Photoreduction

Photoreduction. Only very strong reducing agents are able to perform the one-electron reduction of C02 to C02-, which is difficult to produce by photochemical means. Two-electron reduction, that is, the formation of formic acid (formate), in turn, is energetically highly favorable. 

Photocatalytic reduction of C02 can be accomplished by suspending photosensitive semiconductor powders in aqueous solutions under irradiation, usually using UV light.129156 Photoreduction of C02, however, is in competition with H2 formation due to water decomposition, and leads to mixtures of reduced carbon products. Selectivity, therefore, is one of major problems of these processes. 

Since the concentration of C02 in water is rather low, increasing pressure may enhance the performance of photoreduction. Studies using Ti02 in water showed that hydrocarbons such as methane and ethylene, which were not produced at ambient pressure, were obtained under high pressure.161 166 Methane was formed as the main reduction product when the reduction was performed in isopropyl alcohol, a positive hole scavenger.167 

As mentioned, the photocatalytic activation of Ti02 requires UV irradiation, and hence the semiconductor performance in the solar spectrum is inefficient. A solution to switch the photocatalytic activity to the visible spectral region was described by covalent attachment of an eosin dye monolayer to the semiconductor oxide particles of a Pd-Ti02 catalyst.168 The improved photocatalytic activities, specifically, the efficient formation of formate, are attributed to the effective injection of electrons from the excited dye into the semiconductor conduction band. 

TiOz coated with potassium ferrocyanide proved to be an effective catalyst for the reduction of C02 to formic acid and formaldehyde.169 A very stable and reproducible catalytic system was prepared by immobilizing Ni2+ and Ru2+ complexes into Nation membrane, which was used for the selective reduction of C02 to formic acid.170 Formic acid was again formed when Zn and Co phthalocyanines were adsorbed onto a Nation membrane on irradiation with visible light in acidic aqueous solution containing triethanolamine as a hole scavenger. Cobalt comns (B i2) acting as homogeneous catalysts in acetonitrile-methanol solutions induced the formation of formic acid and CO.172 

The photoreduction of benzophenone in 2-propanol was one of the earliest reported photochemical reactions331 and perhaps the one most familiar to students of organic chemistry.332 Study of this reaction fostered the rapidly mushrooming interest in photochemical mechanisms which has marked the past decade. 

The exact values for the rate constants for hydrogen abstraction by triplet benzophenone are not yet entirely certain. Three groups338-338 have reported a value of 108M-1 sec-1 for abstraction from 2-propanol in concentrated 2-propanol, while the combination of the data of three other groups333,338 339 for dilute benzene solutions yields a value of only 105M-1 sec-1. This discrepancy could well reflect a solvent effect such as that found in studies of the reactivity of alkoxy radicals.340 However, the hundredfold difference between the reported rates for attack of triplet benzophenone on toluene338,338 undoubtedly reflects experimental problems, because both values were measured in aromatic solvents. 

Aliphatic ketones also undergo photoreduction, as evidenced by the facile photoreduction of acetone in cyclohexane341 and in hexane.342 Aromatic ketones are more commonly associated with the reaction because they do not undergo type I cleavages as readily as aliphatic ketones. As already mentioned, photolysis of diethyl ketone 

Photochemical cycloaddition reactions have been discussed in detail by Arnold in another chapter in this volume.343 

Formation of cyclobutanols is closely related to photoelimination.341 For 6-hepten-2-one the total photoreaction in solution is347  

The next example uses a mechanism for photoreductions, in which radicals are formed as intermediates. A typical example is the reaction of benzo-phenone with isopropanol. 

In this specific case the symbols represent A, benzophenone B, isopropanol C, benzpinacol D, acetone E, ketylradical F, alkoxylradi-cal. 

If the Bodenstein hypothesis can be applied to the radical intermediates one obtains 

Since the unstable intermediates appear in negligible concentrations, the following relationship is valid 

Inserting the values listed in the last column of the above scheme into the variables x. the stationary concentrations of the intermediates are given by 

The photodegradation reactions involve not only the photooxidation, but also the photoreduction. For example, Chu W. and Javert C.T. (1994) reported a photoreduction reaction for aromatic compounds in the presence of hydrogen sources in which high reaction quantum yields were observed. Many dyes are also noted for their ease of deco lour ization in the absence of oxygen when a suitable electron donor or hydrogen source is present. For example, in the scheme IV, formation of colourless leuco-form of anthraquinone dye is observed after photo-reduction that the main structural integrity of the dye molecule retains (Rys P. and Zollinger H., 1972). 

