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⋮CZ (Z = COOMe, Z‘ = COOt-Bu,
Z‘ ‘ = COOEt). The reactions were followed under second-order conditions by 1H 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)2Me)(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.