Mechanistic insights into photochemical nickel-catalyzed cross-couplings enabled by energy transfer

Abstract

Various methods that use a photocatalyst for electron transfer between an organic substrate and a transition metal catalyst have been established. While triplet sensitization of organic substrates via energy transfer from photocatalysts has been demonstrated, the sensitization of transition metal catalysts is still in its infancy. Here, we describe the selective alkylation of C(sp³)–H bonds via triplet sensitization of nickel catalytic intermediates with a thorough elucidation of its reaction mechanism. Exergonic Dexter energy transfer from an iridium photosensitizer promotes the nickel catalyst to the triplet state, thus enabling C–H functionalization via the release of bromine radical. Computational studies and transient absorption experiments support that the reaction proceeds via the formation of triplet states of the organometallic nickel catalyst by energy transfer.

Although the combination of photo- and nickel catalysis has garnered intense interest in the preceding years, the mechanisms by which these transformations occur are to some degree unresolved. Here the authors provide computational and spectroscopic support for a Dexter energy-transfer mechanism in the context of a C(sp3)–H alkylation.