Pyridine electron distribution

Besides uracil-6-iminophosphorane, the iminophosphorane component was extended to pyrazole 3 and pyrazolon-4-iminophosphoranes 363 (94JOC3985). In its electron distribution, 363 can be compared with uracil 346. With arylisocyanates, pyridine, or y-picoline, zwitterionic pyrazolo [3, 4 4,5]pyrido[6,l-a]pyrimidines (364) are obtained and with isoquinoline, 365 is formed (Scheme 131). Again, both systems show a typical negative solvatochromism (94JOC3985). 

Nitrogen is more electronegative than carbon, and this influences the electron distribution in the Jt-electron system in pyridine through inductive effects, such that nitrogen is electron rich. In addition, the... 

These results show that the 3pzAO of phosphorus contributes considerably to ring conjugation in X -phosphorins The determining factor is that the highest occupied molecular orbital is of n type in both phosphorin systems. In X -phos-phorin the next lower MO is localized at the P atom to the extent of 60% (as an n MO). In the X -phosphorin system this is not possible, which is in accordance with the observed PE spectral intensities of Fig. 37, p. 115. The very different electron distribution of both X - and X -phosphorins in comparison to that of pyridine is in full accord with the chemistry of these classes of compounds ... 

Even though at present various obstacles prevent a more extensive application of NMR spectroscopy to the study of 77-electron distribution, it seems that it will be possible to make use of the interrelation between the magnitude of the chemical shift of a proton attached at a certain position and the 77-electron density at that position. Among the studies so far published attention should be drawn to the papers dealing with pyridine,111-112 substituted pyridines,113 and pyrimidine and its derivatives.114 Table VII presents data (taken from ref. 112) on experimental and theoretical 77-electron densities in pyridine and the pyridinium cation. The NMR spectra of some azines are briefly discussed by White.114"... 

The presence in azoles of both pyrrole-like and pyridine-like heteroatoms leads to a highly perturbed n-electron distribution. As a result, these molecules often display along with ir-excessive centers, atoms with a rather high rr-deficiency, sometimes even higher than in typical azines. We first consider non-fused azoles (Table 1). 

The application of NMR spectroscopy to structure determination is broad however, in this section the group of studies that allow fundamental properties of the pyridine moiety, particularly electronic distribution, will be discussed. Application to other aspects, such as conformation and tautomerism, is discussed separately in those sections below. 

Molecular orbital calculations of the w-electron distribution in pyridine predict that more 4- than 2-aminopyridine should be formed in the Tschitschibabin reaction.4 The fact that no 4-aminopyridine can be detected when the two positions are allowed to compete for a deficiency of sodamide (see, e.g., Abramovitch et al 268) has led to the suggestion that the observed orientation in this reaction depends on the relative ease of elimination of a hydride ion from C-2 and C-4 and not upon the initial mode of addition (which, by implication, must take place predominantly at C-4 as predicted by the molecular orbital calculations).4 This hypothesis necessitates that the addition step be rapidly reversible and that the second stage, the elimination of hydride ion, be the rate-determining one (Scheme VII). Although it seems reasonable to assume that the hydride ion eliminations are the slow steps in this reaction, the fact that no deuterium isotope effect was observed in the reaction of 3-picoline-2d and of pyridine-2d with sodamide implies that the first stage must be virtually irreversible,268 as was found also in the case of the addition of phenyllithium to pyridine.229 The addition stage must, therefore, be the product-... 

As the table indicates, C6o fullerene solubility is practically the same in nonpolar benzene and polar pyridine. Like benzene, pyridine has a pronounced "aromatic" nature, i.e. electron distribution in a pyridine molecule is identical to that in benzene. However pyridine has a reasonable dipole moment as opposed to benzene. 

C6o solubility in pyridine is identical to that in benzene. Pyridine has a pronounced "aromatic" nature. zi-electron distribution in a pyridine molecule is identical to that in benzene. Pyridine has six mobile 7i-bonds, one of them is formed by an unshared pair of -electrons of a nitrogen atom. Pyridine can be nitrated. A nitro group enters the P-position. Because carbon with the highest electron density is a center for electrophilic substitution, one can make a logical assumption that the reaction center for charge-transfer interaction between pyridine molecules and C6o is also in the P-position or, what is equivalent, in the ortho-position relative to a nitrogen atom (Table 6). 

Because of the presence of nitrogen in the aromatic ring, electrons in pyridine are distributed in such a way that their density is higher in positions 3 and 5 (the P-positions). In these positions, electrophilic substitutions such as halogenation, nitration, and sulfonation take place. On the contrary, positions 2, 4, and 6 (a- and y-positions, respectively) have lower electron density and are therefore centers for nucleophilic displacements such as hydrolysis or Chichibabin reaction. In the case of 3,5-dichlorotrifluoropyridine, hydroxide anion of potassium hydroxide attacks the a- and y-positions because, in addition to the effect of the pyridine nitrogen, fluorine atoms in these position facilitate nucleophilic reaction by decreasing the electron density at the carbon atoms to which they are bonded. In a rate-determining step, hydroxyl becomes attached to the carbon atoms linked to fluorine and converts the aromatic compound into a nonaromatic Meisenheimer complex (see Surprise 67). To restore the aromaticity, fluoride ion is ejected in a fast step, and hydroxy pyridines I and J are obtained as the products [58],... 

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