Steric and electronic factors in the promoting activity of diphosphine ligands in cyclohexene hydrocarbomethoxylation catalyzed by palladium acetate

Cyclohexene hydrocarbomethoxylation catalyzed by Pd(OAc)2-p-toluenesulfonic acid-diphosphine systems has been investigated for a wide range of diphosphine structures and concentrations. The factors controlling the activity of the palladium-containing catalysts include the hydrocarbon moiety of the ligand and the mutual arrangement of the phosphine groups. A comparison between the promoting effects of monophosphine and diphosphine ligands has demonstrated that bridged trans-diphosphines are more efficient in kinetic and concentration terms (TOF and P/Pd ratio, respectively). In particular, the promoting activity of diphosphines is one order of magnitude higher than that of triphenylphosphine, and this effect is attained at 8–65 times lower P/Pd ratios. It is discussed how the catalytic properties of the systems depend on the chelate effect and on the geometric compatibility between the diphosphine structure and the arrangement of vacant s and d orbitals of the palladium center.