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

The dimerization of olefins by well-defined cationic η 3 -allyl−palladium complexes of the type [(C 3 H 5 )Pd(L)(PR 3 )] + [BAr‘ 4 ] - (Ar‘ = [3,5-C 6 H 3 (CF 3 ) 2 ]; L = OEt 2, H 2 O; R = cyclohexyl (Cy), n -butyl ( n Bu)) has been studied. These complexes react with ethylene or methyl acrylate at −80 °C with loss of L to form the η 2 -olefin complexes [(C 3 H 5 )Pd(η 2 -olefin)(PR 3 )] + [BAr‘ 4 ] - (olefin = H 2 C CH 2, CH 2 CHC(O)OCH 3 ). Upon warming, allyl−olefin coupling occurs. The dimerization of ethylene occurs rapidly at 0 °C with an observable ethyl−ethylene intermediate [(C 2 H 5 )Pd(C 2 H 4 ) 2 (PCy 3 )] + [BAr‘ 4 ] - . Methyl acrylate reacts to form a stable acrylate chelate complex, [(CH 3 O(O)CCH 2 CH 2 )Pd(CH 2 CHC(O)OCH 3 )(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.