New and efficient copper(II) complexes as catalysts for allylic oxidation and Ullman-type reactions

In the last decade, to meet the needs of the circular economy, efforts have been made to reuse alkenes in a green methodology, trying to transform them into new raw materials useful from an industrial point of view. One of the processes that could lead to the synthesis of highly functionalized materials is represented by the allylic oxidation of alkenes. The most important allylic radical oxidation reaction is the Kharasch-Sosnovsky, that originally involved the chemistry of Se, Cr, Pd, Rh. We focused on copper species due to their versatile properties, cost-effectiveness, easy-accessibility, and environmentally friendly properties. In recent studies, the attention has been given to heteroscorpionate ligands due to their coordination ability; in particular, transition metal complexes supported by bis(pyrazole-1-yl)carboxylate ligands have shown their potential as catalysts. Based on these experimental data, we designed and synthesized hexyl and isopropyl ester derivatives of bis(pyrazol-1-yl)- and bis(3,5-dimethyl-pyrazol-1-yl)-acetic acids (LOHex, L1iPr, L2OHex and L2iPr, respectively). The related monomeric or dimeric copper(II) complexes were obtained starting from CuCl2.2H2O or CuBr2 acceptors, and tested as catalysts in the allylic oxidation of alkenes. The Cu(II) complexes showed a very promising catalytic activity in allylic oxidations avoiding the use of any external agents and superstoichiometric amounts of reagents, preventing the generation of excessive waste (Figure 1). In addition, studies related to new copper complexes supported by the anions of the sterically hindered β-diketone ligands, 1,3-dimesitylpropane-1,3-dione (HLMes), 1,3-bis(3,5-bis(trifluoromethyl)phenyl)-3-hydroxyprop-2-en-1-one (HLCF3) and the parent 1,3-diphenylpropane-1,3-dione (HLPh) are ongoing, with special attention to their potential catalytical activity on Ullman-type reaction for the coupling of two aryl and heteroaryl halides.