Catalytic asymmetric functionalization of bicyclo[1.1.0]butane boronic esters enabled by 1,2-oxygen migration.

Abstract

The formation of boronate complexes through the coordination of alkoxide salts with organoborons serves as key intermediates in transition-metal-catalyzed coupling reactions. These complexes typically undergo classical transmetallation followed by reductive elimination to form C-C and C-X bonds. However, 1,2-oxygen migration in such boronate complexes remain underdeveloped, particularly in asymmetric catalysis, likely due to competing transmetallation pathways. Successfully achieving this migration potentially establish a fundamentally distinct reaction step, thereby significantly broadening the scope of transition-metal-catalyzed coupling reactions. In this study, we develope an iridium-catalyzed asymmetric alkylation that enables the stereoselective construction of cis-cyclobutanes. Our strategy employs the high ring strain of bicyclo[1.1.0]butane (BCB)-derived boronate complexes to induce 1,2-alkoxy migration. Mechanistic investigations reveal that 1,2-alkoxy migration proceeds stepwise, beginning with C-C bond formation, followed by a distinctive C-B bond rotation and subsequent 1,2-oxygen migration. Notably, exocyclic C-B rotation, which is outcompeted by 1,2-carbon migration in prior studies, can occur in this system because the competing 1,2-oxygen migration has a higher energy barrier. These findings provide crucial insights into the origin of stereoselectivity and the rate-determining step of this reaction.