Benzenediazonium Tetrafluoroborate1

[369-57-3]  · C6H5BF4N2  · Benzenediazonium Tetrafluoroborate  · (MW 191.92)

(much higher stability than the corresponding chlorides;2 shock-insensitive; often used when pure, isolated arenediazonium salts are needed;1 reagent for introducing aryl, arylazo, arylhydrazono, or amino groups; forms fluoroarenes upon heating;3 building block for heterocycles)

Alternate Name: phenyldiazonium tetrafluoroborate.

Physical Data: colorless solid after recrystallization; turns pink at 80 °C; decomposes at 114-116 °C.

Solubility: fairly sol polar solvents such as acetonitrile, acetone, pyridine, DMF, DMSO, and HMPA with decomposition; slightly sol water; insol hydrocarbons and Et2O; solubilized in nonpolar media by crown ethers.

Form Supplied in: 4-nitrobenzenediazonium tetrafluoroborate is widely available (nearly colorless solid). A large variety of arenediazonium BF4 salts are readily prepared from aromatic amines (see below).

Preparative Methods: 1a,1d,3,4 The most commonly used procedures consist in diazotization of an aromatic amine (ArNH2) with NaNO2 in aqueous HCl or H2SO4 followed by precipitation of the salt with added NaBF4 or fluoroboric acid.5 Alternatively, the diazotization can be carried out directly in 40-50% aqueous HBF4.2c Aromatic amines that do not dissolve in aqueous mineral acids can be reacted with NO+BF4- in an anhydrous organic solvent or in liquid SO2.6 This method can also be applied to N-(trimethylsilyl)anilines7 and for the preparation of those salts that are not easily isolated from water.8 An improved, high-yielding one-pot procedure9 employs ArNH2, t-BuONO, and BF3.Et2O in an anhydrous organic solvent, typically CH2Cl2.

Handling, Storage, and Precautions: the dry parent salt can be stored for more than a month at rt or for a few years at -20 °C under N2 in the dark, but decomposes when exposed to direct sunlight. Rapid recrystallization from warm water2b or from acetonitrile-Et2O is possible without decomposition. Although arenediazonium tetrafluoroborates, in contrast to the chloride salts, are renowned in general for their enhanced thermal stability and shock-insensitivity, some care should nevertheless be taken. Some salts are known to decompose while drying, e.g. 3-methoxybenzenediazonium, 2-methylbenzenediazonium, and certain heteroarenediazonium tetrafluoroborates.3,10 Avoid contact with metals.

General Reactivity Patterns.

Similar to the chloride salts, arenediazonium tetrafluoroborates can be transformed into covalent azo compounds by addition of a nucleophile and into various functionalized arenes by displacement of the N2+ group. While the chloride salts are typically prepared in situ in an acidic aqueous or ethanolic solution, the tetrafluoroborates are usually isolated and can be employed as pure compounds in the solvent of choice or in suspension. Their low solubility in nonpolar organic solvents may be a problem, but it can be overcome by phase-transfer techniques.1c,11

Addition to Unsaturated Compounds with Retention of N2.

Diarylazo compounds (Ar1-N=N-Ar2) are obtained by coupling of arenediazonium salts with sufficiently electron-rich (hetero)aromatic substrates. The vast majority of these important azo coupling reactions have been carried out in the reaction media obtained by diazotization of aromatic amines with NaNO2 in aqueous HCl or H2SO4 (see Benzenediazonium Chloride). Comparisons of reactivity between in situ generated arenediazonium chlorides and tetrafluoroborates (used as isolated salts) are rare.12 Thus 4-nitrobenzenediazonium chloride reacts only slowly with indole and its 1-, 2-, and 3-Me derivatives in aqueous neutral solution, whereas the corresponding BF4 salt in H2O-ethanol undergoes azo coupling rapidly and almost quantitatively (eq 1).13a Similarly, several arenediazonium tetrafluoroborates couple smoothly to 2-amino-4-(alkyl or aryl)-1,3-oxazoles in aqueous NaHCO3 solution, whereas neutral or weakly acidic solutions of salts prepared by in situ diazotization give intractable product mixtures.13b In other cases, the use of pure, water-free ArN2+BF4- salts in an organic solvent may be necessary or may simply give better yields. Examples include azo coupling of PhN2+BF4- with 2-dimethylaminopyrazoles in MeCN14 and of l5-phosphinines in MeOH-benzene (eq 2).15 Uncommon substrates such as calixarene,16 ditellurafulvene,17 sesquifulvalene,18 and coordinated cyclooctatetraene19 are also subject to the reaction.

