Relay catalyst

The prototypical photochemical system for CO2 reduction contains a photosensitizer (or photocatalyst) to capture the photon energy, an electron relay catalyst (that might be the same species as the photosensitizer) to couple the photon energy to the chemical reduction, an oxidizable species to complete the redox cycle and CO2 as the substrate. Figure 1 shows a cartoon of the photochemical CO2 reduction system. An effective photocatalyst must absorb a significant part of the solar spectrum, have a long-lived excited state and promote the activation of small molecules. Both organic dyes and transition metal complexes have been used as photocatalysts for CO2 reduction. In this chapter, CO2 reduction systems mediated by cobalt and nickel macrocycles and rhenium complexes will be discussed. 

Photochemical CO2 reduction to CO (and formate in some cases) has been reported in a catalytic system using Ru(bpy)3 + as the sensitizer, nickel or cobalt macrocycles as the electron relay catalyst, and ascorbate as a sacrificial reductive quencher [9, 15, 16]. These systems also produce H2 via water reduction. Although Ni(cyclam) + is an efficient and selective catalyst for electrochemical CO2 reduc-... 

Although sensitizer/relay/catalyst schemes have become very popular, alternative approaches exploiting homogeneous photochemistry are being studied. Reactions in which radical ions or carbanions are generated by illumination have been discussed, and the interesting properties of polynuclear rhodium... 

Cu—Rh redox relay catalysts for synthesis of azaheterocycles via C—H functionalization 12CL1554. 

Examples of electron donors, sensitizers, acceptor relays catalysts which have been found to be quite efficient are and redox listed in... 

It has been our goal for some time to run photochemical energy storage reactions without relay molecules or separate catalysts. We have concentrated on the photochemistry of polynuclear metal complexes in homogeneous solutions, because we believe it should be possible to facilitate multielectron transfer processes at the available coordination sites of such cluster species. 

In order to destabilize the likely unproductive 6-membered chelate structure of type K (Scheme 17) that might be formed if the catalyst reacts with the 4-pen-tenoate entity, the cyclization was run in the presence of a Lewis acid which competes with the evolving carbene for the Lewis basic ester group. Such an additive has to be compatible with the RCM catalyst, should provoke a minimum of acid-catalyzed side reactions, and must undergo a kinetically labile coordination with the relay substituent. Ti(OiPr)4 was found to meet these stringent requirements... 

Davies, Renaud, and Sibi independently reported the chiral relay approach to control the enhanced steric extension inside a substrate to achieve increased asymmetric induction. However, as our study proves, the asymmetric activation of a tropos catalyst clearly differs from the chiral relay approach, in which substrate conformational control is utilized, since asymmetric activation controls the chiral environment of a tropos catalyst by the addition of a chiral external source (a chiral activator). 

Suzuki and co-workers have relayed this methodology into the synthesis of (+)-sappanone B (Scheme 5) [52], The authors found that catalysts previously introduced by Rovis and co-workers led to inferior results iV-Ph catalyst 41 gave significant elimination while Al-C Fj gave low enantioselectivities. By tuning the electronics of the M-aryl substituent these workers identified 49 as providing the optimal mix of reactivity and enantioselectivity. Commercially available 2-hydroxy-4-methoxybenzaldehyde 47 was transformed into aldehyde 48, which upon treatment with triazolium salt 49 in the presence of base was cycUzed to afford (R)-50 in 92% yield and 95% ee and subsequently transformed into (-t-)-sappanone B. 

One of the important applications of mono- and multimetallic clusters is to be used as catalysts [186]. Their catalytic properties depend on the nature of metal atoms accessible to the reactants at the surface. The possible control through the radiolytic synthesis of the alloying of various metals, all present at the surface, is therefore particularly important for the catalysis of multistep reactions. The role of the size is twofold. It governs the kinetics by the number of active sites, which increase with the specific area. However, the most crucial role is played by the cluster potential, which depends on the nuclearity and controls the thermodynamics, possibly with a threshold. For example, in the catalysis of electron transfer (Fig. 14), the cluster is able to efficiently relay electrons from a donor to an acceptor, provided the potential value is intermediate between those of the reactants [49]. Below or above these two thresholds, the transfer to or from the cluster, respectively, is thermodynamically inhibited and the cluster is unable to act as a relay. The optimum range is adjustable by the size [63]. 

