Strain-Induced Bond Localization. The Heteroatom Case

Strain-induced bond localization (SIBL) has been a debated issue for almost 70 years. In the past decade there have been some developments that showed that strain can indeed localize aromatic bonds. However, some compounds that should have exhibited positive Δ R (Mills−Nixon distortion) showed instead negative Δ R (anti-Mills−Nixon distortion), apparently more coherent with π effects (aromaticity−antiaromaticity arguments). Using ab initio methods, the structure of cyclopropabenzenes, cyclobutabenzenes, and benzocyclobutadienes, where the three- and four-membered rings consist of carbon, nitrogen, or boron, were calculated at the B3LYP/6-31G* level of theory. Analyses of these structures show that SIBL and not aromatic factors are responsible for the localization observed in the aromatic moieties, both with positive and negative Δ R .