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Immobilization of homogeneous catalysts on nanoparticles and their application in semi-heterogeneous catalysis

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  • 540 Chemie

Abstract

It was the aim of this work to develop a generally applicable strategy for the immobilization of catalysts on different nanoparticles with particular interest in the �heterogenization� of chiral azabis(oxazolines), which represent a predestined class of ligands for the grafting on solid supports due to their central nitrogen atom. To this end, a copper(I)-catalyzed azide/alkyne cycloaddition (CuAAC) reaction was envisaged to be the most versatile tagging method, allowing even the use of preformed transition-metal complexes after according derivatization with an alkyne moiety. Such a route was expected to provide distinct advantages over the in-situ complexation by adding the equivalent amount of metalsalt to the immobilized ligand, since the exact determination of ligand loading is challenging. This issue was aggravated by recent investigations from Reiser et al., indicating that not only an excess of transition metal has a detrimental effect on the optical yields attained, but also a ligand surplus is capable of diminishing the level of enantioselectivity in certain reactions. Whereas the negative influence of uncomplexed metal centers on the ee-values obtained is apparent, since no stereodiscriminating environment is coined to the catalytic center by a chiral ligand, the elucidation of the mechanism of the unprecedented effect of ligand surplus was considered highly relevant for immobilized ligands on any type of support. Thus, preliminary investigations focused on the understanding of this effect with the aim to develop strategies, which could help circumventing the negative influence of ligand excess in the reactions affected. Azabis(oxazoline) ligands and azabis(oxazoline)-copper(II) complexes respectively were successfully immobilized on superparamagnetic magnetite@silica- and ferromagnetic carbon coated cobalt-nanoparticles using a concise �click� protocol. Propargylated azabis(oxazoline)-ligands and Cu(II)-complexes were equally active in the CuAAC reaction with the azide functionalized core/shell materials, resulting in quantitative conversion of the azide moieties. The material thus obtained, proved to be highly active and selective in the asymmetric monobenzoylation of racemic 1,2-diols. [email protected] supported catalyst could be recycled via magnetic decantation after each run and proved to be active in at least five consecutive batch reactions without any significant drop in selectivity. Co/C-nanoparticle supported catalyst was as active at batch conditions and allowed the implementation of the nanomagnets in a closed circuit-type reactor under continuous-flow conditions. The high saturation magnetization of the ferromagnetic cobalt cores allowed those particles to act as their own nanosized stirrers in a microreactor that was operated vertically between adjacent parallel flanks of two magnetic stir motors.

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