Acyl transfer mediated by complexation. The effect of cyclodextrins on the reaction of nucleophiles with p-nitrophenyl acetate and hexanoate

Abstract

The kinetics-of the cleavage of p-nitrophenyl acetate (pNPA) and p-nitrophenyl hexanoate (pNPH) by trifluoroethanol (TFE), mercaptoethanol, hydroxylamine or imidazole in the presence of α-cyclodextrin, β-cyclodextrin, or hydroxypropyl-β-cyclodextrin (CDs) have been measured in basic aqueous solution. Detailed studies established that TFE reacts with pNPH bound to the CDs and that the bound ester is almost as reactive as the free ester. Similar behaviour has been observed for pNPA and for reaction with the other nucleophiles, with slight retardations or accelerations. There are only minor differences between the effects of the three cyclodextrins and the four nucleophiles. It seems clear that pNPA and pNPH are bound to the CDs in such a way that their carbonyl groups are exposed to the medium and that transition-state binding differs little from substrate binding, even though the orientation may be different for the two esters.