Further evidence for existence of the Mills-Nixon effect— ab initio study of the electrophilic reactivity in heteroanalogs of benzocyclopropene

SUMMARY It is shown that heteroatomic analogs of benzocyclopropene involving NH, O and S groups at position 7 exhibit enhanced selectivity in the electrophilic substitution r eactions. The origin of the different susceptibility of the -a nd-positions toward electrophilic attack is interpreted by the compatibility and incompatibility of two -electron localization modes. The first is induced by the angular strain of the small fused ring and its lone pair(s) electrons thus being a property of the ground state (GS). The second is caused by protonation of the benzene fragment occurring in the formation of the transition structure (TS). The competition between these two antagonistic localization patterns is responsible for enhanced activity of -sites over the -positions, which is in harmony with the original Mills‐Nixon (MN) hypothesis. The actual values of the corresponding proton affinities are affected also by Coulomb interactions between the carbon junction atom(s) and heteroatomic group within the three-membered ring. On the other hand, 7,7- difluorobenzocyclopropene exhibits a reversed behavior: the -site is preferred over the - position, which is in accordance with the predicted anti-MN character of this compound. Chemical relevance of the MN effect is stressed.