Photoinduced NiH Catalysis with Trialkylamines for the Stereodivergent Transfer Semi‐Hydrogenation of Alkynes

Abstract

AbstractWe report a selectivity‐switchable nickel hydride‐catalyzed methodology that enables the stereocontrolled semi‐reduction of internal alkynes to E‐ or Z‐alkenes under very mild conditions. The proposed transfer semi‐hydrogenation process involves the use of a dual nickel/photoredox catalytic system and triethylamine, not only as a sacrificial reductant, but also as a source of hydrogen atoms. Mechanistic studies revealed a pathway involving photo‐induced generation of nickel hydride, syn‐hydronickelation of alkyne, and alkenylnickel isomerization as key steps. Remarkably, mechanistic experiments indicate that the control of the stereoselectivity is not ensuing from a post‐reduction alkene photoisomerization under our conditions. Instead, we demonstrate that the stereoselectivity of the reaction is dependent on the rate of a final protonolysis step which can be tuned by adjusting the pKa of an alcohol additive.