Diazonium fluoroborates diazotized

A standard way to form the aryl diazonium fluoroborate salt is to add fluoroboric acid (HBF4) or a fluoroborate salt to the diazotization medium... 

Diazotization of an arylamine followed by treatment with fluoroboric acid gives an aryl diazonium fluoroborate salt Heating this salt converts it to an aryl fluoride... 

Aryl fluorides also may be prepared from arenamines by way of dia-zonium salts if the procedure is slightly modified. The amine is diazotized with nitrous acid in the usual way then fluoroboric acid or afluoroborate salt is added, which usually causes precipitation of a sparingly soluble diazonium fluoroborate. The salt is collected and thoroughly dried, then carefully heated to the decomposition point—the products being an aryl fluoride, nitrogen, and boron trifluoride ... 

The stabilities of pyridine-2- and -4-diazonium ions resemble those of aliphatic rather than benzenoid diazonium cations. Benzenediazonium ions are stabilized by mesomerism which involves electron donation from the ring, but such electron donation is unfavorable in 2- and 4-substituted pyridines. On formation, pyridine diazonium cations normally immediately react with the aqueous solvent to form pyridones. However, by carrying out the diazotization in concentrated HC1 or HBr, useful yields of chloro- and bromopyridines 752 can be obtained. Iodinated pyridines can be obtained in good yield using the Sandmeyer reaction. Aminopyridazines and -pyrazines, 2- and 4-aminopyrimidines, and amino-1,2,4-triazines behave similarly. Nucleophilic fluorination via the BalzSchiemann reaction of diazonium fluoroborates yields fluoropyridines, including 2-fluoropyridines. Fluoroborates can also be converted into fluoro compounds by ultraviolet irradiation. 

Aminoimidazoles exist as the amino tautomers. Carbonyl compounds condense normally with C-aminoimidazoles, - which can also be diazotized although some amines are rather unstable. The diazonium fluoroborates have been transformed into fluoroimid-... 

In their landmark publication, Balz and Schiemanns reported the syntheses of fluorobenzene, 4-fluorotoiuene, 2,4-dimethylfluorobenzene, 1-fluoronaphthalene, and 4,4 -difluorodiphenyl. Diazotization of the appropriate aromatic amine with nitrous acid (NaNC>2, HC1), followed by the addition of fluoroboric acid gave the corresponding aryl diazonium salts (62-67% yields). Thermal decomposition of the dry aryl diazonium salts gave the aryl fluoride (97-100% yields). Only 2-, 3-, and 4-nitrophenyl diazonium fluoroborates, which were prepared in 74-100% yields, failed to yield the expected nitrofluorobenzenes upon thermolysis. 

The process most widely used for introducing fluorine into an aromatic nucleus is thermal decomposition of diazonium fluoroborates (Schiemann reaction) lc,1242 1244 these salts are usually very stable, even towards deflagration. They are precipitated (i) on addition of hydrogen tetrafluoroborate or NaBF4 to a diazonium solution obtained as usual from the amine in hydrochloric acid or (ii) on diazotization of the amine in HBF4 solution by aqueous... 

Diazonium fluoroborates from some heterocyclic amines are so unstable that they cannot be obtained dry and these are decomposed, whilst still moist with ether, in light petroleum or xylene. Indeed all isolated diazonium fluoroborates should be treated as potentially unstable until their stability has been determined on small samples for instance, an explosion has been reported1250 with 3-pyridinediazonium fluoroborate1249 that was still moist with solvent diazonium fluoroborates from oc- and y-amino N-heterocycles usually decompose at or below 20°, so the amines are diazotized in tetrafluoroboric acid and decomposition is induced by warming the diazotization solution.1246,1247... 

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

Diazotization. Diazonium salts of benzothiazoles have been useful in interconversions involving amino-derivatives of this ring system. 2-Fluoro-benzothiazoles are accessible by the diazotization of the corresponding 2-amino-compounds, followed by cautious pyrolysis of the isolated diazonium fluoroborate with potassium fluoride and sand under reduced pressure. The four isomeric sulphonic acids of 2-methylbenzothiazoles are obtainable from the amino-compounds by diazotization, followed by treatment with acetic acid saturated with sulphur dioxide in the presence of cupric chloride the resulting sulphonyl chlorides serve as source of the... 

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. ... 

Hexfluorophosphate salts behave similarly.108 The diazonium tetrafluoroborates can be prepared either by precipitation from an aqueous solution by fluoroboric acid109 or by anhydrous diazotization in ether, THF, or acetonitrile using r-butyl nitrite and boron trifluoride.110 Somewhat milder reaction conditions can be achieved by reaction of aryl diazo sulfide adducts with pyridine-HF in the presence of AgF or AgNQ3. 

When 2 -methoxy-2-biphenylamine is diazotized in fluoroboric acid, an 85% yield of solid 2 -methoxy-2-biphenylyldiazonium tetrafluoroborate is produced. On heating in dry benzene, this salt loses nitrogen and provides 0-methyldibenzofuranium tetrafluoroborate as an unstable solid. The latter salt, on account of the low basicity of dibenzofuran, reacts with a range of nucleophiles that undergo methylation with the release of dibenzofuran. Pyridine and pentachloropyridine are N-methylated, benzo[fc]thiophene is S-methylated, and tetrahydrofuran and phenetole are 0-methylated. ° Thus it is no surprise that diazotization of 2 -methoxy-2-biphenylamine, followed by boiling of the diazonium salt in acidic solution, provides dibenzofuran in 90% yield.The scope of this method of dibenzofuran synthesis (Scheme 18) has been little explored probably because of the difficulty of preparing the requisite biphenyls, which is usually accomplished by a crossed Ullmann reaction. A similar synthesis of xanthones from 2-amino-2 -methoxybenzophenones is known. ° ... 

The basic Balz-Schiemann reaction involves the preparation of an insoluble arenediazonium tetrafluoroborate 1, either by direct diazotization of arylamines in aqueous tetra-fluoroboric acid, or by addition of tetrafluoroboric acid or alkaline tetrafluoroborates to an aqueous solution of another arenediazonium salt. After filtration and air-drying, this solid diazonium tetrafluoroborate is decomposed either neat or suspended in an inert solvent. 

The weakly basic 2-aminothiazoles are most readily diazotized in concentrated solutions of oxygen containing acids such as sulfuric acid, 40 to 50% (322-326) fluoroboric phosphoric acids (589) phosphoric acid (327, 328) and mixtures of phosphoric and nitric acid (74. 322, 323. 329-331). From strong acid solutions, solid diazonium salts can be isolated (34, 332. 333). 

Burgstahler et al. diazotized the sparingly soluble hydrochloride of 3,4-di-r-butylaniline at O-S", treated the filtered solution with sodium fluoroborate, and obtained the diazonium fluond>orate. This salt underwent slow solvolysis in a mixture of acetic acid — acetic anhydride with formation of the corresponding acetate. 

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