Effect of α-cyclodextrin on the oxidation of aryl alkyl sulfides by peracids

Abstract

Substituent and leaving group effects on the uncatalysed reaction were in good agreement with literature studies. The effect of α-cyclodextrin on the kinetics of aryl alkyl sulfide oxidation by peracids was investigated by studying the following reaction series: (a) a range of aryl alkyl sulfides with three different perbenzoic acids and (b) a range of alkyl peracids and perbenzoic acids with five different aryl alkyl sulfides. For peracids which bind strongly to α-cyclodextrin, the observed second-order rate constant increases to a maximum with increasing cyclodextrin concentration and thereafter non-productive binding of the sulfide causes a decline in rate. Weakly binding peracids, such as peracetic acid show only a decline in rate constant with increasing cyclodextrin concentration. Linear free energy relationships reveal that transition state stabilisation by one molecule of cyclodextrin shows a far greater dependence on the stability of the peracid-cyclodextrin complex than on the stability of the sulfide-cyclodextrin complex, indicating that the principle pathway for the cyclodextrin mediated reaction is that between the peracidcyclodextrin complex and uncomplexed sulfide. Additionally, a linear free energy relationship comparing transition state stabilisation for the α-cyclodextrin mediated oxidation of iodide and methyl 4-nitrophenyl sulfide by peracids indicates a common mechanism of catalysis for both substrates, although the catalysis of sulfide oxidation is more effective. Several possible mechanisms of catalysis are discussed. Transition state stabilisation by two molecules of α-cyclodextrin was observed for those peracids which demonstrate significant 2:1 complex formation. Here the principal pathway is the reaction of the 2:1 cyclodextrin–peracid complex with the unbound sulfide, although the extent of transition state stabilisation by the second cyclodextrin molecule is only about the same as its stabilisation of peracid in the ground state.