An Experimental and Theoretical Investigation into [2π + 2π] Cycloaddition Reactions Catalyzed by a Cationic Cobalt(I) Complex Containing a Strong-Field PCNHCP Pincer Ligand

Abstract

In the past decade, metal-catalyzed [2π + 2π] cycloaddition reactions have gained significant momentum for the synthesis of substituted cyclobutanes and bicyclo[3.2.0]-heptanes. To date, earth-abundant metals that contain redox non-innocent (radical)-type ligands are most commonly used in these cycloaddition reactions, whereby the redox non-innocent ligand plays a crucial role in stabilizing the oxidation and spin-state of the metal center. Classical π-accepting ligands, however, are inactive for these transformations. Here, we report efficient [2π + 2π] cycloaddition reactions that are catalyzed by a cobalt(I) complex containing a traditional π-accepting but redox innocent PC_NHCP pincer ligand. Mechanistic and computational investigations revealed a classical Co(I)–Co(III) redox cycle on the singlet spin manifold, where oxidative cyclization is the rate-limiting step. Overall, the herein developed methodology exhibits a wide substrate scope and low catalyst loadings (<1 mol %) and is compatible with a variety of functional groups while occurring under mild conditions (40–60 °C, N₂) with short reaction times.