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(CH₂CH ₂C(O)Me)(P-P)]BAr′₄ have been studied by in situ high-pressure NMR spectroscopy in CH₂Cl₂; 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(diphenyl-phosphanyl)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.