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

Callum R Woof,Ruth L Webster,Derek J Durand,Natalie Fey,Emma Richards

doi:10.1002/chem.202004980

Callum R Woof, Ruth L Webster + Show 3 more

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https://doi.org/10.1002/chem.202004980

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Abstract

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.

Highlights

Two-electron chemistry within the field of iron catalysis is incredibly rare.[1]
Following a short optimization procedure we found that a cocatalytic quantity (10 mol %) of pinacol borane (HBpin), ammonia borane (H3N·BH3) or dimethylamine borane (Me2HN·BH3) are all competent at transforming allyl benzene (2 a) into b-methylstyrene (3 a) with 5 mol % 1 at 60 8C in 16 h (Table 1, see Supporting Information for further optimization)
We have reported a rare example of iron catalysis likely proceeding through a two-electron oxidation/reduction mechanistic pathway

Summary

Introduction

Two-electron chemistry within the field of iron catalysis is incredibly rare.[1]. We envisioned that if a controlled one electron reduction of an Fe(II) species could be carried out with a welldefined iron pre-catalyst, it might be feasible to access an oxidative addition (OA)/reductive elimination (RE) (i.e. FeI/FeIII) catalytic cycle to afford activity similar to that typically observed with precious metal congeners. Previous catalytic studies within our group using an iron(II) b-diketiminate complex (hereafter referred to as complex 1) have focused on bond forming transformations (for example hydrophosphination, hydrogenation and dehydrocoupling, Scheme 1 a). Chemistry - A European Journal published by WileyVCH GmbH We sought to rule out an alkyl-based mechanism where HBpin or an alternative reagent serves as a hydride source, forming an iron(II) hydride which can undergo addition across a double bond and subsequent b-hydride elimination at the more substituted position. Adding further hydride source to 4 does lead to catalysis, but

Results and Discussion

Conclusions

Conflict of interest