Xanthates, preparation and uses

Xanthate salts add reversibly to electrophihc olefins, but this conjugate addition is reversible [17]. By operating under acidic conditions, the reverse reaction can be prevented and a high yield of the desired xanthates can be secured. In this way, a number of otherwise inaccessible xanthates can be prepared and used in radical addition reactions [18]. Of particular interest are tertiary xanthates, such as compound 14 shown in Scheme 7, since the inter-molecular addition leads to the formation of a quaternary centre. Further-... 

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). 

The most important hazard ia the manufacturiag of xanthates is the use of carbon disulfide (qv) because of its low flash poiat, ignition temperature, and its toxicity. A report on the manufacture of sodium ethyl xanthate at Keimecott Nevada Mines Division discusses the various safety problems and the design of a faciUty (81). A plant layout and a description of the reagent preparations are also given. 

The precise reaction conditions for optimal yields depend upon the specific reagents and both thermal290 and photochemical291 conditions have been developed. Phenyl thionocarbonates are easily prepared and are useful in radical generating reactions.292 A variety of other thiono esters, including xanthates and imidazolyl thiocarbonates also can be used.293... 

There is an enormous literature on thiocarbonyl compounds, due in part to the technical and industrial importance of many of them, including thioamides, thioureas, xanthates, dithiocarbamates and so forth. An excellent, and recent, general review is available.107 There are also specialized reviews germane to the present chapter Griffin, Woods, and Klayman2 discussed the use of thioureas in the synthesis of heterocycles the preparation of thiazoles from thioamides is included in a three-part volume on Thiazoles 108 the use of carbon disulfide in the synthesis of trithiones and related heterocycles has been reviewed by Mayer109 and Huisgen110 has reported numerous examples of 1,3-dipolar cycloadditions in which carbon disulfide was used. 

The synthesis of alkali metal xanthates from trimethylsilyl dithiocarboxylates and alkali metal fluorides317 has been described as convenient and useful for preparative purposes of metal complexes and organometallic derivatives. 

A preparation of quinoline-3-thiol from the diazonium salt, according to prior literature, and using potassium ethyl xanthate, abnormally threw down a solid during extraction of the product into ether. This solid, filtered and dried, exploded on prodding with a metal spatula. The solid had an nmr spectrum consistent with the expected product, an 5-aryl-O-ethyl xanthate. Although the author appears aware of only the arenediazosulfide hazard, this must have been the 5 -arenediazoxanthate. A misprint renders quinoline as quinine. 

Radical deoxygenation of alcohols is important, and the reduction of xanthates prepared from alcohols, with Bu3SnH in the presence of AIBN is called the Barton-McCombie reaction (eq. 2.13) [37-51]. The driving force for the reaction is the formation of a strong C=0 bond from the C=S bond, approximately 10 kcal/mol stronger. This reaction can be used for various types of substrates such as nucleosides and sugars. Though methyl xanthates, prepared from alcohols with carbon disulfide and methyl iodide under basic conditions are very frequently used, other thiocarbonates, as shown in eq. 2.14, can also be employed. 

Xanthates have been used most frequently because they offer the best combination in terms of reactivity, stability, and accessibility. Potassium 0-ethyl xanthate is commercially available and cheap. It is an excellent nucleophile and many xanthates can be made trivially by displacement of a suitable leaving group. The examples of radical additions displayed in Scheme 3 are representative and underscore many of the desirable features discussed above. As can be seen, the xanthate reagent is readily made from the hemiacetal of trifluoroacetaldehyde and adds efficiently to a large assortment of olefins [7]. 1,2-Dichloroethane was used as the solvent but many other solvents can be used. Several related trifluoromethylated xanthates have been prepared and can be used to introduce a trifiuoromethyl group into a complex molecule or to prepare various fiuorinated starting materials [8-10]. 

Xanthate 446 undergoes cyclization in the presence of camphorsulfonic acid via a radical chain reaction initiated by a small amount of lauroyl peroxide to give pyrroloimidazoles 449 in 56% yield. The use of an acid and anhydrous medium inhibits nucleophilic attack of the basic heterocycles at the xanthate moiety and allows radical reactions to occur. Fused heteroaromatic compounds can also be prepared directly from benzimidazole carrying an N-alkenyl substituent and xanthates by a tandem radical addition/cyclization to provide, for example, pyrrolobenzimidazole 453 in 57% yield (Scheme 106) <2002OL4345>. 

Although other xanthates have been used, methyl xanthates (R3 = Me) are by far the most commonly employed in the Chugaev elimination. For the preparation of the xanthate, a variety of bases have been used including sodium hydride, sodium hydroxide, sodium amide, and sodium/potassium metal. In the case of pure chiral alcohol stereoisomers, epimerisation of the alcohol stereocentre, under the basic conditions, can occur (with the corresponding xanthates leading to different products). Purification of the xanthate, prior to pyrolysis, is often a problem and it is usual to pyrolyze the crude xanthate directly. Pyrolysis of the xanthate is often carried out by distillation. Depending on the pyrolysis temperature, pressure, and the boiling point of the olefin, the product will either distill with the other products (COS, thiol) or remain in the reaction flask. 

The third, probably major commercial process used today, forms a material that is known as Viscose rayon. The regenerated cellulose is prepared and sold as a fiber as well as a film, known as cellophane. The viscose, or more properly referred to as the xanthate process, consists of forming cellulose xanthate by reacting alkali cellulose with carbon disulfide ... 

Sec. chlorides. Startg. xanthate (prepared by treating menthol with NaH, CS2, and Mel) in chloroform treated with 1 eq. SO2CI2 in chloroform at 0° for 3 h then at room temp, for 7 h - product. Y 82%. The method is especially useful for preparing hindered chlorides where SOCb is ineffective. F.e.s. A.P. Kozikowski, J. Lee, Tetrahedron Letters 29, 3053-6 (1988). 

Alkali metal xanthates are prepared in high yield from reaction of amyl alcohols with alkah metal hydroxide and carbon disulfide (39—42). The xanthates are useful as collectors in the flotation of minerals and have minor uses in vulcani2ation of mbber and as herbicides (39,41). 

---