Pericyclic reactions general examples

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 ... 

There are numerous reactions in organic chemistry that proceed through cyclic transition states. They may be classified generally as pericyclic reactions. An important and familiar example is the Diels-Alder reaction, in which a conjugated diene cycloadds to an alkene or alkyne ... 

Such rearrangements are quite general for aryl allyl ethers and are called Claisen rearrangements. They are examples of the pericyclic reactions discussed in Section 21-10D. (See Exercise 26-45.)... 

In connection with Eq. (22), yet another important factor differentiates our approach from usual quantum chemical analyses of reaction mechanisms. This difference concerns the fact that while a quantum chemical approach is in principle independent of any external information (all participating species appear automatically as various critical points on the PE hypersurface), in our model that is more closely related to classical chemical ideas some auxiliary information about the structure of the participating molecular species is required. This usually represents no problem with the reactants and the products since their structure is normally known, but certain complications may appear in the case of intermediates. This complication is not, however, too serious since in many cases the structure of the intermediate can be reasonably estimated either from some experimental or theoretical data or on the basis of chemical intuition. Thus, for example, in the case of pericyclic reactions that are of primary concern for us here, the intermediates are generally believed to correspond to biradical or biradicaloid species with the eventual contributions of zwitterionic structures in polar cases. 

When ultraviolet light rather than heat is used to induce pericyclic reactions, our predictions generally must be reversed. For example, the [2 + 2] cycloaddition of two ethylenes is photochemically allowed. When a photon with the correct energy... 

Pericyclic reactions were used successfully, too, to build up macrocycHc ring systems as found, for example, in natural peptides and peptide alkaloids. These reactions are carried out in general at moderate dilution conditions (10 -10 M). An example of a Diels-Alder reaction is the cyclization of 106 to 107, where the basic framework of cytochalasane B (108), a substance found in fungi, is built up in yet 27% yield [79]. 

When ultraviolet light rather than heat is used to induce pericyclic reactions, our predictions are generally reversed. For example, the [2 -i- 2] cycloaddition of two ethylenes is photochemically allowed. When a photon with the correct energy strikes ethylene, one of the pi electrons is excited to the next higher molecular orbital (Figure 15-21). This higher orbital, formerly the LUMO, is now occupied It is the new HOMO, the HOMO of the excited molecule. 

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. 

Pericyclic reactions in general and electrocyclic reactions in particular, for example the ring opening of cyclobutene to 1,3 butadiene as shown in Figure 8.6, have been central to physical organic chemistry and its interpretation by orbital theories of electronic structme. " ... 

The theoretical principles of cycloaddition reactions are well understood and there have been many computational studies (see Pericyclic Reactions The Diels-Alder Reaction). Often the hetero-cycloaddition reaction shows similar characteristics to the carbocyclic analog, but a number of special features have been noted. In heterocyclic chemistry the cycloaddition reactions are often dipolar computational studies show that a concerted mechanism is followed in the gas phase. However, a number of studies have noted that these dipolar cycloaddition reactions become stepwise when solvent effects are included (via the reaction field method), with a consequent loss of stereospecificity." Other characteristics of hetero-cycloaddition reactions which have been studied include the endo/exo selectivity" and the regiose-lectivity (for example, [2-1-2] vs. [2-1-4])." High levels of electron correlation are generally required in order to establish these selectivities. 

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. 

From the foregoing discussion it appears that the frontier orbital method is at once a simple, concise, and accurate method for assessing the stereochemical outcome of pericyclic reactions. Furthermore, it is a method that is equally applicable to symmetrical and to unsymmetrical systems. There are some disadvantages in the theory, however. Firstly, it is necessary to derive the general phase characteristics of the HOMO and LUMO levels. Hiickel molecular calculations can be used for tliis purpose, but there are available a number of approximate methods, for example the electron-in-a-box model, which are usually satisfactory even if they are more difficult to apply to more complex systems. Nevertheless, frontier orbital analysis is quicker and more simple than the formalized correlation diagram approach, and with a little practice one can intuitively arrive at the correct relative phase relationsliips in the HOMO and LUMO levels. 

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