(234e) Homogeneous Catalysts Immobilized On Carbon Coated Cobalt Nanoparticles

The loss of homogeneous catalysts during product work up is a significant processing problem resulting in the leakage of valuable precious metal reagents. Conventional heterogeneous supports (e.g. polystyrene) allow efficient catalyst recycling via filtration, albeit a substantial decrease in activity is frequently observed. Nanoparticles are considered a semi-heterogeneous support since they are readily dispersed in common solvents and exhibit an intrinsically high surface area, which is combined with excellent, almost homogeneous-like accessibility of the surface-bound catalytic sites. The separation of dispersed nanoparticles from solutions is, however, an additional challenge as commonly applied filtering and collection devices are prone to clogging and have very high pressure losses.

We have, therefore, developed highly magnetic nanoparticle carriers which act as support for homogenous catalysts and can be rapidly collected from solution. Within this presentation we will show the applicability of the approach with the aid of two industrially important examples: a) chemoselective oxidation of primary and secondary alcohols utilizing magnetic TEMPO (1), b) asymmetric monobenzoylation using magnetically immobilized azabis(oxazoline)-copper(II) (2). In both cases the catalysts supported on highly magnetic carbon coated cobalt nanoparticles delivered superior results compared to their counterparts immobilized on various conventional supports (silica, iron oxide). In addition, the specific properties of the carbon coating enabled us to design a temperature dependent catch/release-system, that allows a fully homogenous process during the course of the reaction and a complete and rapid collection of the catalyst during product work-up.(3)

Figure caption: Schematic representation of a chiral ligand/transition metal- complex covalently grafted on magnetic Co/C-nanoparticles.

References:

(1) Schatz A, Grass RN, Stark WJ, Reiser O, TEMPO Supported on Magnetic C/Co-Nanoparticles: A Highly Active and Recyclable Organocatalyst, Chem. Euro. J.,2008, 14, 8262.

(2) Schatz A, Grass RN, Kainz Q, Stark WJ, Reiser O, Cu(II)-Azabis(oxazoline) Complexes Immobilized on Magnetic Co/C Nanoparticles: Kinetic Resolution of 1,2-Diphenylethane-1,2-diol under Batch and Continuous-Flow Conditions, Chem. Mater. 2010, 22, 305.

(3) Wittmann S, Schatz A, Grass RN, Stark WJ, Reiser O, A Recyclable Nanoparticle-Supported Palladium Catalyst for the Hydroxycarbonylation of Aryl Halides in Water, Angew. Chem. Int. Ed., 2010, 49, 1867.