Iron Pincer Complexes as Catalysts and Intermediates in Alkyl–Aryl Kumada Coupling Reactions

Iron-catalyzed alkyl–aryl
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.