Blaisdell B.E. (1949) investigated the photoreductive processes of azobenzene and 4-amino-4-nitroazobenzene in isopropanol and isooctane, he concluded that the main products were hydrazobenzene and aniline derivatives after photodisproportionation. 

A photoreductive scheme was proposed in which the triplet state of the ketone (3B) first abstracts a hydrogen atom from the solvent or from a hydrogen donor (YH2) to give a ketyl radical (BH). Hydrogen atom transfer from the ketyl radical to the azo linkage of the dye (D) leads to the formation of a hydrazl radical (DH), which is finally reduced to the colourless hydrazobenzene (DH2) as follow  

This scheme predicts a linear relationship between the reciprocal of the quantum yield for fading and the reciprocal of hydrogen donor concentration. 

Photochemical pinacolization reactions of non-enolizable ketones have been the subject of many investigators.248 Photolysis of benzophenone in 

The radical chain mechanism for the abnormal addition of thiols and hydrogen sulfide (contrary to MarkOwnikofF s rule) is of great practical 

Work on radical addition of thiols prior to 1940 has been summarized by Mayo and Walling.181 The reaction may be initiated photochemi-cally in the absence of oxygen or peroxides even at 0°C.208 light of wavelength 3000 A. is required, but, in the presence of photosensitizers such as acetone, the longer wavelengths transmitted by Pyrex are effective. The reaction steps may be written  

The radical Chain additions of sulfhydryl compounds to al-kenes123,136 163-206,208 293 284 and to cycloalkenes22,62 are currently receiving intensive study. The literature is voluminous, and this discussion is limited to a brief summary of the results which appear to have reasonably unequivocal interpretations. 

Hydrogen sulfide may be added to olefins in the same manner as thiol, but unless an appreciable excess is present the thiol product reacts further to yield a dialkyl sulfide.293 

Electron transfer to certain functional groups can be induced by irradiation with light. When combined with a sensitizer and a hydrogen atom donor, reductions are possible. The basics of photochemical techniques and the use of sensitizers are discussed in Sections ll.lO.B and 13.4. 

In the presence of a hydrogen atom donor (usually an alcohol or an amine), carbonyl derivatives and halides can be photochemically reduced. Photolysis of ketone 585 gave the henzocyclobutane 586 1 hut photolysis in the presence of ec-butylamine gave alcohol 587 as the product. Photoreduction is often less 

Reduction of halides is a useful application of this process and there are variations that make it more useful. Mariano and co-workers showed that the bromide in 591 is removed to give a radical, and this cyclizes (via 592) in the presence of the enaminone moiety to give a mixture of several products on irradiation with Pyrex filtered light.6i3 When a Vycor filter (secs. II.IO.B and sec. 13.4) was used to limit the incident light to 220 nm [130 kcal mol 1, 544.2 kJ mol l] (required for the enaminone moiety), radical cyclization was the major process (sec. 13.7 for a discussion of radical cyclization), giving 593 in 85% yield. 

Ultimately, copper nanoparticles are formed from the reaction of Cu with Cu [Cu(OR) and Cu(OR) are complex intermediates]. 

Despite pure Cu production, photoreduction is not free from either the formation of impurities (i.e. Cu and CU2O composites) or the agglomeration of Cu-NPs. Polymers or surfactants are required to act as protective agents. It is hkely that this radiation-induced synthesis wiU attract further attention in the future. 

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The effects of uv radiation on V/-nitroso compounds depend on the pH and the medium. Under neutral conditions and ia the absence of radical scavengers, these compounds often appear chemically stable, although the E—Z equiUbrium, with respect to rotation around the N—N bond, can be affected (70). This apparent stabiUty is due to rapid recombination of aminyl radicals and nitric oxide [10102-43-9] formed duting photolysis. In the presence of radical scavengers nitrosamines decay rapidly (71). At lower pH, a variety of photoproducts are formed, including compounds attributed to photoelimination, photoreduction, and photo-oxidation (69). Low concentrations of most nitrosamines, even at neutral pH, can be eliminated by prolonged kradiation at 366 nm. This technique is used ki the identification of /V-nitrosamines that are present ki low concentrations ki complex mixtures (72). 

Surface vs Solution Reactions, Anotliei issue of debate in pliotocatalyzed mineialization of oiganic substrates is whether the initial oxidation occurs on the photocatalyst s surface or in solution. Kinetic data of photooxidations and photoreductions have often been fitted to the simple... 

Titanium(III) alkoxides can be produced by photoreduction of the tetraalkyl titanates in the presence of a base, such as pyridine (180), and by reduction of tetraalkyl titanates by organosiUcon compounds containing Si—H groups (181). 

