Pericyclic reactions orbital symmetry correlation diagram

The book opens with an introduction (Chapter 1), which, besides providing background information needed for appreciating different types of pericyclic reactions, outlines simple ways to analyze these reactions using orbital symmetry correlation diagram, frontier molecular orbital (FMO), and perturbation molecular orbital (PMO) methods. This chapter also has references to important published reviews and articles. 

The most important observation in the smdy of pericyclic reactions is the existence of conservation of molecular orbital symmetry throughout the transformation, meaning thereby that the symmetric orbitals are converted into symmetric orbitals whereas antisymmetric orbitals are converted into antisymmetric orbitals. In this approach, symmetry properties of various molecular orbitals of the bonds that are involved in the bond breaking and formation process during the reaction are considered and identified with respect to C2 and m elements of symmetry. These properties remain preserved throughout the course of reaction. Then a correlation diagram is drawn in which the molecular orbital levels of like symmetry of the reactant are related to that of the product by drawing lines. 

While photocycloadditions are typically not concerted, pericyclic processes, our analysis of the thermal [2+2] reaction from Chapter 15 is instructive. Recall that suprafacial-suprafacial [2+2] cycloaddition reactions are thermally forbidden. Such reactions typically lead to an avoided crossing in the state correlation diagram, and that presents a perfect situation for funnel formation. This can be seen in Figure 16.17, where a portion of Figure 15.4 is reproduced using the symmetry and state definitions explained in detail in Section 15.2.2. The barrier to the thermal process is substantial, but the first excited state has a surface that comes close to the thermal barrier. At this point a funnel will form allowing the photochemical process to proceed. It is for this reason that reactions that are thermally forbidden are often efficient photochemical processes. It is debatable, however, whether to consider the [2+2] photochemical reactions orbital symmetry "allowed". Rather, the thermal forbiddenness tends to produce energy surface features that are conducive to efficient photochemical processes. As we will see below, even systems that could react via a photochemically "allowed" concerted pathway, often choose a stepwise mechanism instead. 

Fig. 7. Orbital (a), configuration (b), and state (c, d) correlation diagrams for a typical ground-state symmetry-forbidden pericyclic reaction...

Answer to 2(d). This question illustrates that it is the number of electrons, not the number of nuclei, that is important. The orbital correlation diagram is shown in Figure 14.2. In disrotatory opening, a mirror plane of symmetry is preserved. This correlation is with bold symmetry labels and solid correlation lines. Italic symmetry labels and dotted correlation lines denote the preserved rotational axis of symmetry for conrotatory ring opening. For the cation, the disrotatory mode is the thermally allowed mode. It corresponds to a a2s + 05 pericyclic reaction. 

Three levels of explanation have been advanced to account for the patterns of reactivity encompassed by the Woodward-Hoffmann rules. The first draws attention to the frequency with which pericyclic reactions have a transition structure with (An + 2) electrons in a cyclic conjugated system, which can be seen as being aromatic. The second makes the point that the interaction of the appropriate frontier orbitals matches the observed stereochemistry. The third is to use orbital and state correlation diagrams in a compellingly satisfying treatment for those cases with identifiable elements of symmetry. Molecular orbital theory is the basis for all these related explanations. 

The Woodward-Hoffmann rules arise fundamentally from the conservation of orbital symmetry seen in the correlation diagrams. These powerful constraints govern which pericyclic reactions can take place and with what stereochemistry. As we have seen, frontier orbital interactions are consistent with these features,... 

In view of the demonstrated stereospecificity of at least some cation radical Diels-Alder reactions, it is at least possible that these reactions, like the neutral Diels-Alder, are true pericyclic reactions, i.e., they may occur via a concerted cycloaddition. The results of a variety of calculations, however, make clear that the cydoadditions must at least be highly non-synchronous, so that the extent of the formation of the second bond, which completes the cyclic transition state, is no more than slight [55, 56]. If the cation radical Diels-Alder reaction is nevertheless interpreted as pericyclic and the concept of orbital correlation diagrams is applied to them, it emerges that the cycloaddition is symmetry allowed if the ionized (cation radical) component is the dienophile, but forbidden if it is the diene [39, 55], The former mode of reaction has been referred to as the [4-1-1] mode, and the latter as the [3 -t- 2] mode. Interestingly, the great majority of cation radical Diels-Alder reactions thus far observed seem to represent the formally allowed [4-1-1] mode. An interesting case in point is the reaction of l,l -dicyclohexenyl with 2,3-dimethylbutadiene (Scheme 24) [57]. 

