Coordination-Induced Stereocontrol over Carbocations: Asymmetric Reductive Deoxygenation of Racemic Tertiary Alcohols

The
inherent difficulty in eliciting facial control over carbocations
has limited their utility as intermediates in asymmetric catalysis.
We have now shown that a docking strategy involving the reversible
coordination of a substrate to a chiral transition-metal catalyst
can be used to enable highly stereoselective nucleophilic attack on
intermediate tertiary carbocations. This approach has been implemented
to achieve the first example of enantioselective reductive deoxygenation
of tertiary alcohols. This reduction occurs with high enantio- (up
to 96% ee) and regioselectivity (up to >50:1 rr) by applying a
novel
Hantzsch ester analogue as a convenient hydride source. In-depth mechanistic
studies support the involvement of a tertiary carbocation that is
coordinated to the iridium metal center via the key allene moiety.