Differentiations of Enantiomers via Their Diastereomeric Association Complexes—There Are Two Ways of Shaking Hands

Mannschreck, Albrecht; Kiesswetter, Roland

doi:10.1021/ed082p1034

Cited by 10 publications

(10 citation statements)

References 16 publications

(16 reference statements)

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“…[3][4][5] Chiral distinction 6 implies that a probe can distinguish the enantiomers of a chiral molecule. [7][8][9] Chiral distinction is part of the wider field of molecular recognition concerned with the details by which one molecule recognizes another. 10 The principles underlying the distinction of enantiomers are nevertheless poorly understood.…”

Section: Introductionmentioning

confidence: 99%

Electrostatic chiral distinction: Tetrahedral model dimers

et al. 2009

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Although chiral distinction plays a pervasive role in chemistry, a complete understanding of how this takes place is still lacking. In this work, we expand the earlier described minimal requirement of so called four-point interactions (vide infra). We focus on chiral point charge model systems as a means to aid in the dissection of the underlying, operative principles. We also construct models with defined symmetry characteristics. By considering extensive constellations of diastereomeric complexes, we are able to identify emerging principles for chiral distinction. As previously postulated, all the diastereomeric complexes, regardless of their nominal contact-points, possess a chiral distinction energy. In the comparison of complexes, we find that, contrary to chemical intuition, the magnitude of chiral distinction does not correlate with the stability of the complexes, i.e., consideration of low energy complexes may not be an effective way to evaluate chiral distinction. Similarly, we do not find a correlation between the number of contact-points and chiral distinction. Moreover, favorable interactions and facile chiral distinction appear to be unrelated. We also see some tendency for greater chiral distinction in less symmetric systems, although this may not be general. These studies can now form the basis to fold in higher levels of complexity into the models so as to gain further insights into the nature of chiral distinction.

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Section: Introductionmentioning

confidence: 99%

Electrostatic chiral distinction: Tetrahedral model dimers

et al. 2009

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“…In agreement with NMR, HPLC on a chiral nonracemic sorbent usually shows separate peaks for diastereomers and for enantiomers (4,14). Diastereomeric sorption complexes with the stationary phase are responsible for the splitting.…”

Section: Separations Of Stereoisomersmentioning

confidence: 65%

The Metolachlor Herbicide: An Exercise in Today's Stereochemistry

Mannschreck

Angerer

2009

J. Chem. Educ.

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In the ClassroomMany plants such as cereals are cultivated for the nutrition of humans and animals. The output of these crops would be considerably diminished if they were not protected against a variety of detrimental weeds. The latter compete successfully with the cultivated plants for space, sunlight, water, and nutrients in the soil. Chemicals used to selectively inactivate or kill weeds are called herbicides (1a). One of the most used representatives, (particularly in the United States) is metolachlor (Figure 1), an anilide registered for the protection of many different types of plants worldwide (2). This molecule forms stereoisomers 1 that have been analyzed separately and as a mixture for their bioactivity in reference to weed extermination. Such experiments are required because of environmental aspects (2) and the necessity to optimize the agronomic use. The compositions of stereoisomeric mixtures must also be taken into account for the industrial preparation. The present article deals with the three-dimensional structures, the syntheses, and the bioactivities of metolachlor stereoisomers.An overview of the following topics is beneficial to the comprehension of this manuscript: racemic and chiral nonracemic (3a) samples; zigzag formulas (3b); the principle of NMR and chromatography with a nonracemic auxiliary or sorbent, respectively (4); crystallization of diastereomeric derivatives (5a, 6a); and the elements of enantioselective reactions (7a). The content of this article can be integrated into a lecture on advanced organic agrochemistry. The present manuscript describes facts related to metolachlor, such as its stereoisomers or its syntheses. These specific facts are proposed as examples to illustrate general stereochemical aspects, such as axial and central chirality or enantioselective hydrogenation of achiral substrates. These topics are often taught in the third (or a later) year of university chemistry curriculum. The general aspects will be summarized in the conclusion of the article. Several Metolachlor Stereoisomers Two Elements of ChiralityMetolachlor, 1, has a stereocenter in one of the N substituents but, in addition, it exhibits a second, less usual element of chirality, the axial one (3c, 7b). Before looking at metolachlor, we shall consider a similar molecule, the model anilide 2, showing this element exclusively. We focus for a moment on formulas (M)-and (P)-2 ( Figure 2) representing the enantio mers; the descriptors (M) and (P) will be explained later. (M)-2 contains two planar π systems: the arene ring and the O=C−N fragment 1 with O=C and C−N partial double bonds. When considering rotation about the aryl-nitrogen bond, one sees that conformations with noncoplanar π systems such as (M)-2 are sterically less hindered than coplanar ones. (M)-2 shows neither an internal plane of symmetry nor a center of symmetry; thus two enantiomers must exist (6b, 8). These stereoisomers were experimentally separated and characterized (9).The descriptors (M) and (P) will be briefly explained with the e...

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“…9 It is perhaps an irony that, while stereochemical education emphasizes the role of diastereomeric transition structures in asymmetric syntheses or biological activity resulting from drug-enzyme interactions, 10 research scientists have acquired a particular amnesia overlooking the origin of such molecular interactions. The molecular situation should therefore be denoted as stereoisomer discrimination (vide infra), either enantiomer or diastereomer discrimination.…”

Section: Chirality and Discriminationmentioning

confidence: 99%

Chirality and chemical processes: A few afterthoughts

Cintas¹

2007

Chirality

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Chirality and chiral have become terms that pervade a wide range of disciplines in physical and life sciences. Although such terms are precisely defined, their use often engenders confusion and ambiguity. Perhaps, the most improper use of chirality, yet widely accepted, is related to its association with stereodynamics and physico-chemical transformations, such as chiral discrimination, chiral resolution, chiral recognition, chiral synthesis, and so on. Even though this conceptual perversion has been highlighted by renowned stereochemists, it has become a recurring keyword and a hot message in modern literature. It is timely to renew the correct use and context in forums such as the present journal, adding further reflections that may help both beginners and practitioners. This short article is not intended to criticize or highlight errors, but rather to encourage a level of rigor and the use of statements, which should be universally correct.

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Differentiations of Enantiomers via Their Diastereomeric Association Complexes—There Are Two Ways of Shaking Hands

Cited by 10 publications

References 16 publications

Electrostatic chiral distinction: Tetrahedral model dimers

Electrostatic chiral distinction: Tetrahedral model dimers

The Metolachlor Herbicide: An Exercise in Today's Stereochemistry

Chirality and chemical processes: A few afterthoughts

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