Diazonium fluoroborates amines

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 ... 

In general the required diazonium fluoroborate is obtained as a precipitate when a concentrated solution of sodium fluoroborate is added to a solution of a diazonium salt. In an alternative procedure (e.g. the preparation of p-fluoroanisole, cognate preparation in Expt 6.77), the amine is diazotised in solution in aqueous fluoroboric acid. The diazonium fluoroborates are less sensitive to shock and heat than most diazonium salts and may be prepared and handled in the dry state with relative safety. Most diazonium fluoroborates have definite decomposition temperatures and the rates of decomposition, with few exceptions, are easily controlled. Diazonium fluoroborates containing the nitro group, however, usually decompose suddenly and with violence on heating in such cases the fluoroborate should be mixed with 3-4 times its weight of dry sand and heated cautiously until decomposition commences. 

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-... 

The Balz-Schiemann reaction, the classic synthesis of fluoro-aromatic compounds, involves diazotisation of an aromatic amine, isolation of the diazonium fluoroborate or hexafluorophosphate, then thermal decomposition of the dry salt, usually diluted with sand for safety. It is a useful method, with application to a number of heterocyclic systems, for example methyl 3-aminothiophene-2-carboxylate. ... 

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 ... 

The Schiemann reaction is the most common method for the introduction of fluorine onto an aromatic ring. It is carried out by treatment of a primary aromatic amine with sodium nitrite in aqueous HCl followed by addition of HBF or NaBF. The diazonium fluoroborate salt precipitates and is collected and dried. Heating the dry salt brings about its decomposition to an aryl fluoride, nitrogen, and boron trifluoride. The Schiemann reaction is also thought to involve an aryl cation intermediate. 

Boron fluoride Diazonium fluoroborates from amines and N-nitrosamines 1,2,4-Thiadiazolediazonium fluoroborates... 

MNN is obtained in 3—5% yield by the nitration of naphthalene and is present at this level in coml MNN. It is best prepd by indirect methods for example, by the removal of an amino group from an appropriately substituted nitro-naphthylamine. The amine is treated with Na nitrite and acid to form the diazonium salt which is replaced with a H atom by redn with EtOH (Ref 7). It may also be prepd by treatment of 6-nitro-l, 2,3,4-tetrahydronaphthalene with Br to form a dibromo compn (probably the 1,4-isomer), followed by removal of two moles of H bromide by distn in the presence of base (Ref 10). 2-Naphthalenediazonium fluoroborate... 

ArNHj remains (as it is a weak base), but the concentration is low enough to prevent as yet undiazotised amine undergoing a coupling reaction with the first formed ArNj p. 147). Aromatic diazonium chlorides, sulphates, nitrates, etc., are reasonably stable in aqueous solution at room temperature or below, but cannot readily be isolated without decomposition. Fluoroborates, ArN2 BF4 , are more stable (cf. stabilising effect of BF on other ion pairs, p. 136) and can be isolated in the dry solid state thermolysis of the dry solid is an important preparative method for ffuoroarenes ... 

Research on the fluorination of aromatic amines has provided access to 4-fluoro-2,l,3-benzo-thiadiazole (77). Treatment of 4-amino-2,l,3-benzothiadiazole (1) with nitrosonium tetra-fluoroborate (NOBF4), followed by heating of the resulting diazonium salt, gave the aryl fluoride (77) in 23% yield <92SC73>. 

Since volatile, anhydrous hydrogen fluoride is extremely corrosive and toxic, the fluorodediazo-niatron of aromatic amines with this agent require.s special apparatus and. safety conditions, which are not available in most laboratories. Thus, the decomposition of diazonium tclra-fliioro bo rates, which can be readily prepared from aromatic amines and casier-to-handle tetra-fluoroboric acid, is the commonest fluorodediazoniation process, usually referred to as the Balz-Schiemann reaction (see Vol. E 10a, p 705IT). 

The diazonium salt (70) is prepared from the amine with sodium nitrite in fluoroboric acid. Photolysis of the resulting solution gives the 3-fluoro derivative (71) (77JMC386). 

Nitrous acid (3), which is generated from sodium nitrite and aqueous acid, undergoes dehydration under the acidic conditions to form nitrosyl cation 4. Reaction of 4 with the aryl amine eventually affords the aryl diazonium salt after tautomerization and dehydration. The addition of fluoroboric acid sets the stage for the second step leading to aryl fluoride. 

The preparation of heteroarotinoids (9) and (10) Schemes 1.1 and 1.2) [29,31] began with the conversion of the known amines (18) [71] and (19) [72] to the respective fluoroborate diazonium salts (20) and (21), respectively, by standard techniques. Generation of the aromatic free radical was accomplished in acetone in the presence of the stable free radical TEMPO. Cyclization of hetero-substituted aromatic free radicals generally occurs in a regiospecific manner with the formation of an exocyclic primary radical which then couples with a second molar equivalent of TEMPO [73-75]. However, both exo- and endo-cyclic free radicals were generated from salt... 

Nucleophilic additiot) of the amine to NjOj, is followed by tautomerisation, which permits elimination of H2O to give the diazonium salt product this is converted to the BF salt by treatment with fluoroboric acid. 

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