Plane of chirality

Due to the inherent unsymmetric arene substitution pattern the benzannulation reaction creates a plane of chirality in the resulting tricarbonyl chromium complex, and - under achiral conditions - produces a racemic mixture of arene Cr(CO)3 complexes. Since the resolution of planar chiral arene chromium complexes can be rather tedious, diastereoselective benzannulation approaches towards optically pure planar chiral products appear highly attractive. This strategy requires the incorporation of chiral information into the starting materials which may be based on one of three options a stereogenic element can be introduced in the alkyne side chain, in the carbene carbon side chain or - most general and most attractive - in the heteroatom carbene side chain (Scheme 20). 

The inherent plane of chirality in the metal carbene-modified cyclophane 45 was also tested in the benzannulation reaction as a source for stereoselectivity [48]. The racemic pentacarbonyl(4-[2.2]metacyclophanyl(methoxy)carbene)-chromium 45 reacts with 3,3-dimethyl-1-butyne to give a single diastereomer of naphthalenophane complex 46 in 50% yield the sterically less demanding 3-hexyne affords a 2 1 mixture of two diastereomers (Scheme 30). These moderate diastereomeric ratios indicate that [2.2]metacyclophanes do not serve as efficient chiral tools in the benzannulation reaction. 

The meaning of the defining phrases for the axis and the plane of chirality were now clarified by stating (4) that the axis of chirality is derived by de-... 

In the third chapter, Hans Hirschmann and Kenneth R. Hanson provide a detailed analysis of the principles of stereochemical classification or factorization. In contrast to the system earlier proposed by Cahn, Ingold, and Prelog (and recently extended and modified by Prelog and Helmchen) featuring centers, axes, and planes of chirality, Hirschmann and Hanson here present an alternative scheme not limited to chiral structures. This scheme for the factorization of stereoisomerism uses as principal elements the center and line of stereoisomerism. Numerous examples are given. 

In this section, we shall examine the various approaches by which crown compounds that have their chiral elements associated in some way with fused ring systems can be constructed. A selection of the wide and growing range of saturated chiral diols—many of them derived finom readily available carbohydrates—which have been incorporated, as relatively inexpensive sources of chirality, into crown ether derivatives are displayed in Figure IS. It may be noted that the saturated chiral diols rely for their chirality on centers of the classical type (C abcd)—not so the chiral dihydroxy compounds associated with the unsaturated systems listed in Figure 16. These examples reveal that axes and planes of chirality join with less conventional chiral centers (C aaaa) in being sources of chirality in optically active crown ethers. 

This survey deals with organic structures possessing a plane of chirality (see 1.2.) and reviews the more recent results covering approximately the last ten years. Whereas obviously it would be an impossible task to deal with centro or even axial chiral compounds in one review, the field of planar chiral structures can still be surveyed, especially as there is some excellent background material covering several aspects10 18) (but to date no comprehensive survey has been published). These reviews will serve as the basis for some discussions presented in this article. 

Whereas a center or an axis of chirality can by clearly defined 2> 7) there is still some ambiguity in respect to the specification of a plane of chirality. It therefore seems somewhat difficult to define the scope of planar chiral compounds which at a first glance include mostly rather rigid aromatic compounds of special interest from synthetic, structural, spectroscopic and especially chiroptical points of view. 

Fig. 1. Plane of Chirality Enantio-morphous figures and corresponding torsional angles A—X—Y—Z 7). Molecular examples ([n]paracyclophane, [2.2]meta-cyclophane, bridged[10]anulene and ( )-cyclooctene) with descriptors (/ , S)...

Ferrocenes of type 11 (as well as cyrhetrenes such as 14) are characterized as having two elements of chirality a stereogenic center at the oxazoline ring and a plane of chirality due to the two ortho substituents on the ferrocene core. 

The interconversion of enantiomers can be viewed in general as requiring inversion at a particular atom or twisting about an axis of the molecule. Provided these processes are inhibited to some degree, the chirality of a molecule will be detectable so that any chiral species may be said to contain centres and/or axes and/or planes of chirality.115,116,117,118 The precise meanings and utility of these concepts are, however, a matter of some debate,115,116 129 and they have not been extensively applied to coordination compounds. 

In 2, all three substituents may lie on the same side of the reference plane alternatively, A, B, or C may be on the opposite side from the other two. Isomerism resulting from these four arrangements is analogous to the more familiar cis-trans isomerism. Each of these arrangements is chiral, even in the absence of helicity (e.g., even when all rings are perpendicular to the reference plane), and may exist in two enantiomeric forms which differ in configuration with respect to the reference plane. Thus this plane may be treated as a plane of chirality. This is illustrated in the top portion of Fig. 1 where the two enantiomeric structures each have all three substituents on the same side of the reference plane. There are consequently 4x2=8 stereoisomeric structures, i.e., four dl pairs. 

