Synthesis and Characterization of a Polymer-Supported Salen Ligand for Enantioselective Epoxidation

Abstract

We report the synthesis of a covalently linked salen complex onto the surface of a 98 vol % styrene/2 vol % divinylbenzene polymeric support. The solid-phase method allows manipulation of the substituents on the salen complex in a more versatile manner than can be done for the homogeneous ligand and the production of a support for catalysis that guarantees accessibility to the supported active sites. The ligand was constructed in a stagewise manner that allows the possibility for combinatorial syntheses. Immobilized salen complex 1 was formed by the sequential treatment of the resin with 2,4,6-trihydroxybenzaldehyde, a chiral trans-1,2-diaminocyclohexane, and 3,5-di-tert-butylsalicylaldehyde. After loading the immobilized ligand with Mn, the support effected the enantioselective epoxidation of 1,2-dihydronaphthalene, cis-β-methylstyrene, and styrene as test olefins with enantiomeric excesses of 46%, 79% (cis-epoxide), and 9%, respectively, using aqueous sodium hypochlorite as the oxidant. The heterogeneous catalyst also showed a selectivity toward the formation of the trans-epoxide in the case of cis-β-methylstyrene. The heterogeneous system allows ease of separation and recycle, albeit with diminished enantioselectivity in reuse.

Polymer-supported salen complexes for heterogeneous asymmetric synthesis: Stability and selectivity

“…16,46 Dichloromethane was used as solvent to minimize the competing reac- tion between the aqueous oxidant and olefin to form racemic epoxides. 45 Table I shows that the enantiomeric excesses (ee's) observed with these heterogeneous complexes (2) are lower than those produced by the homogeneous ligand (1) loaded with Mn. While the ee's for dihydronaphthalene and cis-␤-methylstyrene (46% and 79%, respectively) are lower than those produced by the homogeneous complex (ranging from 70 -98% depending on the substituents on the catalyst 16 -18 ), these values are higher than those reported for heterogeneous systems that use two-point attachment of the ligand to a polymeric support (37 and 62%, respectively).…”

“…In our prior work, we synthesized the salen ligand on a styrene/divinylbenzene polymer matrix, loaded it with manganese, and catalyzed reactions between several olefins and an oxidant. 45 In these epoxidation reactions, the catalyst did not maintain its activity and selectivity upon recycle, suggesting a loss of reactive sites either by leaching of the metal center or fracture of the heterogeneous ligand. Subsequent observations under related epoxidation conditions with the heterogeneous as well as the homogeneous catalyst suggested that fracture of the heterogeneous ligand limited recycle.…”

New thiophene-based azo ligands containing azo methine group in the main chain for the determination of copper(II) ions

“…Among the wide range of materials that may be suitable for the electronic and optical applications, salicyaldimine-based ligands are of particular interest. They find applications in preparation of metallomosegens [19], various crystallographic studies [14], optical metal ion detections [20], and enantioselective catalysis [21]. Also, salicylaldehyde gives more easily polymerization reaction of the 3-or 5-position of the structure.…”

Olefin epoxidation with tert-butyl hydroperoxide catalyzed by functionalized polymer-supported copper(II) Schiff base complex

“…In the FT-IR spectrum of polymer-bound aldehyde, a new peak appeared at 1,702 cm -1 that was assigned to carbonyl group stretching vibration of the aldehyde. In the polymer bound aldehyde, the sharp C-Cl peak (due to -CH 2 Cl groups) at 1,264 cm -1 in the starting polymer was seen as a weak band or was absent [38]. The formation of the Schiff base ligand on the polymeric support is indicated by a peak at 1,624 cm -1 assigned to C=N stretching frequency.…”