Mechanistic studies of catalytic olefin dimerization reactions using electrophilic eta(3)-allyl-palladium(II) complexes

The dimerization of olefins by well-defined cationic eta 3-allyl-palladium complexes of the type [(C3H5)Pd(L)(PR(3))](+)[BAr'(4)](-) (Ar' = [3,5-C6H3(CF3)(2)]; L = OEt(2), H2O; R cyclohexyl (Cy), n-butyl ((n)Bu)) has been studied. These complexes react with ethylene or methyl acrylate at -80 degrees C with loss of L to form the eta(2)-olefin complexes [(C3H5)Pd(eta(2)-olefin)(PR(3))](+)[BAr'(4)](-) (olefin = H2C=CH2, CH2=CHC(O)OCH3). Upon warming, allyl-olefin coupling occurs. The dimerization of ethylene occurs rapidly at 0 degrees C with an observable ethyl-ethylene intermediate [(C2H5)Pd(C2H4)2(PCy(3))](+)[BAr'(4)](-). Methyl acrylate reacts to form a stable acrylate chelate complex, [(CH3O(O)CCH2CH2)Pd(CH2=CHC(O)OCH3)(PR(3))(+)[BAr'(4)](-), which is the catalyst resting state for methyl acrylate dimerization which occurs at room temperature to give predominantly trans-dimethyl-2-hexenedioate.