Sulfonium

Short description: Cation of the form [SR3]+

Structure of (CH3)3S+. The C-S-C angles are 102° and C-S bond distance is 177 picometers.[1]

In organic chemistry, a sulfonium ion, also known as sulphonium ion or sulfanium ion, is a positively-charged ion (a "cation") featuring three organic substituents attached to sulfur. These organosulfur compounds have the formula [SR
3]+
. Together with a negatively-charged counterion, they give sulfonium salts. They are typically colorless solids that are soluble in organic solvent.[2][3]

## Contents

* 1 Synthesis
* 2 Inversion
* 3 Applications and occurrence
* 3.1 Biochemistry
* 3.2 Organic synthesis
* 4 See also
* 5 References
* 6 External links

## Synthesis

Sulfonium compounds are usually synthesized by the reaction of thioethers with alkyl halides. For example, the reaction of dimethyl sulfide with iodomethane yields trimethylsulfonium iodide:

CH3–S–CH3 \+ CH3–I → (CH3)3S+I−

The reaction proceeds by a nucleophilic substitution mechanism (SN2). Iodide is the leaving group departs. The rate of methylation is faster with more electrophilic methylating agents, such as methyl trifluoromethanesulfonate.

Dimethylsulfoniopropionate (DMSP), is found in marine phytoplankton and seaweeds.[4]

## Inversion

Sulfonium ions with three different substituents are chiral owing to their pyramidal structure. Unlike the isoelectronic oxonium ions (R3O+), chiral sulfonium ions are resolvable into optically stable enantiomers.[5] [Me(Et)SCH2CO2H]+ is the first chiral sulfonium cation to be resolved into enantiomers.[6] The barrier to inversion ranges from 100 to 130 kJ/mol.[3]

## Applications and occurrence

### Biochemistry

The sulfonium (more specifically methioninium) species S-adenosylmethionine occurs widely in nature, where it is used as a source of the adenosoyl or methyl radicals. These radicals participate in the biosynthesis of many compounds.[7][8]

Structure of S-adenosylmethionine.

Other naturally-occurring sulfonium species are S-methylmethionine (methioninium) and the related dimethylsulfoniopropionate (DMSP).

### Organic synthesis

Sulfonium salts are precursor to sulfur ylides, which are useful in carbon–carbon bond-forming reactions. In a typical application, a R2S+CH2R′ center is deprotonated to give the ylide R2S+CHR−.[9]

Structure of tris(dimethylamino)sulfonium difluorotrimethylsilicate.

Tris(dimethylamino)sulfonium difluorotrimethylsilicate [((CH3)2N)3S]+[F2Si(CH3)3]− is a popular fluoridation agent.[10]

Some azo dyes are modified with sulfonium groups to give them a positive charge. The compound triphenylsulfonium triflate is a photoacid, a compound that under light converts to an acid.

## See also

* Onium compounds

## References

1. ↑ Knop, Osvald; Cameron, T. Stanley; Bakshi, Pradip K.; Linden, Antony; Roe, Stephen P. (1994). "Crystal Chemistry of Tetraradial Species. Part 5. Interaction Between Cation Lone Pairs and Phenyl Groups in Tetraphenylborates: Crystal Structures of Me3S+, Et3S+, Me3SO+, Ph2I+, and 1-Azoniapropellane Tetraphenylborates". Canadian Journal of Chemistry 72 (8): 1870–1881. doi:10.1139/v94-238.
2. ↑ C. J. M. Stirling, ed (1981). The Sulphonium Group: Part 1, Volume 1. PATAI'S Chemistry of Functional Groups. John Wiley & Sons. doi:10.1002/9780470771648. ISBN 9780470771648. C. J. M. Stirling, ed (1981). The Sulphonium Group: Part 2, Volume 2. PATAI'S Chemistry of Functional Groups. John Wiley & Sons. doi:10.1002/9780470771655. ISBN 9780470771655.
3. ↑ 3.0 3.1 Kozhushkov, Sergei I.; Alcarazo, Manuel (2020). "Synthetic Applications of Sulfonium Salts". European Journal of Inorganic Chemistry 2020 (26): 2486–2500. doi:10.1002/ejic.202000249. PMID 32742188.
4. ↑ DeBose, Jennifer L.; Sean C. Lema; Gabrielle A. Nevitt (2008-03-07). "Dimethylsulfoniopropionate as a foraging cue for reef fishes". Science 319 (5868): 1356. doi:10.1126/science.1151109. PMID 18323445. Bibcode: 2008Sci...319.1356D. https://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1339&context=bio_fac.
5. ↑ March, J. “Advanced Organic Chemistry” 5th Ed. J. Wiley and Sons, 1992: New York. ISBN:0-471-60180-2
6. ↑ Barbachyn, Michael R.; Johnson, Carl R. (1984). "Optical Activation and Utilization of Compounds Containing Chiral Sulfur Centers". Asymmetric Synthesis. pp. 227–261. doi:10.1016/B978-0-12-507704-0.50007-6. ISBN 9780125077040.
7. ↑ Layer, G.; Heinz, D. W.; Jahn, D.; Schubert, W.-D. "Structure and function of radical SAM enzymes" Current Opinion in Chemical Biology 2004, volume 8, 468-476. doi:10.1016/j.cbpa.2004.08.001
8. ↑ Perry A. Frey, Olafur Th. Magnusson "S-Adenosylmethionine: A Wolf in Sheep's Clothing, or a Rich Man's Adenosylcobalamin?" Chem. Rev., 2003, 103 (6), pp 2129–2148. doi:10.1021/cr020422m
9. ↑ Mitchell J. Bogdanowicz, Barry M. Trost (1974). "Cyclopropylphenylsulfonium Tetrafluoroborate". Organic Syntheses 54: 27. doi:10.15227/orgsyn.054.0027.
10. ↑ W. J. Middleton (1986). "Tris(dimethylamino)sulfonium difluorotrimethylsilicate". Organic Syntheses 64: 221. doi:10.15227/orgsyn.064.0221.

## External links

* IUPAC definition (short pdf)

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