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I understand that the endo product is, at least in all cases in my textbook, the major product in a Diels-Alder reaction. However, I don't understand why this is the case.

The explanation in my textbook says that the endo product allows for more interaction between the diene and dienophile during the transition state.

I also found the following post, which suggests that secondary orbital interactions are the cause. Stereochemistry of product in Diels-Alder reaction

Then I found the following paper that indicates, at least to my understanding, that steric strain during the transition state is the main reason behind the endo rule (or at least for the example reaction in the article). https://www.ncbi.nlm.nih.gov/pubmed/24449044

So, I'm left wondering what is the reason for the endo rule?

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    $\begingroup$ Your textbook's description is probably a watered-down version of the secondary orbital interactions, which have traditionally been the explanation for the endo rule (see any other org chem textbook for diagrams). Good find on the paper though. $\endgroup$ – orthocresol Dec 10 '16 at 3:49
  • $\begingroup$ related: Endo rule and stereoselectivity in the Diels-Alder reaction $\endgroup$ – ron Dec 10 '16 at 15:28
  • $\begingroup$ Both the explanation of secondary orbital interactions (SOI) and extra configurational strain in exo through transition states (and along reaction coordinates overall) result in endo being favored regularly. Diels-Alder's "endo rule" was born from experiment, and is also not actually a rigid "rule" (see tuning endo/exo selectivity in the two papers, and in the stack exchange related question by @ron), and is only intended as a tool in predicting expected products. Fernandez & Bickelhaupt's work is probably the most thorough description to date, supporting expt & previous theory to some degree. $\endgroup$ – Len_sprague Sep 18 '18 at 4:36
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From what I know, there reason why the endo product is preferentially formed is still not clear and subject to further enlightenment. But it seems to arise from more efficient interactions between the pi electrons of the dienophile and the pi electrons of the developing double bond in the diene during the transition state. In this website you can the both transition states in 3d

http://www.chemtube3d.com/DAendo_vs_exo,cyclopentadiene_and_maleic_anhydride.html

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