Metal-BINOL complex, enantioselective Michael addition

Among metal-BINOL complex catalysts developed by Shibasaki et al., (I )-ALB is found to be the most effective catalyst for the asymmetric Michael addition of malonate to 2-cyclohexen-l-one. In 1998, Shibasaki et al. further fine-tuned the reaction 9onditions by adding the base (e.g., KO-r-Bu) and MS (4 A) to the system. These optimizations accelerate the catalytic asymmetric Michael addition without lowering the enantioselectivity. They suggested that the base was used to activate the Aluminum-Lithium-BINOL (ALB) complex. The 4-A molecular sieve (MS) was used to remove the trace amount of H2O that could otherwise gradually lead to ALB-KO-f-Bu catalyst decomposition. In the presence of ALB (0.3 mol%), KO-f-Bu (0.27 mol%), and 4-A MS, the Michael addition of dimethyl malonate to cyclohex-2-enone proceeded smoothly to give 94% product yield with 99% ee even at room temperature. Particularly noteworthy was that this reaction could be carried out on a 100-g scale without deleterious effect. Later, Xu and co-workers modified and streamlined the work-up procedures... 

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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