Disulfides and Sulfonium Salts

Chemical Shifts and Coupling Constants of Disulfides and Sulfonium Salts (5 in ppm relative to TMS, J in Hz)... 

Ajoene (Spanish, ajo, garlic), 4,5,9-trithiadodeca-l,6,ll-triene-9-oxide 35 (Scheme 12), an antithrombotic compound with other well-defined physiological properties, is formed from allicin.84 Like allicin, ajoene is a sulfoxide but has two further sulfur atoms in a disulfide linkage. E and Z isomeric forms are possible involving the C=C bond at positions 6 and 7. Ajoene is somewhat more stable than allicin. The formation of ajoene probably involves condensation of 2 molecules of allicin forming a sulfonium salt 33, with elimination of propenesulfenic acid. Elimination of a second molecule of propenesulfenic acid... 

The intermediate contains a tricoordinate sulfur cation or sulfonium salt. The chloride ion now attacks the other sulfur atom of this intermediate and two molecules of RSC1 result. Each atom of the original disulfide has formed an S-Cl bond. One sulfur atom was a nucleophile towards chlorine and the other an electrophile. 

The formation of desaurins from ketones, carbon disulfide, and base 1275,1281,1282,1285-1290 believed to involve nucleophilic attack on a thioketene by the dianion of a 1,1-dimercaptoalkene, as shown for the synthesis of 572. Related syntheses involve the use of thiophosgene instead of carbon disulfide and the use of diazoalkanes or phosphonium and sulfonium ylides instead of a ketone and base. Treatment of perfluoroiso-butylene with fluoride ion and elemental sulfur in a dipolar, aprotic solvent ° °° or with sources of anionic sulfur (potassium sulfide, sodium hydrosulfide,potassium thiocyanate,sodium thiosulfate, dithiocarbamate salts, dithiophosphate salts ) give the dimer (573) of bis(trifIuoromethyl)-thioketene. Similarly, other 2,4-bis(methylene)-l,4-dithietanes are obtained by treating 2,2-dichlorovinyl ketones with anionic sulfur re-... 

Thiols, RSH, the sulfur analogs of alcohols, are usually prepared by Sn2 reaction of an alkyl halide with thiourea. Mild oxidation of a thiol yields a disulfide, RSSR, and mild reduction of a disulfide gives back the thiol. Sulfides, RSR, the sulfur analogs of ethers, are prepared by a Williamson-type Sn2 reaction between a thiolate anion and a primary or secondary alkyl halide. Sulfides are much more nucleophilic than ethers and can be oxidized to sulfoxides (R2SO) and to sulfones (R2S02). Sulfides can also be alkylated by reaction with a primary alkyl halide to yield sulfonium salts, RaS. ... 

Chloro(dimethyl)sulfonium salts are extremely reactive, usually losing the chlorine as anion and thus able to transfer the dimethylsulfonium group readily to nucleophilic compounds such as alcohols, compounds containing acidic CH groups, organic sulfides, and disulfides.627... 

They are also good soft electrophiles. Sulfenyl chlorides (RSCl) are easily made from disulfides (RS—SR) and sulfuryl chloride (SO2CI2). This S(V1) chloride has electrophilic chlorine atoms and is attacked by the nucleophilic disulfide to give two molecules of RSCl and gaseous SO2. There s a lot of sulfur chemistry here We start with a nucleophilic attack by one sulfur atom of the disulfide. The intermediate contains a tricoordinate sulfur cation or sulfonium salt. The chloride ion now attacks the other sulfur atom of this intermediate and two molecules of RSCl result. Each atom of the original disulfide has formed an S-Cl bond. One sulfur atom was a nucleophile towards chlorine and the other an electrophile. 

The following compounds are used as inhibitors in acid solutions amines, amino-imidazolines, amino- and nitrophenols, aminotriazole, aldehydes, bezothiazol, dibenzyl sulfoxide, dithiophos-phoric acids, guanidine derivatives, ureas, phosphonium salts, sulfonium salts, sulfonic acids, thio-ethers, thioureas, and thiocarbanoyl disulfides. Amino alcohols, aminobenzimidazole, benzoates, quinoline derivatives, cinnamates, fatty amines, polyether amines, silicates, and triazoles are used as inhibitors in neutral or weakly alkaline solutions, while for strongly alkaline solutions, aldehydes and fatty amines are used. 

Analysis of samples showed that concentrations of methanethiol and dimethyl disulfide were lower in caraway- and TBHQ-treated broccoli florets than in control samples (Table V). Dimethyl tiisulfide was not detected in the refrigerated samples which indicated that its contribution to the objectionable odors of modified atmosphere stored broccoli florets was minimal. Dimethyl sulfide which has a precursor (Sr methyl methionine sulfonium salt 34) different from that for methanethiol-related compounds was present in all samples. 

Introduction. The sulfur-sulfur bond in alkylated disulfides [dialkyl(alkylthio)sulfonium salts] is susceptible to nucleophiles and thus alkylated disulfides are alkylsulfenylating agents, or... 

Examples of additives are the following EA = iodonium salt, e.g. diphenyliodonium hexafluorophosphate (more rarely, a sulfonium salt) and related derivatives [113], alkyl halide, e.g. phenacyl bromide [1.14] and triazine, e.g. 2,4,6-tris(trichloromethyl)-l,3,5-triazine ED = borate disulfide, group IVb dimetal [1.15] HD = alcohol, THE, thiol, benzoxazine, aldehyde, acetal, silane (e.g. tris(trime1hylsilyl)silane = (TMS)3Si-H) [1.16]), germane, borane, stannane, alkoxyamine, silyloxyamine, polymer substrate, etc. EPD = amine [1.17], thiol, etc. The generated radicals (e.g. Ph, R, RsSi, RR C R R in [1.13]-[1.17] ) are the initiating species. Efficient novel or newly modified dye structures in the Dye/amine, Dye/iodonium salt or Dye/silane two-component PISs have been proposed within the last 4 years (see section 1.3.5). 

The less polar nature of disulfides and sulfenamides makes the addition of these reagents to alkenes inefficient. Although thiolates can be generated under reducing conditions,the activation of disulfides and sulfenamides is routinely performed by Lewis acids to enable the introduction of new u bonds other than C—S bonds. As a consequence, the stereospecificity and reactivity observed in the Lewis-acid-promoted addition of disulfides and sulfenamides to alkenes is similar to that of sulfenylation by sulfenyl halides and thi-osulfonium salts. Likewise, the accepted reaction mechanism includes the coordination of Lewis acid, resulting in a thio-sulfonium-like species that can further react with alkene either directly or indirectly by way of dithiosuhonium ions. 

Despite the fact that sulfonium salts are much more easily prepared than oxonium ones, simple sulfur bases appear to be considerably less basic towards protonic acids, hydrogen bond donors, and most Lewis acids than are their oxygen analogs. The exact values for the freezing point depressions of sulfides in sulfuric acid are controversial (134,136,334) but it is clear that if proper allowance is made for sulfonation, a twofold depression is approached magnifying protonsr tion of the sulfur atom. Evidence for the existence of stable sulfonium salts from the interaction of mercaptans, sulfides and disulfides with hydrogen chloride and antimony pentachloride at low temperatures has recently been described (205). 

The tris(methylsulfanyl)cyclopropenylium salt 19 was also prepared from tetrachlorocyclo-propene (3) by the reaction with dirnethyl(methylsulfanyl)sulfonium tetrafluoroborate and dimethyl disulfide." ... 

---