V-Aminoazoles

As was shown for TV-aminobenzimidazoles, /V-aminoazoles are easily deaminated by the action of diazonium salts (79ZOR1108). Unfortunately, this interesting reaction has been poorly investigated so far. Its value lies not so much with deamination but with the formation of aryl azides under mild conditions in high yields. Probably, the intermediates in this reaction are tetrazenes 260 cleaved (probably, via the N-anion) as shown in Eq, (64). 

V-Aminoazoles, for instance 1, react with nitriles or iminoesters to yield amidines 294 (69JPR477 71JPR768). The latter compounds were also obtained on amination of the Schiff bases with potassium amide in liquid ammonia (70JPR669). 

V-Aminoazoles react with the PhjP-Brj complex in the presence of mild bases to give phosphazo compounds 295, which are transformed to a-chlorohydrazones 296 under the action of acylchlorides (88HI935). 

Valerii Kuzmenko and Alexandr Pozharskii (Rostov-on-Don State University, Russia) give us the first comprehensive coverage of V-aminoazoles and bring much needed order into this important field. Much of the earlier literature had assigned structures now known to be inconect. 

A characteristic property of most diazotizations of aminoazoles is the occurrence of a relatively stable transient intermediate (probably the /V-nitrosoamine), in contrast with the diazotization of carbocyclic aromatic amines, where /V-nitrosoamines have been considered to be unstable intermediates. 

Cyclization of an N-amino group with an a-methyl substituent in 7V-aminoazinium salts or in N-aminoazoles is the most frequently used route to pyrazolo-fused systems (92AHC(53)85). Thus, (V-amino-imidazolium and -thiazolium salts (23 Z = NR, S) on heating with anhydrides in the presence of a base, are cyclized to give (25 Z = NR, S). The acyl group in (25) results from acylation of the initially formed pyrazoloazole (24). 

Abstract Synthesis methods of various C- and /V-nitroderivativcs of five-membered azoles - pyrazoles, imidazoles, 1,2,3-triazoles, 1,2,4-triazoles, oxazoles, oxadiazoles, isoxazoles, thiazoles, thiadiazoles, isothiazoles, selenazoles and tetrazoles - are summarized and critically discussed. The special attention focuses on the nitration reaction of azoles with nitric acid or sulfuric-nitric acid mixture, one of the main synthetic routes to nitroazoles. The nitration reactions with such nitrating agents as acetylnitrate, nitric acid/trifluoroacetic anhydride, nitrogen dioxide, nitrogen tetrox-ide, nitronium tetrafluoroborate, V-nitropicolinium tetrafluoroborate are reported. General information on the theory of electrophilic nitration of aromatic compounds is included in the chapter covering synthetic methods. The kinetics and mechanisms of nitration of five-membered azoles are considered. The nitroazole preparation from different cyclic systems or from aminoazoles or based on heterocyclization is the subject of wide speculation. The particular section is devoted to the chemistry of extraordinary class of nitroazoles - polynitroazoles. Vicarious nucleophilic substitution (VNS) reaction in nitroazoles is reviewed in detail. 

Using the cyclic voltamperometric method, oxidation potentials of some 2V-aminoazoles have been measured (Table VII). All compounds thus investigated give only a one-electron oxidation wave. The oxidation process is practically irreversible, and this represents instability in the radical-cations thus formed. Comparison of the oxidation of A -amino- and N-methyl-benzazoles indicates that the A-amino groups decreases the Ep value on the average by 0.2 V. Probably, this is explained by an overlap of the heterocycle 7r-orbitals with that of the unshared electron pair of the amine nitrogen atom, providing some stabilization of the radical-cation. 

Proton Chemical Shifts for /V-Amino Group in JV-Aminoazoles... 

Like all amines, N-aminoazoles are easily added to aryl isocyanates and aryl isothiocyanates, yielding 2V-ureido- or /V-thioureidoazoles 291. To... 

Contrary to the majority of other reactions of A -aminoazoles, oxidation reactions only began to be studied much later, approximately from the 1960s. Since that time, interest has grown, and presently they represent one of the most intriguing and synthetically useful types of N-aminoazole conversions. With the help of these reactions, one can effectively generate cycloalkynes and arynes, synthesize the difficult-to-obtain 1,2,3-and 1,2,4-triazines, /V,N -azoazoles, and other important compounds. 

Tetrazanes could be obtained on cautious oxidation of (V-amino derivatives of some six- and five-membered heterocycles with phenyliodo diacetate [Eq. (85)] (71CC800). The tetrazane products are very unstable and easily decompose with nitrogen elimination and formation of deaminated NH-heterocycles. Probably, on oxidation of N-aminoazoles, deamination occurs via the tetrazane formation. 

Emelina EE, Petrov AA (2009) a-Aminoazoles in the synthesis of heterocycles V. Synthesis of azolo[l, 5-a]pyrimidines from 2-ethoxyvinyl trifluoromethyl ketones and 2,2-diethoxyvinyl trifluoromethyl ketone. Russ J Org Chem 45 417 20... 

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