Oxygenation of aromatics

It is well-recognized that phenols are completely protonated in superacidic solutions.420 This raised the possibility that protonated phenols, once formed in these media, might resist further electrophilic attack. Electrophilic hydroxylations of aromatics with hydrogen peroxide (98%) in superacidic media has been achieved by Olah and Ohnishi617 in Magic Acid, which allows clean, high-yield preparation of monohydroxylated products. Benzene, alkylbenzenes, and halobenzenes are efficiently hydroxylated at low temperatures. The obtained yields and isomer distributions are shown in Table 5.36. Subsequently, Olah et al.618 found that benzene and 

Reactions were performed in HSO3F-SO2CLF solution at -78°C. 

All these reactions involve hydroxylation of aromatics by hydrogen peroxonium ion [Eq. (5.215)]. 

The H202-HF-SbF5 system has been applied by Jacquesy and co-workers in the hydroxylation of a variety of functionalized arenes.624 Hydroxylation of phenyl esters has been shown to afford themeta and para isomers as main products625 [Eq. (5.217)]. Substantial amounts of the deacylated derivatives were obtained in the reaction of phenyl formate and diphenyl carbonate. In the hydroxylation of 2-chlorophenyl and 4-chlorophenyl acetate, regioselectivity is controlled by the chlorine substituent with the hydroxyl entering into the meta position to the ester group.626 A similar effect was observed in the hydroxylation of anilines and anilides. 

The procedure has also been applied for the hydroxylation of aromatic amines. Aniline and its /V-alkyl-substimted derivatives show similar behavior under similar conditions to afford the meta-substi tuted aminophenols as the major hydroxylated product.627 Product formation was interpreted by the attack of protonated hydrogen peroxide on the anilinium ion protected by /V-protonation from oxidation or degradation. Indoles, indolines, and tetrahydroquinoline have also been successfully hydroxylated with H202 in HF-SbF5 with the hydroxyl group meta to the nitrogen function.559,628 Hydroxylation of tryptophane and tryptamine derivatives affords pretonine and serotonine derivatives in 42% and 38% yields, respectively.629 

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Table I. Semiconductor Photocatalyzed Oxygenation of Aromatic Olefins (a)...

Ohkubo, K., Nanjo, T. and Fukuzumi, S. (2005) Efficient photocatalytic oxygenation of aromatic alkene to... 

Tanielian C, Lykakis IN, Seghrouchni R, Cougnona F, Orfanopoulos M. Mechanism of decatungstate photocatalyzed oxygenation of aromatic alcohols. Part I. Continuous photolysis and laser flash photolysis studies. J Mol Catalysis A Chem 2007 262 170-5. 

An interesting dehydrogenation of hydrazones (51) has been reported by Barton which relies on the available oxygen of aromatic nitro groups (equation 22). In a detailed study, quantitative yields were obtained using 4-nitrobenzoic acid as the oxidmt. Whilst this unusual reaction affords some advantages over earlier methods it is unlikely to be the method of choice in most instances. 

Hypervalent iodine species were demonstrated to have a pronounced catalytic effect on the metalloporphyrin-mediated oxygenations of aromatic hydrocarbons [93]. In particular, the oxidation of anthracene (114) to anthraquinone (115) with Oxone readily occurs at room temperature in aqueous acetonitrile in the presence of 5-20 mol% of iodobenzene and 5 mol% of a water-soluble iron(llI)-porphyrin complex (116) (Scheme 4.57) [93]. 2-ferf-Butylanthracene and phenanthrene also can be oxygenated under similar conditions in the presence of 50 mol% of iodobenzene. The oxidation of styrene in the presence of 20 mol% of iodobenzene leads to a mixture of products of epoxidation and cleavage of the double bond. Partially hydrogenated aromatic hydrocarbons (e.g., 9,10-dihydroanthracene, 1,2,3,4-tetrahydronaphthalene... 

The oxygenation of aromatic compounds by Lewis acid-catalyzed decomposition of peroxydicarbonates (19-21) may be formulated as indicated in the following reaction. Saville s rule is applicable to these reactions. 

Igeta, H., Tsuchiya, T., Yamada, M., and Arai, H., Photoinduced oxygenation of aromatics by pyridazine N-oxides, Chem. Pharm. Bull., 16, 767,1968. 

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