Optical activity solvent effects

Recently, the first example of chiral solvation of a polysilane was demonstrated dissolution of the inherently optically inactive poly(methylphenylsilyene), PMPS, and poly(hexylmethylsilylene), PHMS, in the optically active solvents (V)-2-methyl-l-propoxybutane and (V)-(2-methylbutoxymethyl)benzene induced the polymer chains to adopt PSS helical conformations as evidenced by (positive-signed) Cotton effects almost coincident with the UV a-a transition at 340 and 305 nm, respectively.332... 

Mustillo and CiurczakP" presented a paper discussing the spectral effect of optically active solvents on enantiomer mixes. This information was used as a technique to screen for polar modifiers in normal-phase chromatography of racemic mixtures. In 2000, the enantiomeric composition of ibuprofen in solid-state mixtures was performed by Agatonovic-Kurstrin, Beresford, Razzak. ... 

A special type of chemical shift non-equivalence arises when a racemic mixture is dissolved in an optically active solvent. Solvent-solute interactions would be expected to be different for the enantiomers and should show up as non-equivalence of corresponding nuclei in the two forms. The effects so far detected,are disappointingly... 

A recent interesting application of solvent effects has been the use of optically-active solvents in the determination of the optical purity and the absolute configuration of solutes. Work so far has centred on resonances and organic solutes, covering various alcohols, amines, sulphoxides, a-hydroxy- and a-amino-acids, and epoxides (the solvent here being an optically active nematic phase ). There are also reports on disymmetric nickel(II) complexes, and the use of resonances. ... 

The differing effects of hydrogen ion or of base concentration on thiocyanate substitution rates into rans-[CoCl2 enal and into trans-[CoCl(N02)en2]+ in methanol or ethanol support an earlier hypothesis that the mechanisms are not identical for these two complexes, but that there is considerable solvent assistance in the case of the chloronitro-compound. However, one should recall an earlier study in mixed aqueous alcoholic solvents which indicated no diflFerence in substitution mechanisms for these two complexes. Chloride and thiocyanate substitution at cis-[CoCl2diars]+ in methanol both involve formation of the same inter-mediate. Solvolysis of optical and geometrical isomers of [CoCl2en2]+ in an optically active solvent, propane-1,2-diol, results in stereospecific substitution. ... 

To estimate the reactivity ratios of new comonomer pairs, their Q and e values, as summarized in Ref [105], can be compared. The Q and e values are a measure of the reactivities and the polarities in a copolymer system. A special solvent effect has been described in the radical copolymerization of optically active acryloyl-D-phenylglycine methyl ester with MMA or MA in D- or L-ethylmandelate as optically active solvent. The rate of polymerization was higher in the D-ester [106]. 

Solute-solvent interactions can occasionally be put to good use, as in the spectral resolution of enantiomeric pairs. By themselves monostereoisomers have identical spectra but when dissolved in an optically active solvent such as D- (or L-)2-octanol (available commercially) yield slightly different spectra. This is because of the asymmetry of the respective interactions which effectively create diastereoisometric pairs. 

Applications of this method are similar to those of the ORD method since they both are based on the same effect, namely interaction between matter and light. CD allows determining the chirality of a chemical compound as well as enables studying the influence of various effects (solvent effect, temperature effects, and optical activity induced effect by a chiral medium) on its optical activity. 

Studies of reaction mechanisms ia O-enriched water show the foUowiag cleavage of dialkyl sulfates is primarily at the C—O bond under alkaline and acid conditions, and monoalkyl sulfates cleave at the C—O bond under alkaline conditions and at the S—O bond under acid conditions (45,54). An optically active half ester (j -butyl sulfate [3004-76-0]) hydroly2es at 100°C with iaversion under alkaline conditions and with retention plus some racemization under acid conditions (55). Effects of solvent and substituted stmcture have been studied, with moist dioxane giving marked rate enhancement (44,56,57). Hydrolysis of monophenyl sulfate [4074-56-0] has been similarly examined (58). 

The evidence presented so far excludes the formation of dissociated ions as the principal precursor to sulfone, since such a mechanism would yield a mixture of two isomeric sulfones. Similarly, in the case of optically active ester a racemic product should be formed. The observed data are consistent with either an ion-pair mechanism or a more concerted cyclic intramolecular mechanism involving little change between the polarity of the ground state and transition state. Support for the second alternative was found from measurements of the substituent and solvent effects on the rate of reaction. 

As of now no details of the synthesis of optically active tritiated compounds produced under microwave-enhanced conditions have been published. Another area of considerable interest would be the study of solvent effects on the hydrogenation of aromatic compounds using noble-metal catalysts as considerable data on the thermal reactions is available [52]. Comparison between the microwave and thermal results could then provide useful information on the role of the solvent, not readily available by other means. 

Yang12 has effected an intramolecular asymmetric carbonyl-ene reaction between an alkene and an a-keto ester. Reaction optimization studies were performed by changing the Lewis acid, solvent, and chiral ligand. Ligand-accelerated catalysis was observed for Sc(OTf)3, Cu(OTf)2, and Zn(OTf)2 (Equation (6)). The resulting optically active m-l-hydroxyl-2-allyl esters provide an entry into multiple natural products. 

Several optically active glycols were prepared from (+ )-limonene and (+ )-a- and (- )-(J-pinene by oxidation with KMn04 (74). An extensive study of the reduction of acetophenone by a complex of LAH and (+ )-l-hydroxycarvomen-thol (51) was made varying solvents and temperature, and the effect of added... 

The influence of the solvent on chiroptical properties of synthetic polymers is dramatically illustrated in the case of poly (propylene oxide). Price and Osgan had already shown, in their first article, that this polymer presents optical activity of opposite sign when dissolved in CHCI3 or in benzene (78). The hypothesis of a conformational transition similar to the helix-coil transition of polypeptides was rejected because the optical activity varies linearly with the content of the two components in the mixture of solvents. Chiellini observed that the ORD curves in several solvents show a maximum around 235 nm, which should not be attributed to a Cotton effect and which was interpreted by a two-term Drude equation. He emphasized the influence of solvation on the position of the conformational equilibrium (383). In turn, Furakawa, as the result of an investigation in 35 different solvents, focused on the polarizability change of methyl and methylene groups in the polymer due to the formation of a contact complex with aromatic solvents (384). 

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