The following sections present an empirical approach to applying the selection rules. The chapter continues with a basic introduction to the analysis of symmetry properties of orbitals and the application of orbital correlation diagrams to the relatively simply cyclobutene-butadiene interconversion it concludes with some examples of the frontier orbital approach to pericyclic reactions. 

When pericyclic reactions are analyzed in terms of correlation diagrams, all of the 77 and cr molecular orbitals taking part in the reaction are analyzed in terms of their symmetry properties with respect to reflection in a mirror... 

Most known photochemical processes are not pericyclic reactions. Even in many of these cases correlation diagrams can be helpful in estimating the location of minima and barriers on excited-state surfaces. (Cf. Section 4.2.2.) The derivation of these correlation diagrams, however, is often more difficult, not only because of lack of symmetry, but also because it may be difficult to identify any one excited state as the characteristic state, particularly in large molecules. For example, many of the excited states of toluene will have some contribution from the bond orbital excitation... 

The photochemistry of alkenes, dienes, and conjugated polyenes in relation to orbital symmetry relationships has been the subject of extensive experimental and theoretical studyThe analysis of concerted pericyclic reactions by the principles of orbital symmetry leads to a complementary relationship between photochemical and thermal reactions. A process that is forbidden thermally is allowed photochemically and vice versa. The complementary relationship between thermal and photochemical reactions can be illustrated by considering some of the reaction types discussed in Chapter 10 and applying orbital symmetry considerations to the photochemical mode of reaction. The case of [2Tr- -2Tr] cycloaddition of two alkenes, which was classified as a forbidden thermal reaction (see Section 10.1), can serve as an example. The correlation diagram (Figure 12.17) shows that the ground state molecules would lead to a doubly excited state of cyclobutane, and would therefore involve a prohibitive thermal activation energy. 

MOs, while tlie two 7t c orbitals lead to the tt and tt MOs. In the initial stage of (he dimerization, the interaction between two ethylencs is weak so that 7t+ and tt. lie far below the n+ and tt levels, so that only 7t+ and rr are occupied. Of the a orbitals of cyclobutane described earlier, only those related to the tt., 7t1 and nl levels by symmetry are shown in Figure 11.1. Not all the occupied MOs of the reactant lead to occupied orbitals in the product. In particular, tt. correlates with one component of the empty set in cyclobutane. The tt+ combination ultimately becomes one component of the filled set in cyclobutane. So the reaction is symmetry forbidden. The reader should carefully compare the correlation diagram for ethylene dimerization here with the Ho + O2 reaction in ITgure 5.8. flie two correlation diagrams are very similar, as they should be, since in this instance the spatial dfstributions of tt and n " are similar to those of and respectively, in H2. These two reactions are probably the premier examples of symmetry-forbidden reactions. A related symmetry-allowed example is the concerted cycloaddition of ethylene and butadiene, the Diels-Alder reaction. We shall not cover the orbital symmetry rules for organic, pericyclic reactions. There are several excellent reviews that the reader should consult.But it should be pointed out that the orbital symmetry rules have stereochemical implications in terms of the reaction path and products formed. The development of these rules by Woodward and Hoffmann... 

These orbital correlation diagrams subsequently played a very important role in the development of Walsh diagrams and most importantly in the elucidation of the orbital symmetry rules developed by Woodward and Hoffmann which accounted for the stereochemistries of pericyclic reactions of organic molecules [174—184]. 

This is a very powerful rule, and it is especially useful when there are several components to a pericyclic reaction. With several components it is often difficult to identify the appropriate HOMOs and LUMOs for an FMO analysis, and difficult to quickly write an orbital or state correlation diagram. In such cases, aromatic transition state theory, or the generalized orbital symmetry rule, are the easiest approaches for analyzing the reaction. It is your decision as to which works best for you. 

The orbital correlation diagram was introduced by Longuet-Higgins and Abrahamson to predict the allowedness of a pericyclic reaction [6]. In this method, the orbital symmetry properties of both reactants and products are considered. The symmetry elements of the MOs are evaluated and the MOs of reactants and products are arranged in a diagram in two columns. In an allowed pericyclic reaction, the ground-state MO of the reactants and the products has the same element of symmetry. 

In summary, the PMO method when extended to photochemical reactions of the G-type predicts a reversal of aromatic properties from ground state to excited state. This justifies the set of rules for photochemical pericyclic reactions given above. Dougherty s analysis also reveals dangers in the use of orbital and state correlation diagrams for assessing photo-pericyclic reactions because the breakdown in the BO approximation leads to a destruction of symmetry. 

---