Metal-free photoredox-catalyzed reductive dimerization of allenes enabled by heterolytic cleavage of α-bromoesters: access to functionalized 1,5-dienes

Abstract

Selective functionalization of allenes is an actively researched area in synthetic chemistry yet faces challenges, including regio- and stereoselectivity. Herein, we report a metal-free photoredox catalyzed reductive dimerization of allenes with α-bromoesters under blue light irradiation. The reaction proceeds with a wide range of 1,1-disubstituted symmetrical and unsymmetrical allenes along with α-bromoesters to generate a novel series of di-alkyl substituted 1,5-diene molecules. Mechanistic studies, including photophysical studies, radical trapping experiments, intermediate capture experiments, Stern–Volmer experiments, and DFT studies, confirm that the reaction is initiated by selective radical addition of in situ generated alkyl ester radicals to allenes, followed by allyl–allyl radical pairing. The transformation proceeds under mild, environmentally benign conditions, highlighting the potential of organophotoredox catalysis as a green and efficient strategy for constructing complex diene frameworks.