Heteroatom effects, nitrogen

The basicities of the parent azole systems in water are shown in Table 1. When both heteroatoms are nitrogen, the mesomeric effect predominates when the heteroatoms are in the 1,3-positions, whereas the inductive effect predominates when they are in the 1,2-positions. The predominance of the mesomeric effect is illustrated by the pK value of imidazole (82 Z = NH), which is 7.0, whereas that of pyrazole (83 Z = NH) is 2.5 cf. pyridine, 5.2). An fV-methyl group is base-strengthening in imidazole, but base-weakening in pyrazole, probably because of steric hindrance to hydration. When the second heteroatom is oxygen or sulfur the inductive, base-weakening effect increases the pK of thiazole (82 Z = S) is 3.5 and that of isoxazole (83 Z = 0) is 1.3. 

As first disclosed by Blum,294 cyclopropyl products can also be formed if a heteroatom (nitrogen or oxygen) is present at the propargylic position. The observed heteroatom effect has been rationalized, as shown below. This... 

While the term anomeric amides may be used to describe all systems bearing two heteroatoms at nitrogen and thus capable of displaying anomeric effects, not all properties exhibited by these amides may be attributed solely to the operation of such effects . 

The basicities of the parent azole systems in water are shown in Table 1. When both heteroatoms are nitrogen, the mesomeric effect predominates when the heteroatoms are in the 1,3-positions, whereas the inductive effect predominates when they are in the 1,2-positions. The predominance of the mesomeric effect is illustrated by the pATa value of imidazole (91 Z = NH), which is 7.0, whereas that... 

In the so-called non-Kolbe electrolysis, the carboxylate group is substituted by a nucleophile like methanol. This is especially effective if heteroatoms like nitrogen or oxygen are situated in the a-position. A number of amino acid derivatives have thus been transformed into the already mentioned N, (9-acetals as effective amidoalkylation reagents [28] ... 

Both effects 1 and 2 may be more or less obscured by the presence of heteroatoms on nitrogen or other polar groups which may also present solvent-dependent interactions. For instance when different rotamers are present, changing solvent may also affect the rotamer populations (or the equilibrium geometry of a single rotamer) and thus modify the inversion barrier. Such effects may occur, for instance, in hydroxylamine 86> and hydrazine 76> derivatives. 

Fifthly, the bicyclization reaction of nitrogen-containing enynes, i.e. (16)-(18), not only proceeds satisfactorily but also reveals an intriguing and beneficial heteroatom effect. 

It is known that aromatic compounds containing heteroatoms of nitrogen may have light stabilizing effect at polymers irradiation. Some derivates of carbazole, according to literature and patent data, may be CC and antioxidants during oxidative destruction of polymers. 

Coal Hquefaction iavolves raising the atomic hydrogen-to-carbon ratio from approximately 0.8/1.0 for a typical bituminous coal, to 2/1 for Hquid transportation fuels or 4/1 for methane (4). In this process, molecular weight reduction and removal of mineral matter and heteroatoms such as sulfur, oxygen, and nitrogen may need to be effected. 

Mesomeric shifts of the types shown in structures (82) and (83) increase the electron density on the nitrogen atom and facilitate reaction with electrophilic reagents. However, the heteroatom Z also has an adverse inductive effect the pK, of NH2OH is 6.0 and that of N2H4 is 8.0, both considerably lower than that of NH3 which is 9.5. 

A multiply bonded nitrogen atom deactivates carbon atoms a or y to it toward electrophilic attack thus initial substitution in 1,2- and 1,3-dihetero compounds should be as shown in structures (110) and (111). Pyrazoles (110 Z = NH), isoxazoles (110 Z = 0), isothiazoles (110 Z = S), imidazoles (111 Z = NH, tautomerism can make the 4- and 5-positions equivalent) and thiazoles (111 Z = S) do indeed undergo electrophilic substitution as expected. Little is known of the electrophilic substitution reactions of oxazoles (111 Z = O) and compounds containing three or more heteroatoms in one ring. Deactivation of the 4-position in 1,3-dihetero compounds (111) is less effective because of considerable double bond fixation (cf. Sections 4.01.3.2.1 and 4.02.3.1.7), and if the 5-position of imidazoles or thiazoles is blocked, substitution can occur in the 4-position (112). 

---