Formation of a Carbinolamine Intermediate in the Tertiary Amine Catalyzed Enolization of Oxaloacetic Acid. An Alternative Mechanism for Enolization

Article abstract of DOI:10.1021/ja00353a023

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 pK_α 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.