Heterobimetallic lanthanoid

Shibasaki M, Grdger H (1999) Chiral Heterobimetallic Lanthanoid Complexes Highly Efficient Multifunctional Catalysts for the Asymmetric Formation of C-C, C-O and C-P Bonds. 2 199-232... 

Groger, H., Saida, Y., Sasai, H., Yamaguchi, K., Martens, J., and Shibasaki, M., A new and highly efficient asymmetric route to cyclic a-amino phosphonates the first catalytic enantioselective hydrophosphonylation of cyclic imines catalyzed by chiral heterobimetallic lanthanoid complexes, /. Am. Chem. Soc., 120, 3089, 1998. 

Shibasaki, M. and Groger, H. (1999) Chiral heterobimetallic lanthanoid complexes highly efficient multifunctional catalysts for the asymmetric formation of CC CO and CP bonds. Topics in Organometallic Chemistry, 2 (Lanthanides) 199-232. 

In 1992, Shibasaki et al. [8] reported for the first time on the use of recently developed chiral heterobimetallic lanthanoid complexes (LnLB) as chiral catalysts in the catalytic asymmetric Henry reaction (Scheme 1). In the following sections, this efficient concept of an asymmetric nitroaldol reaction, its scope and limitations, and its applications to complex stereoselective synthetic topics are described. 

Scheme 1. Catalytic asymmetric nitroaldol reaction promoted by heterobimetallic lanthanoid catalysis LnLB...

The proposed mechanism for the asymmetric nitroaldol reaction catalyzed by heterobimetallic lanthanoid complexes is shown in Scheme 2 [9]. In the initial step, the nitroalkane component is deprotonated and the resulting lithium nitr-onate coordinates to the lanthanoid complex under formation of the intermediate I [ 10]. Subsequent addition of the aldehyde by coordination of the C=0 double bond to the lanthanoid(III) ionic center leads to intermediate II, in which the carbonyl function should be attacked by the nitronate via a six-membered transition state (in an asymmetric environment). A proton exchange reaction step will then generate the desired optically active nitroalkanol adduct with regeneration of the free rare earth complex LnLB. 

A further example of a diastereoselective nitroaldol reaction using heterobimetallic lanthanoid complexes as catalysts was recently reported by Okamoto et... 

The use of alkali metal-containing, heterobimetallic lanthanoid complexes as catalysts in asymmetric synthesis is reviewed. This new and innovative type of chiral catalyst, which was recently developed by Shibasaki et al., contains a Lewis acid as well as a Bronsted base moiety, thereupon showing a similar mechanistic effect as observed in enzyme chemistry. The heterobimetallic complexes have been successfully applied as highly stereoinducing catalysts in many different types of asymmetric reactions, including the stereoselective formation of C-C, C-O, and C-P bonds. 

In the following, such a desired new and innovative multifunctional catalytic system is reviewed The chiral heterobimetallic lanthanoid complexes, developed by Shibasaki et ah, have recently been shown to catalyze a broad spectra of organic reactions including many classical carbon-carbon bond formations... 

Figure 1. Applications of heterobimetallic lanthanoid complexes in asymetric catalysis...

Asymmetric Catalytic C-C Bond Formation Using Heterobimetallic Lanthanoid Complexes... 

The proposed mechanism for the asymmetric nitroaldol reaction catalyzed by heterobimetallic lanthanoid complexes is shown in Scheme 2 [5]. In the initial step, the nitroalkane component is deprotonated and the resulting lithium nitronate coordinates to the lanthanoid complex under formation of the inter-... 

A further example of a diastereoselective nitroaldol reaction using heterobimetallic lanthanoid complexes as catalysts was recently reported by Okamoto et al. [18] in connection with a novel approach to lc/.,24( R)-dihydroxyvitamin D3... 

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. 

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