Mechanistic pathways in phosphate ester photochemistry
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Abstract This article discusses a number of reactions of potential synthetic interest occurring through photochemically generated organic radicals. Emphasis is placed on the nature of the resulting photoproducts, whose formation is discussed on a mechanistic basis. If available, experimental evidence (obtained by laser flash photolysis or related time‐resolved techniques) is provided. The material is organized according to the reaction types, for instance, homolytic cleavage, mesolytic cleavage, radical addition, and oxidative or reductive electron transfer. In addition to the classical carboncarbon or carbonhydrogen bond breaking processes, such as the Norrish type I reaction, the hydrogen abstraction, and the photo‐Fries or the photo‐Claisen rearrangements, a number of carbonheteroatom and heteroatomheteroatom cleavage processes are also covered. At the end of the article, a short section is included that deals with the experimental techniques for the characterization of photochemically generated radicals.
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ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTOrganochromium(III) macrocyclic complexes. Factors controlling homolytic vs heterolytic cleavage of the chromium carbon bondShu Shi, James H. Espenson, and Andreja BakacCite this: Inorg. Chem. 1990, 29, 21, 4318–4322Publication Date (Print):October 1, 1990Publication History Published online1 May 2002Published inissue 1 October 1990https://pubs.acs.org/doi/10.1021/ic00346a030https://doi.org/10.1021/ic00346a030research-articleACS PublicationsRequest reuse permissionsArticle Views47Altmetric-Citations8LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts
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The production of singlet oxygen by H2O2 disproportionation and via the oxidation of H2O2 by NaOCl in a neutral medium was monitored by spin trapping with 2,2,6,6 tetramethyl-4-piperidone (TMPone). The singlet oxygen formed in both reactions oxidized 2,2,6,6 tetramethyl-4-piperidone to give nitroxide radicals. However the production of nitroxide radicals was relatively small considering the concentrations of H2O2 and NaOCl used in the reaction systems. Addition of electron donating agents: ascorbate, Fe2+ and desferrioxamine leads to an increase in the production of nitroxide radicals. We assumed that a very slow step of the reaction sequence, the homolytic breaking of the O-O bond of N-hydroperoxide (formed as an intermediate product during the reaction of 1O2 with TMPone) could be responsible for the relatively small production of nitroxide radicals. Electron donating agents added to the reaction system probably raise the rate of the hydroperoxide decomposition by allowing a more rapid heterolytic cleavage of the O-O bond leading to a greater production of nitroxide radicals. The largest effect was observed in the presence of desferrioxamine. Its participation in this process is proved by the concomitant appearance of desferrioxamine nitroxide radicals. The results obtained demonstrate that the method proposed by several authors and tested in this study to detect singlet oxygen is not convenient for precise quantitative studies. The reactivity of TMPone towards O2−7HO2' and 'OH has been also investigated. It has been found that both O2−7HO2' and 'OH radicals formed in a phosphate buffer solution (pH 7.4, 37°C), respectively by a xanthine-oxidase/hypoxanthine system and via H2O2 UV irradiation, do not oxidize 2,2,6,6 tetramethyl-4-piperidone to nitroxide radicals.
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Abstract To elucidate the mode of hydrogen–hydrogen bond cleavage at transition metal sites that cooperate with a heteroatom, the hydrogenation of trinuclear μ 3 ‐imido complex (Cp*Ru) 3 (μ 3 ‐NH)(μ‐H) 3 (Cp* = η 5 ‐C 5 Me 5 ) ( 1 ) with deuterium was carried out. The percentage of deuterium content in the amido ligand of (Cp*Ru) 3 (μ‐NH 2 )(μ‐H) 4 ( 2 ), which was generated as an intermediate in the reaction of 1 with dihydrogen, proved that hydrogenation of 1 to 2 proceeded through homolytic cleavage of dihydrogen.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)
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The oxidation of tryptophan photosensitized by PtCl6(2-) has been investigated in aqueous solutions at different pH using nanosecond laser flash photolysis. Cationic and neutral radicals of tryptophan were detected at pH 2.8 and 8.5, respectively. The generation of the radical was attributed to oxidation by Cl2- that was formed from the homolytic bond cleavage in the excited state of PtCl6(2-). The bimolecular rate constant derived from the kinetics analysis, 2.8 +/- 0.2 x 10(9) M-1 s-1, is in good agreement with the value obtained in earlier pulse radiolysis studies. Both the cationic and neutral radicals decayed by second-order kinetics, consistent with the dimerization process.
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Abstract Aus den l‐Naphthylmethylhalogeniden (I) erhält man bei der Photolyse in Methanol die Folgeprodukte (II) bis (IV), in Cyclohexan entstehen (III) sowie die Reduktionsprodukte (V) und (VI).
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Peroxide
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Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.
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The fragmentation of 1 is up to 1020 times faster if it is present as radical ion and not as neutral compound. This acceleration has thermodynamic reasons, which surprisingly lead almost exclusively to a lowering of the energy of the transition state, even for weakly endergonic processes. The term (catio-/anio) mesolytic is proposed for the fragmentations of radical ions.
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