Tert-Butylated phenoxyl

C. J. Orme, J. R. Klaehn, F. F. Stewart, Gas permeability and ideal selectivity of poly[bis-(phenoxy)phosphazene], poly[bis-(4-tert-butyl-phenoxyl)phosphazene] and poly[bis-(3,5-di-tert-butylphenoxy)l,2(chloro)0.8 phosphazene), J. Membr. Sci., 238, 47-55 (2004). 

Fig. 28. Dependence of the rate constant of consumption of tri-tert-butyl-phenoxyl on the concentration of di -ethylphosphorous ester in benzene solution (T = 200°C).

Chemical interaction between transformation products of both the amine and phenol is involved. Derivatives of cyclohexadienone like 77, deeply coloured compounds with structure of indophenols or BQMI 57 are expected to be formed as intermediates [251], The compound 77 is formally analogous to quinol ethers of 1,4-benzoquinone dioxime 163 (R = tert-butyl, methoxy, phenyl) thermally releasing the corresponding phenoxyl and therefore imparting the antioxidant effect [253]. 

Substituted phenols, such as 2,6-di-tert-butyl-p-cresol, readily inhibit autoxidation reactions through formation of the corresponding phenoxyl radicals (ArO ) [54], The latter may undergo either oxygenation to give ketohydroperoxides by a chain... 

Phenoxyls, having bulky substituents in the positions 2,4, and 6 (e. g., tert-butyls), do not dimerize24. If some of the substituents is less sterically demanding (this may be also the case of bridge in multinuclear phenols), C—C or C—0... 

The parallel C—C and C-0 couplings take place via phenoxyl in the photooxidation of 2,4-disubstituted phenols 4,4 -dimethoxy-6,6 -di-tert-butyl-2,2 -bi-phenyldiol and 2-tert-butyl-4-methoxy-6-(2-tert-butyl-4-methoxyphenoxy)phenol were formed from 2-tert-butyl-4-methoxyphenol41 ... 

ROO-CHD XVI is formed in the yield of 5-10 % in the reaction of 2,4-di-tert-butylphenol with tert-Bu0233 via the phenoxyl VI (Scheme 3). Its isolation from the reaction mixture is difficult, because it is cleaved on silica gel to 2-tert-butyl-l,4-benzoquinone. Also the isomeric ROO-CHD XIV is the intermediary formed from the same phenol, but it is immediately cleaved to 3,5-di-tert-butyl-l,2-benzoquinone XV. 2,2 -Biphenyldiol X, originating as a consequence of C-C coupling of phenoxyl VI, is not the final product in the presence of excess of RO 2 and ROO-CHD s XI to XIII are formed (the compound XII in the yield as high as 60 %). Also the phenolic derivative VIII undergoes further oxidation, giving ROO-CHD IX as the final product. 

The tert-butoxy radical is liberated by photolysis of XXXVIa (Scheme 9). Its formation was confirmed by reactions which produce more stable radicals detectable by the ESR method135. The heptane solution of XXXVIa was irradiated by light 360—480 nm in the presence of 2,6-di-tert-butyl-4-methylphenol or 2,4,6-tri-tert-butylphenol. These sterically hindered phenols were transformed by the liberated tert-butoxyl into phenoxyls. Their identification by the ESR method is the indirect evidence of tert-BuO formation. The direct evidence was achieved by means of nitro-sobenzene CVI (R=H) and nitrosodurene CVI (R=Me) as spin-traps. The nitroxide radical CVII (R=H or Me) is formed in both cases and was identified by ESR spectrum135. This spin-adduct is unstable. The intensity of its spectrum gradually decreases during irradiation and the spectrum of another radical appears which is less intense but invariable in time. Judging from its hyperfine structure, it may be formulated as CVIII (R=H or Me). More detailed data on the character of substituents RJ-R2 is not yet known. 

The reactions which can occur with sterically hindered phenols in the presence of 102 and a sensitizer are shown on the example of 2,6-di-tert-butyl-4-methyl-phenol XXIV in Scheme 13. It may be expected that the oxidation of XXIV by the excited sensitizer proceeds to phenoxyl XXV, or that phenol XXIV is attacked by 02 giving either again phenoxyl XXV or, by the 1,4-addition, endoperoxide CXIII, which is transferred to hydroperoxycyclohexadienone CXIV160, The latter compound may also originate via biradical CXV, as stated in161 (Scheme 13). 

XX are formed in the process via phenoxyl XVII and the C—C coupling, or 4-hydro-peroxycyclohexadienone CXXI, or 4,4 -dioxycyclohexadienone CXXII, which are photolyzed to 2,6-di-tert-butyl-1,4-benzoquinone XXII, arises via cyclohexadienonyl CXIX and cyclohexadienonylperoxyl CXX (Scheme 14). 

Phenoxyls are generated by the complex-bonded radicals RO"2 [Co(III)] also from bis(3,5-di-tert-butyl-4-hydroxybenzyl)-sulphide CXCVIIa22l The bridge CH2-S— CH2 completely interrupts the mesomerism between both aromatic nuclei in the phenoxyl formed. However, although the atom S is separated from aromatic nuclei by a methylene group, its stabilization effect on the phenoxyls formed still occurs. This followed from the comparison with ESR spectra of phenoxyls from 4,4 -methylenebisphenols. 

The formation of further compound CLXXXIa may be explained by recombination of phenyl CLXXVIIIa with phenoxyl CLXXVIa27 which is primarily formed from phenol CLXCIIIa by the reaction with RO 2 or ROOH. The probable intermediate was phenol CLXXXa and the final isolated product was 4-[(3-methyl-5-tert-butyl-4-hydroxyphenoxy)-3-methyl-5-tert-butylphenylsulphinyl]-2-methyl-6-tert-butylphenol CLXXXIa. 

The ESR spectra show radical cations of some tert-butylated phenols when they interact with CH3COCI and (013)300 in the presence of AICI3 as well as in the conditions of nitration and bromination. In a number of cases the phenoxyl radicals are formed, probably by the proton loss from the radical cation. Therefore the reactions of tert-butylated phenols with electrophiles have been assumed to follow the scheme of electron transfer ... 

Their effectiveness depends on the number of phenol groups and their steric hindering. Phenols are sterically hindered by substituents such as tert-butyl-groups in position 2,4 and/or 6. Large substituents prevent the reaction of the phenoxyl radicals with the polymer chain and the dimerization of two phenoxyl radicals however, the rate of hydrogen abstraction increases with a decrease in steric hindering. 

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