Metal-Free Synthesis of Indoles from Arylhydrazines and Nitroalkenes at Room Temperature

Abstract

A metal-free cyclocondensation reaction between arylhydrazines and nitroalkenes has been developed for the synthesis of substituted indoles using a combination of I2 (20 mol%) and diethyl phosphite (2 equiv.) at room temperature. Both N-protected and unprotected indoles were obtained from a variety of easily accessible nitroalkenes in good yields. The Medicinally important tetrahydrocarbazole scaffolds were also obtained by employing cyclic nitroalkenes using this strategy. Gram-scale preparation of indole derivative has been successfully demonstrated using this methodology. The reaction likely proceeds through [3, 3]-sigmatropic rearrangements of hydrazone intermediate. Use of safe catalyst, metal and solvent free conditions, room temperature and broad substrate scope have made this indolization protocol advantageous to access these compounds.