Thiocarbonyl compounds reduction

These reactions differ from those of sulfur tetrafluoride with carbonyl compounds in that a formal oxidation-reduction of the sulfur atoms m the thiocarbonyl compound and sulfur tetrafluoride molecule occurs, resulting in the formation of free sulfur and the complete utilization of the fluorine atoms in sulfur tetrafluoride. 

The reaction of a 1,3-dithiolane (336) with n-butyllithium has been shown to result in fragmentation to the corresponding thiocarbonyl compound via the mechanism detailed in Scheme 73 (80JA3S77). The thiocarbonyl compound can react further with excess lithium reagent to provide the product of carbon addition (aldehyde precursors) or reduction (saturated ketones). 

The comparison of their results with data for the open-chain compounds dimethyldithio-carbonate, (CH3S)2C=0 (14)102 and dimethyltrithiocarbonate (CH3S)2C=S (15)103 sheds light on the influence of cyclation on the structure and orbital interactions of thiocarbonyl compounds. A particularly important feature is the decrease in C=0 and C=S bond lengths upon cyclation. This follows from the reduction of the S—C(sp2)—S angles... 

The main feature of thiocarbonyl compounds is their high reactivity. For instance their strong electrophilicity, relative to carbonyl compounds, is related to their low lying LUMO which causes an important reduction of the gap with occupied orbitals of nucleophiles [119]. At the same time, they are more nucleophilic in relation to their high HOMO. A number of examples reported below illustrate this. 

The enzymatic reduction of a thiocarbonyl compound has been investigated [159] for the first time, in order to provide a new route for enan-tiopure thiols, molecules which are currently needed for asymmetric synthesis. Reaction of easily available /1-thioxoesters with baker s yeast under classical conditions did furnish the expected thiols, but with lower enantiomeric purity and moderate conversion rate, due to the competitive hydrolysis of the thioxo group into a carbonyl leading to an alcohol. However, conditions (ethyl acrylate, dry yeast) were found to improve the production of (S)-ethyl 3-mercaptobutanoate. Cyclic thioxo esters led to high stereoselectivity of cis (1S,2S) products, but with moderate chemical yields. 

So far we have considered sulfur-containing carbanions obtained by metallation with appropriate bases of a to sulfur atom(s) acidic C-H bonds. For synthetic purposes this method has been by far the most used. Among the variety of other routes to such carbanions [203] we shall consider two methods which seem to be general and promising the reductive lithiation of phenyl thioethers and the thiophilie addition of organometallics to thiocarbonyl compounds (vide infra Section 4.1.2). 

Radical dehydroxylation is most effective for secondary alcohols, including those derived from carbohydrates, in which traditional methods such as tosylation (or mesylation) and L1A1H4 reduction often fail. The reaction tolerates many different functional groups, as illustrated in the reduction of the thiocarbonyl compound 8 (4.8). ... 

MPc modified CPFs have been employed by several researchers for the analysis of thiols. Carbon paste electrodes incorporating OMo (OH)Pc catalyzed the oxidation of cysteine with a considerable reduction in overpotential Table 7.2. The oxidation of cysteine was mediated by Mo Pc species. CoPc modified CPMF was used as a sensor for analysis of cysteine and glutathionine in urine using electrophoresis " , with detection limits of 3.1 x 10 and 3.0x10 M, respectively. CoPc modified CPE was employed for the potentiometric oxidation of 2-mercaptobenzimidazole and 2-mercaptobenzothiazole . Low peak potentials of 0.45 V for the former and 0.27 V for the latter were obtained . A detection limit of 5 x 10 M was obtained. CPFs modified with CoPc showed catalytic activity for the determination of thiocarbonyl compounds (thiourea, thioac-etamide, thiobenzamide, and dithiooxamide) " and the oxidation of thioglyconic acid " . The latter occurred via a two step process, which leads to the formation of the dimer of the thiol " . A carbon paste electrode constructed from NiTSPc immobilized on silica gel modified with 1102 catalyzed the oxidation of cysteine " . The immobilization caused the increase in the monomeric form of the catalysts. 

Cyclohexyl xanthate has been used as a model compound for mechanistic studies [43]. From laser flash photolysis experiments the absolute rate constant of the reaction with (TMS)3Si has been measured (see Table 4.3). From a competition experiment between cyclohexyl xanthate and -octyl bromide, xanthate was ca 2 times more reactive than the primary alkyl bromide instead of ca 50 as expected from the rate constants reported in Tables 4.1 and 4.3. This result suggests that the addition of silyl radical to thiocarbonyl moiety is reversible. The mechanism of xanthate reduction is depicted in Scheme 4.3 (TMS)3Si radicals, initially generated by small amounts of AIBN, attack the thiocarbonyl moiety to form in a reversible manner a radical intermediate that undergoes (3-scission to form alkyl radicals. Hydrogen abstraction from the silane gives the alkane and (TMS)3Si radical, thus completing the cycle of this chain reaction. 

Under the action of zinc and hydrochloric acid the compound suffers reduction to hydrogen sulphide and hydrogen selenide. Chlorine reacts with it to form thiocarbonyl tetrachloride and selenium tetrachloride bromine acts analogously, except that under certain conditions the compound C 2S2SeBr6 may be formed.6 With ammonia the products of reaction are ill-defined. 

Radical chain chemistry is often employed for the transformation of an alcohol to the corresponding deoxy derivative. The secondary alcohol 1 is first converted into a suitable thiocarbonyl derivative. The first derivatives investigated were thioxobenzoates 2, xan-thates 3, and thiocarbonylimidazolides 4 (Scheme 2). On reduction with tributyltin hydride, these derivatives afforded a good yield of the appropriate deoxy compounds [8-10]. 

Support-bound 1,2-diamines can be readily converted into imidazolidinones by treatment with carbonyl diimidazole [128,129]. The required diamines have been prepared on cross-linked polystyrene by reduction of peptides bound to MBHA resin with borane. Similarly, bicyclic imidazolines have been prepared from triamines and thiocarbonyl diimidazole (Entry 10, Table 14.3). Dehydration of polystyrene-bound monoacyl ethylene-1,2-diamines yields 4,5-dihydroimidazoles (cyclic amidines, Entry 5, Table 13.18). Several groups have reported the synthesis of 2-aminoimidazol-4-ones from resin-bound amino acid derivatives (e.g., Entry 6, Table 15.11). Most of these compounds are, however, unstable, and slowly decompose if dissolved in DMSO (Jesper Lau, private communication). 

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