Photochemically induced reductive

Photochemically induced reduction of alkyl perfluoro carboxylates in hexa-methylphosphoramide (FfMPA) can replace one, two, or (in the case of tn-fluoroacetate) three a-fluonne atoms with hydrogen [7, S] (equation 5) Chlorine-containing fluorocarboxylates preferentially lose a-chlorme and may even lose P-chlorinc in preference to a fluonne [9] (equation 6) Cyclic a-fluoroketones are also defluonnated photochemically [10] (equation 7)... 

A comparison of chemical and photochemically induced reduction of some 2(4),5-dihydro-... 

H. Kamogawa, T. Masui, and M. Nanasawa, Photochemically induced reduction of viologens, Chem. Lett, 1980, 1145-1148. 

Ghosh and co-workers have shown in several studies that Brij 35 and Pol 10 in water provide suitable environments for the photochemically induced reductive dechlorination of chlorobiphenyls [75,77,82,83]. Ghosh and Sayler have demonstrated that photochemistry and microbes work symbiotically to degrade PCB mixtures [76,80]. Photochemistry in a surfactant/water solution converts highly chlorinated biphenyls into much less chlorinated ones. The microbes, which do not oxidize the highly chlorinated biphenyls, then oxidize the less chlorinated biphenyls. For a related study on the combined degradation of a PCB mixture and microbes see [88]. 

Photochemical generation of the bent metallocene (39) has turned out to be very useful for the preparation of 3. Thus, (s-tra j-i7 -butadiene)ZrCp2 (3a) is obtained in good yield from the photochemically induced reductive elimination of biphenyl from diphenylzirconocene (10) at low temperatures in the presence of 1,3-butadiene. This is probably the most versatile and most widely applicable method to prepare (s-fr nj-i7 -diene)zirconocenes. A wide variety of examples of this class of complex [and of (s-cis-diene)zirconocenes 5, as well] has been prepared by this route using various substituted conjugated dienes (22, 23) (Scheme 2). 

The Effect of Light Photochemically Induced Reductive Elimination... 

Photochemically induced reductive elimination can also result from initial photochemical extrusion of a dative ligand. For example, irradiation of [lrH,Cl(CO)(PPhj)J leads to... 

Margulieux GW, Semproni SP, Chirik PJ. Photochemically induced reductive elimination as a route to a zirconocene complex with a strongly activated N2 ligand. Angew Chem Int Ed. 2014 53 9189-9192. 

The photochemically induced reductive eliminations of hydrogen from [IrClH2-(PPh3)a] and [IrHs(PPh3)3] appear to be concerted processes since in the presence of deuteriated analogues no HD is produced. The photoactive excited state is believed to involve population of an iridium-Ha antibonding orbital which explains the easy hydrogen loss. 

In studies of photochemically induced CT, the energy of the donor and acceptor orbitals are close or higher in energy than the bridge states, and occupation of the bridge is understandable. In contrast, our electrochemical measurements employ redox active intercalators with reduction potentials... 

Besides the numerous examples of anionic/anionic processes, anionic/pericydic domino reactions have become increasingly important and present the second largest group of anionically induced sequences. In contrast, there are only a few examples of anionic/radical, anionic/transition metal-mediated, as well as anionic/re-ductive or anionic/oxidative domino reactions. Anionic/photochemically induced and anionic/enzyme-mediated domino sequences have not been found in the literature during the past few decades. It should be noted that, as a consequence of our definition, anionic/cationic domino processes are not listed, as already stated for cationic/anionic domino processes. Thus, these reactions would require an oxidative and reductive step, respectively, which would be discussed under oxidative or reductive processes. 

To date, no examples have been found in the literature for the combination of photochemically induced transformations with reductive/oxidative or enzymatic processes. 

During the past few years, increasing numbers of reports have been published on the subject of domino reactions initiated by oxidation or reduction processes. This was in stark contrast to the period before our first comprehensive review of this topic was published in 1993 [1], when the use of this type of transformation was indeed rare. The benefits of employing oxidation or reduction processes in domino sequences are clear, as they offer easy access to reactive functionalities such as nucleophiles (e. g., alcohols and amines) or electrophiles (e. g., aldehydes or ketones), with their ability to participate in further reactions. For that reason, apart from combinations with photochemically induced, transition metal-catalyzed and enzymatically induced processes, all other possible constellations have been embedded in the concept of domino synthesis. 

