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(CH 2 PPh 2 ) 3 (triphos) has been investigated in the homogeneous phase. Both [Rh(DMAD)(triphos)]PF 6 (DMAD = dimethyl acetylenedicarboxylate) and [Ru(MeCN) 3 (triphos)](SO 3 CF 3 ) 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 3 H -indolium cation, which contains a localized C N bond. The rhodium precursor was more efficient than the ruthenium species and allowed for hydrogenation of the substrate even at 60 °C with turnover 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.