5.5- Dialkyl dithiocarbonates synthesis

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. 

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

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. 

Synthesis of unsymmetrical thioethers from S,S-dialkyl dithiocarbonates... 

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. 

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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