Chiral electron donors

Their controlled formation can be utilized to control the course of the chemical reaction. In this context the chiral discrimination of PET processes of a chiral electron acceptor and (pro)chiral electron donors is of special interest We have observed such a discrimination in case of the isomerization of 1,2-diary Icyclo-propanes [122] and, for the first time, in case of a bimolecular PET process, e.g. the dimerization of 1,3-cyclohexadiene in presence of (+) and (—) l,l -bi-naphthalene-2,2 -dicarbonitrile as chiral electron acceptors [123]. Experiments in the same field are undertaken by Schuster and Kim and have been published recently [124], So far the enantiomeric excesses are small (ca. 15% [124] in toluene at —65 °C) but future efforts will certainly give more information about the applicability of catalytic asymmetric PET reactions. Consequently, the conditions which govern the formation and the fate of radical ion pairs are of central importance both for a better understanding of the mechanism and for synthetic applications. 

Small chiral molecules. These CSPs were introduced by Pirkle about two decades ago [31, 32]. The original brush -phases included selectors that contained a chiral amino acid moiety carrying aromatic 7t-electron acceptor or tt-electron donor functionality attached to porous silica beads. In addition to the amino acids, a large variety of other chiral scaffolds such as 1,2-disubstituted cyclohexanes [33] and cinchona alkaloids [34] have also been used for the preparation of various brush CSPs. 

The [Fe-Cp]-fragment does not only play the role of an additional steric element introducing planar chirality into the otherwise flat pyridine system. Substitution at the pyridine 2-position usually cuts the nucleophilicity of the nitrogen atom thus limiting the possibilities to achieve efficient chirality transfer using nucleophilic pyridine catalysts [84]. Ferrocene, however, functions as a strong electron donor (see Sect. 1) and thus restores the nucleophilicity impaired by substitution. 

While lanthanide triflates have been demonstrated to promote the reaction of indoles with imines <99SL498>, Johannsen has developed a new synthesis of optically active p-indolyl N-tosyl a-amino acids 110 via the enantioselective addition of A-tosylimnio esters of ethyl glyoxylate 109 to indoles 108 bearing both electron-donor and electron-acceptor substituents at C-5 using 1-5 mol% of a chiral copper(I)-Tol-BINAP catalyst <99CC2233>. 

Frequently, the general nature or detailed structure of an intermediate is inferred from reaction products. Radical ions are invoked in reactions between electron donors and acceptors in polar solvents. Probing the fate of chirality, stereochemistry, or an isotopic label or substituent in the products of a chemical transformation exemphfies the classical approach to mechanism. Of course, this approach is not without shortcomings. 

Suzuki et al. reported the photochemical reaction of CT crystals, in which cycloaddition reaction of bis(l,2,5-thiadiazolo)tetracyano-quinodimethane 17 (electron acceptor) and 2-divinylstylene 18 (electron donor) is efficiently induced (Scheme 3). [17] A structural feature of the CT crystal is the asymmetric nature of the inclusion lattice because of the adoption of a chiral space group, P2. The [2 + 2] photoadduct 19 was formed via the single crystal-to-single crystal transformation, and the optically active product with 95% ee was obtained. 

In this enantiodifferentiating photoreduction, the chiral amine plays two roles, as a chiral inductor and as an electron donor. Irradiation of 25 (Scheme 10) in a hexane slurry of unmodified NaY zeolite gave only the intramolecular hydrogen abstraction product 26. However, photolysis of 25 coimmobilized with ephedrine, pseudoephedrine, or norephedrine in NaY supercages afforded the reduction product 27 along with 26. It is clear that the immobilized amine plays the decisive role in the photoinduced electron-transfer reduction of 25, since 27 was not formed in unmodified or (— )-diethyl tartrate-modified zeolites. Consequently, the ee of obtained 27 was independent of the loading level of the chiral inductor. 

We have used chiral cocrystals composed of two different molecules [10]. When one of the two components is chiral, a cocrystal of a chiral space group is necessarily obtained. Combination of an electron donor molecule with an electron acceptor molecule, or a photoinert molecule with a photosensitizer molecule, in a cocrystal can induce new photoreactivity as well as intermolecular photoreaction. 

Absolute asymmetric [2 + 2] photocycloaddition in a crystalline CT complex was also found. A series of 1 1 CT crystals of bis[1,2,5]thiadiazolotetra cyanoquinodimethane (BTDA) 148 as an electron acceptor was obtained by mixing with neat arylolefins such as styrene, o-, m-, or /7-divinylbenzene as electron donors. X-ray crystallographic analyses of the CT crystals revealed the chiral nature of the red crystals formed between 148 and o-divinylbenzene oDV due to their belonging to space group P2, while the other cases were achiral. 

Figure 4 The ratio of the intermolecular to intramolecular hydrogen abstraction products from phenyl cyclohexyl ketone within NaY depended on the nature of the electron donor. The GC traces of the product mixture are shown above. indicates the number of chiral inductor molecules per cage.

For further information on synthesis, properties, and applications of pyrro-lidinofullerenes, the chiral representatives of which were generally prepared and used as racemates, we refer to a recent review by Prato and Maggini.214 Fields in which many fullerene-fused pyrrolidines have been studied are biological and medicinal chemistry351 and, above all, advanced materials science.352,353 In the latter context, dyads and triads used for photoinduced charge separation between a fullerene acceptor moiety and electron donors like porphyrins, tetrathiafulvalenes, ferrocenes, or polyenes, are worth particular mentioning.354-356... 

A similar scheme was reported by Pino, based on the stereoregulating effect of electron donors and on the stereoelectivity obtained in the polymerization of racemic a-olefins in the presence of chiral bases 109). 

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