Selective Hydrogenation of 1, 4-Cyclohexadiene Catalyzed by Nickel Complexes Anchored to Polystyrene

Abstract

Under the mild conditions 1, 4-cyclohexadiene was selectively hydrogenated to cyclohexene by heterogeneous nickel catalysts, Ni(acac)₂-Al₂Et₃Cl₃-phosphinated polystyrene complexes. Two types of phosphine-containing polystyrene beads were prepared; copolymerization of styrene and ρ-(diphenylphosphino)styrene, and phosphination of commercial polystyrene. The results of the hydrogenation were compared with those of the homogeneous system using Ni(acac)₂-Al₂Et₃Cl₃-PPh₃ catalyst. In order to obtain a good selectivity for the hydrogenation, the conditions for the heterogeneous system were desirable to be very similar to those for the homogeneous one. It was noteworthy that the selectivity was good at a low degree of crosslinking and at moderate amounts of phosphine ligands in the polymer, and while it was not so good at high crosslinking. The interesting features observed are discussed in connection with the active species and the active sites.