Carbon-carbon bond activation by B(OMe)3/B2pin2-mediated fragmentation borylation.

Selective carbon-carbon bond activation is important in chemical industry and fundamental organic synthesis, but remains challenging. In this study, non-polar unstrained Csp2-Csp3 and Csp2-Csp2 bond activation was achieved by B(OMe)3/B2pin2-mediated fragmentation borylation. Various indole derivatives underwent C2-regioselective C-C bond activation to afford two C-B bonds under transition-metal-free conditions. Preliminary mechanistic investigations suggested that C-B bond formation and C-C bond cleavage probably occurred in a concerted process. This new reaction mode will stimulate the development of reactions based on inert C-C bond activation.

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

A BF3 ⋅Et2 O-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 BF3 ⋅Et2 O 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.