{"id":11858,"date":"2018-10-01T06:00:51","date_gmt":"2018-10-01T10:00:51","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=11858"},"modified":"2026-05-02T08:53:23","modified_gmt":"2026-05-02T13:53:23","slug":"the-retro-diels-alder-reaction","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2018\/10\/01\/the-retro-diels-alder-reaction\/","title":{"rendered":"The Retro Diels-Alder Reaction"},"content":{"rendered":"

Mechanism and Examples of the Retro Diels-Alder Reaction<\/strong><\/p>\n

In the last post in this series on the Diels-Alder [See: Diels-Alder Reaction: Kinetic and Thermodynamic Control<\/em><\/a>] we saw that if you apply enough heat,\u00a0 the Diels-Alder can operate in both the forward and<\/em> reverse directions!<\/p>\n

\"compressed-summary<\/a><\/p>\n

Table of Contents<\/strong><\/p>\n

    \n
  1. What Is The “Retro” Diels-Alder Reaction<\/a><\/li>\n
  2. Diels-Alder \/ Retro Diels-Alder Reactions With Pyrone<\/a><\/li>\n
  3. The Retro Diels-Alder Of This Molecule Does Not Give Back The Original Reactants!<\/a><\/li>\n
  4. The Pyrone Retro Diels-Alder Is Particularly Favorable Because It Generates An Aromatic Ring And Liberates CO2<\/a><\/li>\n
  5. Summary: The Retro Diels-Alder Reaction<\/a><\/li>\n
  6. Notes<\/a><\/li>\n
  7. Quiz Yourself!<\/a><\/li>\n
  8. (Advanced) References and Further Reading<\/a><\/li>\n<\/ol>\n
    \n

    <\/a>1. What Is The “Retro” Diels-Alder Reaction?<\/strong><\/h2>\n

    The “retro<\/em>-Diels Alder” reaction is the exact reverse of the Diels-Alder and passes through the same transition state.\u00a0\u00a0(We sometimes say it’s the “microscopic” reverse – if you were to play a film of the Diels-Alder reaction occurring, and then reverse the film, it would be indistinguishable from the\u00a0retro-Diels Alder reaction).\u00a0<\/em><\/span><\/p>\n

    In the most common case, where the reverse Diels-Alder is exactly the same as the forward Diels-Alder, all that really happens is that an equilibrium<\/strong> is established between the diene\/dienophile and the Diels-Alder product.<\/p>\n

    \"drawing<\/a><\/p>\n

    However, there\u00a0are<\/strong> cases where the Diels-Alder product can undergo more than one<\/strong> possible retro<\/em>-Diels Alder reaction, and this can result in the formation of products that differ from the “original” diene and dienophile.<\/p>\n

    This situation doesn’t come up very often in introductory organic chemistry courses, but when it does, it’s usually when you’re sitting in an exam hall. What makes the retro<\/em>-Diels Alder reaction exam-question catnip for instructors is\u00a0that it asks you to apply concepts you already know, but in the reverse direction<\/em>.<\/p>\n

    Here, we’ll go through a classic example of a\u00a0retro<\/em>-Diels Alder reaction that shows how the reaction can be used to make useful new<\/strong> products, instead of just reverting to the starting diene and dienophile. (And if that isn’t enough, there’s a bonus, second example of a retro-Diels Alder in the supplemental section ).\u00a0<\/em><\/span><\/p>\n

    <\/a>2. Diels-Alder \/ Retro-Diels-Alder Reactions With Pyrone<\/strong><\/h2>\n

    Pyrone (specifically 2-Pyrone<\/a>)\u00a0 is a cyclic molecule that resembles benzene but is not actually aromatic. Locked in the s-cis<\/em> conformation, pyrone will participate in the Diels-Alder reaction with dienophiles. For reasons that will soon be clear, a useful category of dienophile is electron-poor alkynes such as dimethylacetylene dicarboxylate (DMAD), which results in the following Diels-Alder reaction:<\/p>\n

    \"pyrone<\/p>\n

    The “top view” (above) shows the new six-membered ring as outlined from the (arbitrary<\/em>) numbering scheme.\u00a0It’s also possible to draw the same molecule from the side, which shows this bridged bicyclic molecule in perspective [See: Putting Diels-Alder Products in Perspective<\/a>\u00a0<\/em>]<\/span><\/p>\n

    <\/a>3. The Retro Diels-Alder Of This Molecule Does Not Give Back The Original Reactants!<\/h2>\n

    Now comes the fun part.<\/p>\n

    When this Diels-Alder product is heated, a\u00a0retro<\/em>-Diels Alder reaction occurs, but the final product isn’t 2-pyrone and the alkyne dienophile!<\/p>\n

    Instead, the products are a new\u00a0aromatic<\/a> ring and carbon dioxide (CO2<\/sub>)!<\/p>\n

    \"retro<\/p>\n

    Why doesn’t it fragment to give back the pyrone and alkyne?<\/p>\n

    Well, it’s possible that it does, to some extent (more on that in a second). But the main point is that more than one\u00a0retro-Diels-Alder pathway is possible here!<\/strong><\/p>\n

    The second six-membered ring that can undergo retro<\/em>-Diels Alder fragmentation is highlighted in red below:<\/p>\n

    \"pyrone<\/p>\n

    <\/a>4. The Pyrone Retro Diels-Alder Reaction Is Favorable Because It Generates An Aromatic Ring And Liberates CO2<\/h2>\n

    Note that this\u00a0retro<\/em>-Diels Alder involves the same pattern of bond formation and bond breaking as in the\u00a0retro<\/em>-Diels Alder reaction at the top of the article:<\/p>\n