Organosulfur chemistry is the study of the properties and synthesis oforganosulfur compounds, which areorganic compounds that containsulfur.[1] They are often associated with foul odors, but many of the sweetest compounds known are organosulfur derivatives, e.g.,saccharin. Nature is abound with organosulfur compounds—sulfur is vital for life. Of the 20 commonamino acids, two (cysteine andmethionine) are organosulfur compounds, and the antibioticspenicillin andsulfa drugs both contain sulfur. While sulfur-containing antibiotics save many lives,sulfur mustard is a deadly chemical warfare agent.Fossil fuels,coal,petroleum, andnatural gas, which are derived from ancient organisms, necessarily contain organosulfur compounds,the removal of which is amajor focus ofoil refineries.
Sulfur shares thechalcogen group withoxygen,selenium, andtellurium, and it is expected that organosulfur compounds have similarities with carbon–oxygen, carbon–selenium, and carbon–tellurium compounds.
A classicalchemical test for the detection of sulfur compounds is theCarius halogen method.
Organosulfur compounds can be classified according to the sulfur-containingfunctional groups, which are listed (approximately) in decreasing order of their occurrence.
![Martin's sulfurane with a see-saw structure, like that of SF4[2]](/mt/?noimg=&dark=on&url=https%3a%2f%2fwikipedia.org%2fwiki%2fFile%3aMartinSulfurane.svg)
Sulfides, formerly known as thioethers, are characterized by C−S−Cbonds[3][4] Relative to C−C bonds, C−S bonds are both longer, because sulfur atoms are larger than carbon atoms, and about 10% weaker. Representativebond lengths in sulfur compounds are 183 pm for the S−C single bond inmethanethiol and 173 pm inthiophene. The C−Sbond dissociation energy for thiomethane is 89 kcal/mol (370 kJ/mol) compared to methane's 100 kcal/mol (420 kJ/mol) and when hydrogen is replaced by a methyl group the energy decreases to 73 kcal/mol (305 kJ/mol).[5] The singlecarbon to oxygen bond is shorter than that of the C−C bond. Thebond dissociation energies fordimethyl sulfide anddimethyl ether are respectively 73 and 77 kcal/mol (305 and 322 kJ/mol).
Sulfides are typically prepared byalkylation of thiols. Alkylating agents include not only alkyl halides, but also epoxides, aziridines, andMichael acceptors.[6]
They can also be prepared via thePummerer rearrangement.
In theFerrario reaction,phenyl ether is converted tophenoxathiin by action of elemental sulfur andaluminium chloride.[7]
Thioacetals andthioketals feature C−S−C−S−C bond sequence. They represent a subclass of sulfides. The thioacetals are useful in "umpolung" of carbonyl groups. Thioacetals and thioketals can also be used to protect a carbonyl group in organic syntheses.
The above classes of sulfur compounds also exist in saturated and unsaturatedheterocyclic structures, often in combination with otherheteroatoms, as illustrated bythiiranes,thiirenes,thietanes,thietes,dithietanes,thiolanes,thianes,dithianes,thiepanes,thiepines,thiazoles,isothiazoles, andthiophenes, among others. The latter three compounds represent a special class of sulfur-containing heterocycles that arearomatic. Theresonance stabilization ofthiophene is 29 kcal/mol (121 kJ/mol) compared to 20 kcal/mol (84 kJ/mol) for the oxygen analoguefuran. The reason for this difference is the higherelectronegativity for oxygen drawing away electrons to itself at the expense of the aromatic ring current. Yet as an aromaticsubstituent the thio group is less electron-releasing than the alkoxy group.Dibenzothiophenes (seediagram), tricyclic heterocycles consisting of two benzene rings fused to a central thiophene ring, occurs widely in heavier fractions of petroleum.
Thiol groups contain the functionality R−SH. Thiols are structurally similar to thealcohol group, but these functionalities are very different in their chemical properties. Thiols are morenucleophilic, more acidic, and more readily oxidized. This acidity can differ by 5pKa units.[8]
The difference inelectronegativity between sulfur (2.58) and hydrogen (2.20) is small and thereforehydrogen bonding in thiols is not prominent. Aliphatic thiols formmonolayers ongold, which are topical innanotechnology.
Certain aromatic thiols can be accessed through aHerz reaction.
Disulfides R−S−S−R with a covalent sulfur to sulfur bond are important forcrosslinking: inbiochemistry for the folding and stability of some proteins and inpolymer chemistry for the crosslinking of rubber.
Longer sulfur chains are also known, such as in the natural productvaracin which contains an unusual pentathiepin ring (5-sulfur chain cyclised onto a benzene ring).
