organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...

, a sulfonium ion, also known as sulphonium ion or sulfanium ion, is a positively-charged

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

(a "

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

") featuring three organic

substituents In organic chemistry, a substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety (chemistry), moiety in the resultant (new) molecule. The suffix ''-yl'' is used when naming organic compounds that conta ...

attached to

sulfur Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...

. These

organosulfur compound Organosulfur chemistry is the study of the properties and synthesis of organosulfur compounds, which are organic compounds that contain sulfur. They are often associated with foul odors, but many of the sweetest compounds known are organosulfur der ...

s have the formula . Together with a negatively-charged

counterion 160px, cation-exchange_resin.html" ;"title="Polystyrene sulfonate, a cation-exchange resin">Polystyrene sulfonate, a cation-exchange resin, is typically supplied with as the counterion. In chemistry, a counterion (sometimes written as "counter ...

, they give sulfonium salts. They are typically colorless solids that are

soluble In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubi ...

in polar

organic solvent A solvent (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for p ...

Synthesis

Dimethylsulfoniopropionate Structural Formula V1

Sulfonium compounds are usually synthesized by the reaction of

thioether In organic chemistry, a sulfide (British English sulphide) or thioether is an organosulfur functional group with the connectivity as shown on right. Like many other sulfur-containing compounds, Volatile organic compound, volatile sulfides have ...

s with

alkyl halide The haloalkanes (also known as halogenoalkanes or alkyl halides) are alkanes containing one or more halogen substituents of hydrogen atom. They are a subset of the general class of halocarbons, although the distinction is not often made. Haloalka ...

s. For example, the reaction of

dimethyl sulfide Dimethyl sulfide (DMS) or methylthiomethane is an organosulfur compound with the formula . It is the simplest thioether and has a characteristic disagreeable odor. It is a flammable liquid that boils at . It is a component of the smell produc ...

with iodomethane yields trimethylsulfonium iodide: : The reaction proceeds by a nucleophilic substitution mechanism (S_N2), where iodide is the leaving group. For weakly electrophilic alkyl halides, the reactions can be accelerated by the addition of silver tetrafluoroborate. In that vein, the rate (and irreversibility) of methylation improves with more electrophilic methylating agents such as

methyl trifluoromethanesulfonate Methyl trifluoromethanesulfonate, also commonly called methyl triflate and abbreviated MeOTf, is the organic compound with the formula . It is a colourless liquid which finds use in organic chemistry as a powerful methylating agent. The compoun ...

. These S-alkylations can be reversible, especially when the leaving group is iodide. For example, alkylation of dimethylsulfide with allyl iodide gives trimethylsulfonium iodide. In a related process, secondary alkyl halides react with dimethylsulfide to give mixed thioether, eliminating methyl halide. Below are listed some other synthetic methods, among many: *sulfonium salts can be made by treating secondary and tertiary alcohols with sulfuric acid in the presence of thioethers. *addition of methyl iodide to

dimethyldisulfide Dimethyl disulfide (DMDS) is an organic chemical compound with the molecular formula . It is a flammable liquid with an unpleasant, garlic-like odor resembling that of "leaking gas". The compound is colorless, although impure samples often appear ...

in the presence of a mercuric halide catalyst. *addition of sulfenyl chlorides to alkenes, giving episulfonium salts. *alkylation of thioethers with electrophilic alkenes in the presence of a proton donor.

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. e(Et)SCH₂CO₂Hsup>+ is the first chiral sulfonium cation to be resolved into enantiomers. The barrier to inversion ranges from 100 to 130 kJ/mol.

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 Biosynthesis, i.e., chemical synthesis occurring in biological contexts, is a term most often referring to multi-step, enzyme-Catalysis, catalyzed processes where chemical substances absorbed as nutrients (or previously converted through biosynthe ...

of many compounds. Other naturally-occurring sulfonium species are ''S''-methylmethionine ( methioninium) and the related

dimethylsulfoniopropionate Dimethylsulfoniopropionate (DMSP), is an organosulfur compound with the formula (CH3)2S+CH2CH2COO−. This zwitterionic metabolite can be found in marine phytoplankton, seaweeds, and some species of terrestrial and aquatic vascular plants. ...

(DMSP).

Organic synthesis

Sulfonium salts are precursor to sulfur

ylide An ylide () or ylid () is a neutral dipolar molecule containing a formally negatively charged atom (usually a carbanion) directly attached to a heteroatom with a formal positive charge (usually nitrogen, phosphorus or sulfur), and in which both ...

s, which are useful in carbon–carbon bond-forming reactions. In a typical application, a R₂S⁺CH₂ center is deprotonated to give the ylide R₂S⁺CHR⁻. These ylides add to ketones and aldehydes to give epoxides ( Johnson–Corey–Chaykovsky reaction). Tris(dimethylamino)sulfonium difluorotrimethylsilicate (CH₃)₂N)₃Ssup>+ ₂Si(CH₃)₃sup>− is a popular fluoridation agent. 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. Organic sulfides react with liquid

bromine Bromine is a chemical element; it has chemical symbol, symbol Br and atomic number 35. It is a volatile red-brown liquid at room temperature that evaporates readily to form a similarly coloured vapour. Its properties are intermediate between th ...

to give bromosulfonium bromides, i.e.: :RS + RS⁺Brr⁻

References

External links

IUPAC definition
(short pdf) Cations Sulfur ions