Pericyclic reactions categories

The third major category of pericyclic reactions can be looked upon as involving the migration of a a bond—hence the name—within a 7t-electron framework. The simplest examples involve the migration of a a bond that carries a hydrogen atom. 

By analogy with the categories of pericyclic reactions we have already considered, the feasibility of the migration will then be decided by the relative phase of the terminal lobes, i.e. the symmetry, of the HOMO of the pentadienyl radical (38). As this is a 5ne system, its... 

Within the overall category of pericyclic reactions, it is convenient to divide them into four main classes. These are cycloadditions, electrocyclic reactions,... 

There are, however, also many examples of mixed domino processes , such as the synthesis of daphnilactone (see Scheme 0.6), where two anionic processes are followed by two pericyclic reactions. As can be seen from the information in Table 0.1, by counting only two steps we have 64 categories, yet by including a further step the number increases to 512. However, many of these categories are not - or only scarcely - occupied. Therefore, only the first number of the different chapter correlates with our mechanistic classification. The second number only corresponds to a consecutive numbering to avoid empty chapters. Thus, for example in Chapters 4 and 6, which describe pericyclic and transition metal-catalyzed reactions, respectively, the second number corresponds to the frequency of the different processes. 

As we said in Chapter 5, there are three fundamental reaction types polar reactions, radical reactions, and pericyclic reactions. Let s review each to see how the reactions we ve covered fit the different categories. 

Pericyclic reactions are concerted processes that occur by way of a cyclic transition state in which more than one bond is formed or broken within the cycle. The classic example of such a process is the Diels-Alder cycloaddition reaction, one of the most common and useful synthetic reactions in organic chemistry. Cycloaddition reactions, sigmatropic rearrangements and electrocyclic reactions all fall into the category of pericyclic processes, representative examples of which are given in Schemes 3.1-3.3. This chapter will discuss these reactions and their use in synthesis. 

In 1953, Robert s experiments on the conversion of C-labeled chlorobenzene with KNH2 into aniline gave strong support to the intermediacy of ortho-benzyne in this and related reactions. Additional direct evidence for the existence of ortho-benzyne was provided by the observation of its IR spectrum, sohd-state dipolar NMR spectrum, and NMR in a molecular container, and by UV photoelectron spectroscopy. Even at low temperatures, arynes are extraordinary reactive. The reactions of arynes can be divided into three groups (i) pericyclic reactions, (ii) nucleophilic additions, and (iii) transition-metal catalyzed reactions. The pericyclic reactions can be divided into several categories such as Diels-Alder reactions, [2-f2] cycloadditions, 1,3- and l,4-dipolar cycloadditions, and the ene reactions. Arynes react with practically aU kinds of nucleophiles. More recently, the transition-metal catalyzed reactions of arynes have been studied, in particular those involving palladium. 

The pericyclic reactions can be divided into several categories such as Diels-Alder reactions, [2+2] cycloadditions, 1,3- and 1,4-dipolar cycloadditions, and the ene reactions. 

In order to avoid solvent problems we again chose the pericyclic reactions as the source of our new reaction, since these reactions are usually not solvent dependent. In consideration of the known thermal lability of this class of compounds we only took the photochemically allowed reaction categories into account (those with 4n electrons in the transition state. Figure 10). Photolysis at low temperatures might be the suitable condition for the formation. 

In the course of the reaction a formal lone pair must be formed on carbon. There are only two reaction categories satisfying this restriction, the 3-membered and the 7-membered pericychc reactions. We considered the 7-membered pericyclic reaction to be too large a system to provide an elegant synthesis and concentrated on the 3-membered reaction category. 

The bond changing process must be concerted in order for a reaction to fit the category of pericyclic reactions however, it is often difficult to distinguish between a pericychc and a stepwise mechanism of a cycloaddition. In this chapter the main focus will be on reactions that proceed, or are most likely to proceed, through a concerted mechanism we will, however, include some exceptions for entirety. The majority of stepwise cycloaddition reactions will be discussed in Chapters 10 and 16. This review is not intended to be comprehensive, but instead organocatalytic pericychc reactions will be outlined in the context of their utility in synthetic chemistry. 

Pericyclic reactions may be ring-opening, ring closing or rearrangements and are classified in three major categories which are ... 

Because of condition (iii) all pericyclic reactions may formally be regarded as cyclo-addition processes or their retrogressions, but it is generally more useful to divide pericyclic reactions into a number of more distinct reaction series. These are electrocyclic reactions (e.g. Equations 3.3 and 3.4), cycloaddition reactions (e,g. Equations 3.5 and 3,6), sigmatropic reactions (e.g. Equations 3.7 and 3.8), cheletropic reactions (e.g. Equations 3.9 and 3.10), group transfers (e.g. Equation 3.11), and eliminations (e.g. Equations 3.12 and 3,13). Examples in other categories are less numerous, and will not be considered in this volume. 

Every organic reaction can be classified into one of three, more or less exclusive categories—ionic, radical and pericyclic. Ionic reactions involve pairs of electrons moving in one direction. In a unimolecular reaction, like the ionization of a tertiary alkyl halide, the carbon-halogen bond cleaves with... 

Pericyclic reactions656,657 are the second distinct class of the three, more or less exclusive categories of organic reactions—ionic (Chapters 4 and 5), pericyclic (this Chapter) and radical (Chapter 7). Their distinctive features are that they have cyclic transition structures with all the bond-making and bond-breaking taking place in concert, without the formation of an intermediate. The Diels-Alder reaction and the Alder ene reaction are venerable examples. 

Reactions in Organic Chemistry are broadly classified into three major categories— ionic, radical, and pericyclic. Ionic reactions involve the formation of ionic intermediates by movement of pair of electrons in one direction of a covalent bond. In a unimolecular reaction, it occurs by ionization process and in a bimolecular reaction, it occurs when one component acts as a nucleophile (or electron pair donor) and another component as electrophile (or electron pair acceptor). For example. 

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