Diazonium fluoroborates, reductions

Reduction of diazoniumfluoroborates. Tris(triphenylphosphine)chlororhodium, [(CsHjjjPJjRhCI, and also [(C HjjjPjjRhCUCO) catalyze the reduction of aryl diazonium fluoroborates by DMF (room temperature for 2 days or 80 for 1 day). 

Other hydrogen donors which have been applied to the reduction of diazonium fluoroborates are formamide and A -benzyl-l,4-dihydronicotinamide (equation 97). ... 

Nucleophilic introduction of nitro functions seems to be confined to reactions of diazonium salts which offer a useful entry, particularly to 2-nitroimidazoles (from the diazonium fluoroborates see Section 3.02.7.1.6). This method, though, depends on the ease of synthesis, isolation, and stability of aminoimidazoles. The 4- and 5-amino isomers in particular are frequently unstable in air, and aminoimidazoles are usually made either by ring synthetic methods or by reduction of nitro compounds. Oxidation of amino groups to nitro is seldom a useful option. 

Reduction of diazonium fluvroborates. Diazonium fluoroborates are reduced smoothly by the reagent in ether, THF, or acetonitrile. Triethylsilane (1,1218 2, 433) is equally satisfactory. The reaction therefore provides a method for replacement by hydrogen of an amino group attached to an aromatic ring.7... 

Generation of the radical 243 (Scheme 101) by reduction of the corresponding diazonium fluoroborate by sodium iodide in acetone gives... 

Electrochemical reduction of the trithiapentalenes (804 R = H, Me, or Ph) yields the corresponding radical anions, which are converted into the thiapyran-thiones (807)/" ° 1-Phenyl-6,6a-dithia-l,2-diazapentalene (808) is nitrated mainly in position 3, but the 3,4-dimethyl derivative undergoes nitro-dediazoniation to the dithiole (809). Treatment of compound (808) with nitrous acid yields the phenylazo-oxadithiazapentalene (810) a similar reaction is the transformation of dioxathiapentalene (805) into the oxathiadiazapentalene (811) by the action of nitrobenzene-p-diazonium fluoroborate. ... 

Arylazo tosylates that are obtained fi om aniline derivatives via diazonium fluoroborates in a two-step procedure can be used as alternative starting materials. They react with a broad range of functionalized Grignard reagents under mild conditions. Subsequent allylation of the addition products with allyl iodide, followed by the reductive cleavage of the intermediate hydrazine derivatives, ftimishes polyfunctionalized... 

The Balz-Schiemann reaction has been used for the preparation of the 4-iluoro-derivatives of pyridine and 2,5-, 2,6-, and 3,5-lutidine it has also been used to obtain 5-fluoronicotinic acid, required for conversion into the corresponding pyridylmethanol via LiAlH reduction of the ethyl ester. The introduction of F into fluoroaromatic compounds has been achieved via isotopic exchange in diazonium tetrafluoroborates. U.v. irradiation of aqueous solutions of the appropriate diazonium tetrafluoroborates has been used to procure the first ring-fluorinated imidazoks, e.g. photolysis of the diazonium solution obtained by adding sodium nitrite to 2-amino-imidazole in aqueous fluoroboric acid provides 2-fluoroimidazole contaminated with only a small amount of 2-azidoimidazole, the sole product of thermal decomposition of imidazole-2-diazonium tetrafluoroborate. 

In work recently reported [20] it was found that reduction of aryldiazonium tetra-fluoroborates in nonpolar solution by aqueous hypophosphorous acid and catalytic cuprous oxide, good to excellent yields of arenes were obtained under very mild conditions. The single exception in the group of salts studied was 4-methoxybenzene-diazonium tetrafluoroborate which yielded only 67% of arene. Addition of 7.5 mole-9 of 18-crown-6 increased the yield to 88%. The effect of crown in this system is not understood but believed to involve crown enhanced solubility of the salt (see Eq. 15.6) [20]. 

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