Inherent chirality

Short description: Asymmetry in molecules

Inherently chiral calixarene with XXYZ substitution pattern.

In chemistry, inherent chirality is a property of asymmetry in molecules arising, not from a stereogenic or chiral center, but from a twisting of the molecule in 3-D space. The term was first coined by Volker Boehmer in a 1994 review, to describe the chirality of calixarenes arising from their non-planar structure in 3-D space.

This phenomenon was described as resulting from "the absence of a place of symmetry or an inversion center in the molecule as a whole".[1] Boehmer further explains this phenomenon by suggesting that if an inherently chiral calixarene macrocycle were opened up it would produce an "achiral linear molecule".[1] There are two commonly used notations to describe a molecules inherent chirality: cR/cS (arising from the notation used for classically chiral compounds, with c denoting curvature) and P/M.[2][irrelevant citation] Inherently chiral molecules, like their classically chiral counterparts, can be used in chiral host–guest chemistry, enantioselective synthesis, and other applications.[3] There are naturally occurring inherently chiral molecules as well. Retinal, a chromophore in rhodopsin. exists in solution as a racemic pair of enantiomers due to the curvature of an achiral polyene chain.[4]

## Contents

* 1 History
* 1.1 Calixarenes
* 1.2 Definition
* 2 Molecular symmetry
* 2.1 Chiral plane
* 2.2 Chiral axis
* 3 Other examples
* 4 See also
* 5 References

## History

### Calixarenes

After creating a series of traditionally chiral calixarenes (through the addition of a chiral substituent group on the top or bottom rim of the macrocycle,) the first inherently chiral calixarenes were synthesized in 1982, though the molecules were not yet described as such. The inherently chiral calixarenes featured an XXYZ or WXYZ substitution pattern, such that the planar representation of the molecule does not show any chirality, and if the macrocycle were to be broken open, this would produce an achiral linear molecule.[5] The chirality in these calixarenes is instead derived from the curvature of the molecule in space.[6]

a) 2D representation of corannulene, b) 3D representation of corannulene bowl flip with C5 symmetry.

### Definition

Due to the initial lack of a formal definition after the initial conception, the term inherent chirality was utilized to describe a variety of chiral molecules that don't fall into other defined chirality types. The first fully formulated definition of inherent chirality was published in 2004 by Mandolini and Schiaffino, (and later modified by Szumna):[4]

> inherent chirality arises from the introduction of a curvature in an ideal planar structure that is devoid of perpendicular symmetry planes in its bidimensional representation.

Inherent chirality has been known by a variety of names in the literature including bowl chirality (in fullerene fragments), intrinsic chirality, helicity (see section 3a) residual enantiomers (as applied to sterically hindered molecular propellers,) and cyclochirality (though this is often considered to be a more specific example and cannot be applied to all inherently chiral molecules).[4]

A simple example of inherent chirality is that of corannulene commonly referred to as "bowl chirality" in the literature. The chirality of an unsubstituted corranulene (containing no classic stereogenic centers) cannot be seen in a 2D representation, but becomes clear when a 3D representation is evoked, as the C5 symmetry of corranulenes provides the molecules with a source of chirality (figure 2.) Racemization of these molecules is possible through an inversion of curvature, though some inherently chiral molecules have inversion barriers comparable to a classic chiral center.[4]

## Molecular symmetry

Inherently chiral paracyclophane, rendered chiral by the bromide substituent hindering rotation of the phenylene.

### Chiral plane

Some inherently chiral molecules contain chirality planes, or planes within a given molecules across which the molecule is dissymmetric. Paracyclophanes often contain chiral planes if the bridge across the phenylene unit is short enough, or if the phenylene contains another substituent, not in the bridge, that hinders rotation of the phenylene unit.

### Chiral axis

Similar to chirality planes, chirality axes arise from an axis about which the spatial arrangement of substituents creates chirality. This can be seen in helical molecules (see section 3a) as well as some alkenes.

## Other examples

Spiro compounds (compounds with a twisted structure of two or more rings) can have inherent chirality at the spiroatom, due to the twisting of the achiral ring system.

