Pericyclic reactions molecular orbital theory

The period 1930-1980s may be the golden age for the growth of qualitative theories and conceptual models. As is well known, the frontier molecular orbital theory [1-3], Woodward-Hoffmann rules [4, 5], and the resonance theory [6] have equipped chemists well for rationalizing and predicting pericyclic reaction mechanisms or molecular properties with fundamental concepts such as orbital symmetry and hybridization. Remarkable advances in aeative synthesis and fine characterization during recent years appeal for new conceptual models. 

The first pair of examples we would like to discuss occurs in a field which lends itself naturally to be conquered by theory. Indeed, the past three decades have seen the exploration of mechanistic details of pericyclic reactions as one of the major success stories of computational chemistry. Rooted in qualitative molecular orbital theory, the key concept of... 

Houk, K.N. "Application of Frontier Molecular Orbital Theory to Pericyclic Reactions", in "Pericyclic Reactions", A.P. 

The application of molecular orbital theory to pericyclic reactions has been described, at a level similar to that here, in a number of textbooks T. L. Gilchrist and R. C. Storr, Organic Reactions and Orbital Symmetry, CUP, 2nd Edn. 1972 R. E. Lehr and A. P. Marchand, Orbital Symmetry, Academic Press, New York, 1972 F. A. Carey and R. J. Sundberg, Advanced Organic Chemistry, Plenum, New York, 3rd Edn. 1990 N. Isaacs, Physical Organic Chemistry, Longman, Harlow, 2nd Edn., 1995. 

In this primer, Ian Fleming leads you in a more or less continuous narrative from the simple characteristics of pericyclic reactions to a reasonably full appreciation of their stereochemical idiosyncrasies. He introduces pericyclic reactions and divides them into their four classes in Chapter 1. In Chapter 2 he covers the main features of the most important class, cycloadditions—their scope, reactivity, and stereochemistry. In the heart of the book, in Chapter 3, he explains these features, using molecular orbital theory, but without the mathematics. He also introduces there the two Woodward-Hoffmann rules that will enable you to predict the stereochemical outcome for any pericyclic reaction, one rule for thermal reactions and its opposite for photochemical reactions. The remaining chapters use this theoretical framework to show how the rules work with the other three classes—electrocyclic reactions, sigmatropic rearrangements and group transfer reactions. By the end of the book, you will be able to recognize any pericyclic reaction, and predict with confidence whether it is allowed and with what stereochemistry. 

Up-to-Date Treatment In addition to the classical reactions, this book covers many techniques and reactions that have more recently gained wide use among practicing chemists. Molecular-orbital theory is included early and used to explain electronic effects in conjugated and aromatic systems, pericyclic reactions, and ultraviolet spectroscopy. Carbon-13 NMR spectroscopy is treated as the routine tool it has become in most research laboratories, and the DEPT technique is included in this edition. Many of the newer... 

M. J. S. Dewar, A Molecular Orbital Theory of Organic Chemistry-VIII Aromaticity and Electrocyclic Reactions. Tetrahedron Suppl. 1966,8,75-92 Aromaticity and Pericyclic Reactions. Angew. Chem. Int. Ed. Engl. 1971, 10,761-776 The Molecular Orbital Theory of Organic Chemistry, McGraw-Hill, New York, 1969. 

The mechanisms of pericyclic reactions can be explained by molecular orbital theory. 

This chapter is divided into two sections, largely separating stereospecific reactions from the merely stereoselective. The first deals with the ionic stereospecific reactions, and the explanations based on molecular orbital theory for the sense of the stereospecificity. The second deals with stereoselective reactions, in which a new stereocentre is created selectively under the influence of one or more existing stereochemical features, which is also sometimes a question of how the orbitals interact. The stereospecificity that is such a striking feature of pericyclic reactions is covered in the next chapter. 

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. 

Two studies on the mechanism of this type of [4 + 2] cycloaddition which have led to very di erent interpretations have appeared. Mock and Nugent suggested that the Diels-Alder reactions of N-sulfi-nyl-p-toluenesulfonamide are stepwise, ionic processes. On the other hand, Hanson and Stockbum prefer a concerted, pericyclic mechanism in accord with frontier molecular orbital theory. Both proposals satisfactorily rationalize the observed regioselectivity of these reactions. 

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. 

QUALITATIVE MOLECULAR ORBITAL THEORY AND PERICYCLIC REACTIONS... 

Chapter 12 Qualitative Molecular Orbital Theory Pericyclic Reactions... 

This chapter is an introduction to qualitative molecular orbital theory and pericyclic reactions. Pericyclic reactions have cyclic transition states and electron flow paths that appear to go around in a loop. The regiochemistry and stereochemistry of these reactions are usually predictable by HOMO-LUMO interactions, so to understand them we need to understand molecular orbital theory, at least on a qualitative basis. 

With these three reagents, and their singular properties, we are straying into an unusual borderline region between pericyclic and stepwise reactions. Elsewhere, this is less of a problem, and the existence of a large class of reactions safely called pericyclic is well established. We must now turn to the ideas, based on molecular orbital theory, which have been advanced to explain why the Woodward-Hoffmann rules work so well. 

According to the conservation of orbital symmetry theory, whether a compound will undergo a pericyclic reaction under particular conditions and what product will be formed both depend on molecular orbital symmetry. To understand pericyclic reactions, therefore, we must now review molecular orbital theory. We will then be able to understand how the symmetry of a molecular orbital controls both the conditions under which a pericyclic reaction takes place and the configuration of the product that is formed. 

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