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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 [447] with permission.
Thiiran intermediates were also obtained from carbonyl compounds by the use of thiomethylene transfer reagents. Sulfur extrusion yielded olefins. Meyers’s methylthio-2-oxazoIine is representative of such reagents [305].
90°C. 2 h
64%
The mechanism, which underlines the role of the oxazoline ring in the
4.3.6 Miscellanous reactions
Refoimatsky reagents were shown to undergo carbophilic addition with a variety of thiocarbonyl compounds (thioketones, dithioesters, trithiocarbonates and xanthates). Elimination of sulfur (or of an alkylthio group) led to the formation of carbon-carbon double (or single) bonds [448]. Typically:
1 X
S/'^S CeHfi. reflux s S
\ + BrZn-CH(R)-C02Et —-----------> \ /
61-71%
C02Et
The basc-catalysed condensation of aldehydes with sulfones gives a,p-unsaturated sulfones [449].
4 NaOH 1 9 4
R‘S02CH2R2 + R CHO ----------------------> R S02CR =CHR
2 PhCH2NEt3Cl
20°C. 6 h
4.4 THREE-MEMBERED RING-FORMING REACTIONS. CYCLOPROPANATION, FORMATION OF EPOXIDES
4.4.1 Use of sulfur ylides
The comparison between the reactions of a carbonyl compound with a phosphorus ylide (Wittig’s reagent) on one hand and a sulfur ylide on the other shows clearly the complementarity of these reagents in the strategy of synthesis.
Both reagents transfer a methylene group in efficient and selective pathways. So it is not surprising that sulfur ylides have been widely used as synthetic tools for the preparation of epoxides. The reactions can make use of sulfonium salts under phase transfer catalytic conditions, and the cheap and easily accessible trimethyl sulfonium methyl sulfate and triethylsulfonium ethyl sulfate were found to show a high reactivity under such conditions [450].
Trimethylsulfonium methyl sulfate
© Î
Me2S + Me2S04 ----------> Me3S. MeS04
To dimethyl sulfate (19 ml, 0.2 mol) in acetone (80 ml) was added dimethyl sulfide (15ml. 0.2mol). After a few minutes, the reaction mixture became turbid and warm. It was then cooled with cold water. The salt crystallized slowly, and after 4-5 h it was filtered. The white crystals were washed several times with acetone and dried under vacuum: 26-28 g of the very hygroscopic salt were obtained (70-75% yield), m.p. 100-104°C.
From [450] with permission.
144
4. ORGANOSULFUR COMPOUNDS AS INTERMEDIATES
1
Styrene oxide
PhCHO
© Î Me3S, MeS04
Ph,
><r
50% NaOH/CH2Cl2,2.5 h
H'
To a solution of benzaldehyde (1.06g, lOmmol) in CH2C12 (10ml) were added trimethylsulfonium methyl sulfate (2.17 g, 11.5 mmol) and 50% aqueous NaOH (5 ml). The reaction mixture was magnetically stirred at room temperature for 2.5 h. Water (20-30 ml) was added and the organic phase was decanted. The aqueous phase was extracted twice with ether (2 X 20ml). The combined organic solution was washed twice with water and dried (CaCl2). The solvents were evaporated under vacuum (rotatory evaporator, cold bath) and the epoxide distilled (1.2 g, 80%), b.p. 82°C/38 torr.
From [450] with permission.
It was not necessary to add any phase transfer catalyst. Aldehydes reacted readily at room temperature, and ketones at about 50°C.
A one-pot procedure combines the generation of trimethylsulfonium hydrogensulfate (Me3S* HS04~) from dimethyl sulfide, sulfuric acid and methanol, and its use in situ for oxirane formation with carbonyl compounds [451] (Table 4.5).
Synthesis of epoxides from carbonyl compounds and sulfonium salts
Concentrated sulfuric acid (0.316 mol) was added over lh to dimethyl sulfide (0.316 mol), the temperature of the reaction mixture being kept below 30°C. Methanol (0.156 mol) was then added over 30 min with the temperature below 35°C. After the mixture had been stirred for 5h, t-BuOH (0.048 mol) was added to it followed by potassium hydroxide (U.5S mol) in 10 equal aliquots over 2.5 h. Benzaldehyde (0.15 mol) was added after the seventh KOH aliquot4* and an alkaline pH had been achieved. The mixture was stirred at room temperature for 12 h and subjected to conventional pentane/aqueous work-up. Styrene oxide was isolated in a 55% yield.
“Ketones were added before the first aliquot of potassium hydroxide.
From [451] with permission.
The formation of an oxirane from the reaction of dimethylsulfonium methylide with conjugated carbonyl compounds was applied to
TABLE45
Oxirane formation
Carbonyl compound Yield (%)
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