Dehydrogenating agents ferric chloride

The condensation of a, dicarbonyl compounds (49) with aj3-diamino compounds (50), which proceeds through the dihydropyrazine (51), has been much used for the synthesis of alkyl- and arylpyrazines (52). These reactions are usually carried out in methanol, ethanol, or ether in the presence of sodium or potassium hydroxide. The dihydropyrazines may be isolated, or oxidized directly to the pyrazine. Dehydrogenating agents that have been employed include oxygen in aqueous alkali (329), air in the presence of potassium hydroxide (330), sodium amylate in amyl alcohol (330a), alcoholic ferric chloride (24), and copper chromite catalyst at 300° (331) (see also Section 1). Pyrazines prepared by this method and modifications described below are listed in Table II.8 (2, 6, 24, 60, 80,195, 329-382) and some additional data are provided in Sections VI. 1 A, VlII.lA(l), and IX.4A(1). 

The procedure described here Is by far the most efficient synthesis of terpyridine. Previous preparations include the dehydrogenation of pyridine with ferric chloride, the Ullman reaction of 2-bromopyridine and 2,6-dibromopyridine, the action of copper on 2-bromopyridine and 6-bromo-2,2 -dipyridyl, the reaction of iodine or ferric chloride with 2,2 -bipyridyl and the reaction of 2,2 -bipyridyl with 2-1ithiopyr1d1ne (40% yield). Terpyridine is a very effective chelating agent. 

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