Transient diastereomeric complexes chiral separation

Most chiral chromatographic separations are accompHshed using chromatographic stationary phases that incorporate a chiral selector. The chiral separation mechanisms are generally thought to involve the formation of transient diastereomeric complexes between the enantiomers and the stationary phase chiral ligand. Differences in the stabiHties of these complexes account for the differences in the retention observed for the two enantiomers. Often, the use of a... 

Chiral separations result from the formation of transient diastereomeric complexes between stationary phases, analytes, and mobile phases. Therefore, a column is the heart of chiral chromatography as in other forms of chromatography. Most chiral stationary phases designed for normal phase HPLC are also suitable for packed column SFC with the exception of protein-based chiral stationary phases. It was estimated that over 200 chiral stationary phases are commercially available [72]. Typical chiral stationary phases used in SFC include Pirkle-type, polysaccharide-based, inclusion-type, and cross-linked polymer-based phases. 

Analysis using a CMPA is usually resolved on a nonchiral column. A transient diastereomeric complex is formed between the enantiomer and the chiral component in the mobile phase, similar to the complexes formed with chiral stationary phases. A review by Liu and Liu (2002) cites several papers where addition of CPMAs has been used in analyzing amphetamine-related compounds. Some CPMAs include amino acid enantiomers, metal ions, proteins, and cyclodextrins. Advantages of this method of analysis include the use of less expensive columns and more flexibility in the optimization of chiral separation (Misl anova and Hutta, 2003). 

Cyclodextrins are neutral compounds which migrate at the same rate as the EOF. They have large hydrophobic cavities in their structures into which molecules can fit. The ease with which a molecule fits into the cavity of the cyclodextrin is dependent on its stereoehemistry. Cyelodextrins have been used as additives both in chiral, where opposite enantiomers form transient diastereomeric complexes with the optically active cyclodextrins, and non-chiral separations where the cyclodextrins affect diastereoisomers to a different extent. 

Chiral separations generally rely on the formation of transient diastereomeric complexes with differing stabilities. Complexes are defined as two or more compounds bound to one another in a definite structural relationship by forces such as hydrogen bonding, ion pairing, metal-ion-to-ligand attraction, n-acid/ n-base interactions, van der Waals attractions, and entropic component desolvation. In the following sections, the most important types of molecular interactions in chiral separations are discussed. 

Method development for chiral separation is a multidisciplinary task. It requires knowledge of stereochemistry, organic chemistry, and separation techniques. Separation of enantiomers is not linked to a certain technique (i.e., GC, HPLC, etc.) but rather to an understanding of the specific interactions between the enantiomeric analytes and a certain chiral stationary phase. Knowing these types of relationships will enable one to easily understand the formation of transient diastereomeric complexes between enantiomers and a chiral stationary phase during a chromatographic separation as well as their stereochemical relationship within the complex. Once such dependencies are established, development of a method for the separation of enantiomers becomes an easy process. Based on such a relationship, chiral stationary phases can be divided in five categories [161] ... 

Type IV includes chiral phases that usually interact with the enantiomeric analytes through the formation of metal complexes. There are usually used to separate amino acid enantiomers. These types of phases are also called ligand exchange phases. The transient diastereomeric complexes are ternary metal complexes between a transitional metal (usually Cu +), an amino acid enantiomeric analyte, and another compound immobilized on the CSP which is able to undergo complexation with the transitional metal (see also the ligand exchange section. Section 22.5). The two enantiomers are separated based on the difference in the stability constant of the two diastereomeric species. The mobile phases used to separate such enantiomeric analytes are usually aqueous solutions of copper (II) salts such as copper sulfate or copper acetate. To modulate the retention, several parameters—such as the pH of the mobile phase, the concentration of the copper ion, or the addition of an organic modifier such as acetonitrile or methanol in the mobile phase—can be varied. 

Enantiomers - substances which differ only in their "handedness" and in no other physical property - are Inseparable on conventional, optically-inactive stationary phases such as are commonly used for glc. Since the vapor pressure of both enantiomers is the same, separation is possible only if transient diastereomeric complexes are formed in the stationary phase. For this to happen, the stationary phase must be chiral. The use of a chiral phase alone is insufficient to guarantee the separation of enantiomers and the literature is full of failed examples. In addition to phase chirality, there is a strong requirement that the solute-solvent interactions be chiro- selective. This means that the main interactions must include ones in which the essential difference in handidness is expressed and the more this is true, the larger the difference in retention that will be observed. There has been substantial progress in this area since the late 1960 s when the first authentic separations began to be reported and when relative retention values of 1.008 were common and plate requirements >105 were commonplace. Examples abound in which relative retention values of... 

Most of the aforementioned studies represent quantitative structure-retention relationship studies where a series of analytes are used as probes of enantiodiscrimination. There are, however, a number of atomistic molecular modeling studies where the interactions of chiral guests (analytes) with chiral hosts (CSPs) are explicitly determined. Here guest and host are considered as transient diastereomeric complexes and both liquid and gas chromatographic separations have been modeled. 

In the CMPAs method, enantiomeric separation is accomplished by the formation of a pair of transient diastereomeric complexes between a racemic analyte and the CMPA. Chiral discrimination is due to the differences in the intetphase distribution ration, solvatation in the mobile phase, or binding of the complexes to the achiral/chiral stationary phase. Ion pairing, ligand exchange, inclusion complexes, and protein interactions represent the major approaches in the formation of diastereomeric complexes. 

Enantiomeric separation by using CSP is based on the formation of labile (transient) diastereomeric complexes of solute-CSP between the enantiomers and the chiral molecule that is a part of the stationary phase. The five major CSPclasses based on solute-CSP complexes are as follows [51,52,54] ... 

Both CSPs and CMPAs are based on the formation of transient diastereomeric complexes between the chiral selectors and enantiomers during chromatographic procedure. The diastereomeric complexes can be separated due to their different properties [79]. 

Chiral separation by CE can be achieved by a direct or an indirect method. Direct separation is the more common approach. The chiral selector is dissolved in the running buffer, where it interacts selectively with the enantiomers to form reversible and transient diastereoisomeric or inclusion complexes of differing effective mobility. In indirect chiral CE separation, the enantiomers form covalent diastereomeric derivatives with a chiral reagent. A chiral selector is unnecessary... 

Chromatographic enantiomer separation can be performed in two different ways. The so-called direct method is based, as described above (Section 2.6.2.4), on the formation of a diastereomeric transient complex between the chiral selector and the analyte. The selector can be present in various forms in the column, either linked directly to the silica particle, or as a coating on the carrier particles. However, it can also be dissolved in the mobile phase as an additive. In contrast, the indirect... 

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