Diels-Alder reactions photochemically forbidden

Construct a MO correlation diagram similar to Figure 11.66 (page 737) to show that the photochemical Diels-Alder reaction is forbidden by the principles of orbital symmetry. 

In a photochemical cycloaddition, one component is electronically excited as a consequence of the promotion of one electron from the HOMO to the LUMO. The HOMO -LUMO of the component in the excited state interact with the HOMO-LUMO orbitals of the other component in the ground state. These interactions are bonding in [2+2] cycloadditions, giving an intermediate called exciplex, but are antibonding at one end in the [,i4j + 2j] Diels-Alder reaction (Scheme 1.17) therefore this type of cycloaddition cannot be concerted and any stereospecificity can be lost. According to the Woodward-Hoffmann rules [65], a concerted Diels-Alder reaction is thermally allowed but photochemically forbidden. 

Our initial studies focused on the transition metal-catalyzed [4+4] cycloaddition reactions of bis-dienes. These reactions are thermally forbidden, but occur photochemically in some specific, constrained systems. While the transition metal-catalyzed intermole-cular [4+4] cycloaddition of simple dienes is industrially important [7], this process generally does not work well with more complex substituted dienes and had not been explored intramolecularly. In the first studies on the intramolecular metal-catalyzed [4+4] cycloaddition, the reaction was found to proceed with high regio-, stereo-, and facial selectivity. The synthesis of (+)-asteriscanoHde (12) (Scheme 13.4a) [8] is illustrative of the utihty and step economy of this reaction. Recognition of the broader utiHty of adding dienes across rc-systems (not just across other dienes) led to further studies on the use of transition metal catalysts to facilitate otherwise difficult Diels-Alder reactions [9]. For example, the attempted thermal cycloaddition of diene-yne 15 leads only... 

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. 

It is possible to show very generally that for two olefins, having w, and m2 n electrons, coming together to form a cyclic olefin with (m] + m2 - 4)/2 n bonds, as shown below, the reaction will be thermally allowed when ms + m2 = 4n + 2 (e.g., 4 + 2 = 6 in the case of the Diels-Alder reaction). On the contrary, when m, + m2 = 4n (e.g., 2 4- 2 = 4 for the ethylene dimerization) the reaction is thermally forbidden but photochemically allowed. For a discussion of this generalization the article of Woodward and Hoffmann should be consulted. 

Show that the [4 + 2] Diels-Alder reaction is photochemically forbidden. 

What about the photochemical Diels-Alder reaction The observation that this reaction is most uncommon leads us to the immediate suspicion that there is something wrong with it. Usually, the absorption of a photon will promote an electron from the HOMO to the LUMO. In this case, the lower energy HOMO-LUMO gap is that in the diene partner. Absorption of light creates a new photochemical HOMO for the diene, 3, and now the HOMO-LUMO interaction with the dienophile partner involves one antibonding overlap. Both new bonds cannot be formed at the same time (Rg. 20.22). So this photochemical Diels-Alder reaction is said to be forbidden by orbital symmetry. ... 

Diels-Alder reaction involves /i, j/2, V3 and ]/4 orbitals of 1, 3-butadiene andrt and71 orbitals of ethylene as reactants m.os andCj, CT2,7t, 7i, a3,04 orbitals of cyclohexene which are product molecular orbitals. When these molecular orbitals are arranged in the increasing order of their energies alongwith their symmetries, ground state molecular orbitals of reactants correlate with the ground state molecular orbitals of their product, therefore, reaction is thermally allowed but photochemically forbidden on account of the fact that first excited state of reactant does not correlates with first excited state of product (Fig. 5.6). 

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