Pericyclic Reactions: A Mechanistic and Problem-Solving Approach provides complete and systematic coverage of pericyclic reactions for researchers and graduate students in organic chemistry and pharmacy programs. Drawing from their cumulative years of teaching in the area, the authors use a clear, problem-solving approach, supplemented with colorful figures and illustrative examples.
Written in an accessible and engaging manner, this book covers electrocyclic reactions, sigmatropic reactions, cycloaddition reactions, 1,3-dipolar reactions, group transfer, and ene reactions. It offers an in-depth study of the basic principles of these topics, and devotes equal time to problems and their solutions to further explore those principles and aid reader understanding. Additional practice problems are provided for further study and course use.
- Comprehensive coverage of important topics such as 1,3 dipolar, pyrolytic, and cycloaddition reactions
- Problem-solving approach with clear figures and many worked and unworked problems
- Contents are applicable to advanced students and researchers in organic chemistry
Pericyclic Reactions and Molecular Orbital Symmetry
This book opens with a lucid description of pericyclic reactions and principles of molecular orbital symmetry. Molecular orbitals of alkenes, conjugated alkenes, and conjugated ions or radicals have been adequately explained to provide background for appreciating the subject matter given in the subsequent chapters. It has been shown that the molecular orbitals and their symmetry properties play a key role in understanding various types of pericyclic reactions. The chapter also provides simple ways to analyze the pericyclic reactions using orbital symmetry correlation-diagram, frontier molecular orbital (FMO), and perturbation molecular orbital (PMO) methods.
FMO method; Molecular orbitals; Orbital symmetry correlation-diagram method; Pericyclic reactions; PMO method; Symmetry properties; Woodward-Hoffmann rules
1.1 Classification of Pericyclic Reactions 2
1.2 Molecular Orbitals of Alkenes and Conjugated Polyene Systems 3
1.3 Molecular Orbitals of Conjugated Ions or Radicals 7
1.4 Symmetry Properties of p or s-Molecular Orbitals 11
1.5 Analysis of Pericyclic Reactions 13
1.5.1 Orbital Symmetry Correlation Diagram Method 13
1.5.2 Frontier Molecular Orbital Method 15
1.5.3 Perturbation Molecular Orbital Method 17
Further Reading 19
In organic chemistry, a large number of chemical reactions containing multiple bond(s) do not involve ionic or free radical intermediates and are remarkably insensitive to the presence or absence of solvents and catalysts. Many of these reactions are characterized by the making and breaking of two or more bonds in a single concerted step
through the cyclic transition state
, wherein all first-order bondings are changed. Such reactions are named as pericyclic reactions by Woodward and Hoffmann. The word concerted
means reactant bonds are broken and product bonds are formed synchronously, though not necessarily symmetrically without the involvement of an intermediate. The word pericyclic
means the movement of electrons (p-electrons in most cases) in a cyclic manner or around the circle (i.e., peri
circle or ring). They are initiated by either heat
(thermal initiation) or light
(photo initiation) and are highly stereospecific in nature. The most remarkable observation about these reactions is that, very often, thermal and photochemical processes yield products with different stereochemistry. Most of these reactions are equilibrium processes in which direction of equilibrium depends on the enthalpy and entropy of the reacting species. Therefore, in general, three important points that should be considered while studying the pericyclic reactions are: involvement of p-electrons, type of activation energy required (thermal or light), and stereochemistry of the reaction. There is a close relationship between the mode of energy supplied and stereochemistry for a pericyclic reaction, which can be exemplified by considering the simpler reactions shown in Scheme 1.1
. Scheme 1.1
Stereochemical changes in pericyclic reactions under thermal and photochemical conditions. When heat energy is supplied to the starting material, then it gives one isomer, while light energy is responsible for generating the other isomer from the same starting material.
