Ammoxidation of Heterocyclic Compounds

Methylpyridines are easily converted into the corresponding nitriles. The ammoxidation of 3-methylpyridine to nicotinonitrile has been particularly extensively investigated, mainly because of the importance of the nitrile as precursor of vitamins. The catalysts used are mostly vanadium-containing oxides, e. g. vanadium-titanium oxide [86], vanadium-zirconium oxide [87], or multicomponent systems [30,88]. Yields of more than 90% at nearly complete conversion were recently reported (e. g. 100% selectivity for vanadium oxide on titania (anatase) catalysts with V2O5 loading 3.4 mol% [89]. 

High selectivity was also reported for the ammoxidation of 4-methylpyridine, e. g. over vanadium-molybdenum oxides [90] highly dispersed vanadia on sili-cated alumina [91] or on vanadium-containing molecular sieves (VSAPO, VAPO [92], also used for the ammoxidation of 3-methylpyridine [93,94]). The ammoxidation of 2-methylpyridine leads to the formation of large amounts of pyridine, by total oxidation of the methyl group and subsequent decarboxylation, in addition to the desired nitrile [95]. Yields in excess of 90% can, nevertheless, be achieved, e. g. over vanadium-tin oxide at ca 670 K [23] or over molybdenum phosphates [96]. When the ammoxidation of 2-, 3- and 4-methylpyridine over vanadium phosphates was compared catalyst activity and the nitrile selectivity reflected the reactivity order 4- 3- 2-methylpyridine, probably as a result of different sterie hindranee [41]. 

Pyridines containing more than one methyl group are also ammoxidized, as mentioned for xylenes. The methyl groups are successively converted, e. g. reae-tion of 3,5-dimethylpyridine over vanadium-titanium-tin oxide gives a mixture of 

3-eyano-5-methylpyridine and 3,5-dicyanopyridine (75-85% yield [97]). The ammoxidation of 2-methyl-5-vinylpyridine results in two products 2-cyano-5-vinylpyridine and 2,5-dicyanopyridine [98]. 

The ammoxidation of heteroeyelic compounds is not limited to methylpyridines. Other examples inelude the ammoxidation of 2,5-dimethylpyrazine, over molybdenum-cerium-titanium oxide [99], giving the corresponding mono-and dinitriles, and the ammoxidation of 4-methylthiazole to 4-eyanothiazole [100]. 

---