A convenient preparation of indoline

An efficient method for chemically reducing indole to indoline using zinc dust and 85% phosphoric acid is described. 2, 3-Dimethylindole, 1,2,3, 4-tetrahydrocarbazole and N-methyl-l,2,3,4-tetrahydrocarbazole are also reduced to the corresponding dihydroindoles in somewhat lower yields. Unlike earlier acid-metal reductions of indole little or no polymerization is observed.