Tetrafluoroboric acid

Form Supplied in 48% aq solution diethyl ether complex dimethyl ether complex. 

Purification commercial 50% solutions can be concentrated by evaporation at 50-60 °C75 nrniHg to give a residue of 11N total acidity. Water can also be removed by slow addition to ice-cold acetic anhydride (dangerously exothermic reaction caution is advised ).  

General Considerations. Tetrafluoroboric acid is a useful strong Brpnsted acid with a nonnucleophilic counterion. Its solubility in polar organic solvents makes it a useful strong acid under nonaqueous conditions. 

Arenediazonium salts can be precipitated as their tetrafluorobo-rate salts by addition of a cold solution of HBF4 to a solution of the initial arenediazonium chloride. 4-Methoxybenzenediazonium tetraduoroborate is thus obtained in 94—98% yield. The ary-lamine can be diazotized with Sodium Nitrite in the presence of HBF4, and the diazonium fluoroborate then precipitates directly. 3-Nitroaniline, when treated in this manner, provides a 90-97% yield of 3-nitrobenzenediazonium tetrafluoroborate (eq 1). Diazotizations of arylamines and heteroarylamines in organic solvents can be conveniently conducted using HBF4-OEt2 and alkyl nitrites.  

When di-t-butyl carbonate is treated with DMAP HBF4, water-soluble Boc-pyridinium tetrafluoroborate is formed. This reagent installs the Boc protecting group onto amino acids in aqueous NaOH. Thus L-proline is converted to its A-Boc derivative in nearly quantitative yield in 10 min. 

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Nickel tetrafluoroborate [14708-14-6] Ni(BE 2 prepared by dissolving nickel carbonate in tetrafluoroboric acid [16872-11-0] HBE. Nickel... 

Several variations of this general procedure have been described. The functionalized dithiocarbamate (23) on heating with a mixture of phosphorus pentasulfide and tetrafluoroboric acid gave the 2-amino-substituted 1,3-dithiolylium tetrafluoroborate (24) in moderate to good yield (69CPB1924). 

Hydrogenation reduces the nitro group to amino which is then diazotized using sodium nitrite and tetrafluoroboric acid. The diazotized crown was not isolated but the aq. solution was treated directly with sodium acetate and bis(dibenzylideneacetone)-pal-ladium(O) in acetonitrile solution. Ethylene was then introduced to the autoclave and the solution was allowed to stir for 2 days. 4 -Vinylbenzo-15-crown-5 was isolated (30% from 4 -nitrobenzo-15-crown-5) as a colorless solid (mp 43.5—44.2°) °. The synthesis is illustrated in Eq. (3.16). 

Schiemann reaction (Section 22.17) Preparation of an aryl fluoride by heating the diazonium fluoroborate formed by addition of tetrafluoroboric acid (HBF4) to a diazonium ion. 

Reaction of [Ir( -Cp )Cl2(/A-Cl)2]2 and pyrazole in the presence of potassium hydroxide leads to complex 105 characterized by the dynamic hydrogen bond between three pyrazole nuclei (86AGE1114). 3,5-Dimethylpyrazole in identical conditicHis produces 106 where two pyrazole nuclei share a proton and one is unprotonated. Addition of tetrafluoroboric acid to 106 yields 107, where the second proton is bonded to the nonchelated ligand. Addition of the third proton causes formation of [Ir( -Cp )(Hpz )3](BF4)2. 

Another route to the amido complexes originates from [(>j-Tp )W(CO) (PhC=CMe)(OTf)l and benzylamine and yields [(i -Tp )W(CO)(PhC=CMe) (NHCH2Ph)] (96JA6916). The latter can be protonated with tetrafluoroboric acid to give the amine derivative [(> -Tp )W(CO)(PhC=CMe)(NH2CH2Ph)](Bp4), and this process can be reversed by -butyllithium. Hydride abstraction by silver tetrafiuoroborate, molecular iodine, or PhsCPEe leads to the cationic imine derivatives [(> -Tp )W(CO)(PhC=CMe)(HN=CHPh)]". -Butyllithium deproto-nates the product and gives the neutral azavinylidene species [(> -Tp )W(CO) (PhC=CMe)(N=CHPh)]. The latter with silver tetrafiuoroborate forms the cationic nitrile species [(j -Tp )W(CO)(PhC=CMe)(N=CPh)](Bp4). 

