Oxidative quenching reaction

Table 14 Bimolecular Quenching Rate Constants for the Oxidative Quenching Reactions of 50b with Pyridinium Acceptors...

This reductive quenching reaction is evidenced by the formation of the amine cation R3N immediately subsequent to the flash. It is unlikely that energy transfer plays any role in the quenching process because the triplet energies of tertiary amines are too high for this to occur. A correlation of the quenching rate of Ru(bpy)3 with the redox potential of the amine for a series of aromatic amines is shown in Table As with the oxidative quenching reactions of the bipyridinium ions, there is... 

Allyl aryl ethers undergo accelerated Claisen and [1,3] rearrangements in the presence of a mixture of trialkylalanes and water or aluminoxanes. The addition of stoichiometric quantities of water accelerates both the trimethylaluminum-mediated aromatic Claisen reaction and the chiral zirconocene-catalyzed asymmetric carboalumination of terminal alkenes. These two reactions occur in tandem and, after oxidative quenching of the intermediate trialkylalane, result in the selective formation of two new C-C bonds and one C-0 bond (Eq. 12.70).153 Antibodies have also been developed to catalyze Claisen154 and oxy-Cope155 rearrangements. 

Pt2(P205H2) - (d8-d8), and Mo6Clft ( )6. Two- electron oxidations of Re2Cl and Pt2(P205H2)it have been achieved by one-electron acceptor quenching of the excited complexes in the presence of Cl, followed by one-electron oxidation of the Cl -trapped mixed-valence species. Two-electron photochemical oxidation-reduction reactions also could occur by excited-state atom transfer pathways, and some encouraging preliminary observations along those lines are reported. 

The net quenching reaction in eq. 2, which leads to separated redox products capable of oxidizing and reducing water, relies on a series of electron transfer steps. The basic theme of this account is excited state and related electron transfer events which occur in such systems and the basis that we have for understanding them both experimentally and theoretically. 

In phenolic oxidative coupling reactions, these phenol-derived radicals do not propagate a radical chain reaction instead, they are quenched by coupling with other radicals. Thus, coupling of two of these resonance structures in various combinations gives a range of dimeric systems, as shown. The... 

Diethylbenzo[l,2-i7 4,5- ]bis[l,2,3]trithiole 86 can be oxidized with MCPBA to produce the 1-sulfoxide 100 which undergoes further oxidation on reaction with deuterated sulfuric acid to produce a dication (Scheme 6) <2003EJ04902>. On quenching with water this dication gave a mixture of the T and 2-sulfoxides 100 and 101. 

Fluorescein dyes undergo some interesting oxidation-reduction reactions from their triplet states.449-451 For example, they are reduced by both phenol and phenolate anion. They are quenched by oxgyen... 

In such reactions, both the reductive and oxidative quenching pathways are possible for the photocatalytic reactions between Dred and Aox. In general, one reaction pathway, either reductive or oxidative quenching, becomes dominant when the reactant pair Dox and Aox is fixed. However, acid catalysis can alter the operating quenching pathway by accelerating one pathway and/or retarding the other pathway (vide infra). 

The mechanism of this photochemical reaction is as shown in equations (91)-(94) and relies upon oxidative quenching of [Ru(bipy)3]2+ by the cobalt complex to give [Ru(bipy)3]3+ and highly labile cobalt(II) amine complexes which decompose at a rate which competes with back... 

Because of the much greater driving forces potentially available in reactions between substrates and excited state molecules, difficult—but valuable—electron transfer reactions, such as the oxidation of water or chloride ion, may be accessed through excited state photochemistry. The question of how to separate hole-electron pairs generated in a quenching reaction, how to provide kinetic pathways to lead these two highly reactive species far apart from each another, and how to couple in some useful chemistry are currently of interest. 

