Aryl disulphides

The reductive cleavage of several aryl disulphides to thiols by their reaction with triphenylphosphine in aqueous dioxan has been reported. Several examples have been reported of the cleavage by phosphines and phosphites of S—SandS—N bonds contained in heterocyclic ring systems. Examples include (73) and (74). ... 

The reaction of triphenylphosphine with aryl disulphides occurs by a nucleophilic substitution ... 

The reaction depends on the nature of both the phosphine and disulphide and the rate is influenced by the solvent polarity and the pKa of the thiol corresponding to the displaced thiolate anion. PPhs does not react with either alkyl or aryl disulphides but does react with acyl or aroyl disulphides in boiling benzene. On the other hand, P(NEt2)3 is a very... 

The ease with which the hydrogen atom is detached from the sulphur is noteworthy (analogy with H2S) even atmospheric oxygen gradually converts the mercaptans into aryl or alkyl disulphides, and mild oxidising agents do so at once ... 

Miscellaneous Reactions.—A full report has appeared of the reactions of carbon dioxide and carbon disulphide with tervalent phosphorus aryl esters and amines the products are ureas and thioureas, respectively.74 The suggested mechanism, previously invoked for similar reactions of carboxylic acids, involves the N-phosphonium salt (97). 

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 aryl diazonium tetrafluoroborate (4 mmol) is added portionwise over ca. 20 min to (TEBA)2-MoS4 (2.69 g, 4.4 mmol) in dry MeCN (10 ml) at 0°C. The mixture is stirred at 0°C for 1 h and then at 25°C for ca. 5 h. The solvent is removed under reduced pressure and the residue is extracted with Et20 (5 x 30 ml). The ethereal extracts are evaporated and the product purified by chromatography from silica to yield the diaryl disulphide. 

Although sodium sulphide reacts readily with haloalkanes [2] and activated aryl halides (see Chapter 2) [e.g. 3-5] in the presence of a quaternary ammonium catalyst to form symmetrical thioethers (Table 4.1), a major side reaction results in the formation of disulphides owing to the oxidation of the intermediate thiols under the basic conditions. Consequently, little use has been made of this procedure for the synthesis of thioethers [3, 6], but the corresponding reaction of the a,(0-dihaloalkanes to yield cyclic thioethers has proved to be a valuable procedure for the synthesis of thietanes [7] (Table 4.2). The ring closure with the secondary dihaloalkanes is considerably more difficult to effect than is the reaction of the primary dihaloalkanes. 1,3-Dihydrobenzo[c]thiophene (89%) is produced in the reaction of 1,2-bis(bromomethyl)benzene with sodium sulphide (Scheme 4.1) [8]. The direct... 

Trifluoromethyl thioethers are produced in a fluoride-catalysed one-pot reaction of alkyl or aryl thiocyanates with trifluoromethyl silanes [37]. The reaction is initiated by fluoride ion displacement of the trifluoromethyl anion from the silane the thioether is formed from the thiocyanate by displacement by the trifluoromethyl anion of the cyanide ion, which then perpetuates the reaction. Trifluoromethyl selenoethers are obtained by an analogous route. In a similar manner, disulphides can be converted into trifluoromethyl thio- or selenoethers [38],... 

The catalysed alkylation of l//,4//-pyrazol-5-ones is solvent dependent. In benzene, bis-alkylation occurs at the 4-position whereas, in a carbon disulphide benzene mixture, O-alkylation is observed, although the major product (4, Scheme 5.22) results from nucleophilic attack by the pyrazolone on the carbon disulphide, followed by alkylation of the dithiolate dianion [92]. The catalysed reaction of 2-thiono-3-aryl-thiazolidin-4-ones with alkylating agents under soliddiquid two-phase conditions results in alkylation at the 5-position (60-80%) [93]. The aldol condensation of the thiazolidinones with aryl aldehydes is also catalysed by quaternary ammonium salts. 

Alkyl and glycosyl isocyanates and isothiocyanates are produced in good yield under phase-transfer catalytic conditions using either conventional soluble catalysts or polymer-supported catalysts [32, 33]. Acyl isothiocyanates are obtained under similar conditions [34]. A-Aryl phosphoramidates are converted via their reaction with carbon disulphide under basic conditions into the corresponding aryl isothiocyanates, when the reaction is catalysed by tetra-n-butylammonium bromide [35]. 

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. 

These two classes of compounds can be prepared by treating diphenyl disulphide and aryl sulphonyl chlorides with sodium telluride followed, respectively, by a-halocarbonyl compounds and alkyl halides. 

The cross-coupling reaction proceeds through the initial oxidation of the more reactive thiol, followed by the gradual thiolysis of the formed disulphide by the less reactive thiol. The method is therefore useful for the cross-coupling of the reactive aryl and n-alkyl thiols with the less reactive f-alkylthiols. 

Aminobenzenesulphonic acid (orthanilic acid, Expt 6.64) is readily prepared by the reduction of 2-nitrobenzenesulphonic acid. The latter may be prepared by the hydrolysis of the corresponding sulphonyl chloride which is obtained from di-o-nitrophenyl disulphide. The preparation of this disulphide involves the use of the reactive aryl halide, 2-chloronitrobenzene (cf. Expts 6.93... 

In addition to the reactions discussed in the previous section, the free radical produced photochemically from PhI(02CAd)2 reacted with alkyl or aryl or cyclic disulphides to afford the corresponding adamantyl sulphides in good yield [79] ... 

N-aryl substituted enaminones react similarly with carbon disulphide with attack by the -position, thus opening a way to acridine derivatives14. The use of excess CS2 leads to a benzothiazine derivative. However, application of the reaction to 0-aminophenyl-substituted enaminone affords 3-thioxo-3,5,6,7-tetrahydrobenzo[l,2]dithiol-4-one (equation 10). 

A9.6.4.7 The Nordic Council of Ministers issued a report (Pederson et al, 1995) entitled Environmental Hazard Classification, that includes information on data collection and interpretation, as well as a section (5.2.8) entitled QSAR estimates of water solubility and acute aquatic toxicity . This section also discusses the estimation of physicochemical properties, including log Kow For the sake of classification purposes, estimation methods are recommended for prediction of minimum acute aquatic toxicity, for ...neutral, organic, non-reactive and non-ionizable compounds such as alcohols, ketones, ethers, alkyl, and aryl halides, and can also be used for aromatic hydrocarbons, halogenated aromatic and aliphatic hydrocarbons as well as sulphides and disulphides, as cited in an earlier OECD Guidance Document (OECD, 1995). The Nordic document also includes diskettes for a computerized application of some of these methods. 

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