Methyl Fluoride-Antimony(V) Fluoride1

MeF/SbF5
(MeF)

[593-53-3]  · CH3F  · Methyl Fluoride-Antimony(V) Fluoride  · (MW 34.04) (SbF5)

[7783-70-2]  · F5Sb  · Methyl Fluoride-Antimony(V) Fluoride  · (MW 216.75)

(reactive methylating agent;1 Friedel-Crafts type electrophilic methylations; preparation of onium salts)

Physical Data: MeF: bp -78.2 °C; SbF5: bp 149.5 °C.

Form Supplied in: MeF: gas; SbF5: viscous liquid. Both are commercially available.

Preparative Methods: prepared by mixing equimolar amounts of freshly distilled MeF and Antimony(V) Fluoride in SO2F2 solution at low temperature.2 The reagent can also be prepared in situ in the reaction flask.

Handling, Storage, and Precautions: corrosive, moisture sensitive, and potentially carcinogenic. Thus, caution should be exercised accordingly. Use in a fume hood.

Introduction.

Methyl fluoride/antimony(V) fluoride is the most reactive methylating agent reported.1 It methylates almost all organic bases, including hydrocarbons. The structure of MeF/SbF5 has been extensively studied.2-4 MeF and SbF5 form a strong donor-acceptor complex in SO2F2 solution, but not a free methyl cation, which is too reactive to exist in the condensed state. When more nucleophilic solvents such as SO2 and SO2ClF are used, O-methylation products, Me+O=SO and Me+O=SOClF are, respectively, formed. These systems in their own right are highly effective methylating agents.

Friedel-Crafts Methylation and Related Reactions.

Aromatic compounds are readily alkylated with MeF/SbF5 in high yields (eq 1).2d In the case of toluene, a mixture of isomers was obtained with ortho:meta:para ratio of 54:18:28. Even highly deactivated perfluoronaphthalene is reported to react with MeF/SbF5 to form 1- and 2-methylheptafluoronaphthalene (2:1) in 40-50% yields (eq 2).5

Methyl fluoride/antimony(V) fluoride can add the methyl group to double bonds of alkenes to form carbenium ions (eq 3).2c The reagent also reacts with CO to form the acetylium ion. Hydrolysis of the ion yields acetic acid.2c In the presence of excess MeF, self-condensation to the t-butyl cation takes place via initial formation of ethyl fluoride through s-C-H bond insertion (eq 4).2c

Methylation of Heteroatoms.

Methyl fluoride/antimony(V) fluoride has been widely used for methylation of heteroatoms. Various nitrogen-containing compounds have been studied, including cyanogen halides,6a nitriles (eq 5),6a,6b sulfur difluoride imides,6c sulfur trifluoride imides and derivatives,6d and fluorinated imines (eq 6).6e In most cases, stable methylation salts are isolated. For example, the sulfur(VI) dication [(Me2N)2SF2]2+ is prepared upon treating NSF2NMe2 with MeF/SbF5 in SO2 solution (eq 7).6f

As mentioned, MeF/SbF5 is able to O-methylate SO2. Similarly, methylated oxonium ions are obtained from ethers (eq 8), sulfoxides, nitrates, and ketones.2a,2c,7 It is reported that even weak nucleophiles, such as sulfones, were successfully alkylated with MeF/SbF5 (eq 9).7 At higher temperature, MeF/SbF5 reacts with Copper(I) Oxide to form dimethyl ether (eq 10);8a in the presence of CO the reaction produces methyl acetate.8b

Sulfur-containing compounds are similarly alkylated with MeF/SbF5. Sulfonium salts were prepared from starting materials such as sulfides (eq 11)2d,9a,9b and thiophosphoryl halides (eq 12).9c,9d

Treatment of alkyl halides (with the exception of fluorides) with MeF/SbF5 leads to formation of alkyl(methyl)halonium ions.2c A series of cyclopropyl- as well as vinylhalonium ions have also been prepared (eq 13).10 The preparation of the methyl(trifluoromethyl)iodonium ion is interesting since CF3 is a strongly electron-withdrawing group (eq 14).11 Reaction between Br2 and MeF/SbF5 generates the dimethylbromonium ion. In contrast, the polycationic species (MeI2)nn+(SbF6-)6 are obtained upon the treatment of MeF/SbF5 with I2 (eq 15).12

Related Reagents.

