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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.

    Screen Shot 2022-12-29 at 10.05.56 AM.png

    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?

    Screen Shot 2022-12-29 at 10.06.06 AM.png

    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:

    Screen Shot 2022-12-29 at 10.06.18 AM.png

    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

    Screen Shot 2022-12-29 at 10.06.26 AM.png

    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)

    Screen Shot 2022-12-29 at 10.06.35 AM.png

    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.

    Screen Shot 2022-12-29 at 10.06.40 AM.png


    13.3: Diels-Alder reaction is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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