Iron(II) Active Species in Iron–Bisphosphine Catalyzed Kumada and Suzuki–Miyaura Cross-Couplings of Phenyl Nucleophiles and Secondary Alkyl Halides

While
previous studies have identified FeMes2(SciOPP)
as the active catalyst species in iron–SciOPP catalyzed Kumada
cross-coupling of mesitylmagnesium bromide and primary alkyl halides,
the active catalyst species in cross-couplings with phenyl nucleophiles,
where low valent iron species might be prevalent due to accessible
reductive elimination pathways, remains undefined. In the present
study, in situ Mössbauer and magnetic circular dichroism spectroscopic
studies combined with inorganic syntheses and reaction studies are
employed to evaluate the in situ formed iron species and identify
the active catalytic species in iron–SciOPP catalyzed Suzuki–Miyaura
and Kumada cross-couplings of phenyl nucleophiles and secondary alkyl
halides. While reductive elimination to form Fe­(η6-biphenyl)­(SciOPP) occurs upon reaction of FeCl2(SciOPP)
with phenyl nucleophiles, this iron(0) species is not found to be
kinetically competent for catalysis. Importantly, mono- and bis-phenylated
iron­(II)–SciOPP species that form prior to reductive elimination
are identified, where both species are found to be reactive toward
electrophile at catalytically relevant rates. The higher selectivity
toward the formation of cross-coupled product observed for the monophenylated
species combined with the undertransmetalated nature of the in situ
iron species in both Kumada and Suzuki–Miyaura reactions indicates
that Fe­(Ph)­X­(SciOPP) (X = Br, Cl) is the predominant reactive species
in cross-coupling. Overall, these studies demonstrate that low-valent
iron is not required for the generation of highly reactive species
for effective aryl-alkyl cross-couplings.