Thiosulfinate

In organosulfur chemistry, thiosulfinate is a functional group consisting of the linkage R-S(O)-S-R (R are organic substituents). Thiolsulfinates are also named as alkanethiosulfinic (or arenethiosulfinic) acid esters. They are the first member of a family of compounds containing an oxidized disulfide bond. Other members of this family include thiosulfonates (R-SO₂-S-R), α-disulfoxides (R-S(O)-S(O)-R), sulfinyl sulfones (R-S(O)-SO₂-R), and α-disulfones (R-SO₂-SO₂-R), all of which are known. The thiosulfinate group can occur in cyclic as well as acyclic structures.

Occurrence

A variety of acyclic and cyclic thiosulfinates are found in plants, or formed when the plants are cut or crushed. A well-known thiosulfinate is allicin, one of the active ingredients formed when garlic is crushed. Allicin was discovered in 1944 by Chester J. Cavallito and coworkers. Thiosulfinates containing various combinations of the methyl, ''n''-propyl, 1-propenyl, 2-propenyl, ''n''-butyl, 1-butenyl and 2-butenyl groups are formed upon crushing different ''Allium'' as well as ''Brassica'' species. Crushing the roots of '' Petiveria alliacea'' affords the thiosulfinates ''S''-(2-hydroxyethyl) 2-hydroxyethane)thiosulfinate, ''S''-(2-hydroxylethyl) phenylmethanethiosulfinate, ''S''-benzyl 2-hydroxyethane)thiosulfinate and ''S''-benzyl phenylmethanethiosulfinate (petivericin; PhCH₂S(O)SCH₂Ph). Zeylanoxides are cyclic thiosulfinates containing the 1,2-dithiolane-1-oxide ring, isolated from the tropical weed ''Sphenoclea zeylanica''. These heterocyclic thiosulfinates are chiral at carbon as well as at sulfur. Asparagusic acid ''S''-oxide and brugierol are other natural 1,2-dithiolane-1-oxides occurring in '' Asparagus officinalis'' and '' Bruguiera conjugata'', respectively.

Properties

Allicin, ''S''-benzyl phenylmethanethiosulfinate, and related thiosulfinates show radical-trapping antioxidant activity associated with easy formation of sulfenic acids The acyclic thiosulfinates from ''Allium'' and ''Brassica'' species possess antimicrobial, antiparasitic, antitumor and cysteine protease inhibitory activity while the natural 1,2-dithiolane-1-oxides are growth inhibitors. The thiosulfinates from ''Petiveria'' also exhibit antimicrobial activity. Thiosulfinates feature a S(IV) center linked to a S(II) center, the former being stereogenic. Conversion of simple disulfides to thiosulfinates results in a considerable weakening of the S–S bond from about 70 to 34.5 kcal mol⁻¹ (16.7 to 8.25 kJ mol⁻¹) for the S-S bond in PhS(O)SPh, with the consequence that most thiosulfinates are both unstable and quite reactive. For this reason the mixtures of thiosulfinates from ''Allium'' plants can best be separated by HPLC at room temperature rather than by gas chromatography (GC), although GC has been used with some low molecular weight thiosulfinates. Thiosulfinates can be distinguished from sulfoxides by infrared spectroscopy since they have a characteristic S=O band at about 1078 cm⁻¹ compared to 1030–1060 cm⁻¹ in sulfoxides.

Formation and reactions

The first synthesis of thiosulfinates was reported in 1947 by Cavallito and coworkers by oxidation of the corresponding disulfides. One example of a moderately stable thiosulfinate is the ''tert''-butyl derivative, (CH₃)₃CS(O)SC(CH₃)₃. This thiosulfinate can be obtained in optical purity by catalytic asymmetric oxidation of di-''tert''-butyl disulfide with hydrogen peroxide. Upon heating, (CH₃)₃CS(O)SC(CH₃)₃ decomposes into ''tert''-butanethiosulfoxylic acid (CH₃)₃CSSOH) as shown by trapping studies. In a similar manner racemic methyl methanethiosulfinate (CH₃S(O)SCH₃) can be obtained by peracetic acid oxidation of dimethyl disulfide. Methyl methanethiosulfinate decomposes thermally giving methanesulfenic acid (CH₃SOH), the simplest sulfenic acid, as well as thioformaldehyde (CH₂=S). Methyl methanethiosulfinate can also disproportionate to a 1:1 mixture of dimethyl disulfide and methyl methanethiosulfonate (CH₃SO₂SCH₃) and rearrange via a Pummerer rearrangement to CH₃S(O)CH₂SSCH₃. An unusual three-membered ring thiosulfinate (a dithiirane 1-oxide) has been prepared through rearrangement of a 1,3- dithietane. A related compound, 3-(9-triptycyl)dithiirane-1-oxide, was prepared by the reaction of (9-triptycyl)diazomethane and . The X-ray structure of the dithiirane-1-oxide reveals a significantly lengthened sulfur-sulfur bond (211.9(3)pm). Thiosulfinates have also been invoked as intermediates in the oxidation of thiols to sulfonic acids.

References

{{Functional group

Thiosulfinates