Figure 8 Illustration of the flash-quench technique for measuring intramolecular ET rates. Photoexcitation of Ru(II)(bpy)2(imidazole)(amine)2+, Ru(II)(bpy), bound to ferro-cytochrome c, Fe(II)P, produced an 80-ns lived metal-to-ligand charge transfer (MLCT) excited state, Ru(III)(bpy -), which was oxidatively quenched by bimolecular reaction with Ru(NH3) +. The resulting Ru(III)-complex was then reduced by the Fe(II)P through thermal, protein-mediated ET. Finally bimolecular reaction of the Ru(II)/Fe(III)P product with Ru(NH3) + re-formed the starting Ru(II)-protein-Fe(II)P complex.

On the basis of these data one can clearly state that lumiflavin is submitted to a reductive quenching by semicarbazide, while Ru (II)-tris (2, 2 -bipyridine) reacts via oxidative quenching by molecular oxygen (Navarro et al., 1987b 1987c). Fig. 4 shows a simplified sequence of reactions which leads to oxygen photoreduction, with the concomitant formation of hydrogen peroxide, with electrons from semicarbazide... 

The best evidence for a CT process rather than direct hydrogen abstraction involves the values of kT s-butyl- and ferf-butylamine display much the same value 156> triethylamine and ferf-butyldimethylamine are equally reactive and some 50 times more so than primary amines 155>. Thus the rate constant for reaction is independent not only of the type of C—H bond a to the nitrogen but also of the presence or absence of a-hydrogens. Such evidence demands that abstraction of an a-hydrogen not be involved in the rate-determining quenching reaction. Moreover, the relative reactivity of amines (tertiary > secondary > primary) is proportional to the ease with which they are oxidized. 

In the quenching reaction of A-[Ru(bpy)3]2+ by [Co(ox)3]3- and Co(acac)3, only the homochiral preference was observed in water, whereas the stereoselectivity of the quenching by [Co(ox)3]3 becomes reverse in 80% methanol-water. These results suggest that the stereoselectivity is determined not only by the photoin-duced electron transfer but also by the different elementary step such as the reverse reaction. The photoreduction of the cobalt(III) complex by the ruthen-ium(II) complex involves various elementary steps, as shown in Scheme 11. Considering this scheme, one can easily understand that the overall photoreduction of the cobalt(III) complexes is determined by not only the quenching process but also the reverse reaction between the reduced Co(II) complexes and the oxidized ruthenium(III) complex. This conclusion is essentially the same as that reported by Ohkubo and his collaborators. 

If bi-2-naphthol and Co(acac)3 exist in excess in the reaction solution, the photoexcited ruthenium(II) complex undergoes oxidative quenching by Co(acac)3 to afford the ruthenium(III) complex, as shown by the catalytic cycle of Scheme... 

This value is much larger than that observed in the previously reported deracemizations of [Cr(ox)3]3 and Cr(acac)3. In this reaction, the basic condition is necessary, and the addition of Hacac increases the enantiomer excess, for which the reason will be discussed below. The reaction mechanism shown in Scheme 17 was proposed. In the mechanism, the 3MLCT excited A- [Ru(( — )-men-bpy)3]2+ is oxidatively quenched by Co(acac)3 to form an exciplex with Co(acac)3 followed by electron transfer to Co(acac)3 from A- [Ru(( — )-menbpy)3]2 +, which leads to the formation of a successor complex, [A-Ruin(( — )-menbpy)33 + Con(acac)3 ]. This successor complex dissociates to A-[Rum(( — )-menbpy)3]3+, Co(acac)2, and acac. If the reducing reagent is absent or the reducing reagent does not effectively reduce the ruthenium(III) complex, Co(acac)2 reduces A-[Rum(( — )-menbpy)3]3+ to A-[Run(( — )-menbpy)3]2+ concomitantly with the formation of Co(acac)3. As discussed in Sec. II.A., the photoreduction of Co-(acac)3 occurs stereoselectively. In addition, the oxidation of Co(acac)2 to Co-(acac)3 occurs stereoselectively, because Co(acac)2 reacts with the chiral ruthen-... 

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