Hetero Chiral lanthanide Lewis acid

Recently, some efficient asymmetric Diels-Alder reactions catalyzed by chiral Lewis acids have been reported [67]. The chiral Lewis acids employed in these reactions are generally based on traditional acids such as titanium, boron, or aluminum reagents, and they are well modified to realize high enantioselectivi-ties. Although lanthanide compounds were expected to be Lewis acid reagents, only a few asymmetric reactions catalyzed by chiral lanthanide Lewis acids were reported. Pioneering work by Danishefsky et al. demonstrated that Eu(hfc)3 (an NMR shift reagent) catalyzed hetero-Diels-Alder reactions of aldehydes with si-loxydienes, but enantiomeric excesses were moderate [68]. 

Diels-Alder reactions constitute one of the most important methodologies for the constructuction of a cyclic molecular framework. Lanthanide Lewis acid catalyzed Diels-Alder reaction was pioneered by Danishefsky et al., who revealed that NMR shift reagent Eu(hfc)3 served as chiral catalyst in hetero Diels-Alder reaction of silyloxydiene and aldehydes [32]. Later, although Yb(OTf)3 was first introduced for Diels-Alder reactions as an effective catalyst among lanthanide triflates, scandium triflates (Sc(OTf)3), classified as rare earth metal triflate, has gained popularity as a superior catalyst for Diels-Alder reactions [11, 33]. This section highlights several examples of the reactions where lanthanide triflates displayed preferable performance over scandium triflates. 

Mikami and co-workers reported a hetero Diels-Alder reaetion of butyl glyoxylate using a chiral lanthanide eatalyst reported by Shibasaki s group in 1994 (Sch. 4) [37]. They found that addition of water (11 mol equiv. to catalyst) resulted in the formation of the product in higher yield and ee. Such tolerance of water is never encountered in conventional Lewis acid catalysis. The catalyst (10 mol %) promoted the reaction of Danishefsky s diene with butyl glyoxylate in the presence of water to afford the corresponding product in up to 88% yield and 66% ee. 

Inanaga and coworkers prepared a series of tris[(l )-(-)-l,l -binaphthyl-2,2 -diylphosphato]lanthanides(III) Lnff-lBNPjj as new chiral and stable Lewis acids by the simple treatment of lanthanide(III) chlorides with three equivalent of the optically active sodiirm phosphate at room temperature, and reported the observed catalytic activity [31,32].The asymmetric hetero-Diels-Alder reactions of the Danishefsky diene 5 with benzaldehyde or with 2-naphthaldehyde were successfully performed at 0 °C in the presence of 10 mol % of Sc[(-)BNP]3 to give the corresponding adducts in 77 and 69% chemical yields with 68 and 74% enantiomeric excesses of (l )-(-)-isomers, respectively (Scheme 14). 

Mikami and coworkers also reported the development of lanthanide bis(trif-luoromethanesulfonyl)amides (bistrifylamides) as a new type of asymmetric catalysts for the hetero-Diels-Alder reaction of Danishefsky s diene, wherein the significant effect of water as an additive is observed in increasing not only the enan-tioselectivity but also the chemical yield. Bistrifylamides can be used as effective bidentate Hgands to increase the Lewis acidity of their chiral metal complexes on account of the higher acidity of the conjugated acids than those of afiphatic and aromatic diols, which are commonly used as chiral bidentate hgands [33]. 

The use of lanthanide complexes in asymmetric catalysis was pioneered by Danishefsky s group with the hetero-Diels-Alder reaction,and their utility as chiral Lewis acid catalysts was shown by Kobayashi. The Brpnsted base character of lanthanide-alkoxides has been used by Shibasaki for aldol reactions, cyanosilylation of aldehydes and nitroaldol reactions.The combination of Lewis acid and Brpnsted base properties of lanthanide complexes has been exploited in particular by Shibasaki for bifunctional asymmetric catalysis. These bimetallic lanthanide-main-group BINOL complexes are synthesized according to the following routes ... 

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