Thioesters have general structure R−C(O)−S−R. They are related to regular esters (R−C(O)−O−R) but aremore susceptible to hydrolysis and related reactions. Thioesters formed fromcoenzyme A are prominent in biochemistry, especially infatty acid synthesis.
Asulfoxide, R−S(O)−R, is theS-oxide of a sulfide ("sulfide oxide"), asulfone, R−S(O)2−R, is theS,S-dioxide of a sulfide, athiosulfinate, R−S(O)−S−R, is theS-oxide of a disulfide, and athiosulfonate, R−S(O)2−S−R, is theS,S-dioxide of a disulfide. All of these compounds are well known with extensive chemistry, e.g.,dimethyl sulfoxide,dimethyl sulfone, andallicin (see drawing).
Sulfimides (also called a sulfilimines) are sulfur–nitrogen compounds of structure R2S=NR′, the nitrogen analog of sulfoxides. They are of interest in part due to their pharmacological properties. When two different R groups are attached to sulfur, sulfimides are chiral. Sulfimides form stable α-carbanions.[9]
Sulfoximides (also called sulfoximines) are tetracoordinate sulfur–nitrogen compounds, isoelectronic with sulfones, in which one oxygen atom of the sulfone is replaced by a substituted nitrogen atom, e.g., R2S(O)=NR′. When two different R groups are attached to sulfur, sulfoximides are chiral. Much of the interest in this class of compounds is derived from the discovery that methionine sulfoximide (methionine sulfoximine) is an inhibitor ofglutamine synthetase.[10]
Sulfonediimines (also called sulfodiimines, sulfodiimides or sulfonediimides) are tetracoordinate sulfur–nitrogen compounds, isoelectronic with sulfones, in which both oxygen atoms of the sulfone are replaced by a substituted nitrogen atom, e.g., R2S(=NR′)2. They are of interest because of their biological activity and as building blocks for heterocycle synthesis.[11]
S-Nitrosothiols, also known as thionitrites, are compounds containing a nitroso group attached to the sulfur atom of a thiol, e.g. R−S−N=O. They have received considerable attention in biochemistry because they serve as donors of the nitrosonium ion, NO+, and nitric oxide, NO, which may serve as signaling molecules in living systems, especially related to vasodilation.[12]
A wide range of organosulfur compounds are known which contain one or morehalogen atom ("X" in the chemical formulas that follow) bonded to a single sulfur atom, e.g.:sulfenyl halides, RSX;sulfinyl halides, RS(O)X;sulfonyl halides, RSO2X; alkyl and arylsulfur trichlorides, RSCl3 and trifluorides, RSF3;[13] and alkyl and arylsulfur pentafluorides, RSF5.[14] Less well known are dialkylsulfur tetrahalides, mainly represented by the tetrafluorides, e.g., R2SF4.[15]
Compounds withdouble bonds between carbon and sulfur are relatively uncommon, but include the important compoundscarbon disulfide,carbonyl sulfide, andthiophosgene.Thioketones (RC(=S)R′) are uncommon with alkyl substituents, but one example isthiobenzophenone.Thioaldehydes are rarer still, reflecting their lack of steric protection ("thioformaldehyde" exists as a cyclic trimer).Thioamides, with the formula R1C(=S)N(R2)R3 are more common. They are typically prepared by the reaction of amides withLawesson's reagent.Isothiocyanates, with formula R−N=C=S, are found naturally. Vegetable foods with characteristic flavors due to isothiocyanates includewasabi,horseradish,mustard,radish,Brussels sprouts,watercress,nasturtiums, andcapers.
TheS-oxides of thiocarbonyl compounds are known as thiocarbonylS-oxides: (R2C=S=O, and thiocarbonylS,S-dioxides orsulfenes, R2C=SO2). The thioneS-oxides have also been known assulfines, and whileIUPAC considers this term obsolete,[16] the name persists in the literature.[17] These compounds are well known with extensive chemistry.[18][19] Examples includesyn-propanethial-S-oxide andsulfene.
Triple bonds between sulfur and carbon in sulfaalkynes are rare and can be found incarbon monosulfide (CS)[20] and have been suggested for the compounds F3CCSF3[21][22] and F5SCSF3.[23] The compound HCSOH is also represented as having a formal triple bond.[24]
Thiocarboxylic acids (RC(O)SH) and dithiocarboxylic acids (RC(S)SH) are well known. They are structurally similar to carboxylic acids but more acidic. Thioamides are analogous to amides.
