N-heteroaromatics

Design of self-adapting N-heteroaromatic-substituted claw ligands as E /M" " (E = p-block element, M = main group metal)chargedspacers 97CB1365. 

Mason, S. F., The tautomerism of N-heteroaromatic hydroxy-compounds. Part III. Ionisation constants,/. Chem. Soc. 1958, 674-685. 

Fetzner, S., Enzymes Involve in the Aerobic Bacterial Degradation of N-Heteroaromatic Compounds Molybdenum Hydrolases and Ring Opening 2, 4-Dioxygenases. Naturwis-senschaften, 2000. 87 pp. 59-69. 

N-Heteroaromatic compounds like pyridine, pyridazine, pyrazine, isoquinoline, and their derivatives42,250 react with diphenyl cyclopropenone in a formal (3+2) cycloaddition mode to the C=N bond of the heterocycle. As expected from the results discussed earlier (p. 67), the reaction is initiated by attack of nitrogen at the cyclopropenone C3 position and followed by stabilization of the intermediate betaine 390 through nucleophilic interaction of the Cl/C3 bond with the activated a-site of the heterocycle, giving rise to derivatives of 2-hydroxy pyrrocoline 391—394). In some cases, e.g. diphenyl cyclopropenone and pyridine42, further interaction with a second cyclopropenone molecule is possible under the basic conditions leading to esters of type 392. 

Scheme 6.119 Nucleophilic aromatic substitution reactions involving halo-substituted N-heteroaromatic ring systems.

As a general trend, six-membered mononuclear N-heteroaromatics such as pyridine and derivatives are much less prone to undergo hydrogenation than bi-and trinuclear N-ring compounds (e.g., quinolines, benzoquinolines, acridines) due to their higher resonance stabilization energy. 

Data pertaining to affinity of organic N-bases towards nitrosonium cation are numerous 74-80), however, those for N-heteroaromatic compounds are limited 77-80). 

A linear relationship between calculated An0+ values and experimental values of proton affinity (PA) for N-heteroaromatic compounds was also established (Figure 3). the points 1-4 (compounds 32h, 46, 32d and 31d respectively) have been excluded from the statistical treatment, r = 0.969]. 

Figure 3. Correlation between the affinity of N-heteroaromatic compounds for NO cation (A no ) and proton affinity (PA).

Supported vanadium oxides represent one of the technologically most important class of solid catalysts. These catalysts are useful for partial oxidation of various hydrocarbons 0), ammoxidation of alkyl substituted N-heteroaromatic compounds (2) and most recently for NO reduction (3) For a catalyst to be a successful one in industry, it should exhibit high activity with maximum selectivity, thermal and mechanical stability and long life etc. For getting some of these functionalities, the active component has to be dispersed uniformly on a support material. 

Many anionic o-adducts have been described as obtained from the interaction of N-heteroaromatic compounds with the amide ion in liquid ammonia. The reaction is often followed by decomposition leading to overall substitution (amination). This held has been intensively investigated by van der Plas, Zoltewicz, and co-workers. 

Compounds 108, 109, and the like have served as intermediates for the preparation of j3-alkyl-substituted N-heteroaromatics.136,153,154... 

The Electronic Absorption and Luminescence Spectra of N-Heteroaromatic Compounds and Their Derivatives R. N. Nurmukhametov, Russ. Chem. Rev. (Engl. Transl.), 1967, 36, 693-709. 

The principal tautomeric properties of the fundamental biological pyrimidines—cytosine, uracil, and thymine—are due to the presence in these N-heteroaromatic compounds of electron-donor substituents such as NH2 and OH and of SH in some important analogs. The labile hydrogen may remain attached at the exocyclic 0, N, or S atom or migrate to one of the ring nitrogens, giving rise to three principal types of tautomerism (Scheme 1) ... 

C—H bond alkenylations, 10, 222—223 in cross-coupling polymerization, 11, 687 N-Heteroaromatic compounds, hydrogenation, 1, 792 Heteroaromatic stannanes, preparation, 3, 828 Heteroaryl halides, aminocarbonylatioon, 11, 527 Heteroarylstannanes, preparation, 3, 826 Heteroatoms... 

Table 3. Dicationic species (108-113) arising from diprotonation of N-heteroaromatic compounds.

The observed electrophilic reactivity is indicative of superelectrophilic activation in the dication 173. Other ammonium-carboxonium dications have also been reported in the literature, some of which have been shown to react with benzene or other weak nucleophiles (Table 4).1 42b 57-60 Besides ammonium-carboxonium dications (175-179), a variety of N-heteroaromatic systems (180-185) have been reported. Several of the dicationic species have been directly observed by low-temperature NMR, including 176, 178-180, 183, and 185. Both acidic (175, 180-185) and non-acidic carboxonium (176-177) dicationic systems have been shown to possess superelectrophilic reactivity. The quinonemethide-type dication (178) arises from the important biomolecule adrenaline upon reaction in superacid (entry 4). The failure of dication 178 to react with aromatic compounds (like benzene) suggests only a modest amount of superelectrophilic activation. An interesting study was done with aminobutyric acid... 

Oae, S., Furukawa, N., Heteroaromatic Sulfoxides and Sulfones Ligand Exchange and Coupling in Sulfuranes and Ipso-Substitutions, 48, 1. 

This reaction is based on the proposition that the sensitivity to polar effects in free-radical chemistry is the result of polarity and polarizability of both the radical and the substrate. This means that the polarity of the heteroaromatic base plays a key role in the process. Actually, the nucleophilic character of an alkyl radical, for example, is not so marked as to justify the addition to the N-heteroaromatic base, and in fact either no substitution occurs or low yields and selectivity are observed. 

Scheme 14.3 Mechanism for the carbamoylation of N-heteroaromatic bases with NHPI/Ce(IV) system.

In the last few years we have developed new methods for the photochemical carbamoylation of electron-poor N-heteroaromatic bases. The introduction of Ti02 photocatalysis has supplied new and environmentally friendly synthetic methods that operate under mild conditions (Scheme 14.4) [19, 20]. 

---