Throughout the history of transition-metal catalysis, almost every breakthrough is closely related to the development of ligands. Therefore, the history of transition-metal catalysis roughly parallels the development history of ligands. Therefore, ligand development lies in the heart of transition metal catalysis. Since the beginning of homogeneous catalysis, transition metals have been accompanied by phosphine ligands (Wilkinson's catalyst). Till now, phosphine ligands are one of the most widely used ligand types. There are several key factors for the popularity of phosphine ligands, among which the backbone of ligand plays a decisive role in supporting their stability, activity and selectivity. Many privileged phosphine ligands contain five- or six-membered rings in their core structures, possibly due to their ready-availability, low ring strain, and high stability. Seven- and above membered cyclic structures have flexible frameworks, many stable conformations, and are difficult to synthesize, making them unsuitable as ligand backbones. The cyclobutane structure has relatively strong ring strain, but its conformation can be flipped, and it is difficult to synthesize, making it inappropriate to be used as a ligand skeleton, too. As the smallest all-carbon ring, the three carbon atoms of cyclopropane are located in the same plane. Because of the unique electronic structure and rigidity of cyclopropane, changing a substituent on any one of the carbon atoms of the ring will affect the conformation of the substituents on the other two carbons. And cyclopropane structure is relatively simple to synthesize and modify, making it a promising ligand skeleton. However, it is surprising that phosphine ligands with cyclopropane as the core structure have been rarely studied so far, and their applications need to be further explored, too. Based on types of ligands, this perspective systematically summarizes reported phosphine-containing ligands (including monophosphines, diphosphines, phosphine-heteroatoms and triphosphines) with cyclopropane backbone, and their applications in transition metal catalysis. We hope to draw researchers' attention to the cyclopropane-based phosphine ligands and thus promote the development of transition metal catalysis.

Key words: cyclopropane skeleton, phosphine ligands, catalytic organic synthesis, activity, selectivity