Light‐Induced 1(2 <i>H</i> )‐Isoquinolone Synthesis via Regioselective Denitrogenative [4+2] Transannulation of 1,2,3‐Benzotriazin‐4(3 <i>H</i> )‐ones with Allenes

Abstract We report a regioselective, visible‐light‐mediated synthesis of 1(2 H )‐isoquinolones, which was achieved through photocatalytic denitrogenation of 1,2,3‐benzotriazin‐4(3 H )‐ones followed by regioselective radical addition to allenes. Selective formation of the aromatized 1(2 H )‐isoquinolone product was observed, with DFT studies suggesting that selectivity for this isomer is due to a kinetically favorable nitrogen‐mediated hydrogen atom shift. The mild protocol developed allows for the rapid construction of a broad range of 1(2 H )‐isoquinolones derivatives with good functional group tolerance for N ‐substitution of the 1,2,3‐benzotriazin‐4(3 H )‐ones, although a limited range of allenes were tolerated. Control experiments and mechanistic investigations using UV–vis spectroscopy revealed that photoactive electron donor–acceptor complexes form, although photon absorption by the iridium photocatalyst is the main product‐forming pathway.