Pericyclic reactions allowed stereochemistry

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 Woodward-Hoffmann rules also allow the prediction of the stereochemistry of pericyclic reactions. The Diels-Alder reaction is an example of (re4s + re2s) cycloaddition. The subscript s, meaning suprafacial, indicates that both elements of the addition take place on the same side of the re-system. Addition to opposite sides is termed antarafacial. The Woodward-Hoffmann rules apply only to concerted reactions and are derived from the symmetry properties of the orbitals involved in the transition state. These rules may be summarised as shown in Table 7.1. 

For each class of pericyclic reactions two or more of the following characteristics will be discussed the typical reactions, regioselectivity, stereoselectivity, and stereospecificity. The discussions of typical reactions and stereospecificity will help you recognize when pericyclic reactions are occurring in a particular chemical reaction. The discussions of regioselectivity, stereoselectivity, and stereospecificity will allow you to predict the structures and stereochemistry of the products obtained from pericyclic reactions. 

To sum up the above discussion, we have witnessed that the orbital overlap component of the stereoelectronic effect is indeed a very powerful tool as it controls both the stereochemistry and the rates of a range of pericyclic reactions by allowing exclusively one of the two possible symmetry-allowed pathways for the very simple reason of better overlap of the breaking bonds. 

The application of the general selection rule does not require analyzing a concerted reaction in any particular way. All descriptions of a pericyclic reaction that predict the same stereochemistry in the product will lead to the same conclusion. For example, three descriptions of the Diels-Alder reaction are given in Figure 11.88. In Figure 11.88(a), the reaction is described as a [ 2j + 4J cycloaddition. In this process there is one (4n - - 2)g component and no (4r)a component, so the total (1) is an odd number, and the reaction is allowed. In Figure 11.88(b), each double bond of the diene is considered to be a separate unit, so the reaction is described as a - - 2 -I- 2j process. 

Figure 15.18 shows several examples of electrocyclic processes. Since the reactions are always allowed in either a conrotatory or disrotatory manner, the key issue is the control of stereochemistry. Electrocyclic reactions provide a good example of the power of pericyclic reactions in this regard. In all cases, the reaction proceeds as predicted from the various theoretical approaches. The restrictions placed by the orbital analysis on the reaction pathway are nicely demonstrated by examples D and E in Figure 15.18 only the stereochemistry given is found. An instructive example of the fact that it is the number of electrons that controls the process, not the number of atoms or orbitals, is the conrotatory ring closure of the four-electron pentadienyl cation prepared by protonation of a divinyl ketone (example G). 

In each of the two pericyclic processes so far considered, the course of the reaction was unambiguous. An interesting distinction arises in cases where there is a choice between two stereochemistries for a given pericyclic reaction, one corresponding to a thermally allowed process with a transition state which is aromatic in the ground state, the other to a photochemically allowed process with a transition state which is antiaromatic in the ground... 

The series of reactions below demonstrates a number of pericyclic reactions. For each, indicate the type of reaction, the number of electrons involved, and the allowed stereochemistry [40]. 

Naphthopyran dyes show an interesting photochromic effect. Exposure to UV light converts these colorless compounds reversibly to colored compounds, as shown below. These compounds can be used in self darkening eyeglasses. Identify the pericyclic reaction and describe the allowed stereochemistry [57]. 

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