Photoreductions

One of the earliest photoreactions to be studied was the photoreduction of benzophenone (Ciamician and Silber, 1900)—that is, the conversion of a carbonyl compound into an alcohol by an intermolecular hydrogen abstraction reaction. Intramolecular hydrogen abstraction by the carbonyl group, usually from the y site, is referred to as a Norrish type II reaction. Hydrogen abstraction by olefins and heterocycles has also been observed. 

The correlation diagram for hydrogen abstraction by a ketone has been derived in Example 4.5 (Section 4.2.3). From this diagram it can be concluded that no reaction is to be expected from the ( r,jr ) states for the in-plane 

In addition to the reaction between the no orbital of the carbonyl group and the orbital, another conceivable reaction path for hydrogen abstraction is based on an interaction between the tco and the orbital. This is referred to as a perpendicular attack. 

In the correlation diagram for the perpendicular attack derived in Example 7.6, the state correlates with the ground state of the primary 

The reaction of benzophenone (34) with benzhydrol (35) is a representative example of hydrogen abstraction  

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The effects of uv radiation on V/-nitroso compounds depend on the pH and the medium. Under neutral conditions and ia the absence of radical scavengers, these compounds often appear chemically stable, although the E—Z equiUbrium, with respect to rotation around the N—N bond, can be affected (70). This apparent stabiUty is due to rapid recombination of aminyl radicals and nitric oxide [10102-43-9] formed duting photolysis. In the presence of radical scavengers nitrosamines decay rapidly (71). At lower pH, a variety of photoproducts are formed, including compounds attributed to photoelimination, photoreduction, and photo-oxidation (69). Low concentrations of most nitrosamines, even at neutral pH, can be eliminated by prolonged kradiation at 366 nm. This technique is used ki the identification of /V-nitrosamines that are present ki low concentrations ki complex mixtures (72). 

Surface vs Solution Reactions, Anotliei issue of debate in pliotocatalyzed mineialization of oiganic substrates is whether the initial oxidation occurs on the photocatalyst s surface or in solution. Kinetic data of photooxidations and photoreductions have often been fitted to the simple... 

Titanium(III) alkoxides can be produced by photoreduction of the tetraalkyl titanates in the presence of a base, such as pyridine (180), and by reduction of tetraalkyl titanates by organosiUcon compounds containing Si—H groups (181). 

The tris compounds are highly bridged three-dimensional polymers. Photoreduction of aqueous Ti(IV)-containing alcohols or glycols, but not of ethylene glycol, yields Ti(III) and the aldehyde or ketone corresponding to the alcohol (191,192). A possible mechanism is... 

TTie photoreduction can be quenched by known triplet quenchers. The effecti e quenchers are those which have T] states less than 69kcal/moI above S,. Quenchers with higher triplet energies are ineffective because the benzophenone n-n triplet is then not sufficiently energetic to effect energy transfer. 

The efficiency of reduction of benzophenone derivatives is greatly diminished when an ortho alkyl substituent is present because a new photoreaction, intramolecular hydrogen-atom abstraction, then becomes the dominant process. The abstraction takes place from the benzylic position on the adjacent alkyl chain, giving an unstable enol that can revert to the original benzophenone without photoreduction. This process is known as photoenolization Photoenolization can be detected, even though no net transformation of the reactant occurs, by photolysis in deuterated hydroxylic solvents. The proton of the enolic hydroxyl is rapidly exchanged with solvent, so deuterium is introduced at the benzylic position. Deuterium is also introduced if the enol is protonated at the benzylic carbon by solvent ... 

Reductive dunenzation to form fluorinated benzopinacols proceeds m the partly fluormated case either with zinc or by photolysis but is not observed with perfluorobenzophenone [651 (equation 53). Trifluoroacetophenone is reduced electrochemically in dimethylformamide to a stable radical anion, which, m the presence ot lithium ion, rapidly dunerizes to pinacol in higher yield than that available by photoreduction [66] (equation 54)... 

Another tetrahydroporphyrin derivative, porphodimethene (7), which consists of two dipyr-rylmethene halves is obtained in the photoreduction of magnesium-containing chlorophyll a (6) with hydrogen sulfide3211 42 as reductant and pyridine as base. 

In a study of the photoreduction of zinc(II) or magnesium chlorophyll derivatives 8 with ascorbic acid in the presence of l,4-diazabicyclo[2.2.2]octane (DABC-O), a 2,3-m-hydrogenated isobacteriochlorin 9 is formed which subsequently rearranges to a 3-ethylidenc derivative... 

Ultraviolet spectrophotommetry was used to follow the course of the photoreduction of thiopyrylium salts to their corresponding 4H-thiopyranyl radicals (85BCJ2600) as well as the kinetics of thiopyrylium salt transformations to thiopyrans with various nucleophiles (84JA7082, 84JOC1806 86JA3409). 

Although Ru(bipy)2+ alone will not split water into hydrogen and oxygen, it has been accomplished with Ru(bipy)2+ using various catalysts or radical carriers. Perhaps the most studied system for the photoreduction of water involves using methyl viologen as the quencher, EDTA as an electron donor (decomposed in the reaction) and colloidal platinum as a redox catalyst (Figure 1.19). 

Copper complexes, 5,533-750 acetylacetone hydrolysis, 2,379 photoreduction, 2.384 amidines... 

Iron, tris(hexafluoroacetylacetone)-structure, 1,65 Iron, tris(oxalato)-chemical actinometer, 1,409 photoreduction, 1,471 relief-image-forming systems, 6,125 Iron, tris(l,10-phenanthroline)-absorptiometry, 1,549 racemization, 1,466 solid state, 1,467 structure, 1, 64 lron(III) chloride amino acid formation prebiotic systems, 6,871 Iron complexes acetonitrile. 4,1210 acetylacetone, 2,371 amidines... 

Plutonium spectra showing the photoreduction of plu-tonyl. (I) 0.385 11 Pu022+, some Pu1 "1" and 1.5 M ethanol in 1.32 11 HClOi,. (II) Immediately after the first UV irradiation. (Ill) Twenty-four hours after the first UV irradiation. (IV) Immediately after the second and 25 h after the first UV irradiations (3 ). 

Thus irradiation of benzophenone in toluene gives the photoreduction product 19, benzpinacol, and bibenzyl [equation (59)] of which only 19... 

Figure 2. Photoreduction of 2,3,7,8-tetrachlorodi-benzo- -dioxin (I) (2 mg/liter in methanol) as compared with that of the 2,3,7-trichloro-homolog (II) (20), 1971 by AAAS...

The discussion above refers to the classical dark conditions where the chemical activation is achieved thermally. Fenton requires a moderate thermal activation, resulting in a reaction temperature ranging from 25 to 90 °C. The oxidizing capacity of the Fenton reaction can be increased by UV or UV-vis Hght irradiation [160, 161]. The increase is interpreted by means of the photoreduction ability of Fe ... 

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