| Abstract
| - The effect of aryl substituents on the interconversion of phenylcarbene (PC), bicyclo[4.1.0]hepta-2,4,6-triene (BCT), and 1,2,4,6-cycloheptatetraene (CHTE) has been studied by density functionaltheory. It is found that substituents have a large effect on both the thermochemistry and activationenergy of these rearrangements. For instance, para-substitution yields a range of overall activationenergies for the formation of BCT from PC of 20.3 to 11.7 kcal/mol for the NH2 and NO2 substituents,respectively. In the syn-meta-substituted cases, all of the rearrangements to the substituted CHTEspecies are more exothermic than that of the parent PC. The proximity of the substituent to thecarbene center can also affect the overall chemistry as in the case of ortho-substituted species.Here, formation of bicyclic structures and ylides, which can then rearrange to stable structures,can compete with the ring-expansion process. Also, as calculated herein, the ortho substituentscan, by a combination of mesomeric and steric interactions with the carbene center, affect the overallbarrier to reversible ring expansion. Most notably, in the anti-ortho-substituted species, halogens(F and Cl) raise the activation barrier to ring expansion by ∼5 kcal/mol. This is reminiscent of theeffect of fluorine substitution on the chemistry (inter- and intramolecular) of phenylnitrene.
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