Oxidation photochemically induced

Bolland, Gee, et al. (cf. reference 48) to interpret the oxidation (photochemical, induced, or thermal) of ethylenic hydrocarbons in solution ... 

The combination of electrochemistry and photochemistry is a fonn of dual-activation process. Evidence for a photochemical effect in addition to an electrochemical one is nonnally seen m the fonn of photocurrent, which is extra current that flows in the presence of light [, 89 and 90]. In photoelectrochemistry, light is absorbed into the electrode (typically a semiconductor) and this can induce changes in the electrode s conduction properties, thus altering its electrochemical activity. Alternatively, the light is absorbed in solution by electroactive molecules or their reduced/oxidized products inducing photochemical reactions or modifications of the electrode reaction. In the latter case electrochemical cells (RDE or chaimel-flow cells) are constmcted to allow irradiation of the electrode area with UV/VIS light to excite species involved in electrochemical processes and thus promote fiirther reactions. 

These have already been noted in the context of hydroxyl radical-initiated oxidations, and reference should be made to an extensive review by Worobey (1989) that covers a wider range of abiotic oxidations. Some have attracted interest in the context of the destruction of xenobiotics, and reference has already been made to photochemically induced oxidations. 

Besides the numerous examples of anionic/anionic processes, anionic/pericydic domino reactions have become increasingly important and present the second largest group of anionically induced sequences. In contrast, there are only a few examples of anionic/radical, anionic/transition metal-mediated, as well as anionic/re-ductive or anionic/oxidative domino reactions. Anionic/photochemically induced and anionic/enzyme-mediated domino sequences have not been found in the literature during the past few decades. It should be noted that, as a consequence of our definition, anionic/cationic domino processes are not listed, as already stated for cationic/anionic domino processes. Thus, these reactions would require an oxidative and reductive step, respectively, which would be discussed under oxidative or reductive processes. 

To date, no examples have been found in the literature for the combination of photochemically induced transformations with reductive/oxidative or enzymatic processes. 

During the past few years, increasing numbers of reports have been published on the subject of domino reactions initiated by oxidation or reduction processes. This was in stark contrast to the period before our first comprehensive review of this topic was published in 1993 [1], when the use of this type of transformation was indeed rare. The benefits of employing oxidation or reduction processes in domino sequences are clear, as they offer easy access to reactive functionalities such as nucleophiles (e. g., alcohols and amines) or electrophiles (e. g., aldehydes or ketones), with their ability to participate in further reactions. For that reason, apart from combinations with photochemically induced, transition metal-catalyzed and enzymatically induced processes, all other possible constellations have been embedded in the concept of domino synthesis. 

Relatively few examples of photochemically induced 6n -electrocyclizations of azahexa-l,3,5-trienes to azacyclohexa-1,3-dienes have been documented the reverse process involving ring cleavage, however, is more common. The formation of azalumazin-7-ones 19 from triazahexatrienes 20 under oxidative conditions can be regarded as an example of such a process,18 but... 

Diaryl thioketones are converted by diiron enneacarbonyl into products of orthometallation.141 Oxidative or photochemically induced deligation of these complexes provides an unusual and valuable synthetic entry into compounds in the uncommon isobenzothiophene category142,151 (Scheme 84). Moreover, the photochemical procedure provides the novel complex, (tetracyanoethylene)tetracarbonyl iron. The orthometallated complexes (68) can also be used to prepare isobenzofurans (see Section IV,B,5). 

Photochemically induced oxidation of benzaldehyde was proved to be the chain reaction (4> >> 1) H. Backstrom [51]... 

Autoxidation of secondary acetonitriles under phase-transfer catalytic conditions [2] avoids the use of hazardous and/or expensive materials required for the classical conversion of the nitriles into ketones. In the course of C-alkylation of secondary acetonitriles (see Chapter 6), it had been noted that oxidative cleavage of the nitrile group frequently occurred (Scheme 10.7) [3]. In both cases, oxidation of the anionic intermediate presumably proceeds via the peroxy derivative with the extrusion of the cyanate ion [2], Advantage of the direct oxidation reaction has been made in the synthesis of aryl ketones [3], particularly of benzoylheteroarenes. The cyanomethylheteroarenes, obtained by a photochemically induced reaction of halo-heteroarenes with phenylacetonitrile, are oxidized by air under the basic conditions. Oxidative coupling of bromoacetonitriles under basic catalytic conditions has been also observed (see Chapter 6). 

