Enantioselective ring opening reactions catalysed by heterogenized metal salen complexes

Epoxides are important intermediates for the stereo controlled synthesis of complex organic enantioselective compounds. Their use has increased dramatically with the advent of practical asymmetric catalytic methods for their synthesis. In the early nineties, Jacobsen et al. designed the chiral Schiff base complex, which to date is the most efficient catalyst available for the enantioselective epoxidation of unfunctionalised olefins. By the variation of the metallic centre various active catalysts can be created that enable access to valuable products derived from terminal and meso-epoxides in high enantiomeric purity via asymmetric ring opening reactions. These products are valuable chiral intermediates for the preparation of several pharmaceutical agents. Although the catalytic performance of this type of catalyst is excellent, their use on an industrial scale is limited due to their homogeneous nature. In this work, various cobalt and chromium based Jacobsen’s catalysts were incorporated into M41S and SBA-15 type mesoporous carrier materials using optimised ion-exchange methods and impregnation. The materials were characterised with a large variety of characterisation techniques; their properties were compared with those of the carrier materials and with the metal ion exchanged carriers. The synthesised catalysts were tested on their catalytic performance in several asymmetric ring-opening reactions. The incorporation of the organometallic complexes inside the pores of the carrier materials could be confirmed by several characterisation techniques. The acidity of the protonated aluminium containing support materials was investigated with FTIR on materials that were loaded with pyridine, and with the temperature programmed desorption of ammonia (NH3-TPD). A correlation could be found between the acidic character of the host and the amounts of organometallic complex that could be immobilised inside the mesopores. AlMCM48 could host the highest amount of organometallic complex due to its stronger Brønsted acidic character. Based on the results immobilisation was found to be the most effective way for the incorporation of organometallic complexes without affecting the characteristic properties of the carrier materials. AlMCM48 was found to be the most sensitive support material for immobilisation treatments. The catalytic performance of the heterogenised with Co(II), Co(III)OAc and Cr(III)Cl substituted Jacobsen salen complexes was tested in several asymmetric ring opening (ARO) reactions. All homogeneous catalysts were found to be highly active in the here investigated Kinetic Resolution reactions (styrene oxide with water, cyclohexene oxide with benzoic acid, 1,2-epoxyhexane with phenol and cyclohexene oxide with azido-trimethyl-silane). The here obtained catalytic results were in good correspondence with literature. The catalytic activity of the heterogenised organometallic complexes was found to be connected to their accessible acidic sites and their mesoporous structure. The accessibility of the acidic sites could be optimally maintained by an intact mesoporous structure. AlMCM41 and AlSBA15 possess a 2D hexagonal mesoporous structure; however, the SBA type materials have wider pores and a more stable framework. The highly ordered 3D pore structure of AlMCM48 was shown to have a positive effect in some reactions. The theory of metal-acid bi-functional mechanisms could be confirmed over all materials in all reactions. Generally, among the heterogenised catalysts the best catalytic activity was found for the by immobilisation prepared catalysts. With the immobilisation treatment, more organometallic complex could be incorporated into the framework without significant structural damage than by ion exchange method. The by ion exchange and post ligand loading synthesised catalysts generally showed higher ee’s, which was probably due to a better distribution of metal ions in the framework. In most reactions over the by immobilisation prepared heterogeneous materials, selectivity’s and efficiencies comparable to their homogeneous counterparts were obtained. Several limiting factors might play role in the somewhat affected catalytic performance of the heterogenised complexes, such as i.g. shielding effect of the carrier material or inaccessible acid sites. Finally, the reusability and leaching behaviour of the heterogenised materials was investigated. A wash of the used catalysts prior to the recycle and leaching experiments showed that both substrate and product were occluded in the materials. This means that the real catalytic performance was very likely higher than the found one. The reusability tests showed that all heterogenised catalysts could be reused, although with some loss of activity. The Hot filtration (leaching) experiments showed that all catalysts leached; therefore, catalysis might very well have been homogeneous instead of heterogeneous.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2011

Author dc:creator

• Niederer-Bátorfi, Melinda

Contributors dc:contributor

• Hölderich, Wolfgang F.

• info:eu-repo/classification/ddc/540
• Salen
• Epoxide
• Ringöffnungsreaktion
• Übergangsmetall
• Zeolith M41S
• Asymmetrische Reaktion
• Immobilisierung
• Trägerkatalysator
• Heterogene Katalyse
• Chemie
• metal salen
• Jacobsen complex
• ring opening reactions
• heterogeneous catalysis
• heterogenisation
• enantioselective
• M41S
• mesoporous materials

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-124409%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:62927