Effects of fused benzene rings on tautomerizations and inversions of benzo, azabenzo, and oxabenzocycloheptatrienes at theoretical levels

Abstract

Biologically important bicyclic species, including 6H-, 6H-6-aza-, and 6-oxabenzocycloheptatrienes (in which the benzene moiety is fused meta with respect to the tetrahedral constituents: -CH 2 -, -NH-, and -O-, respectively), show strong shifts of tautomerizations in favor of the corresponding tricyclic benzonorcaradienes In contrast, such shifts are strongly disfavored by the isomeric bicyclic species in which the benzene moieties are fused ortho or para with respect to -CH 2 -, -NH-, and -O-, respectively. Hence for species with ortho benzene rings including 5H-, 5H-5-aza-and 5-oxabenzocycloheptatrienes, tautomerization DH values are 30.76, 31.89, and 25.27 kcal mol -1 , respectively, while for species with para fused benzene moieties including 7H-, 7H-7-aza-, and 7-oxabenzocycloheptatrienes, tautomerization DH values are 24.12, 26.00, and 19.55 kcal mol -1 , respectively. NICS calculations are successfully used to rationalize these results. The calculated energy barriers for inversion of the seven-membered rings of bicyclic species predict a dynamic nature for all the structures except for the virtually planar 6H-6-aza-and 6-oxabenzocycloheptatrienes. Finally, our theoretical data are compared to the experimental results where available.