Lanthanoid-lithium-BINOL complexes

A. Catalytic, asymmetric nitroaldol reaction promoted by the lanthanoid-lithium-BINOL complex (LnLB)... 

Shibasaki and coworkers have developed lanthanoid-lithium-BINOL complexes (LLB catalysts) as efficient catalysts for the asymmetric nitroaldol (Henry) reaction (59-46). The heterobimetallic asymmetric catalysts effectively mediate the reaction of a variety of aldehydes with nitroalkanes to afford the corresponding desired nitroaldols with high enantioselectivity (Scheme 4). We examined the capability of the LLB complexes as asymmetric catalysts for the nitroaldol reaction of 2,2-difluoroaldehydes with nitromethane (47). 

In the lanthanoid-lithium-( )-BINOL-catalyzed nitroaldol reaction of 2,2-difluoroaldehydes, the nitronates were found to preferentially react on the Si face of the aldehydes. On the contrary, (/1)-LLB generdly causes attack with the Re facial selection as ahown in Figure 2 (39,40,42). Therefore, the enantiotopic face selection for 2,2-difluoroaldehydes is opposite to that for nonfluorinated aldehydes. This stereoselectivity is identical with that of jS-oxaaldehydes, suggesting that fluorine atoms at the a position exert a significant influence on the enantioface selection. The fluorine atoms may coordinate with the rare earth or the lithium of LLB complexes. 

The second part of the chapter deals with several kinds of asymmetric reactions catalyzed by unique heterobimetallic complexes. These reagents are lanthanoid-alkali metal hybrids which form BINOL derivative complexes (LnMB, where Ln = lanthanoid, M = alkali metal, and B = BINOL derivative). These complexes efficiently promote asymmetric aldol-type reactions as well as asymmetric hydrophosphonylations of aldehydes (catalyzed by LnLB, where L = lithium), asymmetric Michael reactions (catalyzed by LnSB, where S = sodium), and asymmetric hydrophosphonylations of imines (catalyzed by LnPB, where P = potassium) to give the corresponding desired products in up to 98% ee. Spectroscopic analysis and computer simulations of these asymmetric reactions have revealed the synergistic cooperation of the two different metals in the complexes. These complexes are believed to function as both Brpnsted bases and as Lewis acids may prove to be applicable to a variety of new asymmetric catalytic reactions.1,2... 

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