Electrocyclic Reactions and Cycloadditions

For sigmatropic reactions, as for electrocyclic reactions and cycloadditions, the course of reaction can be predicted by counting the number of electrons involved and applying the selection rules. A comprehensive rationalization of all the stereochemical aspects of these reactions requires application of the frontier orbital or orbital symmetry approaches, and, at this point, we will content ourselves with pointing out the salient features of the more common reactions of this class. 

As emphasized by Fukui, the mechanism of chemical reactions can often be understood in terms of frontier orbitals—the highest occupied molecular orbitals (HOMO S) and lowest unoccupied molecular orbitals (LUMO s) of reacting molecules. Ideally, the frontier orbitals of the reactants interact to form the MO s of the products. And it is in such transformations that orbital symmetry is conserved. We will consider two relevant examples from organic chemistry electrocyclic reactions and cycloadditions. 

Like electrocyclic reactions and cycloadditions, sigmatropic rearrangements are controlled by orbital symmetries. There are two possible modes of reaction migration of a group across the same face of the tt system is suprafacial, and migration of a group from one face of the tt system to the other face is antara-facial (Figure 30.11). 

Some of its special chapters are the Pericyclic Reactions, which includes Cheletropic, Electrocyclic, Sigmatropic and Cycloaddition reactions. The concept of Stereochemistry and Conformation deserve special attention not because they cater to the needs of higher students, but they are immensely useful for candidates trying for UGC and CSIR sponsored competitive examinations, but also those preparing for Union Public Service Commission and State Public Service Commission Exams. The candidates will find the chapters immensely useful and is sure to rouse interest in them in knowing more about mechanistic chemistry. 

The aromaticities of symmetry-allowed and -forbidden transition states for electrocyclic reactions and sigmatropic rearrangements involving two, four, and six r-electrons, and Diels-Alder cycloadditions, have been investigated by ab initio CASSCF calculations and analysis based on an index of deviation from aromaticity. The order of the aromaticity levels was found to correspond to the energy barriers for some of the reactions studied, and also to the allowed or forbidden nature of the transition states.2 The uses of catalytic metal vinylidene complexes in electrocycliza-tion, [l,5]-hydrogen shift reactions, and 2 + 2-cycloadditions, and the mechanisms of these transformations, have been reviewed.3... 

In these cycloaddition reactions which may be intermolecular, or intramolecular, a r -bond is converted into a o-bond. The reverse occurs when ring opening takes place. The closure or opening involves the movement of electrons and atoms but no atoms are gained or lost. Such transformations have been called electrocyclic reactions and may occur thermally or photoche.nically as governed by symmetry considerations. 

The combination of pericyclic transformations as cycloadditions, sigmatropic rearrangements, electrocyclic reactions and ene reactions with each other, and also with non-pericyclic transformations, allows a very rapid increase in the complexity of products. As most of the pericyclic reactions run quite well under neutral or mild Lewis acid acidic conditions, many different set-ups are possible. The majority of the published pericyclic domino reactions deals with two successive cycloadditions, mostly as [4+2]/[4+2] combinations, but there are also [2+2], [2+5], [4+3] (Nazarov), [5+2], and [6+2] cycloadditions. Although there are many examples of the combination of hetero-Diels-Alder reactions with 1,3-dipolar cycloadditions (see Section 4.1), no examples could be found of a domino all-carbon-[4+2]/[3+2] cycloaddition. Co-catalyzed [2+2+2] cycloadditions will be discussed in Chapter 6. 

Reactions in which more than one pair of electrons move simultaneously in a concerted manner, rather than sequentially, are called pericyclic reactions. These can be divided into three principal types, namely electrocyclic, sigmatropic and cycloaddition. 

Pericyclic A reaction type in which the bond breakage and formation steps occur simultaneously without any charged or radical intermediates. There are three main subdivisions, namely electrocyclic, sigmatropic and cycloadditions. 

This cycloaddition works best with substituted oxyallyl cations, like the one from the dibromo-ketone 93 which reacts with the morphol ine enamine of cyclohexanone 95 to give the cyclopentenone 96 in excellent yield.24 You should mark the difference in structure between the Nazarov products, e.g. 70, formed in an electrocyclic reaction, and these cycloadducts, e.g. 96. Each strategy has its part to play and must be chosen according to the structure of the target molecule. 

This simple way of explaining substituent effects is effective, and even gives quite good quantitative correlations for electrocyclic reactions and sigmatropic rearrangements.917 It can also be applied to cycloadditions, although the latter are usually explained by the frontier orbitals discussed in Section 6.5.3. 

The radical cations of conjugated systems can also take part in pericyclic reactions. Examples are known of cycloadditions, electrocyclic reactions and sigmatropic rearrangements. One noticeable feature of some of... 

Notice that electrocyclic reactions and sigmatropic rearrangements occur within a single 7T system—they are intramolecular reactions. In contrast, cycloaddition reactions involve the interaction of two different tt systems—they are usually iniermolecular reactions. The three kinds of pericyclic reactions share the following common features ... 

Electrocyclic, sigmatropic, and cycloaddition reactions are subsequently described in Chapters 2, 3, and 4, respectively. Chapter 5 is devoted to a study of cheletropic and 1,3-dipolar cycloaddition reactions as examples of concerted reactions. Many group transfer reactions and elimination reactions, including pyrolytic reactions, are included in Chapter 6. There are solved problems in each chapter that are designed for students to develop proficiency that can be acquired only by practice. These problems, about 450, provide sufficient breadth to be adequately comprehensive. Solutions to all these problems are provided in each chapter. Finally, in Chapter 7, we have compiled unworked problems whose... 

Interest in reactions of this type has been aroused by the detection of the strongest catalytic effects associated with the inclusion of ion-radical intermediates in the processes of electrocyclic isomerizations and cycloadditions. 

A review detailing computational methods for determining transition-state geometry in stereoselective cycloaddition reactions has been presented. Diels-Alder reactions, 1,3-dipolar cycloadditions, sigmatropic rearrangements, electrocyclic reactions, and ene reactions have been extensively reviewed. Pressure-induced cycloadditions to strained arenes have been reviewed. ... 

Pericyclic reactions, such as Diels-Alder cycloaddition, electrocyclic reactions, and ene reactions, are extremely effective tools in constructing complex products [18]. 

Thermal and photochemical electrocyclic reactions are particularly useful in the synthesis of alkaloids (W. Oppolzer, 1973,1978 B K. Wiesner, 1968). A high degree of regio- and stereoselectivity can be reached, if cyclic olefin or enamine components are used in ene reactions or photochemical [2 + 2]cycloadditions. 

Classify the following reactions as electrocyclizations, sigmatropic rearrangements, cycloadditions, etc., and give the correct symbolism for the electrons involved in each concerted process. Some of the reactions proceed by two sequential processes. 

Scheme 13.1. Some Examples of Photochemical Cycloaddition and Electrocyclic Reactions...

Scheme 13.1 lists some example of photochemical cycloaddition and electrocyclic reactions of the type that are consistent with the predictions of orbital symmetry considerations. We will discuss other examples in Section 13.4. 

Fora [4 + 2 -7r-electron cycloaddition (Diels-Aldei reaction), let s arbitrarily select the diene LUMO and the alkene HOMO. The symmetries of the two ground-slate orbitals are such that bonding of the terminal lobes can occur with suprafacial geometry (Figure 30.9), so the Diels-Alder reaction takes place readily under thermal conditions. Note that, as with electrocyclic reactions, we need be concerned only with the terminal lobes. For purposes of prediction, interactions among the interior lobes need not be considered. 

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