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Intramolecular reaction photoreduction

Compared with photooxygenation, i.e. the photochemical activation of oxygen, the definition of photoreduction is more difficult. Many photochemical processes involve redox reactions where one part of the substrate (in intramolecular reactions) or a second molecule (in intermolecular reactions) is reduced and the original substrate is oxidized (or... [Pg.285]

The triplet state of carbonyl chromophores frequently shows a high reactivity in hydrogen abstraction reactions (l ). These processes can take place intermolecularly (photoreduction) ( l) or intramolecularly, for example in the Norrish Type II process, reaction 1 (.2,3.). [Pg.19]

In the intramolecular photoreduction kinetic scheme, catalytic metallic nuclei are formed in the intramolecular ligand-to-metal electron-transfer process. For example, catalytic metallic Cu nuclei can be formed in the photochemical reaction (A < 3500 A) of cupric acetate (CuA) ... [Pg.150]

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. [Pg.395]

The efficiency of reduction of benzophenones is greatly reduced when an ortho alkyl substituent is present because a new photoreaction, intramolecular hydrogen abstraction, 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. It can be detected, even though no net reaction occurs, by photolysis in deuterated hydroxylic solvents. The proton of the enolic hydroxyl is rapidly exchanged with solvent molecules, so when a deuterated solvent is used, deuterium is introduced at the benzylic position. Deuterium is also introduced if the enol is protonated at the benzyl carbon by solvent. [Pg.476]

The photoreduction efficiency of ortho-zSkyl benzophenone derivatives is greatly reduced by intramolecular enolization process, known as photoenolization reaction. For example, orf/io-ethyl benzophenone 2 on photoirradiation in deuterated hydroxylic solvents gives deuterated ethyl benzophenone 3 by photoenolization without reduction [3]. [Pg.242]

Photohomolysis reactions of cobalt(III) pseudohalide complexes can be used to effect photoreduction to cobalt(II) complexes. Thus, intramolecular photoelectron transfer in the complexes Co(CN)5N3 and Co(NH3)5N3 leads to oxidation of the azide ion to the azide radical, and reduction of the cobalt(III) center to cobalt(II). Evidence for the initial formation of the azide radical comes from the photolysis of solutions containing Co(CN)5N3 and iodide ion, when the iodine anion radical I2 is observed in the solution. This formation of I2 results from the photochemical generation of the azide radical, which then oxidizes the iodide ion to an iodine atom (Scheme 2.1). Subsequently, spin trapping experiments with phenyl-N-t rf-butyl nitrone has been used to verify the formation of azide radicals from the photolysis of Co(CN)5N and Co(NH3)sN3 / ... [Pg.49]

Irradiation of thiobenzophenone with cyclo-octatetraene at A > 340 nm gives a 1,4-adduct (40), but, in contrast, 6,6-diphenylfulvene yields" a 1,1-adduct. The photochemical reaction of o-benzylthiobenzophenone has been examined because of its relevance to the reduction of thiobenzophenone." An intramolecular transfer of hydrogen was postulated to give an enethiol, which could be trapped. It is suggested that the photoreduction of thiobenzophenone involves the initial formation of a radical, PhaCSH. The photochemical behaviour of adamantanethione (41) has also been studied. In the ( ,rr ) state it gives the... [Pg.134]

The solution photochemistry of a,/8-unsaturated ketones has received a great deal of attention. It is found that the course of the photoreactions is sensitive to small variations in the structure of the reactant, the concentration of the reactant, and the solvent. The reactions observed thus far involve structural rearrangements which may be accompanied by reaction with the solvent, photoreduction, both inter- and intramolecular, and dimerization. [Pg.293]

A wide variety of carbonyl compounds undergo photoinduced intramolecular hydrogen atom abstraction to form 1-hydroxy-IvJc-biradicals, which then undergo two common competing reactions (1) coupling to produce cycKc alcohols and (2) disproportionation back to ketone or to various enols. The overall process closely parallels the well-known bimolecular photoreduction of ketones, the most common products of which are formed by radical coupling. These intramolecular hydrogen abstraction processes have been widely studied, and several reviews are available. ... [Pg.1147]


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See also in sourсe #XX -- [ Pg.12 , Pg.283 ]




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