Expanding the Scope of BICAAC Organocatalyst: Hydroboration of Carbonyl and Alkyne Substrates

Abstract

Bicyclic(alkyl)(amino) carbene (BICAAC) (I) has been used as a highly effective catalyst for hydroboration of aldehydes, ketones and alkynes. The catalyst is effective across a broad range of substrates bearing both electron-donating and electron-withdrawing groups, indicating its robust functional group tolerance. Mechanistic investigations, supported by both experimental studies and computational analyses, suggest the initial step involves the reaction of BICAAC (I) with pinacolborane (HBpin), leading to the formation of boron-carbene complex (active catalyst II). This active species facilitates substrate activation. A subsequent hydride transfer from an additional equivalent of HBpin to the activated substrate completes the reduction process. This stepwise mechanism underscores the dual role of HBpin as both a catalyst activator and a hydride donor. The interaction between BICAAC and HBpin is pivotal, as it modulates the electronic environment of the boron center to promote efficient hydride delivery.

Keywords: Carbene; Hydroboration; Organocatalytic; Precatalyst; hydride transfer.