5.5- Dialkyl dithiocarbonates

An alternative procedure for the synthesis of unsymmetrical thioethers, which is equally versatile and also avoids the direct use of thiols, utilizes 0,5-dialkyl [35] or 5,5-dialkyl dithiocarbonates [36] (Scheme 4.5). 

Symmetrical 5,5-dialkyl dithiocarbonates have been obtained by thermal rearrangement of the corresponding (9,5-dialkyl esters in the presence of Aliquat [43]. This procedure is not suitable for the preparation of unsymmetrical 5,5-dialkyl dithiocarbonates, as it has been reported that disproportionation of the products can lead to a mixture of the symmetrical and unsymmetrical esters. Alternatively, they can be prepared by a base-catalysed disproportionation of 5-alkyl-O-methyl dithiocarbonates [44] (Table 4.9). These methods for the synthesis of the 5,5-dialkyl esters are more convenient than the traditional procedures from the thiol and phosgene. 

In a one-pot synthesis of thioethers, starting from potassium 0-alkyl dithiocarbonate [36], the base hydrolyses of the intermediate dialkyl ester, and subsequent nucleophilic substitution reaction by the released thiolate anion upon the unhydrolysed 0,5-dialkyl ester produces the symmetrical thioether. Yields from the O-methyl ester tend to be poor, but are improved if cyclohexane is used as the solvent in the hydrolysis step (Table 4.13). In the alternative route from the 5,5-dialkyl dithiocarbonates, hydrolysis of the ester in the presence of an alkylating agent leads to the unsymmetrical thioether [39] (Table 4.14). The slow release of the thiolate anions in both reactions makes the procedure socially more acceptable and obviates losses by oxidation. 

Selected examples of unsymmetrical thioethers from 5,5-dialkyl dithiocarbonates... 

The haloalkane (20 mmol), 5,5-dialkyl dithiocarbonate (l I mmol) and TBA-Br (0.05 g, 0.15 mmol) in aqueous KOH (30%, 10 ml) are stirred under reflux (Table 4.14). The cooled mixture is extracted with Et20 (3 x 50 ml) and the combined extracts are washed with H20 (3 x 50 ml), dried (Na2S04), and evaporated under reduced pressure to yield the thioether. 

Numerous research activities have focused on the improvement of the protective films and the suppression of solvent cointercalation. Beside ethylene carbonate, significant improvements have been achieved with other film-forming electrolyte components such as C02 [156, 169-177], N20 [170, 177], S02 [155, 169, 177-179], S/ [170, 177, 180, 181], ethyl propyl carbonate [182], ethyl methyl carbonate [183, 184], and other asymmetric alkyl methyl carbonates [185], vinylpropylene carbonate [186], ethylene sulfite [187], S,S-dialkyl dithiocarbonates [188], vinylene carbonate [189], and chloroethylene carbonate [190-194] (which evolves C02 during reduction [195]). In many cases the suppression of solvent co-intercalation is due to the fact that the electrolyte components form effective SEI films already at potential which are positive relative to the potentials of solvent co-intercalation. An excess of DMC or DEC in the electrolyte inhibits PC co-intercalation into graphite, too [183]. 

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]. 

Synthesis of symmetrical S,S-dialkyl dithiocarbonates from potassium O-methyl dithiocarbonate... 

Thermal rearrangement of 0,S-dialkyl dithiocarbonates to give the S,S-dialkyl esters... 

Method A The 0,5-dialkyl dithiocarbonate is prepared by procedure 4.1.14 from O-alkyl potassium dithiocarbonate (50 mmol). The mixture is cooled to 50 °C and, without isolation of the ester, KOH pellets (14 g, 0.25 mol) are added portionwise at <80 °C. The mixture is stirred at 80 °C until GLC analysis indicates the complete disappearance of the ester (ca. 30 min). Petroleum ether (b.p. 40-60 °C, 150 ml) is added and the organic phase is separated, dried (Na2S04), filtered through silica, and fractionally distilled to give the thioether. 

Alkylation of this xanthate salt with an alkyl halide in absolute ethanol leads to an 0,S-dialkyl dithiocarbonate. 

In accord with the special reactivity of a dithiocarboxylate moiety, 0,5-dialkyl dithiocarbonates, ROCSSR, yield 0-alkyl thiourethanes, ROCSNR 2, on addition of amines. Similarly, trithiocarbo-nates, (RS)2CS, yield thioureas. - ... 

Preparation.—The reaction of alcohols with CSj under strongly alkaline conditions remains the standard method for preparing xanthates (0-alkyl dithiocarbonates). Alkylation of xanthates to OS -dialkyl dithiocarbonates is conveniently done by a phase-transfer process, using an alkyl halide or an alkyl methanesulphonate, and... 

Reactions.—Unsymmetrical dialkyl sulphides are obtained by the action of aqueous KOH, at 80 °C, on 05-dialkyl dithiocarbonates (with different alkyl groups) under phase-transfer conditions. The reaction depends on the liberation of a mercaptan and its alkylation by the 0-alkyl group of the diester. Ethoxythiocarbonyl derivatives of amino-acids are made by acylating the amino-acids with the stable reagent bis (ethoxythiocarbonyl) sulphide, (EtOCS)2S. This is prepared by an improved method from potassium ethylxanthate (2 mol) and ethyl chloroformate (1 mol). Esters of amino-acids are acylated by carboxymethyl... 

The substances typically used are sulfides, dialkyl dithiocarbonates or thiodipro-pionates, such as dilauryl thiodipropionate (DLTDP) and distearyl thiodipropionate (DSTDP) as well as disulfides. These products develop their effect mainly in the range of long-term service temperatures, i. e., at moderate temperatures, and they exhibit few process stabilizing effects (with the exception of disulfides). 

General Reactions.—A full account of the synthesis of thiirans from aldehydes and ketones using metalated 2-(alkylthio)-2-oxazolines has appeared. Asymmetric syntheses of chiral thiirans can be effected using either a chiral oxazoline or a chiral dialkyl dithiocarbonate. ... 

The same group of workers also accounted for the thermal rearrangement reaction of di(ethylthio)propyl S-methyl xanthates and the rearrangement of 0,S-dialkyl dithiocarbonates that occurs under Friedel-Crafts reaction conditions. A new, general synthesis of l,3-dithiol-2-ones (451) is based on the acid-catalysed ring-closure reaction of the dithiocarbonates (452). The rearrangements of 2-aminoethyl alkyl trithiocarbonates (453) and alkylthio-thiocarbonyl thiocyanates (433) into dithiocarbonates (454) and alkylthio-thiocarbonyl isothiocyanates (455), respectively, have been described recently. 

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). 

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