Chemistry
Electrocyclic Reactions
Electrocyclic reactions are a type of pericyclic reaction in which a cyclic compound undergoes ring-opening or ring-closing reactions upon exposure to heat or light. These reactions involve the breaking and formation of sigma bonds and are governed by the Woodward-Hoffmann rules. Electrocyclic reactions have important applications in organic synthesis and materials science.
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10 Key excerpts on "Electrocyclic Reactions"
eBook - PDF- Ramesh Chandra, Snigdha Singh, Aarushi Singh(Authors)
- 2020(Publication Date)
- Arcler Press(Publisher)
5.2. PERICYCLIC REACTIONS There have been studies done on an important body of chemical reaction which do differ in a number of respects. There are reactions which do tend to precede by early simultaneous reorganization of bonding electron pairs by way of cyclic transition states; these have been termed as pericyclic reactions. There are five principle classes of pericyclic reauction namely: cycloaddition, electrocyclic, cheletropic reactions, sigmatropic, and ene reactions. All of these reactions are said to be reversible. The reverse of a cycloaddition is referred to as cycloreversion. This is preceded by a ring cleavage and conversion of two sigma-bonds to two pi-bonds. The electrocyclic reaction is a ring forming process. The reverse of the electrocyclic ring opening reactions is done by converting a sigma-bond to a pi-bond. The ene reaction cleaves an unsaturated compound into two unsaturated fragments. Ultimately, sigmatropic bond shifts can involve a simple migration group or it can take place in between two-pi-electron systems. There are a number of reactions which do require the introduction of energy in the form of heat or light having remarkable product dependence on the source of energy, which is used, even though some of pericyclic reactions occur spontaneously. 5.2.1. Cycloadditions A cycloadditions reaction is the reaction which is a concerted combination of two π-electron systems in order to form a ring of atoms which have two new σ bonds and two fewer π bonds. In each component, the number of participating π-electron is provided in brackets preceding the name and the reorganization of electrons can be depicted by a cycle of curved arrows, each of which represent the movement of a pair of electrons. Organic Synthesis and Organic Reagents 144 The most common cycloaddition reaction is the [4π+2π] cyclization which is referred to as the Diels-Alder reaction.
eBook - PDF- R. B. Woodward, R. Hoffmann(Authors)
- 2013(Publication Date)
- Academic Press(Publisher)
5. Theory of Electrocyclic Reactions These intramolecular cycloadditions provided the stimulus for our study of molec-ular orbital symmetry and concerted reactions. We define as Electrocyclic Reactions the formation of a single bond between the termini of a linear system containing k π electrons, and the converse process (41). k-2 (41) In such changes fixed geometrical isomerism imposed upon the open-chain system is related to rigid tetrahedral isomerism in the cyclic array. Λ priori, this relation-ship might be disrotatory or conrotatory (42) . In the former case the transition state is characterized by a plane of symmetry while in the latter a two-fold axis of symme-try is preserved. A . c * ^ b~T^ ^ ^ D 4 --s Disrotatory ^ Conrotatory ^ . (42) C >V -Cs^ 1 *-ÌY-. Consider the essential molecular orbitals in the conversion of cyclobutene to bu-tadiene. These are the four π orbitals of the butadiene χ ΐ9 χ 2 , χ 3 , χ 4 , the π and π* levels of the cyclobutene double bond, and the σ and
eBook - ePub- Rakesh Kumar Parashar(Author)
- 2013(Publication Date)
- Wiley-Blackwell(Publisher)
Chapter 8
Pericyclic Reactions
Pericyclic reactions are a class of reactions that include some of the most powerful synthetically useful reactions such as the Diels–Alder reaction. Pericyclic reactions often proceed with simultaneous reorganization of bonding electron pairs and involve a cyclic delocalized transition state. They differ from ionic or free radical reactions as there are no ionic or free radical intermediates formed during the course of the reaction. They proceed by one-step concerted mechanisms and have certain characteristic properties (although there are some exceptions to all these rules).1. Pericyclic reactions often proceed with a high degree of stereospecificity.2. Although some pericyclic reactions occur spontaneously, most reactions can be frequently promoted by light as well as heat. Normally, the stereochemistry under the two sets of conditions is different. Thus, there may be two main reaction conditions, thermal (in ground state) and photochemical (in excited state).3. Pericyclic reactions are relatively unaffected by solvent changes and can occur in the gas phase with no solvent. Normally, they are unaffected by the presence of electrophilic and nucleophilic catalysts.4. Normally, no catalyst is needed to promote the reactions. But Lewis acids may catalyze many forms of pericyclic reactions, either directly or by changing the mechanism of the reaction so that it becomes a stepwise process and hence no longer a true pericyclic reaction.8.1 Important classes of pericyclic reactions
