Asymmetric Catalytic Hydroboration of Enol Carbamates Enables an Enantioselective Matteson Homologation

A rhodium-catalyzed asymmetric hydroboration of enol carbamates yields E-boryl carbamates with a good enantioselectivity. The enol carbamate starting materials can be prepared with moderate Z-selectivity by using a modified Juila olefination; they can then be used as the resulting E/Z mixtures, taking advantage of the faster reactivity of the major Z-isomers in the directed hydroboration. Optically active E-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.