Selectivity, cation enantiomer

Clay films cast from a pure aqueous colloid appear to form a regular array of microplatelets, thin films of which show selective cation exchange, e.g. segregation of Ru(bipy)i from Na" and methylviologen dication and even partial separation of the enantiomers of Co bipy)3 Thicker films (approx. 3 pm) can be supported by the addition of polyvinyl alcohol additive also aids swelling of the... 

The lipophilicity of the TRISPHAT anion 8 also confers to its salts an affinity for organic solvents and, once dissolved, the ion pairs do not partition in aqueous layers. This rather uncommon property was used by Lacour s group to develop a simple and practical resolution procedure of chiral cationic coordination complexes by asymmetric extraction [134,135]. Selectivity ratios as high as 35 1 were measured for the enantiomers of ruthenium(II) trisdiimine complexes, demonstrating without ambiguity the efficiency of the resolution procedure [134]. 

The cationic rhodium catalysts are useful for asymmetric hydrogenation.152 In this variant, the presence of a chiral phosphine leads to differences in the rates of H2 addition to the two faces of a prochiral alkene. Where the alkene has groups such as C02Me suitably placed to bind to the metal, the selectivity can become very great enantiomeric excesses of the product over its enantiomer can reach 95-98% (equation 67). The mechanism has recently been elucidated by Halpern.153... 

Figure 3.72 Selective binding of the D-enantiomer of the phenylglycinate cation by 3.106. In the L-enantiomer, the carboxylic acid group is brought into close proximity with a methyl substituent of the host.

Similar enantiomer selection at the growing chain end is commonly observed in the cationic polymerizations of bicyclic acetals having bicyclo[3.2.1]octane skeletons. [24]... 

Enantioselective Cyclopropanation of Alkenes. Cationic Cu complexes of methylenebis(oxazolines) such as (1), which have been developed by Evans and co-workers, are remarkably efficient catalysts for the cyclopropanation of terminal alkenes with diazoacetates. The reaction of styrene with ethyl diazoacetate in the presence of 1 mol % of catalyst, generated in situ from Copper(I) Trifluoromethanesulfonate and ligand (1), affords the (rans -2-phenylcyclopropanecarboxylate in good yield and with 99% ee (eq 3). As with other catalysts, only moderate transicis selectivity is observed. Higher transicis selectivities can be obtained with more bulky esters such as 2,6-di-r-butyl-4-methylphenyl or dicyclohexylmethyl diazoacetate (94 6 and 95 5, respectively). The efficiency of this catalyst system is illustrated by the cyclopropanation of isobutene, which has been carried out on a 0.3 molar scale using 0.1 mol % of catalyst derived firom the (R,R)-enantiomer of ligand (1) (eq 4). The remarkable selectivity of >99% ee exceeds that of Aratani s catalyst which is used in this reaction on an industrial scale. 

C. R. Landis and J. Halpern found that cationic rhodium, Rh(I), with the chiral ligand R,R-l,2-bis[(phenyl-o-anisol)phosphino]ethane, or DIPAMP (see Figure 7.2.2), was very selective as a catalyst for the production of the S enantiomer of... 

Polycarboxylate crown ethers such as (205) are suitable ligands for potentiometric studies of mixed-metal complexes of Al3+ and alkali or alkaline-earth cations.303 A similar (+)-18-crown-6-tetracarboxylic acid, chemically immobilized on a chiral stationary phase (CSP), can selectively recognize both enantiomers of some analytes.304 Calixarene polycarboxylates such as (206) and (207) are useful ligands toward alkali-305,306 and also transition-metal ions,307 308 with applications in... 

The process is known to be stereoselective the S enantiomer of squalene epoxide is formed and only the C-20-R isomer of the steroids is formed. All steroids are formed through the protosteroid cation by elimination of protons at C-7, C-9, C-11, or C-19 to give lanostadienol, lanosterol, parkeol and cycloartenol, respectively. All have the same absolute configuration at C-5-C-10, the enzyme being responsible for enantiomeric selection. 

The (35)-enantiomer of a 3,4-disubstituted 4-pentenal derived from limonene preferentially cyclizes with the same catalysts and chiral ligand (-f)-CHDPP to form predominantly the ds-cyclopentanone (ratio up to 8.09 1), while its (37 )-enantiomer surprisingly favors formation of the frms-cyclopentanone (ratio up to 2.57 l)78 80,85. Similar results are obtained with other substituents in the 4-position of the starting pentenal. With ( + )-Diop the same effect, however, with lower selectivities is observed78,80, while with ( + )- or (-)-BINAP a strong dependence on the type of catalyst (neutral or cationic) exists85. 

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