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Author Topic: endo and exo Diels-Alder products  (Read 5156 times)

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mor

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endo and exo Diels-Alder products
« on: May 28, 2007, 11:25:01 PM »

According to the "endo rule", aldehyde substituent on the dienophile directs into the endo Diels-Alder product.

As you can see, when the dienophile is subsituted with OPMB the preference for the "endo product" is 6:1. However, when the dienophile is subsituted with TMS-C(sp) the preference for the "endo product" rises to 20:1. This fact is not reasonable because the endo rule is influenced by steric effect, and the OPMB group is much bulky than TMS-C(sp).
Are there any other effects that can explain the outcome, i.e. the preferable "endo rule"?

thanks!
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Re: endo and exo Diels-Alder products
« Reply #1 on: May 29, 2007, 06:06:56 AM »

The endo rule can also be explained by electronics.  This explanation usually involves "secondary orbital overlap" between the diene and the dienophile.  This effect is disputed, somewhat, but I haven't seen a good alternative explaination.
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