Nucleophilic Substitution of Hydrogen in Nitroarenes: A New Chapter of Aromatic Chemistry

Basic concepts, mechanistic principles and the broad application of nucleophilic substitution of hydrogen in nitroarenes and other electron-deficient arenes in organic synthesis are presented in a concise way. It was shown that the initial process between nucleophiles and (halo) nitroarenes is a fast and reversible addition - in positions occupied by hydrogen - to form anionic sigma(H) adducts that can then be converted into products of nucleophilic substitution of hydrogen in a few ways. Only when these pathways for rapid conversion of the sH adducts into products of nucleophilic substitution of hydrogen are not available, do the sigma(H) adducts dissociate. The subsequent slower formation of sigma(X) adducts, followed by departure of X anions, results in the conventional nucleophilic aromatic substitution of halogens, namely the SNAr reaction. Thus the SNAr reaction is a secondary ipso substitution, whereas nucleophilic substitution of hydrogen is a primary fast process. It is therefore necessary to introduce corrections in appropriate chapters of textbooks and monographs.