Silver(I) Hexafluoroantimonate

AgSbF6

[26042-64-8]  · AgF6Sb  · Silver(I) Hexafluoroantimonate  · (MW 343.62)

(metathetical reagent with organic1 and organometallic halides; mild Lewis acid catalyst for rearrangement of a-halo ketones,3 pinacols,4 epoxides,5 and propargyl esters6)

Physical Data: white crystalline powder; d 4.75 g cm-3.23

Solubility: sol (and forms 1:2 complexes with) benzene, toluene, xylene;24 sol nitromethane, liq SO2;7 insol HF, petroleum ether.7,25

Form Supplied in: anhydrous white powder; commercially available.

Analysis of Reagent Purity: the SbF6- ion can be determined as the nitron salt.26

Preparative Methods: has been made from SbF3 (or Sb2O3), AgCl, and Bromine Trifluoride,27 but is more readily prepared by reacting Antimony(V) Fluoride and Silver(I) Fluoride in anhydrous HF at -78 to 0 °C, washing with pet ether, and drying in vacuo.7

Handling, Storage, and Precautions: this heavy metal salt is hygroscopic, light sensitive, and very corrosive. Storage in tightly sealed, opaque bottles in a moisture-free atmosphere is recommended. Incompatible with strong oxidizing agents; decomposition products include HF.28 Reactions should be performed in a well-ventilated fume hood.

Metathesis.

Silver hexafluoroantimonate provides a useful alternative, as a metathetical reagent, to Silver(I) Perchlorate and Silver(I) Tetrafluoroborate.2 Thus in a similar fashion to the AgBF4 reagent, silver hexafluoroantimonate has been utilized in the generation of stable carbocation salts for solution NMR or crystal structure analysis. Examples include the 4-chloro-1,2,3,4-tetramethylcyclobutenium cation,1 the parent benzenonium cation,8 and the red crystalline salt derived from phenyl(2-chlorophenyl)dichloromethane (eq 1).9 Acylium salts, RCO+SbF6-, are available from treatment of acid halides with AgSbF6 in liq SO2.7 Stable thiobenzoyl hexafluoroantimonate salts are generated in a like manner.10 Application of this silver-assisted ionization method to chloroformates provides an interesting route to highly reactive carbocation species (eq 2).11

Rearrangements.

Silver(I) salts are noted for dehalogenative rearrangements of a variety of halide substrates, and AgSbF6 is no exception. Dehalogenative rearrangement of a-bromo ketones appears to be a particularly effective niche for this salt of a super acid. Select examples include cases of multiple hydride shifts to functionalize remote carbons (eq 3)3 and 1,2-aryl (and vinyl) shifts (eq 4).12 A recent X-ray study of stable crystalline complexes of a-bromo ketones with AgSbF6 and SbCl5 has established an interesting difference in the coordination of the soft Lewis acid (Ag+) with that of SbCl5 which accounts for the preferential activation of the C-Br bond by silver(I).13

In a 1:1 combination, Antimony(V) Chloride and AgSbF6 form a potent Lewis acid catalyst which is superior to either reagent alone in Friedel-Crafts acylations,14 in Beckmann14 and pinacol4 rearrangements, and in the formation of ethers directly from epoxides via a tandem rearrangement-reductive condensation process (eq 5).5 Trityl hexafluoroantimonate is an effective and superior catalyst in the latter case. Thiophenylation of aromatics is also promoted by the mixed catalyst system (eq 6).15

The silver(I) salt-catalyzed allylic rearrangement of propargyl esters to their allenyl counterparts16 has found increasing synthetic development with the discovery17 that extended exposure to Ag+, with heating, promotes further isomerization to the 1,3-dienylic esters. Silver(I) Trifluoroacetate17 and Silver(I) Acetate18 have been employed in this respect, but the hexafluoroantimonate salt appears to be equally effective (eq 7).6 Silver perchlorate is also reported to effect the similar isomerization and subsequent cyclization of butyne-1,4-diol monoacetates (eq 8).19

Although the use of AgClO4 as a weak Lewis acid catalyst is frequently cited, as in the facile rearrangement of substituted cyclopropenes (eq 9),20 it is not to be recommended, especially in stoichiometric quantities (e.g. eq 10)21 in view of the well-known explosive hazard of perchlorate salts in organic solvents. Occasionally, however, there are reports of this salt being the reagent of choice, an example being the macrolactonization of o-hydroxy thiol esters of 2-amino-4-mercapto-6-methylpyrimidine (eq 11).22 Otherwise, the hexafluoroantimonate or tetrafluoroborate salts of silver(I) should constitute acceptable substitutes.

