Polyglycerol‐Supported Chromium‐Salen as a High‐Loading Dendritic Catalyst for Stereoselective Diels–Alder Reactions

Abstract

Abstract:In this paper we demonstrate the application of hyperbranched polyglycerol (PG) 3 as a polymeric support for asymmetric catalysis. A new polyglycerol-supported unsymmetrical salen ligand 4 is described, which was successfully purified by gel permeation chromatography (GPC) or by ultrafiltration. After the insertion of the metal, e.g., chromium, the corresponding polymeric chromium complex was used as catalyst for asymmetric Diels-Alder reactions between Danishefskys diene and benzaldehyde. The catalytic activities (up to 98 % conversion) and enantioselectivities (up to 78 % ee) were comparable to the original catalyst reported by Jacobsen. The soluble polyglycerol-supported catalysts were recovered by dialysis after the catalytic reactions and were recycled two times to afford identical reactivities as in the first run, with slightly reduced enantioselectivities. Moreover, this polymeric support catalyst showed a high retention (99.02 %) in a continuously operated membrane reactor.

Intramolecular Acceleration of Asymmetric Epoxide Ring‐Opening by Dendritic Polyglycerol Salen–CrIII Complexes

“…Synthesis of hPG-C1-CrCl (4). has been used before in our group, [19] but because the salen ligand builds up on the support this approach suffers from ill-defined catalyst species on the polymer, because full conversion of all functional groups on the polymer cannot be guaranteed. Owing to the labile nature of the imine bond, synthesis of the unsymmetrical salen ligand leads to mixtures of symmetrical and unsymmetrical ligands and low yields are obtained.…”

“…[19,31] This polyether polyol can be synthesized on a kilogramscale by ring-opening polymerization of glycidol [32] and contains linear monohydroxy and terminal dihydroxy functional groups, which can be easily converted by standard synthetic methods. Its versatile properties, such as highloading capacity (13.5 mmol g -1 ), good solubility in a wide range of solvents (depending on the functional groups in the periphery), chemical stability, and noncoordinating properties, make it a promising support for reagents and catalysts, especially metal complexes.…”

“…[3,4] Until today, their easy preparation and versatile catalytic performance, depending on the nature of the chelated metal, made them "privileged" ligands. [5] Metalated salens have been prepared with a variety of transition metals, including Mn, Cr, Co, V, Cu, Ti, Ru, Pd, Au, Zn, and Al, and have been used for a multitude of asymmetric organic transformations, for example, the epoxidation of olefins, [4,6,7] the ring-opening of epoxides, [8][9][10][11] hydrolytic kinetic resolution of epoxides, [12][13][14][15] conjugate addition reactions to α,β-unsaturated imides, [16] (hetero)-Diels-Alder reactions, [17][18][19] and many others. [1] Owing to the cost of the catalysts, which usually exceeds the value of the products, and the challenge to simplify product purification from metal residues, it is desirable to [ immobilize salen ligands and several attempts have been made in this field.…”

Continuous Application of Polyglycerol‐Supported Salen in a Membrane Reactor: Asymmetric Epoxidation of 6‐Cyano‐2,2‐dimethylchromene

“…Recently, we reported on the preparation procedure for a new polyglycerol-supported unsymmetrical salen ligand (chemzyme, M n of initial polymer = 8000 g mol À1 ) [52] and its corresponding chromium, cobalt, and manganese complexes. The respective metal complexes were proven as efficient catalysts for the Diels-Alder reaction, hydrolytic kinetic resolution of racemic epoxides, and olefin epoxidation.…”

“…[52] Purification of the crude product via ultrafiltration (as an alternative for preparative GPC) lead to the pure polymer-supported ligand with 1.6 mmol g …”