Counterions achiral

Fig.24 Overview of the synthesis and resolution of solution stable, chiral tetrahedron 6 [119]. a Formation of racemic, homochiral (AAAA)-6 and (AAAA)-6 b Chiral resolution and separation upon addition of s-nic ions (38) by formation of diastereomeric ion-pair (AAAA)-6-38 c Ion-exchange of chiral auxiliary 38 by achiral counterions such as NMe4+ and NEt4+ maintains the chirality of the resolved tetrahedra (AA AA)-6...

The first fundamental studies on this system were published by Styrer and Blout [28] in 1959. They took poly-a-L or poly-a-D glutamic acid as the chiral polyelectrolyte, and a cationic dye such as acridine orange or pseudoisocyanine as the achiral counterion. If the... 

Enantioselective versions of this reaction have been recently developed with achiral substrates. Two different strategies have been applied to obtain an effective chiral catalyst for that purpose the usual chiral phosphine ligand (equation 102) and the novel employment of a chiral counterion of the metal catalyst as the source of chirahty (equation 103). ... 

As in the previous example, the chirality of the ligand is in close proximity to the palladium binding site, while the pendant group is achiral. Introduction of the crown ether was designed to enhance coordination to the counterion. A ternary complex including the crown ether, potassium cation, and the enolate anion was proposed in which the bulky crown ether blocks approach of the enolate to Cl and provides a chiral pocket around carbon C3 [40]. Enantioselectivities up to 75% were reported [40], which were later revised to 65% [38]. 

Aldol reactions of chiral dioxolanones (113) and (114) are summarized in Scheme 6 and Table 9. ° With both (113) and (114), essentially perfect diasterofacial selectivity is observed. The simple dia-stereoselection is modest to good, and is dependent on the enolate counterion. For the lithium and magnesium enolates, the sense of simple diastereoselection is the same as is observed with the achiral dioxolanone (107) and the chiral dioxolanone (110). Use of the zirconium enolate generally reverses the sense of simple diastereoselection, although the isomer ratios are not very high in some cases. 

Unlike enantiomers, diastereomers have different solubilities in an achiral environment. Many chiral counterions, such as arginine and lysine, are commonly used to prepare salts of an ionic drug in order to improve its pharmaceutically relevant properties. However, when the drug is chiral, a chiral counterion should be used with caution. If the drug or the counterion is racemic, diastereomeric salts will be formed, constituting a heterogeneous system. Because the solubilities of diastereomers differ, a diastereomeric mixture of chiral salts is undesirable in a formulation. 

Various achiral and chiral Cu(I) salts have been prepared from mesi-tylcopper(I) and investigated for the diamination of conjugated olefins (200) with 1,3-di-t-butyldiaziridinone (201) as a nitrogen source. It has been found that copper(I) phosphate (202) had a high catalytic activity for the diamination, and encouraging ee values have also been achieved with the chiral phosphate as anionic counterion (Scheme 56). ... 

An example of the use of VCD was provided in a study of ammonium gemini surfactants bearing chiral tartrate counterions such as 23 (Scheme 4) [112], These compounds aggregate into chirally twisted ribbons that express the chirality of the counterions despite the fact that the surfactants are not chiral. However, VCD measurements show induced CD bands in the symmetric and antisymmetric stretching modes of the CH2 groups of the alkyl chains of the achiral cations. This unambiguously demonstrates that the cations adopt chiral conformations in the chiral ribbons, induced by tartrate anions. 

Enantioseparation can be achieved on a conventional achiral stationary phase by the inclusion of an appropriate chiral additive into the mobile phase. It is theoretically predicted that the enantioselectivity in CEC with a chiral additive may be higher than that using a chiral column with the same chiral selector. ° Lelievre et al. compared an HP- 3-CyD column and HP- 3-CyD as an additive in the mobile phase with an achiral phase (ODS) to resolve chlortahdone by CEC. It was demonstrated that resolution on ODS with the chiral additive was superior on the CSP however, efficiency was low. With an increasing amount of acetonitrile, the peak shape was improved and the migration time was decreased. We achieved the separation of salsolinol by the use of CEC with 3-CyD as a chiral additive in the mobile phase containing sodium 1-heptanesulfonate, as shown in Eig. 1. Salsolinol is a hydrophilic amine and is difficult to enantioseparate due to the small k values on the reversed stationary phases. Sodium 1-heptanesulfonate was used as a counterion to improve the retention. 

Besides the utilization of chiral secondary amines to achieve a LUMO-lowering activation as well as face discrimination, the use of achiral secondary amines in combination with a chiral counterion also proved to be highly promising for such transformations. This strategy resembles the use of achiral metal catalysts in combination with a chiral ligand to achieve a stereoselective transformation (206-208). It is due to Benjamin List that the elegant concept of asymmetric counteranion-directed catalysis (ACDC) has found widespread applications in organocatalysis at the present time (209-212). 

Complexation of copper salts with both achiral and chiral ligands offers additional potential for modulation of Lewis acidity, reactivity, and control of stereochemistry. Most notably, the application of chiral copper complexes in enantioselective transformations has steadily increased over the past 15 years. From the extensive investigations of Cu(II)-chiral bisoxazoline complexes to more recent combinations of Cu(I) and Cu(ll) salts with chiral ligands, chiral copper Lewis acids continue to attract considerable attention for several reasons, [3]. The first of which is their ready availability and/or accessibility. Second, chiral copper Lewis acids are moderately Lewis acidic, but more importantly, their Lewis acidity is easily modified by choice of oxidation state, counterion, and ligand. Finally, chiral Cu(l) and Cu(ll) complexes offer predictable and tunable coordination geometries about... 

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