Phosphine Ligands in the Palladium-Catalysed Methoxycarbonylation of Ethene: Insights into the Catalytic Cycle through an HP NMR Spectroscopic Study

Novel cis-1,2-bis(di-tert-butyl-phosphinomethyl) carbocyclic ligands 6-9 have been prepared and the corresponding palladium complexes [Pd(O3SCH3)(L-L)][O3SCH3] (L-L=diphosphine) 32-35 synthesised and characterised by NMR spectroscopy and Xray diffraction. These diphosphine ligands give very active catalysts for the palladium-catalysed methoxycarbonylalion of ethene. The activity varies with the size of the carbocyclic backbone, ligands 7 and 9, containing four- and six-membered ring backbones giving more active systems. The acid used as co-catalyst has a strong influence on the activity, with excess trifluoroacetic acid affording the highest conversion, whereas excess methyl sulfonic acid inhibits the catalytic system. An in operando NMR spectroscopic mechanistic study has established the catalytic cycle and resting state of the catalyst under operating reaction conditions. Although the catalysis follows the hydride pathway, the resting state is shown to be the hydride precursor complex [Pd(O3SCH3)(L-L)][O3SCH3], which demonstrates that an isolable/detectable hydride complex is not a prerequisite for this mechanism.