Enantiomer-selective activation of racemic catalysts

Asymmetric catalysts can be evolved into highly activated catalysts by asso ciation with chiral activators. This asymmetric activation process is parti cularly useful in racemic catalysis through selective activation of one ena ntiomer of the racemic catalysts. Recently, a strategy whereby a racemic ca talyst is selectively deactivated by a chiral additive has been reported to yield nonracemic products. However, we have reported a strategy that is an alternative to asymmetric catalysts but is conceptually opposite, in which a chiral activator selectively activates rather than deactivates one enant iomer of a racemic chiral catalyst. The advantage of this activation strate gy over the deactivation counterpart is that the activated catalyst can pro duce a greater enantiomeric excess (x(act)% ee) in the products than the ee attained by the enantiomerically pure catalyst on its own. Therefore, 'asy mmetric activation' could provide a general and powerful strategy for the u se of not only atropisomeric and, hence, racemic ligands but also chirally flexible and 'pro-atropisomeric' ligands without enantiomeric resolution!.