Aromatic compounds, fused strained

Considerable research effort has been directed towards the generation, detection, and isolation of oxirenes, a highly strained class of antiaromatic heterocycles <83CRV519>. Although the unsaturated oxiranes are called oxirenes, much confusion is generated by the fact that Chemical Abstracts denotes oxiranes fused onto a polycyclic aromatic compound as oxirenes also (e.g., compound (16), Figure 9 <91JHC473 . 

Taylor has collected the above and similar data and compared the ratio of reactivities of the ortho and para positions of compounds of type 19 (expressed as log/odog/p) with the ratio of reactivities of the equivalent positions, a and c, in compounds of type 20 and found that the latter ratio was lower, i.e., a relative increase in the reactivity of the para position (c) has occurred upon ring formation. This fall in the ratio log fa log fc increases along the series X = S < 0 (< NH < CHg) in 20. As this trend parallels the increase in strain in the fused bridging ring it was argued that ring strain was the primary cause of the reduction in ratio. Position a is a-aromatic and position c is j8-aromatic therefore the above concept represents an extension by Taylor of an earlier explanation of the Mills-Nixon effect in indane. Further substitution... 

Recorded a little over 100 years ago, these words and the experiments of W. H. Perkin Jr. provided the foundation for the study of ring-fused aromatics, unquestionably an area of organic chemistry that has stood the test of time. This chapter addresses the chemistry of the cycloproparenes, the most highly strained class of compounds that can ensue from the fusion of a single ring to an aromatic framework and typified by the structure of cyclo-propabenzene (1). 

Of all the rings examined only the non-benzo-fused compound 163 apparently has sufficient resonance energy to overcome the strain involved in acquiring planarity. Fusion of an aromatic ring to a benzene nucleus is well known to reduce its aromaticity and this may account in part for the apparent nonplanarity and nonaromaticity of the fused analogs. 

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