Lanthanides asymmetric catalysis

Inanaga, J. Furuno, H. Hayano, T. Asymmetric catalysis and amplification with chiral lanthanide complexes. Chem. Rev. 2002,102, 2211-2225. 

K. Mikami, M. Terada and H. Matsuzawa, Angew. Chem.,Int. Ed. Engl., 2002, 41, 3554 (asymmetric catalysis by lanthanide complexes). 

M. Shibasaki and N. Yoshikawa, Chem. Rev., 2002, 102, 2187 (lanthanide complexes in multifunctional asymmetric catalysis). 

Lanthanides Containing Multifunctional Heterobimetallic and Heteropolymetallic Asymmetric Catalysis... 

Although the development of a variety of Lewis acids has enabled the reahzation of a wide range of catalytic asymmetric reactions, most of the catalysts have limited activity in terms of either enantioselectivity or chemical yields. The major difference between synthetic asymmetric catalysts and enzymes is that the former activates only one side of the substrate in an intermolecular reaction, whereas the latter not only can activate both sides of the substrate but also can control the orientation of the substrate. If this kind of synergistic cooperation could be realized in synthetic asymmetric catalysis, it would open up a new field in asymmetric synthesis, and a wide range of applications might well ensure. In this section we discuss asymmetric two-center catalysis promoted by chiral lanthanide complexes with Lewis acidity and Brpnsted basicity [44,45]. 

Asymmetric catalysis of hetero Diels Alder reactions using chiral lanthanide complexes 02CRV2211. 

Alnminium alkoxides have long been used as catalysts for the reduction of aldehydes or ketones by secondary alcohols and recently lanthanide alkoxides were reported to act similarly in addition to catalysing the epoxidation of allylic alcohols with tert-butyl hydroperoxide. Aluminium alkoxides have also been used to catalyse the conversion of aldehydes to alkylesters (Tischtchenko reaction). A review gives a fascinating account of the use of heterometal alkoxides in asymmetric catalysis. 

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 ... 

The first catalytic asymmetric aza-Henry reaction appears to have been reported by Shibasaki in 1999, as part of a general research program focused on heterobimetallic lanthanide complexes and their application in asymmetric catalysis [190]. In the event, a 1 1 3 mixture of KOt-Bu, Yb(Oi-Pr)3, and (i )-BINOL afforded an active catalyst suggested to have the structure 302 (Equation 32) [191]. This catalyst was shown to promote enantioselective additions of nitromethane to N-phosphinoylarylimines, including 300, to provide the corresponding products, such as 303, in 79 % yield and 91 % ee. 

The most frequently encountered, and most useful, cycloaddition reactions of silyl enol ethers are Diels-Alder reactions involving silyloxybutadicncs (Chapter 18). Danishefsky (30) has reviewed his pioneering work in this area, and has extended his studies to include heterodienophiles, particularly aldehydes. Lewis acid catalysis is required in such cases, and substantial asymmetric induction can be achieved using either a chiral lanthanide catalyst or an a-chiral aldehyde. 

Cyano compounds liquid crystals, 12, 278 in silver(III) complexes, 2, 241 Cyanocuprates, with copper, 2, 186 Cyano derivatives, a-arylation, 1, 361 Cyanosilanes, applications, 9, 322 Cyclic acetals, and Grignard reagent reactivity, 9, 53 Cyclic alkenes, asymmetric hydrosilylation, 10, 830 Cyclic alkynes, strained, with platinum, 8, 644 Cyclic allyl boronates, preparation, 9, 196 Cyclic allylic esters, alkylation, 11, 91 Cyclic amides, ring-opening polymerization, via lanthanide catalysis, 4, 145... 

Michael-aldol reaction as an alternative to the Morita-Baylis-Hillman reaction 14 recent results in conjugate addition of nitroalkanes to electron-poor alkenes 15 asymmetric cyclopropanation of chiral (l-phosphoryl)vinyl sulfoxides 16 synthetic methodology using tertiary phosphines as nucleophilic catalysts in combination with allenoates or 2-alkynoates 17 recent advances in the transition metal-catalysed asymmetric hydrosilylation of ketones, imines, and electrophilic C=C bonds 18 Michael additions catalysed by transition metals and lanthanide species 19 recent progress in asymmetric organocatalysis, including the aldol reaction, Mannich reaction, Michael addition, cycloadditions, allylation, epoxidation, and phase-transfer catalysis 20 and nucleophilic phosphine organocatalysis.21... 

The LLB catalysts described above served an important role in demonstrating the proof of principle for catalysis with lanthanide-BINOL complexes. In addition, they were the first catalysts for the enantioselective nitroaldol reaction and gave respectable selectivities in synthetically useful yields. However, the reactions required at least 3.3 mol % of the catalysts for efficient conversion, and at that loading the reactions are rather slow. Clearly, the need for more effective catalysts is indicated. Consideration of the mechanism for the catalytic asymmetric... 

Danishefsky and co-workers pioneered the use of chiral lanthanide complexes as catalysts in organic reactions. They found out that Eu(hfc)3, which is used as an NMR shift reagent, promoted hetero Diels-Alder reactions [30] of aldehydes with siloxydienes and induced enantiomeric enrichment (Sch. 1) [31]. Suitable substituents on the dienes were introduced to improve the extent of asymmetric induction. The best result was obtained in the reaction of benzaldehyde with l-methoxy-2-methyl-3-(trimethyl-siloxy)- , 3-butadiene using 1 mol % Eu(hfc)3 the enantiomerie excess was, however, moderate (58%). The authors maintained that the major advantage of lanthanide catalysis lay in the survival of otherwise labile systems used as adducts. 

When the reaction was conducted at room temperature under the catalysis of Yb[(-)BNP]3, the asymmetric induction was improved to 73% ee. The effect of the central metal ion of the chiral catalysts on the optical yield of the product, 2-phenyl-2,3-dihydro-4H-pyran-4-one, is shown in Fig. 2. The degree of enanti-oselection is highly sensitive to and dependent on the ionic radius of lanthanide ions [31]. 

Lanthanide triflates are stable Lewis acids in water and are successfully used in several carbon-carbon bond-forming reactions in aqueous solutions. The reactions proceed smoothly in the presence of a catalytic amount of the triflate under mild conditions. Moreover, the catalysts can be recovered after the reactions are completed and can be re-used. Lewis acid catalysis in micellar systems will lead to clean and environmentally friendly processes, and it will become a more important topic in the future. Finally, catalytic asymmetric aldol reactions in aqueous media have been attained using Ln(OTf)3-chiral crown ether complex as a catalyst. 

Keywords Lanthanide triflate, Lewis acid, Carbon-carbon bond formation, Asymmetric synthesis, Catalysis, Organic synthesis... 

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