R₃O⁺BF₄ ^-: Meerwein’s Salt

Introduction

The discovery of trialkyloxonium salts with the general formula R₃O⁺BF₄ ^- is credited to Meerwein, [1] who also investigated much of their chemistry. [2] Today, many different oxonium salts are known. The most important cations are Me₃O⁺ and Et₃O⁺ whereas the most important anion is the tetrafluoroborate species followed by the more stable SbF₆ ^-, SbCl₆ ^- or PF₆ ^- analogues. [1] [3a] [b] Trialkyloxonium salts are well known for their excellent alkylating properties, particularly when applied to the alklyation of relatively weakly nucleophilic functional groups. Oxonium salts have also been employed as quarternizing agents for a variety of heterocyclic amines. One of the most significant drawbacks of Meerwein salts can be their insolubility in certain organic slovents, in which case, the use of the more soluble MeSO₂CF₃ (magic methyl) can be employed. The electrophilicity of several alkylating reagents have been demonstrated to decrease in the order of Me₂Cl⁺SbF₆ ^- > (MeO)₂CH⁺BF₄ ^- > Me₃O⁺X^- > Et₃O⁺X^-> MeSO₂CF₃ > MeSO₂F > (MeO)₂SO₂ > MeI. [4]

Preparation and Handling of Et3O+BF4 - and Me3O+BF4 - Et3O+BF4 - and Me3O+BF4 - are both commercially available. They can, however, be readily prepared from epi­chlorhydrin and BF3·OEt2.5a b It is recommended that triethyloxonium tetrafluorborate be stored in diethyl ether or dichloromethane at 0-5 °C due to its very hygroscopic properties, whereas the trimethyl salt can be stored neat in a desiccator over drierite at -20 °C for over a year without change in reactivity. Trimethyloxonium salts are non-hygroscopic, and may be easily handled in air for a short period of time.

References

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• 2 Meerwein H. In Houben-Weyl, Methoden der organischen Chemie   Thieme; Stuttgart: 1965.  4th ed. Vol. 6/3. p.325 
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• 3b Szymanski R. Wieczorek H. Kubisa P. Penczek S. J. Chem. Soc., Chem. Commun.  1976,  33 
• 4 Paquette LA. Encyclopedia of Reagents for Organic Synthesis   John Wiley & Sons; New York: 1995.  Vol. 3. p.5105 
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• 5b Curphey TJ. Org. Synth.  1971,  51:  142 
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• 10 Meerwein H. Borner P. Fuchs O. Sasse HJ. Schrodt H. Spille J. Chem. Ber.  1956,  89:  2060 
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