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

Article abstract of DOI:10.1021/ja00176a041

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.