Symmetry properties of ethylene, butadiene, and cyclohexene orbitals with respect to cycloaddition

Electron flow from the diene to the olefin is consistent with the long-recognized experimental fact that electron-withdrawing groups on the olefin greatly enhance its capacity to act as a dienophile. For the reaction of various dienophiles with cyclopentadiene, the relative rates are  

Cyclopentadiene (dimerization) Ethyl acrylate Maleic anhydride T etr acy anoethylene 

Electron-releasing substituents increase the reactivity of the diene. For the reaction of various dienes with maleic anhydride, the relative rates are  

The selection rules for cycloaddition reactions can also be derived from considering the basis set orbitals from which the transition state for the cycloadditions would arise (Fig. 10.11). For [4+2]-suprafacial addition, the transition state is aromatic for [2+2]-suprafacial addition, it is antiaromatic. On the other hand, a [2+2]-addition that is antarafadal in one component is an allowed process. 

The generalized Woodward-Hoffmann rules for concerted cycloaddition can be summarized as follows  

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Fig. 11.9. Symmetry properties of ethylene, butadiene, and cyclohexene orbitals with respect to cycloaddition.

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