Rh-catalyzed asymmetric hydroboration of enol carbamates yields α-boryl carbamates in good enantioselectivity. The enol carbamate starting materials can be prepared with moderate Z selectivity using a modified Juila olefination and used as an E/Z mixture, taking advantage of the faster reactivity of the major Z isomer in the directed hydroboration. Optically active α-boryl carbamates participate in a Matteson-type homologation with Grignard reagents in which the O-carbamate is substituted with high conservation of optical activity to provide enantioenriched secondary boronic esters.
Keywords: Hydroboration; Julia olefination; Matteson homologation; Rh catalysis; boronic esters.