Insertion of 1,1-Me2propadiene across the Pd-C bond of pyridyl-thioether methyl complexes.: A mechanistic study

Novel neutral and cationic palladium methyl complexes containing pyridylthioethers (R'N-SR) as ancillary ligands and different leaving groups were synthesised and characterised by spectroscopic methods and elemental analysis. The reactivity of these complexes toward allene (allene = DMA = 1,1-Me(2)propadiene) insertion across the Pd-C bond was studied under different conditions. The influence of steric and electronic parameters, solvents and leaving groups was explored and an overall mechanistic picture is proposed. The distortion promoted by the ancillary ligand geometry together with the nature of the leaving group proved to be of primary importance in determining the reactivity of the complexes under study. The observed rate constants span several orders of magnitude. On the contrary, only a modest effect is observed when solvents with increasing dielectric constants are employed. The complex [Pd(PPh3)(Me)(MeN-(SBu)-Bu-t)](+) shows a reactivity modulated by the presence of the phosphine which imparts to the substrate a peculiar reactivity owing to the scarce tendency of the phosphine itself to act as a leaving group. (C) 2002 Elsevier Science B.V. All rights reserved.