Chemistry of 1-Fluoro-2,3,4-triphenylcyclobutadiene Dimers

The reaction of 2,4-dichloro-1,1-difluoro-3-phenyl-2-cyclobutene 1 with excess phenyllithium and
subsequent transformations of the products have been reinvestigated. The phenyllithium reaction appears
to proceed through the intermediacy of a fluorotriphenylcyclobutadiene 2 to produce a well-characterized
dimeric trans-hexaphenyldifluorotricyclooctadiene 3a. Subsequent transformations of 3a gave a pentaphenyldihydrodifluoropentalene 4, which on acid hydrolysis formed a pentaphenyldihydropentalenone
5. When 3a was photolyzed in benzene, after purification, it afforded 6, an isomer of 5, probably by way
of 7, an isomer of 4. Thermolysis of 3a also provided, in low yield, a substance believed to be a
pentaphenylfluorophenanthrene 8. Along with isolation of 3a, and probably arising from a different isomer
of the 3 family, was a pentaphenylfluorophenanthrene 9, which was suspected of being an isomer of 8.
Single-crystal X-ray studies were used to derive structures for 4, 5, 6, and 9. Formation of the unusual
and intriguing transformation products has at least been rationalized.