The benzannulation affords arene-Cr(CO)3 complexes possessing a plane of chirality resulting from the unsymmetrical arene substitution pattern. This aspect is relevant to stereoselective synthesis, in which enantiopure arene tricarbonyl chromium complexes play a major role [56]. The benzannulation reaction avoids both harsh conditions incompatible with the retention of chiral information and the cumbersome separation of enantiomers, and is thus attractive for the diastereo- and enantioselective synthesis of arene complexes [17b, 57]. 

The tetrafunctional alcohol pentaerythritol is a popular core in dendrimer chemistry it has been modified into a tetrakis chromium phenylcarbene 85, which underwent a quadruple benzannulation upon reaction with 3-hexyne. The reaction proceeded with only moderate diastereoselectivity in terms of the planes of chirality formed demetalation by mild oxidative work-up gave the tetrakis-hydroquinone derivative 86 (Scheme 33) [76]. 

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). 

Kornienko s group reported the formal synthesis of cydophelitoi (1) via RCM to the diene derived from D-xylose [50]. Their s)mthetic strategy was interestingly focused on the latent plane of chirality present in D-xylose as shown in O Fig. 1, and the enantiodivergent s)mthesis of (-1-)- and (-)-cyclophelitol from D-xylose was achieved (O Fig. 1). 

The property by which a molecule may exist as two mirror forms, which are able to rotate a beam of polarized light in opposite directions, is known as chirality [l].The chiral molecules contain the elements (center, axis or plane) of chirality. The chiral molecule which contains only one element of chirality may exist in two possible configurations which are not superim-... 

Fig. 44a. Stereoselektive complexation of a chiral [2.2]metacyclophane with CifCO)3(NH3)3/THF (i) [45] and regioselective lithiation/electrophilic trapping with PhCOCl (ii) b. [45] X-ray structure of complex 173 containing two elements of chirality (plane of chirality in the phane and the...

Another stereochemical feature is that (achiral) arene ligands bearing two non-identical substituents in ortho or meta position give rise to chiral complexes (Fig. 2) which can be viewed as structures possessing a plane of chirality [12,... 

As a consequence of the unsymmetric substitution pattern encountered in the arene ring (OH versus OR) formed upon benzannulation the chromium complexes bear a plane of chirality. Complexes of this type are powerful reagents in stereoselective synthesis, and approaches directed towards diastereoselective benzannulation will be addressed in section 2.1 of this review. 

The chromium-templated coupling of alkenyl- or arylcarbene, aUcyne and carbonyl ligands generates arene tricarbonylchromium complexes as primary benzannulation products which - based on their unsymmetric substitution pattern - bear a plane of chirality. Chiral arene complexes are powerful reagents in stereoselective synthesis however, the preparation of pure enantiomers is a lengthy and often tedious procedure, and thus diastereoselective benzannulation appears to be an attractive alternative. In order to lure the chromium fragment to one or the other face of the arene formed, chiral information may be incorporated in the carbene complex or the aUcyne. 

For illustrations and examples of both an axis of chirality and a plane of chirality, see Prelog, V. (with Cahn, R. S.) Chem. Brit. 1968, 4,382. See also Prelog, V. Science 1976, 193,17. 

We may consider a plane to be an element of chirality if part of a structure lies in the plane but other parts of the structure leave the plane so as to produce chirality. An example is trans-cyclooctene (28), which is chiral with respect to a plane that includes the two double-bonded carbon atoms and one of the carbon atoms adjacent to the double bond. More than one chain can pass through the double bond, as illustrated by "[8.10]-betwee-nanene" (29). Another example is the [6]paracyclophane 30, in which the plane of chirality can be defined by the three positions marked with an asterisk. In many cases we may consider the chiral element to be either an axis or a plane, and Schlogl has noted the difficulty in defining a plane of chirality. Helical (threaded) structures can also be considered chiral with... 

The R and S nomenclature system can also be used for structures with an axis or plane of chirality if we apply the additional rule that substituents on the end of an axis or on the surface of a plane nearer the observer are arbitrarily given higher priority than those further away. Some authors augment the R and S labels in order to specify that the designation applies to axial or planar chirality, so the terms Ra and Sa (or aR and aS) for axial chirality and Rp and Sp (or pjR and pS) for planar chirality also appear in the literature. ... 

Chapter X contains compounds having axes and planes of chirality. 

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