Dimethyliodonium Hexafluoroantimonate; O-Methyldibenzofuranium Tetrafluoroborate; Methyl Trifluoromethanesulfonate; Trimethyloxonium Tetrafluoroborate.


1. Roberts, R. M.; Khalaf, A. A. Friedel-Crafts Alkylation Chemistry Dekker: New York, 1984.
2. (a) Olah, G. A.; Donovan, D. J. JACS 1978, 100, 5163. (b) Olah, G. A.; Donovan, D. J.; Lin, H. C. JACS 1976, 98, 2661. (c) Olah, G. A.; DeMember, J. R.; Schlosberg, R. H.; Halpern, Y. JACS 1972, 94, 156. (d) Olah, G. A.; DeMember, J. R.; Schlosberg, R. H. JACS 1969, 91, 2112.
3. (a) Calves, J. Y.; Gillespie, R. J. JACS 1977, 99, 1788. (b) Gillespie, R. J.; Riddell, F. G.; Slim, D. R. JACS 1976, 98, 8069. (c) Calves, J. Y.; Gillespie, R. J. CC 1976, 506. (d) Bacon, J.; Gillespie, R. J. JACS 1971, 93, 6914.
4. Peterson, P. E.; Brockington, R.; Vidrine, W. JACS 1976, 98, 2660.
5. (a) Shteingarts, V. D. In Synthetic Fluorine Chemistry Olah, G. A.; Chamber, R. D.; Prakash, G. K. S. Eds. Wiley: New York, 1992; pp 259. (b) Dobronravov, P. N.; Shteingarts, V. D. JOU 1981, 17, 2005.
6. (a) Minkwitz, R.; Meckstroth, W. Z. Anorg. Allg. Chem. 1992, 613, 93. (b) Minkwitz, R.; Meckstroth, W. Z. Anorg. Allg. Chem. 1992, 618, 139. (c) Henle, H.; Mews, R. CB 1982, 115, 2935. (d) Bartsch, R.; Henle, H.; Meier, T.; Mews, R. CB 1988, 121, 451. (e) Henle, H.; Geisel, M.; Mews, R. JFC 1984, 26, 133. (f) Henle, H.; Hoppenheit, R.; Mews, R. AG(E) 1984, 23, 507.
7. Abdel-Malik, M. M.; Dean, P. A. W.; King, J. F. CJC 1984, 62, 69.
8. (a) Olah, G. A.; Bukala, J. JOC 1990, 55, 4289. (b) Olah, G. A.; Bukala, J. JOC 1990, 55, 4293.
9. (a) Minkwitz, R.; Gerhard, V. Z. Anorg. Allg. Chem. 1990, 591, 143. (b) Minkwitz, R.; Gerhard, V. ZN(B) 1991, 46b, 265. (c) Minkwitz, R.; Garzarek, P.; Medger, G. Z. Anorg. Allg. Chem. 1992, 612, 40. (d) Minkwitz, R.; Medger, G. Z. Anorg. Allg. Chem. 1991, 604, 105.
10. (a) Olah, G. A.; Prakash, G. K. S.; Bruce, M. R. JACS 1979, 101, 6463. (b) Prakash, G. K. S.; Bruce, M. R.; Olah, G. A. JOC 1985, 50, 2405.
11. Minkwitz, R.; Gerhard, V. ZN(B) 1991, 46b, 884.
12. Minkwitz, R.; Gerhard, V. ZN(B) 1991, 46b, 1470.

George A. Olah, G. K. Surya Prakash, Qi Wang & Xing-ya Li

University of Southern California, Los Angeles, CA, USA



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