Cycloaddition reaction of zirconacyclopentadienes to alkynes: Highly selective formation of benzene derivatives from three different alkynes

Zirconacyclopentadienes reacted with acetylenecarboxylates in the presence of a stoichiometric amount (2 equiv) of copper chloride to give benzene derivatives in high yields along with the formation of a copper mirror on the wall of the reaction vessel. Reactions of unsymmetrical zirconacyclopentadienes prepared from two different alkynes with acetylenecarboxylates gave benzene derivatives from three different alkynes in high yields with high selectivities. Preparation of unsymmetrical zirconacyclopentadienes and benzene derivatives formation can be done in one-pot, The structure of the reaction product of the unsymmetrical zirconacyclopentadiene having a trimethylsilyl group and a methyl group in the alpha- and beta-position, respectively, with acetylenedicarboxylate was determined by X-ray analysis. It indicated that the regiochemistry of those substituents was maintained during the reaction. When the coupling reaction of zirconacyclopentadienes with acetylenedicarboxylate in the presence of copper chloride was carried out at -78 degrees C, linear trienes were obtained as the intermediates in high yields after hydrolysis. Warming the mixture without hydrolysis gave the desired benzene derivatives in high yields. Use of an unsymmetrical zirconacyclopentadiene afforded a mixture of two isomers of the linear triene as intermediates after hydrolysis. When the mixture of the two isomers was warmed to room temperature without hydrolysis, both isomers gave the same benzene derivative. In the presence of a catalytic amount of copper chloride (10 mol %), this cycloaddition proceeded to give benzene derivatives in good yields without formation of the copper mirror. The reaction under the catalytic conditions at -78 degrees C also gave a linear triene after hydrolysis.