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. 

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Cycloaddition reactions are a very important class of pericyclic reactions in which two unsaturated molecules join by converting two tt-bonds into two new a-bonds between their termini. Although cycloaddition reactions are concerted (no intermediate species are formed), the two new bonds in a few cases may be formed in an asynchronous fashion. Depending on partial charge distribution in both reactants, the formation of one bond may lead to the development of the other. 

Sigmatropic shifts represent another important class of pericyclic reactions to which the Woodward-Hoffmann rules apply. The selection rules for these reactions are best discussed by means of the Dewar-Evans-Zimmerman rules. It is then easy to see that a suprafacial [1,3]-hydrogen shift is forbidden in the ground state but allowed in the excited state, since the transition state is isoelectronic with an antiaromatic 4N-HQckel system (with n = 1), in which the signs of the 4N AOs can be chosen such that all overlaps are positive. The antarafacial reaction, on the other hand, is thermally allowed, inasmuch as the transition state may be considered as a Mobius system with just one change in phase. 

Dienes and polyenes have been a subject of great interest due to their important role in biology, materials science and organic synthesis. The mechanism of vision involves cis-trans photoisomerization of 11 -civ-retinal, an aldehyde formed from a linear polyene. Moreover, this kind of molecule exhibits high linear and non-linear electrical and optical properties. Short polyenes are also involved in pericyclic reactions, one of the most important classes of organic reactions. 

Pericyclic processes comprise a broad and important class of concerted reactions of both theoretical and practical interest. These transformations, which are especially useful in the construction of carbon-carbon bonds,93 include electrocyclic reactions, sigmatropic rearrangements, and cycloadditions. Because they are not typically subject to general acid-general base chemistry but can be highly sensitive to strain and proximity effects, they are attractive targets for antibody catalysis. 

It is important that you do not confuse electrocyclic reactions with pericyclic reactions. Pericyclic is the name for the family of reactions involving no charged intermediates in which the electrons go round the outside of the ring. Electrocyclic reactions, cycloadditians, and sigmatropicrearrangements are the three main classes of pericyclic reactions. 

Before discussing specific reactions, it is important to learn how to describe pericyclic reactions. There are four major classes of pericyclic reactions electro-cyclic reactions (ring openings or ring closings), cycloadditions, sigmatropic rearrangements, and ene reactions. 

An important class of concerted reactions is the pericyclic reactions. A pericyclic reaction is characterized as a change in bonding relationship that takes a... 

Another important type of pericyclic reactions are cycloadditions. As an example of the use of the overlap determinant method for this class of reactions, let us analyze... 

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. 

The last two chapters introduced pericyclic reactions, and the next one will cover reactions of radicals. Together with the ionic reactions which have been the subject of most of this book, these three classes cover all organic mechanisms. But before we move on to consider radicals, we need to fill a gap in our coverage of ionic reactions. You have met the most important types of ionic reactions—additions, substitutions, and eliminations. But two remain and they are closely related, in rearrangements the molecule changes its carbon skeleton and in fragmentations the carbon skeleton splits into pieces. We lead up to these types of reaction by looking at a phenomenon known as participation. 

The conjugated polyenes constitute an important class of organic compounds exhibiting a variety of pericyclic reactions. On the basis of the... 

Importantly, Cl s seems to be involved in many classes of physical, chemical and biological processes, from pericyclic reactions to the complex light harvesting and energy conversion functions of chromophores in proteins (See in this volume) and others amply described in this conference. In contrast, direct experimental information on the passage of the vibrational wavepacket through or near Cl s is less abundant. It mostly concerns, femtosecond pump-probe experiments on isolated organic molecules in the gas phase. 

The mechanism classification and the overall transformation classification are orthogonal to each other. For example, substitution reactions can occur by a polar acidic, polar basic, free-radical, pericyclic, or metal-catalyzed mechanism, and a reaction under polar basic conditions can produce an addition, a substitution, an elimination, or a rearrangement. Both classification schemes are important for determining the mechanism of a reaction, because knowing the class of mechanism and the overall transformation rales out certain mechanisms and suggests others. For example, under basic conditions, aromatic substitution reactions take place by one of three mechanisms nucleophilic addition-elimination, elimination-addition, or SrnL If you know the class of the overall transformation and the class of mechanism, your choices are narrowed considerably. 

Within each class, we will consider the number of electrons in n molecular orbitals in the transition state. Pericyclic reactions may involve either An or 4 + 2 electrons, where is an integer. This distinction is important because the symmetry of the molecular orbitals in thermal or photochemical reactions depends on the number of n electrons in the transition state. The stereochemistry of the products of pericychc reactions depends on the symmetry of these molecular orbitals. We will see that the stereochemistry of a product of a pericychc reaction derived from a 4 7i electron system differs from that of a 4 + 2 7t electron system. Thus, the concepts of orbital symmetry and Hiickle systems that we considered in Chapter 11 and in particular the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) will play a central role in our discussions of pericychc reactions. 

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