we made a reaction between aminorhadanine and isatin in solvent free condition under microwave activation. The single product formed is that resulting from the knoevenagel reaction (addition of aminorhodanine CH2 to the carbonyl of isatin) and not from a Schiff base (addition between NH2 of the aminorhodanine and the carbonyl of isatin). does anyone have an explanation for this regioselectivity?
This experimental fact could be explained as follows.
N-aminorhodanine has two nucleophilic centers, an amino group that is simultaneously basic. Under MW irradiation conditions, it can deprotonate the CH2 group to form a second nucleophilic center. The isatine carbonyl C = O can be attacked by NH2 reversibly, while theinteraction between the carbonyl isatine - carboanion from N-aminorhodanine is irreversible. Thermodynamic stability of the Knoevenagel reaction product, ie 5 - [(Z) -5-chloro-2-oxoindolin-3-ylidene] -3 - {(E) - [(4-hydroxyphenyl) imino] methyl} -2-thioxothiazolidine-4 -one will be higher than the stability of potential Schiff base. Knoevenagel reaction product is therefore a thermodynamically controlled reaction product.
to Pavel Pazdera
thank you for this explanation which seems very plausible. the only concern is that the monitoring of the reaction crude by NMR does not indicate the presence of schiff base. which suggests that the product of knoevenagel is the kinetic and thermodynamic product.
is this conclusion plausible?
Dear friend, yes.
Kinetic and thermodynamic control is about reaction rates and equilibriums, and thus about intermediate concentrations. The sensitivity of NMR measurements is relatively very low, about%. See term "kinetic and thermodynamic control of reaction" (of competitive reactions).
https://www.google.cz/search?client=opera&q=kinetic+and+thermodynamic+control+of+reactions&sourceid=opera&ie=UTF-8&oe=UTF-8
to Pavel Pazdera
thank you dear friend,
According to you, what is the analysis method that you think is best suited to follow this reaction?
We did few experiments with Isatin. We generally monitored the reaction using UV-Vis and TLC.
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