Correlation diagrams symmetry-forbidden

Fig. 11.4. Correlation diagram for cyclobutene and butadiene orbitals (symmetry-forbidden disrotatory reaction).

Correlation diagrams can be constructed in an analogous fashion for the disrotatory and conrotatory modes for interconversion of hexatriene and cyclohexadiene. They lead to the prediction that the disrotatory mode is an allowed process whereas the conrotatory reaction is forbidden. This is in agreement with the experimental results on this reaction. Other electrocyclizations can be analyzed by the same method. Substituted derivatives of polyenes obey the orbital symmetry rules, even in cases in which the substitution pattern does not correspond in symmetiy to the orbital system. It is the symmetry of the participating orbitals, not of the molecule as a whole, that is crucial to the analysis. 

When the orbitals have been classified with respect to symmetry, they can be arranged according to energy and the correlation lines can be drawn as in Fig. 11.10. From the orbital correlation diagram, it can be concluded that the thermal concerted cycloadditon reaction between butadiene and ethylene is allowed. All bonding levels of the reactants correlate with product ground-state orbitals. Extension of orbital correlation analysis to cycloaddition reactions involving other numbers of n electrons leads to the conclusion that the suprafacial-suprafacial addition is allowed for systems with 4n + 2 n electrons but forbidden for systems with 4n 7t electrons. 

The complementary relationship between thermal and photochemical reactions can be illustrated by considering some of the same reaction types discussed in Chapter 11 and applying orbital symmetry considerations to the photochemical mode of reaction. The case of [2ti + 2ti] cycloaddition of two alkenes can serve as an example. This reaction was classified as a forbidden thermal reaction (Section 11.3) The correlation diagram for cycloaddition of two ethylene molecules (Fig. 13.2) shows that the ground-state molecules would lead to an excited state of cyclobutane and that the cycloaddition would therefore involve a prohibitive thermal activation energy. 

Let us turn now to a reaction surface that has been studied in more detail, that is, the surface for the addition of methylene to ethylene (11). Figure 5 shows the various approaches of the two fragments, b) is the most symmetric approach, but the correlation diagram shows that the reaction is symmetry-forbidden for the ground configuration singlet methylene along this path. In Fig. 5 c the levels have been classified as symmetric or antisymmetric with respect to the xz plane, which is the relevant symmetry element for use of the symmetry conservation rales. 

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

We have emphasized that the Diels-Alder reaction generally takes place rapidly and conveniently. In sharp contrast, the apparently similar dimerization of olefins to cyclobutanes (5-49) gives very poor results in most cases, except when photochemically induced. Fukui, Woodward, and Hoffmann have shown that these contrasting results can be explained by the principle of conservation of orbital symmetry,895 which predicts that certain reactions are allowed and others forbidden. The orbital-symmetry rules (also called the Woodward-Hoffmann rules) apply only to concerted reactions, e.g., mechanism a, and are based on the principle that reactions take place in such a way as to maintain maximum bonding throughout the course of the reaction. There are several ways of applying the orbital-symmetry principle to cycloaddition reactions, three of which are used more frequently than others.896 Of these three we will discuss two the frontier-orbital method and the Mobius-Huckel method. The third, called the correlation diagram method,897 is less convenient to apply than the other two. 

Construction of an orbital correlation diagram (7) discloses that concerted cis elimination of R-R from ci -ML4RR is not forbidden by symmetry considerations thus, there appears to be no advantage for the asymmetric elimination... 

The situation is reversed for the tt2s + n4s addition. Figure 11.16 illustrates this case now the bonding orbitals all transform directly to bonding orbitals of the product and there is no symmetry-imposed barrier. As with the electrocyclic processes, the correlation diagrams illustrate clearly the reason for the striking difference observed experimentally when the number of electrons is increased from four to six. The reader may verify that the 4s + 4s reaction will be forbidden. Each change of the total number of electrons by two reverses the selection rule. 

Figure 11.27 State correlation diagram for the forbidden n2s + n2s cycloaddition. The orbital designations are those defined in Figure 11.15. State symmetry designations refer to the mirror planes a and a of Figure 11.15.

The dimerization of acyclic polyenes in which all n bonds are lost would lead to the open structures of (54) and (55). A schematic orbital correlation diagram (Fig. 15) for process (54) shows that allyl dimerization is improbable. The cyclization of higher acyclic polyenes, e.g. to cis-or trans-7 in (55), is subject toa similar prohibition, but the formation of 8 is allowed. In general, processes in which the products retain elements of symmetry inherent in the reactants are symmetry-forbidden the argument used to demonstrate this is analogous to that used for ethylene. One dimerization of 1,3-butadiene, namely to 9, is unique this... 

Inspection of this correlation diagram immediately reveals that there is a problem. One of the bonding orbitals at the left correlates with an antibonding orbital on the product side. Consequently, if orbital symmetry is to be conserved, two ground state ethylene molecules cannot combine via face-to-face approach to give a ground-state cyclobutane, or vice versa. This concerted reaction is symmetry forbidden. ... 

Correlation diagrams have given us a convincing sense of where the barriers come from for those reactions that we have been calling forbidden. In principle, of course, no reaction is forbidden—what these reactions have is a formidable symmetry-imposed barrier, and something very unusual is needed if barriers of this magnitude are to be crossed. 

The original explanation of Woodward and Hoffmann involved construction of an orbital correlation diagram for the reaction under consideration, and then carrying out the reaction in such a manner so that the symmetries of the reactant and product orbitals matched exactly. If the correlation diagram indicates that the reaction may occur without encountering a symmetry-imposed barrier, it is termed symmetry-allowed. If a symmetry is present, the reaction is designated symmetry-forbidden. 

The correlation diagrams for the conrotatory process show that there is a good correlation between the bonding orbitals v 1 and v 2 of butadiene and ct- and TT-orbitals of cyclobutene (Fig. 8.40). Thus, the ring opening of cyclobutene to butadiene or the reverse reaction is thermally allowed and occurs by the conrotatory process. The reaction proceeds with conservation of orbital symmetry. The photochemical conrotatory process in this case will be symmetry forbidden. 

The concepts discussed regarding the symmetry restrictions and their removal can be described in a number of ways. Complete correlation diagrams can be constructed and the forbiddenness illustrated by sharp orbital crossings 20). Although definitive, this approach would not as clearly illustrate the nature of the restraints to reaction. 

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