Phenoxyl-hydrogen bond

The CC and CO vibrations are also sensitive to the molecular environment by virtue of electrostatic and hydrogen bonding interactions. The frequencies of phenoxyl and tyrosyl radicals complexed by macrocyclic hgands and generated in wVo were measured by resonance Raman and FTIR techniques. Thus a selective enhancement of the vibrational CC and CO stretch modes of the phenoxyl chromophores in metal-coordinated radical... 

Mukai and coworkers developed the use of the stable, colored 2,6-di-tcrt-butyl-4-(4 -methoxyphenyl)phenoxyl radical (ArO ) and other para derivatives in stopped-flow measurements of hydrogen bond donating ability of a wide variety of chromanol-type antioxidants, and some of their results are reviewed below. 

The stoichiometric factors of inhibition and the rate constants of the ter-penephenols (TP) with isobornyl and isocamphyl substituents were determined by the reaction with peroxy radicals of ethylbenzene. The reactivity was found to decrease for o-alkoxy compared with o-alkyl substituent caused by the intramolecular hydrogen bond formation that is conformed by FTIR-spectroscopy. The inhibitory activity for mixtures of terpene-phenols with 2,6-di-ferf-butyl phenols in the initiated oxidation of ethylbenzene was also studied. In spite of the similar antiradical activities of terpenephenols with isobornyl and isocamphyl sunstituents, the reactivity of phenoxyl radicals formed from them are substantially different that is resulted from the kinetic data for mixtures of terpenephenols with steri-cally hindered phenols. 

The dimer formed by means of hydrogen bonds does not react with free radicals. When the concentration of the ester is increased, the number of dimer molecules increases, and the rate constant of the reaction of di-tert-phenoxyl with the phosphorous ester decreases. 

Fig. 16 A mechanistic example of a hydrogen-bond relay, capable of transferring a proton to the pyridyl nitrogen while simultaneously removing a proton from the phenoxyl oxygen...

The antioxidant activity of phenol is also increased by the presence of additional hydroxyl group in the ortho or para positions. An example of such an antioxidant is TBHQ. The effectiveness of 1,2-dihydroxybenzene derivatives is attributed to a phenoxyl radical stabilised by an intramolecular hydrogen bond (11-8). The activity of 2-methoxyphenol is lower, because the generated radical cannot be stabihsed by a hydrogen bond. The antioxidant activity of 1,2-and 1,4-dihydroxybenzene is partly caused by the fact that the semi-quinone radical can be further oxidised to the corresponding o-quinone orp-quinone, respectively, by reaction with another lipid radical (Figure 11.7) or may disproportionate to the corresponding quinone and hydroquinone. 

To be effective as autoxidation inhibitors radical scavengers must react quickly with peroxyl or alkyl radicals and lead thereby to the formation of unreactive products. Phenols substituted with electron-donating substituents have relatively low O-H bond dissociation enthalpies (Table 3.1 even lower than arene-bound isopropyl groups [68]), and yield, on hydrogen abstraction, stable phenoxyl radicals which no longer sustain the radical chain reaction. The phenols should not be too electron-rich, however, because this could lead to excessive air-sensitivity of the phenol, i.e. to rapid oxidation of the phenol via SET to oxygen (see next section). Scheme 3.17 shows a selection of radical scavengers which have proved suitable for inhibition of autoxidation processes (and radical-mediated polymerization). 

Fig. 23. Proposed mechanisms for Tyr-Cys cofactor biogenesis. (A) Initial Cu(II) binding leads to the appearance of resonance contributions from a reactive phenoxyl that electrophilically attacks the neighboring Cys-228 thiol. (B) Cu(I) binding produces an oxygen-reactive complex that drives hydrogen abstraction from Cys-228 to form a thiyl free radical which subsequently attacks the neighboring Tyr-272 ring with formation of a carbon-sulfur covalent bond.

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