Sulfonic acids have functionality R−S(=O)2−OH.[25] They are strong acids that are typically soluble in organic solvents. Sulfonic acids liketrifluoromethanesulfonic acid is a frequently used reagent inorganic chemistry.Sulfinic acids have functionality R−S(O)−OH whilesulfenic acids have functionality R−S−OH. In the series sulfonic—sulfinic—sulfenic acids, both the acid strength and stability diminish in that order.[26][27] Sulfonamides,sulfinamides andsulfenamides, with formulas R−SO2NR′2, R−S(O)NR′2, and R−SNR′2, respectively, each have a rich chemistry. For example,sulfa drugs are sulfonamides derived fromaromatic sulfonation. Chiral sulfinamides are used in asymmetric synthesis, while sulfenamides are used extensively in the vulcanization process to assist cross-linking.Thiocyanates, R−S−CN, are related to sulfenyl halides and esters in terms of reactivity.
Asulfonium ion is a positively charged ion featuring three organic substituents attached to sulfur, with the formula [R3S]+. Together with their negatively charged counterpart, the anion, the compounds are called sulfonium salts. An oxosulfonium ion is a positively charged ion featuring three organic substituents and an oxygen attached to sulfur, with the formula [R3S=O]+. Together with their negatively charged counterpart, the anion, the compounds are called oxosulfonium salts. Related species include alkoxysulfonium and chlorosulfonium ions, [R2SOR]+ and [R2SCl]+, respectively.
Deprotonation of sulfonium and oxosulfonium salts affordsylides, of structure R2S+−C−−R′2 and R2S(O)+−C−−R′2. Whilesulfonium ylides, for instance in theJohnson–Corey–Chaykovsky reaction used to synthesizeepoxides, are sometimes drawn with a C=S double bond, e.g., R2S=CR′2, the ylidic carbon–sulfur bond is highly polarized and is better described as being ionic. Sulfonium ylides are key intermediates in the synthetically usefulStevens rearrangement. Thiocarbonyl ylides (RR′C=S+−C−−RR′) can form by ring-opening ofthiiranes, photocyclization of aryl vinyl sulfides,[28] as well as by other processes.
Sulfuranes are relatively specialized functional group that featuretetravalent sulfur, with the formula SR4[2] Likewise,persulfuranes featurehexavalent SR6.
One of the few all-carbon persulfuranes has twomethyl and twobiphenyleneligands:[29]
It is prepared from the corresponding sulfurane1 withxenon difluoride /boron trifluoride inacetonitrile to the sulfuranyl dication2 followed by reaction withmethyllithium intetrahydrofuran to (a stable) persulfurane3 as thecis isomer.X-ray diffraction shows C−Sbond lengths ranging between 189 and 193 pm (longer than the standard bond length) with the central sulfur atom in a distortedoctahedral molecular geometry.
A variety of organosulfur compounds occur in nature. Most abundant are the amino acidsmethionine,cysteine, andcystine. The vitaminsbiotin andthiamine, as well aslipoic acid contain sulfur heterocycles.Glutathione is the primary intracellularantioxidant.[6]Penicillin andcephalosporin are life-savingantibiotics, derived from fungi.Gliotoxin is a sulfur-containingmycotoxin produced by several species of fungi under investigation as anantiviral agent.
Common organosulfur compounds present in petroleum fractions at the level of 200–500 ppm. Common compounds arethiophenes, especiallydibenzothiophenes. By the process ofhydrodesulfurization (HDS) in refineries, these compounds are removed as illustrated by the hydrogenolysis of thiophene:C4H4S + 8 H2 → C4H10 + H2S
Compounds likeallicin andajoene are responsible for the odor ofgarlic.Lenthionine contributes to the flavor ofshiitake mushrooms. Volatile organosulfur compounds also contribute subtle flavor characteristics towine, nuts,cheddar cheese,chocolate,coffee, and tropical fruit flavors.[30] Many of these natural products also have important medicinal properties such as preventingplatelet aggregation or fighting cancer.
Humans and other animals have an exquisitely sensitive sense of smell toward theodor of low-valent organosulfur compounds such as thiols, sulfides, and disulfides. Malodorous volatile thiols are protein-degradation products found in putrid food, so sensitive identification of these compounds is crucial to avoiding intoxication. Low-valent volatile sulfur compounds are also found in areas where oxygen levels in the air are low, posing a risk of suffocation.
Copper is required for the highly sensitive detection of certain volatile thiols and related organosulfur compounds by olfactory receptors in mice. Whether humans, too, require copper for sensitive detection of thiols is not yet known.[31]
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