EFFECT OF ALPHA-CYCLODEXTRIN ON THE OXIDATION OF ARYL ALKYL SULFIDES BY PERACIDS

Substituent and leaving group effects on the uncatalysed reaction were in good agreement with literature studies. The effect of alpha-cyclod extrin on the kinetics of aryl alkyl sulfide oxidation by peracids was investigated by studying the following reaction series: (a) a range o f 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 alpha-cyclodextrin , the observed second-order rate constant increases to a maximum with increasing cyclodextrin concentration and thereafter non-productive bi nding of the sulfide causes a decline in rate. Weakly binding peracids , such as peracetic acid show only a decline in rate constant with inc reasing cyclodextrin concentration. Linear free energy relationships r eveal that transition state stabilisation by one molecule of cyclodext rin shows a far greater dependence on the stability of the peracid-cyc lodextrin complex than on the stability of the sulfide-cyclodextrin co mplex, indicating that the principle pathway for the cyclodextrin medi ated reaction is that between the peracid-cyclodextrin complex and unc omplexed sulfide. Additionally, a linear free energy relationship comp aring transition state stabilisation for the alpha-cyclodextrin mediat ed oxidation of iodide and methyl 4-nitrophenyl sulfide by peracids in dicates 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 alpha-cyclodextrin was observed for those peracid s which demonstrate significant 2:1 complex formation. Here the princi pal pathway is the reaction of the 2:1 cyclodextrin-peracid complex wi th the unbound sulfide, although the extent of transition state stabil isation by the second cyclodextrin molecule is only about the same as its stabilisation of peracid in the ground state.