Mechanistic Studies of Catalytic Olefin Dimerization Reactions Using Electrophilic η³-Allyl−Palladium(II) Complexes

The dimerization of olefins by well-defined cationic η3-allyl−palladium complexes of the type [(C3H5)Pd(L)(PR3)]+[BAr‘4]- (Ar‘ = [3,5-C6H3(CF3)2]; L = OEt2, H2O; R = cyclohexyl (Cy), n-butyl (nBu)) has been studied. These complexes react with ethylene or methyl acrylate at −80 °C with loss of L to form the η2-olefin complexes [(C3H5)Pd(η2-olefin)(PR3)]+[BAr‘4]- (olefin = H2CCH2, CH2CHC(O)OCH3). Upon warming, allyl−olefin coupling occurs. The dimerization of ethylene occurs rapidly at 0 °C with an observable ethyl−ethylene intermediate [(C2H5)Pd(C2H4)2(PCy3)]+[BAr‘4]-. Methyl acrylate reacts to form a stable acrylate chelate complex, [(CH3O(O)CCH2CH2)Pd(CH2CHC(O)OCH3)(PR3)]+[BAr‘4]-, which is the catalyst resting state for methyl acrylate dimerization which occurs at room temperature to give predominantly trans-dimethyl-2-hexenedioate.