Solvent-free oligomerization of phenylacetylene catalyzed by (cyclopentadienyl)nickel complexes

The solvent-free reaction of phenylacetylene at 115 degrees C in the presence of nickelocene, [(eta-Cp)Ni](2)(PhC=CH), [(eta-Cp)Ni(CO)](2), (eta-Cp)Ni(NO), (eta-Cp)Ni(GeBr3)(CO), (eta-Cp)Ni[(P(OMe3)]Cl, (eta-Cp)Ni(Ph3P)Cl, (eta-Cp)Ni((Bu3P)-P-n)I, or (Ph3P)(2)Ni(CO)(2) gives rise to a mixture of cyclotrimers, linear oligomers and poly(phenylacetylene), no reaction being observed in the case of internal acetylenes. Cyclotrimer formation is favoured by the presence of (a) added phosphine (2 equiv.), or (b) (cyclopentadienyl)nickel catalysts bearing a chloro substituent at Ni. A reduction in reaction temperature results in lower conversion but favours linear oligomer and polymer formation. The extent of reaction is greatly reduced in the case of (a) nickelocene in the presence of 2 equiv. PBu3n, (b) (eta-Cp)Ni(GeBr3)(CO), or (c) (eta-Cp)Ni(NO). The main effect of the presence of solvent, regardless of whether it is potentially coordinating (toluene) or not (n-octane), is to suppress almost completely reactions catalyzed by nickelocene.