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

Sz~rnmnr~~ . From a molecular orbital study of model systems we derivc the electronic requiremcnts for the Grob fragmentation. The necessary condition for an allowed fragmentation in an ?I-C,-C,-C,-N system, or the amino cation +C,-C,-C,-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,-C,-cr-bond, and the N-lone-pair. When thc C,-C,-C, and C,-C,-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 i t are met. Ring closure becomes allowed. Some examplcs 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 typc of remote neighbouring group participation operative through bonds and not by direct overlap. The position of equilibria in nitrogen inversion proccsses should also be influenced by remote substituents which are n-acceptors or donors.