Facile Synthesis of 2,3-Disubstituted Indoles by NBS/CuCl Mediated Oxidative Cyclization of N-Aryl Enamines

A one-pot synthesis of 2,3-disubstituted indoles has been accomplished through a facile, mild and efficient oxidative cyclization of N-aryl enamines mediated by N-bromosuccinimide (NBS)/copper chloride (CuCl). This method is applicable to a series of N-aryl beta-enaminoesters and N-aryl beta-amino nitro-olefins containing different substituents such as halides, alkyls, heterocycles, etc. A variety of 2-aryl-3-carboxylate indoles and 2-aryl-3-nitro indoles have been prepared with moderate to quantitative yields in only 20 minutes to 2 hours in refluxing ethanol. Moreover, in order to demonstrate the synthetic utility of this transformation, 2-phenyl-3-nitro indole was subjected in reduction and reduction-alkylation to generate 2-phenyl-3amino indole and N, N-diethyl-2-phenyl-3-amino indole respectively.