TASF sulfonium

TASF tris(diethylamino)sulfonium difluoro(trimethyl)silicate... 

Fluoride ion can also induce reaction of silyl ketene acetals with electrophilic alkenes. The fluoride source in these reactions is fnT-(dimethylamino)sulfonium diflu-orotrimethylsilicate (TASF). 

It was found that treatment of a mixture of 120 and 121 with tris(diethylamino-sulfonium) trimethyldifluorosilicate [TASF(Et)] resulted in smooth addition-elimination to the naphthoquinone to form the y-alkylation product 125 (85 %). TASF(Et) is a convenient source of soluble, anhydrous fluoride ion [47]. It is believed that exposure of 121 to TASF(Et) results in fluoride transfer to generate a hypervalent silicate anion, as depicted in structure 124. The transfer of fluoride between TASF(Et) and 121 may be driven by stabilization of the anionic species 124 by delocalization of the carbon-silicon bond into the LUMO of the unsaturated ketone. 1,4-Addition-elimination of this species to the naphthoquinone 120 would then form the observed product. 

Fluoride ion catalyzes the hydrosilylation of both alkyl and aryl aldehydes to silyl ethers that can be easily hydrolyzed to the free alcohols by treatment with 1 M hydrogen chloride in methanol.320 The most effective sources of fluoride are TBAF and tris(diethylamino)sulfonium difluorotrimethylsilicate (TASF). Somewhat less effective are CsF and KF. Solvent effects are marked. The reactions are facilitated in polar, aprotic solvents such as hexamethylphosphortriamide (HMPA) or 1,3-dimethyl-3,4,5,6-tetrahydro-2(l //)-pyrirnidinone (DMPU), go moderately well in dimethylformamide, but do not proceed well in either tetrahydrofuran or dichloromethane. The solvent effects are dramatically illustrated in the reaction of undecanal and dimethylphenylsilane to produce undecyloxyphenyldimethylsi-lane. After one hour at room temperature with TBAF as the source of fluoride and a 10 mol% excess of silane, yields of 91% in HMPA, 89% in DMPU, 56% in dimethylformamide, 9% in tetrahydrofuran, and only 1% in dichloromethane are obtained (Eq. 164).320... 

Fluorination with Tris(dimethylamino)suIfonium Difluorotrimethyl-silicate (TASF) and Other Tris(dialkylamino)sulfonium Difluorotrimethylsilicates... 

Tris(dimethylamino)sulfonium Difluorotrimethylsilicate (TASF, 1) Typical Procedure 74... 

Tris(dialkylamino)sulfonium difluorotrimethylsilicates are very hygroscopic solids and great care must be taken to work under strictly anhydrous conditions. They are not sufficiently reactive to fluorinate carbonyl compounds or even alcohols. However, they are very soluble in organic solvents and are useful sources of unsolvated fluoride ion. The most commonly used reagent is the commercially available TASF. 

Although tris(dialkylamino)sulfonium difluorotrimethylsilicates are not sufficiently reactive to fluorinate alcohols, they are able to convert trifluoromethanesulfonates to their corresponding fluorides under mild conditions. The mild conditions required to form the ester and to perform the fluorination step, has resulted in TASF (1) and related reagents finding wide application in the syntheses of fluorinated carbohydrates (see Table 10).5"76-78 It is of interest that this process is a modern development of the first reaction ever used to synthesize an organic fluoride in 1835 (see Introduction 1.2.). 

ArOCH(Rl)OR2 —> ArOH.1 Alkoxymethyl aryl ethers are cleaved by P2I4 in CH2C12 at 25° to phenols in 55-90% yield. This reagent is more useful for cleavage of SEM, MOM, and MEM ethers than Bu4NF, BBr3, or tris(diethylamino)sulfonium difluorotrimethylsilicate (TASF). 

However, in the presence of a catalytic amount of tris(dimethylamino)sulfonium difluorotrimethylsilicate ( TASF ), pivaldehyde imine yields the N-unsubstituted adduct, whereas acetone imine leads to the corresponding BSMA amides accompanied with the addition product of THF to DMAD. Formation of amides might be explained by the loss of HC1 from the iminium resulting from condensation of acyl chloride with imine, leading to a vinyl amide which is easily hydrolyzed. No explanation was presented for the formation of the THF adduct.241... 

