Diethylaminosulfur trifluoride

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Diethylaminosulfur trifluoride[1]
IUPAC name	N,N-Diethylaminosuflur trifluoride
Abbreviations	DAST
Identifiers
CAS number	[38078-09-0]
SMILES	FS(F)(F)N(CC)CC
Properties
Molecular formula	C4H103NS
Molar mass	161.19 g mol-1
Appearance	Colourless oil
Density	1.220 g/cm3
Boiling point	30-32 °C at 3 mmHg
Solubility in water	Reacts violently
Hazards
Main hazards	Very hazardous, can be explosive
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Diethylaminosulfur trifluoride (DAST) is a nucleophilic fluorinating reagent used in organic chemistry that was popularized by Bill Middleton at DuPont. It is a colorless liquid that discolors on standing.

Contents 1 Use 2 Synthesis 3 Safety 4 References

[edit] Use

In particular, it will convert alcohols to the corresponding alkyl fluorides and aldehydes and unhindered ketones to geminal difluorides. Carboxylic acids react no further than the acyl fluoride; sulfur tetrafluoride must be used for the transformation —CO₂H → —CF₃. DAST is used in preference to the more classical fluorinating reagent sulfur tetrafluoride, since as a liquid it is easier handled than SF₄ gas, and acid-labile substrates are less likely to rearrangement and elimination. Reaction temperatures are lower as well - alcohols typically react at -78 °C and ketones around 0 °C.

[edit] Synthesis

DAST is prepared by the reaction of diethylaminotrimethylsilane and sulfur tetrafluoride:^[2]

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. Ether may be used instead with no decrease in yield.

[edit] Safety

Upon heating, DAST dismutates to SF₄ and (NEt₂)₂SF₂, a high-boiling and explosive compound. To prevent accidents samples must not be heated above 50 °C when distilling them.

More modern aminosulfur trifluoride reagents, such as bis-(2-methoxyethyl)aminosulfur trifluoride, decompose only at higher temperatures and pose much less of a hazard when purifying them by distillation.

[edit] References