Difluorotrimethylsilicate

Tris(dimethylamino)sulfonium difluorotrimethylsilicate (TSAF), THF, 0°, 5 h, 64% yield. A TES and two phenolic TIPS groups were also cleaved. 

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... 

Tris(diethylamino)sulfoniuni difluorotrimethylsilicate, [(C2H5)2N]3 S (CH3)3 S iF2 (TASF, 1). 

Hiyama and coworkers produced the same allylation compounds via the in situ reaction of l,l-difluoro-3-(dimethylphenylsilyl)propene and aldehydes with catalytic tris(diethyl-aminojsulfonium difluorotrimethylsilicate (TASF) or potassium f-butoxide (equation 85)78-79. 

Because tris(dimethylamino)sulfon1um difluorotrimethylsilicate is very hygroscopic, it is best transferred in a dry atmosphere of nitrogen or argon (dry box or glove bag). 

Tris(dimethylam1no)sulfonium difluorotrimethylsilicate is a source of soluble organic fluoride ion of high anionic reactivity. Fluoride ion from this salt and other tris(dialkylamino)sulfonium difluorotrimethyl si 11 cates has... 

This method has been used to prepare several different tris(dialkyl-amino)sulfonium difluorotrimethylsilicates, including salts with greater organic solubility such as the tris(diethylamino)sulfonium and tris(pyrrolid1ne)sulfon1um2 difluorotrimethylsilicates. The tris(dimethyl-amino)sulfonium salt, however, is highly crystalline and thus has an advantage 1n ease of preparation and purification over these other salts. 

Unactivated aryl iodides undergo the conversion Arl — ArCHj when treated with tris(diethylamino)sulfonium difluorotrimethylsilicate and a palladium catalyst.131 A number of methods, all catalyzed by palladium complexes, have been used to prepare unsymmetrical biaryls (see also 3-16). In these methods, aryl bromides or iodides are coupled with aryl Grignard reagents,152 with arylboronic acids ArB(OH)2,153 with aryltin compounds Ar-SnR3,154 and with arylmercury compounds.155 Unsymmetrical binaphthyls were synthesized by photochemically stimulated reaction of naphthyl iodides with naphthoxide ions in an SrnI reaction.156 Grignard reagents also couple with aryl halides without a palladium catalyst, by the benzyne mechanism.157 OS VI, 916 65, 108 66, 67. 

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

The addition of one of the fluorinating reagents of the bis(dialkylamino)difluoro-24-sulfanes type (see Section 8.5.) to one equivalent of a (dialkylamino)trimethylsilane results in the formation of a tris(dialkylamino)sulfonium difluorotrimethylsilicate. Alternatively, these compounds can be directly synthesized by the addition of sulfur tetrafluoride to 3 equivalents of a (dialkylamino)trimethylsilane, e.g. 1 and 2.71,74... 

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. 

Tris(dialkylamino)sulfonium difluorotrimethylsilicates can convert halides to their corresponding fluorides under mild conditions, e.g. 3 to 4. 

Allyl bromide (3 4.4 mL, 6.1 g, 0.05 mol) was added dropwise to a stirred solution of tris(pyrrolidin-l-yl)sulfonium difluorotrimethylsilicate (2 7.1 g, 0.2 mol) in anhyd MeCN (5 mL), and the reaction was stirred at rt for 2 h. The volatiles were distilled into a dry-ice trap from the solid byproduct tris(pyrrolidin-l -yl)sulfonium bromide. The distillate was fractionated through a low-temperature microcolumn to give the title compound yield 1.0 g (83%) bp — 3 to 0"C. 

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). 

The analogous threo derivatives can be made by use of tris(diethylamino)sulfonium difluorotrimethylsilicate as the catalyst (Note 14). 

Dimethylphenylsilane Silane, dimethylphenyl- (8,9) (766-77-8)8 Tris(diethylamino)sulfonium difluorotrimethylsilicate Sulfur (1+), tris(N-ethyl-ethanaminato)-, difluorotrimethylsilicate (1-) (10) (59201-86-4) 1/treo-1-(3-Hydroxy-2-methy-3-phenylpropanoyl)piperidine, (R, S )- Piperidine, 1-(3-hydroxy-2-methyl-1-oxo-3-phenylpropyl)-, (R, S )- ( )- (11) (99114-35-9)... 

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... 

SCHEME 12.17 Synthesis of a-series ganglioside glycans. TASF, tris(dimethylamino)sul-fonium difluorotrimethylsilicate. 

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