[36121-19-4]  · C13H30FP  · Fluorotributyl(methyl)phosphorane  · (MW 236.40)

(fluorinating reagent1-3 and fluoride source for silicon displacement1)

Alternate Names: tributyl(methyl)phosphonium fluoride; tributylfluoro(methyl)phosphorane.

Physical Data: bp 74-76 °C/0.1 mmHg;4,5 mp 11-13 °C.5

Solubility: sol THF, diethyl ether, petroleum ether, benzene, cyclohexane.

Form Supplied in: not commercially available.

Preparative Methods: can be prepared in three steps from commercially available starting materials. Alkylation of Tri-n-butylphosphine with Iodomethane or Bromomethane gives a phosphonium salt6 which yields tributylmethylenephosphorane after deprotonation with n-Butyllithium in ether and subsequent product distillation.7 The tributylmethylenephosphorane is then treated with Hydrogen Fluoride or KHF2, and purified fluorotributyl(methyl)phosphorane is obtained by distillation.4

Handling, Storage, and Precautions: water sensitive; best kept at -10 °C.4

Fluoride Formation.

Fluorotributyl(methyl)phosphorane has been used as a fluorinating reagent in the displacement reactions of alkyl bromides, chlorides, and tosylates.1-3 For the fluoride displacement of benzyl chloride, fluorotributyl(methyl)phosphorane at -78 °C gives an 80% yield, whereas fluorotriethyl(methyl)phosphorane and fluorotrimethyl(phenyl)phosphorane give only a 20% yield.1

Fluorotributyl(methyl)phosphorane is a milder fluorinating reagent for sensitive functionality than Potassium Fluoride with a crown ether.8 A bromoepoxide will undergo fluoride displacement with fluorotributyl(methyl)phosphorane but will decompose under KF-crown ether conditions (eq 1).2

Secondary alkyl halides undergo elimination to afford alkenes; secondary alkyl tosylates give mixtures of alkyl fluorides and alkenes.2,3 A comparison of fluorotributyl(methyl)phosphorane with other fluorinating reagents in the displacement of the secondary alcohol 2-hydroxyoctanol and derivatives thereof was made (eq 2).3 The highest optical purity in the displacement reaction was obtained with fluorotributyl(methyl)phosphorane (X = OTs); however, this reagent also gave the most alkene (Table 1). (2-Chloro-1,1,2-trifluoroethyl)diethylamine (FAR) gave the best fluoride:alkene ratios. After publication of this comparison of fluorinating reagents, anhydrous Tetra-n-butylammonium Fluoride (TBAF) was shown to give a better fluorooctane yield (54%) and a higher optical purity than fluorotributyl(methyl)phosphorane (+14.2° vs. +13.6°).9

Aldol Condensation.

Fluorotributyl(methyl)phosphorane can act as a desilylating agent in the aldol condensation of the silyl enol ether of cyclohexanone with benzaldehyde (eq 3).1 Similar yields are obtained with the more readily available TBAF.

1. Leroy, J.; Bensoam, J.; Humiliere, M.; Wakselman, C.; Mathey, F. T 1980, 36, 1931 (CA 1981, 94, 121 645p).
2. Bensoam, J.; Leroy, J.; Mathey, F.; Wakselman, C. TL 1979, 353 (CA 1979, 91, 19 801n).
3. Leroy, J.; Hebert, E.; Wakselman, C. JOC 1979, 44, 3406.
4. Schmidbaur, H.; Mitschke, K.-H.; Buchner, W.; Stühler, H.; Weidlein, J. CB 1973, 106, 1226 (CA 1973, 79, 53 457s).
5. Schmidbaur, H.; Mitschke, K.-H.; Weidlein, J. AG(E) 1972, 11, 144.
6. Bestmann, H. J.; Kratzer, O. CB 1962, 95, 1894 (CA 1962, 57, 15 142h).
7. Schmidbaur, H.; Tronich, W. CB 1968, 101, 595 (CA 1968, 68, 59 659d).
8. Liotta, C. L.; Harris, H. P. JACS 1974, 96, 2250.
9. Cox, D. P.; Terpinski, J.; Lawrynowicz, W. JOC 1984, 49, 3216.

Thomas J. Fleck

The Upjohn Company, Kalamazoo, MI, USA

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