Benzene Ring Contractions at High Temperatures. Evidence from the Thermal Interconversions of Aceanthrylene, Acephenanthrylene, and Fluoranthene

Article abstract of DOI:10.1021/ja00252a025

The predicted interconversions of the title compounds (11, 13, and 15) at high temperatures have been verified (Table I) and represent the first examples of thermal rearrangements of aromatic hydrocarbons in which one benzenoid skeleton is transformed into another benzenoid skeleton.Equilibrium concentrations of 11:13:15 were found to be 17:9:74 at 1100 deg C.Acephenanthrylene (13) is the primary product of thermal rearrangement when starting from either aceanthrylene (11) or fluoranthene (15).It is also shown that acephenanthrylene rearranges more rapidly to fluoranthene than to aceanthrylene.These data are summarized in the qualitative energy profile in Figure 1.The mechanism by which these rearrangements occur appears to involve a carbon 1,2-shift (benzene ring contraction) and a hydrogen 1,2-shift, both across the same carbon-carbon bond but in opposite directions.The precise timing of these two shifts relative to each other remains uncertain.The range of possibilities is conveniently illustrated in the More O'Ferrall-Jencks diagram in Figure 2.The demonstration that aceanthrylene, acephenanthrylene, and fluoranthene can be interconverted thermally constitutes the first solid evidence for benzene ring contractions at high temperatures.