Modular arene functionalization by differential 1,2-diborylation.

Aromatic rings, also known as arenes, containing two or more adjacent different substituents are ubiquitously found in small-molecule drugs. Strategies that can rapidly introduce diverse vicinal substituents to readily available precursors would greatly benefit the generation of analogues of biologically active compounds, which, however, remain challenging to realize so far. The existing approaches for preparing vicinal difunctionalized arenes lack modularity, regioselectivity or generality. Here we report a nickel-catalysed arene vicinal diborylation method that can directly install two chemically differentiated boryl groups in a regioselective and site-selective manner using readily available aryl triflates or chlorides as substrates. This reaction operates under simple and mild conditions and is scalable. It also shows a broad substrate scope and excellent functional group tolerance. Given that each boryl group can be independently transformed into various functional groups, this method offers a modular, regioselective and divergent approach to access diverse vicinal difunctionalized arenes, showing promise for constructing analogue libraries. The combined experimental and computational mechanistic studies reveal a highly unusual reaction pathway, involving the formation of a dearomatized gem-diboryl species and 1,2-boron migration. The site-selectivity and regioselectivity of this reaction are proposed to be controlled by steric interactions of the boryl groups with the nickel catalyst. The mechanistic insights gained in this investigation could have broad implications on developing other boron-mediated functionalization reactions.