Pyridine-hydrogen fluonde

A recently discovered variant of the Wallach technique is the silver ion cata lyzed fluorination of aryl diazo sulfides in hydrogen fluonde-pyridine-toluene solvent [57] (equation 12) Electron withdrawing substituents such as acetyl give higher yields of aryl fluoride (71%) than electron donating groups (butyl 39%, methoxy, 2-14%), reductive dediazoniation competes with fluorination... 

Fluorodediazoniation of 2-fIuoro-2-phenylethylammes with sodium nitrite in Olah s reagent (70% anhydrous hydrogen fluonde-30% pyridine) gives high yields of 1,1 -difluoro-2-phenylethanes arising from 1,2 migration of phenyl P] (equation 2)... 

Fluorodediazoniation of alkyl carbamates in hydrogen fluonde/pyridine (70/30) represents a novel route to alkyl fluoroformates [6, 9] (equation 4)... 

Selective fluonnation in polar solvents has proved commercially successful in the synthesis of 5 fluorouracil and its pynmidine relatives, an extensive subject that will be discussed in another section Selective fluonnation of enolates [47], enols [48], and silyl enol ethers [49] resulted in preparation of alpha-fluoro ketones, heto-diketones, heta-ketoesters, and aldehydes The reactions of fluorine with these functionalities is most probably an addition to the ene followed by elimination of fluonde ion or hydrogen fluonde rather flian a simple substitution In a similar vein, selective fluonnation of pyndmes to give 2-fluoropyridines was shown to proceed through pyridine difluondes [50]... 

Nitrosyl chloride [55], nitrosyl fluoride-hydrogen fluonde liquid complexes (NOF3HF, NOF 6HF) [56], nitrous acid-hydrogen fluonde solutions [57, 5 nitrogen tnoxide (prepared in situ from nitric oxide and oxygen) [59] and fert-butyl nitrite-hydrogen fluonde-pyridine [60] have been substituted for sodium nitrite in the diazotization step... 

Thermal stability of arenediazonium fluorides can influence yields of aryl fluorides during decomposition in hydrogen fluoride [52,6/] Use of 70% hydrogen fluoride 30% pyridine (w/w) mixture having a lower pressure of hydrogen fluonde permits higher fluorodediazoniation temperatures and improves yields [6, 61 62, 63 64] This technique has also been extended to complexes of hydrogen fluonde with tertiary amines [65]... 

Various sources of fluoride ion have been investigated, of which highly nucleophilic tetraalkylammomum fluondes are the most effective Thus, fluoro alkyl halides and N (fluoroalkyl)amines are efficiently synthesized by treatment of the corresponding trifluoromethanesulfonic esters with tetrabutylammonium fluoride trihydrate in aprotic solvents [50] (equation 34) The displacement reactions proceed quantitatively at room temperature within seconds, but tail with hydrogen fluoride-pyridine and give reasonable yields only with hydrogen fluo ride-alkylamine reagents... 

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