Enantioselective
addition of trimethylsilyl cyanide (TMSCN) to aldehydes is one of the most extensively
studied organic reactions in asymmetric catalysis. Herein, we report our intensive
kinetic investigation on the asymmetric addition of TMSCN to benzaldehyde,
catalyzed by a covalently bridged dinuclear (salen)titanium complex <b>2</b>, which has been one of the most efficient
artificial chiral catalysts reported so far for this reaction. It was found
that the method of initial rates for kinetic investigation is not appropriate
in this case because of the presence of a significant <a></a><a>incubation </a>period in the catalysis, while the
method of progress rates proved to be more reliable and efficient for judging the
kinetic orders of this catalytic system. The kinetic results revealed that <a>the reaction follows first order with respect to the catalyst</a>
and is nearly independent of concentrations of both benzaldehyde and TMSCN<a>. A detailed catalytic mechanism for cyanosilylation of
benzaldehyde in the presence of <b>2</b>
was proposed, wherein the key active dinuclear species works in a cooperative manner
for dual activation of both reactants.</a>
Asymmetric Catalysis Special Feature Part II: Enantioselective direct aldol reactions catalyzed by L-prolinamide derivatives