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

摘要

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.