Diazonium fluoborates

By adding a concentrated solution of sodium borofluoride to a solution of a diazonium salt, the diazonium fluoborate is precipitated this decomposes into the aryl fluoride when cautiously heated, for example ... 

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 ... 

An interesting application of the diazo reaction is to the preparation of the otherwise difficultly accessible o- and />-dinitrobenzenes o- or p-rutrophenyl-diazonium fluoborates react with sodium nitrite in the presence of copper powder to yield o- or p-dinitrobenzene ... 

It may be mentioned that diazonium fluoborates containing the nitro group usually decompose suddenly and with violence upon heating, hence if o- or p-fluonitrobenzene are required, the fluoborates (in 10-20 g. quantities) should he mixed with 3-4 times their weight of pure dry sand (or barium sulphate or sodium fluoride) and heated cautiously until decomposition commences intermittent heating will be required to complete the reaction. 

The conversion of an aromatic diazonium compound into the corresponding arsonic acid by treatment with sodium arsenite in the presence of a catalyst, such as copper or a copper salt, is called the Bart reaction. A modification of the reaction employs the more stable diazonium fluoborate in place of the diazonium chlorid.i. This is illustrated by the preparation of />-nitrophenylarsonic acid ... 

Dissolve 34 g. of o-nitroaniline in a warm mixture of 63 ml. of concentrated hydrochloric acid and 63 ml. of water contained in a 600 ml. beaker. Place the beaker in an ice - salt bath, and cool to 0-5° whilst stirring mechanically the o-nitroaniline hydrochloride will separate in a finely-divided crystalline form. Add a cold solution of 18 g. of sodium nitrite in 40 ml. of water slowly and with stirring to an end point with potassium iodide - starch paper do not allow the temperature to rise above 5-7 . Introduce, whilst stirring vigorously, a solution of 40 g. of sodium borofluoride in 80 ml. of water. Stir for a further 10 minutes, and filter the solid diazonium fluoborate with suction on a sintered glass funnel. Wash it immediately once with 25 ml. of cold 5 per cent, sodium borofluoride solution, then twice with 15 ml. portions of rectified (or methylated) spirit and several times with ether in each washing stir... 

Dialkylanilines, pure, from commercial products, 572 Di-n-amyl ether, 313 Di-iso-amyl ether, 313 Di-n-amyl ketone, 340 Diazoaminobenzene, 622, 627 Diazoamino-aminoazo rearrangement, 622J, 626, 627 Diazo ketone, 903, 904, 905 Diazomethane, 946, 967, 968, 969-972 determination of, 972 methylation with, 973 precautions in the use of, 968, 969 ring enlargement with, 946J, 947 Diazonium fluoborates, 594, 610, 611, 612, 613... 

It is recommended that the total hydrogen-ion concentration should not be greater than one mole per liter during the precipitation of the diazonium fluoborate. U. S. pat. 1,9x6,327 [C. A. 27, 4539 (1933)]. 

Aromatic diazonium salts on treatment with sodium nitrite decompose to form nitro compounds. This method represents a good procedure for obtaining o- and p-dinitrobenzenes, in 70% and 76% yield, respectively, from the corresponding diazonium sulfates. Improved yields in the preparation of dinitronaphthalenes are obtained when the decomposition of the diazonium sulfates is catalyzed by a cupro-cupri sulfite prepared by the interaction of copper sulfate and sodium nitrite. The procedure is illustrated by the synthesis of 1,4-dinitronaphthalene (60%). Occasionally, diazonium fluoborates are first formed, and these compounds are treated with sodium nitrite in the presence of copper powder, viz.,... 

Deamination of primary aliphatic amino groups in (suitably substituted) peptides may also be possible via diazonium fluoborates and triazenes (White and Scherrer, 1961) ... 

A solution of 2.60 kg. (21.1 moles) of freshly distilled o-anisidine in 5.2 1. of concentrated hydrochloric acid is stirred and cooled to below 0° while a solution of 1.53 kg. (22.2 moles) of sodium nitrite in 2 1. of water is added slowly. Aqueous fluoboric acid solution, prepared [Org. Syntheses Coll. Vol. 2, 295 (1943)] from 4.05 1. of technical 52% hydrofluoric acid and 1.86 kg. of boric acid, is added, and the temperature is held at —10° for 1 hour. The precipitated diazonium fluoborate is filtered off and washed with water, ethanol, and finally ether. The precipitate is dried under vacuum in the presence of concentrated sulfuric acid to give 2.70 kg. (57.5%) of the diazonium fluoborate. The salt is then decomposed in 540-g. portions in accordance with the directions given in Org. Syntheses Coll. Vol. 2, 296 (1943). The distillate is dissolved in ether, and the ethereal solution is washed with dilute aqueous sodium hydroxide, then with water, and dried over sodium sulfate. Distillation gives 820 g. (31%) of o-fluoroanisole boiling at 69-70°/26 mm. 

Replacement of the diazonium group by —F is carried out in a somewhat different way. Addition of huoboric acid, HBF4, to the solution of diazonium salt causes the precipitation of the diazonium fluoborate, ArN2 BF4, which can be collected on a filter, washed, and dried. The diazonium fluoborates are unusual among diazonium salts in being fairly stable compounds. On being heated, the dry diazonium fluoborate decomposes to yield the aryl fluoride, boron trifluoride,... 

Roe, A. Preparation of aromatic fluorine compounds from diazonium fluoborates. The Schiemann reaction. Org. React. 1949, 5, 193-228. 

Schiemann reaction. Formation of diazonium fluoborates by diazotization of aromatic amines in the presence of fluoborates, followed by their thermal decomposition to aryl fluorides. 

The Balz-Schiemann reaction for the introduction of fluorine into an aromatic nucleus involves forming the amine, then the diazonium fluoborate, which in turn decomposes into an aromatic fluoride (104,106,107). One of the reviews (104) of this reaction gives tables of the compounds prepared by this method. 

Roe, A., Preparation of Aromatic Fluorine Compounds from Diazonium Fluoborates The... 

A sintered glass filter should be used for filtering, and the fluoborate stirred well on the filter with each washing, before suction is applied. The diazonium fluoborate is stable and may be dried in a vacuum desiccator over phosphorus pentoxide. 

The Balz-Schiemann (BS) reaction (often called the Schiemann reaction) describes the preparation of an aromatic diazonium fluoborate 1 followed by its thermal decomposition to afford the corresponding aryl fluoride 2.1"4... 

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