Oxidation halophenols

Osborne RL, Coggins MK, Raner GM et al (2009) The mechanism of oxidative halophenol dehalogenation by Amphitrite omata dehaloperoxidase is initiated by H2O2 binding and involves two consecutive one-electron steps Role of ferryl intermediates. Biochemistry... 

Halophenols without 2,6-disubstitution do not polymerize under oxidative displacement conditions. Oxidative side reactions at the ortho position may consume the initiator or intermpt the propagation step of the chain process. To prepare poly(phenylene oxide)s from unsubstituted 4-halophenols, it is necessary to employ the more drastic conditions of the Ullmaim ether synthesis. A cuprous chloride—pyridine complex in 1,4-dimethoxybenzene at 200°C converts the sodium salt of 4-bromophenol to poly(phenylene oxide) (1) ... 

Halophenols are easier to dispose of by extraction. Chlorophenyl esters are resistant to hydrogenation and are hence compatible with benzyloxycarbonyl. Benzotriazolyl esters are unique in that they exist in two different forms (see Section 7.17). They are too reactive for routine use but are often employed without isolation after their preparation, using a carbodiimide or other (see Section 7.20). An exception to this are the benzotriazolyl adducts of. V-lrilylamino acids that are amide oxides (see Section 7.17), which are stable to aqueous sodium hydroxide but undergo aminolysis normally. 

The formation of 3-halophenols in the metabolism of chlorobenzene, bromobenzene, and fluorobenzene215 cannot be explained on the basis of arene oxides as intermediates. These metabolites may represent examples of a direct hydroxylation of the ring. Besides, the magnitude of the isotopic effects observed during the metabolic formation of such meta-substituted phenols... 

Osborne RL, Raner GM, Hager LP, Dawson JH (2006) C.fumago Chloroperoxidase is also a Dehaloperoxidase Oxidative Dehalogenation of Halophenols. J Am Chem Soc 128 1036... 

Oxidative phenolic coupling.1 The vancomycin antibiotics are polypeptides with bridging diphenyl ether groups. Evans et al. have shown in model systems such as 1 that cyclization to o-halophenolic peptides (2) can be accomplished by oxidation with thallium(III) nitrate in THF-methanol or CH2Cl2-methanol followed by CrCl2 reduction of a para-quinol intermediate (a). In three cases the yield of cyclic products was 40-48%. 

The effect of the halogen substituent (fluoro, chloro, bromo and iodo) on the yield and mechanism of 4-halophenol photolysis was investigated by Durand et al. [24], Transient spectroscopy in aerated aqueous solutions indicated the formation of p-benzoquinone O-oxide from each derivative except 4-iodophenol for which no transients were detected p-benzoquinone and hydroquinone were found as photoproducts for all four compounds. It was concluded that the carbene mechanism was valid for the whole series. Under continuous irradiation, the 4-halophenol degradation quantum yields were determined to be

fluorescence lifetimes decreased in the same order, from 2.1 ns for 4-fluorophenol to 0.4 ns for 4-chlorophenol and < 0.1 ns for 4-bromophenol. 

Quinones from phenols Under acidic conditions (70% perchloric acid in HOAc) phenols are converted to p-benzoquinones at room temperature. The oxidation proceeds even with p-halophenols such as 2,4,6-trihalophenols, giving 2,6-dihalo-p-benzoquinones in high yields. 

This idea is based on the assumption that hydration of the radical cation occurs predominantly at the ipso position. This is reasonable on the basis of estimates" of the charge distribution in fluorobenzene radical cation. The mechanism in equation 15 is, however, not likely for oxidative dehalogenation of halophenols" at pH 4.5 since the halophenol radical cations would deprotonate before they had a chance to react with water. With substituted phenols, therefore, the observed semiquinones are probably produced via equation 14. 

Low-level chlorination of 0.5-1.5 mg/1, resulting in a chlorine residual of 0.1 -0.2 mg/1, is used to reduce the degree of biofouling. In sea-water, chlorine oxidizes bromide, present at about 65 mg/1, to bromine, which also contributes to the generation of halogenated by-products [176]. The by-products include hypobromous acid, hypobromite, chloramines, bromamines, trihalomethanes, haloacetonitriles,haloacetic acids, and small amounts of halophenols. However, some haloforms and bromophenols as well as other organobromine compounds are also produced naturally in coastal waters [ 181,182]. Empirical equations for the disappearance of chlorine/bromine derived oxidants from brackish water have been published [183]. 

Oxidation of Phenols to Quinones. Halophenols are conveniently oxidized by CAS to haloquinones in good yields (eq 6). On oxidation with CAS, halonaphthols furnish 1,2-diones as minor products in addition to 1,4-quinones. 

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