Recent advances in transition metal-catalysed cross-coupling of (hetero)aryl halides and analogues under ligand-free conditions

The formation of new bonds is pivotal in organic chemistry and a prerequisite to life because it allows the construction of complex molecules from simple precursors. This gives credence to the intensive study of metal-catalysed C–C bond formation via cross-coupling reactions over the past few decades. However, despite the tremendous success recorded for ligand-supported palladium in the catalytic transformation of (hetero)aryl halides, the challenges in their practical application lie in their cost and difficulty of preparation, sensitivities to air/moisture, toxicity and complete incompatibility with aqueous media, which remarkably limit their industrial applications. Non-ligated or ligand-free metal-catalysed cross-coupling reactions are attractive alternatives to ligand-supported counterparts due to their recyclability, cost-effectiveness, sustainability and full industrial applications. Over the years, various research groups have strived to advance ligand-free catalytic systems for cross-coupling of (hetero)aryl halides because of the aforementioned advantages. This review highlights the progress, challenges and prospects in ligand-free metal-catalysed cross-coupling of (hetero)aryl halides and their analogues.