Inherently chiral alkenes have been synthesized through the use of a "buckle" where in an achiral, linear alkene is forced into a chiral conformation. Alkenes have no classical chirality, so generally, an external stereogenic center must be introduced. However, by locking the alkene into a conformation through the use of an achiral buckle allows for the creation of an inherently chiral alkene. Inherently chiral alkenes have been synthesized through the use of dialkoxysilanes, with a large enough racemization barrier that enantiomers have been isolated.[7]

Chiral spiro ring system.

## See also

* Chirality
* Planar chirality
* Axial chirality

## References

1. ↑ 1.0 1.1 Dalla Cort, Antonella; Mandolini, Luigi; Pasquini, Chiara; Schiaffino, Luca (2004-10-01). ""Inherent chirality" and curvature" (in en). New J. Chem. 28 (10): 1198–1199. doi:10.1039/b404388j. ISSN 1369-9261.
2. ↑ Böhmer, Volker; Kraft, Dagmar; Tabatabai, Moniralsadat (1994-03-01). "Inherently chiral calixarenes" (in en). Journal of Inclusion Phenomena and Molecular Recognition in Chemistry 19 (1–4): 17–39. doi:10.1007/bf00708972. ISSN 0923-0750.
3. ↑ Shirakawa, Seiji; Moriyama, Akihiro; Shimizu, Shoichi (2007-08-01). "Design of a Novel Inherently Chiral Calix[4]arene for Chiral Molecular Recognition". Organic Letters 9 (16): 3117–3119. doi:10.1021/ol071249p. ISSN 1523-7060. PMID 17616144.
4. ↑ 4.0 4.1 4.2 4.3 Szumna, Agnieszka (2010-10-19). "Inherently chiral concave molecules—from synthesis to applications" (in en). Chemical Society Reviews 39 (11): 4274–85. doi:10.1039/b919527k. ISSN 1460-4744. PMID 20882239.
5. ↑ Li, Shao-Yong; Xu, Yao-Wei; Liu, Jun-Min; Su, Cheng-Yong (2011-01-17). "Inherently Chiral Calixarenes: Synthesis, Optical Resolution, Chiral Recognition and Asymmetric Catalysis" (in en). International Journal of Molecular Sciences 12 (1): 429–455. doi:10.3390/ijms12010429. PMID 21339996.
6. ↑ Neri, Placido; Sessler, Jonathan L; Wang, Mei-Xiang, eds (2016) (in en-gb). Calixarenes and Beyond | SpringerLink. doi:10.1007/978-3-319-31867-7. ISBN 978-3-319-31865-3.
7. ↑ Tomooka, Katsuhiko; Miyasaka, Shouji; Motomura, Shougo; Igawa, Kazunobu (2014-06-16). "Planar Chiral Dialkoxysilane: Introduction of Inherent Chirality and High Reactivity in Conventional Achiral Alkene" (in en). Chemistry – A European Journal 20 (25): 7598–7602. doi:10.1002/chem.201402434. ISSN 1521-3765. PMID 24802258.

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* t
* e

Concepts in enantioselective synthesis

Chirality types|

* Chirality
* Stereocenter
* Planar chirality
* C2-Symmetric ligands
* Axial chirality
* Supramolecular chirality
* Inherent chirality

Chiral molecules|

* Stereoisomer
* Enantiomer
* Diastereomer
* Meso compound
* Racemic mixture
* Enantiomeric excess (ee)
* Diastereomeric excess (de)

Analysis|

* Optical rotation
* Chiral derivatizing agents
* NMR spectroscopy of stereoisomers
* Ultraviolet–visible spectroscopy of stereoisomers

Chiral resolution|

* Recrystallization
* Kinetic resolution
* Chiral column chromatography
* Diastereomeric recrystallization

Reactions|

* Asymmetric induction
* Chiral pool synthesis
* Chiral auxiliaries
* Asymmetric catalysis
* Organocatalysis
* Biocatalysis

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