Element of chirality

These polymers, as helical chiral ligand lacking any other elements of chirality, were tested as chiral Hgands for the palladium-catalyzed allyhc substitution, and in the test reaction, enantiomeric excesses of up to 33% were obtained (higher than those reached with monomer 115 as unique Hgand). 

A new class of phosphines (30) containing only an axial element of chirality (atropisomerism) has been made (253, 254). An in situ 1 1 rhodium/2,2-bis(diphenylphosphinomethyl)-1,1 -binaphthyl system (30a) hydrogenated a-acetamidocinnamic acid to a 54% ee (S) using 50 atm H2, the solvent not being recorded (253). The corresponding diphenyl-phosphinite system (30b) in toluene-acetone was particularly effective (76% ee) for hydrogenation (95 atm) of a-acetamidocinnamic and a-acet-amidoacrylic esters (254). 

In Section 2 a brief description of the generally assumed polymerization mechanism and the elements of chirality for the stereospecific olefin polymerization is presented. 

Possible elements of chirality in stereospecific polymerizations will be briefly recalled in order to indicate the used terminology. First of all, upon coordination, a prochiral olefin such as propene gives rise to not superpos-able si and re coordinations.22 According to the mechanism described, the isotactic polymer is generated by a large series of insertions of all si- or all re-coordinated monomers, while the syndiotactic polymer would be generated by alternate insertions of si - and re-coordinated monomers. 

A second element of chirality is the configuration of the nearest tertiary carbon atom of the growing chain, and a third element of chirality is the chirality of the catalytic site, which, in particular, can be of two different kinds ... 

The use of the previously defined procedure (see Section 3.2) for dissecting a molecule into its skeleton and ligand set permits one to enumerate the elements of chirality for monocentric configurations with higher coordination numbers. 

Meso-tartaric acid 10 has no element of chirality, because there is a conceivable conformation 10b with a mirror plane perpendicular to the central C—C-bond. 

The prochirality concept is not necessarily an expression of a precursor-product relationship because there exist stereoselective reactions at pro-chiral elements that do not generate elements of chirality. An illustration of this is the reversible enzymatic dehydration of citric to cu-aconitic acid. In this process two prochiral centers of citric acid disappear and we obtain an achiral line of stereoisomerism that physically coincides with a prochiral plane of prostereoisomerism. 

A set of novel phosphine-phosphoroamidite ligands 16-18 (Fig. 15) possessing two elements of chirality have been prepared through a modular synthetic approach and successfully applied in the Ir-catalyzed asymmetric hydrogenation of... 

Soon thereafter, chirality was recognized as a necessary and indispensible part of synthetic receptor molecule design and function. Predictably, not only has nature s chiral pool been called upon to supply inexpensive and readily available sources of chirality, but the ability of the chemist to resolve optically active precursors from racemic modifications prepared in the laboratory has been exploited ingeniously in a number of different directions. The various elements of chirality centers, axes, planes, as well as helices, have been incorporated into both axially symmetric and asymmetric receptors. 

The chiral 1,2-diols that have been incorporated into crown ether derivatives that cany substituents but have no fused rings associated with the elements of chirality in the macrocycles are listed in Figure 14. We shall now examine some of the ways in which these chiral building blocks have been used. 

The relative importance of the planar and central elements of chirality within the Josi-phos skeleton has also been established. Diastereomeric ligands 13 and 22 bear the same R) central chirality but have the opposite planar chirality (Fig. 9.4). Under standard reaction conditions with methanol as the nucleophile, the (R),(S)-ligand 13 gives 100% conversion after approximately 7 min. Conversely, the (i ),(i )-diastereomer 22 gives incom-... 

Asymmetric polymerization in which the polymer molecules formed contain one (or more) new type(s) of elements of chirality not existing in the starting monomer(s). 

This method251 is based on the attachment of a stereolabile chiral unit X to a compound of unknown configuration (for example IR). An equilibrium mixture of diastereomers differing in configuration at the stereolabile element of chirality results ... 

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. 

An important approach to stereochemical problems is to make use of the concept of chirality. Chirality (7), namely, the phenomenon that a chiral object and its mirror image cannot be superimposed, has been classified according to different elements of chirality. Chiral molecules may contain chiral centers, axes, and/or planes (2, 3). 

The enantiomers +8 and -8 were generated by removal of the planar chirality due to the unsymmetrically substituted arene ring in the separated diastereoisomers (7a and 7b). By treatment with LiAlH4-AlCl3, the ester group was reduced to a methyl group, leaving the asymmetric Cr atom the only element of chirality in the complexes +8 and -8,... 

The most striking feature in diastereoisomerism of metal complexes is the very rapidly growing number of isomers, by introducing different elements of chirality in the basic framework of a coordination compound. The complex formation with l,8-diamino-4-methyl-3,7-diazaoctane (5-Metrien) for example — a ligand with a single asymmetric center — leads not only to four geometric isomers. Three of these show a structure containing all four types of chiral elements mentioned, so that the system offers 28 possibilities of isomers. 

As a result of the limited configurational stability of optically active organotin compounds, in which the chirality is on the tin atom, most advances in enantioselective free-radical reductions involve organostannanes where the elements of chirality are contained in the organic substituents. Selected... 

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