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 measur ed 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 hyd rate, DeltaH = -38.7 kcal mol(-1), despite an unusually low reactivity of b enzene oxide, rate ratio 0.08. The measured enthalpies agree with those cal culated using the B3LYP hybrid functional corrected with solvation energies derived from semiempirical AM1/SM2 calculations. Comparison with the measu red enthalpies of the corresponding reactions of the structurally related 1 ,3-cyclohexadiene oxide (3) and 2-cyclohexenol (4) of DeltaH = -24.9 kcal m ol(-1) (includes a small calculated correction of -1.2 kcal mol(-1)) and De ltaH similar to 0 kcal mol-1, respectively, gives a smaller aromatization e nergy for the benzene oxide than for the benzene hydrate reaction (Delta De lta DeltaH = 6.6 kcal mol(-1)). This suggests that benzene oxide is unusual ly stabilized by a significant amount of homoaromatization as has been prop osed previously (J. Am. Chem. Soc. 1993, 115, 5458). This unusual stability accounts for more than half of the similar to 10(7) times lower than expec ted reactivity of benzene oxide toward acid-catalyzed isomerization. The re st is suggested to originate from an unusually high energy of the carbocati on-forming transition state.