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HCl, rt 6 h) of the initial addition product. At the same time, the
chiral diamide 2a was recovered in high yield. This compares
favorably with other earlier reported organometallic processes
used for such purpose. There is a pronounced enhancement of
enantioselectivity in the reactions of aliphatic aldehydes when
compared to our previously reported method.10
To ascertain the effect of the chirality of the diamide moiety
of 2a on the extent of enantioselectivity, compound 2b lacking
chirality in the diamide function was prepared from 1,2-phenyl-
enediamine following the procedure used for 2a and employed
as a constituent of catalyst in the silylcyanation of benzalde-
hyde. As expected, the enantioselectivity was significantly low
with a maximum of 61% ee under optimum reaction conditions
when the reaction was carried out at 278 °C in dichloromethane
using the complex prepared from 16.5 mol% of 2b and 15 mol%
of Ti(OiPr)4 in the presence of 4 Å molecular sieves. Apparently
chirality in the diamide moiety is facilitating the higher
enantioselectivity.
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In conclusion, a new and efficient catalyst for enantiose-
lective silylcyanation of aromatic and aliphatic aldehydes with
excellent selectivity has been developed. The high degree of
enantioselectivity coupled with the high stability and recovery
rate of chiral component 2a of the catalyst constitutes the major
improvement on existing methods.
We are grateful to the National Science Council, Republic of
China for the finantial support of this work.
Notes and references
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