5-Phenyl-1,3-dihydro-2H-1,4-benzodiazepin-2-ones react under palladium- and visible light photoredox catalysis, in refluxing methanol, with aryldiazonium salts to afford the respective 5-(2-arylphenyl) analogues. With 2- or 4-fluorobenzenediazonium derivatives, both fluoroaryl- and methoxyaryl- products were obtained, the latter resulting from a SNAr on the fluorobenzenediazonium salt ("nuisance effect"). A computational DFT analysis of the palladium-catalysed and the palladium/ruthenium-photocalysed mechanism for the functionalization of benzodiazepines indicated that, in the presence of the photocatalyst, the reaction proceeds via a low-energy SET pathway avoiding the high-energy oxidative addition step in the palladium-only catalysed reaction pathway.
Keywords: C−H activation; DFT; benzodiazepine; palladacycle; photocatalysis.