Methoxycarbonylation of ethene by palladium(II) complexes with 1,1′-bis(diphenylphosphino)ferrocene (dppf) and 1,1′-bis(diphenylphosphino)octamethylferrocene (dppomf)

The square-planar bis(aquo) palladium(II) complexes [Pd(H2O)(2)(dppf)](OTs)(2) and [Pd(H2O)(2)-(dppomf)](OTs)(2) are effective catalysts for the methoxycarbonylation of ethene, yet they exhibit quite different selectivity: the dppf-modified catalyst produces several low molecular weight oxygenates, spanning from methyl propanoate to alternating oligomers of carbon monoxide and ethene, while the dppomf catalyst yields exclusively methyl propanoate (dppf = 1, 1'-bis(diphenylphosphino)ferrocene; dppomf = 1,1'-bis(diphenylphosphino)octa-methylferrocene; OTs = p-toluenesulfonate). In an attempt to rationalize the different selectivities of the two catalytic systems, the methoxycarbonylation of ethene has been carried out under various experimental conditions in both autoclaves and high-pressure NMR tubes. Also, a number of model compounds have been synthesized with the aim of elucidating the structure of intermediate species observed by NMR during catalysis. Model reactions for the initiation, propagation, and chain-transfer steps in either alternating copolymerization or selective production of methyl propanoate have been performed. On the basis of all of these studies, it can be concluded that the behavior of the dppf precursor is similar to that of any other Pd-II catalyst modified with a chelating diphosphine. In particular, the formation of beta-chelate intermediates from either Pd-H or Pd-OMe and their importance in controlling the perfect alternation of monomers have been experimentally demonstrated. The selective production of methyl propanoate with the use of the dppomf-modified catalyst has been attributed to the greater propensity of dppomf versus dppf to form Pd-II complexes with a dative Fe-Pd bond, which forces the P atoms to be trans to each other, yielding Pd-acyl species that do not react with ethene in MeOH. beta-Chelate species are not formed by the dppomf-modified catalyst.