Simple Preparation and Application of TEMPO-Coated Fe3O4 Superparamagnetic Nanoparticles for Selective Oxidation of Alcohols

• Published in: Chemistry : a European journal Vol. 16; no. 42; pp. 12718 - 12726
• Main Authors: Tucker-Schwartz, Alexander K., Garrell, Robin L.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 08.11.2010; WILEY‐VCH Verlag; Wiley; Wiley Subscription Services, Inc
• Subjects: Alcohol; alcohols; Chemistry; Chemistry, Multidisciplinary; heterogeneous catalysis; Nanoparticles; oxidation; Physical Sciences; Science & Technology; supported catalysts; SILICA-SUPPORTED TEMPO; heterogeneous catalysis; oxidation; alcohols; ALDEHYDES; supported catalysts; POLY(ETHYLENE GLYCOL)-SUPPORTED TEMPO; SECONDARY ALCOHOLS; MAGNETITE NANOPARTICLES; nanoparticles; MILD CONDITIONS; AEROBIC OXIDATION; EFFICIENT; TRANSITION-METAL-FREE; NITROXYL RADICAL CATALYST
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200903527

The organic oxidant TEMPO (2,2,4,4‐tetramethylpiperdine‐1‐oxyl) was immobilized on iron oxide (Fe3O4) superparamagnetic nanoparticles by employing strong metal‐oxide chelating phosphonates and azide/alkyne “click” chemistry. This simple preparation yields recyclable TEMPO‐coated nanoparticles with good TEMPO loadings. They have excellent magnetic response and efficiently catalyze the oxidation of a wide range of primary and secondary alcohols to aldehydes, ketones, and lactones under either aerobic acidic MnII/CuII oxidizing Minisci conditions, or basic NaOCl Anelli conditions. The nanoparticles could be recycled more than 20 times under the Minisci conditions and up to eight times under the Anelli conditions with good to excellent substrate conversions and product selectivities. Immobilization of the catalyst through a phosphonate linkage allows the particles to withstand acidic oxidizing environments with minimal catalyst leaching. Clicking TEMPO to the phosphonate prior to phosphonate immobilization, rather than after, ensures the clicked catalyst is the only species on the particle surface. This facilitates quantification of the catalyst loading. The stability of the phosphonate linker and simplicity of this catalyst immobilization method make this an attractive approach for tethering catalysts to oxide supports, creating magnetically separable catalysts that can be used under neutral or acidic conditions. Recycling to a different TEMPO: An extremely simple and economic synthesis of a recyclable 2,2,4,4‐tetramethylpiperdine‐1‐oxyl(TEMPO)‐coated superparamagnetic catalyst is described. The catalyst shows excellent performance in the rapid oxidation of primary and secondary benzylic and aliphatic alcohols by using oxygen and MnII/CuII or biphasic NaOCl/KBr conditions.