Mercury(II) Oxide-Tetrafluoroboric Acid


[21908-53-2]  · HgO  · Mercury(II) Oxide-Tetrafluoroboric Acid  · (MW 216.59) (HBF4)

[16872-11-0]  · BF4H  · Mercury(II) Oxide-Tetrafluoroboric Acid  · (MW 87.82)

(capable of diamination1 and hydroxyamination2 of alkenes, cycloamination of dienes,7,8 and of hydrolysis of alkyl bromides;11 facilitates m-iodination of deactivated arenes9)

Solubility: HBF4: sol ethereal solvents.

Form Supplied in: HgO: red or yellow precipitate; the yellow form is more finely divided and more reactive than the red form. HBF4: available as liquid Et2O or Me2O complexes.

Preparative Methods: reaction of Mercury(II) Oxide (red) with Tetrafluoroboric Acid (Et2O solution) generates Hg(BF4)2 in situ.

Handling, Storage, and Precautions: acute poison; exposure to all mercury compounds is to be strictly avoided. Releases toxic Hg fumes when heated to decomposition. Protect from light. Tetrafluoroboric acid is extremely corrosive.

Electrophilic Additions.

Mercury(II) tetrafluoroborate, generated in situ from HgO and HBF4, appears to be the reagent of choice to promote diamination of alkenes with primary and secondary amines. The reaction occurs in THF at reflux and gives good to excellent yields (eq 1).1 In the presence of alcohols, hydroxyamination is observed.2 The structure of the products is consistent with an overall anti mechanism, involving aminomercuration followed by replacement of HgBF4-. The latter step occurs with overall retention,3 apparently due to the participation of the neighboring nitrogen.4

Allylic alcohols are converted into allylic amines with an overall 1,3-rearrangement (eq 2).5

The reagent differentiates between di- and trisubstituted double bonds. Thus, for example, only the methylene group of limonene reacts (eq 3). Due to the nonnucleophilic anion, a variety of nucleophiles can be introduced in this way: H2O (84% yield), MeOH (87%), AcOH (85%), NaN3 (80%), and MeCN (80%, Ritter reaction).6

Conjugated dienes react with primary amines in the presence of HgO/HBF4 to give products of 1,4-addition as reactive intermediates, which are converted to the corresponding pyrrolidine derivatives via an intramolecular SN2 reaction (eq 4).7 Nonconjugated dienes react with 2 equiv of Hg(BF4)2 to afford the corresponding adduct whose -HgBF4 groups can be replaced by heteroatom nucleophiles such as water in a stereospecific manner (eq 5).8

Allylic Oxidation.

Allylbenzene (PhCH2CH=CH2) is oxidized with HgO/HBF4 in the presence of alcohols to afford exclusively trans-cinnamyl ethers, e.g. PhCH=CHCH2OMe.2a,9

Aromatic Electrophilic Substitution.

Aromatic compounds can be iodinated by I2 in the presence of the HgO/HBF4 reagent. The orientation obeys the rules for aromatic electrophilic substitution: ortho attack is favored over para substitution with activated arenes. The method appears to be particularly useful for meta iodination of deactivated arenes (up to 99% selectivity).10

Hydrolysis of Dithioacetals.

The same reagent, generated in situ from HgO and 35% aq HBF4 in THF, can hydrolyze dithioacetals and hemithioacetals more effectively than HgO/BF3.Et2O.11

Substitution Reactions.

Alkyl bromides are hydrolyzed by HgO/HBF4 in THF, dioxane, or CH2Cl2 to the corresponding alcohols.12 Similarly, ethers are obtained if stoichiometric amounts of alcohols are used. The yields range from 55 to 95% (eq 6).12

1. (a) Barluenga, J.; Alonso-Cires, L.; Asensio, G. S 1979, 962. (b) Barluenga, J.; Pérez-Prieto, J.; Asensio, G. JCS(P1) 1984, 629.
2. (a) Barluenga, J.; Alonso-Cires, L.; Asensio, G. TL 1981, 22, 2239. (b) Barluenga, J.; Alonso-Cires, L.; Asensio, G. S 1981, 376.
3. Barluenga, J.; Perez-Prieto, J.; Asensio, G.; Garcia-Granda, S.; Salvado, M. A. T 1992, 48, 3813.
4. For similar neighboring group effects in the silver(I)-mediated solvolyses of alkyl halides, see: (a) Kočovský, P. JOC 1988, 53, 5816. (b) Kočovský, P.; Pour, M. JOC 1990, 55, 5580.
5. Barluenga, J.; Pérez-Prieto, J.; Asensio, G. T 1990, 46, 2453.
6. (a) de Mattos, M. C. S.; Kover, W. B.; Aznar, F.; Barluenga, J. TL 1992, 33, 4863. (b) Barluenga, J.; Aznar, F.; de Mattos, M. C. S.; Kover, W. B.; Garcia-Granda, S.; Pérez-Carreño, E. JOC 1991, 56, 2930.
7. Barluenga, J.; Pérez-Prieto, J.; Asenio, G. CC 1982, 1181.
8. Barluenga, J.; Pérez-Prieto, J.; Asenio, G.; Garcia-Granda, S.; Salvado, M. A. T 1992, 48, 3813.
9. Barluenga, J.; Alonso-Cires, L.; Compos, P. J.; Asensio, G. T 1984, 40, 2563.
10. Barluenga, J.; Campos, P. J.; Gonzalez, J. M.; Asensio, G. JCS(P1) 1984, 2623.
11. Degani, I.; Fochi, R.; Regondi, V. S 1981, 51.
12. Barluenga, J.; Alonso-Cires, L.; Campos, P. J.; Asensio, G. S 1983, 53.

Pavel Ko&cbreve;ovský

University of Leicester, UK

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