Stereoselectivity in Electron Transfer

To perform a highly stereoselective photoinduced electron transfer reaction, we need an optically pure photosensitizer and a substrate that do not cause photo-induced racemization. In addition, we need the substrate of which the self-exchange reaction occurs slowly or does not occur. If not, the stereoselectivity disappears, as was mentioned above. 

In this chapter, we will discuss what type of chiral photosensitizer is applied to stereoselective photoinduced electron transfer reaction, what type of substrate is used, and what type of chiral functional group is introduced to these compounds to construct a chiral photosensitizer. We will describe also the stereoselective photocatalytic reactions. 

As is well known, the electron transfer reaction deeply relates to the function of metalloenzymes. In this regard, many experimental studies have been carried out concerning the electron transfer reactions of metalloenzymes [55]. Since metalloenzymes involve many amino acid residues, we expect that the stereoselectivity should be observed in electron transfer reactions. In spite of this expectation, reports of the stereoselective electron transfer reactions of metalloenzymes have been limited, so far. Here, we wish to mention several works in which thermal stereoselective electron transfer reaction was investigated. 

The combined systems of the photosensitizer with micelle, LB film, protein, and so on are interesting, because we can expect that new functions will be found in such systems. In some pioneering works, such ideas were applied to stereoselective photoinduced electron transfer reactions. We believe that one can construct the new photoreaction with such combined systems and also expect the enhancement of stereoselectivity in such systems. 

When one thinks about only two-electron reduction of a substrate (A), the reduction and protonation give nine species at different oxidation and protonation states as shown in Scheme 35. Each species can have an interaction with a metal complex (M +-L) and such an interaction can control each redox step. Moreover, the interaction between the ligand L and a substrate has the possibility of controlling not only the reactivity but also the stereoselectivity of the redox reaction. With regard to multi-electron reduction or oxidation of a substrate, the much more redox and protonated or deprotonated states should be considered for the interaction with metal complexes. The scope and the application of catalysis in electron transfer are thereby expected to expand much further in the near future. 

Stereoselective Effects in Electron Transfer Reactions Involving Synthetic Metal Complexes and Metalloproteins... 

Provided electron transfer between the electrode and solute species is not interrupted by the coating, even electroinactive films can offer interesting applications. Thus, a chiral environment in the surface layer may impose stereoselectivity in the follow-up reactions of organic or organometallic intermediates. Furthermore, polymer layers may be used to obtain diffusional permeation selectivity for certain substrates, or as a preconcentration medium for analyzing low concentration species. 

Since the seminal contributions by Nugent and RajanBabu the field of reductive C - C bond formation after epoxide opening via electron transfer has developed at a rapid pace. Novel catalytic methodology, enantio- and stereoselective synthesis and numerous applications in the preparation of biologically active substances and natural products have evolved. In brief, a large repertoire of useful and original reactions is available. These reactions are waiting to be applied in a complex context ... 

The multi-component systems developed quite recently have allowed the efficient metal-catalyzed stereoselective reactions with synthetic potential [75-77]. Multi-components including a catalyst, a co-reductant, and additives cooperate with each other to construct the catalytic systems for efficient reduction. It is essential that the active catalyst is effectively regenerated by redox interaction with the co-reductant. The selection of the co-reductant is important. The oxidized form of the co-reductant should not interfere with, but assist the reduction reaction or at least, be tolerant under the conditions. Additives, which are considered to contribute to the redox cycle directly, possibly facilitate the electron transfer and liberate the catalyst from the reaction adduct. Co-reductants like Al, Zn, and Mg are used in the catalytic reactions, but from the viewpoint of green chemistry, an electron source should be environmentally harmonious, such as H2. 

Epimerization at tin has occured during the substitution process. A one-electron transfer mechanism has been proposed here as for the reactions described in Sections 5.3, 5.4, and 5.8 to account for the absence of stereoselectivity. 

The stereoselective 1,4-addition of lithium diorganocuprates (R2CuLi) to unsaturated carbonyl acceptors is a valuable synthetic tool for creating a new C—C bond.181 As early as in 1972, House and Umen noted that the reactivity of diorganocuprates directly correlates with the reduction potentials of a series of a,/ -unsaturated carbonyl compounds.182 Moreover, the ESR detection of 9-fluorenone anion radical in the reaction with Me2CuLi, coupled with the observation of pinacols as byproducts in equation (40) provides the experimental evidence for an electron-transfer mechanism of the reaction between carbonyl acceptors and organocuprates.183... 

Tetrahydropyrrolo[l,4]oxazine 74, obtained by photoinduced electron-transfer (PET) oxidative activation of substituted prolinol, undergoes nucleophilic substitution of the OH at position C-3 with allyltrimethylsilane in the presence of TiCU (Scheme 8). The reaction was highly stereoselective and produced, after hydrolysis of the resultant amide 75, optically active a-hydroxy acid 76 together with the auxiliary (.S )-prolinol that can be effectively recycled <1998TL7153>. 

The oxazinones 74 and 79, already described as chiral glycine templates in Section 11.11.6.3, have been prepared by the PET cyclisation of 252 by irradiation in the presence of 1,4-dicyanonaphthalene as the electron acceptor and methyl viologen as electron-transfer mediator. When the reaction was carried out under strictly anhydrous conditions, compound 79 was isolated, whereas when the reaction was carried out in wet MeCN, compound 74 was the exclusive product (Scheme 33). In any case, the products were obtained with high stereoselectivity, which is the condition required to use them as chiral auxiliaries <2000EJ0657>. 

Electron transfer sensitization allows either the radical cation or the radical anion of an aromatic alkene to form as desired, which finally results in nucleophile addition with Markovnikov and anti-Markovnikov regiochemistry. In an apolar solvent, the tight radical ion pair undergoes a stereoselective reaction when the electron-accepting sensitizer is chiral (Figure 3.10). ... 

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