Azaaromatic compounds

Cyclization reactions of quaternary salts of azaaromatic compounds 97KGS23. 

As shown in the previous sections, pyridine and related azaaromatic compounds possess a strong electron-withdrawing property equivalent to thep-nitrophenyl group and hence serve as good leaving groups. The facile... 

The chemistry of both organic sulfur compounds and azaaromatic compounds has been widely investigated mainly for industrial purposes for many years, and vast amounts of data have been accumulated. However, many studies lack mechanistic support. 

These results are interesting because amino heterocycles show a quite different behavior under neutral and alkaline conditions, e.g., aminothiazole derivatives are alkylated at the ring nitrogen in weakly acidic and neutral media and at the exo nitrogen in highly alkaline medium.200 Therefore, the use of PTC to promote exo nitrogen alkylation of azaaromatic compounds may be of interest. 

Triazine is a very ir-deficient heterocyclic system. Quantitative data on ir-deficiency of azaaromatic compounds obtained by l3C-NMR spectroscopy indicate that 1,2,4-triazines are more electron-deficient compounds than pyridines, pyrimidines, pyrazines, or pyridazines (Scheme 1) (82QMR192). 

Amination of azaaromatic compounds using liquid ammonia-potassium permanganate (85S884 86MI1) has been found to be a successful method for preparing a number of 5-amino-l,2,4-triazines (Scheme 73). This SNH reaction is based on the ability of 3- and 6-substituted triazines to form amino adducts 22 at C-5 in liquid ammonia (see Section III,A,2 and Scheme 18). These adducts can be oxidized into 5-amino-1,2,4-triazines 126. A number of 1,2,4-triazines containing substituents at C-3 and C-6,... 

Sequential pericyclic reactions between pyridine 7 -oxides and allenes have been evaluated in terms of PM3 calculations (94H(37)257>. A new way to introduce dimethylamino groups into azaaromatic compounds has been devised by reaction of pyridine AT-oxide (69) or picoline A/-oxide with hexamethylphosphoramide (Scheme 36) (94PJC(68)1343). 

Incorporation of one or two fragments of pyrrolidine into triazapyrene in water in the presence of potassium ferricyanide demraistrates how effective is the application of the methodology, which opens new opportunities in the field of amination of azaaromatic compounds (Scheme 35) [73]. 

Stabilities of o -adducts derived from nucleophilic addition of alcohols and alkox-ide ions to azaaromatic compounds are varied to a great extent. A whole number of alkoxy adducts of 1,4-diazinium and 1,2,4-triazinium cations have been registered by NMR [114] however, attempts to isolate them failed. Contrary to that, treatment of 3-aryl-l,2,4-triazin-5-ones with primary or secondary alcohols in the presence of acetic anhydride results in the formation of rather stable 6-aIkoxy-l-acetyl-l,6-dihydro-l,2,4-triazin-5-ones (Scheme 38) [117, 140, 141],... 

Free radical chemistry is another field of application of the Sn reactions. Lithium salts of nitronyl nitroxide radicals proved to be appropriate nucleophilic species to modify the structure of a number of azaaromatic compounds. This reaction provides an access to a new family of free radical compounds (Scheme 44) [148-150]. 

Electron transfer processes, kinetics, and mechanisms for the oxidation of 10-methyl-9,10-dihydroacridine, as the model compound, and other 9-substituted l-methyl-9,10-dihydroacridines have been the subject of many studies [44, 207-211]. In particular, it has recently been found in our laboratory that nanocrystalline Ti02 with CdS nanoparticles embedded in its pores can accelerate the C-H functionalization of azaaromatic compounds. Indeed, the CdS/Ti02 composite proved to be an effective visible-light-driven photocatalyst for the oxidative step of the reaction of A -alkylacridinium salts with arylamines (Scheme 68). 

A variety of other azaaromatic compounds, pteridines [66], pyridazines [67] etc., enter the VNS reactions with the model carbanion of chloromethyl phenyl sulfone or other a-chlorocarbanions. In these reactions azine N-oxides are more active electrophiles than azines themselves. For instance, quinoline fails to enter the VNS reaction with the model carbanion, whereas quinoline N-oxide reacts rather smoothly [68]. Also 3-(chloromethylsulfonyl-amino)pyridine-N-oxide and its quinoline analogue are able to undergo intramolecular VNS reactions [61]. 

Chen Q-Y, Li Z-T (1993) Photoinduced electron-transfer reaction of difluorodiiodomethane with azaaromatic compounds and enamines. J Chem Soc Perkin Trans 1 645-648... 

A rare example of the incorporation of a fluorine atom into azaaromatic compounds is the direct fluorination reaction of 6-azauracyl 13a and 2-(2,3,5-tri-0-acetyl-p-D-ribofuranozyl-l,2,4-triazin)-3,5(2H,4H)-dione 13b which takes place on passing of fluorine through a solution of azauracils 13a,b in acetic acid, thus giving 6-fluoro-l,2,4-triazin-3,5(2H,4H)-diones 14 in 20-55 % yields (Scheme 10) [17,18]. 

Nucleophilic replacement of fluorine atoms in azaaromatic compounds can be performed under rather mild reaction conditions, and this method is certainly one of the most effective approaches to their functionalization. Incorporation of perfluoroisopropyl groups into 2,4,6-trifluoro-l,3,5-triazine 55 proceeds smoothly... 

Bifunctional azaaromatic compounds in which ESIPT is not possible due to tmfavorable arrangement of pyrrole NH and pyridine-type nitrogen atoms may stiU undergo self-assisted" intermolecular excited-state proton transfer if this process is thermodynamically allowed [37]. Examples of known doubly hydrogen-... 

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