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Sulfur reagents in organic - Metzner P.

Metzner P., Thuiller A. Sulfur reagents in organic - Academic press, 1994. - 200 p.
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l_r=N «2™- T2 HaS r I
H© R'^XR2 R^XR2
(3) x=o
(4) X=S
The thionation of esters and thiolesters by thiophosphorus derivatives and mainly by Lawesson’s reagent is another general process (Table 2.2) [136,137].
Synthesis of thionoesters from esters and Lawesson’s reagent
Thionoester (3) Yield Reaction Solvent B.p. Ref.
(%) time (h) (°C/torr)
s 91 25 Xylene 80-85/12 [136]
S 72 36 Toluene [138]
n- CyH OMe
s 74 26 Toluene [138]
6 80 1 Toluene [139]
s 81 22 Toluene [1381
S 87 24 Xylene 110-112/10 [136]
80 26 Toluene [138]
13] x=o
14] X=S
L.R - Lawesson’s reagent:
General procedure for the reactions of esters with Lawesson’s reagent
The ester (0.01 mol) and Lawesson’s reagent (0.012 mol) in anhydrous xylene (10ml) were heated at 140°C until no more ester could be detected (TLC). After cooling to room temperature, the reaction mixture was placed on a silica gel column and the xylene was eluted with light petroleum. On a renewed elution with ether/light petroleum (5:95), the O-substituted thioesters (3) were isolated.
From [136] with permission.
For 5-substituted thioesters the total conversion to dithioesters (4) was obtained in refiuxing toluene, using 0.6 mol of Lawesson’s reagent per mole of substrate (Table 2.3) [136].
A particularly convenient one-step conversion of carboxylic acids into dithioesters [140, 141] makes use of Davy’s reagents [142], in which the R,S —P(=S) structural feature permits the two necessary functional group modifications, first OH substitution by SR2, followed by thionation of the C=0 group.
Synthesis of dithioesters from 5-thioesters and Lawesson’s reagent
Dithioester (4) Yield (%) Reaction time (h) B.p. (°C/torr) or m.p. (°C)
S 90 10 64/0.5
S 99 10 83/0.4
S 97 10 74/10
S 90 15 61
S 97 25 104/0.5
Conversion of propanoic acid into methyl propanedithioate with Davy’s reagent (R2 = Me)
A solution of propanoic acid (5.92 g, 80mmol) in 1,2,4-trichlorobenzene (80 ml) was heated at 80-100°C. Davy’s methyl reagent (12.6 g, 44 mmol), prepared according to [142], was added portionwise in 2 min. The mixture was heated at 140°C during 10 min (caution: operate under a hood and trap obnoxious vapours by bleach). It was then cooled to room temperature. The liquid which separated was distilled to give yellow methyl propanedithioate (5.93 g, 49 mmol, 61%), b.p. 57°C/12 torr.
From [ 141 ] with permission.
Better yields are obtained when the carboxylic acid is heated before addition of the reagent to minimize the formation of trialkyl tetrathiophosphate [142].
Davy’s reagents with R2 = Me, Et, i-Pr or PhCH2 have been prepared [142] and used [140, 141]. The first two are commercially available from Fluka and Aldrich.
Dithioacids themselves (R'CSSH) could be prepared in about a 40-50% yield by addition of Grignard reagents to caibon disulfide in THF as the solvent, followed by acidification in the presence of ether or pentane [143]. Hartke has reported that the yields of dithioacids could be improved by reverse addition of the Grignard reagent to carbon disulfide [144], a protocol previously used by Julia [145] in the preparation of dithioesters from allylic organometallics. A 70-80% yield of dithioacetic acid was thus achieved on a molar scale.
R*Mgx' — > ,Ä. l
H 1 - R^SH
Preparation of dithioacetic acid by addition of methylmagnesium iodide to carbon disulfide
An ether solution of methylmagnesium iodide was prepared from magnesium (24.3 g, 1 mol) and iodomethane (142 g, 1 mol) in ether (300 ml). It was added dropwise to a vigorously stirred solution of carbon disulfide cooled to 0°C (95.0 g, 1.25 mol) in THF (300 ml). It was then stirred at room temperature for 1 h. The heterogeneous mixture was cooled at 0°C and cautiously added to water (300ml) and petroleum ether (100ml). The organic layer was separated and washed with water (100ml). the organic layer was diluted with a 1:1 mixture of diethyl ether and petroleum ether (100ml) and acidified with concentrated hydrochloric acid. The aqueous phase was extracted twice with 1:1 diethyl ether/petroleum ether (2X 100 ml). The organic layers were combined, washed with water (100ml) and dried (Na;S04). The solution was distilled on a short column and under the appropriate reduced pressure to distil below 40°C. The residue (60-80 ml) distilled at 35-37°C/15 ton (65-75 g, 70-80%).
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