Evolution of Ate‐Organoiron(II) Species towards Lower Oxidation States: Role of the Steric and Electronic Factors

Abstract

Ate‐iron(II) species such as [Ar3FeII]− (Ar=aryl) are key intermediates in Fe‐catalyzed couplings between aryl nucleophiles and organic electrophiles. They can be active species in the catalytic cycle, or lead to Fe0 and FeI oxidation states, which can themselves be catalytically active or lead to unwished organic byproducts. Analysis of the reactivity of the intermediates obtained by step‐by‐step displacement of the mesityl groups in high‐spin [Mes3FeII]− by less hindered phenyl ligands was performed, and uncovered the crucial role of both steric and electronic parameters in the formation of the Fe0 and FeI oxidation states. The formation of quaternized [Ar4FeIIMgBr(THF)]− intermediates allows the bielectronic reductive elimination energy required for the formation of Fe0 to be reduced. Similarly, the small steric pressure of the aryl groups in [Ar3FeII]− enables the formation of aryl‐bridged [{FeII(Ar)2}2(μ‐Ar)2]2− species, which afford the FeI oxidation state by bimetallic reductive elimination. These results are supported by 1H NMR, EPR, and 57Fe Mössbauer spectroscopies, as well as by DFT calculations.

Iron-Catalyzed Cross-Coupling of Bis-(aryl)manganese Nucleophiles with Alkenyl Halides: Optimization and Mechanistic Investigations

“…The following catalytic cycle (Scheme 2) can be suggested, which echoes recent reports by Neidig on the Fe-catalyzed alkyl-alkenyl coupling reactions [39], and by ourselves on the benzyl-alkenyl coupling [26]. Thanks to the steric hindrance in the ortho positions, the ate [Mes3Fe II ] -species remains stable for hours at 25 °C [38,41]. Thus, the use of a mesityl nucleophile in the mechanistic experiments discussed earlier prevents any degradation of the Fe II catalyst toward lower oxidation states.…”

“…of PhMgBr, and characterized by its 1 H-NMR signals at 116 and −41 ppm. As we recently reported, [Ph 3 Fe II ] − is stable for more than 1 h at this temperature [40,41]. Its fate upon addition of an excess of the electrophile 3b (10 equiv.)…”

“…was then monitored by paramagnetic 1 H-NMR. Thanks to the steric hindrance in the ortho positions, the ate [Mes 3 Fe II ] − species remains stable for hours at 25 • C [38,41]. Thus, the use of a mesityl nucleophile in the mechanistic experiments discussed earlier prevents any degradation of the Fe II catalyst toward lower oxidation states.…”

“…This suggests that Fe 0 species are formed in situ by 2-electron reductive elimination from [Ph 3 Fe II ] − , which is in agreement with a recent report by some of us demonstrating that the evolution of [Ph 3 Fe II ] − led to the formation of a distribution of Fe 0 and Fe I oxidation states (identified as (η 4 -arene) 2 Fe 0 and [(η 6 -arene)Fe I (Ph) 2 ] − , "arene" being an aromatic ligand present in the bulk medium (e.g., C 6 H 6 or C 6 H 5 -C 6 H 5 coming from the oxidation of PhMgBr). Fe 0 being preponderantly formed [40,41]). Those Fe 0 intermediates would then be trapped by alkene ligands present in the bulk medium, which leads to the observed resonances.…”

“…Following one of our recent procedures, reduction of Fe II into a distribution of Fe 0 and Fe I species was performed, by fast trans-metalation between FeCl 2 and PhMgBr (2.0 equiv.) at room temperature [40,41]. After 10 min, 1.0 equiv.…”