Thioketones reduction

Reduction of benzophenones with FeCl2/NaB(Et)3H63 or LiAlH464 or with NaBH4/CH3S03H65 gives bis-phenylmethanes in low yields. Alternatively, bidentate or monodentate aromatic mercury salts reduce thioketones to diarylmethanes.66... 

Electrochemical reduction of thioketones affords mercaptans under certain conditions. The stereochemistry of this process has not been examined. Reduction of thiocamphor (111) affords a single isomeric thiobomeol 113) but although both isomeric-mercaptans are known compounds, 14>11S>, the stereochemistry of the product was not established n6>. 

In contrast with the normal behaviour of aliphatic thioketones, 3-exo,3 -exo-(lR,l R)-bithiocamphor cannot exist as enethiol, since the latter is obtained by reduction of its 1,2-dithiine, immediately stabilized by 1,5-prototropic rearrangement.6... 

The protocol also tolerated the use of ketones and thioketones, furnishing products 65 and 66, respectively, although a reduction in yield was observed (Fig. 3.6). 

Modifications of the route described earlier are (a) the conversion of [M(CO)6] (M = Mo, W) into the chloro-bridged binuclear trianions 10, which subsequently react with S = C(C6H4OMe-p)2 to give the thioketone complexes, and (b) reduction of [W(CO)6] by sodium amalgam to give the dianion [W2(CO)i0]2 and treatment of the latter with thiobenzophenones, adamantanethione, or thiocamphor (Scheme 3).128... 

As mentioned before, it was with thioketones that the first cases of thiophilic addition were evidenced, but many other modes of reaction have been observed concurrently carbophilic addition, reduction, coupling, double addition, and enethiolization [357]. We will not discuss these reactions here, which have not been used much for synthetic purposes. 

It is necessary to have an excess of the chloride present at all times in order to prevent the formation of dibenzohydryl-disulfide, due to reduction of the thioketone with the hydrosulfide. If the chloride is added to the sodium hydrosulfide solution, a 70 per cent yield of the pure disulfide is obtained and no thioketone is formed. 

Reactions of different organometallic species with thiocarbonyi compounds have been extensively investigated and been shown to proceed both in a carbophilic and a thiophilic fashion. However, other reactions can be observed simultaneously such as reduction, double addition, coupling, deprotonation and formation of enesulfides1,226,423. A complex pattern appears in the reactions of thioketones with lithium or Grignard reagents424. The first application of Reformatsky reagents in C—C bond formation by reaction with... 

Reduction of diaryl thioketones with ytterbium metal affords a mixture of the corresponding thiols and diarylmethanes together with products arising from homocoupling reactions (equation 119)433. 

The following reactions proceed with the participation of the allylic boron system (i) allylboration and protolytic cleavage of organic compounds with multiple bonds, (ii) allylboron-alkyne condensation,598 599 (iii) reductive mono-and trans-a,a -diallylation of nitrogen aromatic compounds, (iv) disproportionation processes between tribut-2-enylborane and BX3 (X = C1, Br, OR, SR). Allylboration of carbonyl compounds, thioketones, imines, or nitriles leads to the homoallylic alcohols, thiols, or amines (Equations (136) and (137). It is most important that 1,2-addition to aldehydes and imines proceeds with high diastereoselectivity so that ( )-allylic boranes and boronates give the anti-products, while -products are formed preferentially from (Z)-isomers. 

There is apparently only one report of the dissolving metal reduction of thioketones thiobenzophe-none (107) has been reduced with excess Na-THF to give dianion (108), which on acidification gave thiol (109). The thiol was not isolated, but was oxidized with iodine to give the corresponding disulfide in 65% overall yield. The mechanism of the reduction is suggested to be the sequential addition of two electrons to the thiocarbonyl, which was confirmed by electron spin resonance spectroscopy studies of the intermediate thioketyl, and trapping dianion (108) with a variety of electrophiles. ... 

Thioketones (6) are slowly oxidised in air to the corresponding ketones, and they are reduced by the majority of common reducing agents to the thiols (29). Sodium borohydride often gives the optimum yield of the thiol. On the other hand, reduction with zinc-hydrochloric acid affords the hydrocarbon (30) the latter reaction is analogous to the Clemmensen reduction of ketones (Scheme 17). 

The preparative reduction, polarography, and cyclic voltammetry of 5-methyl-l,2-dithiole-3-thione have been studied.171 The results obtained are in agreement with a one-electron reduction of the thiocarbonyl group to a radical-anion, which could not be detected by ESR spectroscopy as it dimerizes too fast. The dimer is further reduced by a one-electron reduction to an anion in which the ring has been opened. (Scheme 12). The dianion could be isolated after methylation and was found to be identical with the compounds isolated by Maignan and Vialle by reduction with sodium sulfide.124 The dianion corresponds to the thioketonate dianion obtained by electrochemical reduction of 1,2-dithiolylium salts.172... 

Bis(alkylsulfanyl)cyclopropanes (cyclopropanone dithioacetals) are transformed to a-hydroxy-acetones in hot aqueous formic acid and to acetones in aqueous trifluoroacetic acid at ambient temperature, the latter reaction probably via reduction of an intermediate ot-methyl thioketone by methylsulfide. For example, 7 gave 8 with hot aqueous formic acid and 9 with trifluoroacetic acid. ... 

Isothiazolium salts 23c,27b and 76j-n,281 are masked 2-amino- 1-alkenyl-thioketones and can be converted with thioether (A) and metal hydrides (B, C), in a simple method, by reductive ring opening to p-enaminothioketones 22c,26b and 282a-g (Scheme 94), which are useful intermediates for the synthesis of a wide range of heterocyclic compounds. Isothiazoles are not ring-opened by complex... 

Alkenes lead to 1,4-dihydropyridazines (347) or the spiro derivatives (348). Cyclic alkenes, such as cyclopropenes give rise to diazanorcaradienes (349) or higher homologues, and reaction with alkynes yields pyridazines (350). Hetero-27r-systems such as nitriles, imines, thioketones, and N-sulflnylamines open the way to heterocyclic six-membered systems (351), (353), (357), and (356). (4-1-1) Addition reactions of isocyanides and carbenes make isopyrazoles (354) and (355) available. Reduction processes lead to 1,4- or 1,6-dihydrotetrazines (340) or (352). The reactions of 1,4-dihydrotetrazines (340) were briefly collected in Scheme 57. 

Coates et al. have extended this reaction to a method for gem-dialkylation at a methylene group adjacent to a carbonyl group. a-Phenyl thioketones can be alkylated at the carbon bearing sulfur, and the phenylthio group can be replaced by a second alkyl group by reduction-alkylation. For example, the ot-phenyl thio-ketone (1) is treated with sodium or potassium hydride in THF and then allowed... 

The general reaction mechanism has been shown to involve typical steps for cross-coupling [98, 113]. Oxidative addition of an aryl halide generates a Pd(II) species that undergoes transmetalation to form a Pd(II)-thiolate. C-S reductive elimination provides the aryl sulfide and regenerates the Pd(0) catalyst. More recently, Hartwig reported a detailed mechanistic analysis of the Pd/Josiphos system derived from different Pd precursors. The dominant Pd species were found to be off the catalytic cycle, which accounted for differences in rates between stoichiometric and catalytic reactions [114]. Thioketones are also effective thiolate nucleophiles for C-S bond formation. The reaction involves tandem Pd-catalyzed thioenolate alkylation, followed by 5-arylation (8) [102]. Presumably, the arylation process proceeds by a similar mechanism to related Pd-catalyzed transformations. 

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