Reductive carbonylation thiocarbonylation reactions

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. 

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. 

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. 

Important reactions have included sulfur as nucleophile and leaving group in the Sn2 reaction (illustrated here see also Chapter 17), sulfonation of aromatic rings (Chapter 22), formation and reduction of thioacetals (Chapter 24) and La wesson s reagent for converting carbonyl groups to thiocarbonyl groups (Chapter 44). 

More recently, Wolfrom and Foster found that the d and l enantio-morphs of methyl 3,4-0-isopropylidene-2-0-[(methylthio)thiocarbonyl]-/3-arabinop3Tanoside rearrange, on pyrolysis, to the appropriate 2-methyl-thio)carbonyl] esters. The reductive desulfurization of methyl 3,4-0-isopropylidene-2- S - [(methylthio)carbonylj - 2 - thio -/3 - d - arabinopyranoside (LXVIII) with Raney nickel, to afford a low yield of methyl 2-deoxy-3,4-0-isopropylidene-/3-D-erj/intramolecular rearrangement to LXVIII, and the relationship of the transformation to the Chugaev reaction was discussed. 

Examination of the various steps in the above sequence provides considerable insight. From a thermodynamic standpoint several factors are important. In the first instance, the conceptual link between acylthiohydroxamate chemistry and the elegant Barton-McCombie reductive deoxygenation of xanthates and similar thiocarbonyl derivatives is immediately apparent, inasmuch as the reaction involves formation of the strong carbonyl bond at the expense of a weak thiocarbonyl moiety. The enhancement of the aromatic character which occurs when the pyridine nucleus... 

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