Insertion of substituted Alkynes into the Pd-C bond of methyl and vinyl palladium(II) complexes bearing pyridylthioethers as ancillary ligands. The influence of ligand substituents at pyridine and sulfur on the rate of insertion

The palladium(II) chloro methyl complexes bearing the bidentate 6-R-C5H3N-2-CH2SR' (RN-SR'; R = H, Me, Cl; R' = Me, t-Bu, Ph) and the potentially terdentate 2,6-(CH2SR')(2)-C5H3N (S-N-S(R'); R'= Me, t-Bu, Ph) pyridylthioethers as ancillary ligands were synthesized, characterized, and reacted with substituted alkynes ZC equivalent to CZ (Z = COOMe, Z' = COOt-Bu, Z" = COOEt). The reactions were followed under second-order conditions by H-1 NMR technique, and the reaction rates were determined. The corresponding vinyl derivatives were synthesized, and in the case of the complexes [PdCl(ZC=CZMe)(MeN-SPh)] and [PdCl(ZC= CZMe)(C1N-St-Bu)] (Z = COOMe) reaction rates for alkyne insertion yielding the corresponding butadienyl complexes were also determined. The rate of insertion of the second alkyne on the vinyl complex is more than 3 orders of magnitude lower than the first insertion rate in both the studied complexes, thereby allowing easy separation between vinyl and butadienyl derivatives and an easy preparation of mixed butadienyl esters. Furthermore, the reaction rates are strongly dependent on the steric and electronic features of the ancillary ligands. In particular, the distortion of the complex main coordination plane, induced by the substituent in position 6 of the pyridine ring, was found to significantly influence the substrate reactivity. The structures of the mono-inserted vinyl [PdCl(ZC CZMe)(MeN-St-Bu)] (1) and the bis-inserted butadienyl [PdCl((ZC=CZ)(2)Me)(MeN-St-Bu)] (2) complexes were determined by X-ray diffraction, and the persistence of a structural distortion of the complex skeleton was observed. Moreover, the distortion may be related to facile ancillary ligand displacement, a feature that can be exploited for the synthesis of substrates that would not be easily obtained otherwise.