Reduction photoactivated

Sulfonamides are very difficult to hydrolyze. However, a photoactivated reductive method for desulfonylation has been developed.240 Sodium borohydride is used in conjunction with 1,2- or 1,4-dimethoxybenzene or 1,5-dimethoxynaphthalene. The photoexcited aromatic serves as an electron donor toward the sulfonyl group, which then fragments to give the deprotected amine. The NaBH4 reduces the radical cation and the sulfonyl radical. 

Argyria is a characteristic smoke-gray discoloration of the skin and a bluish discoloration of the mucous membranes reminiscent of cyanosis (12,13). The discoloration is permanent. The deposited pigment may be partly silver sulfide and partly metallic silver, formed by photoactivated reduction. 

Rh(I) complexes. Equations 7.35,74 7.36,75 and 7.3776 show these results. All the examples first involve production of a reactive, electron-rich, 16-electron complex by photoactivated reductive elimination of H2 or loss of a neutral ligand. This is a rather general reaction for a variety of alkanes and arenes that react not only with Ir and Rh complexes, but also with low-valent Ru, Pt, and Os complexes. 

The photochemistry and optical properties of three diarylethenes derived from benzoth-iophene have been reported. A photoactivated reduction of aromatic disulfides in the... 

Yeom and Frei [96] showed that irradiation at 266 nm of TS-1 loaded with CO and CH3OH gas at 173 K gave methyl formate as the main product. The photoreaction was monitored in situ by FT-IR spectroscopy and was attributed to reduction of CO at LMCT-excited framework Ti centers (see Sect. 3.2) under concurrent oxidation of methanol. Infrared product analysis based on experiments with isotopically labeled molecules revealed that carbon monoxide is incorporated into the ester as a carbonyl moiety. The authors proposed that CO is photoreduced by transient Ti + to HCO radical in the primary redox step. This finding opens up the possibility for synthetic chemistry of carbon monoxide in transition metal materials by photoactivation of framework metal centers. 

Fig. 3. Photoactivation of Pt(IV) complexes as a prodrug strategy for metallochemotherapeutics (a) general scheme of prodrug activation by photoreduction (b) photosubstitution and photoisomerization are competing photoreaction pathways, which can result in different reactive species upon reduction (c) an example of a photoactive platinum(IV) diazido complex developed in our lab.

These products are the most important components of the living matter of plants, e.g. cellulose. In these simple terms photosynthesis is the photochemical oxidation of water, and reduction of carbon dioxide, by means of a photoactivated catalyst which in green plants is the molecule chlorophyll. 

Control of the catalyzed reaction, the reductive transformation of a vicinal dibromide to an alkene, was achieved by formation of the doubly reduced, neutral viologen which migrated to the organic phase and functioned as a redox catalyst. After reaction, its dicationic oxidized form migrated rapidly to the aqueous phase where cyclic photoactivation could be attained. 

Less attention has been paid, however, to C02 organometallic chemistry during the past decade. Whilst many reduction or coupling reactions are known to proceed in the presence of stoichiometric or catalytic amounts of transition metal complexes, very few examples remain where the formation of a metal-C02 complex has led to an effective, catalytic reduction reaction of C02. Carbon dioxide complex photoactivation also represents an attractive route to CO bond cleavage, coupled with O-atom transfer. However, progress in the area of C02 utilization requires a better understanding of the reaction mechanisms, of the thermodynamics of reaction intermediates, and of structure-reactivity relationships. 

We have shown how the band structure of photoexcited semiconductor particles makes them effective oxidation catalysts. Because of the heterogeneous nature of the photoactivation, selective chemistry can ensue from preferential adsorption, from directed reactivity between adsorbed reactive intermediates, and from the restriction of ECE processes to one electron routes. The extension of these experiments to catalyze chemical reductions and to address heterogeneous redox reactions of biologically important molecules should be straightforward. In fact, the use of surface-modified powders coated with chiral polymers has recently been reputed to cause asymmetric induction at prochiral redox centers. As more semiconductor powders become routinely available, the importance of these photocatalysts to organic chemistry is bound to increase. 

