2-Aminopyridines as an α-Bromination Shuttle in a Transition Metal-Free One-Pot Synthesis of Imidazo[1,2-a]pyridines

A wide range of imidazo[1,2-a]pyridines are accessible from cheap and readily available 2-aminopyridines and 1,3-dicarbonyl compounds using a unique CBrCl3/2-aminopyridine system for bromination at the alpha-carbon. 2-Aminopyridine is not only the substrate but also acts as a bromination shuttle, transferring the bromine atom from CBrCl3 to the alpha-carbon of the 1,3-dicarbonyl. The reaction mechanism involves a series of reversible steps, including an addition reaction with cyclic transition state, to form a bromo-hemiaminal intermediate. Isolated yields of up to 97% were obtained under mild conditions and at short reaction times in this transition metal-free, one-pot synthesis.