Photoredox/Cobalt‐Catalyzed Chemo‐, Regio‐, Diastereo‐ and Enantioselective Reductive Coupling of 1,1‐Disubstituted Allenes and Cyclobutenes

A dual photoredox/cobalt‐catalyzed protocol for chemo‐, regio‐, diastereo‐ and enantioselective reductive coupling of 1,1‐disubstituted allenes and cyclobutenes through chemo‐, regio‐, diastereo‐ and enantioselective oxidative cyclization followed by stereoselective protonation promoted by a chiral phosphine–cobalt complex is presented. Such process represents an unprecedented reaction pathway for cobalt catalysis that enables selective transformation of the less sterically congested alkenes of 1,1‐disubstituted allenes with cyclobutenes, incorporating a broad scope of tetrasubstituted alkenes into the cyclobutane scaffolds in up to 86% yield, >98:2 chemo‐ and regioselectivity, >98:2 dr and >99.5:0.5 er. Functionalization delivered a variety of enantioenriched cyclobutanes that are otherwise difficult to access. Preliminary mechanistic studies revealed that the reactions proceeded through oxidative cyclization followed by protonation and protonation might be the rate‐determining step.