Reaction of methylphenylacetylene with bis(benzonitrile)palladium chloride (1) in chloroform gave 1,2,4-trimethy 1-3,5,6-triphenylbenzene (2, 58%), l,3,5-trimethyl-2,4,6-triphenylbenzene (3, 39%), and 1,2,3-trimethyl-4,5,6-triphenylbenzene (4, 3%). In benzene, a complex [(PhC2Me)3PdCl2]2 was isolated which readily decomposed to palladium chloride, 2, and 3. 2-Butyne reacted with 1 in benzene to give a complex [Cl(MeC2Me)3-PdCl]2 (6) and in chloroform to give [Cl(MeC2Me)3PdClPdCl2]" (7) and some complex 6. On spectroscopic evidence, both 6 and 7 are assigned structures involving a 2-chloro-3,4,5,6-tetramethyl-2-ira/u,4-cw,6-ira/w-octa-2,4,6triene -bonded at C7 and -bonded at C2,3 to palladium(II). The ligand can take up two positions with respect to the metal, with the coordinated double bond parallel or perpendicular to the coordination plane. Both complexes 6 and 7 decomposed readily to palladium chloride and hexamethylbenzene. A new mechanism for the trimerization of acetylenes is proposed.In 1962 one of us reported3 4a reinvestigation of the reaction, originally described by Malatesta, et al.,* in which diphenylacetylene was dimerized to an ethoxytetraphenylcyclobutenylpalladium chloride complex in the presence of palladium chloride in ethanol. Under slightly different conditions (aprotic solvents) from those used by Malatesta, et al., we observed that diphenylacetylene could also be catalytically trimerized to hexaphenylbenzene. In addition, a tetraphenylcyclobutadienepalladium chloride complex ([Ph4C4( PdCl2)"]z) was formed which eventually deactivated the catalyst.5 These results have been confirmed by a number of workers.6-8These reactions of diphenylacetylene and closely related acetylenes did not readily lend themselves to a more detailed investigation and, since the generality of the reaction was of considerable interest particularly as a very easy route to cyclobutadiene-metal complexes, we began an investigation of the reactions of some other acetylenes with palladium chloride.Our first attempts, using acetylene, propyne, and monophenylacetylene, did not lead to characterizable products. Reactions always proceeded very easily to give mixtures of metal complexes. However, the latter were polymeric and nonstoichiometric in nature and not easily handled. It is probable that the ligands in these complexes are linear polyenes; however, hydrogen-transfer and cyclization reactions are by no means excluded.9

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The solvent was removed from the mother liquors, the oil remaining (ca. 13 ml) was stirred with 15 ml of benzene, and an excess of anhydrous hydrazine was added. Metallic palladium precipitated and was removed by filtration. The solvent was removed and the oil remaining was chromatographed over alumina using light petroleum as solvent to remove impurities and HMB. A pale yellow oil (2.0 g) was obtained and was shown to consist of four fractions by vpc. Samples of each of these fractions were collected by preparative vpc using a 5-ft column (0.25 in.) of 20% SE30 at 185°.Fraction i (ca. 10%, 3 min) was identified by its nmr spectrum as vinylpentamethylcyclopentadiene, the product obtained by hydrazine reduction of 6 (see following paper).Fraction ii (ca. 7%, 4.5 min) was obtained in too small amount for complete characterization. From its mass spectrum, it appeared to be an isomer of fraction iv below.Fraction iii (ca. 25%, 13 min) was an oil with analysis corresponding to Ci6H22.