Multiple pathways in the α-cyclodextrin catalysed reaction of iodide and substituted perbenzoic acids

Abstract

The kinetic rate equation for the title reaction in aqueous acetate buffer has both first-order and second-order terms with respect to cyclodextrin concentration, due to catalysis both by one and by two molecules of cyclodextrin. The stabilisation of the transition state of the iodide–peracid reaction by cyclodextrin is examined using the pseudoequilibrium constant approach of Tee, Carbohydr. Res., 1989, 192, 181. This approach indicates that, depending on the nature of the peracid, the predominant pathway catalysed by one cyclodextrin molecule involves the reaction of either free iodide and a cyclodextrin–peracid complex or free peracid and a cyclodextrin–iodide complex. The latter two pathways are kinetically indistinguishable, but the corresponding terms in the rate equation are separated using the extrakinetic assumption of a Brönsted-type relationship. This assumption is reasonable since the uncatalysed reaction and that catalysed by two molecules of cyclodextrin show Brönsted-type relationships. The mechanism of catalysis is discussed in terms of the effect of cyclodextrin on the nucleophilicity of the iodide and acid catalysis via the protonation of the benzoate leaving group.