Amino Substituents as a Probe of Reactions of Phenyl Acetates With Cyclodextrins

Abstract

The effects of α- and β- cyclodextrin and 6^A-amino-6^A-deoxy-α- and -β- cyclodextrin on the rates of reactions of m- and p-nitrophenyl acetate, in borate buffer at pH 10.0 and 298.2 K, show that the amino substituents of the modified cyclodextrins have only a modest influence on the dissociation constants of the complexes formed with each ester and the first-order rate constants for the reactions of complexed m-nitrophenyl acetate. By contrast, the amino substituents significantly increase the rate constants for the reactions of complexes of p- nitrophenyl acetate. These results indicate that one mode of inclusion of p-nitrophenyl acetate in α- and β- cyclodextrin has the ester oxycarbonyl group in the vicinity of the cyclodextrin primary hydroxy groups. While this mode of inclusion in the unmodified cyclodextrins does not lead to reaction and has not been detected previously for that reason, in analogous complexes of the amino-substituted cyclodextrins, the nucleophilic amino group is proximate to the carboxy group of the ester and reaction proceeds. There is no kinetic evidence for this mode of inclusion of m-nitrophenyl acetate. This interpretation of the kinetic effects is consistent with the extent of acetamide formation in reactions of the esters with the amino-substituted cyclodextrins.