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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: combatwombat on March 04, 2009, 05:31:09 PM

Title: Diels-Alder adducts - why exo?
Post by: combatwombat on March 04, 2009, 05:31:09 PM

I can see how a compound like 1,3-cyclobutadiene would react with maleic anhydride at room temperature to preferentially form the endo product. However I'm not sure why furan and maleic anhydride preferentially form the exo product - is it because of repulsion between oxygen atoms?

Title: Re: Diels-Alder adducts - why exo?
Post by: Vidya on March 07, 2009, 10:14:39 AM

..preference is based on variety of steric and electronic influences on the transition state of the cycloaddition....

Title: Re: Diels-Alder adducts - why exo?
Post by: CopperSmurf on March 07, 2009, 04:39:33 PM

I've asked my professor this before. It has to do with the geometry of the reagents. It's best to make models of them. for example, when 1,3-cyclobutadiene reacts with maleic anhydride you will only get endo product , absolutely no exo product. The compounds can't form bonds to make the exo product, it will have to bend in a different way.

Also, exo vs. endo products also have to do with your reagents being cis or trans. When 1,3-cyclobutadiene reacts with maleic anhydride, one of them is always cis, which is also why you get only endo and not exo. It's much easier to "see it" in 3-D.