Chiral Metal–Organic Framework as a Platform for Cooperative Catalysis in Asymmetric Cyanosilylation of Aldehydes

In
this work, we demonstrate cooperative asymmetric catalysis by
a metal–organic framework (MOF) as exemplified in the context
of catalyzing cyanation of aldehydes with a VO­(salen)-MOF, which after
oxidation affords remarkably increased stereoselectivity (up to >99%
ee) compared to the homogeneous VO­(salen) counterpart as a result
of the pairs of VO­(salen) units in close proximity within its open
channels. The cooperative asymmetric catalysis has been evidenced
by the significantly decreased stereoselectivity and activity when
one VO­(salen) in such pairs of VO­(salen) units is replaced with one
Cu­(salen), which results in blocking the VO–VO synergistic
pathway while prompting unimolecular activation of substrates. The
heterogeneous nature of VO­(salen)-MOF has been verified by the fact
that it can be easily recycled and reused without significant loss
of catalytic activity and enantioselectivity, and its practical utility
as asymmetric cyanation catalysist has been illustrated in the gram-scale
synthesis of the antiviral natural products (R)-
and (S)-enantiomers of tembamide. Our work therefore
advances chiral MOF as an attractive platform for cooperative asymmetric
catalysis in a variety of syntheses.