Chiral coordination

Molecular modeling studies relative to both preinsertion intermediates and insertion states indicate that for all the metallocenes from 1 to 39 of Scheme 1.2 (independent of their structure and symmetry), when a substantial stereoselectivity is calculated for primary monomer insertion, this is mainly due to nonbonded energy interactions of the methyl group of the chirally coordinated monomer with the chirally oriented growing chain. 

The cobalt complex 37 was used in combination with quinine as a chiral coordinating base to hydrogenate l,2-diphenyl-2-propene-l-one in 49% ee (Fig. 29.22) [50]. However, no further studies of this type of catalyst were reported. 

Enantiocontrol using alkyl lithium-chiral coordinating... 

Recently, a more versatile enantioselective variant was accomplished by use of the chiral bis(oxazoline) 27 as a chiral coordinating agent . For example, a rearrangement of dibenzyl ether 25 using the premixed complex t-BuLi (2.0 equivalents)/(5, 5 )-27 (1.0 equivalents) in ether at —78 °C afforded 94% yield of alcohol (S)-26 in 62% ee. Furthermore, an asymmetric catalytic version of this rearrangement has been developed (equation 14). This protocol provides the tertiary alcohol 29 with a relatively high % ee from rac-28 (equation 15). 

Norpseudoephedrine-derived amino ether 81 was also used as a chiral coordinating agent for the enantioselective [2,3]-Wittig rearrangement. The rearrangement of propargyl ether 82 induced by n-BuLi/81 provided allenyl alcohol (5 )-83 in 62% ee (equation 45). In contrast, a similar reaction with (-)-24 provided only 9% ee of (S)-S3. 

Chiral bis(oxazoline) 27 is an effective chiral coordinating agent for enantiocontrol in the [2,3]-Wittig rearrangement. The rearrangement of (Z)-crotyl benzyl ether 84 with f-BuLi/(5, 5)-27 (1.5 equivalents each) in hexane provided [2,3]-shift product (l/ ,25 )-85 in 40% ee (equation 46The feasibility of the asymmetric catalytic version was also examined. In this case, the rearrangement with 20 mol% of 27 in ether was found to provide the same level of enantioselectivity (34% ee). 

A systematic basis can be gained by viewing a tristchelate) complex, the most common species of chiral coordination compounds, down the threefold rotation axis. If the helix thus viewed is right-handed, the isomer is the A-isomer, and its mirror image is the A-isomer.50 The d-. l-, A-. and A-isomers may thus be portrayed as shown in the margin. Note that it is a result of these systems that A A l and A A D, furthering possible confusion. 

To achieve the desired asymmetric induction, chirality must be introduced. There are essentially three ways to do this. One can employ (i) chirally modified substrates (ii) chirally modified nucleophiles or (iii) a chiral reaction medium (chiral coordinating cosolvents, ligands or catalysts). This chapter is organized according to these three approaches. 

Many chiral coordination compounds are very stable, and as such can be isolated, and studied by the various spectroscopic techniques described in Section 5.3. It is a very common situation in solution, however, that chiral coordination compounds may undergo internal rearrangements or ligand exchange with other excess ligand molecules in solution resulting in an interconversion of enantiomers. This process is... 

Keywords Chirality Coordination compounds Guest inclusion ... 

Figure 3.24 Two possible chiral coordinations of a propylene molecule to a metal atom re (rectus) - right-handed si (sinister) - left-handed...

Fig. 11.195Pt-NMR Spectrum of I Pt(, -I5N-Met)2], a metabolite of the cisplatin, showing three sets of triplets for each of the cis- and ir ns-isomers. The three diastereomers for each geometrical isomer arise from slow inversion (on the NMR time scale) of chiral coordinated sulfur. (Adapted from [29]).

N-enriched methionine. The cis-and trans-isomers of this complex have been separated by HPLC and detailed studies of the facile cis-trans-isomerization in aqueous solution have been made by 2D [JH, 15N] HMQC-NMR spectroscopy [30]. NMR data allow characterization of the three diastereomers which arise from the presence of two chiral coordinated S atoms for... 

Within the tris-chelate type of chiral coordination compound, the most extensively studied are the series of diamine chelates of cobalt(III), containing the octahedral [Cot )N6] cluster. The principal d—d transition of interest for the optical activity of these complexes is the Aj -> Tj octahedral excitation near 465 nm (Fig. 2), made up of the three single-orbital promotions, dxy dx -y , and the analogues obtained by the cyclic permutations of the electronic coordinates. The leading moments of the one-electron transition, dxy - , are the z-component of a magnetic dipole... 

The combination of organolithiums with chiral coordination ligands such as sparteine allows the asymmetric polymerization to furnish optically active polymers with very high enantioselectivity. especially when trityl methacrylate is utilized as a monomer [13, 14]. 

The results could be interpreted by assuming exclusive coordination of the N-acyldehydroamino acid moiety with the rhodium complex in which the rest of the molecule, i.e. the a-amino ester moiety, is located in the outer sphere of the chiral coordination site this may be the reason why virtually no double asymmetric induction was observed. However, a simple asymmetric hydrogenation using dppb as achiral ligand (Entry 10) disclosed preferential formation of Bz-( )Phe-( )Phe-OMe with 24.4% asymmetric induction, which is consistent with the result using Ac-APhe-( )Phe-OH as substrate (Entry 20). Accordingly, it seems that the results of using DIOPs are rather exceptional. In this context, we further looked at the effect of the chiral center on the catalytic asymmetric induction... 

Pasteur went on to make many more advances in microbiology. He also realized the importance of making science an international endeavor and advocated for a scientific approach to the betterment of the human condition. He once remarked, Do not put forward anything that you cannot prove by experimentation. Pasteur died in 1893, two years after the first international Pasteur Institute was established in Saigon in what was then French Indochina (now Ho Chi Min City in Vietnam), see also Chirality Coordination Chemistry Isomerism Organic Chemistry. 

Lin and coworkers extended their method to the synthesis of chiral coordination polymers based on the use of rigid bridged ligands derived from the 1,1 binaphtyl framework. Thus homochiral porous coordination networks were prepared however this work will be reviewed in the next chapter of this book. ... 

Alkyllithium-Chiral Coordinating Agent Complex Protocol.225... 

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