Orbital symmetry diagrams/rules cycloadditions

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. 

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... 

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. 

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