Boron Trifluoride-Acetic Acid


[7637-07-2]  · BF3  · Boron Trifluoride-Acetic Acid  · (MW 67.81) (BF3.MeCO2H)

[-]  · C2H4BF3O2  · Boron Trifluoride-Acetic Acid  · (MW 127.86) (BF3.(MeCO2H)2)

[373-61-5]  · C4H8BF3O4  · Boron Trifluoride-Acetic Acid  · (MW 187.91) (MeCO2H)

[64-19-7]  · C2H4O2  · Boron Trifluoride-Acetic Acid  · (MW 60.05)

(mild Lewis acid)

Physical Data: solid monoacetic acid complex; liquid diacetic acid complex, d 1.353 g cm-3.

Solubility: decomposes in water exothermically.

Form Supplied in: liquid diacetic acid complex 98%.

Preparative Methods: boron trifluoride monoacetic acid complex is made by passing Boron Trifluoride into anhydrous Acetic Acid in dichloroethane until a solid is produced; the liquid diacetic acid complex remains in solution. A solid with a composition of ca. 80 mol % absorption can be obtained by passing boron trifluoride into anhydrous acetic acid.2

Handling, Storage, and Precautions: extremely corrosive to the eyes and skin. If swallowed causes severe internal irritation and damage. Toxic irritating vapor. All work must be carried out in an efficient fume hood.

Desilylation Reactions.

These reactions are based on the well-known stabilization of a b-carbenium center by silicon. The conversion of allylsilanes into terminal alkenes using the boron trifluoride-diacetic acid complex has been studied in considerable detail. The reaction occurs by protonation, followed by nucleophile induced desilylation. The principle involved is shown in eq 1.1 The stereochemical implications were considered in a detailed account, and in the deuteration-desilylation sequence shown in eq 2 the structure shown is the major product.2 The method has been used as part of a total synthesis of carbocyclic cytochalasans, where the conversion of an endocyclic alkene into an exocyclic isomer was achieved in an 80% yield.3 Protonation-desilylation of alkynes to give allenes in yields ranging from 70 to 97% evidently can be rationalized as involving a b-vinyl cation. An example is shown in eq 3.4

The formation of b-silicon-stabilized cations from g-hydroxysilanes results from the normal pattern of pinacol rearrangements where hydride and phenyl migration is common. Desilylation then also affords an alkene.5 Where the alkyl group is a ring residue, only hydride migration was observed, as shown in eq 4. Where a suitably positioned aryl residue can cyclize onto a second carbenium center, Friedel-Crafts cyclization takes place (eq 5). The full paper reports an additional number of interesting examples and also indicates that, while precedent might suggest that cyclopropane formation should be possible, none was observed.6

The phenyldimethylsilyl group can be converted in two steps into a hydroxy group and as such can be regarded as a masked hydroxy group. The first step involves the proto-desilylation of the phenyl group, which is then followed by a peracid-mediated rearrangement into a hydroxy group, with retention of configuration.7 The sequence works well for primary, secondary, and tertiary silanes; the presence of ketone and ester functions does not cause a problem. Examples are shown in eqs 6-8.7 The proto-desilylation-oxidation sequence has also been used in a formal synthesis of thienamycin.8

Destannylation Reactions.

The absence of cyclopropane formation in the silicon-based reactions reviewed above prompted an investigation of related tin chemistry. A large number of examples have been reported where cyclopropanes are formed in good yields and with remarkable stereoselectivity.9 The yields are high even when the reacting centers are fully substituted (eq 9) and inversion of configuration occurs at both reacting centers. In the case of secondary benzyl alcohols the cyclopropanes are formed apparently with complete stereospecificity (eq 10).9a

Miscellaneous Reactions.

Many of the other reactions using boron trifluoride/acetic acid complexes have been covered in the article on Boron Trifluoride-Acetic Anhydride, which should be read in conjunction with this article. The hydrolysis of nitriles is an example,10 but in the case of the hydrolysis to benzamide derivatives acylation a to the amide residue is not possible, and so as shown in eq 11, the addition of acetic anhydride would have been superfluous.11 The Fries rearrangement is also discussed in connection with reactions of boron trifluoride and acetic anhydride, but in those cases the liberated phenolic group is acylated. Reaction also occurs using the boron trifluoride diacetic acid complex, as shown in eq 12.12

There are a large number of methods for the cleavage of ethers and esters that are based on boron trifluoride reagents and many of these are covered in other articles. An example that uses the boron trifluoride-acetic acid complex is concerned with protection of carboxy groups in peptides by means of the diphenylmethyl group.13 The S-S bond in cystine is stable to the deprotection conditions.

1. Fleming, I.; Paterson, I. S 1979, 446.
2. Fleming, I.; Lewis, J. J. JCS(P1) 1992, 3267.
3. Vedejs, E.; Reid, J. G. JACS 1984, 106, 4617.
4. Pornet, J.; Damour, D.; Miginiac, L. JOM 1987, 319, 333.
5. Fleming, I.; Patel, S. K. TL 1981, 22, 2321.
6. Fleming, I.; Patel, S. K.; Urch, C. J. JCS(P1) 1989, 115.
7. (a) Fleming, I.; Henning, R.; Plaut, H. CC 1984, 29. (b) Crump, R. A. N. C.; Fleming, I.; Hill, J. H. M.; Parker, D.; Reddy, N. L.; Waterson, D. JCS(P1) 1992, 3277.
8. Fleming, I.; Kilburn, J. D. CC 1986, 1198.
9. (a) Fleming, I; Urch, C. J. TL 1983, 24, 4591. (b) Fleming, I.; Urch, C. J. JOM 1985, 285, 173. (c) Coope, J.; Shiner, V. J. JOC 1989, 54, 4270.
10. Manyik, R. M.; Frostick, F. C.; Sanderson, J. J.; Hauser, C. R. JACS 1953, 75, 5030.
11. Hauser, C. R.; Hoffenberg, D. S. JOC 1955, 20, 1448.
12. Davies, J. S. H.; McCrea, P. A.; Norris, W. L.; Ramage, G. R. JCS 1950, 3206.
13. Hiskey, R. G.; Smithwick, E. L. JACS 1967, 89, 437.

Harry Heaney

Loughborough University of Technology, UK

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