Thermally induced cyclodehydrogenation of biaryls: A simple radical reaction or a sequence of rearrangements?

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