Photoreduction pathways

A recent electron paramagnetic resonance study (13) has shown that I also degrades by a photoreduction pathway in the presence of alcohols or water, through the intermediacy of the corresponding ketyl radical. Since corresponding ketyl radicals and triplet states often have overlapping absorption spectra, it is important to emphasize that the transient observed in our optical flash photolysis experiments is the triplet state (not the ketyl radical), the key evidence being that both the yield and the lifetime of the transient are decreased by triplet quenchers. An important question... 

The mechanism of 3-haloaniline phototransformation was investigated in more detail in a nanosecond flash photolysis and product analysis study in methanolic solution [52], Photosolvolysis, as witnessed by the formation of anisidine, was again the major pathway, in particular for 3-fluoroaniline. A photoreductive pathway yielding aniline was also observed, being most important for 3-bromoaniline transient absorption indicated the formation of anilino radicals by singlet state dehalogenation as an intermediate on this pathway. Homolytic C - Br cleavage was additionally evidenced. The possibility to observe the triplet-triplet absorption spectra of 3-fluoro- and 3-chloroaniline pointed to an unreactive triplet state [52]. 

Photooxidations of Halocarbons. In addition to the photoreduction pathways, other indirect photochemical reactions are significant in the oxidation of halogenated organic compounds. These reactions were reviewed else-... 

Knor, G., Vogler. A., Roffia, S., Paolucci, F,. Balzani, V. (1996).. Switchable photoreduction pathways of antimony(V)tetraphenylporphyrin. A potential mullielectron transfer photosen.siti/.er, y. Chem. Soc.. Chem. Commun., p. 1643. 

This reaction is the reverse of the initial ketyl radical formation by the benzophenone triplet and is therm Q4ynamically favorable. The experiments using optically active alcohols as source of hydrogen atoms show, however, that under normal conditions this reaction is unimportant. This is probably due to other, more efficient pathways for reaction of the ketyl radicals or perhaps to diffusion rates which separate the radicals before reverse transfer can occur. That this reaction can be important in some cases even without the presence of sulfur compounds was shown by studying the photoreduction of benzophenone in optically active ethers.<68) Although the reaction of benzophenone in methyl 2-octyl ether is only 0.17 times as fast as that in isopropanol, ethers can be used as sources of hydrogen atoms for photoreduction ... 

Since transfer of a second hydrogen atom from the ether radical is unreasonable, a pathway available to the acetone ketyl radical in the photoreduction in isopropanol is removed in this system and reverse transfer can occur ... 

Scheme 3. Flavins are capable to undergo both 1 e and 2 e input/output redoxreactions as indicated. Since cytochrome b is a typical 1 e redox reagent, Hemmerich and Schmidt86) suggest a radical mechanism of the sensory transduction (assuming that the cyt b photoreduction is photo-tropically relevant). The nature and fate of the flavin-photosubstrate XH remains obsure. For the case X = cyt b the scheme represents a reversed respiration electron pathway...

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.

A quick survey of the photochemistry of the different complexes described above shows that the mechanism of photoactivation and the subsequent nature of the observed photoproducts varies from complex to complex and from one geometric isomer to another. Photochemical pathways often involve a combination of photosubstitution, photoisomerization, and photoreduction steps. In general, photolysis is rather slow in water and many different products are obtained if the complex is irradiated alone. The presence of nucleophilic biomolecules, on the other hand, can have a major influence, as photoreduction is usually rapid and accompanied by simpler reaction pathways. NMR methods... 

Alternatively, arene displacement can also be photo- rather than thermally-induced. In this respect, we studied the photoactivation of the dinuclear ruthenium-arene complex [ RuCl (rj6-indane) 2(p-2,3-dpp)]2+ (2,3-dpp, 2,3-bis(2-pyridyl)pyrazine) (21). The thermal reactivity of this compound is limited to the stepwise double aquation (which shows biexponential kinetics), but irradiation of the sample results in photoinduced loss of the arene. This photoactivation pathway produces ruthenium species that are more active than their ruthenium-arene precursors (Fig. 18). At the same time, free indane fluoresces 40 times more strongly than bound indane, opening up possibilities to use the arene as a fluorescent marker for imaging purposes. The photoactivation pathway is different from those previously discussed for photoactivated Pt(IV) diazido complexes, as it involves photosubstitution rather than photoreduction. Importantly, the photoactivation mechanism is independent of oxygen (see Section II on photoactivatable platinum drugs) (83). 

Co(phen) and Co(DIP) have been reported to cleave DNA upon irradiation with UV light (k < 320 nm) [117,147]. As no mechanistic studies were performed, the different reactions leading to strand scissions are not known. Photoreduction of the Co(III) species could constitute the initial step of the reaction pathway. 

Here pn is 1,2-diaminopropane and bn is 2,3-diaminobutane. Decomposition of the amine cation radicals obtained by photooxidation of the ligands en, bn, and pn have been discussed by Moeller. The products of Co(en)33+ photolysis can be satisfactorily explained by postulating that carbon-carbon bondbreaking is the principal step in decomposition of the cation radical H2NCH2CH2NH2t.58 Presuming a similar mechanism to obtain in photoreduction of Co(pn)33+, there are then two possible reaction pathways leading to different products. 

As indicated previously, it is frequently assumed that weakly bound oxygen leads to the complete combustion of hydrocarbons. However, another possible pathway to complete combustion, may involve the activation of an oxide ion (59). Kazanskii 60) studied in detail the elementary steps of the reduction, photoreduction, and reoxidation of the surfaces of oxide catalysts. He reported that upon exposure to light some semiconductor oxides undergo a charge transfer from the O2- ion to the cation, as shown ... 

In non-polar solvents, photoreduction of chlorobenzene proceeds by simple homolysis352. Excimers are formed in concentrated solutions of chlorobenzene, but they are either completely unreactive or decompose with much lower efficiency than the monomer353. The preferred pathway for homolysis involves the n,n triplet state353,354. The mechanism can thus be described by equations 88-90. 

The photoreduction of 2- and 3-chloroanisole in alcoholic solvents has been studied and is considered to be best accounted for by invoking methoxyphenyl radicals which abstract hydrogen atoms from the solvent378.4-Chloroanisole probably reacts partly via a homolytic cleavage, but in view of the results of quenching and sensitization experiments, another pathway consists of electron transfer from the solvent (ROH) to excited aryl halide, followed by dissociation of the radical anion into chloride ion and aryl radical. 

This activity is particularly useful for degradation of strongly hazardous substances or recalcitrant pollutants that are difficult to remove in chemical or biochemical processes. In this respect any pathway leading to abatement of chromate(VI) pollution arouses interest. One such pathway seems to be created by cooperation between iron and chromium photocatalytic cycles, which were reported as effective in conversion of chromate(VI) into Crm species [20-23,97]. A synergistic photoreduction of CrVI and Cu11 mediated by Ti02 [98], or photocatalytic reduction of Crvl and oxidation of organic matter by environmental polyoxometallates as photocatalysts [99], may constitute alternative possibilities. 

There are attempts to use other iron nitrosyls, both monomeric complexes and clusters [85-90]. Photochemical liberation of NO follows three main reaction pathways (1) photooxidation-substitution, (2) photoreduction, and (3) ligand rearrangement or decomposition as a result of photoreaction. 

Seemann M, Tse Sum Bui B, Wolff M, Miginiac-Maslow M Rohmer M. isoprenoid biosynthesis in plant chloroplasts via the MEP pathway direct thylakoid/ferredoxin-dependent photoreduction of GcpE/IspG. FEBS Lett. 2006 580 1547-1552. 

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