Approaches to Analyze Cycloaddition Reactions

I. Frontier MO & Transition State Aromaticity Approaches to Cycloadditions

• Frontier MO Approach of [4+2] Addition

This method employs the HOMO of the diene and the LUMO of the dienophile to interact with each other to form a cyclic adduct

What occurs is that the electron density from the diene (Ψ2) flows into an empty orbit of the LUMO (Ψ2) in the dienophile. This occurs when the signs of the orbital lobes align and form 2 new sigma bonds. The orbitals will rotate in a conrotatory fashion (same direction). This requirement is called conservation of orbital symmetry, described by R. B. Woodward and Roald Hoffman at Harvard University.

• Frontier MO Approach of [2+2] Addition

This method employs two ethylene groups interacting with each other to form cyclobutane

*This diagram employs the same principles that we used for the [4+2] addition. However, IT IS WRONG because the orbital phases don't align with one another if the HOMO interacts with the LUMO in this scenario. We need to promote an electron from the HOMO to the LUMO in the diene (left side) which will look like so.

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*This diagram IS RIGHT because the phases of the HOMO orbitals of the excited state interacting with the LUMO of the dienophile. Therefore this reaction is only possible under "hv" (photoirradiated) conditions and will perform a disrotatory process.

• Transition State Aromaticity Approach of [4+2] Addition

This method employs the same compounds of the [4+2] in the Frontier MO Approach

However, the transition state aromaticity approach uses the basis set orbitals of each compound. Meaning, however many pi electrons each compound has, the orbitals used to illustrate the transition state will be the first set (every orbital in the same configuration). This does not change the con- or dis- rotatory character if you use the Frontier MO approach. It is just a different method to visualize the transition state.

II. Hückel and Möbius Transition State Classification

Using the knowledge learned in the "Hückel and Möbius Aromaticity" module, we can classify a system as a Hückel or Möbius system based on the number of phase changes that occur amongst the orbitals.

        - Hückel: Even # phase changes

        - Möbius: Odd # phase changes

III. Allowed vs. Forbidden Reactions

Woodward-Hoffman rules dictate which cycloaddition reactions will occur under certain conditions. A pericyclic transition state involving (4n + 2) electrons with Hückel topology or 4n electrons with Möbius topology is aromatic and allowed, while a pericyclic transition state involving 4n-electrons with Hückel topology or (4n + 2)-electrons with Möbius topology is antiaromatic and forbidden. In Layman's terms if the signs of the HOMO (diene) and the LUMO (dienophile) do not align under thermal conditions, photoirradiation is required to promote an electron to force the signs of the electron lobes to overlap and form the bonds required for a cyclic adduct.