ELECTROPHILIC REACTIVITY IN ANTI-MILLS-NIXON SYSTEMS

Structural properties and the electrophilic substitution susceptibilit y in some anti-Mills-Nixon (anti-MN) systems possessing fused small ri ngs (3-5) are examined by employing HF/6-31G and MP2(fc)/6-31G*//HF/6 -31G theoretical models. The electrophilic substitution is simulated by protonation. It is shown that alpha-Wheland intermediates are energ etically more favourable than their beta-counterparts. This sort of be haviour is antipodal to the electrophilic reactivity exhibited by MN s ystems. The basic mechanism is, however, the same. It is related to th e degree of matching of two distinct pi-electron localization patterns . The first occurs in the ground state (GS). The second type of pi-ele ctron bond fixation is triggered by protonation. Compatibility of thes e two modes of bond localization in the transition structure (TS, the Wheland sigma-complex) determines the directional ability of the small annelated rings in the electrophilic substitution reactions. In anti- MN systems this synaction is greater for alpha-protonation. In additio n, alpha-protonated forms 4a and 5a are energetically prefered because of the increased aromatic character of the fused small rings.