Two-State Reactivity in Iron-Catalyzed Alkene Isomerization Confers σ‑Base Resistance

A low-coordinate,
high spin (S = 3/2) organometallic
iron­(I) complex is a catalyst for the isomerization of alkenes. A
combination of experimental and computational mechanistic studies
supports a mechanism in which alkene isomerization occurs by the allyl
mechanism. Importantly, while substrate binding occurs on the S = 3/2 surface, oxidative addition to an η1-allyl intermediate only occurs on the S = 1/2 surface.
Since this spin state change is only possible when the alkene substrate
is bound, the catalyst has high immunity to typical σ-base poisons
due to the antibonding interactions of the high spin state.