Mechanism of Thermal Decomposition of Dineopentylbis(triethylphosphine)platinum(II): Formation of Bis(triethylphosphine)-3,3-dimethylplatinacyclobutane

Article abstract of DOI:10.1021/ja00542a009

The thermal decomposition of dineopentylbis(triethylphosphine)platinum(II) (1) in cyclohexane solution at 157 deg C yields bis(triethylphosphine)-3,3-dimethylplatinacyclobutane (4) by a reaction which involves dissociation of 1 equiv of triethylphosphine, intramolecular oxidative addition of the C-H bond of a neopentyl methyl group to platinum (3), and reductive elimination of neopentane.Carbon-carbon bond formation resulting in production of dineopentyl is a detectable side-reaction.The overal reaction has Arrenius activation parameters: E_a ca. 49 kcal mol^-1, log A ca. 20.The activation energy for phosphine dissociation is 27 - 35 kcal mol^-1.Transfer of a hydrogen atom from the triethylphosphine group to aneopentyl moiety occurs at a rate approximately 3percent that of transfer of hydrogen from the methyl of one neopentyl group to the methylene of the other.Any processes which abstract α-methylene hydrogens from the neopentyl group occur at less than 1percent the rate of processes which abstract hydrogens from the neopentyl methyl groups.Substitution of deuterium for hydrogen in either the neopentyl methyl groups or the triethylphosphine groups slows the decomposition reactions (k_H/k_D ca. 3.0).The mechanism proposed for generation of 4 is based in part on deuterium-labeling experiments: comparison of results by using different labeling patterns for 1 demonstrates the special utility of "inverted" experiments in which hydrogen transfer from a specific site is examined in a system which is otherwise perdeuterated.The driving force for the conversion of 1 to 4 is not obvious: it may be relief of steric strain in 1 , changes in electronic energy due to reorganization of ligands around platinum, or changes in entropy.