Photoreduction Reactions

Dugger, W. M., and I. P. Ting. The effect of peroxyacetyl nitrate on plants Photoreductive reactions and susceptibility of bean plants to PAN. Phytopathology 58 1102-1107, 1968. 

The irradiation of benzaldehyde in cyclohexene solution gives rise to products of both the photocycloaddition and photoreduction reactions. Naphthalene was added to this reaction mixture in an effort to change the ratio of products, since, if the two reactions result from different excited states, that reaction involving the triplet would be quenched. The rate of product formation was slowed while the ratio of products remained unchanged therefore the photocycloaddition and photoreduction reactions involve the triplet state.37... 

Thus, the observations that (a) dienes quench the photoreduction reaction (b) the isomerization and dimerization of dienes is sensitized by the it,n carbonyl triplet and (c) there is a lack of photocycloaddition products with dienes, taken in conjunction with the relative energy levels of carbonyl compounds and dienes, form a consistent picture. 

An example of how it is possible to influence the excited state attained is presented by the aminobenzophenones and their behavior in the photoreduction reaction. For example, 4-aminobenzophenone is reduced very inefficiently upon irradiation in isopropanol. The excited state attained in this case is believed to have a major contribution of a charge transfer 57 character.68 If hydrogen chloride is added to the... 

The prototype of photoreduction reactions is hydrogen abstraction by carbonyl compounds in presence of suitable H-donors. Such H-atom transfer may be visualized to occur first by transfer of an electron followed by proton transfer. An electron deficient centre is the seat of reaction and the efficiency of the reaction depends on the nucleophilic nature of the donor. 

Hoffman27 recently observed that in the photoreduction reactions of azido-pentaminecobalt(II) cations the predominant first step is photoaquation with loss of ammonia, rather than loss of azide ion. 

Electrochemical techniques are the most widely used methods to obtain nickel(III) complexes. Generally the oxidation of the nickel(II) complexes is performed in acetonitrile solutions under an inert atmosphere using a platinum electrode.3052 A tetraalkylammonium salt, usually the perchlorate, is employed as supporting electrolyte (ca. 0.1 M). The complete procedure is often carried out in the dark at ca. 5°C to prevent possible photoreduction reactions.3053-3055... 

In the presence of electron donors such as amines the photoreduction reaction can take place by direct electron transfer rather than by hydrogen abstraction. 

Lindquist showed that both reversible (quenching) photoreduction reactions and irreversible reactions, leading to free radicals, could occur. For example, the interaction of fluorescein triplet with -phenylenediamine (PPDA) was shown to result in nearly 100% quenching via electron transfer to afford PPDA cation radical and semireduced fluorescein with a rate... 

However, all the available evidence suggests that these photoreduction reactions do not lead to oxygen formation310"312 (although small amounts of H202 may be produced),309,313 but rather hydroxyl radicals, which subsequently attack the somewhat sensitive skeletons of the porphyrin or phthalocyanine rings.311,312... 

The reactivity of the triplet state in photoreduction reactions was used by Nakamaru et al. (1969) to investigate the triplet state basicity of acridine. It should be possible to extend this method to any compounds in which an excited state reaction is affected by protonation. 

A similar model has been advanced by Brown and Rajapakse [186], again based on the work done by Albery et al. [84], They also suggest that electron-hole recombination can be effectively slowed by the presence of interband hole traps, some of which can be attributed to sulphur vacancies, in agreement with conclusions made by Ramsden and Gratzel [169]. Work must be done to refine this model and to identify the nature of the interband hole traps. Nonetheless, the pseudo-first order rate constant for electron loss to them is 0.168 0.052 s 1 a value which has far-reaching implications for the design of photocatalytic systems employing colloidal CdS in photoreduction reactions. 

In 1981, the author s group found large MFEs and MIEs on the radical pair lifetime (Trp) and the escape radical yield (Fe) under magnetic fields of 0 - 70 mT with an ns-laser photolysis at room temperature for the photoreduction reactions of benzophenone (BP), benzophenone-sodium dodecyl sulfate (SDS) solutions [1]. Here, micellar molecules act as hydrogen donors (RH). The scheme of such photoreduction reactions of the benzophenone isotopes (XCO) can be represented by the following reaction scheme ... 

