Inhibiting Effect of 4-Hydroxy-2,5-Dimethylfuran-3-one on the Radical Chain Oxidation of Styrene

The O–H bond strength was calculated by the G4 and the M06-2X/MG3S methods for 2‑hydroxy-3-methylcyclopent-2-en-1-one (maple lactone), 4-hydroxy-2,3-dimethyl-2H-furan-5-one (sotolon), 4-hydroxy-5-methylfuran-3-one, 4-hydroxy-2,5-dimethylfuran-3-one (strawberry furanone), (2R)-2-[(1S)-1,2-dihydroxyethyl]-3,4-dihydroxy-2H-furan-5-one (ascorbic acid), 5-hydroxy-2-(hydroxymethyl)pyran-4-one (kojic acid), 3-hydroxy-2-methylpyran-4-one (maltol), 3-hydroxy-2-ethylpyran-4-one (ethylmaltol), 4-hydroxy-6-methylpyran-2-one, and 5-hydroxy-6-methyl-3,4-dihydro-2H-pyran-4-one (dihydromaltol). The calculations indicated the presence of a weak O–H bond of less than 82.0 kcal/mol in 4-hydroxyfuran-3-one derivatives. The experimental rate constant of the reaction of the compound with the lowest O–H bond strength, 4-hydroxy-2,5-dimethylfuran-3-one, with peroxyl radicals in chlorobenzene and acetonitrile was comparable to the data for dibutylhydroxytoluene, but the stoichiometric coefficient of inhibition was 0.17 (PhCl) and 0.66 (MeCN), which was significantly smaller than for dibutylhydroxytoluene. The activation enthalpy for hydrogen atom elimination from 4-hydroxy-2,5-dimethylfuran-3-one by peroxyl radicals calculated by the SMD(PhCl)-M05/MG3S method correlated well with the data for 5-hydroxy and 5-aminouracil derivatives, which is indicative of the common mechanism of interaction of these compounds with peroxyl radicals.