Classes of Pericyclic Reactions

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. 

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. 

You may have trouble seeing that the last example of an electrocyclic reaction is a two-electron and not a four-electron reaction. The new a bond is formed between the termini of a three-atom -n system. That three-atom system contains two electrons. The lone pairs of O are not part of the three-atom tt system, so they are not included in the electron count of the electrocyclic reaction. 

When x bonds are formed between the ends of two tt systems to give a cyclic product, the reaction is called a cycloaddition. The reverse reaction is called a retro-cycloaddition. Cycloadditions are further classified as [m + n] according to the number of atoms in each component. Again, it is important to note not only the number of atoms but also the number of electrons involved in the process. You are already familiar with the six-electron [4 + 2] cycloaddition, the Diels Alder reaction. Four-electron [2 + 2] cycloadditions are less common, for reasons that will be discussed, but ketenes undergo them readily. The [3 + 2] cycloadditions (or 1,3-dipolar cycloadditions) are a very important class of six-electron cycloadditions that are used to make a wide variety of five-membered heterocycles. Other cycloadditions, including [8 + 2], [4 + 3], and [6 + 4] cycloadditions, are also known. 

Cheletropic reactions (e.g., [2 + 1] cycloadditions and [4 + 1] retro-cycloadditions) are a special class of cycloadditions in which one of the components is a single atom. The one-atom component must have one full and one empty orbital it may be a carbene ( CR2), S02 (0=S+-0 — O-S2 1 ()), or C=0 (C=() — C=0). The [4 + 1] cycloadditions usually proceed in the retro direction with the expulsion of a stable small molecule such as CO or S02. 

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There are several general classes of pericyclic reactions for which orbital symmetry factors determine both the stereochemistry and relative reactivity. The first class that we will consider are electrocyclic reactions. An electrocyclic reaction is defined as the formation of a single bond between the ends of a linear conjugated system of n electrons and the reverse process. An example is the thermal ring opening of cyclobutenes to butadienes ... 

Five classes of pericyclic reactions have been recognized. 

In previous sections we have seen how the CM model may be utilized to generate reaction profiles for ionic reactions, and it is now of interest to observe whether the same general principles may be applied to the class of pericyclic reactions, the group of reactions that is governed by the Woodward-Hoffmann (1970) rules. In other words, the question we ask is whether the concept of allowed and forbidden reactions may be understood within the CM framework. 

Each of the following transformations is the result of two successive pericyclic reactions. Draw the structures of the intermediates A-E, and identify the class of pericyclic reaction to which each step belongs ... 

Electrocyclic reactions are characterized by the creation of a ring from an open-chain conjugated system, with a a-bond forming across the ends of a conjugated system, or, of course, the reverse of this reaction. Unfortunately, the word electrocyclic is sometimes used wrongly by the unwary when they really mean pericyclic. This mistake has come about because the word electrocyclic was introduced before there was any word to describe the whole class of pericyclic reactions, and some people have never caught up. 

Identify the two and the three pericyclic steps involved in the formation of the major and minor products, respectively, in this transformation (hint all four classes of pericyclic reaction are represented) ... 

First, pericyclic reactions are defined, and an example of their unusual stereochemical selectivity is presented. A theoretical treatment of pericyclic reactions requires examination of the MOs for the conjugated molecules that participate in these reactions, so MO theory for these compounds is developed next. Then a theoretical explanation for the selectivity and stereochemistry observed in each of the three classes of pericyclic reactions is presented, along with a number of common examples of reactions of each kind. 

A reaction involving concerted reorganization of electrons within a closed loop of interacting orbitals. Cycloadditions are one class of pericyclic reactions, (p. 692)... 

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. 

Between 1965 and 1969 Woodward and Hoffmann presented rules for each of the different classes of pericyclic reaction. They showed that the allowedness or otherwise of reactions depended critically upon their stereochemistry. We shall go through the rules twice first the rules class-by-class, and then again using the generalised mle that applies to all classes of pericyclic reactions. 

This admirably concise statement is compelling, but we must now see what it means, and learn how to apply it to each of the classes of pericyclic reaction. 

A pericyclic reaction is a type of organic reaction wherein the transition state of the molecule has a cyclic geometry, and the reaction progresses in a concerted fashion (electrons move in a circular fashion and there is no positive and negative charges). Pericyclic reactions are usually rearrangement reactions. The major classes of pericyclic reactions are ... 

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. 

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. 

Dipolar cycloadditions. Interest in 1,3-dipolar cycloadditions increased dramatically during the past 20 years, largely because of the pioneering studies of Huisgen [1, 2] The versatility of this class of pericyclic reactions in the synthesis of five-membered-nng heterocyclic compounds is comparable with that of the Diels-Alder reaction in the synthesis of six-membered-rmg carbocychc systems (equation 1)... 

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. 

Analogously to the example mentioned above, the other classes of pericyclic reactions with even numbers of chemical centers can be related to isomorphic topologies of edges. The structure does not change if we increase the number of atoms taking part during these reactions. The topologies of pericyclic reactions are equal up to isomorphism. 

So far, cycloadditions have been our only examples of pericyclic reactions. There are several other classes of pericyclic reactions, of which the most notable are cheletropic reactions, sigmatropic rearrangements and electrocyclic reactions. In essence, frontier orbital theory treats each of them as a cycloaddition reaction. 

The Alder ene reaction is like a Diels Alder reaction in which one Jt-bond in the diene has been replaced by a C-H bond 121. It does not therefore form a ring and does not fit easily into any of the three classes of pericyclic reaction (cycloaddition, electrocyclic, and sigmatropic). Since a hydrogen atom is transferred from one component to the other it is best described as a group transfer reaction.21 The regioselectivity is determined by the interaction 123 with the Jt-bond of the ene (the HOMO) with the LUMO of the enophile. ... 

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