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 approximately hex-3-yne approximately 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 cyclotrimerization 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.