Water-soluble organometallic compounds. 4. Catalytic hydrogenation of aldehydes in an aqueous two-phase solvent system using a 1,3,5-triaza-7-phosphaadamantane complex of ruthenium

The water-soluble phosphine complex of ruthenium(II), cis-RuCl[sub 2](PTA)[sub 4] (3), has been prepared by the reduction of RuCl[sub 3] in ethanol in the presence of the air-stable phosphine 1,3,5-triaza-7-phosphaadamantane (2). Complex 3 is an effective catalyst for the regioselective conversion of unsaturated aldehydes to unsaturated alcohols using a biphasic aqueous/organic medium with sodium formate as the source of hydrogen, thus affording facile catalyst recovery and recycle. Both aromatic and aliphatic aldehydes were reduced to the corresponding alcohols. The formate ion was shown to directly be the hydrogen source by deuterium labeling experiments. The rate of hydrogenation of benzaldehyde was found to be first order in catalyst concentration, first order in substrate concentration (in the low-concentration regime), and independent of formate concentration at concentrations >2.5 M. The reduction process was greatly retarded by the presence of excess phosphine ligand. The apparent activation energy determined was 23.9 kcal mol[sup [minus]1]. A catalytic cycle was proposed which involves the rapid formation of a ruthenium hydride of a metal alkoxide intermediate and protonation complexes the cycle. Recycling experiments demonstrated the catalyst to be quite robust.