Chapter 4 - Perturbation theory and organic reactions

Abstract

This chapter introduces a molecular orbital approach to rationalizing chemical reactivity in organic reactions. Using the general perturbation expression (Salem–Klopman equation), the stereoelectronic effects are described as the donor–acceptor orbital interaction term contained in the equation. The dominant interactions involve the frontier orbitals. The rules for stereoelectronic effects and their application in ionic reactions using nucleophilic and electrophilic substitution, elimination, nucleophilic and electrophilic addition and fragmentation reactions are described. The Baldwin’s rules for cyclizations are included. The frontier orbital theory for pericyclic reactions such as Diels-Alder reactions, 1,3-dipolar cycloadditions, ketene cycloadditions, carbene cycloadditions, ene reactions, electrocyclic reactions, sigmatropic rearrangements and Cope/Claisen and Wittig rearrangements is described. A diradicaloid model for the Diels–Alder reaction is also included. Frontier orbital effects on the reactivity, regioselectivity and cyclizations in radical reactions are presented. Finally, frontier orbital analysis of photochemical [2+2] cycloadditions and regioselectivity of enone photocycloadditions are provided.