Electronegative nitrogen

Although all the rings in Figure 1 contain six tt-electrons, the accumulation of electronegative nitrogen atoms in the polyaza structures leads to hydrolytic as well as thermal instability. This is noticeable in pyrimidine, and marked in the triazines and tetrazine. Some stability can be conferred by appropriate substitution, as we shall outline later. 

Typical coupling constants for isothiazoles are given in Table 5. The electronegative nitrogen atom reduces 3,4 and V3.5 from the values of 3.50 Hz and 0.27 Hz, respectively, in thiophene. The V values correlate quite well with rr-bond orders calculated by MO methods (74CJC833). 

Methylamine contains an electronegative nitrogen atom with two lone-pair electrons. The dipole moment thus points generally from -CH3 toward NH2. 

For 10-fold 13C labelled retinal, it has been shown that the differences between chemical shifts for polyene chain carbons of the chromophore in its native environment and detergent-solubilised system were small67 Analysis of the environment of the Schiff base has supported the model of stabilisation based on the protonation by a complex counterion. Three factors were responsible for the excessive positive charge in polyene (i) electronegative nitrogen, (ii) protonation and (iii) counterion strength. 

The first spirophosphorane with an azaphosphetidine ring 63 was prepared from the intramolecular cyclization and dehydration of phosphane oxides with DEAD/PPh3 <1996AGE1096, 1996PS489>. Dissolution of 63 in [r/8]-toluene led to some isomerization to the pseudorotamer 64, where the electronegative nitrogen occupies the equatorial position (Scheme 32). Such rotamers were previously unknown for the analogous oxaphosphetanes. 

The driving force for this transformation is the fact that the less electronegative nitrogen atom is a better Tr-donor than oxygen and can form a stronger bond with the carbene carbon atom. Hence displacement of alkoxide by amines and thiols is commonly observed, but the reverse reactions are seldom seen. 

In the second example, N-dealkylation, oxidation of the carbon next to the nitrogen leads to a carbinolamine. This spontaneously leads to formaldehyde and an amine. The mechanism involves loss of a proton with electrons moving toward the electronegative nitrogen atom. The negative charge on the nitrogen is neutralized due to the addition of a proton from the environment. 

Primary and secondary amines contain a polar N-H bond and so have hydrogen bonds between their molecules. No such bonds can be set up between the molecules of tertiary amines because they do not contain a hydrogen atom directly bonded to the highly electronegative nitrogen atom. This is why primary and secondary amines have higher boiling points than their isomeric tertiary amines. 

Note that we can write a similar resonance picture for esters, and we shall actually need to invoke this when we discuss enolate anions (see Section 10.7). However, electron donation from oxygen is not as effective as from the less electronegative nitrogen. We shall also see that this resonance effect in amides has other consequences, such as increased acidity of the amide hydrogens (see Section 10.7) and stereochemical aspects of peptides and proteins (see Section 13.3). In addition, the amide derivatives have... 

The hydride released appears to abstract a proton from the product since the other product of the reaction is gaseous hydrogen. The aminopyridine anion is finally quenched with water. The product is mainly 2-aminopyridine, probably the result of the enhanced inductive effect on carbons immediately adjacent to the electronegative nitrogen. 

We noted above (see Section 11.5.1) that pyrrole, though a very weak base, is potentially acidic (p Ta 17.5). This was because the anion formed by losing the proton from nitrogen has a negative charge on the relatively electronegative nitrogen, but maintains its... 

The stability of azole carbenes can be attributed to electronic factors which operate in both the Tran d CT-frameworks (92JA5530). In the TT-framework, electron donation into the carbene out-of-plane p-orbital by the electron-rich system moderates the typical electrophilic reactivity of carbenes. In the o-framework, additional stability for the carbene electron pair may be gained from the o-electron-withdrawal effects on the carbene center by the more electronegative nitrogens, which moderates the carbene nucleophilic reactivity. The combination of these a- and TT-effects serves to increase the singlet-triplet gap and stabilize the singlet carbene over the more reactive triplet state. For carbenes with bulky substituents (tert-butyl, 1-adamantyl, etc.) steric effects provide additional stabilization. 

As another example, the resonance structure for N2O that places the formal negative charge on the more electronegative oxygen atom rather than on the less electronegative nitrogen atom is probably a more accurate representation of the molecule. 

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