Silver(I) Tetrafluoroborate

AgBF4

[14104-20-2]  · AgBF4  · Silver(I) Tetrafluoroborate  · (MW 194.68)

(mild Lewis acid with a high affinity for organic halides)

Physical Data: mp 200 °C (dec).

Solubility: sol benzene, toluene, nitromethane, diethyl ether, water.

Form Supplied in: white solid; widely available.

Analysis of Reagent Purity: contents of Ag can be assayed conveniently by volumetric titration of AgI.

Preparative Method: can be prepared by reacting Silver(I) Fluoride with Boron Trifluoride in nitromethane.1

Handling, Storage, and Precautions: should be protected from light and moisture; very hygroscopic.

Introduction.

This reagent has replaced Silver(I) Perchlorate to a large extent because of the sensitivity of perchlorates.

Activation of Acyl Chlorides.

In several cases, AgBF4 has been used to increase the reactivity of acyl chlorides towards nucleophiles.2 For example, N-acylammonium salts were prepared for the first time by the reaction of a tertiary amine and an acyl chloride in the presence of AgBF4 (eq 1).3

Nucleophilic Substitution on Alkyl Halides by Heteroatoms.

A number of more or less activated alkyl halides, such as benzyl halides4 and allyl halides,5 undergo substitution reactions mediated by AgBF4 in the presence of a heteroatom nucleophile. For example, treatment of pentamethylcyclopentadienyl bromide with AgBF4 in the presence of a nucleophile gives the corresponding substituted product (eq 2). Thiols, amines, and alcohols have been used as nucleophiles.6

Adenine analogs are prepared stereoselectively from cyclopentene derivatives using a two-step procedure (eq 3). The reaction probably involves a seleniranium salt as an intermediate.7

Intramolecular substitutions mediated by Ag+ do not seem to require activated halides.8 For example, o-chloro amides react with AgBF4, giving products from intramolecular attack of the amide oxygen. Depending on the structure of the amide, imino lactones,9 imino lactonium salts,10 or lactone hydrazones (eq 4)11 are obtained as products. Fluorination of a-bromo ketones using AgBF4 has also been reported.12

Nucleophilic Aromatic Substitution.

In one case, it has been reported that AgBF4 promotes the nucleophilic substitution of an aromatic chloride (eq 5).13 This is not due to activation of the halide, but apparently to suppression of halide-promoted decomplexation of the arene-manganese derivative.

Carbon-Carbon Bond Formation via Cationic Intermediates.

In analogy with the heteroatom substitutions described above, certain aliphatic halides undergo substitution reactions with carbon nucleophiles promoted by AgBF4. For example, Eschenmoser and co-workers used AgBF4 in order to transform a-chloro nitrones into 1,3-dipoles which react with ordinary alkenes in a cycloaddition manner (eq 6).14

Livinghouse and co-workers have shown that acylnitrilium ions, prepared from isocyanides and an acid chloride followed by treatment with AgBF4, are useful intermediates in the synthesis of nitrogen-containing heterocycles (eq 7).15

In certain cases, allylsilanes16 and trimethylsilyl enol ethers17 react with alkyl halides with the formation of a new carbon-carbon bond. a-Bromo imidates18 (eq 8) and b-chloro imines19 have been reported to undergo electrophilic aromatic substitution on relatively electron-rich aromatics in the presence of AgBF4.

Synthesis via Iminium Ions.

a-Cyano amines react with AgBF4 with the formation of an intermediate iminium ion.20 This has been used synthetically as a method for removal of the cyano group either by a consecutive reduction21,22 to the amine (eq 9) or by elimination to the imine23 or enamine.24

Rearrangements.

A number of strained alkyl and/or reactive halides, such as cyclopropyl25 and bicyclic26,27 chlorides, rearrange on treatment with AgBF4. For example, b-bromotetrahydropyrans rearrange to tetrahydrofurans stereoselectively on treatment with AgBF4 (eq 10).28 Other examples include the rearrangement of a-haloalkyl aryl ketones into arylacetic acid derivatives,29 and the rearrangement of a-haloalkylsilanes upon treatment with AgBF4.30

In the presence of strained hydrocarbons, AgBF4 functions as a mild Lewis acid and causes rearrangements.31-33 For example, the tricyclic hydrocarbon (1) rearranges upon treatment with a catalytic amount of AgBF4 to the less strained hydrocarbon (2) (eq 11).34

Numerous examples include the rearrangement of propargyl esters into allenyl esters (see also Silver(I) Trifluoromethanesulfonate)35 or to dihydrofurans,36 the Claisen rearrangement of aryl allenylmethyl ethers,37 and the rearrangement of silyloxycyclopropanes (eq 12, also effected by Copper(II) Tetrafluoroborate).38

Activation of Thiol Esters.

Pyridyl thiol esters are converted into esters on treatment with AgBF4 and an alcohol.39 Acylation of alkynylsilanes can also be carried out using thiol esters in the presence of AgBF4.40

Alkylation of Thioethers.

Thioethers can be methylated by Iodomethane in the presence of AgBF4.41 Benzylation of thioethers in the presence of AgBF4 has also been reported.42

Electrophilic Aromatic Substitution.

Electrophilic nitration using a combination of NO2Cl and AgBF4 has been reported.43 Conversion of arylsilanes into iodides and bromides has been achieved using a combination of the halogen and AgBF4 (eq 13).44

Catalysis of Cycloadditions.

Addition of catalytic amounts of AgBF4 greatly increases the selectivity of [2 + 4] cycloadditions of benzyne.45

Related Reagents.

Dimethyl Sulfoxide-Silver Tetrafluoroborate.


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Lars-G. Wistrand

Nycomed Innovation, Malmö, Sweden



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