Catalyzed photoinduced electron transfer

Trimethylsilyl triflate (McsSiOTf) acts as an even stronger Lewis acid than Sc(OTf)3 in the photoinduced electron-transfer reactions of AcrCO in dichloro-methane. In general, such enhancement of the redox reactivity of the Lewis acid complexes leads to the efficient C—C bond formation between organosilanes and aromatic carbonyl compounds via the Lewis-acid-catalyzed photoinduced electron transfer. Formation of the radical ion pair in photoinduced electron transfer from PhCHiSiMes to the (l-NA) -Mg(C104)2 complex (Scheme 11) and the AcrCO -Sc(OTf)3 complex (Scheme 12) was confirmed by the laser flash experiments [113]. 

Figure 4A. Dependence of kcl on [HC104] for the acid-catalyzed photoinduced electron transfer from [Ru(bpy)3]2 + to acetophenone in the presence of HC104 in MeCN at 298 K [40].

The ke[ values of photoinduced electron transfer reactions from [Ru(bpy)3]2 + to various nitrobenzene derivatives in the presence of 2.0 mol dm-3 HC104 are listed in Table 1, where the substituent effect is rather small irrespective of electron-withdrawing or donating substituents. A similar insensitivity to the substituent effect is also observed in the acid-catalyzed photoinduced electron transfer from [Ru(bpy)3]2+ to acetophenone derivatives [46,47]. The stronger the electron acceptor ability is, the weaker is the protonation ability, and vice versa. Thus, the reactivity of substrates in the acid-catalyzed electron transfer may be determined by two reverse effects, i.e., the proton and electron acceptor abilities, and they are largely canceled out. Such an insensitive substituent effect shows a sharp contrast with the substituent effect on the acid-catalyzed hydride transfer reactions from Et3SiH to carbonyl compounds, in which the reactivity of substrates is determined mainly by the protonation ability rather than the electron acceptor ability. 

Protonated flavin and pteridine analogues formed in the presence of perchloric acid can also function as efficient photocatalysts for the photooxidation of benzyl alcohols by oxygen via H+-catalyzed photoinduced electron transfer [307-309]. In contrast to the case of flavin and pteridine analogues, the triplet excited state of coenzyme pyrroloquinolinequinone (PQQ) by itself can oxidize benzyl alcohols to the corresponding aldehydes via photoinduced electron transfer, since the oxidizing ability of the triplet excited state of PQQ is much higher than that of flavin and pteridine analogues [310],... 

Keywords Mg2+-catalyzed Diels-Alder reactions, anthracenes, p-benzoquin-one, photoinduced electron transfer... 

The oxidative behaviour of the acridinium carbocations 61 was also explored by the group of Lacour in the photoinduced electron transfer reaction [160]. In the amount of 2 mol%, the achiral hindered acridinium salt 61 catalyzed the aerobic photooxidation of the primary benzylic amine to benzylimine in the yield of 74% (Scheme 63). 

Despite these apparent difficulties, there are now a number of examples for photoinduced electron transfer reactions that are significantly catalyzed. It is the purpose of this chapter to present fundamental concepts and the application of catalysis of photoinduced electron transfer reactions. The photochemical redox reactions, which would otherwise be unlikely to occur, are made possible to proceed efficiently by the catalysis on the photoinduced electron transfer steps. First, the fundamental concepts of catalysis on photoinduced electron transfer are presented. Subsequently, the mechanistic viability is described by showing a number of examples of photochemical reactions that involve catalyzed electron transfer processes as the ratedetermining steps. 

A significant enhancement of reactivity of the carbonyl compound by complexation with Mg2+ has also been applied to a novel type of carbon-carbon bond formation via photoinduced electron transfer from unsymmet-rically substituted acetal (5) with benzophenone (6) (Scheme 26) [211]. This photochemical reaction takes place in the absence of Mg2+ in MeCN. However, the yield of the desired carbon-carbon coupling product 7 is only 15% together with radical dimers 8 (28%) and 9 (2%). Addition of Mg(C104)2 to this system results in a much higher yield of 7 (e.g., 78%) at the expense of radical dimer formation [211]. Thus, the initial photoinduced electron transfer may be catalyzed by Mg2+ to produce a radical ion pair (6 "-Mg2+5 +), where 6 is stabilized by the complexation with Mg2+, as shown in Scheme 26 [211]. The efficient C-C bond formation occurs in the radical ion pair, followed by cyclization before and after desilylation to produce various types of products (Scheme 26). 

Metal cation-catalyzed photoadditions of radical species to radical cations of electron rich alkenes have been reported. Lewis found that the radical cation of norbornene generated by photoinduced electron transfer from this alkene to Ag(I) reacts with acetonitrile to produce 2-cyanomethylnorbomane (Scheme 42)... 

