Base-free palladium-catalyzed borylation of enol carboxylates and further reactivity toward deboronation and cross-coupling

Abstract

A series of base-free Pd-catalyzed borylation procedures are reported for a number of alkenyl carboxylates. High-throughput experimentation was used to discover and optimize these reactions using in situ generated catalyst systems. With lactone or lactam substrates, the resulting alkenyl pinacol boronates are hydrolytically unstable, undergoing protodeboronation under even mildly basic aqueous conditions. Optimization of this protodeboronation resulted in a mild, two-step reduction of the C–O bond, achieving net-deoxygenation while leaving the alkene intact. In contrast, use of an alternative tetraalkoxydiboron source – B2EPin2 – was successful in catalysis, and offered improved stability of the resulting organoboron species. This enables further reactivity, such as cross-coupling, while suppressing protodeboronation.