Further N.M.R. Studies of the Dynamic Stereochemistry of π-Allylic Palladium(II) – Tertiary Phosphine Systems

A series of cations of the type 1,2-bis(diphenylphosphino)ethane(π-all)palladium(II) (all = allyl, 2-methylallyl, 2-tert-butylallyl, 1-methylallyl, 1,1-dimethylallyl, syn-1-tert-butyl-2-methylallyl) have been isolated as PF6− and/or BF4− salts. Variable temperature n.m.r. data fully substantiate the proposal that pyridine promoted exchange of syn and anti proton sites in such complexes occurs via a π → σ → π mechanism that involves consecutive SN2 ligand substitutions. Comparison of this variable temperature n.m.r. data with that obtained for the complexes 1,2-bis(diphenylphosphino)ethane(π-all)palladium(II) chloride (prepared in situ) has shown that the chloride ion, functioning as a nucleophile, is also effective in promoting π → σ → π reactions. Studies of bis(dimethylphenylphosphine) (π-1,1-dimethylallyl)palladium(II) chloride in CDCl3 below −30° have shown that rapid consecutive SN2 substitution reactions, in which chloride acts as a nucleophile, are also occurring in systems containing monodentate tertiary phosphine ligands. As such the PR3 promoted syn–syn and anti–anti proton site interchange observed in complexes of the type [(π-2-methylallyl)PdCl(PR3)] has been reinterpreted in terms of consecutive ligand substitution reactions involving PR3 and Cl ligands instead of the original proposal which involved an in plane rotation of the π-allylic ligand.