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.

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“…A tom-and ion-transfer reactions comprise a major class of chemical reactions found in biological systems, notably metabolic pathways (1)(2)(3)(4)(5). Reactions involving the net transfer of a hydrogen atom may be viewed in terms of two coupling limits with intermediate cases implicit (6): movement of a H atom between a donor͞acceptor pair vs. independent proton͞electron-transfer steps involving different electron and proton sites.…”

“…For example, formation of an NOH bond in bis-imidazoline (Hbim) by net transfer of an H atom to Fe(III)(Hbim)(H 2 bim) 2 from a COH bond in dihydroanthracene has a CH͞CD isotope effect of 4 at the lower end (33). A large H͞D KIE of 41 is observed for the redox reaction involving cleavage of the NOH hydrazido bond in the Os(IV) complex (tpy)OsCl 2 [HNN(CH 2 ) 4 O] by quinone to form the hydrosemiquinone (34). Table 1 compares the rate constants and H͞D KIE for Hatom abstraction from pzH by a diverse group of radical oxidants (alkyl, alkoxyl, and peroxyl radicals) and for 1.…”

Synthesis and antioxidant evaluation of novel indole-3-acetic acid analogues

“…In addition to the traditional O-H bond type antioxidants, aromatic amines having N-H bond functions as the antioxidant have attracted much attention, because aromatic amines (Ar2NHs) have always been the central structure in many currently used drugs [11]. From the literature, phenolic compounds and some of the aromatic amines (heterocyclic amines) showed antioxidant properties in vitro and have been discussed from the view of chemical kinetics [12,13].…”

In Vitro Antioxidant Activity of Dibenz[b,f]azepine and its Analogues

“…Now a day, the free-radical scavenging mechanism of aromatic amines (Ar 2 NHs) has been discussed from the view of chemical kinetics 7 .…”