Solvent chiral

A second example of homochiral columns formed by discotics are the complexes of tetrazoles (59 and 60) with l,3,5-tris(4,5-dihydroimidazol-2-yl)benzene (61).74 Four molecules self-assemble to give a supramolecular disc and these discs subsequently form columns in nonpolar solvents. Chiral discs were obtained from the self-assembly of the chiral tetrazole (60) with 61. The chirality of the side chains was found to induce a bias in the helic-ity of the supramolecular assembly. Sergeants-and-soldiers measurements75 were performed for which chiral (60) and achiral (59) molecules were mixed. The experiments showed no amplification of chirality, thus revealing that in these systems chirality transfer from the side chains into the column is... 

Figure 7.15 Chirality transfer from dopant to solvent. Chiral inducer with an (M)-helidty aligned with its biaryl axis parallel to biphenyl axis of solvent can have close contact only with molecules of solvent having same helicity Chirality is therefore transferred from dopant to near solvent molecule and from this to next near one and so on, via chiral conformations.

In principle, separation of resonances of diastereomeric compounds (such as dl and meso isomers) may be increased simply through use of an appropriate achiral solvent. Chiral solvents may in some cases be especially effective in producing a separation, particularly if the diastereomers differ in configuration about a center that is amenable to analysis by the CSA method. Kaehler and Rehse (89) give a detailed account of conditions necessary for measurement of the ratio of meso- and dZ-tartaric acid employing A,N-dimethyl PEA. Bomyl acetate used as solvent for l,2-difluoro-l,2-dichloroethane (90) allows measurement of the diastereomeric composition. Paquette and co-workers (91,92), using TFAE, were able to determine the diastereomeric purity of the recrystallized adducts 47 of... 

The asymmetric epoxidation of several chalcones (39) and other electron-poor olefins in a triphase system (water/organjc solvent/chiral polyamino acid) afford optically active oxirans with optical yields of up to 96%. The influence of the molecular structure of the catalysts and substrates, the solvent, and the temperature on the stereochemistry was investigated by a group of chemists from Italy and Spain 77). 

Like in other chiroptical switches (Section 5.3.1), solvent polarity was found to play an important role. Diastereoselective cyclization was observed in THF and toluene, but not in nonpolar solvents such as n-hexane. Upon photoexcitation, diarylethenes 24 (Scheme 11) can adopt a planar and a twisted conformation, and photocyclization only proceeds through the planar conformation. In the case of chiral diarylethene 27a, there are two diastereomeric planar conformations leading to the diastereomers of the cyclic product 27b. The stereoselectivity in the photocyclization process is enhanced because of a decrease in the excited state energy of the unreactive twisted form, providing a relaxation pathway for the less favorable planar diastereoisomer in more polar solvents. Chiral photochromic diarylethenes are among the most prominent photoswitches known today, featuring nondestructive read-out, excellent reversibility, and the potential for construction of switchable molecular wires and modulation of liquid crystalline phases (see Section 5.5.3).[40,411... 

El-Hachemi Z, Escudero C, Arteaga O et al (2009) Chiral sign selection on the J-aggregates of diprotonated tetrakis-(4-sulfonatophenyl)porphyrin by traces of unidentified chiral contaminants present in the ultra-pure water used as solvent. Chirality 21 408-412... 

Keywords Absolute configuration and conformation determinations Density functional theory Induced solvent chirality Transient VCD measurements Vibrational circular dichroism... 

Modeling Chiral Solute-Water H-Bonding Networks by Induced Solvent Chirality... 

The chiral sources employed include circularly polarized light, chiral sensitizers, chiral solvents, chiral substituents, chiral host-guest assemblies, and chiral crystalline environments. 

Lipases have been extensively used for the kinetic resolution of racemic alcohols or carboxylic acids in organic solvents. Chiral alcohols are usually reacted with achiral activated esters (such as vinyl, isopropenyh and trichloroethyl esters) for shifting the equilibrium to the desired products and avoiding problems of reversibility. For the same reasons, chiral acids are often resolved by using acidolysis of esters. In both cases, the overall stereoselectivity is affected by the thermodynamic activity of water of water favors hydrolytic reactions leading to a decrease in the optical purity of the desired ester. Direct esterifications are therefore difficult to apply since water formed during the reaction may increase the o of the system, favors reversibiUty, and diminishes the overall stereoselectivity. 

The two reagents (periodic acid and lead tetraacetate) are complementary, since periodic acid is best used in water and lead tetraacetate in organic solvents. Chiral lead carboxylates have been prepared for the oxidative cleavage of l,2-diols. ° When three or more OH groups are located on adjacent carbons, the middle one (or ones) is converted to formic acid. 

Another example of intramolecular CT complex formation is provided by trans-4-dimethvlamino-4 -(1-oxobutvl)stilbene Solvent effects on the spectrum give a value of 22D for the excited state dipole moment. The effect of electric field on the fluorescence of 4-(9-anthry1)-N.N.-2.3,5,G-hexamethy1-aniline shows this compound forms an excited state whose dipole moment does not change with solvent . Chiral discrimination in exciplex formation between 1-dipyrenylamine and chiral amines is very weak . In the probe molecule PRODAN (6-propionyl)-2-(dimethylamino)—naphthalene the initially formed excited state converts to a lower CT state as directly evidenced by time-resolved spectra in n-butanol. Rate constants for intramolecular electron transfer have been measured in both singlet and triplet states of covalently porphyrin-amide-quinone molecules . Intramolecular excimer formation occurs during the lifetime of the excited state of bis-(naphthalene)hydrazides which are used as photochemical deactivators of metals in polyethylene . ... 

Bathory, G., Meszaros, G., Gergely, A. Study on the sorption properties of al-acid glycoprotein (AGP)-based stationary phase modified by organic solvents. Chirality, 2003,15, 377-381. 

The reaction can be applied to silyl enol esters as well. Good asymmetric induction can be achieved in the Mukaiyama aldol reaction. The reaction of silyl enol thioether 246 and nonanal, for example, gave 247 in 60% yield and in 93% ee when the (/ )-BINOL-titanium catalyst shown was used. In this work, the reaction was also done in supercritical fluoroform and in supercritical carbon dioxide. A similar reaction was reported using catalysts closely related to 244 and dichloromethane as the solvent.Chiral oxazaborolidine catalysts have also been shown to be effective for enantioselective Mukaiyama aldol reactions. 

In un solvente chirale come due doppietti, uno per ciascuno dei due enantiomeri (in quanto avranno chemical shift diverse)... 

Figure 17.18 (p. 54) depicts transformations 71-72 where account is taken of the chiral influence (chiral solvent, chiral catalyst, "chiral" radiation or other chiral factor). In these... 

Debie E, Jaspers L, Bultinck P, Herrebout W, van der Veken B (2008) Induced solvent chirality a VCD study of camphor in CDQ3. Chem Phys Lett 450 426-430... 

The existence of cellulose as the common material of plant cell walls was first known by Anselm Payen in 1838. It occurs in almost pure form in wood rather than in cotton, where cellulose content was in combination with other materials, such as lignin and hemicelluloses. On the other hand, cellulose is also produced by some bacteria but free in lignin and hemicelluloses content. Cellulose has no taste, odourless, hydrophilic, insoluble in water and most organic solvents, chiral, and biodegradable. It can be broken down chemically into its glucose units by treating it with concentrated acids at high temperature (Charles, 2007). 

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