Axial dissymmetry

The sulfenate-sulfoxide and sulfinate-sulfone rearrangements are very reliable and proceed with complete syn stereoselectivity17, ls. The allenic sulfoxides can be used for the synthesis of chiral alkylallenes with retention of configuration (see Section 1.1.3.). The relative configuration at sulfur in the allenic sulfoxides is not important for further synthetic purposes and racemization at sulfur is often observed without affecting the allenic axial dissymmetry. 

Reactions of lithium and titanium compounds 2, generated in situ by deprotonation of alkynylsilanes with tm-butyllithium, followed by addition of titanium(IV) isopropoxide and an aldehyde result either in a-hydroxyallenes without axial dissymmetry or in /J-hydroxyalkyn-es90 91. 

In another example, the cumulative effect of equatorial attack in prochiral cyclohex-anoneimines with diastereoselectivity induced by a chiral nitrogen substituent allowed the synthesis of spirocyclic oxaziridines with a high induction of axial dissymmetry. The major oxaziridine isomer results from both the favored equatorial attack and oxidation anti to the chiral nitrogen substituent (equation 45)204... 

The enantiomeric atropoisomers of 1,1 -binaphthyl-2,2 -diol (BINOL) and bis-diphenylphosphonate derivatives (BINAP) are completely synthetic molecules that have been developed to exploit the axial dissymmetry induced by the restricted rotation about the biaryl bond (Scheme 1.8) [64]. During the past 15 years, these compounds have become the most widely used ligands for both stoichiometric and catalytic asymmetric reactions, with many analogues and derivatives having been developed recently. 

Katsuki and co-workers have examined AE with Mn(salen) catalysts 12a, b bearing stereogenic centers on the 3- and 3 -positions of the salen ligand [63, 64]. The importance of these substituents was dramatically demonstrated with complex 12a. Even with an achiral ethylenediamine backbone, catalyst 12a effected the epoxidation of frans-stilbene in 61% ee (Scheme 2) [65]. Further optimization of the 3,3 -substituents led to the discovery of the more elaborate catalyst 12b, which incorporates axial dissymmetry into the aromatic portion of... 

Chirality may exist in many molecules that do not possess a chiral center. Such compounds may possess a chiral plane or a chiral axis, and are said to be dissymetric with respect to either that plane or that axis. Certain optically active allenes, biaryls, alkylidenecyclohexanes, and spiranes provide examples of axially dissymmetric molecules (chiral axis), irons-Cycloalkenes exemplify planar dissymmetry in molecules. The configurations of these classes may be specified by the Cahn-Ingold-Prelog convention using the usual R and 5 descriptors. Special subrules, which we will not describe here, are applied to this purpose. The interested reader is referred to references 8 (see p. 43) and 9 for details. Scheme 2.1 presents some molecules that are optically active because of planar or axial dissymmetry, and for which the absolute configurations have been determined. 

Particles which are too small to show a series of maxima and minima in the angular variation of scattered light are frequently studied by measuring the dissymmetry of scattering (usually defined as the ratio of the light scattered at 45° to that scattered at 135°). The dissymmetry of scattering is a measure of the extent of the particles compared with A. If the molecular or particle size is known, it can be related to the axial ratio of rod-like particles or the coiling of flexible linear macromolecules. 

Molecules that do not possess an asymmetric center may still have nonsuperimposable mirror images and exist as enantiomers. These molecules contain a chiral plane or chiral axis and are dissymmetric with respect to either that plane or axis. The structures of the enantiomers of the sedative-hypnotic methaqualone are presented in Fig. 4. In this molecule there is a chiral axis between the nitrogen atom (N-1) and phenyl ring (C-1). The dissymmetry of the two forms of the molecule is a result of hindered rotation around this axis, which is due to steric interactions between methyl groups (M-1 and M-2). Other axially dissymmetric molecules include allene, biaryls, alkylidenecyclohexanes, and spiranes. Planar dissymmetric molecules are exemplified by molecules such as tra s-cycloalkenes. 

Maupin et al. (1998,2000) applied chiral cyclen Yb + complexes 51 and 52 (scheme 14) as CPL probes. As described above (see section 3.1.2), the nona-coordinated complex 51 has a helically twisted ligand arrangement, and shows high emission dissymmetry factors. Since its helical twist angle is sensitively modified by the nature of the axially coordinated ligand, the... 

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