Azo coupling is promoted by various phase-transfer catalysts such as crown ethers,20,21 NaOAc,22 Me4NCl,23 and Na[B(3,5-di-CF3-C6H3)4].24

Arenediazonium BF4- or PF6- salts undergo a facile, probably concerted, [2 + 4] cycloaddition to various acyclic methyl- or aryl-substituted 1,3-dienes, and 1,6-dihydropyridazines are isolated (eq 3).25,26 In contrast, the products isolated with cyclopentadiene result from an initial azo coupling process.27

Azomethine ylides and other 1,3-dipoles also afford cycloadducts with diazonium salts.28 Ene-type reactions with unsaturated lactones have also been reported.29

Addition to Unsaturated Compounds with Loss of N2 (Introduction of Aryl Groups).

Arylation of alkenic compounds by diazonium salts, yielding saturated or unsaturated products, is known as the Meerwein reaction and is usually catalyzed by copper salts. The procedure can be efficiently applied to intramolecular cyclizations (eq 4).30 In the presence of iodide ions the reaction takes place without copper salts.31 Titanium(III) salts also facilitate the addition of alkenic compounds to give reduced products (eq 5).32 Arylation of ferrocenyl-substituted alkynes and alkenes succeeds without such catalysts.33

When the thermal decomposition of ArN2+BF4- is conducted in the presence of pyridine, biaryls are formed in yields up to 30-40% by a radical mechanism, and no ArF is found.34 Much higher yields of unsymmetrical biaryls can be obtained when arenediazonium BF4- or PF6- salts are allowed to react with an aromatic compound in the presence of KOAc and a phase-transfer catalyst35 (eq 6). This procedure often gives better yields than the classical Gomberg-Bachmann-Hey reaction that employs arenediazonium chlorides in a two-phase system. The method also holds promise for the intramolecular version of aryl-aryl coupling (Pschorr cyclization).11 Titanium(III) salts enhance ortho selectivity in the arylation of phenols.36

Diazonium salts add to nitriles with loss of dinitrogen to form N-arylnitrilium ions which further react in situ with nucleophiles or aromatics. Hence, intramolecular cyclizations take place starting from ortho-substituted diazonium salts (eqs 7 and 8); substituents used are azido,37 carboxyl,38 alkylthio,39 hydroxymethyl,40 and aryl.41 In this connection, intermolecular addition of carboxylates to nitrilium ions gives unsymmetrical N-arylimides.42 In place of nitriles, isocyanides also undergo intramolecular cyclizations with ortho-substituted diazonium salts.43 Reactions with carbon monoxide under high pressure44 or with Tetracarbonylnickel45 result in the formation of carboxylic acid derivatives.

Coupling with Nucleophiles with Retention of N2 (Introduction of Azo or Hydrazono Groups).

The coupling of diazonium salts with active methylene compounds is known as the Japp-Klingemann reaction, a process applicable to nucleoside synthesis.46 On the other hand, a-amination of simple esters is achieved through the hydrogenation of azo or hydrazono esters formed from ketene silyl acetals (eq 9).47 The reaction with enamines likewise leads to a-iminio hydrazones which readily rearrange to heterocycles (eq 10).48 The nucleophilic addition of p-Tolylsulfonylmethyl Isocyanide also results in cyclization, affording triazoles.49 In addition, lithium enolates of a-substituted ketones,50 Grignard reagents,50,51 organozinc reagents,52,53 and allylsilanes54 produce corresponding azo or hydrazono compounds. Potassium Cyanide in the presence of 18-Crown-6 yields diazo cyanides that act as efficient dienophiles in the Diels-Alder reaction (eq 11).55

Amines also combine with the terminal nitrogen of diazonium salts to form triazenes (eq 12),56 the utility of which is summarized in a review.57 Couplings with Difluoramine or isopropyl fluorocarbamate yield azides through triazenes as intermediates.58 Reaction with hydrazonomethanesulfonates or guanidine derivatives give five-membered azacycles.59 The addition of sodium sulfinates produces arylazo sulfones.60

Coupling with Nucleophiles with Loss of N2 (Introduction of Aryl Groups).

Arenediazonium salts act as aryl cations toward various nucleophiles. Silyl enol ethers derived from aryl ketones react with diazonium tetrafluoroborates in pyridine to afford a-aryl ketones (eq 13).61

Solvolysis of diazonium tetrafluoroborates in Trifluoromethanesulfonic Acid results in the formation of aryl triflates (eq 14).62 Similarly, decomposition in methanol63 or Trifluoroacetic Acid64 leads to the corresponding ethers or esters; the latter is utilized for phenol synthesis. Pyridine N-Oxide reacts at the oxygen atom to give N-(aryloxy)pyridinium tetrafluoroborates.65 Phenyl ethers and esters can be prepared via the reaction of trimethylsilyl ethers and esters with PhN2BF4.66

Unsymmetrical diaryl sulfides are obtained from diazonium fluoroborates and sodium aryl thiolates in DMSO (eq 15).67 Similarly, aryl thiolesters are formed using thiocarboxylates.68 Diaryl selenides and diaryl tellurides can be prepared by the reaction of arenediazonium salts with Sodium Selenide and Sodium Telluride (eq 16).69

