Photoinduced Transition-Metal-Free Chan-Evans-Lam-Type Coupling: Dual Photoexcitation Mode with Halide Anion Effect

Abstract

Herein, we report a photoinduced transition-metal-freeC(aryl)-N bond formation between 2,4,6-tri(aryl)boroxines or arylboronicacids as an aryl source and 1,4,2-dioxazol-5-ones (dioxazolones) as an amidecoupling partner. Chloride anion, either generatedin situby photo-dissociation of chlorinated solvent molecules or added separately as anadditive, was found to play a critical cooperative role, thereby givingconvenient access to a wide range of synthetically versatileN-arylamidesunder mild photo conditions. The synthetic virtue of this transition-metal-free Chan-Evans-Lam-type coupling was demonstrated by large-scalereactions, synthesis of15N-labeled arylamides, and applicability towardbiologically relevant compounds. On the basis of mechanistic investigations,two distinctive photoexcitations are proposed to function in the currentprocess, in which thefirst excitation involving chloro-boron adductfacilitates the transition-metal-free activation of dioxazolones by single electron transfer (SET), and the second one enables theotherwise-inoperative 1,2-aryl migration of the thus-formedN-chloroamido-borate adduct.