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.