Photochemical reactions involving photo-excited states can also be catalyzed as well as the thermal reactions of ground states. However, the lifetimes of excited states are usually very short, particularly for the singlet excited states, and accordingly reactions of the excited state should be fast enough to compete with the decay of the excited state to the ground state (typically the lifetime is 10" -10 " s). Hence there seems to be little chance of catalysis to accelerate the reactions of excited states, which are already fast. There are many cases, however, such that photochemical reactions can be accelerated by some added substances which act as catalysts in the photochemical reactions [62-65]. Photoinduced electron transfer reactions can also be accelerated by the presence of an appropriate catalyst [52]. 

This chapter is intended to focus on catalysis in both thermal and photoinduced electron transfer reactions between electron donors and acceptors by investigating the effects of an appropriate substance that can reduce the activation barrier of electron transfer reactions. It is commonly believed that a catalyst affects the rate of reaction but not the point of equilibrium of the reaction. Thus, a substance is said to act as a catalyst in a reaction when it appears in the rate equation but not in the stoichiometric equation. However, autocatalysis involves a product acting as a catalyst. In this chapter, a catalyst is simply defined as a substance which affects the rate of reaction. This is an unambiguous classification, albeit not universally accepted, including a variety of terms such as catalyzed, sensitized, promoted, accelerated, enhanced, stimulated, induced, and assisted. Both thermal and photochemical redox reactions which would otherwise be unlikely to occur are made possible to proceed efficiently by the catalysis in the electron transfer steps. First, factors that accelerate rates of electron transfer are summarized and then each mechanistic viability is described by showing a number of examples of both thermal and photochemical reactions that involve catalyzed electron transfer processes as the rate-determining steps. Catalytic reactions which involve uncatalyzed electron transfer steps are described in other chapters in this section [66-68]. 

Acid-catalyzed electron transfer plays an important role in reduction of not only carbonyl compounds but also other substrates such as O2 [90, 91], N02 [92], nitrobenzene derivatives [93, 94], nitrosobenzene derivatives [93, 94] and sulfoxides [95, 96], The ket value for the photoinduced electron transfer from [Ru(bpy)3] + to nitrobenzene increases parabolically with increase in [HCIO4] [94]. This indicates that PhN02 is doubly protonated in the photoinduced electron transfer reaction to give PhN02H2 + (Eq. 9) ... 

Figure 29. Z-scheme of the photoinduced electron-transfer and dark enzymatic reactions operating in the photosynthesis of green plants. Mn = Mn-containing enzyme complex catalyzing water oxidation and O2 evolution Chi a and Chi b = photoactivated primary electron acceptors in photosystems I and II, respectively A and I = primary electron donors in photosystems I and II, respectively ADP = adenosine diphosphate ATP = adenosine triphosphate.

The double bond migration or cis-trans isomerization of linear pentenes catalyzed by a variety of transition metal complexes (Fe(CO)s, Fe3(CO)i2, Ru3(CO)i2) in the presence of irradiation illustrates the operation of case 1.3 [20, 21] (Scheme 3). Case 1.4, which covers photoinduced electron transfer... 

We thank P. Freeman for helpful discussions of photoinduced electron transfer from amines to halocarbons. We thank R. Wever and A. Butler for useful information and exchanges related to haloperoxidase-catalyzed reactions. We thank J. Hoigne and J. Leenheer for providing samples of NOM isolated from the Greifensee in Switzerland and from the Suwannee River in southern Georgia. 

Intramolecular photoinduced electron transfer (PET) may lead to the generation of acid (i.e., protons) from an anthracene-based sulphonium salt. An amphiphilic anthracene-based photoacid generator (An-PAG) has been synthesized and was reported to induce acid-catalyzed hydrolytic degradation of certain lipids upon UV exposure (Shum et al., 2001). At present, the application of this technique is limited by the short wavelength required to trigger the response. 

CONTENTS Preface, Patrick. Mariano. Hole and Electron Transfer Catalyzed Pericyclic Reactions. Nathan L. Baufd. Mechanisms and Catalysis In Electron Transfer Chemistry of Redox Coenzyme Analogues, Shunichi Fukuaumi. Electron T ransfer Chemistry of Monoamine Oxidase. Richard B. Silverman. Photol-yase. DNA Repair by Photoinduced Electron Transfer, Aziz San-car. Index. R T7... 

CONTENTS Preface. Patrick S. Mariano. Recent Advances In Light-Induced Election Transfer Involving Inorganic Systems. Nick Serpone, Rita Terzian and Jean Marie Hermann. Photoinduced Electron Transfer in Dye-Polymer Conjugates, Guilford Jones. Sequential Electron Transfer in Oxidation Reactions Catalyzed by Cytochrome P-450 Enzymes, Peter Guengerich and Timonty L Macdonald. Inner Shell Relaxation Effects on Electron Transfer Reactions of Amino Centered Systems, Stephen F. Nelsen. Index. s s... 

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