This paper presents a thermodynamic study of the rate and equilibria processes involved in the a-cyclodextrin mediated reaction of a range of 4-substituted phenyl methyl sulfides with two peroxyacids of different binding affinities. The results for the inclusion processes show that the formation of 1:1 and 2:1 (host:guest) complexes between a-cyclodextrin and phenyl methyl sulfides are generally enthalpically controlled, particularly so for the 2:1 complexes, as might be expected for a ternary complex. The data from this series of sulfides is presented as enthalpy-entropy compensation plots, yielding slopes of unity for each inclusion process. The formation of a 1:1 complex between cyclodextrin and the strongly associating 3-chloroperbenzoic acid (MCPBA) is also enthapically controlled. The other peroxyacid used, peroxomonosulfate, does not bind to a-cyclodextrin to any measurable degree. As described in our original study of this reaction system (Davies and Deary in J Chem Soc Perkin Trans 2:2423–2430, 1996), catalysis by a-cyclodextrin is effected by activation of the peroxide as a result its inclusion within the cyclodextrin cavity; hence for reactions of phenyl methyl sulfides with MCPBA, catalysis is observed, but is absent for PMS. In this study the reaction rates are analysed using the transition state pseudo equilibrium approach of Tee (Carbohydr Res 192:181–195, 1989), whereby the transition state pseudoequilibrium constant KTS reflects the stabilisation imparted to the transition state by the association with one molecule of cyclodextrin. Enthalpy- entropy compensation plots for KTS give slopes close to unity; this is the first reported example of such plots being applied to transition state pseudoequilibrium constants.
|Journal of Inclusion Phenomena and Macrocyclic Chemistry
|Published - 24 Nov 2011