Ligand modifying reagents

One approach to enantioselective reduction of prochiral carbonyl compounds is to utilize chiral ligand-modified metal hydride reagents. In these reagents, the number of reactive hydride species is minimized in order to get high chemo-selectivity. Enantiofacial differentiation is due to the introduced chiral ligand. 

Major emphasis is placed on the reactions of metal complexes in solution undergoing either inner-sphere ligand substitution or electron transfer to and from the metal center. Such studies relate to the important selective role of metal catalysts in many areas of enzymatic, commercial, and modem synthetic chemistry. Clearly, this field has now matured to the point where basic theoretical considerations, although incomplete, can provide a logical framework for understanding the chemical reactivity of such systems and stimulate the investigation of (1) new and unique reaction pathways, (2) modified reagents, and (3) unorthodox matrices. 

Asymmetric induction in the conjugate addition of achiral residues to a,(3-unsaturated carbonyl compounds can be achieved with chiral ligand-modified organocuprate reagents. Early experiments, em-... 

Presumably, a polypeptide ligand modified near the region which binds to the receptor will be inactivated as most of the bifunctional reagents (Table 5.1) are bulky, and on chemical attachment they often neutralize... 

Fig. 3.26. The interaction of macromolecular photoaffinity reagents with a receptor, a The natural ligand binds tightly to a receptor on a membrane (or in solution), b A ligand modified with a bifunctional reagent close to its site of interaction with the receptor may not bind well. c If the ligand is modified with a short-armed reagent at an alternative site it may not label the receptor but, d if a long-armed reagent is used neighboring or irrelevant polypeptides may...

Cerium(IV) sulfate and a few other ligand-modified CAN reagents have been used for the oxidation. The differences in their oxidation patterns are small, and consequently it is quite safe to replace one particular oxidizing system with another. More rarely employed is cerium(rV) perchlorate. 

With the chiral methyltitanium reagents 39, modified by the bidentate ligands A-sulfonyl-norephedrine 38, a pronounced enantioselectivity is observed in reactions with aromatic aldehydes. Considerably lower enantiosclcction is obtained with aliphatic aldehydes36. 

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