Metalloradical Activation of In Situ-Generated alpha-Alkynyldiazomethanes for Asymmetric Radical Cyclopropanation of Alkenes

alpha-Alkynyldiazomethanes, generated in situ from the corresponding sulfonyl hydrazones in the presence of a base, can serve as effective metalloradicophiles in Co(II)-based metalloradical catalysis (MRC) for asymmetric cyclopropanation of alkenes. With D-2-symmetric chiral amidoporphyrin 2,6-DiMeO-QingPhyrin as the optimal supporting ligand, the Co(II)-based metalloradical system can efficiently activate different alpha-alkynyldiazomethanes at room temperature for highly asymmetric cyclopropanation of a broad range of alkenes. This catalytic radical process provides a general synthetic tool for stereoselective construction of alkynyl cyclopropanes in high yields with high both diastereoselectivity and enantioselectivity. Combined computational and experimental studies offer several lines of evidence in support of the underlying stepwise radical mechanism for the Co(II)-catalyzed olefin cyclopropanation involving a unique alpha-metalloradical intermediate that is associated with two resonance forms of alpha-Co(III)-propargyl radical and gamma-Co(III)-allenyl radical. The resulting enantioenriched alkynyl cyclopropanes, as showcased with several stereospecific transformations, may serve as valuable chiral building blocks for stereoselective organic synthesis.