Table 7-1. Magnetic field and magnetic isotope effects on the radical pair lifetime (Xrp) observed for the photoreduction reactions of benzophenone isotopes in SDS micellar solutions at room temperature under magnetic fields less than 70 mT [1] and under the fields of 0.1 - 1.34 T. (Reproduced from Ref. [2b] by permission from The Japanese Chemical Society)...

A(t) curves observed with a ns-laser photolysis apparatus and a superconducting magnet at 320 nm in the photoreduction reactions of (a) BP and (b) DFBP in Brij 35 micellar solutions at 293 K. (Reproduced from Ref. [15])... 

The author s group studied MFEs on the photoreduction reactions of carbonyl and quinone molecules (XCO) in micellar solutions. The reaction scheme was shown in Chapter 7 as follows ... 

With pulse laser and microwave, Okazaki et al. [17-19] further demonstrated that some kinetic information on radical pairs could be obtained with the pulse-RYDESR method. They employed the three types of pulse sequences as shown in Fig. 14-6. Using this pulse-RYDESR method, they studied the photoreduction reactions of carbonyl and quinone... 

Photoreactions on titanium dioxide have been the focus of considerable interest for some time. Titania offers the opportunity to oxidize organic compounds in polluted environments, and has also been exploited to generate titania-supported nanoparticles of metals (e.g., silver) via photoreduction reactions [85]. While there is not enough room here to thoroughly treat photocatalytic processes, a brief introduction to the subject is presented below. Readers seeking detailed treatments of this subject are referred to a recent review by Yates etal. on titania-facilitated photocatalysis [86]. 

Owing to the great interest in photoreduction reactions, electron transfer processes involving [Co (cage)] + and polypyridine-ruthenium(II) complexes have been extensively investigated. The rate constants for the quenching process range from 2 X 10 to 1 X 10 s at 25 °C. The yields of electron transfer products... 

A similar result is obtained for the photoreduction reaction with participation of... 

Hydrogen abstraction (Section 4.9) by excited carbonyl compounds is one of the most fundamental reactions in organic photochemistry. In a classical photoreduction reaction, an excited carbonyl compound, such as a ketone, undergoes hydrogen abstraction from a hydrogen donor [H] to form the ketyl radicals, which subsequently abstract another... 

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). 

Obviously, low concentration of humics (Ha < 0.0333 pmol/L) can increase the photodegradation quantum yield (O) by about 16% from 1.068 x 10"4 (surfactant only without humic) to 1.238 x Hf1 (surfactant with 0.0333 pmol/L Ha). Likewise, in the presence of low concentration of Soil Extracted Materials (surfactant with 3.96 pmol/L SEM), the quantum yield of DR decay was significantly increased by 239% to 2.554 x 10"4. The quantum yield improvement is likely due to the increment of available hydrogen sources in the solution, which was known critical to the photoreduction reactions. 

The addition of radicals, generated by photoelectron transfer, to an unsaturated functional group has received the most attention. The fate of the intermediate radical does not depend strongly on the method used to produce it. An interesting example of a photoreductive reaction of co-unsaturated ketones in the presence of hexamethylphosphoric triamide (HMPA, neat) or triethylamine (Et3N) in acetonitrile has been reported [33]. The thus formed <5, e-unsaturated ketyl radicals exhibit the same behavior as m-unsaturated radicals [34] and they cyclize to produce cy-clopentanols and cyclohexanols. A typical example is provided in Scheme 18. 

Fluorenol (89) is reduced photochemically by triethylamine to give fluorene in high yield a small amount of 9-ethylidenefluorene is also formed. Other amines are effective, although the yields are generally lower, and 9-acetoxyfluorene also reacts to give fluorene in rather low yield. The photoreduction reaction is related formally to the photoreduction of dicyanopyrazine (84). An apparently straightforward replacement of hydrogen by nitro in the photolysis of butyl p-hydroxybenzoate (90) in aqueous sodium nitrite is shown to be more complex, and it seems possible that the initial attack is by HO radicals, followed by reaction with NO2. 

Platinum.—The cis-trans photo-isomerization and photoreduction reactions of m-[Pt(pn)2Cl2]2+ (pn = propylenediamine) have been studied.120 The isomerization proceeds more rapidly in acidic solution, suggesting that protonation of propylenediamine plays an important role. Isomerization of /ra j,-[Pt(pn)2Cl2]2+ could not be observed. 

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