Diazonium tetrafluoroborates

More recently Gorelik s group determined the structure of 1-phenylsulfonyl-2-pyrazoline-3-diazonium tetrafluoroborate by X-ray crystallography (Gorelik et al., 1989) and calculated (Glukhovtsev et al. 1990) the heats of the dediazoniation reaction of seven cyclic five-membered diazonium ions (including 2.13, R=H) relative to that of the benzenediazonium ion by the MNDO method (see Sec. 8.4). 

In most cases diazonium salts are not isolated, but are converted into products by reactions that can be carried out in situ. Moreover, it is actually recommended not to isolate these salts, not even for purification purposes, as many of them have a tendency to explode. In addition, the high solubility of most diazonium salts in water makes precipitation from this medium difficult. Therefore, to obtain solid diazonium salts the recommended method for many decades was to carry out diazotizations in ethanol followed by precipitation with ether. As inorganic salts of nitrous acid are scarely soluble in ethanol, Knoevenagel recommended alkyl nitrites (ethyl or isopentyl nitrite) as diazotization reagents as long ago as 1890. Various other solvents have subsequently been used for diazotizations with alkyl nitrites (see Saunders and Allen, 1985, p. 23 ff.), but as a method for obtaining solid diazonium salts this has been superseded by the isolation of diazonium tetrafluoroborates and, to a lesser degree, of hexafluorophosphates. 

Ahern and Gokel (1979) claim that diazocyanides can be prepared by stirring arene-diazonium tetrafluoroborates with one equivalent of KCN (solid) and 0.05 equivalent of 18-crown-6 in CH2C12 for some hours. The authors did not, however, investigate whether the products, obtained in good yields, are the (Z)- or the (ii)-isomers. 

With regard to the mechanism of these Pd°-catalyzed reactions, little is known in addition to what is shown in Scheme 10-62. In our opinion, the much higher yields with diazonium tetrafluoroborates compared with the chlorides and bromides, and the low yields and diazo tar formation in the one-pot method using arylamines and tert-butyl nitrites (Kikukawa et al., 1981 a) indicate a heterolytic mechanism for reactions under optimal conditions. The arylpalladium compound is probably a tetra-fluoroborate salt of the cation Ar-Pd+, which dissociates into Ar+ +Pd° before or after addition to the alkene. An aryldiazenido complex of Pd(PPh3)3 (10.25) was obtained together with its dediazoniation product, the corresponding arylpalladium complex 10.26, in the reaction of Scheme 10-64 by Yamashita et al. (1980). Aryldiazenido complexes with compounds of transition metals other than Pd are discussed in the context of metal complexes with diazo compounds (Zollinger, 1995, Sec. 10.1). 

As discussed in Section 10.3, the system consisting of a diazonium ion and cuprous ions can be used for hydroxy-de-diazoniation at room temperature in the presence of large concentrations of hydrated cupric ions (Cohen et al., 1977 see Schemes 10-7 to 10-9). With (Z)-stilbene-2-diazonium tetrafluoroborate under these conditions, however, the major product of ring closure of the initially formed radical was phenanthrene (64%). When the cupric nitrate was supplemented by silver nitrate the yield increased to 86% phenanthrene. Apparently, the radical undergoes such rapid ring closure that no electron transfer to the cupric ion takes place. 

Good yields of chlorides have also been obtained for reaction of isolated diazonium tetrafluoroborates with FeCl2-FeCl3 mixtures.100 It is also possible to convert anilines to aryl halides by generating the diazonium ion in situ. Reaction of anilines with alkyl nitrites and Cu(II) halides in acetonitrile gives good yields of aryl chlorides and bromides.101... 

The reaction can be carried out efficiently using aryl diazonium tetrafluoroborates with crown ethers, polyethers, or phase transfer catalysts.103 In solvents that can act as halogen atom donors, the radicals react to give aryl halides. Bromotrichloromethane gives aryl bromides, whereas methyl iodide and diiodomethane give iodides.104 The diazonium ions can also be generated by in situ methods. Under these conditions bromoform and bromotrichloromethane have been used as bromine donors and carbon tetrachloride is the best chlorine donor.105 This method was used successfully for a challenging chlorodeamination in the vancomycin system. 

Fluorine substituents can also be introduced via diazonium ions. One procedure is to isolate aryl diazonium tetrafluoroborates. These decompose thermally to give aryl fluorides.106 Called the Schiemann reaction, it probably involves formation of an aryl cation that abstracts fluoride ion from the tetrafluoroborate anion.107... 

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. 

In the classical procedure, base is added to a two-phase mixture of the aqueous diazonium salt and an excess of the aromatic that is to be substituted. Improved yields can be obtained by using polyethers or phase transfer catalysts with solid aryl diazonium tetrafluoroborate salts in an excess of the aromatic reactant.177 Another source of aryl radicals is A-nitrosoacetanilides, which rearrange to diazonium acetates and give rise to aryl radicals via diazo oxides.178... 

