VICARIOUS NUCLEOPHILIC-SUBSTITUTION OF HYDROGEN - MECHANISM AND ORIENTATION

Hydrogens located at activated positions in electrophilic arenes, e.g. ortho and para hydrogens in nitrobenzenes, can be replaced with a nuc leophile moiety provided there is at least one nucleofuge X connected to the nucleophilic centre. As the group really leaving in this hydrog en substitution process is not the hydride anion but X, the reaction h as been named vicarious nucleophilic substitution of hydrogen (VNS). T he concepts on the mechanism of the reaction and their experimental ba ckground are presented. Reactivity and orientation - the fundamental q uestions concerning synthetical applications of VNS - are discussed in light of the supposed mechanistic picture.