The sulphoxidation of thioanisole catalysed by lactoperoxidase and Coprinus cinereus peroxidase: Evidence for an oxygen‐rebound mechanism

during turnover and monitoring the presence of native enzymes, compounds I,II and III, led to formation of the sulphoxide in high yield and enantioselectivity. Under those conditions,LPO catalysed the formation of (R) methyl phenyl sulphoxide with a yield of 85% and an enantiomericexcess (e.e.) of 80%. CiP catalysed the formation of (S) methyl phenyl sulphoxide with a yield of 84%and an e.e. of 73%. The enantioselective performance was markedly influenced by the purity of theenzymes used. Presence of compound III during turnover led to rapid inactivation of the peroxidases and,therefore, to both a lower yield of the sulphoxides and a lower enantioselectivity. Stopped-flow kineticdata show that, for both LPO and CiP, the transition of compound I to compound II depends on theconcentration of the methyl phenyl sulphide, suggesting an oxygen-rebound mechanism. In line with thismechanism, a methyl phenyl sulphide radical cation was detected by EPR during turnover for LPO.Keywords: Coprinus cinereus peroxidase; lactoperoxidase; enantioselective sulphoxidation; oxygen-re-bound mechanism.Catalytic oxidative transformations using enzymes to synthe-sise optically active intermediates are of increasing importance;the major reason being the high regio- or enantioselectivity ofprocesses catalysed by enzymes, improving the chemical yieldand decreasing waste stream and enantiomeric ballast.Peroxidases are potentially attractive biocatalysts to meet theindustrial/ecological preferences, and numerous peroxidaseshave been shown to catalyse the enantioselective oxidation ofa variety of alkyl aryl sulphides and dialkyl sulphides [123].Enantiomerically pure sulphoxides are useful chiral auxiliariesin organic synthesis. In addition to the enantioselective sulphoxi-dations, epoxidations [4], benzylic hydroxylations [5] and indoleoxidation [6] have also been reported to be catalysed by theseenzymes. Impediments to the practical application in organicsynthesis include the moderate stability of the peroxidases inturnover due to oxidative inactivation of the haem and the lim-ited solubility of organic reactants in water. The catalytic perfor-mance of peroxidases in sulphoxidation reactions has been im-proved by continuous controlled addition of H