The photolysis of crystalline diazonium tetrafluoroborates and hexa-fluorophosphates affords a convenient route to aromatic fluorides<53) ... 

It has been shown by Zanardi, Nanni and coworkers that, instead of isocyanides, isothiocyanates can be used in the radical process [90]. Treatment of the mixture of the diazonium tetrafluoroborates 3-217 and the isothiocyanates 3-218 with pyridine or a mixture of 18-crown-6 and potassium acetate in ethyl acetate, furnished the benzothienoquinoline derivatives 3-219. In addition, in some cases the rearranged products 3-220 were furnished in low to reasonable yields (Scheme 3.57). 

Several interesting reactions have been described for quinolizine-3-diazonium tetrafluoroborate 121. Thus, its treatment with secondary amines gave the corresponding triazenes 122 <2004ZNB380>, while its reaction with 1,3-dicarbonyl compounds gave the corresponding hydrazones. In the case of alkyl 4-chloro-3-oxobutanoates, the intermediate hydrazone 123 furnished a pyrazole derivative 124, as shown in Scheme 17 <2002H(57)2091>. 

Amino 4-oxo 4//-pyrimido[l,2-4]pyridazin-3-diazonium tetrafluoroborates 73 underwent ring transformation into l-(pyridazin-3-yl)-l//-l,2,3-triazole-4-carboxylates 74 on heating in dry MeOH. Yields dropped drastically when EtOH, instead of MeOH, was used as solvent (Equation 6) <2002ARK143>. 

Juri and Bartsch (1979) have studied the coupling of 4-t-butylbenzene-diazonium tetrafluoroborate with N,N-dimethylaniline in 1,2-dichloroethane solution. The addition of one equivalent (based on diazonium salt) of 18-crown-6 caused the rate constant to drop by a factor of 10, indicating that complexed diazonium is less reactive than the free cation. Benzenediazonium tetrafluoroborate complexes of crown ethers are photochemically more stable than the free salt. The decomposition into fluorobenzene and boron trifluoride is strongly inhibited but no explanation has been given (Bartsch et al., 1977). 

A number of reactions of monocyclic phosphorins have also been described. The P-phosphorins (185) react with diazoalkanes in the presence of protic nucleophiles163 and with diazonium tetrafluoroborates in the presence of alcohols or phenols164 to give the P-phosphorins (186) and (187). 

Solid diazonium tetrachloroborates decompose very vigorously, sometimes explosively, on heating in absence of solvent. Dry o-mtrobenzcncdiazonium tetrachlorob-orate is liable to explode spontaneously during storage at ambient temperature [1]. Hazards involved in drying diazonium tetrafluoroborates have been discussed. The... 

Azidomethylbenzenediazonium tetrafluoroborate, 2708 Benzenediazonium tetrafluoroborate, 2223 2-Chloro-3-pyridinediazonium tetrafluoroborate, 1816 5-(4-Diazoniobenzenesulfonamido)thiazole tetrafluoroborate, 3119 5,6-Diphenyl-l,2,4-triazine-3-diazonium tetrafluoroborate 2-oxide, 3677... 

The aryl diazonium tetrafluoroborate (4 mmol) is added portionwise over ca. 20 min to (TEBA)2-MoS4 (2.69 g, 4.4 mmol) in dry MeCN (10 ml) at 0°C. The mixture is stirred at 0°C for 1 h and then at 25°C for ca. 5 h. The solvent is removed under reduced pressure and the residue is extracted with Et20 (5 x 30 ml). The ethereal extracts are evaporated and the product purified by chromatography from silica to yield the diaryl disulphide. 

Diazonium salts are another useful source of free radicals, and the formation of the reactive species can be achieved by reductive electrolysis or direct treatment with diazonium tetrafluoroborate salts [39]. By this route, several aryl derivatives could be introduced onto the nanotube sidewalls [40]. Aryl groups bearing halogen or alkyne functionalities are particularly interesting as they can be further reacted in Pd-catalyzed coupling reactions (Suzuki, Heck) or in click chemistry reactions to create products with great potential in materials science [41]. 

Industrial emission, auto exhaust, and tobacco smoke Sample collected in NaOH bubbler and derivatized to />-nitrobenzene-diazonium tetrafluoroborate HPLC-UV 0.05 pbb for 150 L sample NG... 

Diazopyrazoles can be converted into very stable diazonium salts (chloride or bromide) upon treatment with the corresponding concentrated acid at room temperature (76JOC3781 84JHC957). Stable diazonium tetrafluoroborate [87JOC5538], platinichloride, and aurichloride [14JCS(105)435] can be isolated. Only in the case of the unstable 3-diazo-pyrazole was it necessary to operate at low temperature and in non aqueous solvents (61CB1036). 

Imidazole-4-diazonium tetrafluoroborates can be prepared, and they show exceptional stability (73JA4619). In the imidazole series, however. 

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