Theoretical study of Wheland intermediates in benzocycloalkenes: vindication of the Mills–Nixon hypothesis

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Abstract
Structural and energetic properties of o-xylene and three benzocycloalkenes involving a three, four or five membered fused carbocycle are considered together with their Wheland intermediates by HF/6-31G* and single point MP2(fc)//HF/6-31G* procedures. It is conclusively shown that protonation of the arylic α-position is energetically less favourable than β-protonation, in accordance with the original Mills–Nixon (MN) hypothesis and the experimental evidence gathered by electrophilic substitution reaction studies. More specifically, it is found that the difference Eα (n)–Eβ (n) is an inverse linear function of the C–C–C angle in the annelated carbocycle, where two carbon atoms of the C–C–C fragment belong to the fused bond. The origin of the different susceptibility of α- and β-sites toward electrophilic attack is analysed by means of a new energy partitioning scheme based on homodesmic reactions, which gives interesting insight into the nature of intramolecular interactions in Wheland complexes. It is shown that the difference in reactivity can be traced down to the compatibility or incompatibility of two π-electron localization modes. The first is related to the ground state and is induced by the angular strain of the annelated carbocycle, thus reflecting a ‘memory’ effect, whereas the other one occurs in the transition structure (TS)(simulated by the Wheland complex) because of protonation and concurrent formation of the sp3 centre within the aromatic fragment. The competition between these two antagonistic localization patterns is responsible for enhanced reactivity of β-sites over the α-positions.