Photochemical reductive coupling

The photochemical reduction of particulate and of dissolved iron(III) is coupled to the oxidation of a reductant, i.e., of dissolved organic carbon. 

In the case of two flavoenzyme oxidase systems (glucose oxidase (18) and thiamine oxidase s where both oxidation-reduction potential and semiquinone quantitation values are available, semiquinone formation is viewed to be kinetically rather than thermodynamically stabilized. The respective one-electron redox couples (PFl/PFl- and PFI7PFIH2) are similar in value (from essential equality to a 50 mV differential) which would predict only very low levels of semiquinone (32% when both couples are identical) at equilibrium. However, near quantitative yields (90%) of semiquinone are observed either by photochemical reduction or by titration with dithionite which demonstrates a kinetic barrier for the reduction of the semiquinone to the hydroquinone form. The addition of a low potential one-electron oxidoreductant such as methyl viologen generally acts to circumvent this kinetic barrier and facilitate the rapid reduction of the semiquinone to the hydroquinone form. 

Cobalt(III) cage complexes can also perform as electron transfer agents in the photoreduction of water.180181 Because of the kinetic inertness of the encapsulated cobalt(II) ion, the cobalt(II)/co-balt(III) redox couple can be repeatedly cycled without decomposition. Thus these complexes are potentially, useful electron transfer agents, e.g, in the photochemical reduction of water, in energy transfer and as relays in photosensitized electron transfer reactions.180,181 The problem of the short excited-state lifetimes of these complexes can be circumvented by the formation of Co(sep)3+ ion pairs, so that the complexes can be used as photosensitizers for cyclic redox processes.182 183... 

A large number of other metal complexes have received long and detailed attention, but activity in recent years has revealed few new principles appropriate for discussion here and some systems have been treated in detail elsewhere.2 Included among these are oxalato complex photochemistry where oxidation of the oxalato ligands is coupled to the central metal reduction Ag(I) photochemistry related to imaging systems uranyl ion photochemical reactions coupled to organic oxidations and aquo ion photoredox reactions. Two specific topics have recently emerged as... 

If H2O2 is added, the photochemical reduction of the Fe(III)-complex will be coupled to a Fenton reaction (Eq. 37) [56,80]. Thus, the use of illuminated mixtures of H2O2 and FeOx is very efficient for the photodegradation of organic contaminants the energy required to treat the same volume of a selected wastewater is ca. 20% of the energy required by the common photo-Fenton system [56,81,82],... 

Photogalvanic cell An electrochemical ceU in which current or voltage changes result from photochemically generated changes in the relative concentrations of reactants in a solution phase oxidation-reduction couple. Compare photovoltaic cell. 

In the pinacol coupling, two ketones are reductively coupled to give a 1,2-diol. (Compare the photochemical pinacol coupling discussed in Section 5.3.1.) The two ketones are usually identical, but intramolecular dimerizations can give unsymmetrical 1,2-diols. The reaction proceeds by electron transfer to the ketone to give a ketyl radical anion. This compound dimerizes to give the 1,2-diol. 

Reduction and reductive coupling. The use of polymer-supported PhjP for debromination of a-bromoketones has been advocated. Intramolecular reductive coupling of bis(acrylic esters) to form carbo- and oxacycles is effected photochemically in the presence of Ph,P and 9,10-dicyanoanthracene. ... 

When oxides in soils or sediments dissolve, substances adsorbed to the oxide surfaces will also be released into solution. Thus, for example, phosphate release into sediment pore waters accompanies the reductive dissolution of iron oxides in anoxic sediments. Release of phosphate into the overlying (oxic) water column is limited by phosphate adsorption on freshly precipitated amorphous iron oxides at the oxic-anoxic interface (9, 10). A similarly coupled cycle of phosphate and iron is observed in surface waters where photochemical reductive dissolution of iron oxides results in increased dissolved concentrations of ferrous iron and phosphate during the day (11). 

In photochemical reduction of CO2 by metal complexes, [Ru(bpy)3] is widely used as a photosensitizer. The luminescent state of [Ru(bpy)3] is reductively quenched by various sacrificial electron donors to produce [Ru(bpy)3] . Metal complexes used as catalyst in the photochemical reduction of CO2 using [Ru(bpy)3] are prerequisites which are reduced at potentials more positive than that of the [Ru(bpy)3] " redox couple (-1.33 V vs SCE) (72). Irradiation with visible light of an aqueous solution containing [Co (Me4(14)-4,ll-dieneNJ], [Ru(bpy)3], and ascorbic acid at pH 4.0 produces CO and H2 with a mole ratio of 0.27 1 (73). Similarly, photochemical reduction of CO2 is catalyzed by the [Ru(bpy)3] /[Ni(cyclam)] system at pH 5.0 and also gives H2 and CO. However, the quantum efficiency of the latter is quite low (0.06% at X = 400 nm), and the catalytic activity for the CO2 reduction decreases to 25% after 4 h irradiation (64, 74, 75). This contrasts with the high activity for the electrochemical reduction of CO2 by [Ni(cyclam)] adsorbed on Hg. 

The pinacol reaction is an example of radical dimerization (Scheme 4.29). Stabihzed free radicals have sufficiently long hfetimes to permit coupling outside solvent cage confinement Scheme 4.30 shows two such coupling reactions. The first is the photochemical reduction of benzophenone to benzopinacol (Scheme 4.30a). The second is an example of the oxidative coupling of phenols, a transformation that is an important step in the biosynthesis of alkaloids (Scheme 4.30b). 

Vishniac, W. and Ochoa, S. (1953) Fixation of carbon dioxide coupled to photochemical reduction of pyridine nucleotides by chloroplast preparations. J. Biol. Chem. 195, 75-93. 

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