{"id":11489,"date":"2018-05-11T07:00:33","date_gmt":"2018-05-11T11:00:33","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=11489"},"modified":"2026-04-18T06:40:44","modified_gmt":"2026-04-18T11:40:44","slug":"endo-vs-exo-in-the-diels-alder-reaction","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2018\/05\/11\/endo-vs-exo-in-the-diels-alder-reaction\/","title":{"rendered":"Why Are Endo vs Exo Products Favored in the Diels-Alder Reaction?"},"content":{"rendered":"

[Advanced] Secondary Orbital Interactions – A Rationale For Why\u00a0Endo\u00a0<\/em>Products Are Favored In The Diels-Alder Reaction<\/strong><\/p>\n

In our last post, we noted that\u00a0endo\u00a0<\/em>products tend to be favored over\u00a0exo<\/em> products in the Diels-Alder reaction<\/a>.\u00a0 [We also introduced a quick-and-dirty rule for telling the difference between\u00a0endo-<\/em> and\u00a0exo-<\/em> products in the Diels-Alder.]<\/p>\n

So\u00a0why<\/strong> are\u00a0endo\u00a0<\/em>products generally favored over\u00a0exo\u00a0<\/em>in the Diels-Alder reaction? That’s what we’re going to learn about today…<\/p>\n

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

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

    \n
  1. Endo<\/em> Products Tend To Be Favored In The Diels-Alder Even Though They Are More Sterically Hindered<\/a><\/li>\n
  2. The Endo\u00a0<\/em>And\u00a0Exo <\/em>Transition States For The Diels-Alder Reaction Between Cyclopentadiene And Maleic Anhydride<\/a><\/li>\n
  3. Another Pair Of Endo<\/em> And Exo<\/em> Diels-Alder Transition States Shows That The\u00a0Endo\u00a0<\/em>Transition State Places The EWG On The Dienophile Over The Diene<\/a><\/li>\n
  4. Secondary Orbital Interactions Are Possible In The Endo Transition State (But Not The Exo)<\/a><\/li>\n
  5. Kinetic and Thermodynamic Control of the 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. Endo Products Tend To Be Favored In The Diels-Alder Even Though They Are More Sterically Hindered<\/h2>\n

    The preference for\u00a0endo<\/em> versus exo<\/em> is especially\u00a0curious since the\u00a0endo\u00a0<\/em>products appear to be more sterically hindered.<\/p>\n

    For example, here’s the Diels-Alder reaction of cyclopentadiene with maleic anhydride. The ratio of\u00a0endo<\/em> to\u00a0exo<\/em> products in this reaction is about four to one:<\/p>\n

    \"the<\/p>\n

    Note that in the\u00a0endo <\/em>product above,\u00a0the anhydride is on the underside of the new six-membered ring, whereas in the\u00a0exo<\/em>, it points away. This is indeed less sterically hindered.<\/p>\n

    \n

    [To jump ahead, here’s a fact we’ll cover in more detail in the next post<\/a>: most exo products are<\/strong> in fact more stable than the\u00a0endo\u00a0products for steric reasons, but the\u00a0endo\u00a0product tends to be formed faster. Furthermore, given enough heat,\u00a0 the Diels-Alder product can revert back to starting materials .\u00a0 Long story short: the Diels-Alder is another example of a reaction that can be run under kinetic or thermodynamic control,<\/a> where the<\/em>\u00a0“endo” is the kinetic product and the “exo” is the thermodynamic product. ]<\/em><\/span><\/p>\n

    \n

    <\/a>2. The Endo\u00a0<\/em>And\u00a0Exo <\/em>Transition States For The Diels-Alder Reaction Between Cyclopentadiene And Maleic Anhydride<\/h2>\n

    So why is the\u00a0endo <\/em>typically\u00a0favored?<\/p>\n

    Here’s one attempt at an explanation, using “secondary orbital effects” [Note 1<\/a>]. Another related explanation is in the footnotes. [Note 2<\/a>].<\/p>\n

    Our last post showed the mechanism of the Diels-Alder reaction<\/a> through the orbital interactions of the highest occupied molecular orbital (HOMO) of the diene and the lowest unoccupied molecular orbital (LUMO) of the dienophile.<\/p>\n

    When the orbital interactions are drawn out in the two different transition states, we see something like this.<\/p>\n

    \"transition<\/p>\n

    Note that in the\u00a0endo\u00a0<\/em>transition state the electron-withdrawing groups point toward the two carbons that will eventually comprise the C2<\/sub>-C3<\/sub> alkene, whereas in the\u00a0exo<\/em> transition state, the electron-withdrawing groups point away<\/em> from the C2<\/sub>-C3<\/sub> alkene:<\/p>\n

    <\/a>3. Another Pair Of Endo<\/em> And Exo<\/em> Diels-Alder Transition States Shows That The\u00a0Endo\u00a0<\/em>Transition State Places The EWG On The Dienophile Over The Diene<\/h2>\n

    Here’s another example with (E,E)-<\/em>2,4-hexadiene and acrolein. Again, note how the\u00a0end<\/em>o\u00a0<\/em>transition state has the electron withdrawing group positioned over the diene:<\/p>\n

    \"drawing<\/p>\n