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Do aromatic dienes undergo Diels-Alder reactions?

My understanding is that the $\pi$ bonds in aromatic compounds are not as localised as they are in other compounds, such as aliphatic dienes. Therefore, the dienophile shouldn't be as willing to go to these compounds. Am I correct?

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Yes, you are correct. Aromatic compounds are stabilized by their resonance energy. This means that the activation energy for reaction of an aromatic compound will typically be greater than that for a similar reaction of a non-aromatic compound.

That said, aromatics will react as the diene component in a Diels-Alder reaction if

  • the dienophile is extremely reactive (o-benzyne for example)

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  • if the aromatic system is destabilized (for example by steric interactions or structural constraints) so that the resonance stabilization is decreased

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  • or if the aromatic system is large enough that loss of one aromatic ring does not significantly destabilize the entire system. Anthracene is such an example where the middle ring will react via a Diels-Alder reaction.

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(image source)

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  • $\begingroup$ But then why does benzyne undergo the Diels Alder reaction with Furan? Both of them are aromatic! Is it because benzyne is highly unstable? $\endgroup$ – Aritra Das Aug 2 '15 at 17:28
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    $\begingroup$ Yes, o-benzyne is a high energy species, therefore extremely reactive. $\endgroup$ – ron Aug 2 '15 at 17:29
  • $\begingroup$ What is this o-benzyne? Is it any different from benzyne or are they just different names for the same thing? $\endgroup$ – Aritra Das Aug 2 '15 at 17:33
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    $\begingroup$ there is ortho-, meta-, and para-benzyne, depending whether the removed hydrogens are located 1,2, 1,3 or 1,4. $\endgroup$ – ron Aug 2 '15 at 17:35
  • $\begingroup$ Does benzyne count when answering this question? Since the pi system involved in the cycloaddition is orthogonal to the aromatic pi system... $\endgroup$ – PCK Nov 15 at 14:53

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