Tertiary Amine-Catalyzed Acyl Group Exchange Reaction of N,O-Diacyl-o-aminophenols. Its Mechanism and Factors Determining the Relative Stability of Acyl Exchanged Isomer Pairs

Article abstract of DOI:10.1246/bcsj.63.3141

Acyl substituent effects on the equilibrium and rate constants for the acyl group exchange reactions of various N,O-diacyl-o-aminophenols have been investigated in solvents with different polarities.It was found that the relative stability of acyl exchanged isomer pairs is determined solely by the inductive effect of acyl groups, provided that the steric hindrance of acyl substituents bonded to amide nitrogen affects the stability to the same extent.The importance of steric hindrance exerted by a bulky acyl group in determining the relative stability was demonstrated by analyzing the correlation between the standard free energy change (ΔG⁰) and pK_a, which were used as the measure of the relative stability of isomer pairs and of the electron-with drawing ability of acyl groups, respectively.On the other hand, the logarithms of catalytic rate constants for the acyl migration reactions were correlated well to the pK_a values.In addition to this finding, a large negative value of activation entropy (ΔS^<*>=-160 J K^-1 mol^-1) and the Broensted coefficient β of 0.65 for the reaction of N,O-(acetyl)-(1-naphthoyl) pair of N,O-diacyl-o-aminophenol provide a definitive evidence for the rate-determining proton transfer from this derivative to amine catalyst in the transition state.