Photocatalytic regeneration of brominating agent in the visible light-mediated synthesis of imidazo[1,2-a]pyridines

A wide array of imidazo[1,2-a]pyridines are available from the coupling of 1,3-dicarbonyls with 2-aminopyridines under visible-light mediated photocatalysis. The inexpensive and commercially available fluorescein dye, erythrosine B, is employed as the photoredox catalyst and KBr as halogenating agent for in situ bromination. This protocol features the use of catalytic amounts of the halogenating agent which is regenerated by the photocatalyst under visible light irradiation. Cyclic voltammetry studies provide supporting evidence of a reductive quenching pathway. The avoidance of stoichiometric excess of the halogen or peroxide oxidants for halide regeneration makes this protocol an environmentally-friendly synthesis with low E-factor.