Highly Stereoselective Cyclopropanation of Alkenes with Unsymmetrical Diazomalonates via Co(II)-Based Metalloradical Catalysis

Abstract

Diazomalonates have been demonstrated, for the first time, as effective radical precursors for asymmetric radical cyclopropanation of alkenes via Co(II)-based metalloradical catalysis (MRC). With an optimized D2-symmetric chiral amidoporphyrin as the supporting ligand, the Co(II)-based metalloradical system can efficiently activate unsymmetrical methyl phenyl diazomalonate (MPDM) for the asymmetric cyclopropanation of alkenes, enabling stereoselective construction of 1,1-cyclopropanediesters bearing two contiguous chiral centers, including at least one all-carbon quaternary stereogenic center. The Co(II)-catalyzed asymmetric cyclopropanation, which operates at room temperature without slow addition of the diazo compound, is generally applicable to a broad range of olefin substrates and tolerates various functionalities, providing a streamlined synthesis of chiral 1,1-cyclopropanediesters in high yields with high level of control in both diastereoselectivity and enantioselectivity. Mechanistic studies on the cyclopropanation reactions, including the use of (E)- and (Z)-b-deuterostyrenes, support the underlying stepwise radical pathway for the Co(II)-catalyzed cyclopropanation. In addition to functioning as effective 1,3-dipoles for stereospecific formation of five-membered ring structures, the resulting enantioenriched methyl phenyl (E)-1,1-cyclopropanediesters serve as useful building blocks for the synthesis of different 1,1-cyclopropanediesters, 1,1-cyclopropaneestercarboxylic acids and 1,1-cyclopropaneesteramides while maintaining the original stereochemistry. Additionally, the enantioenriched (E)-1,1-cyclopropanediesters can be converted to (Z)-diastereomers without affecting the high enantiopurity.