A Publication
of Reliable Methods
for the Preparation
of Organic Compounds
Annual Volume
Org. Synth. 1966, 46, 120
DOI: 10.15227/orgsyn.046.0120
[Oxonium compounds, trimethyloxonium tetrafluoroborate]
Submitted by H. Meerwein1
Checked by O. Vogl, B. C. Anderson, and B. C. McKusick.
1. Procedure
Freshly prepared triethyloxonium fluoborate2 (170 g., 0.90 mole) is dissolved in 500 ml. of anhydrous methylene chloride in a 1-l. three-necked flask equipped with a stirrer, gas-inlet tube, and drying tube (Note 1). The reaction flask is immersed in an ice bath, the stirrer is started, and 138 g. (3.00 moles) of dry dimethyl ether is passed into the solution from a tared cylinder over a period of about 2 hours. The reaction mixture is allowed to stand overnight at room temperature. An hour after the addition of dimethyl ether is complete, trimethyloxonium fluoborate begins to separate. The initially liquid product solidifies slowly.
The stirrer is replaced by a filter stick, and the supernatant methylene chloride is withdrawn from the crystalline mass of trimethyloxonium fluoborate; nitrogen is admitted through a bubbler during this operation to prevent atmospheric moisture from entering the flask. The crystals are washed with three 100-ml. portions of anhydrous methylene chloride. The flask is transferred to a dry box, and trimethyloxonium fluoborate is collected on a sintered-glass filter, dried for 2 hours in a vacuum desiccator at 25° (1 mm.), and bottled in a stream of dry nitrogen. The fluoborate is colorless; yield 114–124 g. (86–94%). Rapidly heated in an open capillary tube, it sinters and darkens, with decomposition, at 141–143° (Note 3).
2. Notes
1. In order to obtain maximum yields, all operations must be carried out under rigorously dry conditions. The apparatus should be dried in an oven at 110°, assembled while hot, and cooled in a stream of dry nitrogen. The checkers dried the methylene chloride over PA 100 silica gel (12–28 mesh) obtained from Davison Chemical Co., Baltimore, Maryland.
2. Trimethyloxonium fluoborate is less hygroscopic and keeps better than triethyloxonium fluoborate, but it should be stored at 0–5° in a tightly closed screw-cap bottle.2 So stored, it can be kept at least a few weeks.
3. The decomposition point varies widely, depending on rate of heating and apparatus. Professor S. H. Pine, California State College at Los Angeles, informed the checkers that he observed decomposition at 210–220°, with the salt totally disappearing and (CH3)2OBF3 forming on the wall of the capillary tube above the bath. This prompted the checkers to study the decomposition by differential thermal analysis. At a heating rate of 30°/min., there was an endotherm peak at 142°, with sample all gone by 200°. At 15°/min., the peak was at 155°, with sample all gone by 180°.
3. Discussion
The procedure used is essentially that described by Meerwein and co-workers.3 The salt has also been prepared from the same reagents in a sealed tube.4
Curphey has described a convenient synthesis from boron trifluoride diethyl etherate, dimethyl ether, and epichlorohydrin.5
4. Merits of the Preparation
This facile preparation is suitable for preparation of large amounts of salt. Like triethyloxonium fluoborate,2 trimethyloxonium fluoborate is a potent alkylating agent. In comparison with trimethyloxonium 2,4,6-trinitrobenzenesulfonate,6 trimethyloxonium fluoborate is easier to make but does not keep quite as well on storage.

References and Notes
  1. Deceased October 24, 1965; formerly at University of Marburg, Marburg, Germany.
  2. H. Meerwein, this volume, p. 1080.
  3. H. Meerwein, P. Borner, O. Fuchs, H. J. Sasse, H. Schrodt, and J. Spille, Ber., 89, 2071 (1956).
  4. H. Meerwein, E. Battenberg, H. Gold, E. Pfeil, and G. Willfang, J. Prakt. Chem., [2] 154, 143 (1939).
  5. T. J. Curphey, Org. Syntheses, 51, 142 (1971).
  6. G. K. Helmakamp and D. J. Pettitt, this volume, p. 1099.

Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)

Epichlorohydrin (106-89-8)

nitrogen (7727-37-9)

dimethyl ether (115-10-6)

methylene chloride (75-09-2)

boron trifluoride diethyl etherate (109-63-7)

Triethyloxonium fluoborate (368-39-8)

trimethyloxonium fluoborate (420-37-1)

Trimethyloxonium 2,4,6-trinitrobenzenesulfonate (13700-00-0)

Oxonium compounds, trimethyloxonium tetrafluoroborate