michael addition as follows. The entangling of
polymer chains to reach the suggested composition of hetero-
bimetallic would concomitantly impose high steric restriction
for the reactants, and this might be responsible for the
observed high asymmetric induction. For poly2b, however,
as the chain length space was higher, the formation of such
heterobimetallic may be incomplete resulting in lower ee.
In an earlier report we had noted that the Michael addition
of thiols to enones gets completed in seconds and that the
stereospecificity is optimum.13 We attempted the same
reaction with LiAl-poly2A catalyst and found that here the
reactions were comparatively slow, 30 min, but the ee’s were
slightly higher (Table 2).
Both poly2a and poly2b were used for promoting asym-
metric Michael addition reactions of nitromethane to the
chalcone (Table 1). The very reaction catalyzed by chiral
Table 1. Michael Addition of Nitromethane with Chalcone
Using Different Ligands
Table 2. Michael Addition of Thiols to Cycloalkenones
% of amino diol
ligand*
in catalysta
yield (%)
ee (%)
1
100
41
28
92
90
90
5.3
51
11.5
P oly2a
P oly2b
a In all cases the ratio, [Al]/[Michael acceptor] was kept at 0.5.
no.
n
R
X
yield (%)
ee %a
ee%b
4
5
6
7
8
9
1
2
1
2
1
2
H
H
CH3
CH3
H
S
S
S
S
95
90
90
90
85
90
41
57
52
33
52c
76c
32
45
26
40
-
alkaloids was done earlier under high pressure and for a
longer time.19 But here the reaction could be completed at
atmospheric pressure and for lesser reaction time of 6 h.
Moreover, the purification process is simple which is just a
quick filtration of the reaction mixture separating the catalyst
from the organics.20
CH2S
CH2S
H
-
a On using LiAl-poly2a. b On using amino diol 1. c By HPLC chiralcel-
OD column.
(13) (a) Manickam, G.; Sundararajan, G. Tetrahedron Asymmetry 1997,
8, 2271-2278. (b) Manickam, G.; Sundararajan, G. Indian J. Chem. 1997,
36A,B, 516-518. (c) Manickam, G.; Sundararajan, G. Tetrahedron 1999,
55, 2721-2736.
Cyclopentenone and cyclohexenone were also used as
Michael acceptors, and the Michael donors were thiophenol,
p-methylthiophenol, and benzyl mercaptan. The yields and
the ee’s are in general high when compared with other
methods.
(14) To a cooled solution of vinyl benzylamine (5 mmol) in 2 mL of
methanol was added (R)-(+)-styrene epoxide (10 mmol) in 4 mL of
methanol at 0 °C and stirred for 1 h. It was then refluxed for 4 h. After
completion of the reaction, the solvent was then removed under reduced
pressure to give a syrupy mass which upon column chromatography
(alumina) using ethyl acetate:hexane (10:90) as eluent gave the product
(yield 30%).
(15) To 1 mol of (R,R)-3-aza-3-(p-vinylbenzyl)-1,5-diphenyl-1,5-dihy-
droxypentane were added 1 mol of divinylbenzene and 4 mol of styrene in
the presence of 10 mg of benzoyl peroxide, and it was sealed in a nitrogen
atmosphere. Then the reaction mixture was heated at 70 °C for 48 h.
(16) (a) Itsuno, S.; Kamahori, K.; Watanabe, K.; Koizumi, T.; Ito, K.
Tetrahedron Asymmetry 1994, 5, 523-526. (b) Frischel, S. J.; Ackerman,
J. J. H.; Keyser, T.; Stille, J. K. J. Org. Chem. 1979, 44, 3152-3157.
(17) (a) Overberger, C. G.; Sannes, K. N. Angew. Chem. In..Ed. Engl.
1974, 13, 99-104. (b) McArthur, C. R.; Worster, P. M.; Jiang, J. L.; Leznoff,
C. C. Can. J. Chem. 1982, 60, 1836-1841. (c) Gong, A.; Liu, W.; Chen,
Y.; Zhang, X.; Chen, C.; Xi, F. Tetrahedron Asymmetry 1999, 10, 2079-
2086.
(19) (a) Funabashi, K.; Saida, Y.; Kanai, M.; Arai, T.; Sasai, H.;
Shibasaki, M. Tetrahedron Lett. 1998, 39, 7557-7558. (b) Matsumoto, K.;
Uchida, T. Chem. Lett. 1981, 1673-1676.
(20) General procedure for the Michael addition reaction: To the polymer
chiral ligand (250 mg) in THF was added slowly a solution of LiAlH4 (10
mg, 0.268 mmol) in dry THF, drop by drop at 0 °C. The mixture was stirred
for 30 min, the mixture was warmed to room temperature, and the Michael
acceptor and donor were added subsequently. After the reaction was over,
it was quenched with 1 N HCl and mixture extracted with ethyl acetate.
The organic layer was washed successively with saturated NaHCO3 solution
and brine and dried over anhydrous Na2SO4. Removal of the solvent under
reduced pressure gave a syrupy mass, which upon flash column chroma-
tography gave the product.
(18) Sherrington, D. C Chem. Commun. 1998, 2275-2286.
Org. Lett., Vol. 3, No. 3, 2001
391
Sundararajan
