A biomimetic S H 2 cross-coupling mechanism for quaternary sp 3 -carbon formation

Bimolecular homolytic substitution (SH2) is an open-shell mechanism that is implicated across a host of biochemical alkylation pathways. Surprisingly, however, this radical substitution manifold has not been generally deployed as a design element in synthetic C–C bond formation. We found that the SH2 mechanism can be leveraged to enable a biomimetic sp3-sp3 cross-coupling platform that furnishes quaternary sp3-carbon centers, a long-standing challenge in organic molecule construction. This heteroselective radical-radical coupling uses the capacity of iron porphyrin to readily distinguish between the SH2 bond-forming roles of open-shell primary and tertiary carbons, combined with photocatalysis to generate both radical classes simultaneously from widely abundant functional groups. Mechanistic studies confirm the intermediacy of a primary alkyl–Fe(III) species prior to coupling and provide evidence for the SH2 displacement pathway in the critical quaternary sp3-carbon bond formation step.