Generation and Application of alpha,alpha-diboryl Radicals via Thiyl Radical Catalyzed Ring Opening of Vinyl Cyclopropyl Diborons: cis-Diastereoselective Borylative Cycloaddition with Alkenes

Carbon-centered radicals stabilized by adjacent boron atoms are underexplored reaction intermediates in organic synthesis. We report the development of vinyl cyclopropyl diborons (VCPDBs) as a versatile source of homoallylic a,a-diboryl radicals via thiyl radical catalyzed diboron-directed ring opening. The diboryl stabilized carbon-centered radicals underwent smooth [3+2] cycloaddition with a variety of olefins to provide highly substituted diboryl cyclopentanes in good to excellent diastereoselec- tivity. In contrast to the trans-diastereoselectivity observed with most of the dicarbonyl activated VCPs, the cycloaddition of VCPDBs showed a remarkable preference for formation of cis-cyclopentane diastereomer which was confirmed by quantitative NOE and 2D NOESY studies. The cis-stereochemistry of cyclopentane products enabled a concise intramolecular Heck reaction approach to rare tricyclic cyclopentanoid frameworks. The mild reaction conditions also allowed a one-pot VCP ring-opening, cy- cloaddition-oxidation sequence to afford disubstituted cyclopentanones. Control experiments and DFT analysis of reaction mecha- nism support a radical mediated pathway and provide a rationale for the observed diastereoselectivity. To our knowledge, these are the first examples of the use of geminal diboron group as activator of VCP ring opening and C-C bond formation reaction of a,a- diboryl radicals.