MODELS FOR SELECTIVITY IN ORGANIC-REACTIONS

Factors contributing to product selection in organic reactions are analyzed using two different approaches. The first involves consideration only of electrostatic preferences upon initial encounter of reagent and substrate, that is, well in advance of the actual reaction transition state. This model, while computationally very simple and applicable to systems of considerable complexity, is capable only of providing a qualitative account of selectivity preferences. The second approach involves direct evaluation of energy differences for transition states leading to different regio- and stereoproducts, based on well designed and calibrated Hartree-Fock and correlated levels of ab initio molecular orbital theory. This is both generally applicable and, subject to the validity of the underlying transition state model, capable of quantitative accuracy, although it is also computationally very demanding and in practice applicable using only the very simplest molecular orbital methods and then only to very simple systems. Examples are provided, among them stereochemistry in electrophilic and nucleophilic additions and regio- and stereochemistry of Diels-Alder cycloadditions. Generalities regarding the factors which infludence product selection in simple organic reactions are discussed.