Trimethyloxonium tetrafluoroborate

Trimethyloxonium tetrafluoroborate is the organic compound with the formula [(CH3)3O]+[BF4]−. (It is sometimes called "Meerwein's salt" after Hans Meerwein.[1][lower-alpha 1]) This salt is a strong methylating agent, being a synthetic equivalent of CH+3. It is a white solid that rapidly decomposes upon exposure to atmospheric moisture, although it is robust enough to be weighed quickly without inert atmosphere protection. Triethyloxonium tetrafluoroborate is a closely related compound.

Trimethyloxonium tetrafluoroborate
Structural formula of trimethyloxonium tetrafluoroborate
Ball-and-stick models of the component ions of trimethyloxonium tetrafluoroborate
Names
IUPAC name
Trimethyloxonium tetrafluoroborate
Other names
Meerwein's salt
Trimethyloxonium fluoroborate
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.006.360
UNII
  • InChI=1S/C3H9O.BF4/c1-4(2)3;2-1(3,4)5/h1-3H3;/q+1;-1
    Key: CZVZBKHWOFJNCR-UHFFFAOYSA-N
  • F[B-](F)(F)F.[O+](C)(C)C
Properties
[(CH3)3O]+[BF4]−
Molar mass 147.91 g·mol−1
Appearance White solid
Melting point 179.6–180 °C (355.3–356.0 °F; 452.8–453.1 K)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Preparation and reactions

The compound is prepared by the reaction of boron trifluoride with dimethyl ether and epichlorohydrin:[1]

4 Me2O·BF3 + 2 Me2O + 3 C2H3OCH2Cl → 3 [Me3O]+[BF4]− + B[(OCH(CH2Cl)CH2OMe]3

The salt hydrolyzes readily:

[Me3O]+[BF4]− + H2O → Me2O + MeOH + H+[BF4]−

Trimethyloxonium tetrafluoroborate is generally ranked as the strongest commercially available reagent for electrophilic methylation, being stronger than methyl sulfonate esters, including methyl triflate and methyl fluorosulfonate ("magic methyl").[2] Only the exotic dimethylhalonium reagents ([Me2X]+[SbF6]−, X = Cl, Br, I), methyl carboranate reagents, and the transiently-generated methyldiazonium cation (MeN+2) are stronger sources of electrophilic methyl.

Due to its high reactivity, it is rapidly destroyed by atmospheric moisture and best stored in an inert atmosphere glovebox at −20 °C. Its degradation products are corrosive, although it is considerably less hazardous than methyl triflate or methyl fluorosulfonate, on account of its lack of volatility.

References
  1. T. J. Curphey (1988). "Trimethyloxonium Tetrafluoroborate". Organic Syntheses.; Collective Volume, vol. 6, p. 1019
  2. Stang, Peter J.; Hanack, Michael; Subramanian, L. R. (1982). "Perfluoroalkanesulfonic Esters: Methods of Preparation and Applications in Organic Chemistry". Synthesis. 1982 (2): 85–126. doi:10.1055/s-1982-29711. ISSN 0039-7881.

Notes
  1. Meerwein's salt classically referred to triethyloxonium tetrafluoroborate. However, in recent years, the trimethyloxonium salt has also been called Meerwein's salt.
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