Molecular modeling of anti-Bredt compounds

The molecular structure and thermochemical stability of 40 bicyclic alkenes and imines containing between five and eight heavy atoms has been investigated computationally by high-level G3/B3LYP method. The same method was also used to study their building blocks: monocyclic alkenes and imines. The relative stability of isomers, standard enthalpies of formation and strain energies are discussed and compared. Our results suggest that molecular mechanics provides an unsatisfactory description of strain energies in bridgehead alkenes. Some bridgehead alkene and imine species represent structures which are not local minima on the potential energy surface and are hence unlikely to be isolated. Furthermore, some bicyclic alkenes and imines have open-shell 1,2-diradical character.