Sulfoxide

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Sulfoxide group

In

sulfides. Examples of important sulfoxides are alliin, a precursor to the compound that gives freshly crushed garlic its aroma, and dimethyl sulfoxide (DMSO), a common solvent.[1]

Structure and bonding

Structure of DMSO (red = O, yellow = S) as determined by X-ray crystallography of PdBr2(bipy)·DMSO.[2]

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 the

dipolar
character, with negative charge centered on oxygen.

Chirality

Enantiomers of methyl phenyl sulfoxide.

A

trigonal pyramidal shape (steric number 4 with one lone pair; see VSEPR theory). When the two organic residues are dissimilar, the sulfur atom is a chiral center, for example, in methyl phenyl sulfoxide. The energy barrier required to invert this stereocenter is sufficiently high that sulfoxides are optically stable near room temperature. That is, the rate of racemization is slow at room temperature. The enthalpy of activation for racemization is in the range 35 - 42 kcal/mol and the corresponding entropy of activation is -8 - +4 cal/mol-K. The barriers are lower for allylic and benzylic substituents.[6]

Preparation

Sulfoxides are typically prepared by

prochiral, thus their oxidation gives chiral sulfoxides. This process can be performed enantioselectively.[9][10]

Symmetrical sulfoxides can be formed from a diorganylzinc compound and liquid sulfur dioxide.[11]

Aryl sulfoxides

In addition to the oxidation routes, di

aryl 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.[12][13]

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.[14] The deoxygenation of dimethylsulfoxide is catalyzed by DMSO reductase, a molybdoenzyme:[15]

OSMe2 + 2 e− + 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:[16]

CH3S(O)CH3 + NaH → CH3S(O)CH2Na + H2

In the

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.[17][18]

CH3S(O)CH2CH2R → CH3SOH + CH2=CHR

The acids are powerful

polymer stabilisers.[20] Structures based on thiodipropionate esters are popular.[21]
The reverse reaction is possible.

Coordination chemistry

Main article: Transition metal sulfoxide complex
cis-RuCl2(dmso)4, a representative metal complex of a sulfoxide. Three DMSO ligands are S-bonded to Ru, one is O-bonded.

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.[22]

Applications and occurrence

racemic
version.

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

  • Gama Á, Flores-López LZ, Aguirre G, Parra-Hake M, Hellberg LH, Somanathan R (2003). "Oxidation of sulfides to chiral sulfoxides using Schiff base-vanadium (IV) complexes". Arkivoc. 2003 (11): 4–15.
    hdl:2027/spo.5550190.0004.b02
    .

References

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  8. doi:10.15227/orgsyn.046.0078
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  10. doi:10.1021/cr00085a002
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  11. LCCN 52-12057
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  13. S2CID 96348273
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  14. doi:10.1016/j.ccr.2014.09.008
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  16. ^ Iwai I, Ide J (1988). "2,3-Diphenyl-1,3-Butadiene". Organic Syntheses; Collected Volumes, vol. 6, p. 531.
  17. doi:10.15227/orgsyn.070.0029.{{cite journal}}: CS1 maint: multiple names: authors list (link
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  18. PMID 11749600
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  19. doi:10.1002/recl.19720911102
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  20. ISBN 978-0-12-803581-8
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  21. doi:10.1016/0014-3057(75)90141-X
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  22. doi:10.1016/j.ccr.2004.02.005
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