Phosphine ligands, ethylene insertion with

Palladium catalysts with simple monodentate phosphine ligands (e.g. PPh3) can catalyze the methoxycarbonylation of ethylene. However, the Lucite process employs a bulky diphosphine, 1,2-( Bu2PCH2)2CgH4, and is highly active and selective under quite mild conditions (10 bar/80°C). Two alternative catalytic cycles are possible, based either upon a palladium hydride or a palladium methoxide complex (Figure 9), and mechanistic and spectroscopic studies indicate that the hydride cycle is dominant. The alkene and CO insertion steps are the same as those in the Pd-catalyzed co-polymerisation of CO and alkenes to polyketones (Section 4.4). 

We shall now revisit the productive cycle in Fig. 14 for alternate CO/ethylene insertion in order to find possible explanations for why Ni(II) systems have such a low activity with respect to CO/ethylene co-polymerization. Our analysis will make use of recent studies of cationic nickel complexes with bidentate phosphine ligands due to Brookhart et al. [44] as well as the observation that nickel is more oxo-philic than palladium. 

---