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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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Dithioester
Yield (%)
CSjMc
-^
CSjMe n-CgHI70
OCHaOMe
CSjMe
OCH2OMe
*
c..
From [258] with permission.
The commercially available diethylaminosulfur trifluoride, Et3NSF3 (DAST), was shown to be a particularly convenient fluorination reagent (for a review, see [259]). The sequence shown here was used for the preparation of a,a-difluoroethers [138].
S F F
Lawessons reagent .i DAST \/
r'^^OR2 toluene. 110 R1 /^OR2 CH2C12. 25C * R1 OR2
Fluorination of thionesters with DAST
A solution of thionester (1.0 mmol) in dry dichloromethanc (5 ml) was placed in a round-bottomed flask with a stir bar and a rubber septum cap. The solution was stirred at room temperature under a nitrogen atmosphere as a solution of DAST in dichloromethanc (1 M, 2.0 ml, 2 eq) was added by a syringe. Stirring was continued for 6 h, after which time the reaction mixture was moled in an ice water bath and quenched by addition of saturated NaHC03 (15 ml). The organic layer was separated, and the aqueous phase was extracted twice with dichloromethane (5 ml). The combined organic layers were dried over Na2S04, and the bulk of the solvent was removed by distillation through a Vigreux column on a steam bath. The residual oil was purified by bulb-to-bulb distillation. Yields: 71-88%.
From [138] with permission.
Thioethers were a-fluorinated by DAST in dichloromethane at ambient temperature [260]. A catalytic amount of SbCl3 was beneficial.
A palladium-catalysed reduction of ethyl thiolesters to aldehydes with triethylsilane has been proposed by Fukuyama [2611. The reaction, coupled with a mild conversion of carboxylic acids to thiolesters (see [261], footnote 2), affords a versatile method for the overall transformation of
carboxylic acids to aldehydes. It was used in the synthesis of natural products, (+)-neothramycin A methyl ether [261] and (+)-leinamycin [262], associated in this last case with a Mukaiyama asymmetric aldol reaction of a thiolester [263].
The conversion of dithioesters into aldehydes is also easily realized through reduction to a mixed dithioacetal by tributyltin hydride [264].
r1sch2r2
DAST or DAST/SbCl3
R1SCHFR2
CH2C12

10% Pd/C
Et3SlH
O
U + EtgSlSEt R'^H
j? n-Bu3SnH f n-Bu3SnSySMe -hydrolysis" r J?
R^^ SMe UV R H cuQ CuCl2 R H
Trimethylsilyl-substituted silanethiols, such as (2) and (3), were shown to be extremely efficient radical-based reducing agents [265]. These agents
(MegSi)3SiSH (MegSiJjSUSHlMe
(2) (3)
take advantage of the reactivities of silyl radicals for abstraction or addition reactions and of thiols as hydrogen donors. A fast intramolecular rearrangement is the key to such a dual behaviour.
A* . ?
RZ + ^Si-SSlMeg ------------ R + Si-SSiMeg
Me3Si Me3Si
Si SH ------> Si-S* + RH
nO. <L
Direct conversions of sulfones to carbonyl compounds have been realized by oxidative desulfonation of the a-carbanions. Molybdenum peroxide (Mo05-Py-HMPA) [266], or bis(trimethylsilyl)peroxide (Mc3SiOOSiMe3, BTSP, Table 3.3) [267] were used as oxidants. Hwus method with BTSP is attractive and was found useful for the preparation of lfiO-lahelled compounds [267]; in this one-pot reaction Me3SiO behaves as a leaving group and a back-attacking species.
TABLE 3.3
Conversion of sulfones to aldehydes or ketones with BTSP
Sulfone Aldehyde or ketone Yield (%)
81
Ph 76
PhCH2SO,Ph PhCHO 66
y~S02Ph o 91
Ph^Cso2Ph 80
^ SP2Ph 90
[^y>-S02Ph o 83
Conversion of sulfones to aldehydes or ketones
A THF solution (4.4 ml) of benzyl phenyl sulfone (0.501 g) was placed in a dried reaction flask under a nitrogen atmosphere and cooled to -78C. After n-BuLi (1.00 ml, 2.6 in hexane) was injected into the reaction vessel, the bright-yellow solution was stirred for 15 min. Neat BTSP (0.465 g) was added, and the colour changed to reddish brown. After the solution was stirred at room temperature overnight, it was poured into an ice-cold, saturated aqueous NaHCO, solution and extracted twice with fresh ether. The combined organic extracts were dried (MgS04) and condensed under reduced pressure. The residue was chromatographed on a silica gel column, eluting with 20% EtOAc/hexanes, to provide pure benzaldehyde (0.208 g, 91%).
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