Iron Pincer Complexes as Catalysts and Intermediates in Alkyl Aryl Kumada Coupling Reactions
Iron-catalyzed alkylaryl Kumada coupling has developed into an efficient synthetic method, yet its mechanism remains vague. Here, we apply a bis(oxazolinylphenyl)amido pincer ligand (Bopa) to stabilize the catalytically active Fe center, resulting in isolation and characterization of well-defined iron complexes whose catalytic roles have been probed and confirmed. Reactivity studies of the iron complexes identify an Fe(II) ate complex, [Fe(Bopa-Ph)(Ph)(2)]-, as the active species for the oxidative addition of alkyl halide. Experiments using radical-probe substrates and DFT computations reveal a bimetallic and radical mechanism for the oxidative addition. The kinetics of the coupling of an alkyl iodide with PhMgCl suggests that formation of the ate complex, rather than oxidative addition, is the turnover-determining step. This work provides insights into iron-catalyzed cross-coupling reactions of alkyl halides.