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 β‐enaminoesters and N‐aryl β‐amino nitroolefins 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‐3‐amino indole and N,N‐diethyl‐2‐phenyl‐3‐amino indole respectively. Intramolecular oxidative cyclization of N‐aryl enamines promoted by NBS/CuCl proceeded smoothly to provide a series of 2,3‐disubstituted indoles in moderate to quantitative yields.