be performed enantioselectively. Aryl sulfoxides In addition to the oxidation routes, diaryl sulfoxides can be prepared by two Friedel–Crafts arylations of sulfur dioxide using an acid catalyst: 2 ArH + SO2 → Ar2SO + H2O Both aryl sulfinyl chlorides and diaryl sulfoxides can be also prepared from arenes through reaction with thionyl chloride in the presence of Lewis acid catalysts such as BiCl3, Bi(OTf)3, LiClO4, or NaClO4. Reactions Deoxygenation and oxygenation Sulfoxides undergo deoxygenation to give sulfides. Typically metal complexes are used to catalyze the reaction, using hydrosilanes as the stoichiometric reductant. The deoxygenation of dimethylsulfoxide is catalyzed by DMSO reductase, a molybdoenzyme: OSMe2 + 2e- + 2 H+ → SMe2 + H2O Acid-base reactions The α-CH groups of alkyl sulfoxides are susceptible to deprotonation by strong bases, such as sodium hydride: CH3S(O)CH3 + NaH → CH3S(O)CH2Na + H2 In the Pummerer rearrangement, alkyl sulfoxides react with acetic anhydride to give migration of the oxygen from sulfur to the adjacent carbon as an acetate ester. The first step of the reaction sequence involves the sulfoxide oxygen acting as a nucleophile: Elimination reactions Sulfoxide undergo thermal elimination via an Ei mechanism to yield vinyl alkenes and sulfenic acids. CH3S(O)CH2CH2R → CH3SOH + CH2=CHR The acids are powerful antioxidants, but lack long-term stability. Some parent sulfoxides are therefore marketed as antioxidant polymer stabilisers. Structures based on thiodipropionate esters are popular. The reverse reaction is possible. Coordination chemistry Sulfoxides, especially DMSO, form coordination complexes with transition metals. Depending on the hard-soft properties of the metal, the sulfoxide binds through either the sulfur or the oxygen atom. The latter is particularly common. Applications and occurrence DMSO is a widely used solvent. The sulfoxide functional group occurs in several drugs. Notable is esomeprazole, the optically pure form of the proton-pump inhibitor omeprazole. Another commercially important sulfoxides include armodafinil. Methionine sulfoxide forms from the amino acid methionine and its accumulation is associated with aging. The enzyme DMSO reductase catalyzes the interconversion of DMSO and dimethylsulfide. Naturally-occurring chiral sulfoxides include alliin and ajoene. Further reading References Functional groups