Fluorinations tetrafluoroborate

Compounds containing fluorine and chlorine are also donors to BF3. Aqueous fluoroboric acid and the tetrafluoroborates of metals, nonmetals, and organic radicals represent a large class of compounds in which the fluoride ion is coordinating with trifluoroborane. Representative examples of these compounds are given in Table 5. Coordination compounds of boron trifluoride with the chlorides of sodium, aluminum, iron, copper, 2inc, tin, and lead have been indicated (53) they are probably chlorotrifluoroborates. 

Boric acid and fluoride ion react to form a series of fluroborates where OH is displaced by (see Fluorine compounds, inorganic—Boron, FLUOROBORic acid). Stepwise formation of the ions fluorotrihydroxyborate [32554-53-3] BF(OH) 3, difluorodihydroxyborate [32554-52-2] BF2(OH) 2 trifluorotrihydroxyborate [18953-00-9] BF3(OH) 3, proceeds rapidly in acidic solutions, but tetrafluoroborate [14874-70-5], forms slowly (56). A... 

Fluorine and nitrogen may be added to olefins with the nitrogen in different oxidation states Fluorine and a nilro group are added by reaction of an olefin with nitryl fluoride [131], nitronium tetrafluoroborate [195] (equation 32), or a combination of nitric acid and hydrogen fluoride [131, 196] (equation 33)... 

The fluoraza reagents consist of two types of compounds one in which a fluorine atom is bound to the nitrogen atom of an amide or, more often, a sulfonamide and one in which a fluorine atom is bound to the nitrogen atom of a tertiary amine such as pyridine, quinuclidine, or triethylenediamine 1,4-diaza-bicyclo[2 2.2]octane. The positive charge on the nitrogen is counterbalanced by a non-nucleophilic anion such as triflate or tetrafluoroborate. 

This topic has been reviewed [2, pp 94, 100-111, 130-134] All of the standard approaches to the synthesis of a compound like methyl 2-fluorostearate from methyl 2-bromostearate result mall yield of the 2-fluoro ester and the unsaturated esters. Although silver fluoride is not a new reagent, its use moist in wet acetonitrile to convert methyl 2-bromostearate to its fluoro ester is a departure from the traditional set of anhydrous conditions (Procedure 6, p 194) [71] In contrast, silver tetrafluoroborate converts a-chloroketones to their respective fluoroketones under anhydrous conditions. The displacement of less activated halogen groups by silver tetrafluoroborate to form their respective fluorides is novel Although silver tetrafluoroborate could not be used to convert an aliphatic terminal dichloromethyl or trichloromethyl group to its corresponding fluoro derivative, it is an effective fluorine source in other situations [72] (Table 8)... 

The Balz-Schiemann reaction continues to attract attention, with much of it generated by the interest in fluoroquinolones, e.g., (7), which is a potential antibacterial. Two approaches to its synthesis are possible—introduction of fluorine prior to or post ring construction. Decomposition of the tetrafluoroborate salt was unsuccessful, whereas the PF6 salt (8) gave only a poor yield (84JMC292). A more successful approach was the introduction of F into the pyridine nucleus prior to formation of the 1,8-naphthyridine ring (84JHC673). A comparison of decomposition media showed that cyclohexane was the best with regard to yield and time. 

On the basis of these redox potentials it seems likely that direct electron release to the benzenediazonium ion takes place only with iodide. This corresponds well with experience in organic synthesis iodo-de-diazoniations are possible without catalysts, light, or other special procedures (Sec. 10.6). For bromo- and chloro-de-di-azoniations, catalysis by cuprous salts (Sandmeyer reaction, Sec. 10.5) is necessary. For fluorination the Balz-Schiemann reaction of arenediazonium tetrafluoroborates in the solid state (thermolysis) or in special solvents must be chosen (see Sec. 10.4). With astatide (211At-), the heaviest of the halide ions, Meyer et al. (1979) found higher yields for astato-de-diazoniation than for iodo-de-diazoniation, a result consistent with the position of At in the Periodic System. It has to be emphasized, however, that in investigations based on measuring yields of final products (Ar-Hal), the possibility that part of the yield may be due to heterolytic dediazoniation is very difficult to quantify. 

The only author to postulate a homolytic mechanism in the last few decades was Deng (1989). His arguments are based on the formation of small amounts of fluorinated bi- and polyphenyls in thermal fluoro-de-diazoniations and in mass-spec-trometric degradations of benzenediazonium tetrafluoroborate and its substituted derivatives. However, he does not include a critical discussion of his work. 

