Study of hydrogenation of olefins catalyzed by polymer-bound palladium (II) complexes

Abstract

A polymer-bound palladium (II) chloride complex has been prepared by the reaction of palladium chloride with a phosphinated polystyrene. Under mild conditions the polymer palladium complex catalyzes the hydrogenation of alkenes and alkynes, particularly the selective hydrogenation of conjugated dienes to monoenes. The catalytic activity for a variety of substrates decreases in the following order: conjugated dienes > nonconjugated dienes > terminal olefins > internal olefins. Oxygen-containing solvents remarkably promote the catalytic activity of the palladium complex. The rates of hydrogenation of cyclohexene, styrene, and 1,3-cyclooctadiene have been studied and the dependence on factors such as substrate concentration, catalyst concentration, pressure, and temperature has been determined. The data can be accommodated by rate expressions of the form: rate = k 1k 2[S][ H 2 ][A] (k −1 + k 1[S] + k 2[ H 2 ] for cyclohexene, and rate = k 2[H 2][ A] for styrene and 1,3-cyclooctadiene, where [ S] and [ A] are the olefin and catalyst concentrations, respectively, and [H 2] is the concentration of hydrogen in solution. A mechanism for hydrogenation is proposed on the basis of the kinetic studies. It is revealed that the reactivities of the polymer palladium complex catalyst and of an analogous catalyst system PdCl 2(PPh 3) 2SnCl 2 reflect the electronic state and the coordination number of the complexes.