Mechanisms of 1,1-Reductive Elimination from Palladium: Elimination of Ethane from Dimethylpalladium(II) and Trimethylpalladium(IV)

Article abstract of DOI:10.1021/ja00404a034

The energies and entropies of activation for the 1,1-reductive elimination of ethane from cis-bis(diphenylmethylphosphine)dimethylpalladium(II) (2a) in polar and nonpolar solvents were determined.The rates of elimination are slower in polar solvents such as Me2SO, acetone, and acetonitrile than in nonpolar solvents such as benzene.The energies of activation in nonpolar solvents are very close (25 kcal/mol) to the calculated values (extended HMO).Lower energies of activation (6-10 kcal/mol) but high negative entropies of activation (ca.45 eu) in polar solvents are consistent with an elimination that produced a coordinatively unsaturated palladium(0) complex and a late transition state having the characteristics of the product, such that solvent coordinates during the transition state.Reaction of 2a or the corresponding bis(triphenylphosphine)dimetnylpalladium(II) complex 2b with methyl iodide yields ethane and the trans-bis(phosphine)iodomethylpalladium(II) complexes (10a,b).The second-order reaction proceeds through a rate-determining oxidative addition of methyl iodide to 2a,b, yielding the bis(phosphine)iodotrimethylpalladium(IV) intermediate, followed by a rapid elimination.In polar solvents, the rates of these reactions are faster than the 1,1-reductive eliminations from 2a,b mostly because of the lower entropies of activation in the oxidative addition step.In nonpolar solvents, the rates are comparable.The reaction of 2a,b with CD3I gave both C2H6 and C2H3D3, the ratios of these isomers in the reaction of 2a being most consistent with the trans oxidative addition reaction followed by statistical reductive elimination from adjacent methyls.