Trimethyloxonium tetrafluoroborate

Trimethyloxonium tetrafluoroborate
Names
	IUPAC name Trimethyloxonium tetrafluoroborate
	Other names Trimethyloxonium fluoroborate; Meerwein's salt
Identifiers
CAS Number	420-37-1 ;
3D model (JSmol)	Interactive image;
ChemSpider	2016863;
ECHA InfoCard	100.006.360
PubChem CID	2735153;
UNII	BFC3NMU8S8 ;
CompTox Dashboard (EPA)	DTXSID20883377 ;
	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 (CH₃)₃OBF₄. (It is sometimes called "Meerwein's salt" after Hans Meerwein.^[1]^[2]) This salt is a strong methylating agent, being a synthetic equivalent of CH+3. It is a white solid that rapidly degrades 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.

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⁺SbF−6, 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[]

^ ^a ^b 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 (2): 85–126. doi:10.1055/s-1982-29711. ISSN 0039-7881.

[OS-1] 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 (2): 85–126. doi:10.1055/s-1982-29711. ISSN 0039-7881.

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