Woodward-Hoffman rules, Diels-Alder reactions

The thermal Diels-Alder reactions of anthracene with electron-poor olefinic acceptors such as tetracyanoethylene, maleic anhydride, maleimides, etc. have been studied extensively. It is noteworthy that these reactions are often accelerated in the presence of light. Since photoinduced [4 + 2] cycloadditions are symmetry-forbidden according to the Woodward-Hoffman rules, an electron-transfer mechanism has been suggested to reconcile experiment and theory.212 For example, photocycloaddition of anthracene to maleic anhydride and various maleimides occurs in high yield (> 90%) under conditions in which the thermal reaction is completely suppressed (equation 75). 

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. 

We shall now leave six-electron cyclodadditions such as the Diels-Alder and ene reactions and move on to some four-electron cycloadditions. Clearly, four is not a (4 -i- 2) number, but when we described the Woodward-Hoffman rules on p. 892 we used the term thermally. All... 

In the all carbon and hydrogen instance, the Alder-Ene reaction is considered to be concerted and is thermally allowed based on Woodward-Hoffman rules. Thus, the Alder-Ene reaction is proposed to be a six-electron process, like the Diels-Alder reaction, having transition states endo and exo) analogous to the Diels-Alder reaction. However, the Alder-Ene reaction is easily modulated by steric effects as secondary electronic stabilizing effects have yet to be clearly identified. For example, Berson reported c/5-2-butene reacted with maleic anhydride to provide about a 4 1 ratio of endo. exo adducts 5 6, while trans-l-hvAeae provided little selectivity at 43 57 ratio of 5 6. In the reaction of maleic anhydride with tra 5-2-butene, the e.xo-TS encounters a steric interaction that the endo-TS does not. Steric effects are... 

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