An Atom‐Economical Approach to 2‐Aryloxypyridines and 2,2′/2,3′‐Diaryloxybipyridines via Ruthenium‐Catalyzed [2+2+2] Cycloadditions

The metal catalyzed cross-coupling reactions are one of the most important transformations in organic synthesis. Copper catalysis has received great attention due to its low toxicity and inexpensive nature. However, traditional Ullmann-type coupling reactions suffered from limited substrate scope and harsh reaction conditions. The introduction of some bidentate ligands such as amino acids, diamines, 1,3-diketones and oxalic diamides during the past two decades has totally changed its scenario, allowing the coupling reactions of aryl halides and nucleophiles to take place at both low reaction temperatures and catalytic loadings. The reaction scope has also been greatly expanded and makes this copper-based cross coupling attractive in both academia and industry. This review will summarize the latest progress in developing the useful reaction conditions for coupling of (hetero)aryl halides with different nucleophiles. Additionally, recent advances on Cu-catalyzed coupling reactions with aryl boronates and Cu-based trifluoromethylations of aromatic electrophiles will be introduced.