Aromatic rings photochemical oxidations

Tetralin. The presence of an aromatic ring in the molecule induces important changes with respect to the saturated homologue (decalin). Firstly, the photochemical oxidation (i.e. in the absence of titania) is very important (see Fig. 1) as expected from the absorption spectrum it produces principally 2-tetralone as the main product and 1 -tetralone. Secondly, the photocatalytic oxidation occurs principally at position 1 (see. Fig. 1 and the following scheme). Note that this scheme refers to both photocatalytic and photochemical products however, because of the role of inner filter played by Ti02, the former products should be predominant. 

Gasoline hydrocarbons volatilized to the atmosphere quickly undergo photochemical oxidation. The hydrocarbons are oxidized by reaction with molecular oxygen (which attacks the ring structure of aromatics), ozone (which reacts rapidly with alkenes but slowly with aromatics), and hydroxyl and nitrate radicals (which initiate side-chain oxidation reactions) (Stephens 1973). Alkanes, isoalkanes, and cycloalkanes have half-lives on the order of 1-10 days, whereas alkenes, cycloalkenes, and substituted benzenes have half- lives of less than 1 day (EPA 1979a). Photochemical oxidation products include aldehydes, hydroxy compounds, nitro compounds, and peroxyacyl nitrates (Cupitt 1980 EPA 1979a Stephens 1973). 

If such reactions were to be coupled with the photochemical generation of organic free radicals or excited molecular states before the H-atoms combined or hydride ions reacted with HaO, the presence of photore-duced products would be explained. The generation of the powerfully nucleophilic peroxy radical-ions and hydroperoxide ions (Equation 8) also could be involved in instances of the oxidative displacement of halide from aromatic rings. 

Iodine in combination with [bis(acyloxy)iodo]arenes is a classical reagent combination for the oxidative iodination of aromatic and heteroaromatic compounds [99], A typical iodination procedure involves the treatment of electron-rich arenes with the PhI(OAc)2-iodine system in a mixture of acetic acid and acetic anhydride in the presence of catalytic amounts of concentrated sulfuric acid at room temperature for 15 min [100,101]. A solvent-free, solid state oxidative halogenation of arenes using PhI(OAc)2 as the oxidant has been reported [102]. Alkanes can be directly iodinated by the reaction with the PhI(OAc)2-iodine system in the presence of f-butanol under photochemical or thermal conditions [103]. Several other iodine(in) oxidants, including recyclable hypervalent iodine reagents (Chapter 5), have been used as reagents for oxidative iodination of arenes [104-107]. For example, a mixture of iodine and [bis(trifluoroacetoxy)iodo]benzene in acetonitrile or methanol iodinates the aromatic ring of methoxy substituted alkyl aryl ketones to afford the products of electrophilic mono-iodination in 68-86% yield [107]. 

One of the main pathways of microbial and photochemical transformation of AHs is oxidation of an aromatic ring to generate hydroxylated products (e.g., phenols and diols). As bioenergetics and metabolic status of microbial communities varies drastically with change in Eh (which follows prevailing... 

The high ion yield of benzene and other aromatic molecules found in low pressure gas phase studies suggests the use of these ions in preparative phtochemistry in solution. The absorption spectrum of benzene iy wat r as a solvent shows a weak band around 250 nm (E = 200 1 mole cm ) corresponding to the S S transition (see sec fon II). The photochemistry fo this molecule exhibits a great manifold of processes. Pure benzene can be transformed by irradiation in this spectral region into various ring isomers, open chain valence isomers and other minor products. In aqueous solution benzene may be photochemically oxidized to formyl-cyclopentadiene-1,3. This oxidation is believed to proceed via the formation of benzvalene in the first step which is rapidly oxidized - with and without oxygen, eqn. (1) - to the aldehyde... 

Highly selective formation of phenyl acetate was observed in the oxidation of benzene with palladium promoted by heteropoly acids.694 Lead tatraacetate, in contrast, usually produces acetoxylated aromatics in low yields due to side reac-tions. Electrochemical acetoxylation of benzene and its derivatives and alkoxylation of polycyclic aromatics789 790 are also possible. Thermal or photochemical decomposition of diacyl peroxides, when carried out in the presence of polycyclic aromatic compounds, results in ring acyloxylation.688 The less reactive... 

The observed photochemical ring contraction of monocyclic aromatic iV-oxides can also be interpreted as proceeding through an intermediate oxazirane. Pyridine 1-oxide and 2-methylpyridine... 

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