Pyrrole-like nitrogen

A pyrrole-like nitrogen in a ring iV-methyl group gives a peak around -220 p.p.m. if adjacent to two carbon atoms one or two neighboring nitrogens shift the peak downfield by about 40 and 90 p.p.m., respectively. 

Electron donation from pyrrole-like nitrogen, or to a lesser extent from analogous sulft or oxygen atoms, helps electrophilic attack at azole carbon atoms, but as the number c heteroatoms in the ring increases, the tendency toward electrophilic attack at both C an N decreases rapidly. 

Note that nitrogen atoms have different roles depending on the structure of the molecule. The nitrogen atoms in pyridine and pyrimidine are both in double bonds and contribute only one tt electron to the aromatic sextet, just as a carbon atom in benzene does. The nitrogen atom in pyrrole, however, is not in a double bond and contributes two tt electrons (its lone pair) to the aromatic sextet. In imidazole, both kinds of nitrogen are present in the same molecule— a double-bonded "pyridine-like" nitrogen that contributes one v electron and a pyrrole-like" nitrogen that contributes two. 

Just as there are heterocyclic analogs of benzene, there are also many heterocyclic analogs of naphthalene. Among the most common are quinoline, iso-quinoline, indole, and purine. Quinoline, isoquinoline, and purine all contain pyridine-like nitrogens that are part of a double bond and contribute one electron to the aromatic it system. Indole and purine both contain pyrrole-like nitrogens that contribute two - r electrons. 

Indole has a nonbasic, pyrrole-like nitrogen and undergoes electrophilic substitution more easily than benzene. Substitution occurs at C3 of the electron-rich pyrrole ring, rather than on the benzene ring. 

I Practice spotting basic and nonbasic I nitrogen atoms. Most nonbasic (pyrrole-like) nitrogens have three single bonds, and a lone pair in a I p orbital. Most basic (pyridine-like)... 

Purine has an imidazole ring fused to a pyrimidine ring. Purine has three basic nitrogen atoms and one pyrrole-like nitrogen. 

The initial nitration may occur at either of the remaining sites on the ring with the electrons coming from the pyrrole-like nitrogen atom. Tautomerism after nitration gives the mixture. 

There are two triazoles, and each has one pyrrole-like nitrogen and two pyridine-like nitrogens. Both triazoles have the possibility of tautomerism (in 1,2,3-triazole the tautomers are identical) and both give rise to a single anion. 

Now, what about this ring system—is it aromatic It s certainly highly delocalized and your answer to the question clearly depends on whether you include the nitrogen electrons or not. In fact, if you ignore the pyrrole-like nitrogen atoms but include the pyridine-like nitrogens and weave round the periphery, you have nine double bonds and hence 18 electrons—a An+ 2 number. Most people agree that these compounds are aromatic. 

The alkylation is regioselective because the methylated nitrogen must become the pyrrole-like nitrogen atom and the molecule prefers the longest conjugated system involving that nitrogen and the ester. 

Indole has a pyrrole-like nitrogen and undergoes electrophilic aromatic substitutions in the heterocyclic ring. 

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