Reductive quenching cycle

A number of rhodium(III) complexes can be used effectively in place of viologens as relays. Thus photolysis of a solution containing Ru(bpy)32+ as the photosensitizer, ascorbate as the electron donor and [Rh(dpm)3Cl]3 (dpm = diphenylphosphinobenzene-m-sulfonate) as the electron relay leads to nett formation of hydrido-rhodium species via a reductive quenching cycle. The hydrido-rhodium product acts a two-electron carrier for the reduction of NAD-i- to NADH. In place of NADH, synthetic nicotinamide analogues such as N-benzyl nicotinamide or N-alkylnicotinamides can be similarly reduced in the photosystem [68]. The sequence of cyclic redox reactions can be extended by the addition of an enzyme. In the presence of... 

Ru"(bpy)3]2+ reductive quenching cycle [Ru"(bpy)3]2+ ground state triplet excited state... 

Scheme 6 [2+2] Enone cycloadditions through the reductive quenching cycle...

In 2010, Stepehenson and coworkers developed aza-Henry reactions of tetrahydroisoquinolines 14 on the assumption that electron-rich tertiary alkylamines serve as electron donors to be converted into iminium ion through SET photoredox processes [50]. They showed that the Ir photocatalyst is more efficient than the Ru photocatalyst, [Ru(bpy)3]Cl2. Proposed reaction mechanism based on the reductive quenching cycle is illustrated in Scheme 10. The photoex-cited Ir species undergoes SET from tetrahydroisoquinoline 14 to give the... 

A plausible mechanism for the alkylation, involving reductive quenching cycles, has been proposed by Stephenson and coworkers (Scheme 13.24). First, the Ru(II) photocatalyst was excited by visible light, generating excited Ru(II), which was reductively quenched by 185 to afford Ru(I) and the radical ammonium cation... 

Figure 10.1 Oxidative and reductive quenching cycles for photocatalysts.

The initial step in the photocatalytic cycle has been illuminated by laser flash photolysis experiments. These experiments have shown that reductive quenching of the triplet metal-to-ligand charge transfer ( MLCT) excited state by a sacrificial donor occurs to generate a one-electron reduced (OER) species of the rhenium... 

The mixed system of 16, [Ni(cyclam)] (23, cyclam = 1,4,8,11-tetraazacyclo-tetradecane), and ascorbic acid (H2A) was irradiated tmder a CO2 atmosphere using 340-600-nm light. This mostly evolved H2 along with a small amount of CO. The TN and quantum yield for CO production were TNco < 1 and co = 0.0006 [39, 40]. Control experiments carried out in the dark or without 16, 23, H2A or CO2 produce no CO [39]. A labeling experiment using " C02 revealed that the CO is produced from CO2 [40]. This photocatalytic reaction cycle is also initiated by reductive quenching of the MLCT excited state of 16 by HA. ... 

The overall process, schematically represented in Figure 5, comprises two interconnected catalytic cycles a photochemical one involving [Ru(bpy)3]2+ and a thermal one involving the bis-bpy or mono-bpy ruthenium(II) complex. The reduced [Ru(bpy)3] complex was photogenerated by reductive quenching of the photosensitizer excited state by TEOA (eq. 5), with a rate constant of 1.7 10 M s. ... 

Such efficiency is the result of the intermolecular redox process in which Q reductively quenches the metal-to-ligand charge-transfer (MLCT) excited state of the palladium complex. The resulting radical cation Q+ rearranges to its most stable isomer N+, which then oxidizes another molecule Q to restart the cycle. 

Fig. 9. The MoFe protein cycle of molybdenum nitrogenase. This cycle depicts a plausible sequence of events in the reduction of N2 to 2NH3 + H2. The scheme is based on well-characterized model chemistry (15, 105) and on the pre-steady-state kinetics of product formation by nitrogenase (102). The enzymic process has not been chsiracter-ized beyond M5 because the chemicals used to quench the reactions hydrolyze metal nitrides. As in Fig. 8, M represents an aji half of the MoFe protein. Subscripts 0-7 indicate the number of electrons trsmsferred to M from the Fe protein via the cycle of Fig. 8.

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