Nitrogen heterocycles, basicity

Poisoning occurs by strong adsorption of stable molecules on the active sites of catalysts. Basic heterocyclic nitrogen compounds can usually poison the acidic sites. Adsorption of... 

CAS 91-22-5. C9H7N. A basic heterocyclic nitrogenous compound occurring in coal tar and obtained from it, but more frequently by synthesis. 

Halogenide complexes of heavy metals such as mercury bismuth, cadmium and antimony can be used in the titration of some basic heterocyclic nitrogen compounds < and among others, thiamineS > In this latter example, thiamine was titrated... 

With heteroaromatic substrates it is possible to prepare, for example, thiazolyipyridines. It is noteworthy that basic solvents (e.g., heterocyclic nitrogen compounds) increase the yield of substitution by a cage effect (see Tables III-37 and III-38) (208). 

Reaction with vatious nucleophilic reagents provides several types of dyes. Those with simple chromophores include the hernicyanine iodide [16384-23-9] (20) in which one of the terminal nitrogens is nonheterocyclic enamine triearbocyanine iodide [16384-24-0] (21) useful as a laser dye and the merocyanine [32634-47-2] (22). More complex polynuclear dyes from reagents with more than one reactive site include the trinuclear BAB (Basic-Acidic-Basic) dye [66037-42-1] (23) containing basic-acidic-basic heterocycles. Indolizinium quaternary salts (24), derived from reaction of diphenylcyclopropenone [886-38-4] and 4-picoline [108-89-4] provide trimethine dyes such as (25), which absorb near 950 nm in the infrared (23). 

Spectroscopic evidence indicates that protonation of 2-fluoro-and 2-chloro-quinoline is not appreciable in O.OlJf aqueous hydrochloric acid. Protonation becomes evident in more strongly acidic solution in the case of the chloro compound without any accompanying decomposition, but the fluoro compound hydrolyzes to carbostyril under the latter conditions. The hydrolysis is acid-catalyzed, but it is doubtful whether protonation on the heterocyclic nitrogen is responsible, owing to its low basicity (presumably below that for the chloro compound). An alternative explanation in this case would be hydrogen bond formation with fluorine, Ar—F. .. H-O+H2. 

While in most reported cases the nucleophiles were amines, there were few examples involving heterocyclic nitrogens [40], alcoholic oxygens [27] or carbon nucleophiles [42, 43] too. Figure 4 shows a recent example of tandem Ugi-Dieckmann protocol [42]. Ugi convertible isocyanide 2, which requires a basic activation, was used, allowing a domino activation-cyclization of the intermediate 20 to give pyrrolidinediones (tetramic acids) 22. 

These compounds exist as the amino tautomers and normally the imino forms are not observed. All react with acids at the heterocyclic nitrogen atom, giving salts. The protonated forms of both 2- and 4-aminoquino-lines are resonance hybrids (Scheme 3.10), but 4-aminoquinoline is more basic than 2-aminoquinoline, possibly because the nitrogen atoms that carry the charge between them in the corresponding cation are more widely separated. 

These empirical relationships are intended to provide quantitative and conceptual frameworks for the rationalization of structural effects on the basicity of nitrogen heterocycles. 

Undoubtedly, pyridine, C5H5N (2), is the best-known heterocyclic nitrogen ligand and its coordination chemistry has been studied in great detail, as have its simple derivatives bearing a non-coordinating substituent. For the physical properties, the reader is referred to the heterocyclic literature.1 3,5,9 The basic properties of pyridine have been mentioned above. Alkyl-substituted derivatives are slightly more basic [pA (base) values of about 5-7]. 

Heptane-4,6-dione, decafluoro-metal complexes decomposition, 385 Heptane-2,4,6-trione metal complexes, 399 Heteroallenes coordinated reactivity, 579 metal complexes, 579 Heterocyclic nitrogen compounds basic pKa... 

The basic monomers of nucleic acids are nucleotides which are made up of heterocyclic nitrogen-containing compounds, purines and pyrimidines, linked to pentose sugars. There are two types of nucleic acids and these can be distinguished on the basis of the sugar moiety of the molecule, Ribonucleic acids (RNA) contain ribose, while deoxyribonucleic acid (DNA) contains deoxyribose. The bases cytosine (C) adenine (A) and guanine (G) are common in both RNA and DNA. However, RNA molecules contain a unique base, uracil (U), while the unique DNA base is thymidine (T). These differences in the base structure markedly affect the secondary structures of these polymers. The structures of DNA and RNA are outlined in Appendix 5.2. 

The aromatic sextet is completed by delocalisation of the lone pair from the second heteroatom, 4.4a-e. Consequently, as in pyridine, the nitrogen atoms of the 1,2-azoles have a lone pair available for protonation. However the 1,2-azoles are significantly less basic than the 1,3-azoles because of the electron-withdrawing effect of the adjacent heteroatom. Isoxazole and isothiazole are essentially non-basic heterocycles (pAas <0), and even pyrazole (pAa=2.5) is a much weaker base than the corresponding 1,3-azole imidazole (pAa=7). 

We have just mentioned the protonation of aziridine, and you might imagine from what we said earlier about the comparative nucleophilicity and basicity of nitrogen heterocycles and their acyclic counterparts that aziridine will be even more nucleophilic than pyrrolidine, and about as basic. Well,... 

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