Nhc-boryl radical catalysis for cycloisomerization with c-c triple bond reorganization

Abstract

Our present study unveils a new and efficient Nheterocyclic carbene (NHC)-boryl radical-catalyzed cycloisomerization of N-(2-ethynylaryl)arylamides. This catalytic process is triggered by the addition of an NHC-boryl radical to the alkynyl moiety, followed by a radical cascade comprising of an intramolecular cyclization, successive 1,5- and 1,2-aryl migrations, and the reorganization of a C-C triple bond. Eventually, the catalytic cycle is achieved through a radical β-fragmentation reaction, from which the starting NHC-boryl radical is regenerated, and a structurally important quinolinone framework is assembled. A 13C-labeling experiment was carried out to verify the C-C triple bond reorganization, and the proposed catalytic pathway was supported further by density functional theory calculations. Our identification of NHC-boryl radical as a catalyst, as well as its mode of operation via radical addition/elimination mechanism, would stimulate the advancement of design of radical catalysis and the discovery of novel enantioselective radical-catalyzed processes.