Q.-S. Hu et al. / Tetrahedron Letters 43 (2002) 927–930
929
OH
References
5 mol% Chiral Catalyst
+
Et2Zn
PhCHO
Toluene, r.t.
Ph
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We have used 1 and 2 as macromolecular chiral cata-
lysts for the asymmetric addition of diethylzinc to
benzaldehyde (Scheme 2)14 and have compared the
catalytic properties of these dendronized polymers with
their corresponding linear polymeric and dendritic chi-
ral catalysts. We find that 1 and 2 are more efficient
than their corresponding linear polymeric and dendritic
chiral catalysts. In the presence of 5 mol% of 1 or 2
(based on the polymer repeat unit) in toluene, diethylz-
inc adds to benzaldehyde to afford the addition product
(R)-1-phenyl-1-propanol in 75% e.e. and 73% e.e.,
respectively.15 The enantioselectivity of 1 and 2 is as
good as that of polymers 7 (76% e.e.) and 8 (74% e.e.),
and dendrimer 9 (78% e.e.) for the same reaction.6a,11c,12
99% conversion of benzaldehyde is observed after 12 h
for both 1 and 2. However, 24 h are needed to achieve
the same conversion when 8 was used to catalyze the
reaction.12 Compared to 9 which was recovered by flash
chromatography after the reaction,6a 1 and 2 can be
easily recovered by filtration and reused. The recovered
1 shows the same reactivity and enantioselectivity (99%
conversion of benzaldehyde after 12 h, 76% e.e.).
Although 1 only shows 76% enantioselectivity for the
asymmetric addition of Et2Zn to benzaldehyde due to
the nature of ephedrine chiral units, it can be antici-
pated that highly enantioselective dendronized poly-
mers can be developed when highly enantioselective
monomeric chiral units are used for the construction of
dendronized polymers.
In summary, the first optically active ephedrine-bearing
dendronized polymers as a new type of macromolecular
chiral catalysts for asymmetric catalysis have been
designed and synthesized. The combination features of
more catalytic sites, higher solubility and nanoscopic
dimensions of optically active dendronized polymers
make them more efficient than their corresponding
linear polymeric and dendritic chiral catalysts. This
study provides a new direction for the design and
synthesis of highly efficient macromolecular chiral
catalysts.
Acknowledgements
10. Recent examples of dendronized polymers with rigid
polymeric cores: (a) Wyatt, S. R.; Hu, Q.-S.; Yan, X.-L.;
Bare, W. D.; Pu, L. Macromolecules 2001, 34, 7983; (a)
Marsitzky, D.; Vestberg, R.; Blainey, P.; Tang, B. T.;
Hawker, C. J.; Carter, K. R. J. Am. Chem. Soc. 2001,
123, 6965; (b) Bo, Z.; Schlu¨ter, A. D. Chem. Eur. J. 2000,
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This work was supported by the Department of Chem-
istry, College of Staten Island-City University of New
York (CUNY). Partial support from Professional Staff
Congress-CUNY Research Award Program is grate-
fully acknowledged. This work also benefited from the
NSF-REU program at CUNY-College of Staten
Island. We also thank Professor Howard Haubenstock
for allowing us to use the polarimeter.