Tris(dimethylamino)sulfonium Difluorotrimethylsilicate1

(R2N)3S+Me3SiF2-
(R = Me)

[59218-87-0]  · C9H27F2N3SSi  · Tris(dimethylamino)sulfonium Difluorotrimethylsilicate  · (MW 275.55) (R = Et)

[59201-86-4]  · C15H39F2N3SSi  · Tris(diethylamino)sulfonium Difluorotrimethylsilicate  · (MW 359.73)

(anhydrous fluoride ion source; synthesis of C-F compounds by nucleophilic displacement of sulfonates;3 promoter for electrophilic reactions of silyl enolates of ketones and esters;4-6 source of sulfonium cation capable of stabilizing or imparting high nucleophilic reactivity to other anions;46,8a,10a activator of vinylsilanes in Pd-catalyzed cross-coupling reactions;7 also used for generation8 and reactions10 of a- and b-halo carbanions; hydrosilylation11 and cyanomethylation14 of ketones)

Alternate Name: TASF.

Physical Data: R = Me, mp 98-101 °C; R = Et, mp 90-95 °C.

Solubility: R = Me, sol MeCN, pyridine, benzonitrile; partially sol THF. R = Et, sol THF, MeCN. Both react slowly with MeCN.

Form Supplied in: R = Me, white crystalline solid, ~90% pure; major impurity is tris(dimethylamino)sulfonium bifluoride [(Me2N)3S+ HF2-].

Analysis of Reagent Purity: mp; 19F NMR d (at 200 MHz, CFCl3 standard) TASMe3SiF2 (CD3CN) d -60.3; TASHF2 -145.8 (d, JHF = 120 Hz).

Preparative Methods: the methyl derivative is prepared by the reaction of dimethylaminotrimethylsilane and Sulfur Tetrafluoride at -70 °C to rt in ether; the precipitated solid is filtered off.1a The ethyl derivative is best prepared by the reaction of N,N-Diethylaminosulfur Trifluoride (DAST) and diethylaminotrimethylsilane.1b,11b

Handling, Storage, and Precautions: because of the extreme hygroscopic nature of this compound, it is best handled in a dry box or a polyethylene glove bag filled with high purity nitrogen. Use in a fume hood.

Introduction.

The acronym TASF has been used to refer to both (Me2N)3S+F2Me3Si- and (Et2N)3S+F2Me3Si-. To eliminate confusion, the sulfonium salt containing dimethylamino groups is referred to as TASF(Me), and the sulfonium salt containing diethylamino groups is referred to as TASF(Et). Reactions of both reagents are similar. Since both of these salts can be prepared in a rigorously anhydrous state, they have an advantage over quaternary ammonium fluorides which usually contain some water. TASF(Me) has a slight advantage over TASF(Et) in that it is highly crystalline and easier to prepare in a high state of purity, whereas TASF(Et) has an advantage over TASF(Me) in that it has greater solubility in organic solvents. The tris(dialkylamino)sulfonium cation is often referred to by the acronym TAS.

TASF is a source of organic soluble fluoride ion2 with a bulky noncoordinating counter ion (eq 1).9

Fluoride Ion Source in Nucleophilic Displacements.

TASF(Me) can be used to prepare fluorides from halides1b and sulfonates3 under relatively mild conditions (eq 2).

Generation of Enolates and Enolate Surrogates from Enol Silanes.

Enol silanes react with TASF(Et) to give highly reactive naked enolates which have been characterized by NMR and electrochemical measurements.4 These enolates, generated in situ, can be regioselectively alkylated without complications from polyalkylation or rearrangements of the alkylating agent (eq 3).

In the presence of excess Fluorotrimethylsilane, TASF(Et) catalyzes aldol reactions of silyl enol ethers and aldehydes.4 The stereochemical course of the reaction (syn selectivity, independent of the enol geometry, (Z)- or (E)-(1), eq 4), has been interpreted as arising from an extended transition state in which steric and charge repulsions are minimized.

