Chiral backbone

In 2008, Grisi et al. reported three ruthenium complexes 65-67 bearing chiral, symmetrical monodentate NHC ligands with two iV-(S)-phenylethyl side chains [74] (Fig. 3.26). Three different types of backbones were incorporated into the AT-heterocyclic moiety of the ligands. When achiral triene 57 was treated with catalysts 65-67 under identical reaction conditions, a dramatic difference was observed. As expected, the absence of backbone chirality in complex 65 makes it completely inefficient for inducing enantioselectivity in the formation of 58. Similarly, the mismatched chiral backbone framework of complex 66 was not able to promote asymmetric RCM of 57. In contrast, appreciable albeit low selectivity (33% ee) was observed when the backbone possessed anti stereochemistry. 

Figure 1. The transmission of backbone-chirality to the reaction environment by way of the differing spatial orientation of the P-phenyl substituents in a C2-symmetric ligand.

All approaches to the design of enantiomerically pure rhodium(II) catalysts had depended on the attachment of enantiomerically pure ligands to the rhodium core. In collaboration with Professor Pascual Lahuerta of the University of Valencia, Spain, we undertook a complementary strategy, the preparation of rhodium(I I)-dimers (P)-56 and its enantiomer (M)-56, having backbone chirality [23]. Using the approach outlined above, we calculated that the transition state 52 should be favored over the transition state 53 by 4.2 kcal moU. ... 

Polymeric backbone Chiral smectic Active Matrix Flat panel... 

In a comparative study, an unfunctionalised wingtip group chiral NHC, a backbone chiral saturated NHC and the hydroxyethyl functionalised saturated NHC (chiral centre on the... 

There is no reason to believe that this protocol is limited to (L)-vaUne. It is likely to be a general route to the synthesis of backbone chiral, saturated NHC ligands, although certain... 

The popularity of (l/ ,2/ )-1,2-diaminocyclohexane and (l/ ,2R)-diphenylethylen-ediamine as starting materials for these chiral imidazolium salts originates from their ready availability from commercial sources. The question remains to be answered as to whether a different substituent on the backbone (the substituent that makes the backbone chiral) improves the properties of the ligand and thus ultimately increases the chiral resolution in the product of asymmetric catalysis. No study has been proposed yet, probably because such a remote group is not expected to influence the performance of any... 

It is unlikely that the chiral configurations of the linear portions of the chains contribute to the asymmetry of the cavity since no asymmetric cyclocopolymerization is possible for the template monomer 7. With other types of template monomers, though, such contributions of backbone chiral portions of the polymer might be expected (see Section 2.2). 

Obviously, catalyst structure is the key to determining enantiocontrol in the phospho-aldol reaction. In the lanthanide heterobimetaUic systems, backbone chirality within the binaphthol leads to generation of a stereocentre at the metal itself and does so with complete diastereospecificity. This is yet another example of the concept of chirality tra s/er, wherein fixed stereochemistry at one site leads to control over stereochemistry at a more remote site and is one of the most powerful concepts in catalyst design. It is the presence of such a highly... 

Figure 2. Possibilities for the placement of backbone chiral centres (tacticity) in poly(4-methyl cyclopentene)...

Of the 20 amino acids that are commonly found in proteins, with the exception of glycine, all have a chiral centre of L-configuration at their a-carbon atoms, and two, isoleucine and threonine, also have a chiral centre in their side-chains. The biological properties of proteins and peptides are critically dependent on the configuration of the backbone chiral centres, so maintaining the integrity of these centres is of paramount importance in peptide synthesis. 

As for other catalysts, chiral Rh(III) porphyrin catalysts 23a were also foiuid to catalyze asymmetric cyclopropanation of alkenes with EDA (84). Although moderate enantiocontrol (<60% ee) was observed, the reaction was cis-selective tic = 1 2.5) with >1800 turnover number. Other newly developed Rh(III) porphyrin catalysts 23b gave slightly improved enantioselectivity (68% ee) with moderate diastereoselectivity (85). Similar to their achiral covmterparts, a perpendicular approach of alkene to rhodium-carbene was proposed as a mechanism. Apart from porphyrins, phosphine catalysts also gave interesting results. Catalyst 24, which contains backbone chirality, catalyzed the cyclopropanation of styrene with EDA to give 91% ee for the cis-isomer and 90 10 of the cis trans ratio (86). 

This is still more difficult to do in the case of the diphosphine ligands with backbone chirality, when all four substituents on the phosphorus atoms are phenyls. The conformation of the chelate cycle (fixed by the backbone substituents) makes these foin phenyls pairwise nonequivalent... 

However, in the case of the ligands with backbone chirality the asymmetric environment reverses when changing from a Rh(I) square planar to a Rh(III) octahedral complex. This is demonstrated in Figure 1.18 (bottom left), showing that in the octahedral complex the area above the "quasi"-equatorial phenyl becomes more attractive for building a chelate cycle. ... 

Comparing the sense of enantioselection obtained with P-chiral ligands generating a clearly defined asymmetric environment with that observed with ligands bearing backbone chirality, - one can conclude that the stereoselection in Rh-catalyzed asymmetric hydrogenation takes place in octahedral Rh(III) complexes. ... 

If the ligand has a backbone chirality like in 16, computations are required to determine, whether the stereo-discriminating substituent on the oxazoline ring lies "below" or "above the ligand plane. ... 

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