Dithiocarbonates formation

Titanium complexes have been shown to be active for the synthesis of cyclic carbonates or either di- or trithiocarbonates from epoxides and either carbon dioxide or carbon disulfide (Scheme 5.6). Titanium-catalysed synthesis of cyclic carbonates has been recently reviewed by North and coworkers. Titanium-salen complexes find application as catalysts, in combination with tetrabutylammonium bromide or tributylamine, for the synthesis of di- or trithiocarbonates from epoxides and carbon disulfide. It is worth highlighting that the catalyst loading can be reduced to 0.5 mol%, although 1 mol% of catalyst was required in order to achieve quantitative yields. The catalyst system showed a preference for dithiocarbonate formation for most of the epoxides studied. 

Most attempts to differentiate these hydroxyl groups with conventional derivatives resulted in the formation of a tetrahydrofuran. The dithiocarbonate can also be prepared by phase-transfer catalysis (Bu4N HS04T, 50% NaOH/H20, CS2, Mel, rt,. 5h) ... 

Quebrachitol was converted into iL-c/j/roinositol (105). Exhaustive O-isopropylidenation of 105 with 2,2-dimethoxypropane, selective removal of the 3,4-0-protective group, and preferential 3-0-benzylation gave compound 106. Oxidation of 106 with dimethyl sulfoxide-oxalyl chloride provided the inosose 107. Wittig reaction of 107 with methyl(triphenyl)phos-phonium bromide and butyllithium, and subsequent hydroboration and oxidation furnished compound 108. A series of reactions, namely, protection of the primary hydroxyl group, 0-debenzylation, formation of A-methyl dithiocarbonate, deoxygenation with tributyltin hydride, and removal of the protective groups, converted 108 into 7. 

Pohl, L.R., Branchflower, R.V., Highet, R.J., et al. 1981. The formation of diglutathionyl dithiocarbonate as a metabolite of chloroform, bromotrichloromethane, and carbon tetrachloride. Drug. Metab. Dispos. 9 334—338. 

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]. 

An unusual reaction of methoxythiocarbonyl chloride with tetra-n-butylammo-nium iodide in the presence of sodium thiosulphate leads to the formation of 0,5-dimethyl dithiocarbonate [49], The reaction appears to involve a reduction step, with the iodide anion being regenerated from the released iodine by the thiosulphate ions (Scheme 4.7). In the absence of the thiosulphate ions, the thiocarbonyl chloride decomposes to yield chloromethane and carbonyl sulphide. 

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). 

Treatment of FC(0)SCFs with CsF affords bis(trifluoromethyl)-dithiocarbonate, (CFsS)2CO, with the evolution of COF2 (CFsSlgCS is also formed as a by-product. The following reaction mechanism explains the formation of the products isolated ... 

Reaction of norbornane trithiolane (24) with both dichloro- and dibromocarbene afforded the trithiocarbonate (94), which upon further reaction with the dihalocarbene yielded the corresponding dithiocarbonate <90JOC1146>. The mechanism of formation of (94) from (24) is postulated to proceed by addition of the carbene to S-2 of the trithiolane, followed by a ring-expansion reduction sequence as outlined in Scheme 22. 

The precursor dithiocarbonates were also used to prepare mesomorphic derivatives of TTF (tetrathiafulvalene), a strong electron donor compound used in the formation of highly conductive charge transfer complexes. Attempts to prepare CT complexes between the mesomorphic dithiolenes as acceptors and these mesomorphic TTF donors showed that the electron acceptor strength of this type of dithiolene does not suffice to form strong donor-acceptor complexes mixtures of the two components form mixed crystals without any visible degree of charge transfer.208... 

Pilcher73 has recently reviewed the experimental data for a variety of carbonyl and thio-carbonyl compounds. Enthalpies of formation for dithiocarbonic acids, R2N—C(=S)SH, thioamides and thioureas were determined by standard calorimetric methods. For an important, older review on the thermochemistry and thermochemical kinetics of sulfur-containing compounds, see work of Benson74. 

Adewuyi and Carmichael (73) observed the reaction of CS2 with OH- to form a dithiocarbonate complex as the rate-determining step for the overall reaction 74 above. The formation of sulfate was found to be dependent on the rate of hydrolysis and to be preceded by long induction periods. As shown in Figure 3, the formation rate of sulfate was also found to be pH dependent and to increase exponentially with time. 

Hu, Sun and Scott described recently an efficient synthesis of some taxadiene derivatives266. In their approach, alcohol (66) is treated with carbon disulphide followed by methyl iodide under standard conditions for the formation of xanthates, to afford the dithiocarbonate (67) in a process that presumably involved a Claisen rearrangement (Scheme 9). Interestingly, 67 undergoes smooth reaction with B SnH to provide the required products (68, 69)266. 

Like alkoxide ions, thiolate, dithiocarbamate and O-ethyl dithiocarbonate anions easily undergo nucleophilic addition at the 2-position of 1,3-dithiolylium salts (31) in boiling ethanol or acetonitrile with formation of derivatives (47) (80AHC(27)151). 

The preparation of viscose rayon fibers and cellophane proceeds via the xanthate, which therefore is an extremely important derivative of cellulose. Treatment of alkali cellulose with carbon disulfide results in the formation of cellulose xanthate (dithiocarbonate) ... 

The conversion of the dithiocarbonates into alkenedithiolates involves base hydrolysis, which is usually effected with sodium alkoxides in alcohol. With the dianion in hand, the synthesis of complexes follows the usual course, as described above. Obviously, oxophilic metal centers, for example, Ti(IV) and Nb(V) (62), are incompatible with the usual alcohol solutions of in situ generated alkenedithiolates. In such cases, the anhydrous salts Na2S2C2R2 are employed in nonhydroxylic solvents, although after complex formation protic solvents are typically employed for cation exchange. 

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. 

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