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When I was reading about Diels-Alder reactions, I was confused about the cis and trans rules for dienophiles, as shown below.

enter image description here

The text told me that "a cis dienophile will generate a ring with cis substitution, and trans dienophiles will generate a ring with trans substitution."

I'm a bit confused.

Why is this the case?

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To answer you question, imagine (or: build a molecular model) that the newly formed six-membered ring may be simplified as a flat disk. Then

  • either you start with a (trans)-configurated diene, as in the first example drawn by you. Since the Diels-Alder simultaneously forms the new $\sigma$-bonds on the expense of $\pi$-bonds and moves the location of the remaining double bond, the substitutents R are either both below, or both above this reference plane. Relatively to each other, these substitutents R in the product conserved their (cis)-relationship from the dienophile.

  • or, if your dienophile is (trans)-configurated, yielding a product were one of the substitutents is above, and the other is below this reference plane; corresponding to the second example drawn by you where are R conserved their (trans)-relationship to each other.

On paper, the observed conservation of sterochemical information is both the synchronicity of the events (pushing the pairs of electrons, absence of [charged] intermediates as e.g. in nucleophilic substitutions), as well as the principle of conserving the symmetry of molecular orbitals envolved here.

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You are right, this process and many others that involve 3 pairs of electrons are concertated reacctions, and this happens as before having the final bonds filled, there exist a combination of sigmas and pi wave functions so that the other bonds that have been previous have no time to take other positions. After profesor Eliel, and also my own experience doing this in 500 l reactor using methylpentadiene as diene and acrolein or crotonic aldehyde and having huge GLC showing the different products.

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