The chiral plane

A plane of chirality is encountered in molecules in which a molecular plane is desymmetrized by a bridge (ansa compounds and analogs). Examples include the paraphane derivatives XXXV and XXXVI, and wwts-cycloalkenes (XXXVII). 

The R and S nomenclature has also been extended to the chiral plane (see for example [44]). Thus, structure XXXV has the (S) absolute configuration. Some confusion between chiral axes and chiral planes has led to the proposal of a new definition of the chiral plane and of a new procedure for specifying planar chirality [45]. 

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Planar Chirality. Planar chirality arises from the desymmetrization of a symmetric plane in such a way that chirality depends on a distinction between the two sides of the plane and on the pattern of the three determining groups. In the definition of this chiral system, the first step is the selection of a chiral plane the second step is to identify a preferred side of the plane. The chiral plane is the plane that contains the highest number of atoms in the molecule. 

Fortunately, the original assignment rule for the chirality plane is identical to the helieity assignment defined above with descriptors aRjaS and PjM, respectively, corresponding. However, the rule for the chirality axis was based on an elongated tetrahedron as the stereogenic unit and the descriptors aR and P or aS and M are not equivalent ... 

Recently new definitions for the chiral plane were put forward 6,21,27) and appropriate procedures for the application of the descriptors (R)p and (S)p proposed 21,27). They will not be discussed in this article since we shall follow the revised and refined proposal as presented in Ref. 7) 1,2... 

Chirality (handedness, from Greek cheir = hand) is the term used for objects, including molecules, which are not superposable with their mirror images. Molecules which display chirality, such as (S)-(+)-lactic acid (/, Fig. 1) are called chiral. Chirality is often associated with a chiral center (formerly called an asymmetric atom ), such as the starred carbon atom in lactic acid (Fig. 1) but there are other elements that give rise to chirality the chiral axis as in allenes (see below) or the chiral plane, as in certain substituted paracyclophanes.1,2)... 

The compound possesses a chirality plane. Before establishing the configuration, it is necessary to determine the pilot atom. This is the atom of highest priority according to the CIP rules outside the chirality plane that is directly bonded to an atom in the plane. This is indicated in the formula below with an arrow. A view from this atom onto the chirality plane shows that the atoms on the way to the cyano group will be reached by a counter-clockwise curve. The configuration is therefore Sp. 

V/101, and V/103, have been prepared in 80, 50, and 74 % yield, respectively, by [2.3] sigmatropic rearrangement of the ylides, see Scheme V/16. The ring expansion of a mixture of trans- and cis-l-ethyl-2-vinylthiolanium hexafluoro-phosphates (V/105) gave a mixture of three sulfides, (Z)-2-methyl-thiacyclooct-4-ene (V/106) and the two diastereoisomerically related ( )-(SR,RS)- and ( )-(RR,SS)-2-methylthiacyclooct-4-enes (V/107) and (V/108)3 Scheme V/17. The existence of the two (E) isomers is evidence for the structure of the molecule, holding two elements of chirality, a chiral center, and a plane. The dia-stereoisomers are stabile because of restricted conformational inversion around the chiral plane [45] [60]. 

Figure 12.23. Comparison of simulated (on the left) and observed polarizing microscope patterns for the RO structure, viewed from the direction (a) normal to both the electric field and the -line and (b) along the field. On the photograph in (a) one of the chiral planes is emphasized in order to perceive the others. The radial /-line is visible in (b), whereas in (a) it causes a left-right asymmetry (from [14]).

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