There are four major classes of pericyclic reactions: cycloaddition, electrocyclic, sigmatropic and ene reactions. All these reactions are potentially reversible. A general illustration of each class is given below.8.1.1 Cycloaddition reactions
A cycloaddition reaction involves the concerted formation of two σ-bonds between the termini of two π-systems. The reverse reaction involves the concerted cleavage of two σ-bonds to produce two π-systems. The simplest example being the hypothetical combination of two ethene molecules to give cyclobutane. This does not occur under normal heating, but the cycloaddition of 1,3-butadiene to ethene does, and this is an example of the Diels–Alder reaction
eBook - PDF- David R. Klein(Author)
- 2020(Publication Date)
- Wiley(Publisher)
FIGURE 17.23 The phases of the frontier orbitals align properly in a photochemical [2+2] cycloaddition. 17.9 Electrocyclic Reactions An Introduction to Electrocyclic Reactions An electrocyclic reaction is a pericyclic process in which a conjugated polyene undergoes cyclization. In the process, one π bond is converted into a σ bond, while the remaining π bonds all change their location. The newly formed σ bond joins the ends of the original π system, thereby creating a ring. Two examples are shown. Both reactions are reversible, but the position of equilibrium is different. The first example favors the cyclic product, while the second example disfavors formation of the cyclic product as a result of the ring strain associated with a four-membered ring. When substituents are present at the termini of the π system, the following stereochemical outcomes are observed: H H CH 3 CH 3 CH 3 H CH 3 H Heat hν hν Heat CH 3 CH 3 En CH 3 CH 3 + Note that the configuration of the product is dependent not only on the configuration of the reac- tant but also on the conditions of ring closure. That is, a different outcome is observed when the Conceptual CHECKPOINT 17.21 Consider the following [4+4] cycloaddition process. Would you expect this process to occur through a thermal or photochemical pathway? Justify your answer with MO theory. . + HOMO of excited state of ethylene LUMO of ground state of ethylene Phases now line up... ...symmetry allowed 772 CHAPTER 17 Conjugated Pi Systems and Pericyclic Reactions reaction is performed under thermal conditions (using heat) or under photochemical conditions (using UV light).
eBook - PDFOrbital Symmetry
A Problem - Solving Approach
- Roland Lehr(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
The three general reaction types are cited in Fig. 1.1, with one example of each. In this chapter, we shall define each of the three types of reactions and discuss the stereochemical features of each. We shall present rules that have been derived which correlate their behavior and apply them to predict transformations within each set of reactions. Finally, in Section D, we shall find that electrocyclic, sigmatropic, and cycloaddition reactions are all representatives of a larger class of concerted reactions termed pericyclic reactions, and we shall discuss them in that context. 3 4 I. I N T R O D U C T I O N Ρ > = = < μ- jj [1,5] sigmatropic shift Cycloaddition reactions: Diels-Alder reaction Electrocyclic Reactions: C H 3 C H 3 ^ C H 3 / / / c H 3 H butadiene ^ cyclobutene interconversion Fig. 1.1. Sigmatropic, cycloaddition, and Electrocyclic Reactions. Some reactions, termed symmetry forbidden, confront very large energy barriers to reaction, whereas others, termed symmetry allowed, proceed with relative ease. In Chapter II, we shall explore the theoretical basis which dif-ferentiates these two classes of reactions, and thereby lay the foundation for problems which the reader will encounter in succeeding chapters. A. Electrocyclic Reactions Electrocyclic Reactions were the first to be treated by Woodward and Hoffmann in their classic series of articles. The reactions are defined as involving the cyclization of an η pi-electron system to an (n — 2) pi- + 2 sigma-electron system or the reverse process (Fig. 1.2). Usually, the reactions are reversible and the Fig. 1.2. Generalized representation of an electrocyclic reaction. observance of ring opening or ring closure will depend upon the thermodynamic stability of the open and closed forms. An example of an electrocyclic process is the conversion of 1,3,5-hexatriene to 1,3-cyclohexadiene [Eq. (1.1)]. Sigmatropic reactions:
eBook - ePub- Metin Balcı(Author)
- 2021(Publication Date)
- Wiley-VCH(Publisher)
10 Pericyclic ReactionsIn organic chemistry, reactions can be classified according to their mechanisms into three groups.- Ionic or polar reactions
- Radical reactions
- Pericyclic reactions
Ions and radicals occur as intermediates in polar and radical reactions. Solvent polarity significantly affects the reaction. Pericyclic reactions have characteristic features compared to polar and radical reactions.- The reactions proceed in a concerted manner and bond cleavage and bond forming occur simultaneously in a single step.