C6H5(CH3)2SiH, in the presence of Bu4NF or tris(diethylamino)sulfonium difluo-rotrimethylsilicate (TASF, 13, 336) in aprotic polar solvents, particularly HMPT. The actual reagent may well be a hexavalent fluorosilicate involving HMPT. CsF and KF are far less effective than Bu4NF. With proper control of all the parameters, near quantitative yields can be achieved, and high stereoselectivity is also possible, as in the case with methylcyclohexanone. 

A limited number of vinyltrimethylsilanes and acetylenic trimethylsilanes couple with aryl and vinyl iodides in the presence of TASF (tris(diethylamino)sulfonium difluo-romethylsilicate) or TBAF (tetrabutylammonium fluoride) and ( 3-C3H5PdCl)2 catalyst249. Acetylenic trimethylsilanes couple with vinyl bromide under even milder conditions. In the absence of an organic trimethylsilane, TASF itself transfers a methyl group in reasonable yields to the aryl halide250. 

TASF tris(diethylamino)sulfonium difluoro(trimethyl)silicate TBAF tetrabutylammonium fluoride Tf trifluoromethanesulfonyl THF tetrahydrofuran tol tolyl... 

The reagent tris(dimethylamino)sulfonium difluorolrimcthylsilicate [TASF(Me)], which is commercially available, albeit expensive, has recently found application in carbohydrate chemistry as a source of nucleophilic fluoride for the substitution of triflates. For example,... 

Tris(dimethylamino)sulfonium difluorotrimethylsilicate [TASF(Me)] has found very little use in the ring-opening fluorination of epoxides. From the experimental conditions that were successfully applied in one ease (THF. reflux. 4d). it can be concluded that TASF(Mc) is not very reactive towards epoxides. 

It is a palladium-catalyzed cross-coupling reaction between organosilanes (vinyl, ethynyl and allylsilanes) and organic halides (aryl, vinyl and allyl halides). Allylpal-ladium chloride dimmer [( ri -C3H5PdCl)2] and either tris(diethylamino)sulfonium difluorotrimethylsilicate (TASF) or tetra-n-butylammonium fluoride (TBAF) are used as catalysts. Fluoride ion acts as an activator for the coupling, forming an intermediate hypervalent anionic silicon species, which can then transmetallate with palladium as a preliminary reaction to coupling. 

Trimethyl(trifluoromethyl)silanc is easily fluorodesilylated by tris(dimethylamino)sulfonium difluorotrimethylsilicate [TASF(Me)], tetrabutylammonium fluoride, or tris(diethylamino) (methylamino)phosphonium hydrogen difluoride and, if arenesulfonyl fluorides are used to trap the trifluoromethyl anion, trifluoromethyl sulfones arc formed in high yield. Trisfdiethyl-amino)(methylamino)phosphonium phenolate is also an effective catalyst for this reaction, but TASF(Me) is favored as it can achieve the quantitative transformation of ArS02F into aryl trifluoromethyl sulfones.115-117... 

MEK methyl ethyl ketone TASF tris(diethylamino)sulfonium... 

In the presence of a fluoride ion generating reagent such as tris(diethylamino)sulfonium difluorotri-methylsilicate (TASF, 99) and a palladium catalyst, vinyltrimethylsilane couples with vinyl and aryl iodides." For substituted vinylsilanes, the fluorodimethylsilyl group is particularly effective in the coupling reaction, which takes place with retention of configuration of both substrates (Scheme 17). Silicon-based C—C bond formation is as useful as that which employs organoborons " for the synthesis of stereo-defined conjugated dienes. 

SMDA sodium dihydrobis(2-methoxyethoxy)aluminate TASF tri(dimethylamino)-sulfonium difluorotrimethylsilicate TFAA trifluoroacetic anhydride... 

A variety of fluoride ion sources have been used in an effort to improve product yields in deoxyfluoro sugar synthesis. These salts include cesium fluoride, tetrabutylammonium fluoride, and tetrabutylammonium difluoride (BU4NHF2) [37,38]. And currently, the most successful fluoride source is tri(dimethylamino)-sulfonium difluorotrimethylsilicate (TASF), which is soluble in many organic solvents and produces an anhydrous fluoride ion [39]. 

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