Traditionally, metal catalyzed cross-coupling chemistry has been carried out using precious metals, but lower cost, lower toxicity, and rich redox chemistry make iron an attractive alternative. In iron cross-coupling chemistry, many additives like bisphosphines, N-heterocyclic carbenes (NHCs), and N-Methyl-2-pyrrolidone (NMP) have helped to reduce homocoupled product yield, but little is understood about the role of these ligands and how structural perturbations directly influence product distributions. In iron cross-coupling with NHCs specifically, mechanistic insight is limited to investigations using stoichiometric amounts of electrophile. In this work, reactive iron species in catalytic alkyl-alkyl cross-coupling with Fe(OAc)<sub>2 </sub>and the NHC IMes●HCl are directly probed using in situ freeze trapped Mössbauer and electron paramagnetic resonance (EPR) spectroscopy. While previous studies suggest the formation of an iron(I) precatalyst, this work identifies an iron(II) bisalkyl complex as the key reactive iron species in catalysis. The use of a dioxane group on the alkyl Grignard is able to stabilize this unique precatalyst for rapid product formation. Overall, this study extends the importance of iron(II) active species to systems utilizing NHC ligands and alkyl nucleophiles.

Structure, reactivity, and mechanism in alkyl-alkyl cross-coupling with iron-NHCs

254th National Meeting (2017)

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