Asymmetric Activation and Deactivation of Racemic Catalysts

Department of Applied Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo 

New Frontiers in Asymmetric Catalysis, Edited by Koichi Mikami and Mark Lautens Copyright (g) 2007 John Wiley Sons, Inc. 

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ASYMMETRIC ACTIVATION AND DEACTIVATION OF RACEMIC CATALYSTS (a) Aldol reaction... 

Combination of the asymmetric activation and asymmetric deactivation protocols as asymmetric activation/deactivation can be achieve the difference in catalytic activity between the two enantiomers of racemic catalysts can be maximized through selective activation and deactivation of enantiomeric catalyst, respectively (Scheme 8.15). 

While non-racemic catalysts thns developed can generate non-racemic products with or without the non-linear relationship in enantiomeric excesses between catalysts and products [8], racemic catalysts inherently give only a racemic mixture of chiral products. Recently, a strategy whereby a racemic catalyst is selectively deactivated by a chiral molecule has been reported to yield non-racemic products (Fig. 7-3). However, we have reported a conceptually opposite strategy to asymmetric catalysts in which a chiral activator selectively activates one enantiomer of a racemic chiral catalyst (Fig. 7-2). The advantage of this activation strategy over the deactivation counterpart is that the activated catalyst can produce a greater enantiomeric excess (Xact% ee) in the products, even with the catalytic amount of the chiral activator to the chiral catalyst, than can the enantiomerically pure catalyst on its own (x% ee). 

Another interesting issue is the possibility of creating optically active compounds with racemic catalysts. The term chiral poisoning has been coined for the situation where a chiral substance deactivates one enantiomer of a racemic catalyst. Enantiomerically pure (R,R)-chiraphos rhodium complex affords the (iS )-methylsuccinate in more than 98% ee when applied in the asymmetric hydrogenation of a substrate itaconate.109 An economical and convenient method... 

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