Pyrazolopyridine Ligands in Transition-Metal-Catalyzed C-C and C-Heteroatom Bond-Forming Reactions

Pyrazole-substituted pyridines have emerged as versatile bidentate ligands in transition-metal catalysis, providing opportunitiesto fine-tune reactivity and selectivity beyond what conventional bipyridine ligands can achieve. This review focuses on two representative pyr-azolopyridine ligands: 2-(1H-pyrazol-1-yl)pyridine (1-PzPy) and 2-(1H-pyrazol-3-yl)pyridine (3-PzPy). The 1-PzPy series, characterized by apyrazole ring serving as a weakly coordinating Lewis basic ligand, offerflexibility in ligand binding. Alternatively, the 3-PzPy series provide bothL2- and LX-type binding modes, functioning as hydrogen bond donors and sigma-donors, respectively. The structural diversity of pyrazolopyridine ligands enables the development of various synthetic strategies, facilitating cross-coupling, cycloaddition, photocatalytic, and asymmetric reactions. This review highlights the roles of these ligands in advancing transition-metal-catalyzed C-C and C-hetero atom bond-forming reactions.1 Introduction2 Synthesis of Pyrazolopyridine Ligands3 Applications of 1-PzPy Ligands4 Applications of 3-PzPy Ligands5 Conclusion