Steric effects chiral complexes

Because the steric effect contributes to the complex formation between guest and host, the chiral resolution on these CSPs is affected by the structures of the analytes. Amino acids, amino alcohols, and derivatives of amines are the best classes for studying the effect of analyte structures on the chiral resolution. The effect of analyte structures on the chiral resolution may be obtained from the work of Hyun et al. [47,48]. The authors studied the chiral resolution of amino alcohols, amides, amino esters, and amino carbonyls. The effects of the substituents on the chiral resolution of some racemic compounds are shown in Table 6. A perusal of this table indicates the dominant effect of steric interactions on chiral resolution. Furthermore, an improved resolution of the racemic compounds, having phenyl moieties as the substituents, may be observed from this Table 6. ft may be the result of the presence of n—n interactions between the CCE and racemates. Generally, the resolution decreases with the addition of bulky groups, which may be caused by the steric effects. In addition, some anions have been used as the mobile phase additives for the improvement of the chiral resolution of amino acids [76]. Recently, Machida et al. [69] reported the use of some mobile phase additives for the improvement of chiral resolution. They observed an improvement in the chiral resolution of some hydrophobic amino compound using cyclodextrins and cations as mobile phase additives. 

Acyclic -butadiene Fe(CO)3 complexes have repeatedly demonstrated their enormous value for organic synthesis in the last few years [1], In this context, both the changed reactivity of the ligand and the steric effect(s) of the Fe(CO)3 fragment have been exploited for the stereocontrolled generation of new chirality centers in the neighborhood of the butadiene-Fe(CO)3 unit. It is important to note that unsym-metrically substituted complexes (e.g. of type A with R / R ) are chiral. 

Chiral auxiliaries may also be used to effect diastereoselective complexations of acyclic dienes. For chiral dienamides (225a-d), the best case employed the sterically demanding (5 )-2-(diphenylhydroxymethyl)pyrrolidine auxiliary. Diene (225d) was complexed with excellent diastereoselectivity but in modest yield (Scheme 64). For substrates (217a-c), diastereomer ratios of the corresponding complexes were poorer (1.5 1 to 4.6 1), likely a result of the increased distance between the auxiliary s chiral center and the diene. 

Retention of Rohrschneider-McReynolds standards of selected chiral alcohols and ketones was measured to determine the thermodynamic selectivity parameters of stationary phases containing (- -)-61 (M = Pr, Eu, Dy, Er, Yb, n = 3, R = Mef) dissolved in poly(dimethylsiloxane) . Separation of selected racemic alcohols and ketones was achieved and the determined values of thermodynamic enantioselectivity were correlated with the molecular structure of the solutes studied. The decrease of the ionic radius of lanthanides induces greater increase of complexation efficiency for the alcohols than for the ketone coordination complexes. The selectivity of the studied stationary phases follows a common trend which is rationalized in terms of opposing electronic and steric effects of the Lewis acid-base interactions between the selected alcohols, ketones and lanthanide chelates. The retention of over fifty solutes on five stationary phases containing 61 (M = Pr, Eu, Dy, Er, Yb, n = 3, R = Mef) dissolved in polydimethylsiloxane were later measured ". The initial motivation for this work was to explore the utility of a solvation parameter model proposed and developed by Abraham and coworkers for complexing stationary phases containing metal coordination centers. Linear solvation... 

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