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Author: Subject: Lewis Acid/Base Interactions and Product Stabilization(Solvent Effects)
smaerd
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[*] posted on 10-5-2014 at 19:06
Lewis Acid/Base Interactions and Product Stabilization(Solvent Effects)


I have a weird theoretical organic chemistry inquiry. It arose when I was reading about 1,3-diketones and 1,3-diesters acidity. Specifically with Meldrum's acid and Dimedone's 'unusually' high acidity. I have read that the there appears to be a correlation between the solvents lewis basicity (donicity) and the magnitude of K (Tautomerization coefficient) for six membered cyclic 1,3-dicarbonyls (Christian Reichardt. Solvents and Solvent Effects in Organic Chemistry. Third Edition. Wiley-VCH. Pg 109.)

Now I am familiar with how hydrogen bonding between a product and solvent can afford appreciable stability. A common motif in biochemical mechanisms, sure. However, can lewis acids and lewis bases without reacting, 'coordinate' with one another to a meaningful extent(through bulk interactions) to stabilize a product? Similar to maybe a pi stacking interaction or a charge transfer complex?

Back to the original paragraph I guess. For example. Could it be that the lewis basic solvents interacts favorably with the π electron deficient enone moiety (lewis acid site in dimedone) to further reduce it's dipole/repulsion after tautomerization. Aside from the coulombic repulsion of the carbonyl oxygens, could such an interaction explain why dimedone favors the enol tautomer more than meldrum's acid which contains a cyclic acrylate ester-like moiety (lewis base) in more lewis basic solvents. (picture attached corresponds to the moeities I'm referring too). For reference the donicities of Chloroform (DNb 4), DMSO (DNb 29.8).

Hopefully this isn't worded too awfully. Please ask if there's any confusion for what I am trying to say.

acrylate and enone.png - 21kB

[Edited on 11-5-2014 by smaerd]




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[*] posted on 14-5-2014 at 10:09


Quote:
However, can lewis acids and lewis bases without reacting, 'coordinate' with one another to a meaningful extent(through bulk interactions) to stabilize a product? Similar to maybe a pi stacking interaction or a charge transfer complex?


Yes, absolutely, this is ubiquitous - it's why, for example, NaBH<sub>4</sub> and Zn(BH<sub>4</sub>;)<sub>2</sub> can exhibit such divergent reactivity.




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