Diethylaminosulfur trifluoride explained

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.

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, SF₄, effects the same transformation but will also convert the acyl fluoride to the trifluoromethyl derivative. For laboratory-scale operations, DAST is used in preference to SF₄, which is far less expensive but less easily handled. A slightly thermally more stable compound is morpho-DAST.^[1] 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:^[2]

Et₂NSiMe₃ + SF₄ → Et₂NSF₃ + Me₃SiFThe original paper calls for trichlorofluoromethane (Freon-11) as a solvent.^[3] Diethyl ether is a green alternative that can be used with no decrease in yield.^[4] 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^[5] was one of the few suppliers of the chemical but a number of companies now sell DAST. Carbolabs was acquired by Sigma-Aldrich in 1998.

Safety and alternative reagents

Upon heating, DAST converts to the highly explosive (NEt₂)₂SF₂ with expulsion of sulfur tetrafluoride. To minimize accidents, samples are maintained below 50 °C.^[6] Bis-(2-methoxyethyl)aminosulfur trifluoride (trade name: Deoxo-Fluor) and difluoro(morpholino)sulfonium tetrafluoroborate (trade name: XtalFluor-M) are reagents related to DAST with less explosive potential.^{[7] [8] [9]} XtalFluor-E has been jointly developed by OmegaChem Inc. and Manchester Organics Ltd. in 2009–2010.

See also

• Ishikawa reagent
• Fluorination with aminosulfuranes

Notes and References

1. Markovskii. LN. Pashinnik. VE. KIRSANOVA. NA. 1975. Sulfur Bis(dialkylamino Difluorides. Zhurnal Organicheskoi Khimii. 11. 74–77. Web of Science.
2. W. J. Middleton, E. M. Bingham "Diethylaminosulfur Trifluoride" Organic Syntheses, Coll. Vol. 6, p.440; Vol. 57, p.50. Online version
3. Middleton. William J.. 1975-03-01. New fluorinating reagents. Dialkylaminosulfur fluorides. The Journal of Organic Chemistry. 40. 5. 574–578. 10.1021/jo00893a007. 0022-3263.
4. L. N. Markovskij . V. E. Pashinnik . A. V. Kirsanov . Synthesis . 787–789 . 1973 . 10.1055/s-1973-22302 . 1973 . 12 . Application of Dialkylaminosulfur Trifluorides in the Synthesis of Fluoroorganic Compounds.
5. http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv9p0446 REACTION OF SULFOXIDES WITH DIETHYLAMINOSULFUR TRIFLUORIDE: FLUOROMETHYL PHENYL SULFONE, A REAGENT FOR THE SYNTHESIS OF FLUOROALKENES
6. 10.1016/S0022-1139(00)83974-3. Aminosulfur Trifluorides: Relative Thermal Stability . Journal of Fluorine Chemistry. 42. 1. 137–143. 1989. Messina . P. A. . Mange . K. C. . Middleton . W. J..
7. Gauri S. Lal . Guido P. Pez . Reno J. Pesaresi . Frank M. Prozonic . 10.1039/a808517j . Bis(2-methoxyethyl)aminosulfur trifluoride: a new broad-spectrum deoxofluorinating agent with enhanced thermal stability . 1999 . Chemical Communications . 215–216 . 2.
8. 10.1021/jo100504x. Aminodifluorosulfinium Salts: Selective Fluorination Reagents with Enhanced Thermal Stability and Ease of Handling†,‡. The Journal of Organic Chemistry. 75. 10. 3401–3411. 2010. l'Heureux . A. . Beaulieu . F. . Bennett . C. . Bill . D. R. . Clayton . S. . Laflamme . F. O. . Mirmehrabi . M. . Tadayon . S. . Tovell . D. . Couturier . M. . 20405933. 2869536 .
9. 10.1021/ol902039q. Aminodifluorosulfinium Tetrafluoroborate Salts as Stable and Crystalline Deoxofluorinating Reagents. Organic Letters. 11. 21. 5050–5053. 2009. Beaulieu . F. . Beauregard . L. P. . Courchesne . G. . Couturier . M. . Laflamme . F. O. . l’Heureux . A. . 19799406 . 2770860.