engleski

Electrophilic reactivity in anti-Mills-Nixon systems

Structural properties and the electrophilic substitution susceptibility in some anti-Mills-Nixon (anti-MN) systems possessing fused small rings (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 energetically more favourable than their beta-counterparts. This sort of behaviour is antipodal to the electrophilic reactivity exhibited by MN systems. The basic mechanism is, however, the same. It is related to the degree of matching of two distinct pi-electron localization patterns. The first occurs in the ground state (GS). The second type of pi-electron bond fixation is triggered by protonation. Compatibility of these two modes of bond localization in the transition structure (TS, the Whelandsigma-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 addition, alpha-protonated forms 4a and 5a are energetically prefered because of the increased aromatic character of the fused small rings.

ab initio calculation ; anti-Mills-Nixon molecule

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