Chiral induction displaying

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. 

Fig. 6 Chiral and achiral rr/.s-amides that display chiral induction phenomena and a representation of the n-stacked aggregates that the) form. (From Ref. [17].) The authors thank Dr. "Bert" Meijer for supplying these images.

Treatment of a number of covalent polymers substituted with molecular recognition capability with suitable guests leads to chiral induction. The same crown ether-amino acid complementary pair described for the rosettes above was employed in the form of crown-ether pendant cis-transoidai poly(phenylacetylene). When the achiral polymer is treated with amino acids (in the form of their hydroperchlorate salts in acetonitdle) a large induced CD signal is observed in the backbone of the polymer. The polymer is sensitive to small enantiomeric excesses in the amino acid, as little as 0.005% enantiomeric excess of alanine can be detected. In a similar vein, c/5-transoidal poly(carboxyphenylacetylene) shows induced circular dichroism when treated with nonracemic chiral amines In addition, the system displays chiral memory, in that treatment of the complex with achiral amino alcohols results in retention of the chiral polymer backbone. 

SUPRAMOLECULAR ASSISTANCE IN REACTIONS DISPLAYING CHIRAL INDUCTION... 

Finally, the catalytic activity of chiral polymer poly(+)(MASBF)4PFeCl bearing four chiral SBF units in meso position of the porphyrin ring was investigated. However, this first example of chiral porphyrin based on SBF scaffold does not display any enantioselectivity (ee < 5 %) and even displays a low epoxidafion yield of 30 % [73]. Different alkenes such as styrene, 4-chlorostyrene, 4-methylslyrene or cyclooctene were tested but no chiral induction was observed. This might be due to the structure of the SBF unit, which leads in both monomer and polymer to a large chiral cavity and thus no asymmetric synthesis was observed. 

From these experiments it appeared that only the axial chirality of the binaphthyl scaffold of L5-a is responsible for the chiral induction. Therefore, the simplified ligand L5-b could be developed which displays an achiral gem-dimethyl substituted oxazoline ring (Fig. 10.7). This specifically designed ligand performs as good as, or even better than the analogous phosphino-oxazoUnes bearing two chirality elements. Authors also demonstrated that the double substitution of the 4-position of the oxazoline unit in L5-b is essential to the stereochemical control of the cycloisomerization reaction. 

Ricci and co-workers introduced a new class of amino- alcohol- based thiourea derivatives, which were easily accessible in a one-step coupling reaction in nearly quanitative yield from the commercially available chiral amino alcohols and 3,5-bis(trifluoromethyl)phenyl isothiocyanate or isocyanate, respectively (Figure 6.45) [307]. The screening of (thio)urea derivatives 137-140 in the enantioselective Friedel-Crafts reaction of indole with trans-P-nitrostyrene at 20 °C in toluene demonstrated (lR,2S)-cis-l-amino-2-indanol-derived thiourea 139 to be the most active catalyst regarding conversion (95% conv./60h) as well as stereoinduction (35% ee), while the canditates 137, 138, and the urea derivative 140 displayed a lower accelerating effect and poorer asymmetric induction (Figure 6.45). The uncatalyzed reference reaction performed under otherwise identical conditions showed 17% conversion in 65 h reaction time. 

The degree of stereochemical control displayed by the first chiral center usually depends on how close it is to the second —the more widely separated they are, the less steric control there is. Another factor is the degree of electronic control. If all the groups are very much the same electrically and steri-cally, not much stereochemical control is to be expected. Even when the chiral centers are close neighbors, asymmetric induction is seldom 100% efficient in simple molecules. In biochemical systems, however, asymmetric synthesis is highly efficient. 

Among the many classes of chiral molecules, helical systems are particularly fascinating. Their structure is relevant to proposed mechanisms of handedness induction in relation to chiral amplification [76], Helicenes ([A]-H) are helical molecules formed from A-ortho-fused benzene rings (Fig. 8) which display considerable rotatory power [77]. Helicenes are presently the subject of intense synthesis efforts that try to functionalize these molecules in order to attain enhanced electric, magnetic, and optical properties [78, 79]. Phenylenes ([A]-P), or heliphenes, constitute another class of helical aromatic compounds for which syntheses have recently been reported [80, 81]. They are made up of N benzene rings fused together with N - 1 cyclobutadiene rings (Fig. 8). 

The DlOP-rhodium(I) complex attached to organic polymers , e.g., polystyrene resin and poly(methyl vinyl alcohol), exhibits good catalytic activity as a chiral catalyst comparable to the corresponding homogeneous catalyst. In contrast, the rhodium(I) complexes anchored on inorganic supports display only a low efficiency . Studies show that the steric requirements for a match of the chiral ligand, a hydrosilane and a ketone are of definite importance in bringing about effective asymmetric induction. 

The most obvious stimulus to ehiral induction is the use of stereogenie eenters, but external perturbation, such as physical fields ean also be used to induce chirality. The term is applied to traditional asymmetric synthesis through covalent bonds as well as to supramolecular synthesis, which is our interest here. Mention the word "chirality" to any chemist, and thoughts will be conjured of the right-handed B-DNA double helix, the right-handed a-helices formed by peptides of the natural L-amino acids, of thalidomide, and other emblematic and dramatic examples of the importanee of stereochemistry. It is clear that the chirality of the eomponents of biological systems play a key role in their function, in which induction of chirality through noncovalent bonds is inherent. But beyond natural systems and related phenomena, there is a wealth of unnatural chemical systems that display remarkable and important properties. 

The induction of chiral structure in supramolecular systems at equilibrium was widely studied, partially because of surging interest in the hierarchical nature of the self-assembly of chiral aggregates. It is also of utmost relevance in the study of chiral information in unnatural systems displaying new properties. 

The efficiency of synthesized chiral azolium salts (260)-(262), derived from (5)-pyroglutamic acid, as carbene precursors was evaluated in the [Rh(cod)Cl]2-catalysed asymmetric transfer hydrogenation of aromatic ketones in isopropanol, acting as the hydrogen donor, and KOH as promoter to the corresponding alcohol. It was reported that the use of (262) displayed the highest activity and asymmetric induction for the transfer hydrogenation. The yield was up to 94% and enantioselectivities up to 90% ee were observed. ... 

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