The Role of Lone Pairs in Heteroatomic Chemistry. Graphical Analysis and Ab Initio Calculations of Oxirane and Aziridine Ring Opening

The »overlap graph«, where vertices represent atomic orbitals and edges, their in-phase or out-of-phase overlaps, suggests that, . in the thermal electrocyclic opening of oxirane, the second oxygen lone pair has little effect on the allowed conrotatory mode but lowers the activation energy of the forbidden disrotatory mode. This effect does not exist in the electrocyclic opening of aziridine, which is one reason why the energy gap between the allowed and forbidden processes should remain substantial. In the course of an aziridine ring opening, the nitrogen's hydrogen may remain in the plane of the heavy atoms or may move out into either of the two half-spaces defined by this plane. Three different disrotatory pathways are thus conceivable. They are compared by a perturbational treatment using simple Hiickel theory. Localization of the transition states on these three reaction paths - i. e. determination of their »early« or »late« character, as measured by the magnitude of the rotation of the two methylene groups, for example - can be made with the help of »natural correlation diagrams«. This provides a simple criterion for predicting the breakdown of the perturbational treatment, when pathways of similar energies are compared: if one transition state is much earlier (later) than the other(s), no safe conclusion can be reached. These qualitative arguments are fully supported by ab initio calculations (ST0-3G + (3 X 3) CI and 4-31G + extensive CI). Substituent effects and possible cases of violation of orbital symmetry requirements are discussed.