TEMPO in metal complex catalysis

Nitroxyl radicals, illustrated by 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), constitute a versatile class of stable free radicals that are extensively used in metal complex catalyzed synthetic transformations of a great interest. The Cambridge Structural Database reveals more than 260 hits, concerning crystal structures of metal complexes of N-oxyl ligands and most of them (ca. 240) relate to complexes comprising TEMPO and its derivatives. In order to prepare multifunctional catalysts, supramolecular architectures, drugs, magnetic materials, etc., the para-position of TEMPO has been functionalized with -OH, -COOH, -CN, -NH2 or -N-3, leading to an increase of the coordination sites beyond the N-O. synthon. Herein, we highlight the coordination mode and reactivity of TEMPO ligands in the design of coordination compounds, which can be applied in catalysis. The review mainly focuses on the auxiliary/crucial role of TEMPO in functionalization of alkanes, alkenes, alkynes, aromatics, heterocycles, aldehydes, ketones and alcohols, catalyzed by metal complexes. (C) 2020 Elsevier B.V. All rights reserved.