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Magnetic TEMPO

Von Dipl. Chem. Alexander Schätz

The selective oxidation of primary and secondary alcohols into the corresponding carbonyl compounds is one of the most important transformations in organic chemistry. Common reagents for these oxidations are usually toxic chromium(VI) salts in stoichiometric amounts causing a severe environmental problem. Therefore, the development of systems displaying high atom efficiency using comparatively harmless oxidants such as oxygen, peroxide or hypochlorite is desired. The stable nitroxyl radical 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) exhibits benign properties such as low toxicity and a reversible redox behavior which motivated its application as a organocatalyst for the oxidation of alcohols to the corresponding alcohols in combination with a primary oxidant. Whichever oxidant is used, product isolation and catalyst recovery remain key issues. Moreover, although low catalyst concentrations are required (typically 1-2.5 mol%), TEMPO is quite expensive. Consequently, it is highly desirable to separate and reuse the catalyst after the oxidation reaction. Nanoparticles are being increasingly recognized as supports for catalysts. In particular, the use of magnetic nanobeads promises easy recycling of the catalyst by magnetic separation (Figure 1) with the advantages of a nanostructured powder.

Magnetic Nanoparticles[Bildunterschrift / Subline]: Figure 1: Magnetic Nanoparticles can be dipersed in and separated from solvents with the aid of an external magnet.

In our efforts to create recyclable magnetic nanobead supported TEMPO we found carbon coated cobalt nanoparticles (Figure 2) recently developed by Prof. W. Stark at the ETH Zürich to be both excellent in terms of stability and magnetic properties. We have designed a simple and efficient covalent functionalization route for these particles utilizing a copper-catalyzed azide/alkyne cycloaddition (“Click”-reaction) as a general tagging method (Figure 3). The resulting magnetic TEMPO emerged as a highly stable and easy separable heterogeneous catalyst showing neither loss of activity nor chemoselectivity even after multiple oxidation reactions.

Transmission electron image of carbon coated cobalt nanoparticles.[Bildunterschrift / Subline]: Figure 2: Transmission electron image of carbon coated cobalt nanoparticles.
Grafting of the diazonium salt of 4-aminobenzyl alcohol onto carbon coated cobalt particles[Bildunterschrift / Subline]: Figure 3: Grafting of the diazonium salt of 4-aminobenzyl alcohol onto carbon coated cobalt particles and subsequent substitution of the alcohol against an azide under modified Mitsunobu conditions affords (azidomethyl) benzene functionalized particles yielding after a copper(I)-catalyzed “click”-reaction with propargyl ether TEMPO in Magnetic TEMPO