syn–anti Isomerism in the 1,3‐dipolar cycloaddition to cis‐3,4‐disubstituted cyclobutenes. 4. Intramolecular interactions

Ab initio (4‐31G) molecular geometry optimizations on cis‐3,4‐disubstituted cyclobutenes show that the extent and direction of their ground‐state pyramidalization depend on the type of substituents and on whether these substituents are part of a ring condensed on to the cyclobutene moiety. Thus, electron‐withdrawing substituents such as Cl and OMe bring about an out‐of‐plane anti bending of the olefinic hydrogens, whereas cyclobutenes condensed on to small carbocyclic rings exhib high syn bending. Intramolecular interactions are analysed in the frame of a new model where the interactions arise from the energy matrix elements between the hybrid atomic orbitals involved in the different bonds. The analysis is carried out by C‐INDO calculations on deformed molecules (with the olefinic hydrogens forced out‐of‐plane by ± 20°). These latter calculations reveal that non‐planarity of the double bond occurs in the direction of increasing interactions between the hybrid atomic orbitals involved in the σ‐allylic bonds and in the π‐bond. The increase in this kind of interaction can also explain non‐planarity in norbornene, 7‐oxanorbornene, 5,6‐dioxanorbornene and 5,6‐dioxabicyclo [2.2.2] oct‐2‐ene. The results strongly suggest that pyramidalization and the related energy asymmetry of out‐of‐plane bending of olefinic hydrogens are important factors in governing facial selectivity of 1,3‐dipolar cycloadditions to cis‐3,4‐disubstituted cyclobutenes.