For the introduction of fluorine into aromatic and heteroaromatic compounds the photolytic fluoro-de-diazoniation sometimes has advantages compared with the corresponding thermal dediazoniation (Balz-Schiemann reaction, see Sec. 10.4). For aromatic substrates the reaction was studied by Rutherford et al. (1961), Christie and Paulath (1965), Petterson et al. (1971), and Becker and Israel (1979). Hexafluorophos-phates sometimes give better yields than tetrafluoroborates (Rutherford et al., 1961). In analogy to Balz-Schiemann reactions in solution (Fukuhara et al., 1987), photolytic fluoro-de-diazoniations of benzene derivatives with electron-withdrawing substituents give lower yields. 

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... 

When the fluorine used for synthesis contained traces of oxygen, the solid behaved as a powerful oxidant (causing 2-propanol to ignite on contact) and it also exploded on impact. Material prepared from oxygen-free fluorine did not show these properties, which were ascribed to the presence of traces of dioxy geny 1 tetrafluoroborate (above). 

A representative series of alk-l-yn-l-ylxenonium tetrafluoroborates were synthesized (eq 4). Perfluoroorganoethynylxenonium tetrafluoroborates, [RFC=CXe][BF4], were isolated in 30-98 % yield, and non-fluorinated organo-... 

Koppe, K. Frohn, H.-J. Mercier, H. P. A. Schrobilgen, G. J. Difluoro-pentafluorophenylxenon (IV) Tetrafluoroborate, [C6F5XeF2][BF4], an Improved Synthesis and Detailed Structural Study. In Abstracts, 18th International Symposium on Fluorine Chemistry, Bremen, Germany, Jul 30-Aug 4,2006 p 219. 

Fluorination of B(, H(, 2 is difficult in contrast to halogenation by Cl, Br, and I. However, B HsF2- is obtained by using the tetrafluoroborate of 1-chlormethyl-4-fluoro-l,4-diazabicyclo[2.2.2]-octane, a reaction that is described formally in Eq. (58) [82],... 

Gillespie, R. J., Hartman, J. S. Change of Sign of the Boron-Fluorine Spin-spin Coupling Constant in the Tetrafluoroborate Anion. J. Chem. Phys. 45, 2712 (1966). 

The above book [1] deals, in 26 chapters in 5 sections, with theoretical and practical aspects of the use and safe handling of powerful oxidants and their complementary reactive fuels. Materials include nitrogen pentaoxide, perfluoroam-monium ion and salts, nitronium tetrafluoroborate, hydrazinium mono- and diperchlorates, nitronium perchlorate, tricyanomethyl compounds, difluoramine and its alkyl derivatives, oxygen difluoride, chlorine trifluoride, dinitrogen tetraoxide, bromine trifluoride nitrogen fluorides, and liquid ozone-fluorine system. 

In order to use the potentiality of the Balz Schiemann reaction and to increase the specific radioactivity, the tetrafluoroborate anion was substituted for the tetrachloroborate. The fluorination of p-toluidyl diazonium tetrachloroborate. 

Methylene-l,3-benzoxazin-2-ones 194 were found to be susceptible to addition to the exocyclic double bond. On heating under reflux in ethanol, oxazinones 194 were transformed to the 4-ethoxy derivatives 195. Treatment of 194 with l-chloromethyl-4-fluoro-l,4-diazabicyclo[2.2.2]octane bis(tetrafluoroborate) (F-TEDA-BF4), an electrophilic fluorinating reagent, in the presence of methanol, led to the 4-fluoromethyl-4-methoxy-substituted compounds 196 (Scheme 34) <2003T8163>. 

Two routes to the electrophilic fluorination of pyrrolo[2,3-3]pyridine A -oxide lead to the 4-fluoro derivative. The Balz-Schiemann reaction route, via a diazonium tetrafluoroborate salt, or a lithium/halogen-exchange reaction followed by quenching with an electrophilic fluorine source, generates the 4-fluoro product in moderate yields <20030L5023>. 

Nishida, T., Tashiro, Y., and Yamamoto, M., Physical and electrochemical properties of l-alkyl-3-methylimidazolium tetrafluoroborate for electrolyte. Fluorine Chem., 120,135,2003. 

Diazotization in the presence of boron trifluoride enables diazonium tetrafluoroborates to be isolated from the reaction mixture and purified. Subsequent controlled decomposition produces the required fluoroaromatic. Although explosion hazards and the toxicity of the isolated salts are significant concerns with this process, known as the Balz-Schiemann process, 4,4 -di-fluorobenzophenone (BDF. 6) has been prepared by this route as a monomer for the production of the engineering plastic poly(ether ether ketone) , or PEEK , by condensation with 1,4-dihydroxybenzene in the presence of potassium carbonate. BDF 6 is superior to its chlorine analog because in aromatic systems the nucleophilic displacement of fluorine is more facile than that of chlorine, leading to a shorter polymerization time and a better quality product containing less degradation impurities. 

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