Chiral electron acceptors

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. 

Fluorescence studies have shown that the radiative lifetime of 25f increases upon increasing DP, suggesting that the mobile excitons move through the supramolecular polymers and relax at their ends.139 Insertion of electron acceptors between the triphenylenes accounts for the formation of longer polymers and increases the order within the column. An X-ray diffraction ring with a diffraction spacing of 3.5 A indicates a short intermolecular distance, a feature not present for undoped samples.140 A chiral electron acceptor resulted in the formation of a cholesteric mesophase. 

Perhaps one of the most important applications of chiral induction is in the area of liquid crystals. Upon addition of a wide range of appropriate chiral compounds, the achiral nematic, smectic C, and discotic phases are converted into the chiral cholesteric (or twisted nematic), the ferroelectric smectic C and the chiral discotic phases. As a first example, we take the induction of chirality in the columns of aromatic chromophores present in some liquid-crystalline polymers. " The polymers, achiral polyesters incorporating triphenylene moieties, display discotic mesophases, which upon doping with chiral electron acceptors based on tetranitro-9-fluorene, form chiral discotic phases in which the chirality is determined by the dopant. These conclusions were reached on the basis of CD spectra in which strong Cotton effects were observed. Interestingly, the chiral dopants were unable to dramatically influence the chiral winding of triphenylene polymers that already incorporated ste-reogenic centers. 

Acetyl naphthalene 24 on photoirradiation undergoes [4 + 2]-cycloaddition reaction with chiral electron acceptor alkene, (5)-(2-methoxy methyl-1-... 

By using the chiral electron acceptor (-)-TAPA, even the cholesteric versions of the nematic discotic phase Ng or of the nematic columnar one Ncoi can be obtained, for example, by doping a nematic discotic pentayne ether with up to 30 mol% (-)-TA-PA or by inducing the N oi phase in a ternary mixture composed of pentayne ether (4) (n = 16) with TNF and (-)-TAPA the latter electron acceptor does not itself lead to the induction of a columnar mesophase with such pentayne donors [26, 59 d]. 

Pu, L.S., A new chiral electron acceptor for nonlinear optical materials, J. ChertL Soc. Chem. CommurrL, 429, 1991. 

Lyotropic nematic phases (see Section A) can also be produced by preparing, for instance, binary or ternary mixtures of organic disc-like compounds in suitable solvents such as hydrocarbons [20]. In linear saturated [20,21] or, as found recently [21], even better in cyclic saturated hydrocarbons, preferably cyclohexane [21], almie or in such a solvent plus an achiral or a chiral electron acceptor compound, induction of lyotropic Ncd or N coi phases, respectively, can occur. Sometimes, an Ncd phase can be formed in addition to a columnar phase [21]. Furthermore, it has also been observed that even two different No>i phases can be induced in diat way in the same system [22,23] showing a nematic-nematic phase transition [22-24] due to a diffa ence in the construction of their columns. In one of these Ncoi phases the constituent discs of the columns spontaneously formed are tilted with respect to the column axis, but in the second, parallel Ncoi phase they are untilted [22,23]. However, reliable data about the length of the columns in Ncoi phases do not yet seem to exist... 

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. 

Induction of chirality in No phases was also shown to be possible using charge transfer interactions, via jt-jt stacking. The binary mixture of mesogen 8, which is electron rich, with chiral electron-deficient molecule 9 was shown to induce a twist in the mesophase.15 Furthermore, nonmesogenic 8 gave rise to a cholesteric mesophase, denoted as being of the columnar type (N ), when present in a ternary system with TNF (trinitrofluorenone, an electron acceptor)... 

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>. 

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>. 

In this paper, I propose a promising new electron acceptor of cyclobutenedione for nonlinear optical materials to prevent centrosymmetric crystal structures by the introduction of chirality and hydrogen bonding property into the acceptor itself. Compared with electron donative groups, electron acceptor is not yet well studied for nonlinear optical materials. The most commonly used electron acceptor is nitro (NO2) group. Therefore, we evaluated the possibility of cyclobutenedione as a new electron acceptor for nonlinear optical materials. One of the most simple cyclobutenediones is squaric acid. Squaric acid is known to be soluble in water and show very strong acidityQ2), as squarylium anion formed in water has a stable 2n delocalized electron system as shown below. 

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. 

Kimura, Shirai and coworkers used two chiral dimeric porphyrins 95 and 96 to investigate their self-assembling behavior [162,163]. While incorporation into fibers made of the alkylamide derivatives of (fl,fl)-DACH, 95 formed stable well-resolved fibrous assemblies as visualized by transmission electron microscopy, the fluorescence of which was not quenched by external electron acceptors [162]. However, the induced CD was not detected indicating an inability of 95 to form chirally orientated aggregates under the applied conditions. In contrast, 96 was able to produce optically active inter molecular self-assemblies with an enhanced chiroptical response through the //-oxo bridging in an alkali solution, while intramolecular //-oxo dimer formation was excluded on the basis of steric reasons [163]. 

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