The development of catalysts that are not only enantioselective and high yielding but also useful from a practical standpoint persists as a challenging goal in asymmetric synthesis. In the ideal case, a catalyst should be readily available or easily synthesized on any scale and should display both high reactivity (turnover frequency) and durability (turnover number). In this context, substantial progress has been made over the past several years in the discovery of chiral salen-metal-based catalysts (H 2 salen bis(salicylidene)ethylenediamine) for the asymmetric ring-opening of epoxides, and attention has focused recently on the development of these catalysts from a practical perspective. [1] We describe herein a significant advance in this regard, with the development of easily synthesized and highly active oligomeric [(salen)Co] catalysts for the asymmetric hydrolysis of meso-epoxides and kinetic resolution of terminal epoxides.
We reported recently the preparation of mixtures of cyclic oligomeric [(salen)Co] complexes (1), which were designed to enforce the cooperative bimetallic mechanism common to many epoxide ring-opening reactions. [2] Catalyst system 1 displayed substantial improvements in reactivity and enantioselectivity relative to monomeric analogues, with kinetic behavior consistent with cooperative reactivity within the