Heterobimetallic Binaphthoxide Catalysts

Ees in these reactions increased with the electrophilic super-delocalizability [Sj j at the carbonyl oxygen, a measure of susceptibility to nudeophUic attack, consistent with the idea that Lewis add coordination of the aldehyde is important for enantiocontrol [24]. 

Sterically bulky 3,3 substituents reduced the enantioselectivity, while coordination between La and O atoms of ortho-substituents improved ees. 6,6 -diphenyl-BI-NOL gave the best results (69% ee for para-tolualdehyde) it was proposed that the phenyl substituents affected the Lewis acidity of the catalyst via electronic effects. With this catalyst, ee and yield depended strongly on solvent, THE being the most 

In contrast, much lower ees were obtained for hexanal and cyclohexanecarbox-aldehyde. 

The Al-Me complexes 28a-b were catalyst precursors for the reaction, which was not affected by air or water and did not require dry or degassed reagents. This system gave high yields but ees ranged only from 10 to 54% with 28a. In contrast, the t-Bu-substituted SALEN complex 28b gave racemic products at a slower rate [32]. 

A Hammett plot for para-substituted benzaldehydes showed that electron-rich aldehydes gave higher ees (r = -0.4). As in Shibuya s related results (Section 5.3.3.1 above), this indicates that aldehyde coordination is important in enantiodifferentia-tion, but the lower rvalue (compared to Shibuya s r = -1.30) suggests a weaker electronic influence, probably due to the relative Lewis acidities of A1 and La. For ortho-substituted aldehydes, lower ees were observed, presumably due to steric effects. Although Al-Cl and Al-triflate complexes 29-30a-b did not catalyze the reaction, they 

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Shibasaki and co-workers reported an elegant asymmetric total synthesis of 11-deoxy-PGFic 19 using the Al-Li bis(binaphthoxide) complex (ALB) 21 [15], a member of a novel class of heterobimetallic chiral catalysts showing dual behavior as both a Bronsted base and a Lewis acid (Scheme 12.5) [16]. 

Shibasaki and coworkers have conducted extensive research on the use of hetero-bimetallic complexes as catalysts for asymmetric synthesis [11]. The reactions are catalyzed by heterobimetallic complexes that function as both a Lewis acid and a Bronsted base. Among these, LaLi3tris(binaphthoxide) catalyst 1 (LLB) was proven to be an effective catalyst in direct asymmetric aldol reactions (Fig. 1) [12]. On the basis of this research, Shibasaki et al. reported the first report of a direct catalytic asymmetric Mannich reaction [13],... 

The successful achievement of the (/ )-LSB catalyst in asymmetric Michael addition suggested that the metal centers other than rare earths might lead to a novel heterobime-talhc asymmetric catalyst with unique properties. With this foundation, the same group further developed a new heterobimetallic chiral catalyst (/ )-ALB consisting of aluminum, lithium, and (/ )-BINOL in 1996 (Table 9.3). They reported that this type of catalyst could be more efficiently prepared from LiAlH with two equivalents of (/ )-BINOL. When this AlLibis(binaphthoxide) complex (/ )-ALB was employed as catalyst, up to 99% ee and 88% yield of products could be obtained in the reaction of dibenzyl malonate to 2-cyclohexen-l-one. Notably, both dimethyl and diethyl malonates furnished the 1,4-adducts with more than 90% of enantioselectivities. In particular, the catalytic asymmetric tandem Michael-aldol reactions were also achieved in the presence of (/ )-ALB. This protocol provides a usefid method for the catalytic asymmetric synthesis of complex molecules. 

Heterobimetallic asymmetric complexes contain both Bronsted basic and Lewis acidic functionalities. These complexes have been developed by Shibasaki and coworkers and have proved to be highly efficient catalysts for many types of asymmetric reactions, including catalytic asymmetric nitro-aldol reaction (see Section 3.3) and Michael reaction. They have reported that the multifunctional catalyst (f )-LPB [LaK3tris(f )-binaphthoxide] controls the Michael addition of nitromethane to chalcones with >95% ee (Eq. 4.140).205... 

When ethyl cyanoformate was used as the cyanide source and a heterobimetallic YLi3(binaphthoxide) complex (YLB) (Figure 5.15) was used as catalyst, asymmetric cyanoethoxycarbonylations of aldehydes were achieved in high yields and enantioselectivities in the presence of three achiral additives water, tris(2,6-... 

Heterobimetallic asymmetric complexes developed by Shibasaki et al. are known as effective catalysts for asymmetric Michael additions. They achieved a catalytic asymmetric protonation in Michael additions of thiols to a,P-unsaturated carbonyl compounds using LaNa3tris(binaphthoxide) and SmNa3tris(binaphthoxide) complexes (SmSB) 37 [42]. For instance, treatment of thioester 48 with 4-fert-butyl(thiophenol) and 0.1 equivalents of (P)-SmSB 37 (Ln=Sm) in CH2C12 at -78°C gave the adduct 49 in 93% ee and in 86% yield (Scheme 4). The high enantiomeric ratio is considered to be attributable to an... 

In 2002, Sasai et al. reported the synthesis of dendrimer heterobimetallic multi-funchonal chiral catalysts, containing up to 12 chiral BINOL units at the periphery (Figure 4.36) [107]. The insoluble dendrihc heterobimetallic mulhfunchonal chiral AlLibis(binaphthoxide) (ALB) complexes were obtained by treahng these dendrimer ligands with AlMcj and n-BuIi. The resulhng dendrimer-supported ALB... 

In a one-pot synthetic method for chiral 2,2-disubstituted oxetanes, a sequential addition reaction of a sulfur ylide to ketones and intermediate epoxides was promoted by the heterobimetallic catalyst LaLi3tris(binaphthoxide) (LLB). Chiral amplification was observed in the second reaction and was the key to affording the desired oxetanes with excellent enantioselectivity (Scheme 14). 

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