Lanthanoid catalysts

Groeger, H., Saida, Y., Arai, S., Martens, J., Sasai, H., and Shibasaki, M., First catalytic asymmetric hydrophosphonylation of cyclic imines highly efficient enantioselective approach to a 4-thiazolidinylphosphonate via chiral titanium and lanthanoid catalysts,Tetrahedron Lett., 37, 9291, 1996. 

The addition of oxygen nucleophiles (peroxides) to a,(i-unsaturated ketones is also catalyzed by the lanthanoid catalysts, leading to the formation of the corresponding epoxides with up to 96% ee (Scheme 8D.19) [41]. This reaction shall be reviewed in another chapter. 

The asymmetric catalytic hydrophosphonylation is an attractive approach for the synthesis of optically active a-amino phosphonates [84]. The first example of this type of reaction was reported by the Shibasaki group in 1995 using heterobimetal-lie lanthanoid catalysts for the hydrophosphonylation of acyclic imines [85a]. This concept has been extended to the asymmetric synthesis of cyclic a-amino phosphonates [85b—d]. Very recently, the Jacobsen group developed the first organocatalytic asymmetric hydrophosphonylation of imines [86], In the presence of 10 mol% of thiourea-type organocatalyst 71, the reaction proceeds under formation of a-amino phosphonates 72 in high yield (up to 93%) and with enantioselectivity of up to 99% ee [86], A selected example is shown in Scheme 5.42. Di-o-nitrobenzyl phosphite 70 turned out to be the preferred nucleophile. 

Previous studies on asymmetric Michael reactions with a chiral lanthanoid catalyst are on record. Scettri et al. have reported the use of (+)-Eu(tfc)3[tris(3-(trifluoromethylhydroxymethylene)-(+)-camphoralo)europiuin(ni)] as a catalyst with modest enantioselectivities.8... 

Selective ketallzation. Ketalization with lanthanoid catalysts can distinguish between aliphatic and aromatic ketones the latter remain unaffected. Also selective ketalization of aldehydes in the presence of ketones is usually possible. CeCI., and ErCU are more efficient than the heavier rare earth ions. 

Keywords HeterobimetaUic lanthanoid catalysts. Asymmetric nitroaldol reaction, Henry reaction, Enantioselective and diastereoselective reactions, syn-Selectivity... 

Concerning the heterobimetallic potassium lanthanoid catalysts (Pj-LnPB, a very recent NMR spectroscopic as well as FAB and ESI mass spectrometric study of isolated (Rj-YbPB complexes provided a clear insight into the assembly of this heterobimetallic complex in solution. At first, the assumption that the proposed... 

The catalytic asymmetric nitroaldol reactions promoted by LLB or its derivatives require at least 3.3 mol% of asymmetric catalysts for efficient conversion. However, even in the case of 3.3 mol% of catalyst, reactions are rather slow. Attempts were made to reduce the required catalytic amount and accelerate the reactions, which led to a second-generation heterobimetallic lanthanoid catalyst (LLB-II), prepared from LLB, 1 mol equiv of H20, and 0.9 mol equiv of butyllith-ium. The use of only 1 mol% of LLB-II efficiently promoted catalytic asymmetric nitroaldol reactions and additionally LLB-II (3.3 mol%) accelerated these reactions [32]. A comparison of the efficiency of LLB (or LL(B-a)) and the second-generation catalysts LLB-II (or LL(B-a)-II) is given in Scheme 9. The structure of LLB-II has not yet been unequivocally determined. However, it appears that it is a complex of LLB and LiOH. 

Although the development of a range of catalytic asymmetric aldol-type reactions has proven to be a valuable contribution to asymmetric synthesis [35—37], in all of these reactions pre-conversion of the ketone moiety to a more reactive species such as an enol silyl ether, enol methyl ether, or ketene silyl acetal has been an unavoidable necessity. However, quite recently Shibasaki et al. reported that a direct catalytic asymmetric aldol reaction, which is known in enzyme chemistry, is also possible in the presence of heterobimetallic lanthanoid catalysts [38]. Using fR)-LLB (20 mol%), which shows both Lewis acidity and Bron-sted basicity similar to the corresponding aldolases, the desired optically active aldol adducts were obtained with up to 94% ee. A variety of aldehydes and unmodified ketones can be used as starting materials (Scheme 11). 

A further application of the heterobimetallic lanthanoid catalysts of the LLB type to the field of catalytic asymmetric Diels-Alder reactions [47,48] was also achieved by Shibasaki et al. [49]. In general, LLB type complexes are multifunctional asymmetric catalysts, showing both Bronsted basicity and Lewis acidity. Nevertheless, in this study the use of LLB type catalysts acting as asymmetric Lewis acids alone was examined and led to the development of an LLB (type) catalyzed asymmetric Diels-Alder reaction [49]. Representative results for the catalytic asymmetric Diels-Alder reactions using 48 and cyclopentadiene in toluene as a solvent are shown in Scheme 16. 

Also under the rubric of reaction acceleration, the carboxylic acid imidazolides 34 were found to undergo epoxidation 12 times faster than the corresponding esters, using lanthanoid catalysts of type 32. The resultant epoxy peroxyesters (35) could be converted to alkyl esters 36 under mild conditions <01JA9474>. 

It is supposed that the diene polymerization with lanthanoid catalysts proceeds via the... 

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