Orbital Theory of Heterolytic Fragmentation and Remote Effects on Nitrogen Inversion Equilibria

Abstract

Abstract From a molecular orbital study of model systems we derive the electronic requirements for the Grob fragmentation. The necessary condition for an allowed fragmentation in an X‐C 1 ‐C 2 ‐C 3 ‐N system, or the amino cation + C 1 ‐C 2 ‐C 3 ‐N is the level ordering A below S . This in turn is set by maximal through‐bond coupling of the empty cation orbital and the nitrogen lone pair. The conformational dependence of through‐bond coupling is exactly that derived by Grob , namely parallel orientation of the cation orbital (or the C‐X bond), the C 2 ‐C 3 ‐σ‐bond, and the N‐lone‐pair. When the C 1 ‐C 2 ‐C 3 and C 2 ‐C 3 ‐N angles are small, the through‐space interaction dominates, reversing the level ordering to S below A , and consequently makes the fragmentation forbidden even though the conformational requirements for it are met. Ring closure becomes allowed. Some examples exploiting this result are presented, as well as procedures for enhancing through‐bond coupling and thus fragmentation. The through‐bond‐effect has also kinetic consequences, allowing the definition of a new type of remote neighbouring group participation operative through bonds and not by direct overlap. The position of equilibria in nitrogen inversion processes should also be influenced by remote substituents which are π‐acceptors or donors.