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

The presence of a rate maximum rather than simple saturation-type kinetics in a study of the effect of alpha-cyclodextrin on the hydrolysis of p-nitro phenyl acetate (PNPA) indicates that alpha-cyclodextrin forms not only 1 : 1 but also 2:1 complexes with PNPA. This is confirmed using a spectrophotom etric method to determine binding constants directly for PNPA, giving value s of 46 +/- 9 and 66 +/- 19 dm(3) mol(-1) for the first and second binding steps respectively. These results contradict the majority of literature stu dies of this reaction in which it is assumed that only a 1 : 1 complex is f ormed. Formation of a 1 : 1 complex with cyclodextrin increases the reactiv ity of PNPA towards hydrolysis, as has been widely reported, whereas the ad dition of a second cyclodextrin molecule to the complex results in the PNPA taking up a less reactive configuration. The effect of alpha-cyclodextrin on the reaction between PNPA and the anions of hydrogen peroxide, peroxomon osulfate, peracetic acid, perbenzoic acid, 4-methylperbenzoic acid, 4-nitro perbenzoic acid, 4-sulfonatoperbenzoic acid, 3-chloroperbenzoic acid and 4- tert-butylperbenzoic acid is described. Linear free energy studies for tran sition state stabilisation of the reaction by one molecule of cyclodextrin reveal that the main pathway involves the bound PNPA reacting with free per oxide anions, although for m-chloroperbenzoic acid an alternative pathway m ay be significant. This is in contrast to the behaviour observed for the al pha-cyclodextrin-mediated reaction of the molecular acid form of these pero xides with a series of aryl alkyl sulfides in which the main pathway involv es nucleophilic attack of the free sulfide on the cyclodextrin-peracid comp lex. With the exception of the m-chloroperbenzoic acid anion there is no ev idence of transition state stabilisation of the title reaction by two molec ules of cyclodextrin.