Controlled thermal decomposition of dry arenediazonium tetrafluoroborates affords fluoroarenes in normally good to high yield (Balz-Schiemann reaction3) (eq 17). Typically, the reaction is carried out with the solid salt,5 but decomposition in suspension or in solution2b,3,70 has also been reported. Nitroarenediazonium BF4- salts are usually mixed with sand, NaF, or BaSO4 to avoid violent decomposition.3 The BF3 etherate complex may be useful for an efficient transformation.71

The formation of fluoroarenes proceeds via an aryl cation intermediate that reacts with the BF4- ion in the ion pair,72 and is accelerated by photolysis.73 In some cases, the use of hexafluoroantimonates or hexafluorophosphates affords better yields.74 On the other hand, aryl iodides are obtained via the reaction with Potassium Iodide/Iodine in DMSO.75 The reactions with trimethylsilyl halides or Azidotrimethylsilane yield aryl halides or aryl azides, respectively (eq 18).76 Organic halides such as BrCCl3 and Iodomethane also act as halogen sources.77

Diazonium fluoroborates transform iminophosphoranes into phosphonium salts.78 Similarly, sulfones are converted to sulfoxonium salts which can be reduced in situ to sulfoxides with Sodium Borohydride.79

Nucleophilic substitution of diazonium salts in the presence of CuI salts is known as the Sandmeyer reaction. Nitro-dediazoniation of diazonium tetrafluoroborate with Sodium Nitrite in the presence of copper powder occurs smoothly (eq 19).2c An important access to arenephosphonic acids is provided by the CuI-catalyzed reaction of ArN2+BF4- salts with Phosphorus(III) Chloride in an organic solvent, followed by hydrolysis (eq 20).80 The method tolerates a wide range of substituents.81 The thermal decomposition of arenediazonium salts in dilute aqueous solution in the presence of Cu2O and a large excess of Cu(NO3)2 affords phenols in good yield; notably, this method does not require a strongly acidic medium.82

Palladium-Catalyzed Reactions.

Palladium-catalyzed reactions of diazonium salts proceed with nitrogen evolution and provide a mild method to introduce aryl groups.

While the classical, copper salt-catalyzed arylation of alkenes (Meerwein arylation) requires alkene activation by an electron-withdrawing substituent83 (see Benzenediazonium Chloride), the Pd-catalyzed modification succeeds well not only with acrylic aldehydes and esters, but also with styrene, ethylene, and other non-activated acyclic and cyclic alkenes (eqs 21 and 22).84 Coupling reactions with isolated PhN2+BF4- in general give much better yields than with the in situ generated salt PhN2+X- (X = Cl or OAc). In an analogous reaction, silyl-, germyl-, or stannyl-substituted alkenes result in the loss of heteroatom substituents (eq 23).85-87

The palladium-catalyzed carbonylation of an arenediazonium tetrafluoroborate with CO in the presence of Et3SnH or a poly(methylhydrosiloxane) affords substituted benzaldehydes (eq 24).88 Formation of ArH is sometimes a minor side-reaction, but predominates (81%) in the 2-nitro case. In related Pd-catalyzed reactions, diaryl ketones (eq 25),89 aryl alkyl ketones,89 substituted benzoic acids,90 and mixed arylalkylcarboxylic anhydrides (eq 26)91 can be obtained. Thermal disproportionation of the latter can be used to prepare homo arenecarboxylic anhydrides.91

Reduction to Arenes.

The removal of an NH2 function by reductive dediazoniation is an important synthetic operation on the aromatic nucleus. As with other benzenediazonium salts, a number of methods exist for the BF4- salts, but the radical-chain reaction with H3PO2 is a particularly good method (eq 27).92 Hydro-dediazoniation with NaBH4 in methanol93 is also quite convenient, but does not always occur cleanly.94 Other effective and easily available reducing agents are Thiophenol,95 hydrosilanes,96 and hydrostannanes.96 Decomposition in HMPA97 or in the presence of crown ethers98 also gives good results. Catalysis by rhodium phosphine complexes in DMF does not seem very effective.99

Benzeneselenol is a unique reagent for producing arylhydrazines from diazonium fluoroborates (eq 28); the reaction is applicable to the synthesis of indazolone from ortho-carbamoyl diazonium salts.100

Radical Initiation.

Diazonium fluoroborates can be used as radical initiators. Applied examples are alkylation of heteroaromatics with alkyl iodides101 and polymerization of vinylic compounds.102


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Gerhard Maas

University of Kaiserslautern, Germany

M. Tanaka & Toshiyasu Sakakura

National Institute of Materials & Chemical Research, Tsukuba, Japan



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