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Electron withdrawing/donating groups and Diels-Alder

johnwandering

Full Member
10+ Year Member
May 18, 2009
436
14
    I've been looking through this stuff for some time now, and it doesn't make a lick of sense.

    http://imgur.com/CBj0e

    I understand that when X is an electron donating/withdrawing group, there is a definitive partial positive or negative charge at the attacking end of the diene.

    But the dienophile doesn't make any sense.
    1.) having methane on Y should make the molecule have a definitive partial positive charge at the region being attacked (at carbon 4).

    2.) but having amine on Y should make the molecule have LESS of a partial positive charge because an electron pair is being pushed into carbon 4, and hence a full resonance structure is enabled from top to bottom.

    Having established that the partial positive charge on carbon 4 of the dienophile is greater when we have a methane, NOT when we have a lone pair donating substituent, why in the world does having -NHCH3 as Y make it MORE selective for product A?
    If anything, it should be far less selective.
    (I know that N is more "electronegative" than methane, which is donating, but it's clear that the contributions of resonance structures should far far outweigh induction).



    So, what is going on here?
     
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