Boron(III)-Catalyzed C2-Selective C-H Borylation of Heteroarenes

A BF3Et2O-catalyzed C2-selective C-H borylation of indoles with bis(pinacolato)diboron was developed to afford indole-2-boronic acid pinacol esters. A variety of functional groups were tolerated, and other heteroarenes like pyrrole and benzo[b]thiophene were also suitable substrates. An electrophilic substitution mechanism was proposed based on the preliminary mechanistic studies. This novel transformation utilizes simple and cheap BF3Et2O as catalyst and exhibits unusual C2 regioselectivity, providing a significant non-transition-metal-catalyzed C-H borylation and an efficient method towards the synthesis of C2-functionalized heteroarenes.