Related Reagents.

Antimony(V) Chloride; Antimony(V) Fluoride; Hydrogen Fluoride-Antimony(V) Fluoride; Silver(I) Acetate; Silver(I) Tetrafluoroborate; Silver(I) Trifluoroacetate.


1. Katz, T. J.; Hall, J. R.; Neikam, W. C. JACS 1962, 84, 3199.
2. Liston, D. J.; Lee, Y. J.; Scheidt, W. R.; Reed, C. A. JACS 1989, 111, 6643.
3. (a) Bégué, J.-P.; Charpentier-Morize, M. ACR 1980, 13, 207. (b) Bégué, J.-P. JOC 1982, 47, 4268.
4. Harada, T.; Mukaiyama, T. CL 1992, 81.
5. Harada, T.; Mukaiyama, T. CL 1992, 1901.
6. Hollinshead, D. M.; Howell, S. C.; Ley, S. V.; Mahon, M.; Ratcliffe, N. M.; Worthington, P. A. JCS(P1) 1983, 1579.
7. Olah, G. A.; Kuhn, S. J.; Tolgyesi, W. S.; Baker, E. B. JACS 1962, 84, 2733.
8. Olah, G. A. JACS 1965, 87, 1103.
9. Laube, T.; Bannwart, E.; Hollenstein, S. JACS 1993, 115, 1731.
10. Olah, G. A.; Surya Prakash, G. K.; Nakajima, T. AG(E) 1980, 19, 812.
11. (a) Beak, P. ACR 1976, 9 230. (b) Beak, P.; Harris, B. R. JACS 1974, 96, 6363.
12. (a) Kume, T.; Hideharu, I.; Yamamoto, Y.; Akiba, K. TL 1988, 29, 3825. (b) See: Pelter, A.; Ward, R. S.; Balasubramanian, M. CC 1976, 151, for a similar rearrangement in the 3-bromoflavanone series.
13. Laube, T.; Weidenhaupt, A.; Hunziker, R. JACS 1991, 113, 2561.
14. Harada, T.; Ohno, T.; Kobayashi, S.; Mukaiyama, T. S 1991, 1216.
15. Mukaiyama, T.; Suzuki, K. CL 1993, 1.
16. Oelberg, D. G.; Schiavelli, M. D. JOC 1977, 42, 1804.
17. Cookson, R. C.; Cramp, M. C.; Parsons, P. J. CC 1980, 197.
18. Banks, R. E.; Miller, J. A.; Nunn, M. J.; Stanley, P.; Weakley, T. J. R.; Ullah, Z. JCS(P1) 1981, 1096.
19. Saimoto, H.; Hiyama, T.; Nozaki, H. JACS 1981, 103, 4975.
20. Padwa, A.; Blacklock, T. J.; Loza, R. JACS 1981, 103, 2404.
21. Shimizu, N.; Tanaka, M.; Tsuno, Y. JACS 1982, 104, 1330.
22. Nimitz, J. S.; Wollenberg, R. H. TL 1978, 3523.
23. Bode, H. Z. Anorg. Allg. Chem. 1951, 267, 62.
24. Sharp, D. W. A.; Sharpe, A. G. JCS 1956, 1855.
25. Clifford, A. F.; Beachell, H. C.; Jack, W. M. J. Inorg. Nucl. Chem. 1957, 5, 57.
26. Woolf, A. A.; Emeléus, H. J. JCS 1950, 1050.
27. Woolf, A. A.; Emeléus, H. J. JCS 1949, 2865.
28. The Sigma-Aldrich Library of Chemical Safety Data, 2nd ed.; Lenga, R. E., Ed.; Sigma-Aldrich: Milwaukee, 1988; Vol. 2, p 3095.

Merle A. Battiste

University of Florida, Gainesville, FL, USA



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