Xanthates, ester

Xanthate esters are prepared by reaction of isopropyl alcohol and carbon disulfide [75-15-0]. Isopropyl xanthates have wide use ia mineral flotation (qv) processes, and sodium isopropyl xanthate [140-93-2], C4HyOS2Na, is a useful herbicide for bean and pea fields (see Herbicides) (30). 

Esters derived from the primary alcohols are the most stable and those derived from the tertiary alcohols are the least stable. The decomposition temperature is lower in polar solvents, eg, dimethyl sulfoxide (DMSO), with decomposition occurring at 20°C for esters derived from the tertiary alcohols (38). Esters of benzyl xanthic acid yield stilbenes on heating, and those from neopentyl alcohols thermally rearrange to the corresponding dithiol esters (39,40). The dialkyl xanthate esters catalytically rearrange to the dithiol esters with conventional Lewis acids or trifluoroacetic acid (41,42). The esters are also catalytically rearranged to the dithiolesters by pyridine Ai-oxide catalysts (43) ... 

A number of catalysts of Pd(II), Pt(II), Rh(I), and Ir(I) induce rearrangements of 0-a11y1ic-.9-methy1 dithiocarbonates at 25°C (45). In a relatively low temperature procedure, olefins readily form from certain classes of xanthate esters (46) ... 

The reactions of the xanthate esters with some soft electrophiles proceed with good yields (47) ... 

The Reformatskii reaction was carried with the dimethyl xanthate ester (58) ... 

Organic chemists have been aware of reversible addition-fragmentation involving xanthate esters in organic chemistry for some time. It is the basis of the Barton-McCombie process for deoxygenation of alcohols (Scheme 9.37).402 404... 

The addition of alcohols to carbon disulfide in the presence of a base produces xanthates. The base is often OH, but in some cases better results can be obtained by using methylsulfinyl carbanion (MeSOCH ) If an alkyl halide RX is present, the xanthate ester ROCSSR can be produced directly. In a similar manner, alkoxide ions add to CO2 to give carbonate ester salts ROCOO. ... 

Alcohols can be dehydrated via xanthate esters at temperatures that are much lower than those required for acetate pyrolysis. The preparation of xanthate esters involves reaction of the alkoxide with carbon disulfide. The resulting salt is alkylated with methyl iodide. 

The sulphidization process, which was first successfully applied on a commercial scale on lead carbonate ores, is currently the most popular method used during treatment of oxide copper ores that contain malachite and carbonaceous gangue. The commonly used sulphi-dizers are Na2S 9H20 and NaHS, with xanthate or xanthate ester. 

One of the earlier methods was to treat cellulose with sodium hydroxide and carbon disulfide to obtain xanthate esters which could be dispersed in water and cast into sheets or spun into fibers. Subsequent treatment with acid decomposed the xanthates and gave regenerated cellulose, either in fiber or film form. The fibers were called viscose rayon and the films were named cellophane. Cellophane is still used as a wrapping film and some of it is still manufactured by the xanthate process. By treatment with nitric acid, cellulose was converted to a trinitric acid ester, which could be cast into units which were satisfactory for making gun cotton for a smokeless powder for either artillery shells or shotgun ammunition. It was quite insoluble but it could be converted to a jelly-like mass, which could be shaped into a desired form for ammunition use. Under milder conditions, a lower nitrate... 

Salts of O-alkyldithiocarbonatcs ( xanthates ) are hazardous as dusts, forming explosive suspensions in air. The lower-alkyl salts are claimed to be explosive in the solid state when dry [1]. Explosions reported when drying hydrated xanthate salts are probably the consequence of release of carbon disulphide to form an inflammable atmosphere of very low autoignition temperature in the oven [2], Xanthate esters are thermally unstable by a variety of eliminations and rearrangements, all distinctly exothermic and many evolving extremely flammable gases and vapours. Free xanthic acids, which may be isolated on acidification, decompose autocatalytically and perhaps explosively [3],... 

The standard conversion of alcohols into their xanthate esters through reaction with carbon disulphide and an alkylating agent is improved by the addition of a quaternary ammonium salt [69]. Excellent yields (>90%) are obtained in relatively short reaction times under mild conditions. The formation over relatively short reaction times of O-glycosyl xanthates from partly protected glycosides has been described using a stoichiometric amount of the quaternary ammonium salt [70]. 

Another method that provides chlorides from alcohols with retention of configuration involves conversion to a xanthate ester, followed by reaction with sulfuryl chloride. This method is thought to involve collapse of a chlorinated adduct of the xanthate ester. The reaction is useful for secondary alcohols, including stoically hindered structures.8... 

A variety of thiocarbonyl derivatives, in addition to xanthate esters,... 

These include thiobenzoates, thiocarbonylimidazolides, and phenyl thionocarbonate esters.6 The S-methyl xanthate ester is a particularly convenient intermediate to prepare because of its ease of formation and the low cost of the reagents. Its use is precluded, however, by the presence of base-labile protecting groups and, in such cases, the thiocarbonylimidazolide or phenyl thionocarbonate ester will generally prove satisfactory. Additional methods for the radical deoxygenation of alcohols are described in a review by Hartwig.7... 

Pyrolyses of esters (60) and xanthate esters (61), either in the gas phase or in solution, give 1,2-elimination (Equations 7.46 and 7.47). These reactions are... 

A very elegant, simple, versatile and high yielding synthesis of fused l,3-dithiol-2-ones has been described recently. In a representative example, treatment of 4-benzoyloxy-l-bromo-2-butyne with potassium O-methyl xanthate in methanol gave the expected xanthate ester in excellent yield. Refluxing of this ester in chlorobenzene for one hour in the presence of dimethyl fumarate gave 1 in quantitative yield. 

S-dialkyldithiocarbonates (xanthate esters, e.g. O-ethyl S-ethyl dithiocarbon-ate (3)]. 

The xanthate esters are of interest in that if the O-alkyl group contains at least one a-hydrogen atom, pyrolysis produces an olefin, a thiol and carbon oxysul-phide (the Chugaev reaction, Section 5.2.1, p. 489). 

Isolation of the intermediate xanthate salts in the preparation of the xanthate esters is not essential. The formation of the latter may be achieved, in one step, by converting the alcohol into the corresponding alkoxide by reaction with a potassium derivative of a tertiary alcohol (e.g. potassium t-pentoxide), followed by reaction successively with carbon disulphide and the alkyl halide. Both preparative procedures are illustrated in the following section. 

The conversion of alcohols to xanthate esters, and treatment of the latter with DFIT provides access to 1°- and 2°-alkyl fluorides, probably through intermediates of the type shown in Scheme 38 [108]. 

J. Nicholas Kirwan, B. P. Roberts, and C. R. Willis, Deoxygenation of alcohols by the reactions of their xanthate esters with triethylsilane An alternative to tributyltin hydride in the Barton-McCombie reaction, Tetrahedron Lett., 31 (1990) 5093-5096. 

Three more crystal structures are included in this section for the sake of completeness. The molecular structure of the only crystallographically determined structure of a xanthate ester is shown in Fig. 5 (25). In fact, this is a diester in that two isobutylxanthates are connected via an ethylene bridge. 

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