Kinetic Evaluation of Cooperative [Co(salen)] Catalysts in the Hydrolytic Kinetic Resolution of rac‐Epichlorohydrin

Abstract

A wide variety of [Co(salen)] catalysts with different structures designed to enhance salen–salen cooperative interactions have been reported as catalysts for the hydrolytic kinetic resolution (HKR) of epoxides. However, the myriad catalysts have been evaluated under inconsistent experimental conditions, at different catalyst loadings, at different temperatures, with various epoxides and in several different laboratories, making rigorous comparisons between the catalysts impossible. To this end, 12 representative catalysts from our studies, as well as catalysts originally described by others, are reported in the HKR of epichlorohydrin under rigorously identical conditions. The comparison of the reactivity and selectivity of the different catalysts indicates that the soluble, oligomeric [Co(salen)] catalysts with cyclic frameworks impart the highest activity and enantioselectivity (>99 % ee, krel>99) in this representative reaction, requiring very short reaction times (<30 min), including the most active catalyst reported to date for the target reaction. In general, it is found that soluble polymer or oligomer‐supported catalysts are more active than insoluble supported catalysts under the conditions used. Among the insoluble catalysts, a polymer resin supported [Co(salen)] catalyst with excellent enantioselectivity (>99 % ee, krel>99) and medium reaction time (222 min), is the most active and selective of the class. The lessons demonstrated herein regarding design of active and selective catalysts offer useful insights into refining design strategies for other related cooperative catalytic reactions, suggesting emphases on catalyst flexibility and solubility under reaction conditions.

Poly(styrene) Resin‐Supported Cobalt(III) Salen Cyclic Oligomers as Active Heterogeneous HKR Catalysts

“…We evaluated our supported catalysts under previously established conditions using epichlorohydrin, 0.1 mol% catalyst, 0.6 equiv. water, 25 °C, and acetate as the counterion 14. Using catalyst 13a 99% ee was reached in less than 20 min, which is comparable to the activity of the homogeneous cyclic oligomers.…”

Enantioselective ring opening of epoxides with TMSN3 by macrocyclic oligomer-supported Cr(III)–salen catalysts

“…5,15,28,29 The bimetallic mechanism was confirmed. 30,31 The three macrocyclic oligomer supported Cr(III)-salen catalysts herein showed an opposite trend in the reaction rate and enantioselectivity in the asymmetric ring opening of cyclohexene epoxide.…”

“…17 Weck et al developed a series of macrocyclic oligomeric cyclooctene supported salen complexes showing high activity in various asymmetric catalytic reactions. [23][24][25][26] Macrocyclic oligomeric cyclooctene supported Al(III)-salen with linkers of three different lengths (3-, 7-or 10-bond) were tested in the bimetallic mechanism based cyanide addition to unsaturated imides. The catalyst with the longest linker afforded both the highest yield and the highest enantioselectivity.…”

A Computational Study of Solvent and Electric Field Effects on Propylene Oxide Ring‐Opening Reaction

“…Co(salen), a chiral salen‐Co‐based homogeneous catalyst, shows good performance in the ring‐opening reaction of epoxides . Investigations have been carried out to improve the activity, selectivity and lifetime of Co(salen) …”