Lithium Tetrafluoroborate1


[14283-07-9]  · BF4Li  · Lithium Tetrafluoroborate  · (MW 93.75)

(fluoride source for cleavage of silyl ethers;2 mild Lewis acid capable of promoting acetal hydrolysis,3 rearrangement of oxaspiropentanes,4 facilitating the addition of arenesulfenyl chlorides to alkenes;5 catalysis of Diels-Alder cycloadditions;6 used as an electrolyte in oxidative cleavage of 2-oxo-1-cyclohexanones7)

Physical Data: mp 5 °C.

Solubility: sol methanol, acetonitrile.

Form Supplied in: anhydrous white solid; also available as a 1 M solution in acetonitrile. Drying: anhydrous material is obtained by overnight drying in an oven at 80-90 °C.1

Handling, Storage, and Precautions: extremely hygroscopic; should be handled and stored under an inert atmosphere; contact irritant.

Cleavage of Ethers.

LiBF4 is an effective reagent for the removal of silyl ethers.2,8,9 An excess of the salt in acetonitrile at ambient temperature effects cleavage of t-butyldimethylsilyl ethers (eqs 1-3).

Use of this reagent under more vigorous conditions results in removal of (trimethylsilyl)ethoxymethyl (SEM) ethers and methoxymethyl (MOM) ethers (eqs 4, 5).11

LiBF4 is particularly effective in the deprotection of b-trimethylsilylethyl protected glycosides (eq 6).12 Notably, the b-anomers undergo hydrolysis more rapidly than the corresponding a-anomers. Benzylidene acetals are cleaved under these conditions (eq 7).

Hydrolysis of Acetals.

Treatment of acetals with LiBF4 in wet acetonitrile provides carbonyl compounds in moderate yield.3 Aliphatic acetals (eq 8) give the corresponding carbonyl products in near quantitative yield. Cyclic dioxolane derivatives are hydrolyzed at a slower rate (eq 9). Substitution of the dioxolane further hinders the reaction (eq 10). Aliphatic aldehydes protected as 1,3-dioxanes are inert to these hydrolysis conditions.

Lewis Acid Promoted Reactions.

LiBF4 promotes a high yielding rearrangement of oxaspiropentanes to cyclobutanones (eq 11).4 In this regard, LiBF4 is reported to be superior to Lithium Perchlorate.

LiBF4 is an effective catalyst for the intramolecular Diels-Alder reaction. A single cis-fused cycloadduct was obtained in quantitative yield from the triene (eq 12).6 The use of other Lewis acids gives poor selectivity and lower yields of the bicyclic product.

LiBF4 has been employed as an electrolyte in the oxidative cleavage of 2-oxy-1-cyclohexanones, which gives good yields of the ring-opened dicarbonyl compound (eq 13).7

The Lewis acid character of LiBF4 enhances the electrophilicity of arenesulfenyl chlorides in their additions to carbon-carbon double bonds.5 In appropriately disposed bicyclic systems (eq 14), transannular bond formation is observed.

1. Shapiro, I.; Weiss, H. O. JACS 1953, 75, 1753.
2. Metcalf, B. W.; Burkhart, J. P.; Jund, K. TL 1980, 21, 35.
3. Lipshutz, B. H.; Harvey, P. F. SC 1982, 12, 267.
4. Trost, B. M.; Preckel, M. JACS 1973, 95, 7862.
5. Zefirov, N. S.; Koz'min, A. S.; Kirin, V. N.; Zhdankin, V. V.; Caple, R. JOC 1981, 46, 5265.
6. Smith, D. A.; Houk, K. N. TL 1991, 32, 1549.
7. Torii, S.; Inokuchi, T.; Oi, R. JOC 1982, 47, 47.
8. Grisenti, P.; Ferraboschi, P.; Mazocchi, A.; Santaniello, E. TL 1992, 48, 3827.
9. Magnus, P.; Mugrage, B.; DeLuca, M. R.; Cain, G. A. JACS 1990, 112, 5220.
10. Ireland, R. E.; Varney, M. D. JOC 1986, 51, 635.
11. Watanabe, H.; Mori, K. JCS(P1) 1991, 12, 2919.
12. Lipshutz, B. H.; Pegram, J. J.; Money, M. C. TL 1981, 22, 4603.

Paul J. Coleman

Indiana University, Bloomington, IN, USA

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