Determination of key intermediates for homogeneous water-gas shift reaction and hydrocarbonylation of ethylene to diethyl ketone catalyzed by the ‘Pd(OAc) 2-PPh 3-CF 3COOH/H 2O’ system

Abstract

The homogeneous water-gas shift reaction (WGSR) and hydrocarbonylation of ethylene to diethyl ketone (DEK) catalyzed by the ‘Pd(OAc) 2-PPh 3-CF 3COOH/H 2O’ system have been studied. The main kinetic regularities of these reactions have been elucidated, and the catalytically active palladium species have been detected and characterized by 1H, 13C and 31P NMR. The key palladium intermediate for both catalytic reactions is the cationic hydride complex [(PPh 3) 3PdH] + 1. In the case of the WGSR, this complex serves as a source of dihydrogen, and in the case of the hydrocarbonylation of ethylene it undergoes transformations via consecutive rapid insertions of C 2H 4 into a Pd-H bond and then of CO into a Pd-C bond to form, respectively, palladium ethyl and propionyl complexes. All the kinetic and NMR data obtained support the mechanism involving the phosphinepalladium(II) complex reduction with CO to form CO 2 as the rate-determining step for the WGSR, and the mechanism involving formation of the species 1 and the palladium propionyl complex interaction with C 2H 4 as slow steps for the hydrocarbonylation of ethylene with CO/H 2 to give DEK. The possible reasons for the relative stability of the palladium hydride, ethyl and propionyl complexes in strongly acidic medium and the high selectivity of DEK formation in the hydrocarbonylation reaction are discussed.