Data on the nitration, nitrosation, azo-coupling, and electrophilic amination of potentially tautomeric methyl and methylene derivatives of a series of pyridines, pyridazines, pyrimidines, and pyrazines in media of varying acidity and basicity are reviewed systematically.Interest in alkyl and alkylidene derivatives of six-membered nitrogen-containing heterocycles is almost always linked to the possibility of using them as synthons for the preparation of a variety of functionally substituted heterocyclic systems, including condensed heterocycles [1]. The large number of chemical conversions of the potentially tautomeric alkyl-and alkylideneazines opens up the possibility of changing the chemoselectivity of their reactions via the aromatic or more reactive methylene tautomer or by the intermediate generation of mesomeric anions.In this review we have systematized data on the reactions of potentially tautomeric methyl (A) and methylene (B) derivatives of azines for a series of pyridines (I, II), pyridazines (III, IV), pyrimidines (V-VII), and pyrazines (VIII) with Nelectrophiles in various media. Nitration, nitrosation, azo-coupling, and electrophilic amination are examined, leading to products with aromatic or ylidene structures, in which regioselectivity of substitution is determined by the reagents and the nature of the substrate under the reaction conditions.It was noted earlier [1] that it is characteristic for potentially tautomeric methyl and methylene derivatives of azines that, under conditions of kinetic control, the electrophile attacks the pyridine nitrogen atom in the aromatic tautomer (A) but it attacks the exocyclic a-carbon atom in the ylidene form (B). Further conversions during the course of the reaction can lead to the thermodynamically stable product in which the electrophile is found in an electron deficient position of the heterocycle or is contained in its substituent.
NITRATION OF METHYL-AND METHYLENEAZINESNitration of methyl and methylene derivatives of azines has been carried out with a variety of nitrating agents in various media, with the nitro groups inserting only into the heterocycle or only into the side chain. Depending on the acid-base properties of the substrates their interactions with nitrating agents in various media may occur in the form of: a) the free base of either tautomeric form (in neutral or even strongly acidic media with a substrate of low basicity); b) conjugated acids in the aromatic or ylidene form (in acid media although the form of the substrate may change with change in the pH or H o of the media); c) an anion generated by bases.Criteria based on kinetic data for the reaction have been used to identify the nature of the reacting particles of the substrate [2]. For example, when 2,4,6-trimethylpyridine and 1,2,4,6-tetramethylpyridinium cation were nitrated with acid nitrifying mixture (conc. HNO 3 in H2SO 4) to give the corresponding 5-nitro derivatives of pyridine the same prbfile of rates was observed, passing through a maximum at 90% H2SO 4. It was therefore co...