The trans-chlorometalation of hetero-substituted alkynes:: A facile entry to unsymmetrical palladium YCY′ (Y, Y′ = NR2, PPh2, OPPh2, and SR) "pincer" complexes

A simple and efficient method for the preparation of unsymmetrical palladium YCY'PdCl (Y, Y, = NR2, Py, PPh2, OPPh2, and SR) "pincer" complexes has been disclosed from the chloropalladation of hetero-substituted alkynes. This method tolerates a variety of alkyne functional groups (amines, pyridine, thioethers, phosphines, and phosphinites) and allows the preparation of palladacycles containing different metalated ring sizes. Thus the reaction of Li2PdCl4 with hetero-substituted alkynes Me2NCH2C=CCH2CH2Y (Y = S-t-Bu, NMe2, PPh2, and OPPh2) 1-4 affords the "pincer" pallaclacycles (Me2NCH2(Cl)C=CCH(2)CH(2)YkappaN,kappaC,kappaY)PdCl 7-10, in high yields. Under the same reaction conditions the chloropalladation of o-MeSC6H4C=CCH2NMe2, 5, and o-NC5H4C=CCH2CH2S-t-Bu, 6, yields (C6H4(o-MeS)C= C(Cl)CH2NMe2-kappaS,kappaC,kappaN)PdCl, 11, and (t-BuSCH2CH2C=C(Cl)(o-NC5H4)-kappaS,kappaC,kappaN)PdCl, 12, respectively. The molecular structures of compounds, 7 and 11 have been ascertained by means of X-ray diffraction analyses. IR and NMR spectroscopic investigation of the species involved in these reactions suggests that the chloropalladation reaction proceeds through the coordination of only one donor group followed by coordination of the C=C bond to the metal center. Selective intermolecular chloride, nucleophilic,, addition on this activated unsaturated bond affords the more thermodynamically stable palladacyclic ring. Finally, coordination of the second donor group to the Pd center yields the "pincer" palladacycles.