Another example concerning the reduction of carbonyl compounds also relates to the salt effect theme. Shaefer and Peters (1980), Simon Peters (1981,1982,1983,1984), Rudzki et al. (1985), and Goodman and Peters (1986) described photoreductions of aromatic ketones by amines. In this case, the addition of excess NaC104 results in considerable retardation, even prevention, of final product formation. The two fundamental steps in this photoreduction consist of rapid electron transfer from the amine to the photoactivated ketone (in its triplet state), followed by the slow transfer of proton from the amine cation radical to the carbonyl anion radical ... 

Ferredoxin is believed to function as the first electron acceptor of photoactivated chlorophyll Tagawa and Arnon 99) San Pietro 85)). In net electron flow, the electrons for reducing ferredoxin originate from water, pass to chlorophyll and then to ferredoxin. The reduction of ferredoxin is coupled to the evolution of oxygen Arnon, Tsujimoto, and McSwain (8)). It is emphasized that oxygen evolution end ferredoxin reduction are closely associated and, in the presence of ADP and Pi, they... 

It is possible that the hydroquinone-reduced species with its trapped Mn(II) corresponds to an intermediate in the photoactivation process. There appears to be one oxidized Mn dimer in the hydroquinone-reduced sample, as indicated by the 2.7-A feature in the EXAFS. This feature could correspond to the Mn that is photooxidized in the photoactivation process. Oxidation and photoligation of the first two Mn are the rate-limiting steps in photoactivation. Thus, as long as hydro-quinone reduction does not affect these two critical Mn, the effects of hydroquinone should be easily and rapidly reversed, as is observed. In... 

The role of methylation on substrate function remains incompletely understood due to the large number of substrates and the wide range of functions carried out by these substrates. However, many genetic and biochemical studies have identified important roles for Icmt in the function of specific substrates. Early studies utilized a pig liver esterase to demethylate the py subunit of transducin, the retinal heterotrimeric G protein (T) [50]. These studies demonstrated a two-fold reduction in the level of GTPyS exchange by the unmethylated Tpy-OH subunit in the presence of T and photoactivated membrane-bound rhodopsin compared to the methylated Tpy-OCHs form [51,52], and a 10-fold reduction in the ability to activate... 

Light absorption modifies the driving force for electron transfer processes in all kinds of materials. As photoactivated species are always better oxidants and reductants than their ground state equivalents, an enhanced redox reactivity is usually observed in the excited state. Photoreactions are therefore ideally suited to trigger, study, and mimic bioinorganic electron transfer. 

However, these reaction conditions (refluxing methanol or photoactivation at room temperature) are useful only for the reduction of highly activated double bonds. Nevertheless, it was found that the reaction is promoted by silica gel, broadening the scope of reducible enone substrates. The method is highly chemoselective as no alcoholic products are observed, and saturated carbonyls, nitro, cyano, sulf-inyl and sulfonyl groups remain intact under the reaction conditions (Scheme 77). 

Even if pulse radiolysis has been used [13], most studies of the initial electron entry into cytochrome oxidase are based on the finding that at low ionic strength cytochrome c forms a strong electrostatic complex with the oxidase [14]. Cytochrome c itself cannot be photoactivated, but in the complex with the enzyme it can be reduced by photochemically generated radicals, such as that of 5-diazariboflavin [15]. Another approach has been to employ a photoinduced uroporphyrin/NADH reduction system [16]. 

For practical photoinduced synthetic biocatalyzed transformations, it is important to integrate biocatalysts in immobilized matrices that allow the recycling of the photosystems. The fact that bipyridinium sites act as electron mediators for various redox enzymes was used to develop two paradigms for the electrical contacting and photoactivation of the biocatalyst (Figure 39). By one approach, the bipyridinium electron relays are tethered by covalent bonds to the protein backbone (Figure 39A). These electron relays act as oxidative quenchers of the excited state of the dye and, upon photoreduction of the electron acceptor units, they act as electron carriers that activate the reductive functions of the enzyme. As an example, the... 

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