In the synthesis of 3,4-dihydropyrimidin-2(1H)- ones, in the last step of the reaction mechanism, why does an imine be produced not an enamine? (see attached file)
Hirbod,
The second nitrogen of the urea/thiourea group condenses with the ketone in an intramolecular fashion to first give the imine. However, acid-catalyzed tautomerization allows the double bond of the enamine to be conjugated with the ester carbonyl, making it the more stable of the two and the dominant form. Hope this helps.
Hirbod,
The second nitrogen of the urea/thiourea group condenses with the ketone in an intramolecular fashion to first give the imine. However, acid-catalyzed tautomerization allows the double bond of the enamine to be conjugated with the ester carbonyl, making it the more stable of the two and the dominant form. Hope this helps.
The enamine form has more resonance structures that can be imagined, so it is more stable. See word document I sent attached here.
If you look at the structure of the end product, it is a enamine which in turn is conjugated with ester carbonyl which is stable .
Look the conjugated adduct that forms the enamine; the electron pair of the -NH group, extend it; in the imine this electrons are already attached to the carbon, and there is no extended conjugation with the ester group. Take a look at this old review about the hundred years of the Biginelli reaction.
Good luck
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