Reactivity of ketones in homogeneous catalytic hydrogenation with cationic rhodium complexes
The catalytic hydrogenation of several ketones with cationic rhodium complexes has been investigated at 30 °C with hydrogen at atmospheric pressure. The rate of the reaction depended very much on the structure of both the ketone and phosphine ligand, with triethylphosphine giving the highest activity among the ligands used. An electron-withdrawing substituent in the ketone was found to increase its reactivity among both alkyl and aryl ketones; the rates of reaction of the former ketones were generally faster. Benzyl methyl ketone showed a very high reactivity among aryl ketones, suggesting that the phenyl group may be in a position to interact with the rhodium atom to enhance the reactivity. Unsaturated ketones are hydrogenated catalytically to yield saturated ketones first, which are further hydrogenated to corresponding saturated alcohols. No trace of unsaturated alcohol was observed in this hydrogenation, even when the hydrogenation rate of the olefinic bond in the unsaturated ketone was much smaller than that of the carbonyl group of the corresponding saturated ketone in the consecutive hydrogenation. The reactivities of ketones are discussed in connection with their co-ordination and the hydrogenation mechanisms.