Nitrogen as Heteroatom

Nitrogen as Heteroatom. Amides are protonated on the carbonyl oxygen atom (cation 371) in superacid media at low temperatures, as shown first 

Protonation of aroyl azides and a-ketonitriles in superacid media (HS03F-SbF5, 78°C) occurs exclusively on the oxygen to yield a-azidocarboxonium ions 374688 and a-cyanocarboxonium ions 375,689 respectively. 

Diprotonated /V,/V-bis(carboxyl)-1,2-diaminoethane 383 was also generated by protolytic ionization694 [Eq. (3.108)]. The carbocationic carbon resonance found at 

813C 161.5 in HSO3F-SO2CIF solution is only 0.9 ppm shielded from that of protonated carbamic acid. 

Since nitrogen is a better electron donor than oxygen, in cationic derivatives of imines the contribution of the aminomethyl cation structure 385b, compared with that of the iminium ion resonance form 385a, is small.691 695 The same also applies to amidines (386) and guanidines (387). 

It was claimed that protonation of ethyl N,N-diisopropylcarbamate, a hindered amide, takes place on nitrogen, and not on oxygen182. A reinvestigation, however, established, that at low temperature first O-protonation is taking place (kinetic control) with the O-protonated amide subsequently rearranging to the more stable N-protonated form (thermodynamic control)183.  

The possibility of observing the protonated amide linkage in strong acid media has particular relevance in the study of peptides and proteins184,18S.  

In protonated nitriles, however, the contribution from the iminomethyl cation resonance form is important187,188.  

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With nitrogen as heteroatom Ito A, Ono Y, Tanaka K (1998) New J Chem 22 779 With silicon as heteroatom Yoshida M, Goto N, Nakanishi F (1999) Organometalhcs 18 1465... 

As would be expected, the majority of the compounds described in this chapter can be considered as indoles with one or more nitrogens as heteroatoms in the six-membered ring. The four pyrrolopyridines are commonly referred to as azaindoles, terms more elegant than the IUPAC nomenclature. These compounds are also occasionally called diazaindenes. The following synonyms are frequently met ... 

IV. Five-Membered Rings Containing Only Nitrogen as Heteroatom 286... 

V. Compounds from Six-Membered Ring Heterocycles Containing Only Nitrogen as Heteroatom... 

Volume 43 of Advances in Heterocyclic Chemistry is composed of six chapters by an international set of authors. David Hewitt (Australia) provides us with the first available comprehensive review of heterocyclic six-membered rings containing phosphorus and nitrogen as heteroatoms the azophosphorins. 

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