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 system s that benzenes fused to carbocycles and possessing a beta-hydroxy sub stituent exhibit a characteristic electrophilic regioselectivity, whic h is a linear function of the size of the annelated ring. This directi ve property, which determines the susceptibility of various positions within the aromatic fragment towards electrophilic substitution, is ra tionalized in terms of the degree of matching of two pi-electron local ization patterns, one occurring in the ground state of the molecule an d the other in the transition structure (Wheland sigma-complex formed by protonation). The overwhelming influence, however, is exerted by th e OH group, which substantially activates its ortho positions. The rol e of hyperconjugation seems to be small but not negligible. The releva nce of the present result in interpreting the Mills-Nixon effect is br iefly discussed.