Reactions of carbonyl compounds in basic solutions. Part VI. The mechanism of the alkaline hydrolysis of methyl 8-acetyl-, 8-propionyl-, and 8-isobutyryl-1-naphthoate. Neighbouring-group participation by carbon acids

Abstract

Rate coefficients have been measured for the alkaline hydrolysis of methyl 8-acetyl-, 8-propionyl-, and 8-isobutyryl-1-naphthoate, as well as the 8-[²H₃]acetyl ester, in 70%(v/v) dioxan–water at several temperatures. The immediate products of these reactions are the corresponding phenalene-1,3-diones and the rate-determining step for this cyclisation was found to be the base-catalysed ionisation process. Thus the kinetic isotope effect, k^H/k^D, observed for the 8-acetyl ester was ca. 5 to 6. All the diones eventually hydrolyse to give the anions of the corresponding 8-acyl-1-naphthoic acids. Whereas 2,3-dihydrophenalene-1,3-dione and 2-methyl-2,3-dihydrophenalene-1,3-dione ionise in base and are relatively stable in this anionic form, 2,2-dimethyl-2,3-dihydrophenalene-1,3-dione hydrolyses relatively rapidly. The rates of ring fission for the latter dione have been measured in 70%(v/v) dioxan–water at several temperatures. The entropies and enthalpies of activation for the cyclisation and fission processes are evaluated. Solvent (aqueous dimethyl sulphoxide) and solvent isotope effects have been studied for the 8-acetyl ester. The hydrolysis rates of this and related esters are correlated with an acidity function. The evidence indicates a mechanism of ester hydrolysis with neighbouring-group participation by carbon acid for these three 8-acyl esters. The implications of the results are discussed.