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

Abstract

The palladium(II) chloro methyl complexes bearing the bidentate 6-R-C 5 H 3 N-2-CH 2 SR′ (RN-SR′; R ) H, Me, Cl; R′ ) Me, t-Bu, Ph) and the potentially terdentate 2,6-(CH 2 SR′) 2 -C 5 H 3 N (S-N-S(R′); R′ ) Me, t-Bu, Ph) pyridylthioethers as ancillary ligands were synthesized, characterized, and reacted with substituted alkynes ZCtCZ (Z ) COOMe, Z′ ) COOt-Bu, Z′′ ) COOEt). The reactions were followed under second-order conditions by 1 H NMR technique, and the reaction rates were determined. The corresponding vinyl derivatives were synthesized, and in the case of the complexes [PdCl(ZCdCZMe)(MeN-SPh)] and [PdCl(ZCd 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(ZCdCZMe)(MeN-St-Bu)] (1) and the bis-inserted butadienyl [PdCl((ZCdCZ) 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.

Synthesis of novel heteroditopic carbene–pyridine palladium(II) chloro vinyl complexes. Comparative reactivity of different palladium vinyl derivatives toward transmetalation with alkynyl stannane

“…Thus, we resorted to our long-standing protocol exploiting the peculiar reactivity of palladium complexes bearing substituted pyridyl-thioethers as spectator ligands. In particular, the complex [Pd(MeN-SPh)(Me)Cl] (MeN-SPh = 2-methyl-6-phenylsulfanylmethyl-pyridine) displays a remarkable reactivity toward Pd\C bond insertion due to the distortion of the main coordination plane of the complex imposed by the methyl group in position 2 of the pyridine ring [6]. It is noteworthy that complex 2D does not show similar reactivity despite the presence of the 2-methyl substituted pyridine in the ligand scaffold since in this case the increased flexibility of the six-membered ring probably reduces the distortion.…”

“…It is noteworthy that complex 2D does not show similar reactivity despite the presence of the 2-methyl substituted pyridine in the ligand scaffold since in this case the increased flexibility of the six-membered ring probably reduces the distortion. Thus, we have reacted the silver carbene complexes 1 and the vinyl palladium derivatives of MeN-SPh synthesized according to the literature [6] in anhydrous CH 2 Cl 2 under strictly 1:1 stoichiometric conditions (Scheme 2).…”

“…10 ppm in the 1 H NMR spectra disappears whereas 13 C NMR experiments display the signals due to the carbene carbon coordinated to the metal at ca. 180 ppm.Furthermore, we have synthesized the complex 2D by reacting the silver carbene derivative 1D with the complex [Pd(COD)(Me)Cl] [13] in order to exploit its supposed high reactivity in promoting the alkyne insertion and consequently the direct synthesis of the vinyl species, thanks to the activating presence of a methyl substituent in position 6 of the pyridine ring [6]. Formation of complex 2D is clearly detected from precipitation of AgCl and its NMR spectrum displaying all the signals attributable to the carbene ligand together with those related to the protons of the methyl group coordinated to palladium at ca.…”

“…In the past we have been involved in the study of insertion of alkynes in methyl complexes of Pd(II) bearing pyridylthioether as ancillary ligands giving vinyl derivatives [6]. We therefore decided to investigate the possibility of obtaining new palladium vinyl species with NHC as spectator ligands.…”

The role of ancillary ligands and of electron poor alkenes and alkynes in stabilizing Pd(0) derivatives: A comparative study

“…The synthesis of the ligands 8-diphenylphosphanylquinoline (DPPQ) [14] and 8-diphenylphosphanyl-2-methylquinoline (DPPQ-Me) [15], the alkenes cis-and trans-1,2-bis[ (4-methylphenyl) …”