Stability of 1∶1 and 2∶1 α-cyclodextrin–p-nitrophenyl acetate complexes and the effect of α-cyclodextrin on acyl transfer to peroxide anion nucleophiles

Abstract

The presence of a rate maximum rather than simple saturation-type kinetics in a study of the effect of α-cyclodextrin on the hydrolysis of p-nitrophenyl acetate (PNPA) indicates that α-cyclodextrin forms not only 1∶1 but also 2∶1 complexes with PNPA. This is confirmed using a spectrophotometric method to determine binding constants directly for PNPA, giving values of 46 ± 9 and 66 ± 19 dm³ mol^–1 for the first and second binding steps respectively. These results contradict the majority of literature studies of this reaction in which it is assumed that only a 1∶1 complex is formed. Formation of a 1∶1 complex with cyclodextrin increases the reactivity of PNPA towards hydrolysis, as has been widely reported, whereas the addition of a second cyclodextrin molecule to the complex results in the PNPA taking up a less reactive configuration. The effect of α-cyclodextrin on the reaction between PNPA and the anions of hydrogen peroxide, peroxomonosulfate, peracetic acid, perbenzoic acid, 4-methylperbenzoic acid, 4-nitroperbenzoic acid, 4-sulfonatoperbenzoic acid, 3-chloroperbenzoic acid and 4-tert-butylperbenzoic acid is described. Linear free energy studies for transition state stabilisation of the reaction by one molecule of cyclodextrin reveal that the main pathway involves the bound PNPA reacting with free peroxide anions, although for m-chloroperbenzoic acid an alternative pathway may be significant. This is in contrast to the behaviour observed for the α-cyclodextrin-mediated reaction of the molecular acid form of these peroxides with a series of aryl alkyl sulfides in which the main pathway involves nucleophilic attack of the free sulfide on the cyclodextrin–peracid complex. With the exception of the m-chloroperbenzoic acid anion there is no evidence of transition state stabilisation of the title reaction by two molecules of cyclodextrin.