Reductive Elimination of a Carbon-Carbon Bond from Bis(trialkylphosphine)-3,3-dimethylplatinacyclobutanes Produces Bis(trialkylphosphine)platinum(0) and 1,1-Dimethylcyclopropane

Article abstract of DOI:10.1021/ja00377a011

The thermal decompositions of several platinacyclobutanes-bis(triethylphosphine)-(1), bis(triisopropylphosphine)-(2), and bis(tricyclohexylphosphine)-3,3-dimethylplatinacyclobutane (3) -have been examined in cyclohexane solution.Decomposition of 1 proceeds heterogeneously: it products platinum metal, neopentane, 1,1-dimethylcycloprorane, and ethilene and seems to be autocatalytic.Decompositions conducted using reaction mixtures containing mercury(0) proceed homogeneously and produce predominately 1,1-dimethylcyclopropane; any platinum metal produced is apparently amalgamated by Hg(0) and rendered catalytically inactive.Thermal decompositions of 2 and 3 proceed homogeneously, even in the absence of Hg(0), and yield 1,1-dimethylcyclopropane, bis(trialkylphosphine)platinum(0), and small quantity of neopentane.The rates of the decomposition decreases by addition of trialkylphosphine, and a study of these dependence of the rate on concentration of added phosphine for 2 indicates that one phosphine dissociates to produce a three-coordinate platinum(II) intermediate prior to reductive elimination of 1,1-dimethylcyclopropane.The Arrhenius activation parameters for decomposition of 2 and 3 are respectively E_a=46+/-3 kcal mol^-1, log A=23+/-2 and E_a=42+/-2 kcal mol^-1, log A=21+/-1.Substitution of deuterium for hydrogen in the triisopropylphosphine groups of 2 produces no change in the rate of the reaction.We cannot distinguish between mechanisms in which reductive elimination of 1,1-dimethylcyclopropane occurs directly from the three-coordinate intermediate and those in which oxidative addition of a C-H bond to platinum bycyclometalation of the phosphine produces a Pt(IV) intermediate in a step preceding rate-limiting reductive elimination of 1,1-dimethylcyclopropane.