Highly active oligomeric (salen)co catalysts for asymmetric epoxide ring-opening reactions.

Cooperative bimetallic catalysis has been documented in several recently reported asymmetric epoxide opening reactions that display second-order kinetic dependence on catalyst, specific requirements for multiple metal ions, and/or pronounced catalyst nonlinear effects.1 In such systems, one metal is proposed to serve as Lewis acid for epoxide activation and another as counterion for the nucleophile. In this mechanistic context, complexes that contain multiple metal centers in appropriate relative proximity and orientation can provide improved reactivity relative to monometallic catalysts. For example, chiral metal salen complexes such as 1 are effective catalysts for asymmetric epoxide ringopening reactions, and operate by a second-order mechanism.2 Linking these catalysts as dimers3 or to polymeric4 or dendrimeric5 frameworks leads to catalytic systems with similar enantioselectivity and substantially enhanced reactivity relative to 1. The synthetic utility of multimeric catalysts prepared in this manner is limited, however, by the inefficiency of their syntheses.6 We report here new, easily synthesized oligomeric analogues of 1 that exhibit not only remarkably enhanced reactivity, but also significantly higher enantioselectivity relative to 1.