A new, high-yield synthesis of perchlorodiphenylacetylene (2) by dechlorination of cis- perchlorostilbene (la) with Cu at 310" C is described. The photochemical reaction of tolane 2 with C1, or tetrachloroethylene affords trans-perchlorostilbene (lb) or mixtures of perchloro- 1, 2-diphenylcyclobutene (3) and perchloro-1, 2-dihydro-cyclobuta [l] phenanthrene (4), respectively. The thermal dimerization of tolane 2 at 350" C gives a mixture of perchloro-1, ...