The chemistry of vinyl nitroxides

Abstract

Vinyl nitroxides (VNs) generated by oxidation of nitrones or the corresponding hydroxylamines are very reactive radicals. In contrast to VNs disubstituted at the β-position of the vinyl group those which are monosubstituted could not be observed directly by ESR with a few exceptions. Instead, their spin adducts with the precursor nitrones were detected. Dimerization of VNs occurs usually by bond formation between the two β-carbon atoms, frequently followed by a secondary reaction step. An unsymmetrical dimer formed by bonding between the oxygen and the β-carbon atom could be isolated in a single case. However, such OC-dimers are frequently formed in a kinetically controlled reaction. Being usually less stable compared to the CC-dimers, their intermediate formation becomes conspicuous if they can undergo a subsequent reaction that gives a thermodynamically stable product. β-Acyl-β-aryl-disubstituted VNs form either bi- or tricyclic dimers. The latter dissociate in solution, usually even at room temperature, regenerating the VNs by breaking of four bonds. © 1990 De Gruyter