Very potent carbon nucleophiles formally equivalent to ester enolates are generated by the interaction of TASF(Me) with unhindered trialkylsilyl ketene acetals. In contrast to lithium enolates, these TAS enolates add 1,4 (nonstereoselectively) to a,b-unsaturated ketones. These adducts can be alkylated in situ to form two new C-C bonds in one pot, or they can be hydrolyzed to give 1,5-dicarbonyl compounds (eq 5).5a

Conjugated esters undergo sequential additions to form polymers (group transfer polymerization).5b The molecular weight and the end group functionality of the polymer can be controlled by this method. Mechanistic studies indicate an associative intramolecular silicon transfer process via (2), with concomitant C-C bond formation during the polymer growth (eq 6).

Silyl enol ethers and ketene silyl acetals add to aromatic nitro compounds in the presence of TASF(Me) to give intermediate dihydro aromatic nitronates which can be oxidized with Bromine or 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone to give a-nitroaryl carbonyl compounds;6a the latter are precursors for indoles and oxindoles.6b The reaction is widely applicable to alkyl-, halo-, and alkoxy-substituted aromatic nitro compounds, including heterocyclic and polynuclear derivatives (eq 7).

Cross-Coupling Reactions.

TASF(Et) activates vinyl-, alkynyl-, and allylsilanes in the Pd-mediated cross-coupling with vinyl and aryl iodides and bromides.7 As illustrated in eqs 8-10, the reaction is stereospecific and chemoselective. This cross-coupling protocol is remarkably tolerant towards a variety of other functional groups such as carbonyl, amino, hydroxy, and nitro. Vinylsilanes can be synthesized from Hexamethyldisilane and vinyl iodides in the presence of TASF(Et) (eq 10) via cleavage of a Si-Si bond.7a Aryl iodides can also be synthesized by this method. TASF is superior to Tetra-n-butylammonium Fluoride for these reactions. In the absence of a vinylsilane reagent, one of the methyl groups from the difluorotrimethylsilicate is substituted for the halide (eq 11).7d

Generation and Reactions of Unusual Carbanions.

Both a- and b-halo carbanions are generally labile species and their generation and reactions require extremely low temperatures. TASF(Me) has been used to prepare several stable and isolable perfluorinated carbanions (eq 12)8a or alkoxides.8b As compared to the corresponding metal salts, the TAS+ counterion has little coordination to the fluorines of the anion,9 and this presumably slows the decomposition of the TAS salts to carbenoids or alkenes. Addition of a- and b-halo carbanions to carbonyl compounds may be achieved by the in situ generation of these species by the reaction of TASF(Et) with the corresponding silylated derivatives (eq 13).10

Other Applications.

TASF(Et) catalyzes the addition of Dimethyl(phenyl)silane to a-alkoxy, -acyl or -amido ketones to give the corresponding anti aldols (eq 14).11 This complements the acid-catalyzed reduction which gives the syn isomer.

Dimethyl(phenyl)silane will reduce aldehydes and ketones to hydroxyl compounds under very mild conditions in the presence of a catalytic amount of TASF(Et).11c

Aryl or vinyl anions can be generated by the reaction of the corresponding vinyl iodide with Bu3Sn anion, which in turn is produced from TASF(Et) and Bu3SnSiMe3. With an appropriately placed carbonyl group, an intramolecular cyclization ensues (eq 15).12

A useful variation of the Peterson alkenation relies on the generation of a-silyl carbanions from geminal disilyl compounds containing an additional stabilizing group (CO2R, SPh, SO2Ph, OMe, CN, Ph) at the a-carbon.13 A related reaction is the cyanomethylation of ketones and aldehydes with Trimethylsilylacetonitrile in the presence of TASF(Me).14 TASF(Me) was found to be the best fluoride ion source for the synthesis of aryl trifluoromethyl sulfones from the corresponding sulfonyl fluorides and Trifluoromethyltrimethylsilane or Me3SnCF3 (eq 16).15

The carbanions formed by scission of a C-Si bond with TASF can also be oxygenated. Benzylic trimethylsilyl groups can be converted to hydroxyl groups in 20-95% yield by reaction with TASF(Me) in the presence of oxygen and Trimethyl Phosphite (eq 17).16 No other source of fluoride ion was found that could replace TASF.


