Fluorination, halogen exchange

Alkali metal hexafluoroantimonates arc used, like hexafluorophosphate or tetrafluoroborate salts, to effect halogen-exchange fluorination in organosilanes both in the presence and in the absence of solvents. Fluorotriphenylsilane and difluorodiphenylsilane are obtained in 87 and 95 % yield by heating the corresponding chlorides with sodium hexafluoroantimonate in tetra-ethylene glycol dimethyl ether (tetraglyme).102 Tetrafluoroborates react with chlorosilanes faster than hexafluorophosphate or hexafluoroantimonate salts. 

Orthorhombic, deliquesc crystals. Poisonousf Irritates the skin. dj j 4.379. mp 292". bp 376". Also reported as bp 319" (Andersen). Soly in water (g/ ]00 ml) 443 (20°) 562 (30"). Dissolves in water with limited hydrolysis, readily forms complexes such as [SbFJ- and many so] salts. For the purpose of halogen exchange fluorination, the compd must be dry Henne. U.S, pot. 2,082,161 (1937). May be stored in glass vessels or steel drums. Keep well closed. 

Interhalide Conversions.- Almost quantitative halogen exchange fluorination of cyclic and tertiary halides (secondary halides... 

Scheme 15.60 Sequential Li/halogen exchange/fluorination in flow.

Halogen exchange with KF is not successful ia acetic acid (10). Hydrogen bonding of the acid hydrogen with the fluoride ion was postulated to cause acetate substitution for the haUde however, the products of dissolved KF ia acetic acid are potassium acetate and potassium bifluoride (11). Thus KF acts as a base rather than as a fluorinating agent ia acetic acid. 

Qua.driva.Ient, Zirconium tetrafluoride is prepared by fluorination of zirconium metal, but this is hampered by the low volatility of the tetrafluoride which coats the surface of the metal. An effective method is the halogen exchange between flowing hydrogen fluoride gas and zirconium tetrachloride at 300°C. Large volumes are produced by the addition of concentrated hydrofluoric acid to a concentrated nitric acid solution of zirconium zirconium tetrafluoride monohydrate [14956-11-3] precipitates (69). The recovered crystals ate dried and treated with hydrogen fluoride gas at 450°C in a fluid-bed reactor. The thermal dissociation of fluorozirconates also yields zirconium tetrafluoride. 

Fluorinated and iodinated derivatives are usually prepared by halogen exchange reactions, although the Baltz-Schiemann reaction has been applied to the synthesis of 2-fluoroquin-oxaline (66JHC435>. 

Vinyl and phenyl mfluoromethyl groups are reactive in the presence of aluminum chloride [10] Replacement of fluorine by chlorine often occurs Polyfluori-nated trifluoromethylbenzenes form reactive a,a-difluorobenzyl cations in antimony pentafluoride [11] 1 Phenylperfluoropropene cyclizes in aluminum chloride to afford 1,1,3-trichloro 2 fluoroindene [10] (equation 10) The reaction IS hypothesized to proceed via an allylic carbocation, whose fluoride atoms undergo halogen exchange... 

A. Introduction of Fluorine into a Heterocycle 1. Nucleophilic Halogen Exchange... 

Fluorinated olefins, 77 866 Fluorinated organics, production of, 77 847 Fluorinated polyimides, 20 278 Fluorinated thermoplastics, 10 8 Fluorinated titanocycle, 25 118 Fluorinating agents, electrophilic, 77 847 Fluorination, 9 281, 678-679 aromatic ring, 77 866 chlorocarbons, 6 235 direct, 77 864, 882 electrochemical, 77 864—865, 882 of germanium, 72 552 halogen exchange, 77 861-864 selective, 77 831... 

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>. 

The synthetic protocol for many of these reagents is detailed elsewhere9. Note that stereochemistry in EIZ alkenes is preserved in the formation of both the fluorinated vinyl-lithium and the fluorinated vinylzinc reagents, and that hindered bases permit selective metallation vs. metal/halogen exchange. 

Meinert demonstrated that fluorination of pyridine at low temperatures gives the ionic salt N-fluoropyridinium fluoride, a compound that was reported to be explosive at 0°C (Fig. 59) [154]. However, direct fluorination of variously substituted pyridines is possible and good yields of the corresponding 2-fluoro-pyridines (Figs. 60 and 61) [155] are obtained, offering an attractive alternative to the usual halogen-exchange and Balz-Schiemann routes to these products. These reactions probably proceed via N-fluoropyridinium salts (Fig. 62) which are activated towards nucleophilic attack. 

Starr Finger Chem. Ind. (London) 1962, 1328 Shiley Dickerson Finger J. Fluorine Chem. 1972,2. 19 Kimura Suzuki Tetrahedron Lett. 1989, 30. 1271. For the use of phase transfer catalysis in this reaction, sec Yoshida Kimura Chem. Lett. 1988, 1355. For a review of the preparation of aryl fluorides by halogen exchange, sec Dolby-Glover Chem. Ind. (London) 1986, 518-523. 

The exchange of halogen for fluorine with sulfur tetrafluoride, due to the high temperatures generally required, is of limited use. Carbon tetrachloride and carbon tetrabromide react with sulfur tetrafluoride at 225-325 C to give mixtures of halofluoromethanes, while hexachloro-ethane under similar conditions gives l,1.2,2-tetrachloro-l,2-difluoroethane (59%) as the main product.191... 

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