Model calculations on the electrophilic reactivity of fused aromatics. Influence of the OH substituent

It is shown by MP2(fc)/6-31G**//HF/6-31G* calculations on model systems that benzenes fused to carbocycles and possessing a β-hydroxy substituent exhibit a characteristic electrophilic regioselectivity, which is a linear function of the size of the annelated ring. This directive property, which determines the susceptibility of various positions within the aromatic fragment towards electrophilic substitution, is rationalized in terms of the degree of matching of two π-electron localization patterns, one occurring in the ground state of the molecule and the other in the transition structure (Wheland σ-complex formed by protonation). The overwhelming influence, however, is exerted by the OH group, which substantially activates its ortho positions. The role of hyperconjugation seems to be small but not negligible. The relevance of the present result in interpreting the Mills-Nixon effect is briefly discussed.