Chapter 10: Conjugation in Alkadienes and Allylic Systems

Molecular Orbital analyzis of Diels-Alder reaction

• Review Diels-Alder reaction ?
• Electron donating groups (e.g. -R, -OR) on diene promote reaction. Conclusion diene = nucleophile
• Electron withdrawing groups (e.g. -CO₂R, -CN) on dienophile promote reaction. Conclusion dienophile = electrophile
• Nucleophile characterised by the HOMO
• Electrophile characterised by the LUMO

Notice how the phases of the orbitals at C1 and C4 of the HOMO of butadiene match the phases of the orbitals of C1 and C2 in the LUMO of ethene (i.e. the colours match). This means there is a favourable bonding interaction at each of these positions, which corresponds to the positions where the two new s bonds are formed. Therefore the reaction is said to be a "symmetry allowed" - this is good news, since we know the reaction works !

A more complete analyzis using molecular orbital methods (beyond our scope) can also explain:

1. The preference for the endo product
2. Substituent effects on the rate of reaction
3. Substituent effects on the regiochemistry of the reaction
4. Catalysis by Lewis acids