Fluoboric acid

The controlled thermal decomposition of dry aromatic diazonium fluoborates to yield an aromatic fluoride, boron trifluoride and nitrogen is known as the Schiemann reaction. Most diazonium fluoborates have definite decomposition temperatures and the rates of decomposition, with few exceptions, are easily controlled. Another procedure for preparing the diazonium fluoborate is to diazotise in the presence of the fluoborate ion. Fluoboric acid may be the only acid present, thus acting as acid and source of fluoborate ion. The insoluble fluoborate separates as it is formed side reactions, such as phenol formation and coupling, are held at a minimum temperature control is not usually critical and the temperature may rise to about 20° without ill effect efficient stirring is, however, necessary since a continuously thickening precipitate is formed as the reaction proceeds. The modified procedure is illustrated by the preparation of -fluoroanisole ... 

If it is desired to employ fluoboric acid HBF4, it can be prepared by adding 100 g. of A.R. boric acid in small proportions to 325 g. of A.R. hydrofluoric acid (40 p>er cent. HF) cooled in ice the hydrofluoric acid is contained in a Bakelite beaker, a beaker coated with wax or in a lead vessel. A simple container may also be prepared by cutting of the neck of the wax bottle (in which the hydrofluoric acid is supplied) with a large e.g., a butcher s ) knife which has been slightly warmed. One-third of the above solution should be employed in the preparation. Handle unth great care. 

The fluoboric acid may be prepared by adding 92 g. of A.R. boric acid slowly and with constant stirring to 250 g. of hydrofluoric acid (40-48 per cent.) in a copjier, lead or a waxed-lined beaker. A lead rod may be used for stirring. All operations should be carried out in a fume cupboard. 

The first question can be answered relatively simply (although not completely exactly) from the available factual material. If the pK value of the lactam is taken as a criterion, the border of reactivity lies at about pK 12. Valerolactam and caprolactam react just noticeably with diazomethane (yields 14 and 7%). However, it should be noticed that catalysts are frequently necessary in order to initiate the reaction (methanol, water, aluminum isopropylate," fluoboric acid " ). For example, phthalimide does not react at all with diazomethane in ether, but a smooth reaction occurs if some methanol is added. 

Bor-fluorid, n. boron fluoride borofluoride. fiuorwasserstoffsaure, /., -fiuorwasserstoff, m. fluoboric acid, hydrofluoboric acid, HBFi. -flusssSure,/. fluoboric add. 

The metal to be plated is first cleaned carefully and then activated with a weak acid. Steel can be treated with 3-5% HCl, whilst a 10% fluoboric acid solution is suitable for copper alloys. It is then ready for the electrodeposition process. 

Electrodeposited chromium, both decorative and hard , is produced with the use of a solution of chromic acid containing a small amount of catalyst which is usually sulphuric acid, although fluosilicic or fluoboric acid may be used. A typical electrolyte contains 250-400 g/1 of chromic acid and... 

The ice-cold fluoboric acid solution is added rather rapidly, with stirring, to the finished tetrazo solution, the temperature being kept below io°. A thick paste of 4,4 -biphenylene-bis-diazonium borofluoride forms. The mixture is stirred at io° for twenty to thirty minutes. It is then collected on a 19-cm. Buchner funnel, and washed consecutively with about 200 cc. of cold water, 200 cc. of cold commercial methyl alcohol, and 200 cc. of commercial ether the cake is sucked as dry as possible between washings. It is then dried in a vacuum desiccator over concentrated sulfuric acid (sp. gr. 1.84). The yield of the dry solid is 393 400 g. (68-69 Per cent of the theoretical amount). The product decomposes at 135-1370. 

Fluoboric acid is now a commercial product. For this size preparation, 355.5 g. of the commercial 40 per cent fluoboric acid is required. 

Absolute ethanol (400 ml.) is vigorously stirred in a 1-1. widenecked Erlenmeyer flask immersed in an ice bath (Note 4). The tropylium hexachlorophosphate-tropylium chloride double salt4 is separated from the reaction mixture by suction filtration, washed briefly with fresh carbon tetrachloride, and transferred as rapidly as possible into the cold, well-stirred ethanol (Note 5). The salt dissolves rapidly and exothermally to give a reddish solution. Fifty milliliters (0.39 mole) of 50% aqueous fluoboric acid is added rapidly to the cold stirred solution (Note 6). The dense white precipitate of tropylium fluoborate that forms is separated by suction filtration, washed with a little cold ethanol and with ether, and air-dried at room temperature (Note 7) weight 34-38 g. (80-89%) decomposition point about 200° A j, HC1 218 mja (log e 4.70), 274 m/i (log e 3.61). The product is 98-100% pure (Notes 8 and 9). 