Treatment of aniline 1 with nitric acid in the presence of tetrafluoroboric acid leads to a relatively stable benzenediazonium tetrafluoroborate 2 by the usual diazotization mechanism. There are several variants for the experimental procedure. Subsequent thermal decomposition generates an aryl cation species 4, which reacts with fluoroborate anion to yield fluorobenzene 3 " ... 

Determination of borate as nitron tetrafluoroborate Discussion. Boric acid (100-250 mg) in aqueous solution may be determined by conversion into tetrafluoroboric acid and precipitation of the latter with a large excess of nitron [see Section 11.11(E)] as nitron tetrafluoroborate, which is weighed after drying at 110°C. The accuracy is about 1 per cent. 

H- and 3//-Azepines are generally unstable in aqueous acid solution and the few examples of simple azepinium salts, namely perchlorates,77 bromides,105 picrates35201 and a solitary iodide,105 have been prepared under nonaqueous conditions. The fractional crystallization of oxalate salts has been used for the separation of mixtures of 4- and 6-substituted 3f/-azepines,66 and 3,6-di-tm-butyl- and 2,5-di-tert-butyl-3//-azepine, on treatment with tetrafluoroboric acid in acetonitrile, are converted quantitatively into their crystalline tetrafluoroboratc salts.70... 

Treatment of tricarbonylh4-1-(ethoxycarbonyl)-l//-azepine]iron(0) (30) with acetic anhydride and tetrafluoroboric acid at 0 C (Method A) yields the 3-acetyl derivative 31.226 The acetyl derivative is also formed, but in lower yield, by the action of acetic anhydride and tin(IV) chloride (Method B) on complex 30. The 3-propanoyl derivative (20% mp 95-96 C) can be prepared similarly, whereas formylation to give aldehyde 32 is successful under Vilsmeier conditions. 

Reaction of iminophosphorane 1006 with ethoxycarbonyl isocyanate gave (91T6747) regioselectively thiadiazolotriazine derivatives 1007, whereas treatment of 1006 with aromatic isocyanates in ethanol or in the presence of tetrafluoroboric acid afforded 1008 and 1009, respectively. Similarly, 1006 with methyl isocyanate gave 1010 (91T6747) (Scheme 187). 

Hydroxy-4//-selenopyrans 113 have been reported to be air-sensitive intermediates in the transformations of 100c to salts 73a and 73 (R = Me) by successive action of lithium aluminium hydride or methyllithium and then tetrafluoroboric acid (90AG450). 4//-Selenopyranone 100c was reduced to 4//-selenopyran 101c with DIBAL-H (88MI1). 

These salts can be made easily since tetrafluoroboric acid (HBF4) and hexa-fluorophosphoric acid (HPF6) are commercially available. However, the main advantage of the diazonium salts with the anions of these acids is their stability, which is significantly higher than that of probably all other diazonium salts. 4-Nitrobenzenediazonium tetrafluoroborate is nowadays even a commercial product. Preparative diazotization methods with these two acids can be found in Organic Syntheses (tetrafluoroborate Starkey, 1943 hexafluorophosphate Rutherford and Redmont, 1973). 

Transformation of bromocriptine free base 2 into water soluble salt -mesylate, is the only way to obtain a suitable therapeutical form. Crystallization of mesylate using alcohol as a solvent in the presence of excess of strong acid, e.g. methanesulphonic acid can induce formation of 12 -0-alkyl-derivative 2. Until now this derivatisation of ergot molecule has been practically unknown. In continuation we developed the preparative method for obtaining these compounds, (using tetrafluoroboric acid as a catalyst) (ref. 20). 

Bromocriptine 2 (0.65 g, 1 mmol) was dissolved in 100 ml of dry ethanol and 60 ml of tetrafluoroboric acid / diethylether complex (85 %) was added while stirring. After standing overnight at RT the solvent was evaporated and the raw product isolated by extraction in the system dichloromethane 12% ammonia in water and evaporated to the dry residue. This residue was applied to the chromatographic column (I.D. = 2 cm, lenght = 20 cm) packed with silicagel and eluted with dichloromethane / ethylacetate =1 1. The fractions containing 2 were evaporated to the dry residue and crystallized from alcohol. 

