Enhanced Cooperativity through Design: Pendant Co^IIISalen Polymer Brush Catalysts for the Hydrolytic Kinetic Resolution of Epichlorohydrin (Salen=N,N′‐Bis(salicylidene)ethylenediamine Dianion)

Abstract

The Co(III)--salen-catalyzed (salen=N,N'-bis(salicylidene)ethylenediamine dianion) hydrolytic kinetic resolution (HKR) of racemic epoxides has emerged as a highly attractive and efficient method of synthesizing chiral C(3) building blocks for intermediates in larger, more complex molecules. HKR reaction rates have displayed a second order dependency on the concentration of active sites, and thus researchers have proposed a bimetallic transition state for the HKR mechanism. Here we report the utilization of pendant Co(III)--salen catalysts on silica supported polymer brushes as a catalyst for the HKR of epichlorohydrin. The novel polymer brush architecture provided a unique framework for promoting site-site interactions as required in the proposed bimetallic transition state of the HKR mechanism. Furthermore, the polymer brushes mimic the environment of soluble polymer-based catalysts, whereas the silica support permitted facile recovery and reuse of the catalyst. The polymer brush catalyst displayed increased activities over the soluble Jacobsen Co--salen catalyst and was observed to retain its high enantioselectivities (>99 %) after each of five reactions despite decreasing activities. Analysis indicated decomposition of the salen ligand as an underlying cause of catalyst deactivation.

Recyclable Polymer‐ and Silica‐Supported Ruthenium(II)‐Salen Bis‐pyridine Catalysts for the Asymmetric Cyclopropanation of Olefins

“…In the proposed bimetallic transition state of the Co-salencatalyzed hydrolytic kinetic resolution (HKR) mechanism, this flexibility greatly affected the catalyst performance. [14,21] In the monometallic mechanism of the Ru-salen cyclopropanation reaction, this flexibility could be less important. However, the linker does impact the steric hindrance of the polymer backbone near the catalytically active site, which can be important for polymer-supported catalysts in general.…”

“…Synthesis of polymer resin-supported Ru(II)-salen bis-pyridine catalysts [a: 5 was synthesized via thiol coupling of mercaptopropyltrimethoxysilane (MPTMS) with styryl-salen 2a at 46% isolated yield. [14,25] Compound 5 was immobilized on 60 mesoporous SBA-15 silica by condensation with surface hydroxy groups forming material 6. Unreacted silanol groups were capped with hexamethyldisilazane (HMDS) to form material 7 prior to the metallation procedure with n-butyllithium and ruthenium dimer.…”

“…A silane-modified salen species 5 was synthesized using procedures developed within our research group. [14] The salen was post-grafted to the SBA-15 support by refluxing silane-modified salen 5 (290 mg) and SBA-15 (1.7 g) in anhydrous toluene (16 g) under argon (24 h). The resulting solid was recovered via filtration and washed with copious toluene, DCM, hexane, and ether.…”

“…Building on past work in our research group focused on development of supported Co-salen catalysts, [14][15][16][17] alternative techniques are developed here to provide the first report of heterogenized Ru-salen complexes covalently grafted to solid supports. The goal of this work was to develop a solid catalyst, stable over multiple cycles, that retained the high activity, diastereoselectivity, and enantioselectivities exhibited by the homogeneous analogue in the asymmetric cyclopropanation of olefins with ethyl diazo-A C H T U N G T R E N N U N G acetate (EDA).…”

Catalytic hydrogen evolution from a covalently linked dicobaloxime

“…It is of interest in this context that binuclear analogues of many catalysts that rely on bimetallic interactions show enhanced activity when compared to monomeric species, examples include certain Ru and Os diporphyrin H 2 evolving catalysts with cofacial orientation (24,25). In one noteworthy case, Jones and coworkers (26,27) showed that a "bisalen" analogue of Jacobsen's Co-(salen) catalyst for hydrolytic kinetic resolution of racemic epoxides (a reaction that also shows a second-order dependence on catalyst concentration) maintained activity when linked to a solid support, whereas the immobilized monomer was essentially inactive (26)(27)(28). Because one of our goals is construction of a catalytic cobaloxime-modified electrode for water reduction, we have investigated the reactivity of a covalently-linked dicobalt complex.…”