Xanthation side-reaction

The overall heat of reaction for potassium isopropyl xanthate is 48.5 kj /mol (11.6 kcal/mol) (64). The presence of water favors the principal side reaction ... 

The kinetics of the xanthation of sucrose were studied in the same year by Cherkasskaya, Pakshver, and Kargin, who determined potentiometric-ally the concentrations of the dithiocarbonate derivative and also of inorganic sulfide and trithiocarbonate. The rate of formation of 0-(sodium thiol-thiocarbonyl)sucrose was found to pass through a maximum with increasing alkali concentration, presumably due to a shift of the equilibrium in favor of side reactions in strongly alkaline solution. This result appears to parallel the qualitative findings of Lieser and Hackl for polysaccharides. 

Limitations in possibility of chemical modifications of starch result from steric hindrance of reaction sites, solubility, viscosity of reaction medium, and susceptibility to side reactions among them, depolymerization almost always accompanies intended modification. As a rule, polysaccharides are soluble, although frequently only sparingly, in water and dimethyl sulfoxide. Polysaccharides solubilize on xanthation, i.e., on reaction with CS2 in alkaline medium, to form syrups of xanthates. On acidification polysaccharides could be recovered. Such procedure was utilized for several decades for production of artificial silk from cellulose. 

In practice, a degree of substitution of 0.5-0.6 xanthate groups per glucose unit is sufficient to yield a soluble xanthate. Hydrolysis of carbon disulfide by alkali is an unavoidable side reaction which consumes 20-30 per cent of the carbon disulfide charged. 

Usually, the time and temperature of reaction are used to control the extent of xanthation. Carbon disulfide also reacts with alkali-forming sulfur-containing by-products such as sodium trithiocarbonate and sulfide, but other by-products also form by interaction of CS2, xanthate, by-products, water, and alkali. The side reactions result, in part, from the instability of cellulose xanthate in the alkaline environment. 

Meantime, some side reactions occur during the preparation of xanthates. 

Xanthate Precursor Route. Son et al. [994] published a modification of the Wessling precursor route. The sulfonium groups were replaced by xanthate groups in order to avoid typical side reactions of the Wessling approach (Fig. 81). The precursor is stable at room temperature and soluble in most common organic solvents. Elimination with formation of the polyconjugated system takes place between 160 and 250° C. 

The xanthation process is complex because in addition to Reaction 10.2, many side reactions occur simultaneously upon the addition of CS. The following are some important side reactions ... 

A final consideration is that Z should not cause any side reactions. With xanthates (4, Z = OR ) it is important that R be a poor homolytic leaving group. Otherwise fragmentation with loss of R (irreversible chain transfer) will compete with the desired RAFT process. This requires that R be primary alkyl or aryl. Similar design consideration apply in the case of unsymmetrical trithiocarbonates (4, Z = SR )- ... 

Eq. 3.7 shows the cyclization of the xanthate ester (20) formed from homopropargyl alcohol, through 5-exo-dig manner of the formed carbon-centered radical. This reaction is not a deoxygenation reaction of alcohol, but the cyclization of the carbon-centered radical formed from the addition of Bu3Sn to the thiocarbonyl-sulfur atom, onto the side-chained triple bond to form thionolactone, the hydrolysis of which readily creates lactone (21) [21-23]. 

The corresponding 9,1 l-dioxa-10-thiahomoprostanoid 263 was synthesized by reaction of the diol 258 with thionyl chloride. When the reaction with thionyl-chloride was carried out with the erythro diol 264 the isomeric 9,1 l-dioxa-10-thiahomoprostanoid 265 with cis side chains was obtained. The 9,11-dithiahomo-prostanoid 267 was prepared by reaction of the epoxide 266 with potassium methyl xanthate. 

Starch thiols can have the SH group directly on the pyranose ring or in a side chain. There are several methods of synthesizing starch thiols of the first type. One of them is based on the pyrolysis of starch xanthates (32), but the reaction proceeds in two parallel routes one producing thionates (35) and the other producing thiols (37).2675,2677 Reduction of starch xanthates with NaBPU in alkali is another approach to thiols. Thiols prepared in this manner were subjected to graft polymerization with vinyl polymers. Nucleophilic substitution of the chlorine... 

The slow step in the deoxygenation process is the collapse of the radical adduct 2. Under certain circumstances, this intermediate may be intercepted by the stannane, causing the formation of unwanted side-products arising mostly from further reactions of the thioformate 5 [2, 10). The various possibilities are depicted in Scheme 4, which also includes an example of formation of a methyl ether as a co-product in the reaction of a xanthate with tributylstannane [10b]. The yield is variable, depending strongly on the exact reaction conditions. 

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