The methoxycarbonylation of vinyl acetate catalyzed by palladium complexes of [1,2-phenylenebis(methylene)]bis[di(tert-butyl)phosphine]

Palladium complexes of [1,2-phenylenebis(methylene)]bis[di(tert-butyl)phosphine] (1) catalyze the methoxycarbonylation of vinyl acetate (=ethenyl acetate) in the presence of methanesulfonic acid (Scheme 1). High selectivities to ester products can be obtained if free phosphine ligand is in excess over the amount of added acid (Table 1). Selectivities to methyl 2-acetoxypropanoate, a precursor to lactate esters, can be as high as 3.6:1 at low temperature and pressure (Table 2). Replacing Bu-t by Pr-i groups leads to less-active catalysts and lower selectivities to the branched product. Replacing the phenylene moiety by a naphthalenediyl moiety also gives lower activity, but with similar selectivity to the phenylene-based analogues. Linear hydrocarbon-chain linkers as the backbone instead of the phenylenebis(methylene) linker leads to poor catalysis, except for a propane-1,3-diyl linker, which gives good rates but poor branched selectivity (Table 5). The effect of different reaction conditions on the catalysis is discussed. The syntheses of the new xylene-based diphosphines 2-5 with one to four Pr-i groups replacing the Bu-t groups at the P-atoms of 1 and of the ligands 6 and 7 based on 1,2- and 2,3-dimethylnaphthalene are also described (Schemes 2 and 3).