Since the reduction potential of MV2+/MV is low enough (—0.44 V at pH 7) to reduce protons, the presence of platinum as a catalyst in the solution containing MV 7 brings about hydrogen formation. Scheme 1 is a typical model of photo-induced charge separation and electron relay to yield H2. It also represents the half reaction cycles of the reduction site for the photochemical conversion shown in Fig. 3. 

Oxidation of water to evolve 02 is an important reaction for water photolysis as described in Sect. 2. Oxygen evolution from water by Ru(bpy)f+ oxidation with Ru02 catalyst was studied 41). The authors established an electron relay system for water oxidation (Scheme 3) by the polymer Ru(bpy) + complex and Ru02 catalyst with Pb02 as an oxidant40). 

A photoinduced electron relay system at solid-liquid interface is constructed also by utilizing polymer pendant Ru(bpy)2 +. The irradiation of a mixture of EDTA and water-insoluble polymer complex (Ru(PSt-bpy)(bpy) +, prepared by Eq. (15)) deposited as solid phase in methanol containing MV2+ induced MV 7 formation in the liquid phase 9). The rate of MV formation was 4 pM min-1. As shown in Fig. 14, photoinduced electron transfer occurs from EDTA in the solid to MV2+ in the liquid via Ru(bpy)2 +. The protons and Pt catalyst in the liquid phase brought about H2 evolution. One hour s irradiation of the system gave 9.32 pi H2 after standing 12 h and the turnover number of the Ru complex was 7.6 under this condition. The apparent rate constant of the electron transfer from Ru(bpy)2+ in the solid phase to MV2 + in the liquid was estimated to be higher than that of the entire solution system. The photochemical reduction and oxidation products, i.e., H2 and EDTAox were thus formed separately in different phases. Photoinduced electron relay did not occur in the system where a film of polymer pendant Ru complex separates two aqueous phases of EDTA and MV2 9) (see Fig. 15c). 

Whilst [ Ru(bipy)3]2t itself is incapable of splitting water, its electron-transfer properties have been utilized for hydrogen production in a series of reactions involving cocatalysts (see equations 21 to 26). The first step involves electron transfer from the excited state complex to an electron relay (R), which in its reduced form is capable (in the presence of a suitable heterogeneous redox catalyst) of reducing protons to hydrogen. The [Ru(bipy)3]3+ which is formed is then capable of... 

Crucial to the success of reactions designed to produce hydrogen via intermolecular electron transfer reactions is the addition of an efficient redox catalyst which allows reduction of protons by the reduced form of the relay (e.g. MV+) formed by the initial photochemical electron transfer. 

With the semiconductor oxidation catalyst, however, the surface becomes activated only upon photoexcitation. At low light intensities, the possibility that many holes are formed in the valence band is remote, so that the irradiated semiconductor powder becomes an effective one-electron oxidant. Now if the same chemistry ensues on the photochemically activated TiC>2 surface, then the reaction will proceed as in the bottom route of eqn 9. Thus, the carboxy radical is formed, producing an alkyl radical after loss of carbon dioxide. Since the semiconductor cannot continue the oxidation after the first step, the radical persists, eventually recapturing the conduction band electron, either directly or through the intervention of an intermediate relay, perhaps superoxide. The resulting anion would be rapidly protonated to product. 

Figure 1. Basic features of sacrificial water reduction systems. (A) Homogeneous solution, with sensitizer, S, electron relay, R, sacrificial electron donor, D, and metal catalyst. (B) Catalyst-coated colloidal semiconductor dispersion, obviating the need for electron relay.

The design of such artificial photosynthetic systems suffers from some basic limitations a) The recombination of the photoproducts A and S+ or D+ is a thermodynamically favoured process. These degra-dative pathways prevent effective utilization of the photoproducts in chemical routes, b) The processes outlined in eq. 2-4 are multi electron transfer reactions, while the photochemical reactions are single electron transformations. Thus, the design of catalysts acting as charge relays is crucial for the accomplishment of subsequent chemical fixation processes. 

The intramolecular alkylation of the enolate derived from phenylalanine derivatives 22a,b to form P-lactams 23a,b has also been achieved using Taddol as a chiral phase-transfer catalyst (Scheme 8.11) [23]. In this process, the stereocenter within enantiomerically pure starting material 22 is first destroyed and then regenerated, so that the Taddol acts as a chiral memory relay. Taddol was found to be superior to other phase-transfer catalysts (cinchona alkaloids, binol, etc.) in this reaction, and under optimal conditions (50 mol % Taddol in acetonitrile with BTPP as base), P-lactam 23b could be obtained with 82% et. The use of other amino acids was also studied, and the... 

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