Solvating agent

Chiral Shift Reagents and Chiral Solvating Agents 

Sulfoxides form hydrogen-bonded complexes with a-methoxyphenylacetic acid, which results in differential shielding of the two substituents.  

---

The ion pair extraction by flow injection analysis (FIA) has been used to analyze sodium dodecyl sulfate and sodium dodecyl ether (3 EO) sulfate among other anionic surfactants. The solvating agent was methanol and the phase-separating system was designed with a PTFE porous membrane permeable to chloroform but impermeable to the aqueous solution. The method is applicable to concentrations up to 1.25 mM with a detection limit of 15 pM [304]. 

The lack of solvent separated pairs raises the question whether some alternative mode of solvation should be considered. The ester group of the penultimate unit of the polymer, or the one preceeding it, could act as a solvating agent. The idea of intramolecular solvation was proposed by several workers in the field 37) and it is supported by the results of nmr studies of polymethyl methacrylate formed under various experimental conditions 38). Hypothetical structures such as those depicted below were proposed 39 h... 

Cryptands are powerful solvating agents developed by Lehn42). The symbol [222] denotes the following cryptand structure ... 

In dimethoxyethane DME, a more powerful solvating agent than THF, solvation by solvent molecules competes with the intramolecular solvation, increasing the reactivity of ion-pairs. Indeed, the propagation constants of Na+ and Cs+ salts of polymethyl methacrylate are higher in that solvent than in THF, although again both salts are nearly equally reactive 39) as shown in Fig. 5. 

Korotkov offered an ingenious explanation for this phenomenon. The monomers were treated as solvents, with butadiene believed to be a less reactive monomer than styrene, but treated as the preferential solvating agent for Li+. Thus butadiene was expected to be present virtually exclusively in the vicinity of the growing polymer ends, and hence it polymerizes preferentially, albeit slowly. On its exhaustion styrene reaches the reactive centers and, being assumed to be the more reactive monomer, it polymerizes rapidly speeding up the reaction. 

Kagan and coworkers determined and studied in detail the crystal structure of a 1 1 molecular complex between (i )-methyl p-tolyl sulfoxide (10) and S)-N-(3,5-dinitrobenzoyl)-l-phenylethylamine (11). They suggested that amide 11, which had been used as a chiral solvating agent for sulfoxides, might find use as a resolving agent for these compounds. ... 

In contrast to the allyl system, where the reduction of an isolated double bond is investigated, the reduction of extensively delocalized aromatic systems has been in the focus of interest for some time. Reduction of the systems with alkali metals in aprotic solvents under addition of effective cation-solvation agents affords initially radical anions that have found extensive use as reducing agents in synthetic chemistry. Further reduction is possible under formation of dianions, etc. Like many of the compounds mentioned in this article, the anions are extremely reactive, and their intensive studies were made possible by the advancement of low temperature X-ray crystallographic methods (including crystal mounting techniques) and advanced synthetic capabilities. 

The properties of organic liquids relevant to their use as solvating agents have also been reviewed [76]. The ability of liquids to solvate a solute species depends mainly on their polarity and polarizability properties, ability to hydrogen bond, and cohesive electron density. These molecular properties are best measured by the Kamlet-Taft solvatochromic parameters, and the square of Hildebrand s solubility parameter. 

Nuclear Magnetic Resonance Spectroscopy Measured in a Chiral Solvent or with a Chiral Solvating Agent. One method of NMR analysis for enantiomer composition is to record the spectra in a chiral environment, such as a chiral solvent or a chiral solvating agent. This method is based on the diastereomeric interaction between the substrate and the chiral environment applied in the analysis. 

The vinyl ethers, being polar, are much better solvating agents than alkenes, so that both cations and anions have a stabilizing solvation shell. (Hayashi et al. 1971). 

The situation would be completely different for oxycarbenium ions in a highly polar solvent such as sulphur dioxide which could compete effectively as solvating agent with the DCA and their polymers. In such systems one could envisage that both the solvent-solvated oxycarbenium ions and also the solvent-solvated teJt.-oxonium ions could coexist in a true equilibrium, and that each would react according to its own characteristics. This is an area which remains very largely unexplored, although Penczek has made a start in this direction and these considerations arose from discussions with him of his exploratory experiments with sulphur dioxide as solvent. 

There are three types of chiral auxiliary that are used chiral derivatizing agents (CDAs), chiral lanthanide shift reagents (CLSRs) and chiral solvating agents (CSAs)75. Chiral derivatizing agents (CDAs), such as the enantiomers of o -methoxy-o -(trifluoromethyl)phenylacetic acid (MTPA, 83)76, require the separate formation of discrete... 

Related to the NMR use of CLSRs is the application of chiral solvating agents (CSAs), so-called because the first report of the determination of enantiomeric excess by NMR was that of a partially racemic sample of 2,2,2-trifluro-l-phenylethanol (98),... 

S )-Ar- -Butyl-2-(phenylcarbamoyloxy)propionamide (193) was used as chiral solvating agent in the NMR determination of the enantiomer composition of the N-(3,4-dinitrobenzoyl) derivative of amino acid ethyl esters400. 

In Table 6 the differences of free enthalpies of solvation for several anion ligands in a donor solvent D and in AN are given. HMPA shows very weak solvation whereas water is a very strong solvating agent for anions. The free enthalpies of solvation of halide and pseudohalide ions are by 4 to 15 keal/mol more negative than in aprotic donor solvents. 

In conclusion, the most important result is that the use of permethylated cyclodextrin as chiral solvating agent for NMR spectroscopy not only affords a simple and practical way for the determination of the stereochemical purities of trisubstituted allenes, but also allows one to simultaneously determine their absolute configuration. Indeed, TRIMEB induced only positive complexation shifts of all the allene protons, which are greater for the (S )-enantiomer than for the (R)-enantiomer, independent of the structure of the allene. This empirical correlation seems to be reliable since it has been satisfied by a large number of trisubstituted allenes. 

Barretta and coworkers63 reported a direct determination of the enantiomeric purity of chiral trisubstituted allenes by using permethylated cyclodextrin as a chiral solvating agent. They found that the heptakis ft-cyclodextrin TRIMEB discussed above can be successfully used as a chiral solvating agent (CSA) for the NMR determination of the enantiomeric purity of trisubstituted allenes llOa-f. An accurate analysis of the experimental conditions (molar ratio aliene/TRIMEB, temperature and solvent) required to optimize the enantioseparation has been carried out. The XH NMR spectra of TRIMEB, allenes llOa-f, and the mixtures TRIMEB/allene have been recorded at 300 MHz in CD3OD as solvent. 

Consequently, the splitting observed is due to the fact that TRIMEB induces nonequivalence in the proton nuclei of the two enantiomers of the allenes, thus enabling one to determine the enantiomeric purities by using a chiral solvating agent. In all cases examined the extent of the nonequivalence, i.e. the difference of the proton chemical shifts of the two enantiomers in the presence of TRIMEB, can be increased by increasing the... 

The detailed structure of the small bacteriocin (isolated from the culture broth of the Gram-negative bacterium Rhizobium leguminosarum) as N-[(3R)-hydroxy-7-ns-tetradecenoyl]-L-HSL 14 was elucidated largely by the 1-D and 2-D and 13C spectroscopy. The absolute configuration of both asymmetric carbon atoms in the molecule was determined by the use of chiral solvating agents (S)-(+)- and (R)-(-)-2,2,2-trifhioro-l-(9-anthryl)ethanol [11]. 

---