6153-53-3Relevant academic research and scientific papers
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.
Formation of a Carbinolamine Intermediate in the Tertiary Amine Catalyzed Enolization of Oxaloacetic Acid. An Alternative Mechanism for Enolization
Bruice, Paula Yurkanis
, p. 4982 - 4996 (1983)
The keto.enol and keto-hydrate equilibria of oxaloacetic acid have been investigated in aqueous solution in the presence of lyate species and in the presence of both oxygen base actalysts and tertiary amine catalysts.Lyate species and oxygen based catalyze the interconversion of keto-enol tautomers by the well accelerated acid and base catalyzed mechanisms of eq 1 and 2; Bronsted β=0.32, α=-0.43. the second order rate constants for catalysis by ternary amines are from 800 to 2200 (depending on the pKα of the ternary amine) times greater than oxygen base second-order rate constants, a rate acceleration too great to be attributed to the enhanced ability of ternary amines in proton removal.The ternary amine buffer dilution plots exhibit a nonlinear dependence of rate on amine concentration at low ternary amine concentrations.The intercepts at zero amine concentration of the linear portions of the ternary amine buffer dilution plots are more than 100-fold greater than can be attributed to lyate species catalyzed keto-enol interconversion and more than 10-fold greater than can be attributed to lyate species catalyzed keto-hydrate interconversion, suggesting that in the ternary amine catalyzed enolization reaction lyate species is operating not on keto, enol, or hydrate, but some reactive intermediate.In order to account for these observations, the ternary amine catalyzed interconversion of keto-enol teutomers is proposed to occur through the mechanism of eq 3, which involves the formation of a carbinolamine intermediate followed be amine catalyzed elimination of a proton and ternary amine.The β value for the reaction of ternary amines with oxaloacetic acid is 0.24.Severely sterically hindered ternary amines do not follow the nucleophilic addition-elimination mechanism of eq 3 but, like oxygen bases, catalyze enolization via the general base catalyzed mechanism of eq 2.Enolate protonation rates were also determined; Bronsted α=-0.59 for oxygen acids and -0.78 for protonated ternary amines.
