Pyridine electrostatic field

The more recent results of Scheme 18 can be summarized as follows the less hindered isomer is formed unless the substituent has a lone pair, the electrostatic field of which enhances the nucleophilicity of the adjacent nitrogen atom. This is true not only for pyridine and pyrazole, but probably also for CHO and C02Et. 

The pKj of the aminopyridines attached to the polymer (Table 3.3) are invariably lower than those of the corresponding isolated small molecules. The cationic character of polyethylenimine produces an electrostatic field that weakens the basicity of the nucleophile by 2-3 pK units. A lowering of an additional unit ocurrs when a long apolar group is attached to the aminopyridine moiety. Evidently, in this molecule the apolar environment favors further the uncharged, nonprotonated form of the pyridine. 

Fig. 5 Pyridine-water hydrogen-bonded complex embedded in the electrostatic field of the remaining water molecules...

Physicochemical properties rather than reactivities were also explored. Molecular electrostatic potential (MEP) was calculated for the [l,2,4]triazolo[4,3- ]pyridine fragment 23, according to the CHELPG algorithm. This afforded a prediction of its H-bond acceptor ability in view of the synthesis of p38 MAP kinase inhibitors <2005JME5728>. Tautomerism was also examined for compound 24, also postulated as two possible acyclic structures. The ab initio self-consistent field (SCF)-calculated energies support 24a as the most stable tautomer <1999MRC493>. 

In the case of pyridine attached to cations, it was shown by Ward [678] that the band position linearly depends on the electrostatic Coulomb field, q/r, which is often useful for the identification of the cation (cf. Fig. 46). 

Fig. 46. Wavenumber of pyridine adsorbed on monovalent cations (C-sites) as a function of the electrostatic Coulomb field of the cations determined by the cation charge and radius see text (adopted from [677])...

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