Aryl-Alkyl Cross-coupling $versus$ Homo-coupling: A Mechanistic Study On The Catalytic Performance Of A Bulky Silylamide Iron(II) Complex

Lidie Rousseau (1, 2) , Guillaume Lefèvre (2)

1 LCMCE - Laboratoire de Chimie Moléculaire et de Catalyse pour l'Energie (ex LCCEF)
2 CSB2D - Catalysis, Synthesis of Biomolecules and Sustainable Development

Iron catalysis has raised a great interest in the organometallic field since the 1970s and the first successful Fe-catalyzed cross-coupling reactions developed by Kochi, as it is a cheaper, more earth-abundant and less toxic alternative to many other transition metals. Kumada-type reactions between aryl Grignard reagents and organic electrophiles are of high importance, but a significant issue remains in the competition between effective cross-coupling, and homocoupling of the Grignard reagent affording unwanted symmetrical biaryl byproducts. Nakamura proposed an explanation for the large presence of this byproduct in the presence of several aryl groups on the iron center resulting from multiple Mg-to-Fe transmetallations, and proposed the addition of a source of fluoride anions to block this possibility. However, our study of the catalytic behavior of the bulky silylamide iron(II) complex [Fe(N(SiMe$_2$Ph)$_2$)$_2$] in an aryl-alkyl cross-coupling reaction refutes the hypothesis that the multi-transmetallations are the only cause of homocoupling. The steric hindrance of the ligands effectively prevents more than one transmetallation on the iron atom, affording a stable Ar-Fe bond, but it does not quite impede the homocoupling. Stoichiometric and catalytic tests involving 1H paramagnetic NMR, EPR spectroscopy and cyclic voltammetry were lead in order to discuss the mechanism of the reaction, evidencing transient Fe III species responsible for the formation of the homocoupling product.

Domaines: Catalyse; Chimie de coordination

Lidie Rousseau, Guillaume Lefèvre. Aryl-Alkyl Cross-coupling $versus$ Homo-coupling: A Mechanistic Study On The Catalytic Performance Of A Bulky Silylamide Iron(II) Complex. GreenCat 2020 - 14th International Green Catalysis Symposium, Mar 2020, Rennes, France. ⟨cea-02482667⟩