Reactivity frontier orbital treatments

A more complete analysis of interacting molecules would examine all of the involved MOs in a similar wty. A correlation diagram would be constructed to determine which reactant orbital is transformed into wfiich product orbital. Reactions which permit smooth transformation of the reactant orbitals to product orbitals without intervention of high-energy transition states or intermediates can be identified in this way. If no such transformation is possible, a much higher activation energy is likely since the absence of a smooth transformation implies that bonds must be broken before they can be reformed. This treatment is more complete than the frontier orbital treatment because it focuses attention not only on the reactants but also on the products. We will describe this method of analysis in more detail in Chapter 11. The qualitative approach that has been described here is a useful and simple wty to apply MO theory to reactivity problems, and we will employ it in subsequent chapters to problems in reactivity that are best described in MO terms. I... 

Five Standard Frontier Orbital Treatments of Reactivity Absolute Reactivity... 

The high reactivity towards nucleophiles, as compared to carbonyl compounds, is easily explained by frontier orbital treatment [119]. However the regioselectivity of addition, whether on sulfur or on carbon atoms, has not yet been fully rationalised. However this has not prevented numerous successful applications [1]. 

It should be no surprise that in a full MO treatment, the HOMO of B5H9 is the degenerate ring-cap tt MO set and the LUMO is the non-bonding ring 8 orbital. For the binary boron hydrides, the frontier orbitals and, by implication, the reactivity... 

Three levels of explanation have been advanced to account for the patterns of reactivity encompassed by the Woodward-Hoffmann rules. The first draws attention to the frequency with which pericyclic reactions have a transition structure with (An + 2) electrons in a cyclic conjugated system, which can be seen as being aromatic. The second makes the point that the interaction of the appropriate frontier orbitals matches the observed stereochemistry. The third is to use orbital and state correlation diagrams in a compellingly satisfying treatment for those cases with identifiable elements of symmetry. Molecular orbital theory is the basis for all these related explanations. 

More recently, molecular orbital theory has provided a basis for explaining many other aspects of chemical reactivity besides the allowedness or otherwise of pericyclic reactions. The new work is based on the perturbation treatment of molecular orbital theory, introduced by Coulson and Longuet-Higgins,2 and is most familiar to organic chemists as the frontier orbital theory of Fukui.3 Earlier molecular orbital theories of reactivity concentrated on the product-like character of transition states the concept of localization energy in aromatic substitution is a well-known example. The perturbation theory concentrates instead on the other side of the reaction coordinate. It looks at how the interaction of the molecular orbitals of the starting materials influences the transition state. Both influences on the transition state are obviously important, and it is therefore important to know about both of them, not just the one, if we want a better understanding of transition states, and hence of chemical reactivity. 

This chapter describes the structures of aromatic heterocycles and gives a brief summary of some physical properties. The treatment we use is the valence-bond description, which we believe is appropriate for the understanding of all heterocyclic reactivity, perhaps save some very subtle effects, and is certainly sufficient for a general textbook on the subject. The more fundamental, molecular-orbital description of aromatic systems is less relevant to the day-to-day interpretation of heterocyclic reactivity, though it is necessary in some cases to utilise frontier orbital considerations, however such situations do not fall within the scope of this book. 

There are several major reaction types available to radicals coupling, addition, substitution, fragmentation, rearrangement, and oxidation. The frontier molecular orbital treatment shown here for addition to alkenes can, with modification, be used to predict reactivity in virtually all radical reactions, if the molecular orbitals for the reactive species are known. The following sections will describe the various radical reaction types. 

Frontier molecular orbital (FMO) theory 62) has provided new insights into chemical reactivity. This, and the simplicity of its application, has led to its widespread use, particularly in the treatment of pericyclic reactions 63). An FMO treatment depends on the energy of the highest occupied (HOMO) and lowest unoccupied molecular... 

The last years have seen a great development of the hetero Diels-Alder reactions of thiocarbonyl compounds, due to their rather high reactivity, as reported by many groups, including those of Kirby, Bonini, Capozzi, DeglTnnocenti, Vallde, Okazaki, Koizumi, Saito and Huisgen. A simple orbital frontier treatment allows comparison of the reactivity of C=S vs C=0 [119]. 

---