Dialkyl disulphides

Organosulphur compounds represent the most common class of molecules exploited for formation of monolayers [1-8]. Many different kinds of similar molecules, e.g., aUcanethiols, dialkyl disulphides, dialkyl sulphides, thiophenes (Ths), thiophenols, and cysteines, have been reported to form monolayers on different substrates mainly consisting of (1) metals (e.g., Au, Ag, Cu, Pt, or Hg) or (2) semiconductors, such as indium tin oxide (ITO) and GaAs. Among the possible systems, the most extensively studied are those based on thiols adsorbed on Au. Although fundamental studies have also been carried out on the analogous Se and Te molecules, these have led to a limited number of applications in electroanalysis. 

A pletliora of different SA systems have been reported in tire literature. Examples include organosilanes on hydroxylated surfaces, alkanetliiols on gold, silver, copper and platinum, dialkyl disulphides on gold, alcohols and amines on platinum and carboxyl acids on aluminium oxide and silver. Some examples and references can be found in [123]. More recently also phosphonic and phosphoric esters on aluminium oxides have been reported [124, 125]. Only a small selection out of tliis number of SA systems can be presented here and properties such as kinetics, tliennal, chemical and mechanical stability are briefly presented for alkanetliiols on gold as an example. 

The dialkyl disulphides may be prepared by the oxidation of an alkaline solution of a mercaptan with iodine, for example ... 

Rajanikanth and Ravindranath44 have recently published a deoxygenation reaction for sulphoxides that uses metallic lithium in refluxing dimethoxyethane. Dialkyl and alkyl phenyl sulphoxides were reduced cleanly in yields around 70%, even if sterically hindered, but benzyl sulphoxides gave mixtures of products. For example, benzyl phenyl sulphoxide gave frans-stilbene (33%), benzyl phenyl sulphide (20%) and diphenyl disulphide (47%). These products can be rationalized by reaction pathways such as in equation (17) ... 

Phosphinyl radicals, obtained by hydrogen abstraction from dialkyl phosphites, have been trapped with t-nitrosobutane and the resultant nitroxyl radicals examined by e.s.r, The reaction of phosphinyl radicals, e.g. (5) and (6), with olefins has been shown to occur with retention of configuration at phosphorus. " These radicals have also been postulated as intermediates in the reactions of dialkyl disulphides and diaryl disulphides with phosphinates. " From the reaction of diphenyl disulphide... 

Powdered KOH (0.17 g, 3 mmol) is added to the freshly prepared thioiminium halide [MeC(SR)NH2+Cl- or McC(SR)NMc2+C1 ] (3 mmol) and TEBA-C1 (0.12 g, 0.5 mmol) in CH2C12 (30 ml). The mixture is stirred at room temperature until the reaction is complete, as shown by TLC analysis. The organic phase is separated, washed with H20 (2 x 25 ml), dried (Na2S04), and the solvent evaporated under reduced pressure to yield the alkyl aryl thioether and the dialkyl disulphide, which can be separated by chromatography from silica. 

The profitable conversion of thiouronium salts into dialkyl thioethers (see Section 4.1) is less successful for synthesis of diaryl thioethers. For example, l-chloro-4-nitrobenzene reacts with bis-thiouronium salts of the type (H2N)2CS(CH2) SC(NH2)2+ under soliddiquid and liquidrliquid conditions to produce the desired bis-thioethers, ArS(CH2) SAr, (20-35%), together with the diaryl sulphide, Ar2S (5-15%). Higher yields of the diaryl sulphide are observed under liquiddiquid conditions whereas, under solidiliquid conditions, the diaryl disulphide, (ArS)2, (20%) is also formed [56], Diaryl disulphides are the sole products (>65%) from the stoichiometric reaction of aryl diazonium salts with benzyltriethylammonium tetrathiomolybdate [57],... 

The 0,5-dialkyl dithiocarbonates (Table 4.8) are readily prepared under phase-transfer catalytic conditions by the reaction of an alkylating agent with potassium O-alkyl dithiocarbonate [35, 39], which can be obtained from carbon disulphide and the appropriate potassium alkoxide [cf. 40]. Monosaccharides are converted into 5-glycosyl dithiocarbonates via the in situ formation of the tosylate, followed by reaction with potassium O-alkyl dithiocarbonate (Scheme 4.6) [41], In a similar manner, 5-glycosyl 7V,7V-diethyldithiocarbamates are obtained from the monosaccharide and A.A-diethyldithiocarbamate (see 4.3.2) [42]. 

