Diazonium hexafluorophosphates

A. o-Bromobenzenediazonium hexafluorophosphate. A solution of 95 ml. of 12N hydrocMoric acid in 650 ml. of water is added with stirring to 60 g. of o-bromoaniline (0.35 mole Note 1) in a 2-1. three-necked flask equipped with stirrer and thermometer. Solution is effected by heating the mixture on a steam bath (Note 2). A solution of 29 g. (0.42 mole) of sodium nitrite in 75 ml. of water is added with stirring while the mixture is maintained at — 5° to —10° by means of a bath of ice and salt or of dry ice and acetone. At the end of the addition there is an excess of nitrous acid, which can be detected with starch iodide paper. Seventy-four milliliters (134 g., 0.60 mole) of 65% hexafluorophosphoric acid (Note 3) is added in one portion, with vigorous stirring, to the cold solution of the diazonium salt. Cooling and slow stirring are continued for an additional 30 minutes, and the precipitated diazonium hexafluorophosphate is then collected on a Bilchner funnel. The diazonium salt is washed on the funnel with 300 ml. of cold water and with a solution of 80 ml. of methanol in 320 ml. 

A very convenient and efficient modification of the diazonium hexafluorophosphate technique has been reported 274 it involves the decomposition of diazonium trifluorotris(per-fluoroalkyl)phosphates. These salts precipitate quantitatively when aqueous solutions of diazonium chlorides and alkaline trifluorotris(perfluoroalkyl)phosphates are mixed. These very stable latter reagents are obtained quantitatively by addition of an alkaline fluoride to a di-fluorotris(perfluoroalkyl)-A5-phosphane. 

The diazonium hexafluorophosphates, prepared similarly from the appropriate diazonium chloride solution and hexafluorophosphoric acid, may in general be used instead of the fluoroborates with advantage. The thermal decomposition of diazonium hexafluorophosphates to aryl fluorides generally proceeds smoothly and in better yield.29... 

Triphenylmethyl, tropylium, xanthylium, acylium and diazonium salts have also been used successfully in the study of the polymerization of THF [50]. In fact the true equilibrium conversions were first demonstrated using p-chlorophenyl diazonium hexafluorophosphate [41, 115], and at the same time the polymerizations were shown to be living . The lower equilibrium yields [50] observed when other counter-ions are employed, e.g. SbClg, appear to arise because of a termination mechanism associated with the anions. Kinetic studies [50] of the polymerization of bulk THF with Phg C SbCl have established the apparent rate law... 

Diazonium Salts. The first use of a diazonium salt as an initiator for cationic polymerization of cyclic ethers was reported in 1965 by Dreyfuss and Dreyfuss (1 ), who announced the polymerization of tetrahydrofuran initiated by thermal decomposition of a "new catalyst" which they identified as benzene-diazonium hexafluorophosphate. In a later publication (13) this material was correctly identified as p-chlorobenzenediazomum hexaf1uorophosphate). 

Dreyfuss and Dreyfuss (1 ) reported that p-chlorobenzene-diazonium hexafluorophosphate was not a very effective initiator for polymerization of epoxides, based on an observation that thermal decomposition of the diazonium salt in the presence of propylene oxide did not yield a high molecular weight polymer. 

More prevalent than diazonium fluorosilicates in so-called modified Baiz-Schiemann reactions have been diazonium hexafluorophosphates.16 In some cases these latter diazonium salts give higher yields of aryl fluorides. Some examples are shown below, with the normal Balz-Schiemann reaction yields using diazonium tetrafluoroborates given in parentheses.163... 

Polymerizations of tetrahydrofuran with the aid of chlorobenzene diazonium hexafluorophosphate [343], triphenylmethyl hexachloroantimonate [344], or phosphorus pentafluoride [445]. 

Hexafluorophosphates. There is a great deal of interest in the hexafluorophosphate anion [1691-18-8], mostly as organic hexafluorophosphates for catalysis in photopolymerization. A number of the compounds are diazonium compounds (see Photoreactivepolymers). 

Protonic initiation is also the end result of a large number of other initiating systems. Strong acids are generated in situ by a variety of different chemistries (6). These include initiation by carbenium ions, eg, trityl or diazonium salts (151) by an electric current in the presence of a quartenary ammonium salt (152) by halonium, triaryl sulfonium, and triaryl selenonium salts with uv irradiation (153—155) by mercuric perchlorate, nitrosyl hexafluorophosphate, or nitryl hexafluorophosphate (156) and by interaction of free radicals with certain metal salts (157). Reports of "new" initiating systems are often the result of such secondary reactions. Other reports suggest standard polymerization processes with perhaps novel anions. These latter include (Tf)4Al (158) heteropoly acids, eg, tungstophosphate anion (159,160) transition-metal-based systems, eg, Pt (161) or rare earths (162) and numerous systems based on tri flic acid (158,163—166). Coordination polymerization of THF may be in a different class (167). 

Arenediazonium hexafluorophosphates represent a promising alternative to diazonium fluoroborates because of decreased water solubility and, in some instances, higher yields of fluoroaromatics [27, 28] (equation 6)... 

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. 

These salts can be made easily since tetrafluoroboric acid (HBF4) and hexa-fluorophosphoric acid (HPF6) are commercially available. However, the main advantage of the diazonium salts with the anions of these acids is their stability, which is significantly higher than that of probably all other diazonium salts. 4-Nitrobenzenediazonium tetrafluoroborate is nowadays even a commercial product. Preparative diazotization methods with these two acids can be found in Organic Syntheses (tetrafluoroborate Starkey, 1943 hexafluorophosphate Rutherford and Redmont, 1973). 

