Hydride-free reduction of propargyl electrophiles: a nickel-catalyzed photoredox strategy for allene synthesis

Abstract

Herein we report a catalytic reduction of propargyl carbonates to allenes mediated by a nickel molecular catalyst. The catalytic system uses light as the driving force and amine as the sole hydrogen source. Contrary to other catalytic approaches, the process proceeds without the intermediacy of metal hydride species. The commonly observed pathway in transition metal catalyzed reductive processes is replaced by a pathway involving a sequence of electron transfer and proton transfer. Using this catalytic approach, a wide range of allenes could be obtained under mild conditions. Experimental investigations support the dual role of trialkylamine as a reductant and a proton source and have revealed the key mechanistic features of the reaction. A key protodenickelation step of the Ni(II) allenyl intermediate is proposed to account for the reduction process. Finally, we also demonstrate that the selective S_N²′ reduction process can be also efficiently driven by an electrochemical approach.