Synthesis of C5-boryl indoles via a borane-catalyzed borylation/hydride transfer cascade

The C–H functionalization of indoles on the benzenoid moiety in preference to the more reactive pyrrolic unit is a significant challenge. Here, a borane-catalyzed C–H borylation of indoles toward the synthesis of important C5-boryl indoles has been developed. Enabled by a versatile B(C6F5)3 catalyst, various indoles reacted with HBcat to form C5-borylated indoles as well as C3, C5-diborylated indoles, which were further converted to C5-borylated indoles after protodeboronation at the C3 position under acidic conditions. Mechanistic investigations revealed a borane-catalyzed borylation/hydride transfer cascade mechanism. The synthetic potential of this transformation is also demonstrated by the concise syntheses of biorelevant C5-substituted indoles and diverse diaryl indoles.