Ruthenium-Catalyzed Cycloisomerization of 2,2'-Diethynyl- biphenyls Involving Cleavage of a Carbon-Carbon Triple Bond.

A ruthenium complex catalyzes a new cycloisomerization reaction of 2,2'-diethynylbiphenyls to form 9-ethynylphenanthrenes, thereby cleaving the carbon-carbon triple bond of the original ethynyl group. A metal-vinylidene complex is generated from one of the two ethynyl groups, and its carbon-carbon double bond undergoes a [2+2] cycloaddition with the other ethynyl group to form a cyclobutene. The phenanthrene skeleton is constructed by the subsequent electrocyclic ring opening of the cyclobutene moiety.