Intramolecular Aromatic 1,5-Hydrogen Transfer in Free Radical Reactions. 1. Unprecedented Rearrangements in Pschorr Cyclization, Sandmeyer, and Hydro-, Hydroxy-, and lododediazoniation Reactions

摘要

Radical-induced hydrogen migration is very common when the migrating hydrogen originates from an aliphatic carbon or from a heteroatom. Normally, the initially formed less stable radical rearranges to a more stable one. On the other hand, rearrangements where the migrating hydrogen originates from an aromatic carbon have not been generally recognized. It is commonly assumed that aromatic hydrogens cannot be ed by radicals. In fact, benzene is an often used solvent in free radical reactions, because it is a poor hydrogen donor. In this paper we report that the chemistry of benzophenone radicals, generated from the corresponding diazonium salts, is dominated by very rapid 1,5-hydrogen transfer leading to a partial equilibrium between the radicals. The results are summarized in Table 1, following the rationale depicted in Scheme 1.