Asymmetric synthesis by enantiomer-selective activation of racemic catalysts

Asymmetric catalysis of organic reactions to provide enantiomerically enriched products is of central importance to modern synthetic and pharmaceutical chemistry1–3. While non-racemic catalysts can generate non-racemic products, racemic catalysts inherently produce only a racemic mixture of chiral products. But a strategy whereby a racemic catalyst is selectively deactivated by a chiral molecule has been shown to yield non-racemic products4–6. Here we describe an alternative, conceptually opposite strategy to asymmetric catalysis in which a chiral activator selectively activates one enantiomer of a racemic chiral catalyst. Our catalyst is a titanium(IV) complex for which a chiral additive acts as the chiral activator. The advantage of this approach over the deactivation strategy is that the activated catalyst can produce a greater enantiomeric excess in the products than can the enantiomerically pure catalyst on its own, as our results demonstrate.