The cleavage of 1- and 2-naphthyl acetates by cyclodextrins in basic aqueous solution

Abstract

The reactions of 1- and 2-naphthyl acetate (1-NA and 2-NA) with four cyclodextrins (CDs): α-CD, β-CD, γ-CD, and ‘hydroxypropyl-β-CD’(Hp-β-CD), in basic aqueous solution, all show saturation kinetics. Even though the strength of substrate binding varies appreciably, the limiting accelerations at high [CD] are all relatively modest (3 to 30-fold). The effects on the two isomeric esters are generally similar but there are significant differences between the CDs. These differences are discussed in terms of the relative importance of transition-state and initial-state binding, and in relation to structural variations in the four cyclodextrins. Except for one case, the second-order rate constants (k₂) for the reactions of 1-NA and 2-NA with the CDs are appreciably greater than those for reaction with trifluoroethanol (TFE), as expected for stabilization of the transition state by inclusion of part of the ester in the CD cavity. For 2-NA reacting with a-CD, k₂ is virtually the same as that for reaction with TFE, suggesting that inclusion is not a significant factor in this particular case. Consistent with this suggestion, the cleavage of 2-NA by α-CD is not competitively inhibited by alcohols, it is mediated by them. The cleavage of 1-NA by β-CD can also be mediated by alcohols and by alkanesulfonate ions, but the analogous reaction of 2-NA is not as susceptible.