Scandium bipyridine catalysts

Using same strategy, a catalytic, enantioselective addition of amines to meso-epoxides was established employing a scandium-bipyridine complex in emulsion (Scheme 8.12). In this work, Kobayashi and coworkers demonstrated that high yields and excellent enantioselectivities could be achieved using an amphiphilic Lewis acid combined with chiral Hgand as the catalyst in water. This work offered a green method for the synthesis of a-amino alcohols, which are important... 

In recent years, many chiral catalysts for the enantioselective synthesis of optical active 1,5-dicarbonyl compounds have been developed, such as chiral crown ethers with potassium salt bases and chiral palladium complexes, including bimetallic systems. Nakajima and coworkers reported on enantioselective Michael reactions of S-keto esters to a,/3-unsaturated carbonyl compounds in the presence of a chiral biquinoline N,N dioxide-scandium complex, which catalyzed the additions in high yields and with enan-tioselectivities up to 84% ee . Kobayashi and coworkers found that the combination of Sc(OTf)3 with the chiral bipyridine ligand 149 (equation 41) was also effective as a chiral catalyst for asymmetric Michael additions of 1,3-dicarbonyl compounds 147 to a,/3-unsaturated ketones 148. The corresponding Michael adducts 150 were obtained in good to high yields with excellent enantiomeric excesses in most cases (Table 10). 

Scandium trifiate was found to be an effective catalyst for the aldol reactions of silyl enol ethers with aldehydes in aqueous solvent/micellar systems (205). While the reactions proceeded sluggishly in water, remarkable enhancement of the reactivity was observed in the presence of a small amount of a surfactant (206). In related asymmetric version, scandium trifiate (Sc(OTf)3) catalyzed asymmetric aldol of formaldehyde (hydroxymethylation) could be conducted with highly enantioselectively in the presence of chiral bipyridine based ligand (Scheme 53) (207). A significant progress was also made by Feng and co-workers recently a C-2-symmetric iV,iV -dioxide-Sc(III) complex has been developed to asymmetric catalytic aldol reaction of a-ketoesters and diazoacetate... 

Highly enantioselective Michael reactions of indanone and tetralone -ketoesters derivatives with a,P-unsaturated ketones promoted by a chiral scandium catalyst have been developed [123]. In the presence of Sc(OTf)3 and bipyridine (3), the reactions proceeded smoothly in dichloroethane at 40 °C to give the corresponding adducts in moderate to excellent yields with excellent enantioselectivities in most cases (Scheme 12.47). It was found that a lower concentration of the reaction mixture was key to attaining high selectivity. 

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