- Generally, a cyclic transition complex is formed.
- No intermediates are formed in pericyclic reactions.
- Heat or light is required to start the reactions.
- Reactions are stereoselective.
- Solvent polarity generally does not affect the reaction rate or product distribution.
Electrocyclic Reactions: An electrocyclic reaction is a unimolecular process in which a new σ bond across the end of a conjugated π system is formed. During this process, the double bonds are displaced and a cyclic compound is formed containing one more σ bond and one fewer π bond than the reactant. The conversion of 1,3,5-hexatriene to 1,3-cyclohexadiene is one of the most typical examples of these reactions.The reverse or retroelectrocyclic reaction can also occur. In the reverse reaction, a σ bond of a cyclic compound breaks to form a conjugated product with one more π bond, such as the conversion of 1,3-cyclohexadiene to 1,3,5-hexatriene. An additional example is the opening of a cyclobutene ring by heat or photochemical reaction. As the σ bond in the ring opens, a new double bond is formed.Cycloaddition reactions
eBook - PDF- Arthur Scott(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
A pericyclic reaction is a reaction in which all bond-making and bond-breaking processes occur in concert (simultaneously) on a closed curve. Pericyclic reac-tions may be further subdivided into cycloadditions, electrocycliza-tions, sigmatropic shifts, chelotropic reactions, group transfers, and some types of additions and eliminations, each of which will be treated in detail. A. CYCLOADDITIONS Cycloaddition reactions are formally pericyclic reactions in which the product is the sum of the reactants and a new ring is formed. Examples of some common types of cycloadditions are shown (Figs. 1-4). The reversals of these reactions, in which rings are broken, are known as cycloreversions. Woodward and Hoffmann (1970) intro-duced a formal classification of pericyclic reactions in which the type of reaction is classified by the number and type of electrons in bonds broken in the reaction. Electrons in bonds which are broken during a reaction are designated as follows: Designation Type of bond or orbital σ Sigma (single) 7 Pi (double) ω Nonbonding The photochemical formation of oxetanes from ketones and alkenes (Fig. 1), known as the Paterno-Büchi reaction, is formally classified as a [^2 + ^2] reaction, indicating that the reaction involves two π electrons on each addend. That is, two π bonds are broken in the reaction, while two σ bonds are formed. Similarly, the Diels-Alder reaction is a [^4 + ^2] reaction (Fig. 2). The 1,3-dipolar cycloaddition, of which the addition of diazomethane to an alkene (Fig. 3) is an example, is also a [^4 + ^2] cycloaddition, since four π electrons of diazomethane are involved in the reaction (Fig. 5). The [ 2 + 2] cycloaddition (Fig. 4) is an example of the addition of a σ bond to a π bond. Strictly speaking, all these reactions are pericyclic reactions only if they are concerted —that is, if both new bonds are made simultaneously. Cycloadditions may involve more than two systems, as in the
eBook - PDF- David R. Klein(Author)
- 2016(Publication Date)
- Wiley(Publisher)
In the process, one π bond is converted into a σ bond, while the remaining π bonds all change their locations. • The use of thermal conditions vs. photochemical conditions has a profound impact on the stereochemical outcome of the reaction. • Conservation of orbital symmetry determines whether an electrocyclic reaction occurs in a disrotatory fashion or a con- rotatory fashion. SECTION 16.10 • A sigmatropic rearrangement is a pericyclic reaction in which one σ bond is formed at the expense of another. • A [3,3] sigmatropic rearrangement is called a Cope rear- rangement when all six atoms of the cyclic transition state are carbon atoms. • The oxygen analogue of a Cope rearrangement is called a Claisen rearrangement. SECTION 16.11 • Compounds that possess a conjugated π system will absorb UV or visible light to promote an electronic excitation called a π→π * transition. • A standard UV-Vis spectrophotometer will irradiate a sample with UV and visible light and will generate an absorption spec- trum, which plots absorbance as a function of wavelength. • The most important feature of the absorption spectrum is the λ max (pronounced lambda max), which indicates the wave- length of maximum absorption. • The amount of UV light absorbed at the λ max for any com- pound is described by the molar absorptivity ( ε) and is related to absorbance by an equation called Beer’s law. • Compounds with a greater extent of conjugation will have a longer λ max . • The region of the molecule responsible for the absorption (the conjugated π system) is called the chromophore, while the groups attached to the chromophore are called auxochromes. • The λ max for a simple compound can be predicted with Woodward–Fieser rules. SECTION 16.12 • When a compound exhibits a λ max between 400 and 700 nm, the compound will absorb visible light, rather than UV light. Compounds that absorb light in this range will be colored.
