Non-planar and Still Aromatic

Let us comment briefly on these reactions. The rearrangement of 189a to 190 proceeds extremely easily under the action of catalytic amounts of CF3COOH at 20 °C, while most usually alkylated aromatic compounds are able to undergo such rearrangements under much harsher conditions. Upon irradiation of 189a a formally similar transformation occurs, but special experiments have shown that the latter reaction proceeds via formation of a tricyclic intermediate of the benzvalene type 190a. Diels-Alder reaction with conventional benzene 

The discrepancy between physicochemical and reactivity criteria for aromaticity was known long ago. Yet the data procured by the studies of compounds like 186 or 189 shed a new light on this problem. The observed deviation from planarity did not cause any substantial changes in the spectral parameters of these compounds. Thus they still should be considered aromatic from the point of view of physical chemists. At the same time, this distortion of geometry so dramatically affected their reactivity pattern that, in accordance with this criterion, cyclophanes 186 or 189 should best be referred to as non-aromatic compounds, derivatives of 1,3,5-cyclohexatriene.  

All previous examples referred to the anomalies due to out-of-plane distortions of the aromatic rings. Properties of aromatic compounds could also be affected by in-plane angle distortions caused by a fusion of a small ring and an aromatic residue. Among various compounds which have been designed and synthesized for investigations into these effects, the most interesting data were obtained for tris(benzocyclobutadieno)benzene 197, easily prepared by the 

---