Hydroquinones aromatization reagents

August et al. (1998) conducted kinetic studies for the reaction of chlorobenzene (0.2 mM) and other monocyclic aromatics with Fenton s reagent (8 mM hydrogen peroxide [Fe" ] = 0.1 mM) at 25 °C. They reported a reaction rate constant of 0.0820/min. The following intermediates were identified during this reaction chlorophenol, chlorohydroquinone, chloroquinone, hydroquinone, and quinone. 

In an aqueous solution, 4-nitrophenol (100 pM) reacted with Fenton s reagent (35 pM). After 15 min into the reaction, the following products were identified 1,2,4-trihydrottybenzene, hydroquinone, hydroxy-p-benzoquinone, p-benzoquinone, and 4-nitrocatechol. After 3.5 h, 90% of the 4-nitrophenol was destroyed. After 7 h, no aromatic oxidation products were detected. The pH of the solution decreased due to the formation of nitric acid (Lipczynska-Kochany, 1991). In a dilute aqueous solution at pH 6.0, 4-nitrophenol reacted with excess hypochlorous acid forming 2,6-dichlorobenzoquinone, 2,6-dichloro-4-nitrophenol, and 2,3,4,6-tetrachlorophenol at yields of 20, 1, and 0.3%, respectively (Smith et al., 1976). 

The quinones may be regarded as unsaturated diketones derived from cycloolefins, and closely related to aromatic compounds. They exhibit great reactivity with a variety of reagents. They are readily reduced and oxidized, and undergo both olefinic and carbonyl reactions. Equimolecular proportions of p-benzoquinone and hydroquinone yield a crystalline molecular compound, quinhydrone. 

The oxidation of phenols or o- and p-hydroquinones with stoichiometric [bis(acyloxy)iodo]arenes to the corresponding benzoquinones is one of the most typical synthetic applications of hypervalent iodine reagents (Section 3.1.11). The catalytic version of this reaction was first reported by Yakuraand Konishi in 2007 [52], The reaction of p-alkoxyphenols 50 with a catalytic amount of 4-iodophenoxyacetic acid in the presence of Oxone as the terminal oxidant in aqueous acetonitrile at room temperature affords p-quinones 51 in high yields (Scheme 4.26) [52]. 4-Iodophenoxyacetic acid is a readily available and water-soluble aromatic iodide that has a particularly high catalytic activity in this reaction. 

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