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Sulfur reagents in organic - Metzner P.

Metzner P., Thuiller A. Sulfur reagents in organic - Academic press, 1994. - 200 p.
Download (direct link): sulfarreagents1994.djvu
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From [128] with permission.
Di-t-butyl thioketone
Di-t-butyl ketone imine (5.64 g) in dry light petroleum under dry nitrogen was treated with ethereal methyllithium (1.00 mol. eq). When methane evolution had ceased, dry carbon disulfide (3.2 ml) was added and the mixture stirred for 3h. The solvent was removed under reduced pressure at room temperature and the solid brown residue heated at 80-100C under vacuum, and practically pure thione was distilled into a solid C02-acctonc trap (100%). The residue is yellow when the distillation is complete. Redistillation afforded the pure, deep-violet, thioketone, b.p. 61C/14 torr.
From [128] with permission.
The H2S/HC1 method of thionation of ketones was efficiently applied in the syntheses of aromatic thioketones, such as thiobenzophenone [129] and hindered aliphatic thiones, e.g. thiocamphor [128, 130] and thiofenchone [128, 131]. The following convenient procedure uses trimethyl orthoformate as a co-reactant [128].
()- Thiocamphor
(+)-Camphor (15.2 g) and trimethyl orthoformate (15.9 g) in dry methanol (50 ml) were cooled to 0C and simultaneously saturated with streams of hydrogen sulfide and hydrogen chloride gases over 1.5 h. Pouring onto ice (200 ml) afforded orange crystals of the crude thione. Traces of ketone were removed by dissolving the thione in warm petroleum and chromatography on silica gel (10 g) (elution with petroleum). Concentration under reduced pressure and sublimation at 85C/20 torr afforded the pure thione (88%), m.p. 145-146C, [<x]D24 = -21.4 (c 3.0 in EtOAc).
From [128] with permission.
(+)-Thiofenchone was obtained similarly [128] from (+)-fenchone (yield 88%, deep-orange crystals, m.p. 22-24C, b.p. 105C/15 torr, [a]D21 =46.1 (c 1.2 in EtOAc)).
For simpler, less hindered aliphatic thioketones, the method of Mayer and Berthold [132] of thionation of acetals by H2S was shown [133] to lead to tautomerically pure thioketones when the transformation was carried out under sufficiently mild conditions. Thus the reaction at 0C in ethanol or THF in the presence of 0.05-0.01 eq of zinc chloride yielded reasonable yields of thioketones (2) devoid of the isomeric enethiols (2').
A solution of 4,4-dimethoxy-2,6-dimethylheptane (10 g, 53.5 mmol) and zinc chloride (0.72 g, 5.35 mmol) in ethanol (40 ml) was cooled to 0C. H2S was bubbled through the solution during 30 min. After a few minutes a red colour appeared. The extraction was executed by partition between pentane and a 0.1 N HC1 solution. The organic solution was then washed with water, dried onmgS04 and concentrated. The crude material was purified by flash chromatography with pentane as the eluent, giving 2,6-dimethylheptanethione (2d) (5.09 g, 32 mmol, 60%).
From [133] with permission.
Deprotonation of compounds (2) with LDA [133, 134] led to (Z)-cnethiolatcs which were S-silylated with trimethylsilyl chloride to give silyl
0C, 30 min
(2a) R1 = R2 - Me, R3 = i-Pr
(2b) R1 - x-C4Hs, R2 - H, R3 - -5̈́
(2c) R1 - -5, R1 - H, R3 - -QH13
(2d) R1 = i-Pr, R2 = H, R3 = CH2-i-Pr
Yield - 67%
MeOH R 20C, 1 h

vinyl sulfides, which by methanolysis yielded selectively and quantitatively enethiols (2'), compounds which are tautomerically stable under the neutral conditions used.
Selective synthesis of enethiols
n-Butyllithium (0.6-1 mmol) was added dropwise to a THF solution of diisopropylamine (1 eq) cooled to 78C. The solution was stirred for 15 min. The thioketone (2) (1 eq) was added dropwise. The red colour disappeared. After stirring for 5-10 min, trimethylsilyl chloride (2 eq) was added in one portion. The mixture was stirred for 15 min at -78C and then allowed to warm at room temperature. It was quenched with a saturated NH4C1 aqueous solution and extracted with pentane. The organic layer was washed with water, dried over mgS04 and concentrated. The silyl vinyl sulfide thus obtained was stirred with methanol (4 ml) at room temperature for lh. Essentially pure enethiol (2') was obtained quantitatively by concentration of the methanol solution.
From [133] with permission.
It must be noted that this selective synthesis of enethiols from thioketones is restricted to cases for which regio- and stereoselective deprotonation occurs, as was achieved above with the symmetrical thiones (2a)-(2d).
2.8.3 Thionoesters and dithioesters
These compounds are reasonably stable and surprisingly easy to prepare and to handle. Conjugation between the double bond of the thiocarbonyl group and the lone pair on the a heteroatom leads to a good balance between stability and reactivity for these functional groups.
A ,A 2
R1 OR2 R1 SR2 (3) (4)
As mentioned previously, there are reviews which cover the many synthetic methods available [119, 120], and only a few specific aspects of some of the main routes are discussed here: for both (3) and (4) the most important method of preparation relies upon the reactivity of imidooesters and imidothioesters towards hydrogen sulfide [135].
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