Nitrogen homocycles

Unlike carbocyclic (or homocyclic) hydrocarbons which have been discussed previously, heterocyclic compounds contain in their rings other atoms, such as nitrogen, oxygen, or sulfur, in addition to carbon. Some examples are shown below ... 

In intramolecular arylations, a new bond is created between two aromatic moieties of the same molecule or between an aromatic nucleus and an atom of a side-chain. Many intramolecular arylation reactions of homocyclic and heterocyclic aromatic halides have been studied mainly in view of their synthetic applications, and it is not always clear which mechanistic pathway is followed. The reaction may start with homolytic or heterolytic dissociation of the carbon-halogen bond and proceed by attack of the aryl radical or aryl cation on another part of the molecule. Electrocyclization followed by elimination of hydrogen halide is another possibility. Especially when heteroatoms such as nitrogen, sulphur or phosphorus are involved, the initial step may be a nucleophilic attack on the carbon atom bearing the halide atom. 

In most of the cyclic compounds that we have studied so far—benzene, naphthalene, cyclohexanol, cyclopentadiene—the rings are made up only of carbon atoms such compounds are called homocyclic or alicyclic compounds. But there are also rings containing, in addition to carbon, other kinds of atoms, most commonly nitrogen, oxygen, or sulfur. For example ... 

Nitrogen-containing heterocycles can pose problems that are not encountered in their homocyclic analogues. Obviously, heterocycles involving NH groups, such as pyrrol and its annellated and aza derivatives, will destroy an organomagnesium-halide reagent formed from their halo derivatives. Another example is that of... 

The homoljdic amination is of less use with heterocyclic than with homocyclic aromatic compounds because either the heteroccylic compounds are too deactivated (protonated heteroaromatic bases) or they are unstable in the strongly acidic medium usually required by the reaction. Thus, quinoline cannot be aminated because the protonated heterocyclic nitrogen deactivates both rings. In the... 

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