Rhodium Tolane Complex and Cyclotrimerization of Substituted Acetylenes Catalyzed by Transition Metal Complexes

Cyclotrimerization of tolane and phenylacetylene has been studied from the viewpoint of the reaction intermediates and the composition of reaction products. Rh4(CO)12 and Co4(CO)12 have been found to catalyze the trimerization of tolane and phenylacetylene to yield some substituted benzenes. By the reaction of Rh4(CO)12 with tolane, an unknown complex was obtained. The triphenyl phosphine derivative of this complex has been determined to be Rh2(CO)3(C6H5C≡CC6H5)2P(C6H5)3. From infrared spectroscopic study, it was concluded that the two tolane molecules in the complex are connected to rhodium atoms separately without completion of dimerization. On the basis of the ratio of 1,2,4-triphenylbenzene to 1,3,5-triphenylbenzene, the reaction course of trimerization is discussed.