Dimethyliodonium Hexafluoroantimonate

Me2X+ Y-
(Me2I+ SbF6-)

[24400-13-3]  ·  C2H6F6ISb  · Dimethyliodonium Hexafluoroantimonate  · (MW 392.73) (Me2Br+ Sb2F11-)

[24400-14-4]  ·  C2H6BrF11Sb2  · Dimethylbromonium Undecafluorodiantimonate  · (MW 562.48) (Me2Cl+ Sb2F11-)

[24400-15-5]  ·  C2H6ClF11Sb2  · Dimethylchloronium Undecafluorodiantimonate  · (MW 518.03) (MeF.SbF5)

[24400-12-2; 35873-33-7]  ·  CH3F6Sb  · Fluoromethane-Antimony Pentafluoride  · (MW 250.79) (Me2F+)

[64710-12-9]  ·  C2H6F  · Dimethyliodonium Hexafluoroantimonate  · (MW 49.08) (MeOSO+)

[59970-36-4]  ·  CH3O2S  · Dimethyliodonium Hexafluoroantimonate  · (MW 79.11) (MeOSO+ SbF6-)

[85611-60-5]  ·  CH3F6O2SSb  · Methoxysulfoxonium Hexafluoroantimonate  · (MW 314.86)

(powerful methylating agents1)

Physical Data: Me2I+ SbF6- dec. 153-157 °C. Me2Br+ Sb2F11- dec. 74-79 °C. Me2Cl+ Sb2F11- mp 108-113 °C, dec. 116 °C.2 IR and Raman spectra indicate that R2X+ have C2v geometry and are sp3 hybridized.1 Most reactions of these species have been monitored by NMR.1 In 1H and 13C NMR spectra, the shielding order is as expected, F < Cl < Br < I. dH: d, 5.68 (MeF.SbF5/SO2F2); s, 5.63 (MeOSOClF+/SO2ClF); s, 5.50 (MeOSO+/SO2); s, 4.20 (Me2Cl+/SO2); s, 4.13 (Me2Br+/SO2); s, 3.60 (Me2I+/SO2). dC: 96.8 (MeF.SbF5/SO2F2); 81.9 (MeOSOClF+/SO2ClF); 74.9 (MeOSO+/SO2); 48.9 (Me2Cl+/SO2); 37.6 (Me2Br+/SO2); 9.5 (Me2I+/SO2).

Solubility: sol SO2, SO2ClF, etc. React with protic and heteroorganic solvents.

Preparative Methods: MeF.SbF5 and related reagents: the Me2F+ ion does not exist in solution, and the MeF.SbF5 complex is only obtained in SO2F2 and CH2F2 solution.3 When MeF and SbF5 are mixed in other solvents, solvent-derived species are obtained. For example, addition of MeF in SO2 to SbF5 in SO2 at -78 °C gives the MeOSO+ ion.3 The reaction of MeOSOCl with SOCl2.SbCl5 at -78 °C also generates the MeOSO+ ion and has the advantage of using the cheaper and safer SbCl5 and MeOSOCl (conveniently prepared from MeOH/SOCl2).4 Solutions of MeF.SbF5, etc. are not stable: on warming to 0 °C MeOSO+ SbF6- (MeF.SbF5/SO2) forms FS(OMe)2+ slowly,3 whereas MeOSO+ SbCl6- decomposes to MeCl and SO2.4

Me2X+ salts (X = Cl, Br, I): there are a large number of methods of preparing the Me2X+ ions, usually as perfluoro- or halofluoroantimonate salts. The four principal methods involve the addition of the appropriate MeX to an SO2 solution of (i) SbF5, (ii) HSbF6, (iii) AgSbF6, or (iv) MeF.SbF5 at -78 to -68 °C.1 Method (i) is generally preferred, since this leads to fewer impurities and side products. New methods to generate Me2X+ have been reported recently.5,6

Handling, Storage, and Precautions: these reagents should all be regarded as extremely hazardous, but no data on toxicity have been reported. Me2X+ (X = Cl, Br, I) can be isolated as crystalline, highly deliquescent salts,1,2 and are stable at rt under dry N2. The MeF.SbF5 complex can only be obtained in SO2F2 and CH2F2 solution,3 but MeOSO+ salts can be isolated by evaporation of SO2 solutions.3 Use in a fume hood.

Reactivity.

The general reactivity order is MeF.SbF5 >> Me2Cl+ > Me2Br+ > Me2I+; MeF.SbF5 is the strongest methylating agent known. The ability of MeF.SbF5 solutions to O-methylate the solvent (SO2, SO2F2, SO2ClF) has already been mentioned. At the other extreme, Me2I+ will not effect the same methylations as the other halonium ions below 0 °C, and often reacts less quickly than Methyl Trifluoromethanesulfonate (MeOTf) towards n-donors (heteroatoms).

s-Donors (i.e. C-alkylation).

