Iron Catalyzed Double Bond Isomerization: Evidence for an FeI/FeIII Catalytic Cycle

Iron‐catalyzed isomerization of alkenes is reported using an iron(II) β‐diketiminate pre‐catalyst. The reaction proceeds with a catalytic amount of a hydride source, such as pinacol borane (HBpin) or ammonia borane (H3N⋅BH3). Reactivity with both allyl arenes and aliphatic alkenes has been studied. The catalytic mechanism was investigated by a variety of means, including deuteration studies, Density Functional Theory (DFT) and Electron Paramagnetic Resonance (EPR) spectroscopy. The data obtained support a pre‐catalyst activation step that gives access to an η2‐coordinated alkene FeI complex, followed by oxidative addition of the alkene to give an FeIII intermediate, which then undergoes reductive elimination to allow release of the isomerization product. Using deuterium labelling, electrochemistry, EPR and DFT, an iron catalyzed double bond isomerization reaction was disclosed that is likely to proceed through on‐cycle FeI and FeIII intermediates. The reaction uses an FeII pre‐catalyst that undergoes a one‐electron reduction to get to the first on‐cycle species. These studies on mechanism, along with synthetic scope of the reaction, are presented.