General Survey of SNH Substitution

There are three groups of substrates for which SNH reactions are especially characteristic (i) neutral azines and azoles (ii) azinium and azolium salts and (iii) nitro-arenes. Their electron-deficiency and, thus, their ability to react with nucleophiles strongly differ from each other. Azinium salts are the more electrophilic and are able to add even neutral nucleophiles very easily. Triazines,. v-tetrazines and polynitro-arenes also possess high electrophilicity. At the same time substrates such as pyridine (82JHC1285, 72JA682) and cinnoline (03CHE87), at a low temperature (e.g. in liquid ammonia as a solvent), do not react even with sodium amide. It should also be 

spontaneous elimination of hydride ion commonly demands a high temperature and is characteristic for anionic aH-complexes only. 

Oxidation of aH-complexes with air oxygen is also possible, though for neutral adducts it demands hours and even days for completion. Alkylamination of 5-azacinnoline serves as a typical example (77KGS1554). In nitroarenes the N02 group itself was often found to be an acceptor of hydride ion (see Section III.D.l). Similarly, the ring C = N bond on amination of azaheterocycles can also intercept hydride ion (78RCR1042). 

The theory and practice of vicarious nucleophilic substitution of hydrogen (VNS) were examined in detail mostly by Mqkosza and his co-workers (87ACR282, 

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