Lithium catalysts chiral heterobimetallic

By virtue of a deep understanding of his LnM3tris(BINOLate)3 complexes (Ln = rare-earth metal, M = alkali metal) based on evidence from X-ray analysis and other experiments, Shibasaki developed chiral heterobimetallic yttrium(in) lithium(i) tris(binaphtholate) complex 22, which can promote the catal) ic enantioselective aza-Michael reaction of metho g lamine to enones in excellent yields with up to 97% ee as a Lewis-acid-Lewis-acid cooperative catalyst (Scheme 2.17). Transformation of the 1,4-adducts 23 afforded the corresponding optically active aziridines 24 in high yields. 

Shibasaki developed the first catalytic enantioselective hydropho-sphonylation of aldimines with the use of chiral heterobimetallic lantha-num(iii) potassium(i) tris(binaphtholate) 89, which provides optically active a-amino phosphonates with high enantioselectivities (Scheme 2.50). Similar to lithium catalyst 26 and sodium catalyst 67, potassium catalyst 89 acts as an acid-base bifunctional catalyst to activate both nucleophiles and electrophiles. In particular, in this reaction, deprotonation of dimethyl phosphite by more basic potassium catalyst 89 was essential for increasing the reactivity and enantioselectivity, while less basic lithium catalyst 26 and sodium catalyst 67 were not effective. 

This idea was realized very successfully by Shibasaki and Sasai in their heterobimetallic chiral catalysts [17], Two representative well-defined catalysts. LSB 9 (Lanthanum/Sodium/BINOL complex) and ALB 10 (Aluminum/Lithium/BINOL complex), are shown in Figure 8D.2, whose structures were confirmed by X-ray crystallography. In these catalysts, the alkali metal (Na, Li, or K)-naphthoxide works as a Br0nsted base and lanthanum or aluminum works as a Lewis acid. 

The first chiral aluminum catalyst for effecting asymmetric Michael addition reactions was reported by Shibasaki and coworkers in 1986 [82], The catalyst was prepared by addition of two equivalents of (i )-BINOL to lithium aluminum hydride which gave the heterobimetallic complex 394. The structure of 394 was supported by X-ray structure analysis of its complex with cyclohexenone in which it was found that the carbonyl oxygen of the enone is coordinated to the lithium. This catalyst was found to result in excellent induction in the Michael addition of malonic esters to cyclic enones, as indicated in Sch. 51. It had previously been reported that a heterobimetallic catalyst prepared from (i )-BINOL and sodium and lanthanum was also effective in similar Michael additions [83-85]. Although the LaNaBINOL catalyst was faster, the LiAlBINOL catalyst 394 (ALB) led to higher asymmetric induction. 

A highly q -selective and enantioselective direct Mannich-type reaction of aldimines 32 and trichloromethyl ketones 33 was developed by Shibasaki using a chiral Pybox (31)-lanthanum(m) lithium(i) heterobimetallic catalyst (Scheme 2.23). The 2-thiophenesulfonyl moiety of 32 can be readily removed by using magnesium metal. Moreover, a trichloromethyl ketone moiety can be transformed to ester, dithiane, and syn- and anti-trichloromethyl carbinols in high yields. 

Scheme 2.23 Sjw-selective and enantioselective direct Mannich-type reaction of aldimines with trichloromethyl ketones with the use of chiral I box-lanthanum(ni) lithium(i) heterobimetallic catalyst.

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. 

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