Six-membered heteroaromatic ring

The main chain of these polymers contains, as the principal component, five- or six-membered heteroaromatic rings, ie, imides, which are usually present as condensed aromatic systems, such as with benzene (phthalimides, 3) and naphthalene (naphthalimides, 4) rings. 

Reactions involving monocyclic six-membered heteroaromatic rings have not been studied sufficiently extensively to allow a quantitative treatment of substituent effects. However, comparison with aza-naphthalene reactivities indicates that aza- and polyaza-benzene systems must also be highly selective. 

The diazotization of amino derivatives of six-membered heteroaromatic ring systems, particularly that of aminopyridines and aminopyridine oxides, was studied in detail by Kalatzis and coworkers. Diazotization of 3-aminopyridine and its derivatives is similar to that of aromatic amines because of the formation of rather stable diazonium ions. 2- and 4-aminopyridines were considered to resist diazotization or to form mainly the corresponding hydroxy compounds. However, Kalatzis (1967 a) showed that true diazotization of these compounds proceeds in a similar way to that of the aromatic amines in 0,5-4.0 m hydrochloric, sulfuric, or perchloric acid, by mixing the solutions with aqueous sodium nitrite at 0 °C. However, the rapidly formed diazonium ion is hydrolyzed very easily within a few minutes (hydroxy-de-diazonia-tion). The diazonium ion must be used immediately after formation, e. g., for a diazo coupling reaction, or must be stabilized as the diazoate by prompt neutralization (after 45 s) to pH 10-11 with sodium hydroxide-borax buffer. All isomeric aminopyridine-1-oxides can be diazotized in the usual way (Kalatzis and Mastrokalos, 1977). The diazotization of 5-aminopyrimidines results in a complex ring opening and conversion into other heterocyclic systems (see Nemeryuk et al., 1985). 

Volume 26 contains three chapters. C.A. Ramsden reviews heterocyclic betaines derived from six-membered heteroaromatic rings. These compounds have been intensively studied over the past 10 years and have not previously been comprehensively reviewed. O. Meth-Cohn and B. Tamowski summarize the chemistry of the thiocoumarins, a somewhat neglected group of heterocycles. Finally, W. Friedrichsen has reviewed the chemistry of the benzo[c]furans which, unlike the aza and thia analogs, have not previously been considered in this series. Here again we have a group of compounds the chemistry of which has advanced considerably over the last decade. 

Table 8 H NMR Spectral Data for Six-membered Heteroaromatic Rings with Exocyclic Carbonyl or Thione Groups1 ...

Six-membered heteroaromatic rings show bands characteristic of in-plane CH bending in the region 1300-1000 cm1 (Table 21), and of out-of-plane CH bending below 1000 cm 1 (Table 22). 

The difference in the reactivities of the same substituents on heteroaromatic nuclei and on benzene rings are a measure of the influence of the heteroatom(s). For six-membered heteroaromatic rings, the typical effect of the heteroatom(s) is to attract electrons away from the carbon atoms of the ring. This influence is relatively small when the heteroatom is p to the substituent, but large for the a- and y-orientations. 

Substituents attached to a nitrogen atom in a six-membered heteroaromatic ring are to be found in compounds of the following types ... 

Table 8 1H NMR spectral data for six-membered heteroaromatic rings with exocyclic carbonyl or thione groups...

Six-membered heteroaromatic rings have received a great deal of attention from theoretical and experimental standpoints. In particular, the molecular orbital energy levels and the frontier electron densities of these compounds... 

In addition one or two carbon atoms in the chain of (4) may be replaced by nitrogen, finally forming a six-membered heteroaromatic ring compound. 

Polyazines are polymers with six-membered heteroaromatic rings in the main chain, whereby these rings contain at least one tertiary nitrogen. Compounds which can be conceived by partial hydrogenation of the hetero rings are related to these. Of the very great number of possible different compounds, those with quinoxaline (I), quinazoline dione (II), triazine (III), melamine (IV), and isocyanurate (V) groups have found use in industry ... 

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