1,1,1,3,3,3-Hexakis(dimethylamino)-1l5,3l5-diphosphazenium Fluoride

[137334-99-7]  · C12H36FN7P2  · 1,1,1,3,3,3-Hexakis(dimethylamino)-1l5,3l5-diphosphazenium Fluoride  · (MW 359.49)

(anhydrous stable organic fluoride salt; source of homogeneous, extremely basic solutions of naked fluoride ions of unprecedented reactivity and selectivity in E2 elimination reactions;1 readily cleaves C-Si bonds, generating extremely reactive naked carbanions1)

Alternate Name: P2-F.

Physical Data: mp ca. 152 °C (dec); d 1.193 g cm-3.

Solubility: very sol benzene, fluorobenzene (with partial deactivation); sol with deactivation in protic solvents, acetonitrile, DMSO; sparingly sol toluene, dioxane, HMPA, THF; reacts rapidly with partially halogenated alkanes and slowly with CCl4.

Form Supplied in: commercially available as a 0.5 M solution in benzene.

Preparative Methods: a solution of 1,1,1,3,3,3-hexakis(dimethylamino)-1l5,3l5-diphosphazenium tetrafluoroborate1 (12.8 g, 30.0 mmol) in anhydrous MeOH (20 mL) is treated with a solution of KF (1.83 g, 31.5 mmol, dried for 12 h at 240 °C in a high vacuum) in cold MeOH (25 mL) under N2 and stirred vigorously for 10 min. The KBF4 is removed by suction. The flask is then equipped with a glass-covered stirring bar, the solution concentrated in vacuo at 40-50 °C bath temperature, purged with N2, and anhydrous i-PrOH (5 mL) is added. This sequence is repeated twice. The flask is then fitted to the cold trap of a (diffusion pump) vacuum apparatus, connected by a 10-mm flow cross-section stop-cock. The solution is concentrated in vacuo with stirring, the bath temperature being brought to 80 °C within 1 h and kept for 1 h at 80 °C and 1 h at 100 °C. For the following operations, rigorous exclusion of moisture (glove box) is essential. The completely solidified product is scratched from the surface of the flask and dried for 1 h at 120 °C. The product is slurried in absolute THF (20 mL), larger lumps are crushed with a pestle, the crystals filtered off with a D4 glass suction filter, washed with absolute THF (15 mL) and dried in a high vacuum, providing 9.7 g (90%) of colorless crystals, 98-99% in naked fluoride by titration; ca. 1% of inorganic material can be removed by filtration of the benzene solution.

Analysis of Reagent Purity: 1H NMR (benzene-d6) d 2.60; by titration in benzene with solid HPF6 salt of 1,8-bis(dimethylamino)naphthalene, with triphenylmethane as indicator.

Handling, Storage, and Precautions: store and handle so as to rigorously exclude moisture. It is thermally stable up to ca. 100 °C and insensitive to dry oxygen.

E2 Eliminations.

P2-F is the first reported, soluble source of naked fluoride.1d For substrates that tolerate its high basicity (P2-F deprotonates DMSO in equilibrium), it is one of the most effective E2 elimination bases known. Due to formation of FHF-, 2 mols are required for elimination of 1 mol of acid. Workup of reaction mixtures is achieved by precipitation of the salts formed with ether or benzene, by extracting the product from aqueous solution with diethyl ether or hydrocarbons, or by filtration over silica gel. All halides of the cation can be converted to tetrafluoroborates by precipitation from aqueous solution with NaBF4.

Very high selectivity (low nucleophilicity) and very high reactivity are the main advantages of P2-F over conventional bases. In general, reactivity is only slightly reduced when heterogeneous conditions are employed1d,2 (e.g. THF, eq 1). These conditions have the advantage of slowing down double bond migration considerably. However, reaction of primary alkyl fluorides at rt is rapid only in a homogeneous benzene solution, affording a largely equilibrated mixture of alkenes.2 Other primary alkyl halides react in THF at -78 °C, affording isomerically pure 1-alkenes1d (eq 2) (see also Potassium t-Butoxide).

Strongly shielded protons are readily attacked,3 even when other bases fail completely (eq 3).

Highly strained alkenes like dodecahedrene4 are formed with great ease (eq 4) in almost quantitative yield.

Epoxides are isomerized to allylic alcohols under relatively mild conditions (eq 5).2 The stereochemistry differs considerably from that of the reaction with Lithium Diethylamide, with trans-diaxial elimination predominating.

In benzene solution, unactivated aryl halides afford mixtures of regioisomeric aryl fluorides,2 presumably via arynes, but only with higher phosphazenium fluorides2 is partial reduction to the arene effectively suppressed.

Cleavage of Carbon-Silicon Bonds.

Allylsilanes are cleaved at -78 °C, forming naked allyl anions, which can be alkylated in situ (eq 6).1d

Related Reagents.

Phosphazene Base P4-t-Bu.

1. (a) Most efficacious procedure: Link, R.; Schwesinger, R. AG(E) 1992, 31, 850; compare also: (b) Marchenko, A. P.; Shaposhnikov, S. I.; Koidan, G. N.; Kharchenko, A. V.; Pinchuk, A. M. ZOB 1988, 58, 2230; (c) Schwesinger, R.; Schlemper, H. AG(E) 1987, 26, 1167; (d) Schwesinger, R.; Link, R. AG(E) 1991, 30, 1372.
2. Schwesinger, R.; Link, R., unpublished.
3. Wollenweber, M.; Pinkos, R.; Leonhardt, J.; Prinzbach, H., AG(E) 1994, 33, 117.
4. Prinzbach, H.; Sackers, E.; Weiler, A., unpublished.

Reinhard Schwesinger

University of Freiburg im Breisgau, Germany

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