Chiral dilution effect

Let us consider an N phase doped with BSMs, where BSMs interact with chiral host molecules. The interaction energy between left-handed bent-core conformation and the chiral host molecule f/LH is different from that between right-handed bent-core conformation and the same chiral host molecule t/RH- The nonzero difference AU = f/LH Crh induces finite ee in BSMs, resulting in increased chiral molecules in the system. At the same time, we have to consider the dilution effect. 

The possible role of SEs in conjunction with the above high energy polarized electron experiments was first noted by Walker. He pointed out that any chiral effects of the high energy electrons would be seriously diluted by the unpolarized SEs produced in the samples [84]. Subsequent years saw the research into electron-induced chiral-specific reactions shift to the use of low energy electrons. 

The chirality of the pretransition liquid crystal is the same as that of the dilute solution of the respective polymer. As the temperature is increased, the optical activity is reduced in a continuous fashion to that observed in dilute solution, without change of sign. This effect is observed in CH2CI2, 1,2-dichloroethane (EDC), pyridine, and 2-chloropyridine. 

The chirality of the pretransition phase is opposite to that of the dilute solution. As the temperature increases, ORD curves reverse sense from predominantly positive to negative for PBLG solutions, and from predominantly negative to positive for PBDG solutions. This effect is observed in dioxane, DMF, tetrahydrofuran, 1,1,2,2-tetrachloroethane, and 1,3-dioxolane. 

Columnar stacks of 3,4,5-trialkoxy-benzoylated 3,3 -diamino-2,2 -bipyridine triamide derivatives of benzene-1,3,5-carboxylic acid (87, 88) have recently been studied in dilute alkane (dodecane) and chloroform solution (Chart 14). In chloroform, neither the derivative with hexyloxy tails (88) nor the one with chiral citronellol tails (87) showed Cotton effects in the CD spectrum. TTie chiral derivative 87 showed such effects in dodecane, whereas the achiral derivative 88 showed them in the chiral solvent, (/7)-2,6-dimethyloctane. Interestingly, addition of a small amount (2.5%) of chiral 87 to stacks of achiral 88 in hexane induced a strong Cotton effect, similar in magnitude to that of the pure chiral derivative in hexane. This is an example of the sergeant and soldiers effect in a noncovalent assembly, and it is similar to the effects already established for stiff helical polymers like polyiso-... 

Determination of enantiomeric purity. Three mL of the solution was diluted in 50 mL of acetone and treated with 0.3 g of potassium permanganate and 0.32 g magnesium sulfate to effect oxidation of the product aldehydes to their respective acids. The mixture was stirred at room temperature for 30 minutes after which time the solvent was removed under reduced pressure. The residue was extracted three times with 50 mL of hot water. The three aqueous solutions were then combined, filtered, and washed with 50 mL of chloroform. The aqueous layer was acidified with hydrochloric acid to a pH of 2 and then extracted with 50 mL of chloroform. The chloroform was removed in vacuo and the resulting residue dissolved in 0,5 mL of toluene. This solution was analyzed by GC on a chiral /3-cyclodextrin column which separated the two enantiomers of the resulting 2-arylpropanoic acid. This analysis indicated a 91 9 ratio of the S and R enantiomers for an ee of 82%. 

Use has also been nude of the fact that chain fragments within a crosslinked polymer have restricted mobility. Under certain conditions active species attached to die polymer can thus be effectively isolated from each other at relatively high concentrations, providing the advantages of high dilution and specificity along with rapid kinetics. In other cases, properties of die backbone itself such as polarity, pore size and chirality were utilized to achieve unique reactions, the polymer providing a specifrc microenvironment for the reaction. These aspects of PRs have been extensively reviewed (6.7). 

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