Oxidative quenching cycle

Consequently, the Rovis group identified a productive dual-catalysis mode that enables the asymmetric a-acylation of tertiary amines with aldehydes facilitated by the powerful combination of chiral NHC catalysis and photoredox catalysis. m-DNB (dinitrobenzene) is likely to induce an oxidative quenching cycle of [Ru(bpy)3] under these conditions, with adventitious oxygen likely being the terminal oxidant (Scheme 7.14). 

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... 

When the ruthenium(II) complex is irradiated under an oxygen atmosphere, the oxidative quenching of the photoexcited ruthenium(II) complex by an oxygen molecule rapidly occurs, to afford the ruthenium(III) complex. Because the ruth-enium(III) complex is a powerful oxidizing reagent, it can oxidize some substrate. Such a catalytic cycle was applied to the asymmetric photosynthesis of Co(acac)3 from Co(acac)2 + acac . A- and A-[Ru(( — )-menbpy)3]2+ and A- and A-[Ru(R(or S)-PhEtbpy)3]2+ were used as photosensitizers for such asymmetric photosynthesis [38,39]. 

The viologen reduction by EDTA in reverse micelles in the presence of Ru(bpy)3 is another example of vectorial photoinduced electron transfer [106], The accumulation of photoproducts is associated with the catalytic cycles depicted in Fig. 10(b). The oxidative quenching of the ruthenium complex occurs at the micelle outer boundary, while the regeneration of the dye takes place by the oxidation of EDTA in the inner core of the micelle. The reduction of the final product 4-dimethylaminoazobenzene is further mediated by the acceptor 1-benzylnicotinamide (BNA ). In Fig. 10(c), the photocatalytic reduction of methyl benzoylformate (MBF) by thiosulfate is described in the presence of the porphyrin ZnTPPS and the mediator quinolinium-3-carboxiamide (DCA ) [107]. This sequence of reactions occurs only in micelles such as those formed by hexadecyl-trimethylammonium bromide, which contain in the interior the ultimate donor acceptor. Finder illumination, ZnTPPS photoreduces DCA to DCQ, which is subsequently extracted into the micelle core. Within the microenvironment, DCA is regenerated via reduction of MBF, while the oxidized porphyrin is reduced by thiosulfate outside the micelle. 

Under circumstances where the sensitizer is efficiently reduced by the donor in the excited state, (in comparison to oxidative quenching by the acceptor relay), then one obtains the same products via a reductive cycle. The proflavine-sensitized (also ZnTMPyP sensitized under high donor concentration) reduction of MV with EDTA has been shown to involve a reductive cycle... 

A possible catalytic cycle for this transformation, involving an oxidative quenching pathway, was proposed by the authors (Scheme 13.38). Single-electron transfer from photoexcited Ir(III) to trifluoromethylacyloxyphthalimide 261 generates the A/ -centered radical (PhthN) 263, trifluoroacetate anion, and Ir(IV). The addition of radical 263 to a heteroarene forms the carbon radical intermediate 264, which is then oxidized by Ir(IV) with subsequent deprotonation providing the aminated product 262a. 

Figure 10.1 Oxidative and reductive quenching cycles for photocatalysts.

Substrates with a heteroatom in proximity to the carboxylic acid, benzylic and homobenzylic carboxylic acids were transformed more rapidly due to faster CO2 extrusion, and it was found that only two equivalents of Select-fluorwas needed for these substrates. Fluoride elimination was not observed under the basic conditions, even for substrates which might be expected to have a propensity to display this type of reactivity. Mechanistic studies support an oxidative quenching pathway in which initial reduction of the N-F bond of Selectfluor initiates the photoredox cycle with oxidation/dec-arbo)q lation of the carboxylates leading to the intermediate allq l radicals. 

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