Unraveling the o ‐Methoxy Effect in the CO/Ethene Copolymerization Reaction by Diphosphanepalladium(II) Catalysis

Relevant steps in the CO/ethene copolymerization reaction, namely the migratory insertion of [Pd(Me)(CO)(P‐P)]BAr′ 4 and the carbonylation of the β‐keto chelates [Pd(CH 2 CH 2 C(O)Me)(P‐P)]BAr′ 4 have been studied by in situ high‐pressure NMR spectroscopy in CH 2 Cl 2 ; P‐P = 1,2‐bis[bis(2‐methoxyphenyl)phosphanyl]ethane or 1,3‐bis[bis(2‐methoxyphenyl)phosphanyl]propane. This study, backed by batch catalytic reactions in the same solvent, has contributed to rationalizing the higher catalytic activity of Pd II catalysts modified with o ‐methoxy‐substituted diphosphane ligands as compared to analogous Pd II catalysts with 1,2‐bis(diphenylphosphanyl)ethane (dppe) and 1,3‐bis(diphenylphosphanyl)propane (dppp) ligands. It was found that o ‐MeO substituents on the phosphorus aryl rings ease the opening of the β‐chelate ring by CO and affect the kinetics of the overall CO/ethene copolymerization. Indeed, unlike the catalysts with the dppe and dppp ligands, the rate of carbonylation of the o ‐MeO‐modified β‐keto chelates is limited by the Pd(alkyl)(CO) migratory insertion, which makes the overall copolymerization process independent of the CO pressure. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)