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

Abstract

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.

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

“…We suggest that the doublet spin state of the complex is largely preserved throughout the catalytic cycle, although we have also investigated the other spin states, with a particular focus on the trans ‐selective route. While the quartet 1 B was calculated to be more stable than the doublet by 21.0 kcal mol −1 , we found, similarly to Smith, [13] that the activation barrier for the trans selective oxidative addition on the quartet surface (Δ G ≠ =45.0 kcal mol −1 ) was far in excess of what might be considered accessible under the experimental conditions used (see Supporting Information). In addition to this, our EPR results do not support the presence of a quartet Fe I species, or a quartet Fe III species.…”

“…Mechanistic studies revealed that the reaction is likely to proceed via an iron‐hydride with subsequent olefin insertion and β‐hydride elimination [12] . Pertinent to this study, Smith has employed an Fe I species, exploiting the propensity for spin crossover, to achieve iron catalyzed double bond isomerization via a two‐electron catalytic cycle [13] …”

“…[12] Pertinent to this study, Smith has employed an Fe I species, exploiting the propensity for spin crossover, to achieve iron catalyzed double bond isomerization via at wo-electron catalytic cycle. [13] From the synthetic studies we have already disclosed in hydrofunctionalizationc hemistry, [14] we postulated that access to at wo-electron isomerization catalytic cyclew ould be possible by limiting the quantityo fh ydrides ource employed in order to access Fe I ,m eanwhilep reventing competitive Fe II hydroboration. We herein report evidencet os upport this hypothesis, using pre-catalyst 1.…”

Parts–per–million of ruthenium catalyze the selective chain–walking reaction of terminal alkenes

“…All these results, together, point to a π–allyl (1,3–hydrogen shift) rather than a σ–alkyl (1,2–hydrogen shift) mechanism (Supplementary Fig. 20 ) 48 . Figure 6 shows the plausible mechanism for the Ru–catalyzed alkene isomerization reaction reported here.…”

Effects of 2,6‐Dichlorophenyl Substituents on the Coordination Chemistry of Pyridine Dipyrrolide Iron Complexes

“…This process involves a square‐planar transition state which requires crossing to the lower energy paramagnetic surface(s) at this geometry, introducing small paramagnetic contributions. The involvement of different spin‐surfaces during the geometric interconversion is akin to “two‐state reactivity”, which has been extensively studied computationally to explain the reactivity of high‐valent iron‐oxo complexes [37–39] and organometallic iron compounds [40–42] …”