Mechanistic Facets of the Competition between Cross-Coupling and Homocoupling in Supporting Ligand-Free Iron-Mediated Aryl-Aryl Bond Formations

ACS Org Inorg Au. 2022 Aug 3;2(4):359-369. doi: 10.1021/acsorginorgau.2c00002. Epub 2022 Apr 29.

ABSTRACT

In the context of cross-coupling chemistry, the competition between the cross-coupling path itself and the oxidative homocoupling of the nucleophile is a classic issue. In that case, the electrophilic partner acts as a sacrificial oxidant. We investigate in this report the factors governing the cross- versus homocoupling distribution using aryl nucleophiles ArMgBr and (hetero)aryl electrophiles Ar’Cl in the presence of an iron catalyst. When electron-deficient electrophiles are used, a key transient heteroleptic [Ar2Ar’FeII]- complex is formed. DFT calculations show that an asynchronous two-electron reductive elimination follows, which governs the selective evolution of the system toward either a cross- or homocoupling product. Proficiency of the cross-coupling reductive elimination strongly depends on both π-accepting and σ-donating effects of the FeII-ligated Ar’ ring. The reactivity trends discussed in this article rely on two-electron elementary steps, which are in contrast with the usually described tendencies in iron-mediated oxidative homocouplings which involve single-electron transfers. The results are probed by paramagnetic 1H NMR spectroscopy, experimental kinetics data, and DFT calculations.

PMID:35942278 | PMC:PMC9354087 | DOI:10.1021/acsorginorgau.2c00002

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