13.3: Diels-Alder reaction

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Dienes participate in a [4+2] cycloaddition reaction to generate cyclic alkenes. It is an incredibly useful way of making six-membered rings. Remember that for cycloadditions, there is a concerted movement of electrons in the transition state such that bonds are being formed and broken simultaneously.

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Key features of the Diels-Alder reaction:

1. concerted cycloaddition

2. 6\(π\) electron transition state, which is especially stable (aromatic)

3. reversible in principle

4. thermal (\(Δ\)) or photochemical (h\(ν\))

5. stereospecific

6. thermodynamically favored (exchange 2 \(π\) bonds for 2 \(σ\) bonds), but entropically unfavorable.

The reacting partners for the Diels-Alder reaction are:

diene (4\(π\) electrons) – in order for a reaction to occur, it must be able to adopt the s-cis conformation

dienophile (2\(π\) electrons) – literally, “diene-loving,” a simple alkene

Here are several examples of Diels-Alder reactions – can you draw a mechanism?

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Looking at the reactions above, you may be able to draw a mechanism but are still curious why the products have the observed regioselectivity and stereoselectivity. So, let’s take a look at the molecular orbital description for the simplest Diels-Alder reaction, between 1,3-butadiene and ethylene:

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In order for a reaction to occur, we need orbital overlap that reinforces bonding interactions. In other words, they need to be “in phase.” We call this control by orbital symmetry and we say that the following processes are “symmetry allowed.”

1. Normal electron demand Diels-Alder 2. Inverse electron demand Diels-Alder

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While these two processes are allowed theoretically, the energy gap between HOMO and LUMO is sometimes prohibitively larger and will require harsh conditions (high pressures and temperatures). So, how do we improve this energy difference?

Normal electron demand Diels-Alder – add electron density to the diene (raise HOMO) and remove electron density from the dienophile (lower LUMO).

- add electron-donating groups to the HOMO: -OMe, -NR₂, -R (things with lone pairs or ability to \(σ\)-donate, which push electron density into the \(π\) system, raising HOMO)

- add electron-withdrawing groups to the LUMO: -NO₂, -CN, -CF₃, -C(O)X (anything electronegative or in conjugation, which pulls electron density out of the \(π\) system, lowering LUMO)

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Inverse electron demand Diels-Alder – add electron density to the dienophile (raise HOMO) and remove electron density from the diene (lower LUMO). The types of electron-donating and electron-withdrawing groups are the same as above.

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