Aroxyls

Typical auxochromes are hydroxyl, alkoxyl and aroxyl, amino, alkyl-amino and arylamino, all of which promote conjugation with lone pairs on oxygen or nitrogen atoms. 

Dimroth, K. Arylated Phenols, Aroxyl Radicals and Aryloxenium Ions Syntheses and Properties. /29, 99-172 (1985). 

Rate Constants, Equilibrium Constants, Enthalpies, and Entropies of Aroxyl Radicals Dimerization in Hexane at T = 293 K... 

The mechanism of inhibitory action of aryl phosphites seems to be relatively complex. Phosphites reduce hydroperoxide and thus decrease chain autoinitiation. The formed peroxyl and alkoxyl radicals react with phosphites to form aroxyl radicals. The latter terminates the chains by reaction with peroxyl radicals. On the other hand, phosphites are hydrolyzed with... 

There seems to be a ratio of one free radical per quinone oxygen. Donnet et al. (101) concluded that either paraquinones or aroxylic radicals were present, which would react according to the following schemes. 

Since the number of free radicals calculated from the dilference in nitrogen uptake (1.13 102 /gm) agreed well with the number of unpaired electrons as determined by e.s.r. (0.80-10 /gm), the aroxylic structure seemed very likely. The reaction of oxidized carbon black with styrene can be explained on this basis 102). 

The procedure for preparing 2,4,6-triphenylphenoxyl is based on the method described by Dimroth and co-workers. This method represents the commonly used preparation of aroxyl radicals by oxidation of the corresponding phenol. The chemistry of stable phenoxyl radicals has been reviewed. ... 

Other aroxyl radicals, especially those with i-butyl groups at the phenyl ring, are described by Cook and by Miiller. ... 

In some cases, cation-radicals are formed from neutral organic molecules by the action of neutral organic acceptors such as tetracyanoethylene, tetranitrofluorenone, quinones, and free radicals—aroxyls, nitroxyls, and hydrazyls. 

When coniferyl alcohol is dehydrogenated, it loses its phenolic hydrogen atom to form first an aroxyl radical Ra (XI), which is in eflFect also present as the mesomeric radicals Rb (XII), Rc (XIII), and Rd (XIV). Of these limiting structures, Rb is the most favored. The existence of the radicals in these forms is recognized by their reaction products. In very dilute dioxane-water solution (1 1 vol.), the half-life of the radicals is about 45 seconds 13). 

Similar to aroxyl-benzo[b]pyrylium systems (78ZOR1643), there is no detectable delocalization of the unpaired electron on the benzo[c]pyrylium ring in 301-303 (87RRC417). The stability of the radical cation is determined in this case by the position of the aroxyl substituent in the cation, and the most stable is 301. The formation of diradical species 303 together with the monoradical cation, was indicated on oxidation of the corresponding 2-benzopyrylium salt. 

The reactions between free radicals and flavonoids (or polyphenols) are assumed to form aroxyl radicals (PO) (reaction 8). The stability of these secondary radical species is an important element to be considered in their antioxidant actions. Flavonoids with similar reduction potentials can originate radicals with very different reactivity toward other molecules present in biological systems. While a stable and relatively nonreactive PO is also nonreactive to propagate the chain reaction, a high reactive PO would propagate rather than interrupt a chain reaction. 

To compare the stability of aroxyl radicals derived from different flavonoids, the rate constants for the disproportionation reaction between two PO to render nonradical species (reaction 15) can be used ... 

When a series of flavonoids were compared through the analysis of the second-order rate constants (ko), it was concluded that (i) ko values differ considerably, ranging from 105 M-1 s 1 (the most stable) to 10s M 1 s 1 (the less stable), and (ii) these data, together with the spectral characteristics of transient aroxyl radicals, were used to establish the structural requirements for... 

If alkyl or aryl radicals at the silicon atom should be replaced with alkoxyl or aroxyl, tetraethoxysilane is re-etherified with triethanolamine in the presence of alcohols or phenols. 

Fig. 7.1.1. Reaction sequences for the formation of a-hydroxy and a-aroxyl groups in lignins...

Arylated Phenols, Aroxyl Radicals and Aryloxenium Ions... 

This review includes many hitherto unpublished results concerning aryl substituted phenols, their synthesis and their chemical and physical properties, which will supplement the many available publications of t-butyl substituted aroxyls by Cook11 and Muller, 2> or other authors 13"18) and the many reviews of this type of sterically hindered phenoxyls19). 

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