Keto-Enol Equilibrium of Methyl Fluorene-9-thionocarboxylate and Acidity Constants of the Enol and the Keto Forms of This and Related Oxygen and Sulfur Esters in Aqueous Solution

Determination of the pKaof Ethyl Acetate: Brønsted Correlation for Deprotonation of a Simple Oxygen Ester in Aqueous Solution

“…The only other acidity constants available for analogous oxygen and thiol esters are those for the 9-acylfluorenyl derivatives ( 4-X ) recently reported by Kresge et al . (Table ) ), which shows that the 9-fluorenyl nucleus provides considerable resonance stabilization of the enolate. ,, The data in Table show that the effects of alkylthio and alkoxy substituents at the carbonyl group on the carbon acid acidities of 4-X are much smaller than those for simple acyl derivatives, so that these substituent effects are strongly attenuated by the 9-fluorenyl group.…”

“…Kresge and co-workers have pioneered the generation of enols in larger than equilibrium amounts, as products of the decay of reactive intermediates generated by laser flash photolysis. − These techniques provided the carbon acid acidities of mandelic acid ( 2 , p K a K = 22) , and α-cyano-α-phenylacetic acid ( 3 , p K a K = 8.22) in aqueous solution. The recently reported acidity constants of 9-acylfluorenyl derivatives, 4-X , determined using conventional stopped-flow techniques, provides a comparison of the thermodynamic barrier to formation of the enolates of related oxygen (X = OMe, p K a K = 11.5) and thiol (X = SMe, p K a K = 10.5) esters in aqueous solution. However, these enolates are stabilized by the strongly acidifying 9-fluorenyl group ( cf .…”

Flash Photolysis of 10-Diazo-9(10H)-phenanthrenone in Aqueous Solution. Hydration of Fluorenylideneketene and the Fluorene-9-carboxylic Acid Keto−Enol System

“…37 This, coupled with the expectation that acidities of carboxylic acid and ester enols ionizing as oxygen acids might be similar as well, based upon the close similarity of Q a E for mandelic acid and methyl mandelate mentioned above, suggests, by virtue of the relationship pQ a K ) pK E + pQ a E , that enol contents of carboxylic acids and their esters might be similar also. Carboxylic acids could therefore serve as useful surrogates for esters in studies of enol chemistry; this would be especially advantageous in the case of investigations conducted in aqueous solution, where hydrolysis of the ester is often a troublesome competing reaction.…”

“…It is interesting that the acidity of fluorene-9-carboxylic acid ionizing as a carbon acid, eq 18, R ) H, pQ a K ) 11.67, is practically the same as that of its methyl ester, eq 18,R ) Me, pQ a K ) 11.52. 37 This, coupled with the expectation that acidities of carboxylic acid and ester enols ionizing as oxygen acids might be similar as well, based upon the close similarity of Q a E for mandelic acid and methyl mandelate mentioned above, suggests, by virtue of the relationship pQ a K…”

Enolization and Hydrolysis of 7-Nitroisochroman-3-one in Aqueous Solution: Generation of a Relatively Stable Lactone Enolate

“…Direct examination of the enolization of carboxylate esters in aqueous solution is possible, but complicated by two factors: the inability to generate appreciable concentrations of the ester enolates (enol(ate)s of simple esters are highly unstable with respect to their keto tautomers) and the susceptibility of esters to hydroxide ion-promoted hydrolysis. To date, studies of the enolization of carboxylate esters in aqueous solution have typically utilized enolates stabilized by “secondary” groups (i.e., β-ketoesters and fluorene-9-carboxylate esters) …”