Cycloadditions state correlation diagram

The photochemical dimerization of unsaturated hydrocarbons such as olefins and aromatics, cycloaddition reactions including the addition of 02 ( A ) to form endoperoxides and photochemical Diels-Alders reaction can be rationalized by the Woodward-Hoffman Rule. The rule is based on the principle that the symmetry of the reactants must be conserved in the products. From the analysis of the orbital and state symmetries of the initial and final state, a state correlation diagram can be set up which immediately helps to make predictions regarding the feasibility of the reaction. If a reaction is not allowed by the rule for the conservation of symmetry, it may not occur even if thermodynamically allowed. 

Figure 11.26 State correlation diagram for the allowed iris + tt4s cycloaddition. Refer to Figure 11.16 for the orbital labeling and orbital correlation.

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 state correlation diagram for a [2+2] cycloaddition showing the symmetry-imposed barrier E... 

From the orbital correlation diagram derived by Bryce-Smith [38], it was deduced that the ortho cycloaddition is forbidden from the lowest excited singlet state of benzene and the ground state of ethene. Van der Hart et al. [189] have constructed molecular orbital and state correlation diagrams for the ortho photocycloaddition of benzene to ethene. The molecular orbital correlation diagram differs from that given by Bryce-Smith, because natural correlations have been used. From a topological point of view, it seems less desirable to correlate the tt... 

Figure 8 State correlation diagrams for the ortho cycloaddition of ethylene to benzene.

In contrast, the state correlation diagram for the forbidden cycloaddition (Fig. 6.20) is not so simple. The ground state of the starting materials on the left, %2%2, is overall symmetric, because both terms are squared. Following the... 

Epiotis criticized state correlation diagrams because they neglect electronic repulsion and molecular orbital interactions, which may lead to chemical intermediates and energy barriers [39]. These diagrams are also inappropriate to deal with bicentric reactions, like cycloadditions to olefins, which may be initiated by the carbonyl group a or n electrons [40]. 

State correlation diagram for suprafacial-suprafacial [2 -f 2] cycloaddition. 

From a more detailed analysis of MO and state correlation diagrams. Woodward and Hoffmann presented a set of selection rules for cycloaddition reactions, which are summarized in Table 11.1. ° Here p and q are the number of electrons in the two n systems imdergoing the cycloaddition reaction. When the sum of p and g is a member of the 4n series, then the reaction is thermally allowed to be suprafacial with respect to one of the n components and antarafacial with respect to the other one. When the sum of p and qisa member of the 4n -h 2 series, then Ihe reaction is thermally allowed when it is either suprafacial with respect to both components or antarafacial with respect to both. As usual, the selection rules are reversed for photochemical reactions. 

We begin our analysis of state symmetry by examining the [2-1-2] cycloaddition of two ethylenes. To start a state correlation diagram, we must know how to assign symmetries to... 

The state correlation diagram for the [2+2] cycloaddition of Eq. 15.1. The intended crossing of states witli like symmetry is shown with dotted lines, while the avoided crossing is shown as a solid colored line. 

It can be seen that state correlations provide, in a sense, a justification for the simpler orbital symmetry analysis. Clearly, the origin of the avoided crossing seen in the state correlation of the [2+2] cycloaddition can be traced back to the original orbital correlation diagram. It will generally be true that the predictions from an orbital correlation will carry over to the state correlation diagram. Thus, it is rare that practicing chemists construct a state correla-... 

An oft-cited dichotomy is that if a reaction is thermally forbidden, it is photochemically allowed and vice versa. In fact, photochemical [2 + 2] cycloadditions are well known (see Chapter 16), and other examples of thermally forbidden processes that proceed photochemically can be found. A justification for this binary aspect of pericyclic reactions can be gleaned from the orbital or state correlation diagrams. Figure 15.4 shows a direct correlation between the first excited states of reactants, produced by photolysis, and products for the... 

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. 

State correlation diagram for a [2+2] cycloaddition. There is a substantial barrier on the ground state energy surface, but the first excited state surface approaches the ground state surface, and a funnel forms that allows the excited state to exit to the ground state, facilitating the reaction. 

How do orbital symmetry requirements relate to [4tc - - 2tc] and other cycloaddition reactions Let us constmct a correlation diagram for the addition of butadiene and ethylene to give cyclohexene. For concerted addition to occur, the diene must adopt an s-cis conformation. Because the electrons that are involved are the n electrons in both the diene and dienophile, it is expected that the reaction must occur via a face-to-face rather than edge-to-edge orientation. When this orientation of the reacting complex and transition state is adopted, it can be seen that a plane of symmetry perpendicular to the planes of the... 

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. 

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