STEREOSPECIFIC PROTONATION OF COORDINATED ALKYNES

The reaction of [V(eta(5)-C5H5)(2)(eta(2)-PhCCPh)] with anhydrous HCl in tetrahydrofuran gives [V(eta(5)-C5H5)(2)Cl-2] together with cis-PhC HCHPh (ca. 85%) and trans-PhCHCHPh (ca. 15%). Kinetic studies indicate that the formation of cis-PhCHCHPh involves initial protonation of th e metal followed by migration of the hydride on to the alkyne to give the cis-vinyl species. Subsequent further protonation gives cis-PhCHCH Ph. trans-PhCHCHPh is formed by direct protonation of the coordinated alkyne to form the trans-vinyl species which, upon further protonation , gives the corresponding alkene.