Heterogeneous Catalytic Hydrogenation of Unprotected Indoles in Water: A Green Solution to a Long-Standing Challenge

An environmentally benign procedure for the hydrogenation of unprotected indoles is described. The hydrogenation reaction is catalyzed by PVC and activated by p-toluenesulfonic acid in water as a solvent. The efficacy of the method is illustrated by the hydrogenation of a variety of substituted indoles to their corresponding indolines which were obtained in excellent yields.