Bond Dissociation Energies of the N−H Bond and Rate Constants for the Reaction with Alkyl, Alkoxyl, and Peroxyl Radicals of Phenothiazines and Related Compounds

Abstract

The results of a detailed thermodynamic and kinetic investigation on the homolytic reactivity of phenothiazine, phenoxazine, and phenoselenazine, of several substituted phenothiazines, and of related tricyclic aromatic amines are reported. All these compounds give, by hydrogen atom abstraction from the N−H group, persistent aminyl radicals. Equilibration of each of these radicals with the parent amine and a reference compound having an easily abstractable hydrogen allowed us to determine, by using EPR spectroscopy, the N−H Bond Dissociation Energies (BDE) of the amines. These are characterized by low BDE values (in some cases lower than the O−H bond strength of α-tocopherol, i.e 78.3 kcal/mol) and therefore are very good hydrogen atom transfer reagents. To check the efficiency of tricyclic amines as antioxidants and as polymerization inhibitors, absolute rate constants were determined for their reaction with alkyl, alkoxyl, and peroxyl radicals by using competitive techniques in the first two cases and by autoxidation studies under controlled conditions in the last one. All amines have been found to be highly reactive toward these radicals which makes them very good autoxidation and polymerization inhibitors.