In addition to the dark oxidation of S(IV) on surfaces, there may be photochemically induced processes as well. For example, irradiation of aqueous suspensions of solid a-Fe203 (hematite) containing S(IV) with light of A > 295 nm resulted in the production of Fe(II) in solution (Faust and Hoffmann, 1986 Faust et al., 1989 Hoffmann et al., 1995). This reductive dissolution of the hematite has been attributed to the absorption of light by surface Fe(III)-S(IV) complexes, which leads to the generation of electron-hole pairs, followed by an electron transfer in which the adsorbed S(IV) is oxidized to the SO-p radical anion. This initiates the free radical chemistry described earlier. 

Photochemically induced epoxidation of tetrafluoroethene by oxygen with improved yields (71-76%, conversion 21-62%) is achieved in the presence of radical generators such as tri-bromofluoromethane, 1,2-dibromotctrafluorocthane, ethyl nitrite or 1 H, /F,5//-octafluoropen-tyl nitrite.36 The oxidation of tetrafluoroethene with oxygen can also be catalyzed with bis(trifluoromethyl)diazene an undistillable viscous oil with peroxide composition is formed initially which can be quantitatively converted into carbonyl fluoride when heated.37-38... 

Dibenzo-fused dithiepins undergo photochemically induced ring contractions. Thus, irradiation of 2-phenyldibenzo-[df [l,3]dithiepin 1-oxide 72 <1999T5027> and 9-phenyl-4,8,9-trithiadibenzo[i2/,9]azulene 8-oxide 70 <1996CL655> using a high-pressure mercury lamp (400 W) gave disulfides 99 and 100 (Scheme 22). 

Temperature will affect the degradation rate of different organic pollutants. Weir et al. (1987) reported that benzene and hydrogen peroxide are insensitive to temperature because photochemically induced reactions often have low activation energies. Koubek (1975) stated that temperature has little effect on the oxidation of refractory organics however, Sundstrom et al. (1986) observed that the decomposition rates of some halogenated aliphatics increased with temperature. 

Iron carbonyls have been used in stoichiometric and catalytic amounts for a variety of transformations in organic synthesis. For example, the isomerization of 1,4-dienes to 1,3-dienes by formation of tricarbonyl(ri4-l,3-diene)iron complexes and subsequent oxidative demetallation has been applied to the synthesis of 12-prostaglandin PGC2 [10], The photochemically induced double bond isomerization of allyl alcohols to aldehydes [11] and allylamines to enamines [12,13] can be carried out with catalytic amounts of iron carbonyls (see Section 1.4.3). 

Oxidative addition, such as that exemplified by addition of H-S1R3 to photo-generated Fe(C0)4, reaction (17), is a very important, large thermal reaction class. Generally, it can be said that oxidative addition to low valent, coordinatively unsaturated metal complexes is common. Recently, Geoffroy, Hammond, and Gray188 have established that reductive elimination can be photochemically induced. A typical example is shown in reaction (67). Other small molecules can be reductively elimi-... 

Aggregation of metallotriazole hemiporphyrazines possessing crown ethers was induced by alkali cations [95CC419] and the first examples of 2,2 -tethered bifluorenylidene crown ethers were synthesized using an oxidative photochemical cyclization process [95JOC7380]. 

Iron bearing clays and oxide minerals, because of their partially occupied Fe(3d) orbitals, may participate in a number of redox processes in nature. For example, photochemically induced... 

Ghosh and co-workers have shown in several studies that Brij 35 and Pol 10 in water provide suitable environments for the photochemically induced reductive dechlorination of chlorobiphenyls [75,77,82,83]. Ghosh and Sayler have demonstrated that photochemistry and microbes work symbiotically to degrade PCB mixtures [76,80]. Photochemistry in a surfactant/water solution converts highly chlorinated biphenyls into much less chlorinated ones. The microbes, which do not oxidize the highly chlorinated biphenyls, then oxidize the less chlorinated biphenyls. For a related study on the combined degradation of a PCB mixture and microbes see [88]. 

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