An Orbital Approach to Predicting Regiochemistry

Regiochemical issues that arise in the Diels-Alder reaction when substituted dienes and dienophiles are involved. 

Analysis of olefin substituent effects relevant to the Diels-Alder reaction. A. The familiar mixing diagram for the formation of the tt and ir orbitals of ethylene, which serves as a reference for the rest of the figure. 

Mixing diagram to develop the molecular orbitals of an olefin substituted with a donor substituent (D). 

Once again these perturbations of coefficients and energies of the ethylene orbitals can be anticipated using resonance. Eq. 15.8 shows the expectations for substitution of an electron wi thd rawing group on an ethylene. The 3-carbon is electrophilic. Therefore, the LUMO should he polarized to the unsubstituted carbon as seen in orbital (n). The reverse polarization is predicted for the HOMO, and this is seen in orbital (m). 

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