Regioselective 1,2-Alkylboration of Benzylidenecyclopropanes: Access to Csp-Enriched Cyclopropyl Boronic Esters.

We describe a novel, regioselective alkylboration of versatile (hetero)benzylidenecyclopropanes with β-H-containing alkyl iodides and bis(pinacolato)diboron enabled by copper catalysis. This three-component method allows for consecutive B-Csp and Csp-Csp bond formation to access Csp-enriched diverse tertiary cyclopropyl boronic esters with broad functionality tolerance, and the so-formed C-B bond is amenable to further structural diversification. Radical clock experiment, Hammett analysis, and DFT calculation suggest a mechanism of polar, rather than radical manifold, and S2-type C-C bond formation was found to be the rate-limiting step instead of migratory alkene insertion.