Effect of the structure and concentration of diphosphine ligands on the rate of hydrocarbomethoxylation of cyclohexene catalyzed by palladium acetate/diphosphine/TsOH system

Cyclohexene hydrocarbomethoxylation catalyzed by the Pd(OAc)2 – p-toluenesulfonic acid – diphosphine systems with broad variation of diphosphine structure and concentration was studied. It was shown that the hydrocarbon part of the structure and the mutual arrangement of the phosphine groups are the factors that control the activity of palladium-containing catalysts. By comparison of the promoting effects of mono and diphosphine ligands, it is demonstrated that bridging trans-diphosphines show higher efficiency with regard to both the kinetic (TOF) and concentration factors (low P/Pd ratios). In particular, their promoting activity is an order of magnitude higher than that for triphenylphosphine at lower P/Pd ratios (8–65 times). The results were interpreted from the standpoint of chelation effect and the geometric matching of the diphosphine structure to the arrangement of vacant s,d-orbitals of the Pd centre.

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► Present work represents first systematic study devoted to a comparative investigation of the most conventional bis-diphenyl-diphosphines in cyclohexene hydrocarbomethoxylation taken as a model reaction, aiming to study the optimal backbone of the diphosphine for the process.
► Rate of the process depends strongly on the geometry of the diphosphine added (related TOFs are given for all studied diphosphines).
► Diphosphines which work well in ethylene hydrocarbomethoxylation are almost inactive in cyclohexene hydrocarbomethoxylation.
► The most performant diphosphine has bicyclic structure – trans-2,3-bis-(diphenylphosphinomethyl)-norbornane.