1.1. Classification of Pericyclic Reactions
Pericyclic reactions are mainly classified into the four most common types of reactions as depicted in Scheme 1.2
. Scheme 1.2
Common types of pericyclic reactions. In an electrocyclic reaction
, a cyclic system (ring closure) is formed through the formation of a s-bond from an open-chain conjugated polyene system at the cost of a multiple bond and vice versa (ring opening). These reactions are unimolecular in nature as the rate of reactions depends upon the presence of one type of reactant species. Such reactions are reversible in nature, but the direction of the reaction is mainly controlled by thermodynamics. Most of the electrocyclic reactions are related to ring closing process instead of ring opening due to an interaction between the terminal carbon atoms forming a s-bond (more stable) at the cost of a p-bond. Sigmatropic rearrangements
are the unimolecular isomerization reactions in which a s-bond moves from one position to another over an unsaturated system. In such reactions, rearrangement of the p-bonds takes place to accommodate the new s-bond, but the total number of p-bonds remains the same. In cycloaddition reactions
, two or more components containing p-electrons come together to form the cyclic system(s) through the formation of two or more new s-bonds at the cost of overall two or more p-bonds, respectively, at least one from each component. Amongst the pericyclic reactions, cycloadditions are known as the most abundant, featureful, and valuable class of the chemical reactions. The reactions are known as intramolecular when cycloaddition occurs within the same molecule. The reversal of cycloaddition in the same manner is known as cycloreversion
. There are some cycloaddition reactions that proceed through the stepwise ionic or free radical mechanism and thus are not considered as pericyclic reactions. These reactions are further extended to cheletropic and 1,3-dipolar reactions, which shall be discussed in detail in Chapter 5
. Group transfer reactions
involve the transfer of one or more atoms or groups from one component to another in a concerted manner. In these reactions, two components join together to form a single molecule through the formation of a s-bond. It is very important to note that in studying the pericyclic reactions, the curved arrows can be drawn in clockwise or anticlockwise direction (Scheme 1.3
). The direction of arrows does not indicate the flow of electrons from one component or site to another as in the case of ionic reactions; rather, it indicates where to draw the new bonds. Scheme 1.3
Clockwise and anticlockwise direction of the curved arrows in pericyclic reactions.
1.2. Molecular Orbitals of Alkenes and Conjugated Polyene Systems
In order to understand and explain the results of the various pericyclic reactions on the basis of different theoretical models, a basic understanding of the molecular orbitals of the molecules, particularly those of alkenes and conjugated polyene systems and their symmetry properties, is required. According to the molecular orbital theory, molecular orbitals (MOs) are formed by the linear combination of atomic orbitals (LCAO) and then filled by the electron pairs. In LCAO when two atomic orbitals of equivalent energy interact, they always yield two molecular orbitals, a bonding and a corresponding antibonding orbital. The bonding orbital possesses lower energy and more stability while antibonding possesses higher energy and less stability as compared to an isolated atomic orbital. Let us consider the simplest example of H2 molecule formed by the combination of 1s atomic orbitals (Figure 1.1
). Figure 1.1
Formation of molecular orbitals in the case of an H2 molecule. The bonding molecular orbital is a result of positive (constructive) overlap, and hence electron density lies in the region between two nuclei. However, an antibonding molecular orbital is formed as a result of negative (destructive) overlap and, therefore, exhibits a nodal plane in the region between the two nuclei. The bonding and antibonding molecular orbitals exhibit unequal splitting pattern with respect to the atomic orbitals because a fully filled molecular orbital has higher energy due to interelectronic repulsion. We now consider molecular orbital theory with reference to the simplest p-molecular system, ethene. As already discussed, the number of molecular orbitals formed is always equal to the number of atomic orbitals combining together. Similarly, in the case of an ethene molecule, sideways interaction between p
-orbitals of the two individual carbon atoms results in the formation of the new p bonding and p* antibonding molecular orbitals that differ in energy (Figure 1.2
). In the bonding orbital of ethene, there is a constructive overlap of two similar lobes of p
-orbitals in the bonding region between the nuclei. However, in the case of an antibonding orbital, there is destructive overlap of two opposite lobes in the bonding region. Each p
-orbital consists of two lobes with opposite phases of the wave function. We ignore s-bond skeleton in this treatment as sigma molecular orbitals remain unaffected during the course of a pericyclic reaction. The conjugated polyenes constitute an important class of organic compounds exhibiting a variety of pericyclic reactions. On the basis of the number of p-electrons, such compounds are classified into two categories bearing 4n or...