The Mechanism of Nickel-Catalyzed Ethylene Hydrocyanation. Reductive Elimination by an Associative Process

Article abstract of DOI:10.1021/ja00277a022

The complex (c2H4)L(CN)(C2H5)Ni^II has been identified at -40 deg C by 1H, 31P, and 13C NMR spectroscopy as the primary intermediate in the catalytic hydrocyanation of ethylene.Reaction of this intermediate with L causes reductive elimination of propanenitrile and produces (C2H4)L2Ni which reacts with ethylene and hydrogen cyanide to regenerate the intermediate.Measurements of second-order rate constants at -50 to -10 deg C result in ΔH^exct=8.8 +/- 0.9 kcal/mol and ΔS^exct=-34 +/- 4 eu (ΔG^exct=16.7 +/- 0.1 kcal/mol at -40 deg C).At higher L concentrations, these nickel species are also in equilibrium with (C2H4)L3Ni, L4Ni, and HNi(CN)L3 which remove nickel from the productive catalytic cycle.Equilibrium constants relating these species and corresponding thermodynamic parameters have been determined.The rates of dissociation of L from L4Ni and HNi(CN)L3 have also been determined.The former reaction is very slow with ΔH^exct=20.1 +/- 1.6 kcal/mol and ΔS^exct=7 +/- 7 eu (ΔG^exct=18.5 kcal/mol at -40 deg C) whereas the latter is about 1E3 faster with ΔH^exct=18.8 +/- 2.7 kcal/mol and ΔS^exct=17 +/- 10 eu (ΔG^exct=14.8 kcal/mol at -40 deg C).