Hydrocarbonylation of enol esters catalyzed by a palladium(II) complex

The palladium(II)-catalyzed carbonylation of various enol esters in the presence of methanol under pressurized carbon monoxide was investigated. As typical results, the carbonylation of isopropenyl acetate afforded a cyclocarbonylation product, 2-methoxy-2,5,5-trimethyl-1,3-dioxolan-4-one, in 46% yield under carbon monoxide of 150 atm at 100 degrees C, whereas vinyl acetate selectively gave an alpha-hydroesterification product, methyl 2-acetoxypropionate, in 56% yield under 250 atm at 100 degrees C. On the contrary, the reaction of 3,3-dimethyl-1-buten-2-yl acetate gave exclusively a beta-hydroesterification product, methyl 3-acetoxy-4,4-dimethylpentanoate, in 64% yield. The yields of these products show a maximum as a function of the base/palladium ratio and of the carbon monoxide pressure. The effects of the reaction variables, such as the MeOH/substrate ratio, the structure of the base, the base/Pd ratio, the CO pressure, and the reaction temperature, were examined for optimizing the process. The mechanisms for the unique cyclocarbonylation and highly regioselective hydrocarbonylation of enol esters involving hydridopalladium(II) intermediate are discussed.