Chiral rare-earth metal triflates

Scheme 13. Chiral rare-earth metal triflates.

While Ln(OTf)3 are the first metal salts which were found to catalyze aldol reactions of aldehydes with silyl enol ethers efficiently in aqueous media, it has been difficult to realize asymmetric versions of Ln(OTf)3-catalyzed reactions in such media. Recently, the first example of this type of reaction using chiral bis-pyridino-18-crown-6 (Structure 4) has been developed (Eq. 8) [38]. In the reaction of benzal-dehyde 5 with water-ethanol (1/9), the cation size of rare earth metal triflates including Ln(OTf)3 strongly affected the diastereo- and enantioselectivities of the... 

A heterodinuclear rare earth metal SchifF base catalyzes enantioselective desymmetrization of cyclic and acyclic weyo-aziridines with malonates (Scheme 13). The combined use of two rare earth metal sources with different properties, Bronsted basic rare earth metal alkoxide and Lewis acidic rare earth metal triflate, was important to promote the desired ring-opening reactions. The La(0 Pr)3/Yb(OTf)3/SchifFbase complex gave chiral cyclic and acyclic y-amino esters in high yields and high enantiomeric excesses. 

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. 

After screening suitable metal combinations for Schiff base 2, Ga(0-/Pr)3/Yb (OTf)3 afforded promising results. The chiral diamine backbone affected both the reactivity and enantioselectivity, and the best reactivity and selectivity were achieved using Schiff base 2c with an anthracene-derived diamine unit (Table 1, entry 3 94% ee). After optimization of the solvent (CH2Q2) and Ga/Yb ratio (1 0.95), product was obtained in > 95 % conversion and 96% ee after 24 h (entry 4). To confirm the utility of the Ga(0-iPr)3/Yb(0Tf)3 combination, several cmitrol experiments with the best ligand 2c were performed (entries 5-13). Neither Ga-2c alone nor Yb-2c alone efficiently promoted the reaction (entries 5-6). With Ga(0-/Pr)3 and other rare earth metal triflates, the reactivity decreased in correlation with the Lewis acidity of the rare earth metals (Yb > Gd > Nd > La) [17], while good to excellent enantioselectivity... 

Hamada T, Manahe K, Ishikawa S, Nagayama M, Shiro M, Kobayashi S. Catalytic asymmetric aldol reactions in aqueous media using chiral his-pyridino-18-crown-6-rare earth metal triflate complexes. J. Am. Chem. Soc. 2003 125 2989-2996. 

---