Is benzene oxide homoaromatic? A microcalorimetric study.

Rate constants and heats of reaction for the aromatization of benzene oxide (1) and the acid-catalyzed aromatization of benzene hydrate (2) in highly aqueous solution giving phenol and benzene, respectively, have been measured by heat-flow microcalorimetry. The measured heat of reaction of benzene oxide, DeltaH = -57.0 kcal mol(-1), is much larger than that of benzene hydrate, DeltaH = -38.7 kcal mol(-1), despite an unusually low reactivity of benzene oxide, rate ratio 0.08. The measured enthalpies agree with those calculated using the B3LYP hybrid functional corrected with solvation energies derived from semiempirical AM1/SM2 calculations. Comparison with the measured enthalpies of the corresponding reactions of the structurally related 1,3-cyclohexadiene oxide (3) and 2-cyclohexenol (4) of DeltaH = -24.9 kcal mol(-1) (includes a small calculated correction of -1.2 kcal mol(-1)) and DeltaH approximately 0 kcal mol(-1), respectively, gives a smaller aromatization energy for the benzene oxide than for the benzene hydrate reaction (DeltaDeltaDeltaH = 6.6 kcal mol(-1)). This suggests that benzene oxide is unusually stabilized by a significant amount of homoaromatization as has been proposed previously (J. Am. Chem. Soc. 1993, 115, 5458). This unusual stability accounts for more than half of the approximately 10(7) times lower than expected reactivity of benzene oxide toward acid-catalyzed isomerization. The rest is suggested to originate from an unusually high energy of the carbocation-forming transition state.