Titanium-Catalyzed Reductive Umpolung Reactions with a Metal-Free Terminal Reducing Agent

A new method for titanium‐catalyzed reductive umpolung reactions is reported that overcomes the traditional requirement for a stoichiometric metallic reductant. With N,N′‐disilylated tetramethyldihydropyrazine as a potent organic reducing agent, reductive carbonyl–nitrile, enone–acrylonitrile and pinacol coupling reactions can be achieved in good yields and stereoselectivities. [Cp2TiI2] is a superior catalyst to [Cp2TiCl2], which is rationalized by a faster generation of the active catalyst [Cp2TiI]. A mechanism is proposed that is in agreement with the experimental results. Replacing zinc: A protocol for titanium(III)‐catalyzed reductive umpolung reactions is presented that enables the title reactions in the presence of an N,N′‐disilylated tetramethyldihydropyrazine as an organic sacrificial reducing agent. It is successfully applied to carbonyl–nitrile, enone–acrylonitrile and pinacol coupling reactions. A remarkable effect of the titanocene counterion renders titanocene diiodide a superior catalyst.