Tetra-n-butylammonium fluoride
Tetra-n-butylammonium fluoride, commonly abbreviated to TBAF and n-Bu4NF, is a quaternary ammonium salt with the chemical formula (CH3CH2CH2CH2)4N+F−. It is commercially available as the white solid trihydrate and as a solution in tetrahydrofuran. TBAF is used as a source of fluoride ion in organic solvents.[1]
Preparation and properties
TBAF can be prepared by passing hydrofluoric acid through an ion-exchange resin, followed by tetrabutylammonium bromide. Upon evaporation of the water, TBAF can be collected as an oil in quantitative yield.[1]
Preparing anhydrous samples is of interest as the basicity of fluoride increases by more than 20 pK units on passing from aqueous to aprotic solvent.Script error: No such module "Unsubst". However, heating samples of the hydrated material to 77 °C under vacuum causes decomposition to the hydrogen difluoride salt.[2] Similarly, samples dried at 40 °C under high vacuum still contain 10-30 mol% of water and some 10% of difluoride.[3] Instead, anhydrous TBAF has been prepared by the reaction of hexafluorobenzene and tetrabutylammonium cyanide. Solutions of the salt in acetonitrile and dimethyl sulfoxide are stable.[4]
Reactions and uses
Because the fluoride ion is such a strong hydrogen bond acceptor, its salts tend to be hydrated and of limited solubility in organic solvents. As a fluoride ion source, TBAF solves this problem, although the nature of the fluoride is uncertain because TBAF samples are almost always hydrated, resulting in the formation of bifluoride (HF2−) hydroxide (OH−) as well as fluoride. Many applications tolerate heterogeneous or ill-defined fluoride sources.
As a fluoride source in organic solvents, TBAF is used to remove silyl ether protecting groups. It is also used as a phase transfer catalyst and as a mild base. As a deprotecting agent, TBAF in DMSO will convert O-silylated enolates into carbonyls. With C-Si bonds, TBAF gives carbanions that can be trapped with electrophiles or undergo protonolysis.[1][5]
TBAF is also used, when in an organic solvent, as an activator compound to allow super glue to stick to low surface energy polymers such as polyethylene and polypropylene. [6]
References
Further reading
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- ↑ Nina Gommermann and Paul Knochel "N,N-Dibenzyl-N-[1-cyclohexyl-3-(trimethylsilyl)-2-propynyl]-amine from Cyclohexanecarbaldehyde, Trimethylsilylacetylene and Dibenzylamine" Org. Synth. 2007, 84, 1. Script error: No such module "doi".
- ↑ European patent application/publication EP0333448A2