Electrocyclic reactions, and

This is an example of an electrocyclic reaction, and involves rotation of the terminal methylene groups either in the same way ( conrotatory ) or in opposite ways ( disrotatory ). 

Wiest, Montiel, and Houk (1997) have studied carefully a large number of TS structures for organic electrocyclic reactions and, based on comparison to experiment (particularly including kinetic isotope effect studies) and very high levels of electronic structure theory. 

The ring opening of cyclopropyl cations (pp. 345, 1076) is an electrocyclic reaction and is governed by the orbital symmetry rules.389 For this case we invoke the rule that the o bond opens in such a way that the resulting/ orbitals have the symmetry of the highest occupied orbital of the product, in this case, an allylic cation. We may recall that an allylic system has three molecular orbitals (p. 32). For the cation, with only two electrons, the highest occupied orbital is the one of the lowest energy (A). Thus, the cyclopropyl cation must... 

Figure 4.39 (a) The principle of a concerted electrocyclic reaction and (b) an example of a photochemical process... 

In contrast, applying frontier orbital theory to unimolecular reactions like electrocyclic reactions and sigmatropic rearrangements is inherently contrived, since we have artificially to treat a single molecule as having separate components, in order to have any frontier orbitals at all. Furthermore, frontier orbital theory does not explain why the barrier to forbidden reactions is so high—whenever it has been measured, the transition structure for the forbidden pathway has been 40 kJ mol-1 or more above that for the allowed pathway. Frontier orbital theory is much better at dealing with small differences in reactivity. 

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. 

Biosynthetic and biomimetic electrocyclic ring openings and ring closures have been comprehensively reviewed 67 New evidence for the similarity of the transition states of electrocyclic reactions and cationic [1, ]-proton shifts has been obtained by both generalized population analysis and quantum molecular similarity indices 68... 

In the skin of animals, 7-dehydrocholesterol is converted to vitamin D, by the reaction sequence that follows. The first step in this process, the conversion of 7-dchy-drocholesterol to pre-cholecalciferol, requires light. This is an electrocyclic reaction and must occur by a conrotatory motion to avoid the formation of a highly strained trans double bond in one of the rings. Conrotation involving three pairs of electrons must occur photochemically to be allowed. 

One final example of a name reaction presented within the text of this book is the cation-tt cyclization. This reaction, illustrated in Scheme 8.15, returns to the previously described reaction classes that include electrocyclic reactions and rearrangements. Inclusion of this reaction complements the various nucleophiles used throughout the examples of this book by highlighting the nucleophilic nature of double bonds. 

A hetero Diels-Alder reaction of a precursor 1-9 may be involved in the biosynthesis of the lignane carpanone 1-8 (Fig. 1-3), however, there is no proof for such an assumption [32]. On the other hand, it is well known that pericyclic reactions such as electrocyclic reactions and sigmatropic rearrangements occur in nature e.g. in the biosynthesis of vitamine D, vitamine B12 [33-35] and ecto-carpene [36]. 

What made his proposal so convincing was that the stereochemistry of the endiandric acid D is just what you would expect from the requirements of the Woodward-Hoffmann rules. The first step from the precursor is an 87t electrocyclic reaction, and would therefore be conrotatory. 

One of the five tc orbitals involved is empty—so the cyclization is a 4ji electrocyclic reaction, and the orbitals forming the new c bond must interact antarafacially. Loss of a proton and tautomerism gives the cyclopentenone. 

In general, polyenes with odd numbers of double bonds undergo disrotatory thermal electrocyclic reactions, and polyenes with even numbers of double bonds undergo conrotatory thermal electrocyclic reactions. 

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. 

The book is divided into three parts. Part I deals with typical complex organic reactions such as (i) reactions involving carbocations and carbanions, (i/) Pericyclic and electrocyclic reactions and (ii/) Sigmatropic and Chelotropic reactions. This part also includes material useful for characterization of products from structural point of view such as Geometrical isomerism, Stereochemistry and Conformation. Part II is concerned with spectroscopic methods of structure determination such as U.V.,... 

In this paper, we shall discuss, first by a polymerization of unsaturated side-groups (side-chains), second by the polymer-analogous condensation of suitable functional groups, third by ring-closures (cyclization) via electrocyclic reactions and, fourth by cyclization via electrophilic substitution reactions. 

Although the pericyclic chemistry of anion radicals has been much slower to emerge than that of cation radicals, the number of intriguing examples now available suggests that this could be an attractive area for future development in electron transfer chemistry. Reaction types which have been exemplified include cyclobutanation, retrocyclobutanation, Diels-Alder addition, electrocyclic reactions, and retroelec-trocyclic reactions. 

The cyclobutene-butadiene interconversion involves four v electrons and is designated a process. Note that by the principle of microscopic reversibility, the number of tt electrons involved in the transformation is the same for ring opening as for ring closing. Once we know the number of tt electrons involved in an electrocyclic reaction and the method of activation, the stereochemistry of the process is fixed according to the rules outlined in Table 6.1. 

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. 

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