NITRILE HYDRATION CATALYZED BY PALLADIUM(II) COMPLEXES

The palladium(I) 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(di en)(H2O)](2+), which contain aqua, ethane-1,2-diamine ten), methionine methyl ester (Met-OMe), 1,5-dithiacyclooctan-3-ol (dtcol), and diethy lenetriamine (dien) ligands, catalysed selective hydration of various nitriles, yielding the corresponding carboxamides. Further hydrolysis to carboxylic acids was not detected. The catalysed reactions are enha nced as much as 10(6)-fold over the uncatalysed ones. Equilibrium cons tants for co-ordination of nitriles to palladium(II) complexes were de termined or estimated. Since carboxamides do not detectably co-ordinat e to palladium(II) in solutions containing water, the product of hydra tion does not inhibit the reaction. Carboxamidate anion, however, co-o rdinates to palladium(II) in acetone solution. Kinetic effects of the following factors were examined: catalyst concentration, substrate con centration, pD value, water concentration, electrophilicity of the nit rile group in the substrate, and electron donation and trans effect of the ancillary ligands in the catalyst. In the reaction catalysed by t he four chelate complexes no intermediates were detected. In the hydra tion of CHCl2CN catalysed by [Pd(H2O)(4)](2+) palladium(II)-iminol com plexes were observed as intermediates. In aqueous solutions only biden tate iminol coordination was detected. In acetone solutions the more l abile unidentate iminol co-ordination was observed as well. The substr ate (nitrile), the product (carboxamide), and an iminol intermediate w ere 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 invo lving and not involving the iminol intermediate. Internal attack on th e nitrile ligand by the aqua (not hydroxide) ligand and external attac k on the nitrile ligand by solvent water occur at similar rates. A gen eral scheme of catalysis is devised on the basis of the kinetic experi ments.