Aqueous–Organic Biphasic Hydrogenation of trans‐Cinnamaldehyde Catalyzed by Rhodium and Ruthenium Phosphane‐Free Porphyrin Complexes

Abstract

AbstractThe synthesis and spectroscopic characterization of new rhodium and ruthenium complexes with a porphyrin, possessing carboxylic groups in the arms of the porphyrin ring, is described. The complexes are completely soluble in water in the presence of a base (KOH or triethylamine). These phosphane‐free and air‐stable complexes were evaluated as catalysts in the aqueous hydrogenation of trans‐cinnamaldehyde. Catalysis was more efficient with the ammonium salts of the complexes rather than the corresponding potassium carboxylate salts, preferably in water/toluene rather than in neat water. The effect of catalyst concentration, reaction temperature, hydrogen pressure and methanol addition on the catalytic activity of the complexes in the presence of triethylamine in an aqueous–organic biphasic system was investigated. The catalysts were presumably homogeneous, and could be recycled and reused without any significant difference in activity and selectivity. This is the first time that porphyrin–metal complexes have been used as catalysts for hydrogenation of α,β‐unsaturated aldehydes.