Strain induced bond localization in strained aromatic compounds with extended π systems

The influence of small rings annulated to aromatic rings is usually additive. However, in some systems the behavior is nonadditive, for example, in tricarbonyliron‐cyclobutadienobenzene versus tris(tricarbonylironcyclobutadieno)benzene. A study of dimethylenecyclobutabenzene, 1,3‐bis(dimethylenecyclobuta)benzene, tris(dimethylenecyclobuta)benzene, and their bora, imino, and oxo derivatives is presented. The behavior of these systems ranges from an almost perfect additivity (in the bora derivatives) to nonadditive behavior (in the oxo derivatives) where the single ring annulated system shows a negative difference between the bonds endocyclic and exocyclic to the annulated small ring (negative Δ R ; anti‐Mills–Nixon distortion), the doubly annulated system shows Δ R =0, and the triply annulated system shows positive Δ R (Mills–Nixon distortion). The geometrical properties of the systems are analyzed in terms of the strain, bond curvature, and electronegativity of the exocyclic substituent. These atomic and σ properties are found to be responsible for the structural properties whereas π factors are of minor or no importance.