Quatemisation heterocycles

The introduction of a nitro group at C-1 in isoquinolines can be achieved using a mixture of potassium nitrite, dimethylsulfoxide and acetic anhydride. The key step is the nucleophihc addition of nitrite to the heterocycle previously quatemised by reaction at nitrogen with a complex of dimethylsulfoxide and the anhydride. 

As with the azoles, oxa- and thiadiazoles are very weak bases due to the inductive effects of the extra heteroatoms, although A-quatemisation reactions can be carried out. For similar reasons, electrophilic substitutions on carbon are practically unknown, apart from a few halogenations and mercurations - it is an intriguing paradox that mercurations, with what is generally thought of as a weak electrophile, are often successful in electron-poor heterocycles. Another important difference from other azoles is of course the absence of A-hydrogen, so that A-anion-mediated reactions are not available. 

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