The S-alkyl-O-methyl dithiocarbonate, prepared by procedure 4.1.14 from MeOCS2K (48.26 g, 0.33 mol), is extracted from the reaction mixture with Et20 (3 x 25 ml). The ethereal extracts are washed with H20 (3 x 100 ml) and evaporated under reduced pressure at 40°C. Aqueous Na2C03 (2%, 300 ml) is added and the aqueous mixture is stirred at 50°C for ca. 3 h. The mixture is then extracted with EtjO (3 x 50 ml) and the dried (Na2S04) extracts are evaporated under reduced pressure at 70 °C. The temperature of the residue is maintained at 70°C for 30 min and the 5,5-dialkyl ester, which is contaminated with the 0,5-dialkyl ester, the dialkyl sulphide and the disulphide, is isolated by chromatography from silica. 

As indicated above, the traditional base-catalysed hydrolysis of 0,5-dialkyl thio-carbonates for the synthesis of thiols is generally unsatisfactory, as oxidation leads to the formation of disulphides. Under phase-transfer conditions, the procedure produces thioethers to the virtual exclusion of the thiols, as a result of the slow release of the thiolate anions in the presence of the electrophilic ester. However, a simple modification of the reaction conditions provides an efficient one-pot reaction [50] from haloalkanes (Table 4.15) via the intermediate formation of the thermally labile (9-/ert-butyl-5-alkyl dithiocarbonates (Scheme 4.8). 

In the reaction of the phenylacetonitrile carbanion with thiocyanates, a major side reaction leads to the formation of the dialkyl disulphides, as a result of the base-catalysed decomposition of the thiocyanate. This side reaction is reported to be insignificant in the reactions of the other carbanions. Phenylacetonitrile reacts with 1,2-ethanyl bisthiocyanate to produce 2-cyano-2-phenyl-1,3-thiolanes [52] under conditions analogous to those used for the synthesis of the thioethers (Scheme 4.12). 

Carbon disulphide should not be used as the solvent for alkylation of fS-diketones, as the carbanion reacts preferentially with the solvent to generate the dithiocarboxyl-ate derivative, which undergoes mono- and dialkylation (Scheme 6.9) [95]. Ketene thioacetals have also been isolated from acetophenones (60-80%) and cyclopenta-diene (80%) using an ultrasound technique in carbon disulphide [96] and, in a similar manner, pyrazol-5-ones form pyrazole-4-dithiocarboxylic esters [97]. 

Although there are other more cost effective and efficient procedures, the periodate oxidation of dialkyl sulphides to the sulphoxides has been shown to proceed in high yield [24, 25, 29] thiols are oxidized to disulphides (95-100%) [29]. The potency of the quaternary ammonium periodate for the oxidation of sulphides is improved by the addition of wieso-tetraphenylporphinatoironflll) chloride [TPPFe(III)Cl] [27]. In contrast with the oxidation conducted in the absence of TPPFe(III)Cl [24,25], aryl sulphides are oxidized more rapidly than alkyl sulphides. 

Disulphides, 1078 dialkyl, 496, 498 Dithizone, 955 Di-p-tolyl ketone, 930, 931 Dixon gauze rings, 97 n-Dodecane, 237 n-Dodecyl bromide, 282 n-Dodecyl chloride, 275 n-Dodecyl p-toluenesuiphonate, 825 Doebner reaction, 463, 465, 710, 711, 719... 

Dialkyl disulphides, of which (S)-cystine above is an example, may be prepared from thiols by mild oxidation, usually with iodine in the presence of alkali. A convenient synthesis of unsymmetrical analogues results when a symmetrical disulphide is heated with thiol to establish an equilibrium mixture. By careful choice of the reactants, fractional distillation then removes the more volatile thiol leaving the mixed disulphide as a residue.240... 

The oxidation of thiols to disulphides by DIB proceeds through the intermediacy of PhI(SR)2, which are homolytically converted into iodobenzene and dialkyl disulphides. These intermediates from 2,3,5,6-tetrafluoro-thiophenol yielded with various terminal alkynes adducts as mixtures of E and Z isomers of varying composition [111] ... 

Sulphur halides and dialkyl disulphides have been used to obtain a variety of aryithk) and alkytthio derivatives of phenols generally by electrophilic substitution. For example, phenol was treated with zirconium tetrachloride, heated to 156°C under nitrogen, kept at this temperature overnight and after reduction of this to 100°C, diethyidisulphide was added. The mixture was heated to remove ethanethiol and the product, 2-(ethylthio)phenol, isolated in 41% yield (ref. 116). 

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