Strong acids tetrafluoroboric acid

For very strong acids, it is usually possible to use a solvent of a more conventional kind thus, for example, the acid HBF, tetra fluoroboric acid, is extremely strong, because attachment of the hydrogen to the tetrafluoroborate group BF is essentially ionic, H BF and hence dissociation to an acid is very easy. Hence HBF behaves as a strong acid in, for example, an organic solvent, in which it can be used. 

The existence of the nitronium ion in sulfuric-nitric acid mixtures was demonstrated both by cryoscopic measurements and by spectroscopy. An increase in the strong acid concentration increases the rate of reaction by shifting the equilibrium of step 1 to the right. Addition of a nitrate salt has the opposite effect by suppressing the preequilibrium dissociation of nitric acid. It is possible to prepare crystalline salts of nitronium ions, such as nitronium tetrafluoroborate. Solutions of these salts in organic solvents rapidly nitrate aromatic compounds. ... 

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). 

There are numerous similarities between ammonia and phosphine, but the latter is a much weaker base (see Chapter 9). In fact, phosphonium salts are stabilized by large anions that are also the conjugates of strong acids. Accordingly, the most common phosphonium salts are the iodides, bromides, tetrafluoroborates, and so forth. Phosphine and substituted phosphines are good Lewis bases toward soft Lewis acids, and many coordination compounds of this type are known. 

Electron-rich carbyne complexes can react at the carbyne carbon atom with electrophiles to yield carbene complexes. Numerous examples of such reactions, mostly protonations, have been reported [519]. Depending on the nucleophilicity of the carbyne complex, such reactions will occur more or less readily. The protonation of weakly nucleophilic carbyne complexes requires the use of strong acids, such as triflic [533], tetrafluoroboric [534] or hydrochloric acid [535,536]. More electron-rich carbyne complexes can, however, even react with phenols [537,538], water [393,539], amines [418,540,541], alkyl halides, or intramolecularly with arenes (cyclometallation, [542]) to yield the corresponding carbene complexes. A selection of illustrative examples is shown in Figure 3.25. 

Nitronium tetrafluoroborate is a very efficient reagent for the D-nitration of alcohols and glycols containing both primary and secondary hydroxy functionality (Equation 3.7). Yields of nitrate ester are high and frequently quantitative. 0-Nitration with nitronium salts produces a strong acid and so an acid-binding agent should be present for acid sensitive substrates. 

The initiator may be a Lewis acid, an oxonium salt or precursor ( ), or an ester of a strong acid (9 ). The cinion A in the formula scheme below may designate, e.g., tetrafluoroborate... 

If H3BO3 is treated with HE, tetrafluoroboric acid HBF4 is slowly produced in solution. This strong acid has not been isolated from solution, but numerous salts, tetraflouroborates, have been prepared. These salts are similar in solubilities to perchlorates (salts C104 ). 

Thus, the .v-,V-phosphorylimidazole 2 is fluorinated with tetrafluoroboric acid etherate (TFBA) to yield the corresponding fluorophosphorinanes cis- and turns-A. It is also possible to fluorinate the A -phosphorylbenzimidazole 3 in chloroform with 100% inversion.58 The stereochemical results depend strongly on the solvent used (Table 8).58... 

As diazonium tetrafluoroborates are more soluble in strongly acidic media, the precipitation of the diazonium tetrafluoroborate from an auxiliary diazonium salt and alkaline tetrafluoroborate is preferred when the diazonium tetrafluoroborate is thought to be too soluble an increase of the ionic strength tends to decrease its solubility.3-129 However, some alkaline chloride can co-precipitate with the diazonium tetrafluoroborate. Consequently, the diazonium tetrafluoroborate yields can be a little optimistic and some chlorinated aromatics can be produced together with fluorinated aromatics during the dediazoniation step.3,171 To prevent such contamination, the diazonium tetrafluoroborate is washed with the minimum amount of chilled water, then with another polar solvent, such as an alcohol, and finally with a water-insoluble solvent, such as diethyl ether. When their decomposition points are not too low, diazonium tetrafluoroborates can also be purified by rccrystallization (in water or acetic acid) or by dissolution in acetone then salting out with diethyl ether.3... 

The parent system is prepared by oxidation of tetrahydrothiopyran with triphenylmethane and perchloric acid (66HCA2046) in high yield, and the method is applicable to the preparation of virtually all simple alkyl or aryl derivatives. In more complex cases, especially where the strong acid might cause elimination or isomerization side reactions, triphenylmethyl tetrafluoroborate may be used with the thiin precursor (equation 113) (78CL723). 

Borate is selectively concentrated on Amberlite XE-243 ion exchange resin and converted to tetrafluoroborate using 10% hydrofluoric acid. Tetrafluoroborate is strongly retained by the resin, thus allowing excess fluoride to be eluted without loss of boron. The tetrafluoroborate is eluted with lmol L 1 sodium hydroxide and is determined in the eluent by ion chromatography. Boron is quantified to a lower limit of 0.05mg L 1. 

Mootz D, Oellers E-J, Wiebcke M (1987) First examples of type I clathrate hydrates of strong acids polyhydrates of hexafluorophosphoric, tetrafluoroboric, and perchloric arid. J Am Chem Soc 109 1200-1202... 

Although tetrafluoroboric and hexafluorophosphoric adds do not exist as such, but only in the form of oxonium salts, for example, H502BF4, in nonaqueous solvents such as carboxylic anhydrides or ethers there can be strong acid behavior. Tetrafluoroboric acid in diethyl ether is a useful strong add, [Et2OH]+BF4. 

A general method of replacement of the primary aromatic amino group by hydrogen is reduction of aromatic diazonium salts, prepared by diazotization, typically by treatment of the primary amine or its salts with sodium or potassium nitrite in strongly acidic aqueous media. Conversion to the diazonium salts may also be accomplished in nonaqueous media using acetic acid, or using alkyl nitrites (equation 86). Most diazonium salts decompose at temperatures above 0-5 C. Only certain salts such as naphthalene-1,5-sulfonates,tetrafluoroborates ° ° and hexafluorophosphates ° ° are stable at room temperature. 

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