Enantioselective Synthesis of Oxazaborolidines by Palladium-Catalyzed N-H/B-H Double Activation of 1,2-Azaborines

Abstract

A palladium-catalyzed N-H/B-H double activation of 1,2-dihydro-1,2-benzazaborines proceeded via cycloaddition with vinyl ethylene carbonate to produce polycyclic oxazaborolidines in 31-96 % yield. The key step in this process is the release of molecular hydrogen from a borate intermediate. Using a SPINOL-derived phosphoramidite as a chiral ligand, chiral oxazaborolidines were synthesized in good to high yields with excellent enantioselectivity (up to 95 % ee). The vinyl group of the resulting oxazaborolidine underwent metathesis, Heck reaction, and Wacker oxidation without affecting the oxazaborolidine framework.

Keywords: 1,2-Azaborines; Asymmetric Synthesis; BN Heterocycles; Cycloaddition; Palladium.