Abbreviations DAST Formula C4H10F3NS | Density 1.22 g/cm³ Appearance colourless oil | |
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Diethylaminosulfur trifluoride (DAST) is the organosulfur compound with the formula Et2NSF3. This liquid is a fluorinating reagent used for the synthesis of organofluorine compounds. The compound is colourless; older samples assume an orange colour.
Contents
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 —CO2H → —CF3). DAST is used in preference to the more classical gaseous SF4, since as a liquid it is more easily handled. A slightly thermally more stable compound is morpho-DAST. Acid-labile substrates are less likely to undergo 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.
Synthesis
DAST is prepared by the reaction of diethylaminotrimethylsilane and sulfur tetrafluoride:
Et2NSiMe3 + SF4 → Et2NSF3 + Me3SiFThe 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. Because of the dangers involved in the preparation of DAST (glass etching, possibility of exothermic events), it is often purchased from a commercial source. At one time Carbolabs was one of the few suppliers of the chemical but a number of companies now sell DAST.
Safety and alternative reagents
Upon heating, DAST converts to SF4 and (NEt2)2SF2, a high-boiling and explosive compound. To minimize accidents, samples are maintained below 50 °C. Bis-(2-methoxyethyl)aminosulfur trifluoride (Deoxo-Fluor) and morpholinosulfur trifluoride (morpho-DAST or MOST) are related reagents which are more thermally stable.
Recent alternatives have been manufactured by OmegaChem based on DAST but crystalline and with better handling properties. The XtalFluor range offers XtalFluor-E (diethylamine) and XtalFluor-M (morpholine) difluorosulfonium salts (tetrafluoroborates).