1. (a) Middleton, W. J. OS 1985, 64, 221. (b) Middleton, W. J. U.S. Patent 3 940 402, 1976 (CA 1976, 85, 6388j). See also Ref. 11(b).
2. For a review of applications of fluoride ion in organic synthesis, see: Clark, J. H. CRV 1980, 80, 429.
3. (a) Card, P. J.; Hitz, W. D. JACS 1984, 106, 5348. (b) Doboszewski, B.; Hay, G. W.; Szarek, W. A. CJC 1987, 65, 412.
4. (a) Noyori, R.; Nishida, I.; Sakata, J. JACS 1983, 105, 1598. (b) Noyori, R.; Nishida, I.; Sakata, J. TL 1981, 22, 3993.
5. (a) RajanBabu, T. V. JOC 1984, 49, 2083. (b) Webster, O. W.; Hertler, W. R.; Sogah, D. Y.; Farnham, W. B.; RajanBabu, T. V. JACS 1983, 105, 5706.
6. (a) RajanBabu, T. V.; Reddy, G. S.; Fukunaga, T. JACS 1985, 107, 5473. (b) RajanBabu, T. V.; Chenard, B. L.; Petti, M. A. JOC 1986, 51, 1704.
7. (a) Hatanaka, Y.; Hiyama, T. TL 1987, 28, 4715. (b) Hatanaka, Y.; Hiyama, T. JOC 1988, 53, 918. (c) Hatanaka, Y.; Fukushima, S.; Hiyama, T. H 1990, 30, 303. (d) Hatanaka, Y.; Hiyama, T. TL 1988, 29, 97.
8. (a) Smart, B. E.; Middleton, W. J.; Farnham, W. B. JACS 1986, 108, 4905. (b) Farnham, W. B.; Smart, B. E.; Middleton, W. J.; Calabrese, J. C.; Dixon, D. A. JACS 1985, 107, 4565.
9. For a discussion of structural aspects of TAS salts, see: (a) Farnham, W. B.; Dixon, D. A.; Middleton, W. J.; Calabrese, J. C.; Harlow, R. L.; Whitney, J. F.; Jones, G. A.; Guggenberger, L. J. JACS 1987, 109, 476. (b) Dixon, D. A.; Farnham, W. B.; Heilemann, W.; Mews, R.; Noltemeyer, M. HC 1993, 4, 287.
10. (a) Fujita, M.; Hiyama, T. JACS 1985, 107, 4085. (b) Hiyama, T.; Obayashi, M.; Sawahata, M. TL 1983, 24, 4113. See also: de Jesus, M. A.; Prieto, J. A.; del Valle, L.; Larson, G. L. SC 1987, 17, 1047.
11. (a) Fujita, M.; Hiyama, T. JOC 1988, 53, 5405. (b) Fujita, M.; Hiyama, T. OS 1990, 69, 44. (c) Fujita, M; Hiyama, T. TL 1987, 28, 2263.
12. Mori, M.; Isono, N.; Kaneta, N.; Shibasaki, M. JOC 1993, 58, 2972.
13. Palomo, C.; Aizpurua, J. M.; García, J. M.; Ganboa, I.; Cossio, F. P.; Lecea, B.; López, C. JOC 1990, 55, 2498. Also see: Padwa, A.; Chen. Y.-Y.; Dent, W.; Nimmesgern, H. JOC 1985, 50, 4006.
14. Palomo, C.; Aizpurua, J. M.; López, M. C.; Lecea, B. JCS(P1) 1989, 1692.
15. Kolomeitsev, A. A.; Movchun, V. N.; Kondratenko, N. V.; Yagupolski, Yu. L. S 1990, 1151.
16. Vedejs, E.; Pribish, J. R. JOC 1988, 53, 1593.

T. V. (Babu) RajanBabu

The Ohio State University, Columbus, OH, USA

William J. Middleton & Victor J. Tortorelli

Ursinus College, Collegeville, PA, USA



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