Masami Terasawa

A coordinatively unsaturated, polymer-bound palladium(0) complex. Synthesis and catalytic activities

Abstract A coordinatively unsaturated palladium(0) complex was prepared by the reduction of a polymer-bound palladium(II) chloride complex, which was prepared by the reaction of poly-4-diphenylphosphinomethylstyrene with palladium chloride, with hydrazine in ethanol in the presence of triphenylphosphine. Catalytic activities of the polymerbound palladium(0) complex were examined for three representative types of palladium(0)-induced reactions involving oxidative addition of halides to the metal: (i) vinylic hydrogen substitutions with aryl halides, (ii) acetylenic hydrogen substitutions with aryl halides, (iii) vinylic halogen substitutions with Grignard reagents. Use of the catalyst resulted in formation of corresponding products in good yields. The catalytic activity is comparable to that of analogous homogeneous catalysts, yet is not remarkably lowered on being recycled.

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

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 = k1k2[S][H2][A](k−1 + k1[S] + k2[H2] for cyclohexene, and rate = k2[H2][A] for styrene and 1,3-cyclooctadiene, where [S] and [A] are the olefin and catalyst concentrations, respectively, and [H2] 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 PdCl2(PPh3)2SnCl2 reflect the electronic state and the coordination number of the complexes.

Selective hydrogenation of acetylenes to olefins catalyzed by polymer-bound palladium(II) complexes

Abstract Semihydrogenations of 15 acetylenes to olefins catalyzed by polymer-bound palladium(II) complexes have been studied synthetically and mechanistically, and the hydrogenation of phenylacetylene has been studied kinetically. In the hydrogenation of isolated acetylenes, the catalyst generated corresponding olefins in high selectivities (above 92%). In the case of conjugated acetylenes, the catalyst generated corresponding conjugated olefins in relatively low selectivities (71–85%), whereas phenylacetylene was hydrogenated to styrene in a high selectivity (93%). A high activity of the catalyst was observed in oxygen-containing solvents such as dimethylformamide, tetrahydrofuran, dimethyl sulfoxide, and ethanol. The catalytic activity is affected more strongly by the π-acidity of acetylenes than their steric factor. The hydrogenation rate of phenylacetylene is expressed by the form: R = k 2 [H 2 ][ A ], where [H 2 ] and [ A ] are hydrogen and the catalyst concentrations, respectively. A mechanism for the hydrogenation is proposed on the basis of kinetic studies. Finally, it is summarily discussed what factors control the activity and the selectivity of the polymer catalyst for the hydrogenation of carbon-carbon double and triple bonds. This polymer-bound palladium complex was shown to be comparable in selectivity to cationic rhodium and the Lindlar catalysts.

Catalysis of Polymer-Bound Palladium Complexes

Heterogeneous reactions with metal catalysts have been studied extensively, however, the design of the catalysts is not very easy because the active sites of the catalysts are not well-defined. On the other hand, homogeneous complex catalysts have well-defined active sites, consequently the steric and electronic environments of the metal atom can, at least in principle, be varied easily. But homogeneous catalysts have so far been used only in limited chemical processes because of many practical problems including corrosion, deposition of the catalysts on the wall of the reactors, and recovery of the catalysts from the reaction mixture. In order to combine the advantages of both homogeneous and heterogeneous catalysts, methods for chemically bonding metal complexes to an insoluble organic or inorganic polymer have been developed in recent years (1). The catalysts, prepared in such a manner, are situated between those usually classified as “heterogeneous” and those classified as “homogeneous”, and can be regarded as a new class of catalysts.

Study of the distribution of metals on the surface of polymer-bound palladium(II) complexes by X-ray photoelectron spectroscopy

The distribution of metals on the surface of aminated polystyrene-bound palladium(II) complexes was found to be constant and independent of the metal loading by X-ray photoelectron spectroscopy.