Benzyltrimethylammonium fluoride

It would appear that when fluoride ion is to be used in stoichiometric amounts, benzyltrimethylammonium fluoride is the preferred source on the other hand, tetra-n-butylammonium fluoride, commercially available as its trihydrate, is more convenient in catalytic situations. However, there are difficulties. (79) in successfully dehydrating the latter source without inducing Hofmann elimination. 

A solution of benzyltrimethylammonium hydroxide (Triton B, 10 ml, 40% in MeOH) was treated with aqueous HF (ca. 8.6ml, 4.7%) until the pH reached 8-7. The solvent was removed in vacuo (ca. 1 mm), and the residue was dried at 50 °C/0.5 mmHg for 20 h. The resulting highly hygroscopic solid was powdered, and then stored in a desiccator over P2Os. 

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A mixture of finely ground benzyltrimethylammonium fluoride (125 mmol) and molecular sieves (50 g, 4 A) in THF (50 ml) was stirred for 14 h at ambient temperature, and then the mixture was cooled to 0°C. A solution of l-(trimethylsilyloxy)cyclohexene (115 mmol) and methyl bromoacetate (105 mmol) in THF (50 ml) was added with stirring over 10 min. After stirring for a further 10 min at 0 °C, stirring was continued for 11 h at ambient temperature. The reaction mixture was filtered through Celite. concentrated and distilled, to give the ketoester (75 mmol, 65%), b.p. 90-100°C/1.2mmHg. 

Similarly, benzyltrimethylammonium fluoride cleaves trimethylsilyl enol ethers to give quaternary ammonium enolates and Me3SiF (Equation Si3.5). 

Alkyl aryl ketones. The adducts (2) of aromatic aldehydes and cyanotrimethylsilane can be deprotonated with LDA (THF or ether, - 78°). The resulting anions can be alkylated by alkyl iodides, bromides, and tosylates in good yield. The products are usually not isolated but are hydrolyzed by dilute hydrochloric acid or benzyltrimethylammonium fluoride to ketones in yields mainly of 80-90%. 

A solution of 3-[l-(trimethylsilyl)cyclopropyl]cyclohex-2-enone (0.19 g, 0.91 mmol) in anhyd THF (10 mL) was added dropwise to a magnetically stirred slurry of benzyltrimethylammonium fluoride (0.18 g, 1.07 mmol) and benzaldehyde (0.24 g, 2.3 mmol) in THF (5 mL) at 0°C over 2 h. The reaction mixture was stirred for an additional 30 min and HjO (2mL) was added. The reaction mixture was transferred to a separatory funnel, diluted with EtjO (100 mL), and washed with brine (50 mL) and sat. NaHCOj (50 mL) prior to drying. Solvent evaporation gave a clear yellow oil which was chromatographed on silica gel (20% EtOAc/petroleum ether) yield 0.16 g (73%) as a clear oil which solidified to a white solid on standing at rt overnight mp 92 94.5 C. 

The reactions between the cyclopropyl anions and the aldehydes and ketones are clearly influenced by the fluoride salt used. Tetrabutylammonium fluoride gives product mixtures whose composition varies from batch to batch because the reactivity of this fluoride is sensitive to the presence of water and degradation products from the salt. These problems are largely overcome by using the more stable benzyltrimethylammonium fluoride, which in many cases substantially improves the yield of the addition products. For example, treatment of 1 -trimethyl-silylcyclopropanecarbonitrile with fluoride in the presence of benzaldehyde gave 1-[hydroxy (phenyl)methyl]cyclopropanecarbonitrile (4) in 47% yield when tetrabutylammonium... 

A number of addition reactions have been reported. For example, diethyl vinylphosphonate reacts with the enolates of cyclohexanone and 2-methylcyclohexanone, generated in situ by the treatment of silyl enol ethers with benzyltrimethylammonium fluoride, to give the corresponding Michael adducts in 31% and 44% yields, respectively. ... 

In fluoride ion catalyzed reactions of allylsilanes, aldehydes are the only carbon electrophiles that work well protodesilylation is unavoidable with most other electrophiles, even in the most rigorously dried media. The most commonly used fluoride ion source, which only needs to be present in catalytic amounts, is TBAF in THF at room temperature or under reflux, benzyltrimethylammonium fluoride may be better, and cesium fluoride in DMF, potassium fluoride with 18-crown-6 in THF, and TASF in polar... 

Benzyltrimethylammonium fluoride, C6H5CH2N(ch3)3 f . Mol. wt. 169.25 hygroscopic (manipulate in a drybox or under argon). 

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