THERMALLY-INDUCED CYCLODEHYDROGENATION OF BIARYLS - A SIMPLE RADICAL REACTION OR A SEQUENCE OF REARRANGEMENTS

Comprehensive BLYP/6-311G* calculations identify three distinct react ion pathways relevant to the thermally induced cyclodehydrogenation of 1-phenylnaphthalene (and, by analogy, other aromatic hydrocarbons). T he first pathway, which is expected to predominate at lower temperatur es, involves the formation of biarylyls, followed by their rapid cycli zation to transient radical intermediates that ultimately lose hydroge n to yield fluoranthene. The second reaction channel commences with th e dehydrogenation of biarylyls to arylarynes, which then either direct ly cyclize to fluoranthene or rearrange to cyclopentadienylidenecarben es. Rapid intramolecular trapping ultimately converts the latter speci es to cyclopentafused isomers of fluoranthene. Direct equilibration of these isomers and fluoranthene itself, which constitutes the third re action pathway, is associated with very high activation energies. For this reason, such equilibration is not expected to be operative under normal pyrolytic conditions.