The nucleophilic substitution reactions of halopyridines with sulfur, oxygen and carbon nucleophiles under microwave irradiation was complete within several minutes with yields up to 99%. The method ...using microwave irradiation is superior to those conducted under conventional heating processes.
Graphic
Solvent-free reactions of 2- and 3-halopyridines with PPh
3
, PBu
3
, and PCy
3
were studied under conventional heating, as well as under microwave irradiation. No difference was observed in the ...reaction course between classical and microwave reactions. 2-Bromopyridine gave quantitative yields of 2-pyridyltriphenylphosphonium bromide within few minutes at 190°C. Equimolar amounts of some inorganic salts (LiPF
6
, LiOTf, LiBr, NaPF
6
, KPF
6
) were necessary for the reactions of the other 2-halopyridines. 3-Halopyridines did not react with PPh
3
even in the presence of LiPF
6
. Their reactions with PCy
3
in the presence of LiPF
6
resulted in the quantitative formation of dicyclohexylphosphine oxide.
Cyclic voltammetry and controlled-potential electrolysis, along with quantitative determination of all electrolysis products, have been employed to investigate the electrochemical reduction of a ...series of mono- and dihalopyridines at glassy carbon cathodes in dimethylformamide containing tetramethylammonium perchlorate. Two reduction waves are observed for 2-bromo-, 3-bromo-, 2-chloro-, 3-chloro-, and 3-iodopyridine; the first wave is due to two-electron cleavage of the carbon-halogen bond, and the second wave is attributable to reduction of pyridine. Bulk electrolyses of these monohalopyridines at potentials corresponding to the first voltammetric wave give pyridine exclusively. Cyclic voltammograms for the reduction of 2,5-dibromo-, 2,6-dibromo-, 3,5-dibromo-, 2,3-dichloro-, 2,5-dichloro-, and 2,6-dichloropyridine exhibit three waves arising from sequential cleavage of the two carbon-halogen bonds as well as reduction of pyridine. Controlled-potential electrolytes of the dihalopyridines at potentials for the first stage of reduction lead to one or two monohalopyridines, whereas electrolyses at potentials corresponding to the second stage of reduction afford only pyridine.
