Inorganic chemistry, asulfoxide, also called asulphoxide, is anorganosulfur compound containing asulfinyl (>SO)functional group attached to twocarbon atoms. It is apolar functional group. Sulfoxides are oxidizedderivatives ofsulfides. Examples of important sulfoxides arealliin, a precursor to the compound that gives freshly crushed garlic its aroma, anddimethyl sulfoxide (DMSO), a commonsolvent.[1]
Sulfoxides feature relatively short S–O distances. In DMSO, the S–O distance is 1.531 Å. The sulfur center is pyramidal; the sum of the angles at sulfur is about 306°.[3]Sulfoxides are generally represented with the structural formula R−S(=O)−R', where R and R' are organic groups. The bond between thesulfur andoxygen atoms is intermediate of adative bond and a polarizeddouble bond.[4] The double-bond resonance form implies 10 electrons around sulfur (10-S-3 inN-X-L notation). The double-bond character of the S−O bond may be accounted for by donation of electron density into C−S antibonding orbitals ("no-bond" resonance forms in valence-bond language). Nevertheless, due to its simplicity and lack of ambiguity, the IUPAC recommends use of the expanded octet double-bond structure to depict sulfoxides, rather than the dipolar structure or structures that invoke "no-bond" resonance contributors.[5] The S–O interaction has anelectrostatic aspect, resulting in significantdipolar character, with negative charge centered on oxygen.
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Alone pair of electrons resides on the sulfur atom, giving it tetrahedral electron-pair geometry andtrigonal pyramidal shape (steric number 4 with one lone pair; seeVSEPR theory). When the two organic residues are dissimilar, the sulfur atom is achiral center, for example, inmethyl phenyl sulfoxide. Theenergy barrier required to invert thisstereocenter is sufficiently high that sulfoxides are optically stable near room temperature. That is, the rate ofracemization is slow at room temperature. The enthalpy of activation for racemization is in the range 35 to 42 kcal/mol and the corresponding entropy of activation is −8 to +4 cal/(mol·K). The barriers are lower for allylic and benzylic substituents.[6]
Sulfoxides are typically prepared byoxidation ofsulfides, sometimes referred to assulfoxidation.[7]hydrogen peroxide is a typical oxidant, but periodate has also been used.[8] In these oxidations, care is required to avoid over oxidation to form thesulfone. For example,dimethyl sulfide is oxidized todimethyl sulfoxide and then further todimethyl sulfone. Unsymmetrical sulfides areprochiral, thus their oxidation gives chiral sulfoxides. This process can be performed enantioselectively, through e.g., a variant of theShi epoxidation[9] orbiotransformation.[10]
Symmetrical sulfoxides can be formed from adiorganylzinc compound and liquidsulfur dioxide.[11]
In addition to the oxidation routes, diaryl sulfoxides can be prepared by twoFriedel–Crafts arylations ofsulfur dioxide using an acid catalyst:
Both aryl sulfinyl chlorides and diaryl sulfoxides can be also prepared from arenes through reaction withthionyl chloride in the presence of Lewis acid catalysts such as BiCl3, Bi(OTf)3, LiClO4, or NaClO4.[12][13]
Sulfoxides undergodeoxygenation to give sulfides. Typically metal complexes are used to catalyze the reaction, using hydrosilanes as the stoichiometric reductant.[14] The deoxygenation of dimethylsulfoxide is catalyzed byDMSO reductase, a molybdoenzyme:[15]
The α-CH groups of alkyl sulfoxides are susceptible to deprotonation by strong bases, such assodium hydride:[16]
In thePummerer rearrangement,alkyl sulfoxides react withacetic anhydride to give migration of the oxygen from sulfur to the adjacent carbon as anacetate ester. The first step of the reaction sequence involves the sulfoxide oxygen acting as anucleophile:
Sulfoxide undergo thermal elimination via anEi mechanism to yield vinylalkenes andsulfenic acids.[17][18]
The acids are powerfulantioxidants, but lack long-term stability.[19] Some parent sulfoxides are therefore marketed as antioxidantpolymer stabilisers.[20] Structures based on thiodipropionate esters are popular.[21] The reverse reaction is possible.
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Sulfoxides, especially DMSO, formcoordination complexes with transition metals. Depending on thehard-soft properties of the metal, the sulfoxide binds through either the sulfur or the oxygen atom. The latter is particularly common.[22]
DMSO is a widely used solvent.
The sulfoxide functional group occurs in several drugs. Notable isesomeprazole, the optically pure form of the proton-pump inhibitoromeprazole. Another commercially important sulfoxides includearmodafinil.
Methionine sulfoxide forms from the amino acidmethionine and its accumulation is associated with aging. The enzymeDMSO reductase catalyzes the interconversion of DMSO and dimethylsulfide.
Naturally occurring chiral sulfoxides includealliin andajoene.
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