Trimethyloxonium tetrafluoroborate

Trimethyloxonium tetrafluoroborate
Names
	IUPAC name Trimethyloxonium tetrafluoroborate
	Other names Trimethyloxonium fluoborate
Identifiers
CAS Number	420-37-1 ;
3D model (JSmol)	Interactive image;
ChemSpider	2016863;
ECHA InfoCard	100.006.360
PubChem CID	2735153;
	InChI InChI=1S/C3H9O.BF4/c1-4(2)3;2-1(3,4)5/h1-3H3;/q+1;-1 Key: CZVZBKHWOFJNCR-UHFFFAOYSA-N;
	SMILES F[B-](F)(F)F.[O+](C)(C)C;
Properties
Chemical formula	C3H9BF4O
Molar mass	147.91 g·mol−1
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

Trimethyloxonium tetrafluoroborate is the organic compound with the formula Me₃OBF₄. It is sometimes called the "Meerwein salt" after Hans Meerwein.^[1]^[2]) This salt is a strong methylating agent, being a synthetic equivalent of CH₃(+). It is a white solid that rapidly degrades upon exposure to atmospheric moisture, although it is robust enough to be weighed and dispensed quickly without the benefit of inert atmosphere protection. Triethyloxonium tetrafluoroborate is a closely related reagent.

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 C₂H₃(O)CH₂Cl → 3 Me₃O⁺BF⁻
₄ + B[(OCH(CH₂Cl)CH₂OMe]₃

The salt hydrolyzes readily:

Me₃OBF₄ + H₂O → Me₂O + MeOH + HBF₄

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').^[3] Only the exotic dimethylhalonium reagents (Me₂X⁺SbF₆^–, X = Cl, Br, I), methyl carboranate reagents, and the transiently-generated methyldiazonium cation (MeN⁺≡N) 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 fluorosulfonate, on account of its lack of volatility.

References

1 2 T. J. Curphey (1988). "Trimethyloxonium Tetrafluoroborate". Organic Syntheses. ; Collective Volume, 6, p. 1019
↑ Meerwein's salt classically referred to triethyloxonium tetrafluoroborate. However, in recent years, the trimethyloxonium salt has also been called Meerwein's salt.
↑ Stang, Peter J.; Hanack, Michael; Subramanian, L. R. (1982). "Perfluoroalkanesulfonic Esters: Methods of Preparation and Applications in Organic Chemistry". Synthesis. 1982 (02): 85–126. doi:10.1055/s-1982-29711. ISSN 0039-7881.

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[OS-1] 1 2 T. J. Curphey (1988). "Trimethyloxonium Tetrafluoroborate". Organic Syntheses. ; Collective Volume, 6, p. 1019

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

[Stang-3] Stang, Peter J.; Hanack, Michael; Subramanian, L. R. (1982). "Perfluoroalkanesulfonic Esters: Methods of Preparation and Applications in Organic Chemistry". Synthesis. 1982 (02): 85–126. doi:10.1055/s-1982-29711. ISSN 0039-7881.