Direct conjugate alkylation of α,β-unsaturated carbonyls by Ti^III-catalysed reductive umpolung of simple activated alkenes

The titanium(III)-catalysed cross-selective reductive umpolung of Michael-acceptors represents a unique direct conjugate β-alkylation reaction. It allows the cross-selective preparation of 1,6- and 1,4-difunctionalised building blocks without the requirement of stoichiometric organometallic reagents. In this full paper, the development and scope of the titanium(III)-catalysed cross-selective reductive umpolung of Michael-acceptors is described. Based on the observed selectivities and additional mechanistic experiments a refined mechanistic proposal is presented.