In a variation of this procedure that gives a nearly quantitative yield of good material, the intermediate salt is dissolved in 250 ml. of glacial acetic acid in a 2-1. beaker, and 100 g. of 50% fluoboric acid is added with stirring. When the evolution of gas has stopped, 11. of ethyl acetate is added to precipitate tropylium fluoborate. The fluoborate is separated by filtration, washed successively with ethyl acetate and ether, and dried in an oven at 40°.4... 

Ferrocenylacetonitrjle, 40, 46 Fluoboric acid as catalyst for diazomethane etherifications, 41, 9, 10 9-Fluobenone, 2,4,5,7-tetranitro-, 42,95... 

The fluoboric acid-catalyzed aza-Diels-Alder reaction of aldimine and Danishefsky s diene proceeds smoothly to afford dihydro-4-pyridones in high yields [90] (Equation 4.16). Unstable aldimines generated from aliphatic aldehydes can be prepared in situ and allowed to react under one-pot reaction conditions. This one-pot Bronsted acid-catalyzed three-component aza-Diels-Alder reaction affords the adducts in good to high yields. 

Fluoboric acid is also an efficacious promoter of cyclic oxo-carbenium ions (Scheme 4.24) bearing an activated double bond which, in the presence of open-chain and cyclic dienes, rapidly undergo a Diels-Alder reaction [91]. Chiral a, -unsaturated ketones bearing a -hydroxy substituents, protected as acetals, react with various dienes in the presence of HBF4, affording Diels-Alder adducts that were isolated as alcohols by hydrolysis of the acetal group by TsOH. Some examples of reactions with isoprene are reported in Table 4.23. The enantios-electivity of the reaction is dependent on the size of the substituent R on the of-carbon high levels of asymmetric induction were observed with R = z-Pr (90 1) and R = t-Bu (150 1) and low levels with R = Me (2.7 1) and R = Ph (3.0 1). Scheme 4.24 shows the postulated reaction mechanism. 

Fluoboric acid was also found to form a cement of some strength (106 MPa) but of poor hydrolytic stability. 

The use of sodium fluoborate solution supersedes the less convenient fluoboric acid and permits the preparation to be carried out in ordinary glass vessels. 

The effect of pH on both clay swelling and fines production has been widely discussed(89-95). Little consensus is found in this literature. Suggested treatments range from application of fluoboric acid(96) to 15% KOH(92) solutions — both treatments are believed to create a protective silicate film that inhibits release of fines. Polyacrylate polymers can provide protection against swelling of smectite clays and shales(97-100). 

Ethereal fluoboric acid can be prepared as follows 19 ml. (19 g., 0.95 mole) of anhydrous hydrofluoric acid, b.p. 19.4° (760 mm.) [Caution Hydrofluoric acid in contact with the skin produces extremely painful hums. It is therefore necessary to use eoery precaution to protect exposed parts of the body, especially the hands and eyes. Cf. Org. Syntheses, Coll. Vol. 2, 295 1943), Note 3 Org. Syntheses, 46, 10 1966), Note 1 is added in small portions with shaking or stirring to 126 ml. (142 g., 1.00 mole) of distilled boron trifluoride etherate, b.p. 126° (760 mm.), contained in a 500-ml. polyethylene flask that is cooled in an ice bath to 0°. The concentration of the resulting yellowish solution of fluoboric acid in ether is about 52% by weight ca. 6.6 moles per 1.). 

In a 1-1. four-necked flask (or a three-necked flask with a Y-tube connector) equipped with a mechanical stirrer, a reflux condenser, a dropping funnel, and a thermometer are placed 208 g. (1.00 mole) of benzalacetophenone (Note 1), 60 g. (58.5 ml., 0.50 mole) of acetophenone, and 350 ml. of 1,2-dichloroethane. The contents of the flask are warmed to 70-75°, and 160 ml. of a 52% ethereal solution of fluoboric acid (Note 2) is added from the funnel with stirring during 30 minutes. With the first addition the mixture becomes orange subsequently the color changes to brownish yellow. After the addition is completed, the mixture is stirred and heated under reflux for 1 hour (Note 3). The fluorescent mixture is allowed to stand overnight in a refrigerator. The crystalline product that separates is collected on a Buchner funnel and washed well with ether. By addition of 250 ml. of ether (Note 4) to the mother liquor an additional quantity of 2,4,6-triphcnylpyrylium tctrafluoroboratc is obtained. A total yield of 125 135 g. (63 68%) of yellow crystals... 

Another important variant of the preceding approach is the cycloaddition reaction between monocarbonyl iodonium salt 47 and an alkene to give dihydrofuran 48 (88TL3703 89JOC2605). The iodonium salt 47 is generated by the oxidation of acetophenone silyl enol ether (46) with iodosobenzene in the presence of fluoboric acid. 

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