The co-cyclotrimerization of hexynes and phenylacetylene with niobium(V) chloride as catalyst

Various alkynes were cyclotrimerized with NbCl₅ as catalyst and C₂H₅AlCl₂ as co-catalyst at an Al/Nb molar ratio = 2 in CCl₄ at room temperature and pressure. The tendency of the alkynes to cyclotrimerize decreased in the following order: phenylacetylene > hex-1-yne > hex-2-yne ~ hex-3-yne ~ oct-4-yne If phenylacetylene (X) was co-cyclotrimerized with another alkyne (Y), four different cyclotrimers were found, as represented by the following equation: X + Y → X₃ + X₂Y + XY₂ + Y₃ If the amount of cyclotrimers formed is plotted against the phenylacetylene mole fraction, a statistically random distribution of the products is observed. Differences with regard to the ideal random case are attributed to a polymerization reaction. During the cyclo-trimerization process there is therefore a statistically random coordination of the alkynes on the catalytic species, after which ring closure occurs to form the cyclotrimeric products.