Diethylaminosulfur trifluoride (DAST) is the organosulfur compound with the formula Et2NSF3. This colorless liquid is a fluorinating reagent used for the synthesis of organofluorine compounds.
Use in organic synthesis
DAST converts alcohols
to the corresponding alkyl fluorides as well as aldehydes and unhindered ketones to geminal difluorides. Carboxylic acids react no further than the acyl fluoride (sulfur tetrafluoride
effects the transformation —CO2
H → —CF3
). DAST is used in preference to the more classical gaseous SF4
, since as a liquid it is more easily handled. Acid-labile substrates are less likely to rearrangement and elimination since DAST is less prone to contamination with acids. Reaction temperatures are milder as well - alcohols typically react at -78 °C and ketones around 0 °C.
DAST is prepared by the reaction of diethylaminotrimethylsilane and sulfur tetrafluoride:
- Et2NSiMe3 + SF4 → Et2NSF3 + Me3SiF
The Organic Syntheses protocol calls for trichlorofluoromethane as a solvent, a compound that has been banned under the Montreal Protocol and is no longer available as a commodity chemical. Diethyl ether may be used instead with no decrease in yield.
Safety and alternative reagents
Upon heating, DAST converts to SF4
, a high-boiling and explosive compound. To minimize accidents samples are maintained below 50 °C. bis
-(2-methoxyethyl)aminosulfur trifluoride is more thermally robust.