Poly(styrene)‐Supported Co–Salen Complexes as Efficient Recyclable Catalysts for the Hydrolytic Kinetic Resolution of Epichlorohydrin

Abstract

Here we describe an unprecedented synthetic approach to poly(styrene)-supported chiral salen ligands by the free radical polymerization of an unsymmetrical styryl-substituted salen monomer (H2salen = bis(salicylidene)ethylenediamine). The new method allows for the attachment of salen moieties to the polymer main chain in a flexible, pendant fashion, avoiding grafting reactions that often introduce ill-defined species on the polymers. Moreover, the loading of the salen is controlled by the copolymerization of the styryl-substituted salen monomer with styrene in different ratios. The polymeric salen ligands are metallated with cobalt(II) acetate to afford the corresponding supported Co-salen complexes, which are used in the hydrolytic kinetic resolution of racemic epichlorohydrin, exhibiting high reactivity and enantioselectivity. Remarkably, the copolymer-supported Co-salen complexes showed a better catalytic performance (>99 % ee, 54 % conversion, one hour) in comparison to the homopolymeric analogues and the small molecule Co-salen complex. The soluble poly(styrene)-supported catalysts were recovered by precipitation after the catalytic reactions and were recycled three times to afford almost identical enantiomeric excesses as the first run, with slightly reduced reaction rates.

Enhanced Cooperativity in Hydrolytic Kinetic Resolution of Epoxides using Poly(styrene) Resin‐Supported Dendronized Co‐(Salen) Catalysts

“…The oxidation process was followed by a color change from deep red to dark brown which is well documented in the literature. [25] We investigated the HKR of the following structurally diverse racemic epoxide sub- [a] Catalyst loadings are reported on a per cobalt basis relative to racemic epoxide. [b] Enantiomeric excess of the remaining epoxide was determined by GC analysis using a Chiraldex G-TA column.…”

“…[26,40] It has been postulated that the HKR of epoxides using (salen)Co catalysts follows a bimetallic pathway where two catalytic centers with proper proximity are needed for the catalysis to occur. [47][48][49][50][51] Examples of promoting high local concentrations of (salen)Co for beneficial effects on catalytic activity for HKR have been shown previously on dendrimers, [38] polymers [25,26,[41][42][43][44] and silica. [49] In contrast to soluble supports, it is more difficult to generate cooperative activation on a solid material due to the absence of elaborate control over the proper proximity and relative conformation of active centers on a surface, porous material or insoluble polymer.…”

“…[31,32] Among these reactions, the hydrolytic kinetic resolution (HKR) of racemic epoxides by (salen)Co catalysts is of particular interest, since enantiopure epoxides are versatile intermediates for asymmetric organic syntheses in the pharmaceutical and fine chemical industries. A variety of groups have reported the immobilization of (salen)Co catalysts on polymer supports or membranes, [25][26][27][28][33][34][35][36][37][38][39][40][41][42][43][44][45][46] but only two reports [45,46] describe the use of insoluble polymer resins as supports. However, these resin-supported catalysts require high catalyst loadings in HKR when compared to soluble and optimized polymer-supported catalysts.…”

Highly Active Polymer‐Supported (Salen)Al Catalysts for the Enantioselective Addition of Cyanide to α,β‐Unsaturated Imides

“…[31,[34][35][36] Our research group has previously demonstrated that the selectivity and activity of sa-A C H T U N G T R E N N U N G len(Co) and salen(Mn) catalysts for the hydrolytic kinetic resolution (HKR) and asymmetric epoxidation reactions can be improved significantly in a modular fashion by varying their catalyst site density along a polymeric backbone. [37,38] In all cases, the polymersupported catalysts could be precipitated easily after completion of the reaction and in some cases reused up to four times. For example, we have reported macrocyclic oligomeric salen(Co) catalysts with excellent activities for the HKR reaction of epoxides that allow for the reduction of catalyst loadings to 0.01 mol% for a wide variety of substrates without reducing the yields or enantioselectivities of the reaction.…”

Polymer and Silica Supported Tridentate Schiff Base Vanadium Catalysts for the Asymmetric Oxidation of Ethyl Mandelate – Activity, Stability and Recyclability

“…This research predominantly focused on the immobilization of cobalt salen catalysts for application in the hydrolytic kinetic resolution (HKR) of epoxides, and a number of successful polymer supported catalysts have resulted from these efforts. [14][15][16] Since the tridentate Schiff base vanadium catalysts used in the oxidative kinetic resolution reaction resemble "half" salen functionalities, we set out to apply the knowledge gained from the immobilization of the cobalt salen HKR catalysts to the development of immobilized vanadium oxidative kinetic resolution catalysts. Even though the activities of the homogeneous catalysts are well-established, there are, to the best of our knowledge, no reports of analogous immobilized catalysts derived from salicylaldehydes and tert-leucinol for the oxidative resolution reaction.…”