Stereoselective 1,2-Metalate Rearrangement of α-Chloroalkylboronic Esters with gem-Diborylalkanes.

Abstract

Highly diastereoselective 1,2-metalate rearrangements of α-haloalkylboronic esters that simultaneously construct contiguous stereogenic centers remain limited. Herein, we report a highly diastereoselective and stereospecific 1,2-metalate rearrangement of enantioenriched α-haloalkylboronic esters using gem-diborylalkanes as pronucleophiles. Controlled in situ generation of α-borylcarbanions, achieved by tuning the boron activator, enables efficient boron-ate complex formation and subsequent Lewis acid-promoted 1,2-metalate rearrangement to furnish vicinal diboronic esters in high yields with excellent diastereoselectivity. Mechanistic studies, including in situ11B NMR spectroscopy and density functional theory calculations, support the formation of a boron-ate intermediate and suggest that activator-dependent carbanion generation and minimization of alkyl-alkyl steric repulsion contribute to the high diastereoselectivity. The synthetic versatility of the products is demonstrated through diverse derivatizations and the concise synthesis of enantioenriched 1,2-dicyclohexylethane-1,2-diol.