- Prof. Scott A. Snyder(Author)
- 2015(Publication Date)
- Thieme(Publisher)
3.5 Alkenation Followed by Electrocyclization As Electrocyclic Reactions are mainly based on the movement of conjugated ð -electrons to create one new ó -bond that closes a ring, a major concern in using this type of approach in synthesis is the way in which the required polyene system will be installed. One of the most direct ways to generate this series of C = C bonds is to start from a carbonyl group bearing a polyunsaturated side chain and to perform a classical alkenation reaction, such as a Wittig, Julia–Kocienski, or Horner–Wadsworth–Emmons reaction. Logically, the reaction can be performed in the opposite way by using a simple carbonyl group and an alkenation reagent bearing several conjugated C = C bonds. Surprisingly though, this latter strategy has not been widely used, and only a small number of examples have been reported. Because of the very rare use of this methodology, only two recently pub-lished approaches will be presented. These examples have been selected as they involve two different types of alkenation and are followed by two distinct types of electrocycliza-tion. The first method reported by Kim and co-workers [57] is based on Wittig alkenation of a benzaldehyde derivative with a phosphonium salt bearing two C = C bonds (synthesized in situ from the Morita–Baylis–Hillman adducts 78 of cinnamaldehyde analogues [58] ). This se-quence leads to triene 79 , which then undergoes 6 ð -electrocyclization to form various ortho -terphenyl derivatives 80 under an oxidative aerobic atmosphere (Scheme 28). 2.1. 3 Domino Transformations Involving an Electrocyclization Reaction 123 for references see p 156 Scheme 28 Synthesis of ortho -Terphenyl Derivatives through a Wittig Alkenation/6 ð -Elec-trocyclization/Oxidation Sequence [57] R 1 CO 2 Me Br 1.
eBook - PDF- Metin Balcı(Author)
- 2021(Publication Date)
- Wiley-VCH(Publisher)
This reveals that the disrotatory ring closure of the hexatriene molecule under thermal conditions is an allowed process according to the Woodward–Hoffmann rules. Excited state Ground state E π 1 A S A S A A S Σ π 2 π 3 π 1 π 2 σ orbital σ∗ orbital S S A A S Σ π 5 π 6 * * π 4 * π 3 * π 4 * Figure 10.17 Correlation diagram for the disrotatory conversion of hexatriene to cyclohexadiene. 10.4 Cycloaddition Reactions As discussed, cycloaddition reactions are reactions in which two compounds having π bonds add to one another, yielding new cyclic compounds with two new σ bonds and two fewer double bonds. Cycloadditions are among the reactions in organic chemistry that have been well known for many years. Reactions are classified according to the number of π bonds present in the reactants and directly involved in the reaction. The numbers of participating π electrons in both compo- nents are given in brackets. For example, the most common cycloaddition reaction is the [4 + 2] cycloaddition reaction, known as the Diels–Alder cycloaddition [22]. The German chemists Otto Paul Hermann Diels (1876–1954) and Kurt Alder 516 10 Pericyclic Reactions (1902–1958) were awarded the Nobel Prize in Chemistry in 1950 for their work in this field. Cycloaddition reactions, like Electrocyclic Reactions, may be thermally or photochemically allowed or forbidden. In this section, we will focus on the application of the Woodward–Hoffmann rules to these reactions. One of the most classic examples of cycloaddition reactions is the conversion of butadiene to cyclohexene by reacting with ethylene. In this reaction, two new σ bonds are formed as a result of the reaction of the three π bonds, while the location of one π bond is displaced. + Diene Dienophile New σ bonds Chemical reactions are electron exchange reactions. Electron transfer occurs from the HOMO of one of the reactants to the LUMO of the other.
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