Substrate-Controlled Divergent Synthesis of Benzimidazole-Fused Quinolines and Spirocyclic Benzimidazole-Fused Isoindoles

A Rh(III)-catalyzed annulation of 2-arylbenzimidazoles with α-diazo carbonyl compounds via C–H activation/carbene insertion/intramolecular cyclization is explored. The switchable product selectivity is achieved by the use of distinct α-diazo carbonyl compounds. Benzimidazole-fused quinolines are obtained through [4 + 2] annulation exclusively when 2-diazocyclohexane-1,3-diones are used, where they act as a C2 synthon. Alternatively, diazonaphthalen-1(2H)-ones merely function as a one-carbon unit synthon to generate a quaternary center through [4 + 1] cyclization to afford spirocyclic benzimidazole-fused isoindole naphthalen-2-ones. A thorough mechanistic study reveals the course of the reaction.