Hydrogenation of Indole by Phosphine-Modified Rhodium and Ruthenium Catalysts

The selective hydrogenation of indole (IN) to indoline (INE) by rhodium and ruthenium catalysts modified with the tripodal polyphosphine ligand MeC(CH2PPh2)3 (triphos) has been investigated in homogeneous phase. Both [Rh(DMAD)(triphos)]PF6 (DMAD = dimethylacetylene dicarboxylate) and [Ru(MeCN)3(triphos)](SO3CF3)2 have been found to generate effective catalysts for the conversion of IN to INE in THF in the presence of triflic acid. The protic acid is required to generate the 3H-indolium cation that contains a localized C=N bond. The rhodium precursor was more efficient than the ruthenium one and allowed for hydrogenation of the substrate already at 60 °C with turn-over-frequencies as high as 100. Catalytic experiments in autoclaves and in high-pressure sapphire NMR tubes have provided valuable information on the hydrogenation mechanism of IN.