The first diazonium-salt-crown-ether adduct was isolated and identified as a 1 1 complex by Haymore et al. (1975). Unfortunately Haymore never published the X-ray structural analysis of benzenediazonium hexafluorophosphate with 18-crown-6 which he performed in 1980. ORTEP drawings with measured bond angles and lengths from Haymore s investigation can be found in a review chapter by Bartsch (1983, p. 893). A few data from Haymore s work (e.g., R = 0.064) were also mentioned by Cram and Doxsee (1986, footnote 7). Groth (1981) published the results of his X-ray investigation of 4-methoxybenzenediazonium tetrafluoroborate and 21-crown-7 (R = 0.042) and Xu et al. (1986) those of 4-methoxybenzenediazonium tetrafluoroborate and dibenzo-24-crown-8 (R = 0.086). 

B. l-Bromo-2-fluorobenzene. Cautionl This step should be carried outm a hood because the PFS evolved on thermal decomposition of the diazonium salt is poisonous. The apparatus consists of a 1-1., three-necked, round-bottomed flask equipped with a thermometer, a condenser, a magnetic stirrer (optional), and a 250-ml. Erlenmeyer flask that is attached by means of a short rubber Gooch connecting tube. The dry powdered hexafluorophosphate salt is placed in the Erlenmeyer flask, and 300 ml. of heavy mineral oil is placed in the round-bottomed flask. The mineral oil is heated to 165-170° by means of an oil bath or electric heating mantle and maintained at this temperature while the salt is added rapidly in portions over a period of 30 minutes. The flask is cooled rapidly to room temperature, the side flask is removed, and 400 ml. of 10% aqueous sodium carbonate is added slowly through the condenser. The mixture is steam-distilled until no more oil is visible in the distillate. 

As mentioned above, the conventional diazonium salts have good optical properties as CEL dyes and negative working sensitizers for the two-layer resist system. However, almost all diazonium salts are stabilized with metal-containing compounds such as zinc chloride, tetrafluoroborate, hexafluoro-antimonate, hexafluoroarsenate, or hexafluorophosphate, which may not be desirable in semiconductor fabrication because of potential device contamination. To alleviate the potential problem, new metal-free materials have been sought for. 

Hexafluorophosphate salts behave similarly.101 The diazonium tetrafluoroborates can be prepared either by precipitation from an aqueous solution by fluoroboric acid102 or by anhydrous diazotization in ether, THF, or acetonitrile using /-butyl nitrite and boron trifluoride.103 104 Somewhat milder conditions can be achieved by reaction of aryldiazo sulfide adducts with pyridine-HF in the presence of AgF or AgN03. 

Attempts to prepare fluorothiophenes from diazonium salts (the fluoro-borate and hexafluorophosphate salts) have met only variable success. Methyl 3-fluorothiophene-2-carboxylate (32) was obtained in 89% yield by this method (Scheme 12), but the 3-diazonium salt of the corresponding 4-ester could not be isolated. Furthermore, the methyl ester of 2-diazothiophene-3-carboxylic acid coupled before decomposition could be attempted [85H(23) 1431]. 

Difluorotris(perfluoroalkyl)-25-phosphancs (Rf)3PF2 give stable salts with cesium fluoride, potassium fluoride, sodium fluoride or arenediazonium fluorides, i.e. Cat + [(RF)3PF3] , where Cal+ = Cs +. K+, Na +, ArN2+. The last are difficult to dissolve and they decompose at much lower temperatures (60-90 C) than tetrafluoroborates and hexafluorophosphates. which allows more complete precipitation of diazonium salts from water solutions and their conversion into aryl fluorides 6 without tar formation.6... 

Diethylamino)benzenediazonium hexafluorophosphate also decomposes more rapidly and more efficiently than the corresponding diazonium tetrafluoroborate under UV irradiation (350 nm) at room temperature (yield 72% vs 53-55%).247... 

Decomposition of the diazonium trifluorotris(perfluoroalkyl)phosphates proceeds mildly, without formation of tars (since no strong Lewis acid evolves), and with high yields of the substituted fluorobenzenes. For example, l-fluoro-4-nitrobenzene is obtained in a far better yield (82%) than from 4-nitrobenzenediazonium hexafluorophosphate or tetrafluoroborate [cf. formation of 2 (R = 4-N02)]. The coproduced difluorotris(perfluoroalkyl)-A5-phosphanes are not soluble in the resulting fluorobenzene derivatives and, because of their high density, form a lower layer which is readily separated. Thus, these difluoro-25-phosphanes can be regenerated quantitatively and used repeatedly. 

Diazonium hexafluoroantimonates are the best alternative to diazonium tetrafluoroborates. The preparation of the hexafluoroantimonate salts is. in general, as practicable as that of the corresponding tetrafluoroborates or hexafluorophosphates, as they are insoluble in water and stable to both light and air. Furthermore, thermal decompositions of hexafluoroantimonates can usually be achieved at lower temperatures than decompositions of the corresponding tetrafluoroborates and hexafluorophosphates (see Table 10). This is of particular interest as the severity of the decomposition (fluorodediazoniation) step can be decreased. Moreover, electron-withdrawing groups (Table 10. entries 5-10) do not reduce the yield of the fluoro derivatives. ... 

Introduction of a fluorine atom is achieved using the Schiemann reaction. Originally, the reaction involved gently heating the solid diazonium fluoroborate (Scheme 8.18), but improved yields result from the thermal decomposition the hexafluorophosphate, ArN2 PF, or hexa-fluoroantimonate, ArN2 SbFg, salts. 

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