Neat MeF.SbF5 rapidly self-condenses at rt to t-Bu+ with concomitant formation of HF.7 MeF.SbF5.SO2ClF reacts with alkanes slowly at -78 °C.8 The major product is always CH4 via hydrogen transfer. Propane and higher straight-chained hydrocarbons also form straight and branched higher homologs. The reactions are thought to proceed via 2-electron-3-center bound carbenium ions and hydrogen abstraction.

p-Donors (i.e. C-alkylation): Arenes.9

Friedel-Crafts-type methylation of benzene and toluene has been studied in SO2ClF at low temperatures. MeF.SbF5 reacts at -78 °C in <1 min, forming benzenium ions.10 Me2X+ (X = Cl, Br) reacts quickly at -50 °C, whereas Me2I+ requires temperatures above 0 °C.2 Quenching yields the methylation product. The o:m:p ratios for methylation of toluene with all the ions are similar to those obtained from MeOSO2F/SbF5 or conventional Friedel-Crafts reagents at higher temperatures.2

p-Donors: Alkenes.

Methylation of alkenes in SO2 with any of these reagents leads initially to carbenium ion formation. The reaction of MeF.SbF5.SO2 with highly substituted alkenes gives stable carbenium ions, whereas mono- or 1,2-dialkyl-substituted alkenes react further, resulting in polymerization and fluorination.10 In contrast, Me2X+ ions (e.g. Me2Br+)2,11 cause rapid polymerization under the reaction conditions studied.

n-Donors.

Reaction of Me2X+ (X = Cl, Br)2,12 and MeF.SbF510 with a range of heteroorganic compounds in SO2 or SO2ClF at -78 °C is rapid and quantitative. Methylation with Me2I+ usually requires temperatures >0 °C. In addition to methylations effected by MeOTf (often at higher temperatures), Me2X+ species also methylate RNO2, R2SO2, RCO2H, R1CO2R2, and enolizable aldehydes and ketones. The greater reactivity of MeF.SbF5 is exemplified by the formation of MeOSO2F from MeF.SbF5 and SO3.10 MeF.SbF5 has also been reported to effect Cl-, Br-, I-, N-, O-, P-, and S-methylations with a large variety of heteroorganic and inorganic substrates.10,13

Other Uses.

Dimethylhalonium ions (X = Br, I) have been used as lactone polymerization catalysts.14 1-Alkyl-2-methyleneaziridines have been successfully N-methylated with Me2Cl+ at -78 °C, whereas reaction with MeOSO2F in CHCl3 (-60 °C) results in polymerization.15 MeF.SbF5/SO2 (i.e. MeOSO+) has been reported to undergo the ene reaction with vinyl halides at -65 °C to yield b,g-unsaturated methyl sulfinates after the addition of MeOH (70-80%).16 The reaction is reversed by the addition of base.

Related Reagents.

Dimethoxycarbenium Tetrafluoroborate; O-Methyldibenzofuranium Tetrafluoroborate; Methyl Trifluoromethanesulfonate; Triethyloxonium Tetrafluoroborate.


1. Olah, G. A. Halonium Ions; Wiley: New York, 1975; Chapter 3.
2. Olah, G. A.; DeMember, J. R.; Mo, Y. K.; Svoboda, J. J.; Schilling, P.; Olah, J. A. JACS 1974, 96, 884.
3. Olah, G. A.; Donovan, D. J. JACS 1978, 100, 5163 and references therein.
4. Christie, J. J.; Lewis, E. S.; Casserly, E. F. JOC 1983, 48, 2531.
5. Minkwitz, R.; Gerhard, V. ZN(B) 1991, 46, 1470; (CA 1992, 116, 74 736e).
6. Minkwitz, R.; Gerhard, V. ZN(B) 1991, 46, 561 (CA 1991, 115, 48 794g).
7. Olah, G. A.; DeMember, J. R.; Schlosberg, R. H. JACS 1969, 91, 2112.
8. Olah, G. A.; DeMember, J. R.; Shen, J. JACS 1973, 95, 4952.
9. Olah, G. A.; AG(E) 1993, 32, 767 provides a recent review.
10. Olah, G. A.; DeMember, J. R.; Schlosberg, R. H.; Halpern, Y. JACS 1972, 94, 156.
11. Olah, G. A. Makromol. Chem. 1974, 175, 1039.
12. Olah, G. A.; DeMember, J. R. JACS 1970, 92, 2562.
13. Olah, G. A.; DeMember, J. R. JACS 1969, 91, 2113.
14. Hofman, A.; Slomkowski, S.; Penczek, S. Makromol. Chem. 1987, 188, 2027; CA 1987, 107, 176534u.
15. Jongejan, E.; Steinberg, H.; DeBoer, T. J. RTC 1978, 97, 145.
16. Peterson, P. E.; Brockington, R.; Dunham, M. JACS 1975, 97, 3517.

Roger W. Alder & Justin G. E. Phillips

University of Bristol, UK



Copyright 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.