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| - On the Addition of •OH Radicals to the Ipso Positions of Alkyl-Substituted Aromatics: Production of 4-Hydroxy-4-methyl-2,5-cyclohexadien-1-one in the Radiolytic Oxidation ofp-Cresol
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| - 4-Hydroxy-4-methyl-2,5-cyclohexadien-1-one has been conclusively identified by its 1H and 13C NMR spectraas a significant initial product in the radiolysis of aqueous solutions of p-cresol. This product is formed as theresult of oxidation of the hydroxycyclohexadienyl radicals produced by addition of ∼12% of the •OH radicalsto the aromatic ring at the ipso position adjacent to the methyl group. It has a strong absorption band at 228nm that is similar to the 246 nm band of p-benzoquinone. Its proton NMR spectrum exhibits strong couplingbetween two pairs of ring protons. A quartet is observed in the spectrum of the methyl-13C labeled product,confirming that a methyl group is attached to the dienone ring.13C chemical shifts and 13C−H spin−spinsplittings of the dienone are reported. Although DFT calculations of the proton NMR parameters are in verygood agreement with the experimental values, the calculated chemical shifts of the ring 13C carbons are 4−8ppm too high, indicating that the DFT calculations do not properly take into account the dienone π system.This conclusion is confirmed by parallel experimental and theoretical studies of 4H-pyran-4-one that providesa model system closely related to the dienone. The observation of 4-hydroxy-4-methyl-2,5-cyclohexadien-1-one as a product in the radiolysis of aqueous solutions is important in demonstrating that addition of •OHradicals to aromatic rings at positions substituted with alkyl groups can be of appreciable importance in the•OH oxidation of aromatic substrates.
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