The tris compounds are highly bridged three-dimensional polymers. Photoreduction of aqueous Ti(IV)-containing alcohols or glycols, but not of ethylene glycol, yields Ti(III) and the aldehyde or ketone corresponding to the alcohol (191,192). A possible mechanism is... 

TTie photoreduction can be quenched by known triplet quenchers. The effecti e quenchers are those which have T] states less than 69kcal/moI above S,. Quenchers with higher triplet energies are ineffective because the benzophenone n-n triplet is then not sufficiently energetic to effect energy transfer. 

The efficiency of reduction of benzophenone derivatives is greatly diminished when an ortho alkyl substituent is present because a new photoreaction, intramolecular hydrogen-atom abstraction, then becomes the dominant process. The abstraction takes place from the benzylic position on the adjacent alkyl chain, giving an unstable enol that can revert to the original benzophenone without photoreduction. This process is known as photoenolization Photoenolization can be detected, even though no net transformation of the reactant occurs, by photolysis in deuterated hydroxylic solvents. The proton of the enolic hydroxyl is rapidly exchanged with solvent, so deuterium is introduced at the benzylic position. Deuterium is also introduced if the enol is protonated at the benzylic carbon by solvent ... 

Reductive dunenzation to form fluorinated benzopinacols proceeds m the partly fluormated case either with zinc or by photolysis but is not observed with perfluorobenzophenone [651 (equation 53). Trifluoroacetophenone is reduced electrochemically in dimethylformamide to a stable radical anion, which, m the presence ot lithium ion, rapidly dunerizes to pinacol in higher yield than that available by photoreduction [66] (equation 54)... 

Another tetrahydroporphyrin derivative, porphodimethene (7), which consists of two dipyr-rylmethene halves is obtained in the photoreduction of magnesium-containing chlorophyll a (6) with hydrogen sulfide3211 42 as reductant and pyridine as base. 

In a study of the photoreduction of zinc(II) or magnesium chlorophyll derivatives 8 with ascorbic acid in the presence of l,4-diazabicyclo[2.2.2]octane (DABC-O), a 2,3-m-hydrogenated isobacteriochlorin 9 is formed which subsequently rearranges to a 3-ethylidenc derivative... 

Ultraviolet spectrophotommetry was used to follow the course of the photoreduction of thiopyrylium salts to their corresponding 4H-thiopyranyl radicals (85BCJ2600) as well as the kinetics of thiopyrylium salt transformations to thiopyrans with various nucleophiles (84JA7082, 84JOC1806 86JA3409). 

Although Ru(bipy)2+ alone will not split water into hydrogen and oxygen, it has been accomplished with Ru(bipy)2+ using various catalysts or radical carriers. Perhaps the most studied system for the photoreduction of water involves using methyl viologen as the quencher, EDTA as an electron donor (decomposed in the reaction) and colloidal platinum as a redox catalyst (Figure 1.19). 

Copper complexes, 5,533-750 acetylacetone hydrolysis, 2,379 photoreduction, 2.384 amidines... 

Iron, tris(hexafluoroacetylacetone)-structure, 1,65 Iron, tris(oxalato)-chemical actinometer, 1,409 photoreduction, 1,471 relief-image-forming systems, 6,125 Iron, tris(l,10-phenanthroline)-absorptiometry, 1,549 racemization, 1,466 solid state, 1,467 structure, 1, 64 lron(III) chloride amino acid formation prebiotic systems, 6,871 Iron complexes acetonitrile. 4,1210 acetylacetone, 2,371 amidines... 

Plutonium spectra showing the photoreduction of plu-tonyl. (I) 0.385 11 Pu022+, some Pu1 "1" and 1.5 M ethanol in 1.32 11 HClOi,. (II) Immediately after the first UV irradiation. (Ill) Twenty-four hours after the first UV irradiation. (IV) Immediately after the second and 25 h after the first UV irradiations (3 ). 

Thus irradiation of benzophenone in toluene gives the photoreduction product 19, benzpinacol, and bibenzyl [equation (59)] of which only 19... 

Figure 2. Photoreduction of 2,3,7,8-tetrachlorodi-benzo- -dioxin (I) (2 mg/liter in methanol) as compared with that of the 2,3,7-trichloro-homolog (II) (20), 1971 by AAAS...

The discussion above refers to the classical dark conditions where the chemical activation is achieved thermally. Fenton requires a moderate thermal activation, resulting in a reaction temperature ranging from 25 to 90 °C. The oxidizing capacity of the Fenton reaction can be increased by UV or UV-vis Hght irradiation [160, 161]. The increase is interpreted by means of the photoreduction ability of Fe ... 

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