Nitrile hydration catalysed by palladium(II) complexes

Abstract

The palladium(II) complexes [Pd(H₂O)₄]²⁺, cis-[Pd(en)(H₂O)₂]²⁺, cis-[Pd(Met-OMe)(H₂O)₂]²⁺, cis-[Pd(dtcol)(H₂O)₂]²⁺ and [Pd(dien)(H₂O)]²⁺, which contain aqua, ethane-1,2-diamine (en), methionine methyl ester (Met-OMe), 1,5-dithiacyclooctan-3-ol (dtcol), and diethylenetriamine (dien) ligands, catalysed selective hydration of various nitriles, yielding the corresponding carboxamides. Further hydrolysis to carboxylic acids was not detected. The catalysed reactions are enhanced as much as 10⁶-fold over the uncatalysed ones. Equilibrium constants for co-ordination of nitriles to palladium(II) complexes were determined or estimated. Since carboxamides do not detectably co-ordinate to palladium(II) in solutions containing water, the product of hydration does not inhibit the reaction. Carboxamidate anion, however, co-ordinates to palladium(II) in acetone solution. Kinetic effects of the following factors were examined: catalyst concentration, substrate concentration, pD value, water concentration, electrophilicity of the nitrile group in the substrate, and electron donation and trans effect of the ancillary ligands in the catalyst. In the reaction catalysed by the four chelate complexes no intermediates were detected. In the hydration of CHCl₂CN catalysed by [Pd(H₂O)₄]²⁺ palladium(II)–iminol complexes were observed as intermediates. In aqueous solutions only bidentate iminol co-ordination was detected. In acetone solutions the more labile unidentate iminol co-ordination was observed as well. The substrate (nitrile), the product (carboxamide), and an iminol intermediate were monitored in the cycle catalysed by [Pd(H₂O)₄]²⁺. This reaction was analysed in terms of the Michaelis–Menten model of enzyme kinetics, and microscopic rate constants were determined for the pathways involving and not involving the iminol intermediate. Internal attack on the nitrile ligand by the aqua (not hydroxide) ligand and external attack on the nitrile ligand by solvent water occur at similar rates. A general scheme of catalysis is devised on the basis of the kinetic experiments.