Similar photo-induced reductive dissolution to that reported for lepidocrocite in the presence of citric acid has been observed for hematite (a-Fe203) in the presence of S(IV) oxyanions (42) (see Figure 3). As shown in the conceptual model of Faust and Hoffmann (42) in Figure 4, two major pathways may lead to the production of Fe(II)ag i) surface redox reactions, both photochemical and thermal (dark), involving Fe(III)-S(IV) surface complexes (reactions 3 and 4 in Figure 4), and ii) aqueous phase photochemical and thermal redox reactions (reactions 11 and 12 in Figure 4). However, the rate of hematite dissolution (reaction 5) limits the rate at which Fe(II)aq may be produced by aqueous phase pathways (reactions 11 and 12) by limiting the availability of Fe(III)aq for such reactions. The rate of total aqueous iron production (d[Fe(aq)]T/dt = d [Fe(III)aq] +... 

Borgarello E, Serpone N, Emo G, Harris R, Pelizzetti E, Minero C (1986) Light-induced reduction of rhodium (III) and palladium (II) on titanium dioxide dispersions and the selective photochemical separation and recovery of gold (III), platinum (IV) and rhodium (III) in chloride media. Inorg Chem 25 4499-4503... 

In addition to the dark oxidation of S(IV) on surfaces, there may be photochemically induced processes as well. For example, irradiation of aqueous suspensions of solid a-Fe203 (hematite) containing S(IV) with light of A > 295 nm resulted in the production of Fe(II) in solution (Faust and Hoffmann, 1986 Faust et al., 1989 Hoffmann et al., 1995). This reductive dissolution of the hematite has been attributed to the absorption of light by surface Fe(III)-S(IV) complexes, which leads to the generation of electron-hole pairs, followed by an electron transfer in which the adsorbed S(IV) is oxidized to the SO-p radical anion. This initiates the free radical chemistry described earlier. 

The presence of the hydroxyl groups makes photochemically-induced reactions possible, e.g., the decomposition of water into hydrogen and oxygen and the reduction of nitrogen to ammonia and hydrazine (see also Section 2.1.5) [2.8],... 

Oxidative addition, such as that exemplified by addition of H-S1R3 to photo-generated Fe(C0)4, reaction (17), is a very important, large thermal reaction class. Generally, it can be said that oxidative addition to low valent, coordinatively unsaturated metal complexes is common. Recently, Geoffroy, Hammond, and Gray188 have established that reductive elimination can be photochemically induced. A typical example is shown in reaction (67). Other small molecules can be reductively elimi-... 

One method for the synthesis of hydroxyalkyl-substituted P-lactams is by the Staudinger reaction, the most frequently used method for the synthesis of P-lactams.86 This method for the preparation of 4-acetoxy- and 4-formyl-substituted P-lactams involves the use of diazoketones prepared from amino acids. These diazoketones are precursors for ketenes, in a diastereoselective, photochemically induced reaction to produce exclusively tram-substituted P-lactams. The use of cinnamaldimines 96, considered as vinylogous benzaldimines, resulted in the formation of styryl-substituted P-lactams. Ozonolysis, followed by reductive workup with dimethyl sulfide, led to the formation of the aldehyde 97, whereas addition of trimethyl orthoformate permitted the production of the dimethyl acetal 98 (Scheme 11.26). 

Photochemically induced electron ejection from anions has been used in the preparation of radical anions in the cavity of an ESR spectrometer. Subtle structural differences between the radical anions prepared by photoirradiation and by other ways were outlined [50, 51], Two modes of radical anion photoinitiated formation were used. An electron is photoejected from the excited fluorene anion and adds to a neutral fluorene molecule in a subsequent step (A) or a dianion obtained from the potassium mirror reduction of tetrabenzocyctloocatetraene is directly photooxidized to the corresponding radical anion (B). This last method was also used to generate the otherwise inaccessible radical anion of pentalene, starting from the corresponding dianion irradiated in an ESR probe [52],... 

The photosensitized electron transfer reaction forms the reduced lipophilic electron acceptor BNA which is ejected into the continuous organic phase and thus separated from the oxidized product. In order to monitor the entire phase transfer of the reduced acceptor, BNA, a secondary electron acceptor, p-dlmethyl-amlnoazobenzene (dye),was solubilized in the continuous oil phase. The photochemically induced electron transfer reaction in this system results in the reduction of the dye (0 = 1.3 x 10 3). Exclusion of the sensitizer or EDTA or the primary electron acceptor, BNA, from the system resulted in no detectable reaction. Substitution of the primary acceptor with a water soluble derivative, N-propylsulfonate nicotinamide, similarly results in no reduction of the dye. These results indicate that to accomplish the cycle formulated in Figure 6A the amphiphilic nature of the primary electron acceptor and its phase transfer ability in the reduced form are necessary requirements. 

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