peri-Substituted imidazo[1,2-a]pyridines. A new reductive elimination reaction

Abstract

A new reductive elimination reaction of 3,5-disubstituted imidazo[1,2-a]pyridines (3) with hydrazine is reported. Thus on treatment of the 3,5-dibromo (7a), 5-bromo-3-nitro (7b), and 3,5-dinitro (4) derivatives with hydrazine hydrate in hot ethanol, the bromine and nitro groups are replaced by hydrogen. A mechanism based on the conjugated relationship of these peri-substituents is proposed and used to explain the reported conversion of 1,3,5-trichloro-2,4,6-trinitrobenzene (9) into 1,3-dichloro-4,6-dinitrobenzene (10). A variety of other 3-nitro- 5-substituted imidazo[1,2-a]pyridines (15)–(18) is described, but these could not be cyclised to 1,2,4-triazacyclopentindenes. The 3-amino-5-methoxycarbonyl derivative (19a) cyclises to the triazacyclopentindenone (20) with sodium methoxide.