Copper(ii) phenalenyl complexes: structures, redox properties and catalytic activity towards Sonogashira coupling reactions

Abstract

Transition metal-based cross-coupling reactions have set their value in myriad applications in organic synthesis for the construction of carbon–carbon bonds. Among the various cross-coupling reactions, the alkynylation is considered to be one of the most significant and useful cross-coupling reactions. Classically, a Pd/Cu-based bimetallic catalyst was considered an efficient and convenient system for this reaction (widely known as the Sonogashira coupling). However, due to the high cost of Pd and its toxicity, less toxic 3d-metal-based catalysts have emerged as a suitable substitute for Pd in alkynylation reactions. We herein introduce three Cu(II) complexes (1, 2 and 3) bearing bidentate phenalenyl ligands as efficient catalysts for the Sonogashira reactions of terminal alkynes with aryl halides (iodides and bromides) in the presence of KOH in dimethylsulfoxide at 110 °C under inert conditions. All these complexes showed excellent catalytic activity toward Sonogashira coupling reactions, resulting in the isolation of the corresponding coupling products in good to excellent yields (50 examples). Structural, spectroscopic, electrochemical, several mechanistic studies and theoretical calculations were carried out to understand the reactivity of complexes 1–3 towards Sonogashira coupling reactions.