Kanji Omura

Antioxidant synergism between butylated hydroxyanisole and butylated hydroxytoluene

Decay of the 2,6-di-tert-butyl-4-methylphenoxy radical [butylated hydroxytoluene (BHT)-radical] in the presence of butylated hydroxyanisole (BHA) was investigated in 1,2-dimethoxyethane with or without triethylamine. BHT-radical was conveniently generated by dissociation of its unstable dimer in solution. The products were BHT, 3,3′-di-tert-butyl-5,5′-dimethoxy-2,2′-dihydroxybiphenyl (BHA-dimer), 2,6-di-tert-butyl-p-quinone methide (QM), 1,2-bis(3,5-di-tert-butyl-4-hydroxyphenyl)ethane, and 3,3′,5,5′-tetra-tert-butyl-4,4′-stilbenequinone. The reaction without added triethylamine gave larger quantities of the last two products and BHA (recovery), whereas the reaction with it provided larger quantities of the first two products. The marked difference in the product distribution can be accounted for by a series of reactions including reversible dehydrogenation of BHA with BHT-radical, which generates 2-tert-butyl-4-methoxyphenoxy radical (BHA-radical) and BHT, reversible dimerization of BHA-radical, which affords an intermediarybis(cyclohexadienone), and spontaneous and base-catalyzed prototropic rearrangement of the intermediate into BHA-dimer. Products of coupling between BHT-radical and BHA-radical were not obtained. BHA was found to undergo facile acid-catalyzed addition to QM, providing two isomericbis(hydroxyphenyl)methanes. The results help to elucidate the mechanism of antioxidant synergism between BHA and BHT and may suggest that the synergism can be affected by base or acid.

On the structures of the intermediates from reversible coupling between hindered phenoxy radicals

Abstract Hindered phenoxy radicals 1 and 2 are found to undergo reversible, C-C rather than C-O cross-coupling, and give bis(cyclohexadienone)s 14 and 17. These primary products are not isolable but are recovered as phenolic cyclohexadienones 15 and 18, respectively, after treatment with Et3N or as biphenols 16 and 12, respectively, after treatment with TFA. The other products obtained after treating the reaction mixture with Et3N or TFA are phenol 5 and 4,4′-diphenoquinone 13 alone. Dienones 14 and 17 are interconvertible with each other via dissociation into the parent radicals, and 14 appears to be thermodynamically more stable than 17. Phenoxy radical 1 and other, less hindered 2,6-dialkylphenoxy radicals 24 also form intermediates of reversible cross-coupling. Treatment with TFA of the mixtures containing the intermediates provides 2,4′-biphenols 25 preferentially.

Oxidation of 4,4′-diphenoquinones giving p-benzoquinones

4,4′-Diphenoquinone 1a can be oxygenated to give p-benzoquinone 2a when treated with PbO2 in a solvent containing an aqueous acid. The conversion of 1a is facilitated when the acid is strong and the solvent is of low basicity. The concentration of H2O in the medium also has a profound effect on the rate of conversion of 1a. These facts may indicate that protonation of 1a is the first step in the conversion into 2a. 4,4′-Diphenoquinones 1b and 1c can be analogously converted into p-benzoquinones 2a and 2b, respectively. Benzoquinones 2 can thus be the ultimate products of both oxygenation and dehydrogenative dimerization of phenols 3.

Chemistry on the decay of the phenoxy radical from butylated hydroxytoluene

Decay of unstable 2,6-di-tert-butyl-4-methylphenoxy radical (2) was investigated in various solvents. Radical 2 was conveniently generated by dissociation of bis(methylcyclohexadienone) (1a), unstable dimer of 2, in solution. The products were butylated hydroxytoluene (3), 1,2-bis(4-hydroxyphenyl)ethane (7), 4,4′-stilbenequinone (8), and 4-(4-hydroxybenzyl)cyclohexadienone (5). Unidentified products also were formed. The formation of 5 was favored in polar solvents, but was not subject to catalysis with Et3N or HCl. In contrast, the rates of formation of 7 and 8 appeared to be independent of solvent polarity. The mechanism of formation of the dimeric productsvia reactive intermediate quinone methide 4, generated from 2 by disproportionation, is discussed. Gradual disintegration of 5 in solution into 3 and 4 was investigated.

Synthesis of Acid-Resistant P-Quinone Monoketals and Thermally Stable 4,4-Dialkoxycyclohexa-2,5-Dien-1-OLS

Abstract Preparation and high resistance to acid hydrolysis of sterically hindered p-quinone monoketals 1a, 1b and 1c are described. Thermal stability of ketal alcohols 2a and 2b derived from 1a and 1c, respectively, is also mentioned.