Sulfonium

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Sulfonium ion

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 [SR3]+. Together with a negatively charged counterion, they give sulfonium salts. They are typically colorless solids that are soluble in organic solvent.

Synthesis[edit]

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:

CH
3
–S–CH
3
+ CH
3
–I
(CH
3
)
3
S+
+ I

Before the above reaction, the sulfur atom has two lone electron pairs. One of these lone pairs links to the methyl group. At the same time, as part of a concerted nucleophilic substitution mechanism (SN2), the iodide leaving group departs. This leaves a positively charged trimethylsulfonium ion, whose charge is balanced by the iodide. The rate of reaction is even faster with stronger methylating agents, such as methyl trifluoromethanesulfonate.

Structure[edit]

The compounds are pyramidal at sulfur. Thus, Me3S+ is isostructural and isoelectronic to trimethylphosphine. Sulfonium compounds wherein the three substituents differ are chiral and optically stable.[1]

Applications and occurrence[edit]

Biochemistry[edit]

The sulfonium (more specifically methioninium) species S-adenosylmethionine occurs widely in nature, where it is used as a source of the adenosoyl radical. This radical participates in the biosynthesis of various compounds.[2][3]

Structure of S-adenosylmethionine.

Another, sulfonium (methioninium) species found in nature is S-methylmethionine.

Organic synthesis[edit]

Sulfonium salts are precursor to sulfur ylides, which are useful in C-C forming reactions. In a typical application, a R2S+CH2R′ center is deprotonated to give the ylide R2S+CHR.[4]

Tris(dimethylamino)sulfonium difluorotrimethylsilicate [((CH3)2N)3S]+[F2Si(CH3)3] is a popular fluoridation agent.[5]

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[edit]

References[edit]

  1. ^ March, J. “Advanced Organic Chemistry” 5th Ed. J. Wiley and Sons, 1992: New York. ISBN 0-471-60180-2.
  2. ^ 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
  3. ^ 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
  4. ^ Mitchell J. Bogdanowicz, Barry M. Trost (1988). "Cyclopropylphenylsulfonium Tetrafluoroborate". Org. Synth. ; Coll. Vol., 6, p. 364 
  5. ^ W. J. Middleton (1990). "Tris(dimethylamino)sulfonium difluorotrimethylsilicate". Org. Synth. ; Coll. Vol., 7, p. 528 

External links[edit]