(101b) Combining Chemical and Enzymatic Catalysis for the Enantioselective Oxidation of Sulfides

Chloroperoxidase (CPO) from Caldariomyces fumago is a versatile and highly selective catalyst for the sulfoxidation of thioethers preferably using hydrogen peroxide as oxidizing agent.¹ However, inactivation of CPO in the presence of high concentration of H₂O₂ is an inherent problem. Various strategies have been advocated viz. alternative oxidants than H₂O₂^2,3, fed-batch addition⁴, electrochemical generation⁵, and immobilization of enzyme⁶. We report the synthesis of optically pure sulfoxide combining the direct generation of hydrogen peroxide from hydrogen and oxygen⁷ with enzymatic catalysis using CPO (Scheme-1). This reaction is carried out using the biphasic media H₂O/supercritical carbon dioxide. Thus, the otherwise hazardous stoichiometric combination of hydrogen and oxygen for the formation of hydrogen peroxide and the enzymatic conversion can be carried out conveniently in one pot. The results of our investigation towards this cascade reaction will be presented. Scheme-1 1. Van Rantwijk F.; Sheldon R A. Selective oxygen transfer catalysed by heme peroxidases: synthetic and mechanistic aspects. Curr. Opin. Biotechnol. 2000, 11, 554-564. 2. Piero P.; Giacomo C.; Enrico M.; Nicoletta G.; Stefano C. Chloroperoxidase-catalyzed enantioselective oxidation of methyl phenyl sulfide with dihydroxyfumaric acid/oxygen or ascorbic acid/oxygen as oxidants, Biotechnol. Bioeng. 1999, 62 , 489-493, 3. Van de Velde F.; Van Rantwijk F.; Sheldon R. A. Selective oxidations with molecular oxygen, catalyzed by chloroperoxidase in the presence of a reductant. J. Mol. Cat. B: Enzymatic, 1999, 6, 453-461. 4. Van Deurzen M. P. J.; Seelbach K.; Van Rantwijk F.; Kargl, U.; Sheldon R.A. Chloroperoxidase: use of hydrogen peroxide-stat for controlling reactions and improving enzyme performance. Biocatal. Biotransform. 1997. 15, 1-16. 5. Kohlmann C.; Lütz S. Electroenzymatic synthesis of chiral sulfoxides. Eng. Life Sci., 2006, 6, 170?174, 6. Trevisan V.; Signoretto M.; Colonna S.; Pironti V.; Strukul G. Microencapsulated chloroperoxidase as a recyclable catalyst for the enantioselective oxidation of sulfides with hydrogen peroxide, Angew. Chem. 2004, 116, 4189-4191 7. Beckman E. J. Production of H₂O₂ in CO₂ and its use in the direct synthesis of propylene oxide,Green Chem., 2003, 5, 332-336,