Cyclotrimerization of Phenylacetylene Catalyzed by Halides of Niobium and Tantalum

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Abstract
Halides of niobium and tantalum (NbX5, TaX5; X=Cl, Br, F) catalyzed the cyclotrimerization of phenylacetylene in hydrocarbon and chlorinated hydrocarbon solvents at 0–90 °C. When NbCl5 and TaCl5 were used as catalysts, two cyclic trimers, 1,2,4- and 1,3,5-triphenylbenzenes were exclusively formed, and their ratio varied from 17:83 to 94:6 depending on reaction conditions. The cyclotrimerization was more selective than those by other known catalysts. The cyclotrimerization by NbBr5 and TaBr5 occurred in a similar manner, but the reaction was less selective than that catalyzed by NbCl5 and TaCl5. Not only cyclotrimerization but also linear oligomerization proceeded by NbF5 and TaF5 as catalysts. The results obtained were discussed together with the linear polymerization of phenylacetylene catalyzed by MoCl5 and WCl6.