6153-53-3Relevant articles and documents
Role of the acidity of the ketone in determining the mechanism of enolization via proton abstraction from ketone, carbinolamine, or imine. Catalysis of the enolization of 2,4-pentanedione and 3-methyl-2,4-pentanedione by oxyanions and by primary, secondary, and tertiary amines
Bruice, Paula Yurkanis
, p. 7361 - 7368 (1990)
Interconversion of the keto and enol tautomers of 2,4-pentanedione and of 3-methyl-2,4-pentanedione was studied in the presence of primary amines, secondary amines, tertiary amines, and oxyanion catalysts. All four classes of bases were found to catalyze keto-enol interconversion by the base-catalyzed proton-abstraction mechanism. This is unlike what was found for keto-enol interconversion of oxaloacetate where only oxyanions were found to follow the base-catalyzed proton-abstraction mechanism; primary, secondary, and tertiary amines followed the nucleophilic addition-elimination mechanism which involves a carbinolamine intermediate. The rate of primary and secondary amine catalyzed enolization of 2,4-pentanedione and 3-methyl-2,4-pentanedione is 103- to 104-fold faster than the rate of imine or enamine formation with these ketones, indicating that C-H bond labilization of the ketones is a very facile process and occurs much faster than imine formation. It is suggested that acidic ketones enolize via the base-catalyzed α-proton abstraction mechanism since removal of a proton is a more facile process than attack on the carbonyl carbon; ketones of medium acidity and non-acidic ketones enolize via the nucleophilic addition-elimination mechanism since nucleophilic attack on the carbonyl carbon is more facile than α-proton abstraction; for ketones of medium acidity, enolization via the carbinolamine intermediate is faster than imine formation while the reverse is true for non-acidic ketones. Imines, therefore, are useful intermediates for α-carbon bond labilization only in the case of non-acidic ketones.
