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Formation of low-valent Fe$^0$ and Fe$^l$ species in Fe-catalyzed cross-coupling chemistry : key role of ate-Fe$^{ll}$ intermediates

Abstract : $ate$-iron(II) species such as [Ar$_3$Fe$^{ll}$]$^-$ (Ar = aryl) are key intermediates in Fe-catalyzed cross-coupling reactions between aryl Grignard reagents (ArMgX) and organic electrophiles. They can be active species in the catalytic cycle, or lead to Fe$^0$ and Fe$^I$ oxidation states.[l] In sorne cases, these low oxidation states can be themselves active in the coupling process.[2] However, their presence in the reaction medium most often leads to unwished organic byproducts. The development of new and efficient Fe-mediated cross-coupling processes therefore requires new frameworks enabling a fine control of the iron species distribution in the reaction medium, giving the possibility to inhibit the formation of unwanted oxidation states. So far, the formation of Fe$^0$ and Fe$^I$ species in cross-coupling mediums can be hardly controlled. A deep understanding of the formation mechanism of these oxidation states is thus fundamental if one wants to finely monitor their presence in the reaction medium. We report in this work our efforts related to the understanding of the elementary steps connecting ale [Ar$_3$Fe$^{ll}$]$^-$ species (Ar = Mes, Ph), formed by transmetallation of aryl Grignard reagents with FeCl$_2$, to the formation of Fe$^o$ and Fe$^l$ low oxidation states. A particular focus is put on the role of the steric and electronic effects of the aryl group in the reduction process. We demonstrate that a steric decompression induced by progressive substitution of the mesityl groups in [Mes$_3$Fe$^{ll}$$^- $ by phenyl anions leads to the formation of [Mes$_n$Ph$_{3-n}$Fe$^{II}$]$^-$ species (n = 0-2; observation by IH NMR). Thanks to this steric decompression, formation of Fe$^o$ and Fe$^l$ oxidation states is concomitantly observed by EPR and Méissbauer spectroscopies (Scheme 1).
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Guillaume Lefèvre, Lidie Rousseau, Christian Herrero, Martin Clémancey, Geneviève Blondin, et al.. Formation of low-valent Fe$^0$ and Fe$^l$ species in Fe-catalyzed cross-coupling chemistry : key role of ate-Fe$^{ll}$ intermediates. International Symposium on Metal Complexes 2019, Jun 2019, Debrecen, Hungary. ⟨cea-02329104⟩

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