Classification of Pericyclic Reactions

For a classification of pericyclic reactions, see Hendrickson, J.B. Angew. Chem. Int. Ed. Engl., 1974, 13, 47. Also see Fleming, I. Pericyclic Reactions-, Oxford University Press Oxford, 1999. 

Figure 12.2. Three classifications of pericyclic reactions, with examples of thermally allowed reactions. Cheletropic is a special case of electrocyclic.

In addition, Dewar demonstrated that the classification of pericyclic reactions has nothing to do with symmetry. The nature of the reaction is defined by the AO overlap topology in a pericyclic transition state and not by the MO symmetry. 

Write short note on classification of pericyclic reactions. 

Discuss Molecular Orbital Synunetry and Classification of Pericyclic Reactions. 

The participation of the lone-pair orbital in the cyclization process allows its classification as a so-called pseudopericyclic reaction (76JA4325 97JA4509), which is a subset of a general type of pericyclic reactions... 

Although such an understanding of the reaction mechanism is in principle applied in the theory of pericyclic reactions, the above general picture is in this case slightly complicated by the specific (introduced in the course of historical development) classification of reaction mechanisms in terms of concertedness and/or nonconcertedness. Concerted reactions are intuitively understood as those reactions for which the scission of old bonds and the formation of the new ones is synchronised, whereas for nonconcerted reactions the above bond exchange processes are completely asynchronised. Moreover, since the above asynchronicity is also intuitively expected to induce the stepwise nature of the process, the nonconcertedness is frequently believed to require the presence of intermediates, whereas the concerted reactions are believed to proceed in one elementary step. 

This chapter examines reactions that involve molecular rearrangements and cycloadditions. The use of these terms will not be restricted to concerted, pericyclic reactions, however. Often, stepwise processes that involve a net transformation equivalent to a pericyclic reaction are catalyzed by transition metals. The incorporation of chiral ligands into these metal catalysts introduces the possibility of asymmetric induction by inter-ligand chirality transfer. The chapter is divided into two main parts (rearrangements and cycloadditions), and subdivided by the standard classifications for pericyclic reactions e.g., [1,3], [2,3], [4-1-2], etc.). The latter classification is for convenience only, and does not imply adherence to the pericyclic selection rules. Indeed, the first reaction to be described is a net [1,3]-suprafacial hydrogen shift, which is symmetry forbidden if concerted. 

This example illustrates the difficulties that are associated with a proper investigation of the reaction mechanisms. In addition, it clearly demonstrates that the outcome of a pericycHc cascade can often be described by several alternative reaction mechanisms diflfering in the type and order of pericyclic reaction(s) involved. Thus, the classification made in this chapter might be taken with care. We (as well as the authors of the papers we rely on) commonly tried to provide plausible mechanistic scenarios however, often only the products of a transformation are what we really know. 

For the reason of comparison and the development of new domino processes, we have created a classification of these transformations. As an obvious characteristic, we used the mechanism of the different bond-forming steps. In this classification, we differentiate between cationic, anionic, radical, pericyclic, photochemical, transition metal-catalyzed, oxidative or reductive, and enzymatic reactions. For this type... 

An example of a reaction, first presented in Section 20.4, that falls under the pericyclic classification is the decarboxylation of /3-ketoacids produced in the malonic and acetoacetic ester syntheses ... 

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. 

Classification based on the Number of Electrons General Rule for Pericyclic Reactions... 

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. 

As shown in the example of pericyclic six centered reactions, the dynamic graph D consists of graphs in all cases and the graph S provides for the classification of reactions (see Figure 3.1) (18,24 26). 

C—C) bonds with the 7t-system (hyperconjugation, see p. 80) to supply electrons to the conjugated system.39 The effect is like that of the lone pairs it is usually much smaller, but quite noticeable. We shall be using this classification again in Chapter 4, on pericyclic reactions, the subject into which it was first introduced by Houk.40... 

On the basis of the classification introduced by professors Woodward and Hoffmann pericyclic reactions are divided into several basic types. 

Pericyclic reactions are routinely classified as"allowed"or"forbidden"with a particular structure for the transition state. In practice, this classification means that one geometry for the reaction has a low energy transition state (allowed) or that a different geometry has a very high energy transition state (forbidden). To determine whether a reaction is allowed or forbidden, a handful of approaches exist. We will examine one approach frontier molecular orbital theory. 

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