Semicorrins metal complexes

Sodium borohydride (160) was found to serve as a hydrogen donor in the asymmetric reduction of the presence of an a,pi-unsaturated ester or amide 162 catalyzed by a cobalt-Semicorrin 161 complex, which gave the corresponding saturated carbonyl compound 163 with 94-97% ee [93]. The [i-hydrogen in the products was confirmed to come from sodium borohydride, indicating the formation of a metal enolate intermediate via conjugate addition of cobalt-hydride species (Scheme 2.17). 

Inspired by the structures of corrinoid and porphinoid metal complexes, which in nature play a fundamental role as biocatalysts, we have developed a route to chiral C2-symmetric semicorrins 1, a class of bidentate nitrogen ligands specifically designed for enantioselective catalysis [7]. These ligands possess a number of structural features that make them attractive ligands for the stereocontrol of metal-catalyzed reactions. 

Cu catalysts for metal carbene transformations are active as Cu(I) complexes and not as Cu(II). Although in the distant past there was some disagreement with this proposition, bis-oxazoline, semicorrin, and even the Aratani catalysts are active only when Cu is in its +1 oxidation state [6,34,39,40], The chiral Cu(I) catalysts have been produced from the correspond-... 

Oxazolines are excellently suited for the com-plexation of metals. Based on this knowledge, a variety of metal-oxazoline complexes have been synthesized in recent years and used with outstanding results. In 1986 Pfaltz developed the semicorrin ligands 27 and thereby laid the foundation for future developments [13]. However, the more recently developed bis(oxa-zoline) ligands are more easily accessible and have therefore found wider application (Fig. 5). 

Semicorrins form stable chelate complexes with a variety of metal ions such as Co , Rh Pd , or Cu . Depending on the metal ion, the ligand structure, and the reaction conditions, mono-or bis(semicorrinato) complexes are obtained. - ... 

The substitution of nickel for palladium catalyst precursors has, in principle, two advantages the lower price of the metal and the lower tendency to plate. The effectiveness of nickel in the carbonylation of ethene to low molecular weight polyketones has been known for a long time [4]. Only recently, however, have nickel catalyst precursors with a reasonably high catalytic activity been discovered. The newest compounds are nickel complexes, which are modified by semicorrine-type ligands (Scheme 8.10). 

Only a few chiral catalysts based on metals other than rhodium and ruthenium have been reported. The titanocene complexes used by Buchwald et al. [109] for the highly enantioselective hydrogenation of enamines have aheady been mentioned in Section 3.4 (cf. Fig. 32). Cobalt semicorrin complexes have proven to be efficient catalysts for the enantioselective reduction of a,P-unsaturated carboxylic esters and amides using sodium borohydride as the reducing agent [ 156, 157]. Other chiral cobalt complexes have also been studied but with less success... 

The cyclopropanation with diazo compounds via decomposition is amenable to asymmetric induction using chiral metal chelates. Rhodium complexes of 3(S)-phthalimido-2-piperidinone and Al-(arenesulfonyl)proline 44 are typical. The latter catalyst is suitable for generating alkenylcarbenoids. For intramolecular reactions, the cognate complex 45 and the semicorrin-copper 46 are effective. 

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