Tetrasubstituted phosphinous amides of the type R2NPPh2 have been successfully arylated at phosphorus by the action of bromobenzene, in a process catalyzed by NiBr2, to give the aminophosphonium bromides [R2NPPh3] Br [109]. Other representative members of this class form phosphane-borane complexes [62], are aminated at phosphorus by chloramine to yield bis(amino)phos-phonium salts [110] and have been claimed to be protonated at phosphorus by ethereal tetrafluoroboric acid, as determined by NMR analysis [111]. 

Acetic anhydride was introduced into a container containing an aqueous solution of 48% tetrafluoroboric acid in order to obtain this acid in the anahydrous state. Although the operation was carried out at 0°C, it gave rise to a violent detonation. 

Commercially available 54wt% tetrafluoroboric acid solution in diethyl ether was used in all transformations. 

Boyd, G. V. etal., J. Chem. Soc., Perkin Trans. 1, 1978, 1346 Interaction to give /V-fert-butylphthalisoimidium tetrafluoroborate was very violent, possibly because of exothermic hydrolysis of the anhydride by the 40% aqueous tetrafluoroboric acid. 

See Sodium carbonate hydrogen peroxidate Acetic anhydride Tetrafluoroboric acid... 

A compound with tetrafluoroboric acid, previously claimed to be a useful safe oxidant, proved not to be on preparation at 0.5 molar scale. It exploded spontaneously while drying in vacuo at room temperature. Chem. Abs. (112 98095p) identifies it with this title compound, which was claimed to be too unstable to explode by the original users. It seems more likely that the explosive was the anhydride , oxybisphenyliodonium tetrafluoroborate. Several related compounds have also proved to be explosive. 

The use of powdered zinc in hydrochloric acid has been reported to reduce 4(5)-nitroimidazole (27 R = H) to 4(5)-aminoimidazole (25 R = H), but the yield was not recorded [56JBC(223)985]. In another study, treatment of 4(5)-nitroimidazole (27 R = H) with zinc dust in tetrafluoroboric acid solution followed by in situ diazotization of the amine (25 R = H), which was presumed to be formed, gave 4-fluoroimidazole (17%) (73JA4619, 73JOC3647). 

JOC3570). Diazotization of l-methylimidazole-5-carbohydrazide (103) followed by treatment with r-butanol gave the carbamate (104), which was dissolved in cold tetrafluoroboric acid when evolution of gas had ceased, the solution was treated with sodium nitrite and irradiated. However, unlike the 4-aminoimidazoles (Section III,A,2), this reaction produced none of the desired 5-fluoro-l-methylimidazole (105) (Scheme 10). Although 5-amino-1-methylimidazole (96 R = Me, R2 = H) was almost certainly formed, ultraviolet spectral analysis showed only traces of a diazo-nium chromophore after addition of nitrite, indicating compound (96 R1 = Me, R2 = H) to be extremely unstable under the acidic reaction conditions (78JOC3570). 

An interesting hydroiodination reaction occurs when a mixture of cyclohexene and triethylsilane in dichloromethane is treated with a mixture of bis(pyridine) iodonium tetrafluoroborate and tetrafluoroboric acid in diethyl ether (Eq. 125). A 50% yield of iodocyclohexane is produced after one hour at 20°.268... 

As shown in Scheme 199, the 5-aminopyrimidine stmcture may be also incorporated into a more complex bicyclic system. Thus, diazotization of 3-amino-4-oxo-4//-pyrimido[ 1,23 lpyndazincs 1198 followed by treatment with 50% aqueous tetrafluoroboric acid results in precipitation of salts 1199. When heated with alcohols, nucleophilic attack on the carbonyl group opens the pyrimidine ring. The obtained species 1200 assume conformation 1201 that is more suitable for bond formation between the opposite charged nitrogen atoms. Alkyl l-(pyridazin-3-yl)-l//-l,2,3-triazole-4-carboxylates 1202 are obtained in 31-66% yield <2002ARK(viii)143>. 

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