Revisiting Claims of the Iron‑, Cobalt‑, Nickel‑, and Copper-Catalyzed Suzuki Biaryl Cross-Coupling of Aryl Halides with Aryl Boronic Acids

Intrigued by recent
reports on the surprisingly excellent activity
of a range of cobalt-, iron-, copper-, and nickel-based catalysts
in the Suzuki biaryl cross-coupling of simple aryl boronic acids with
aryl halides, we undertook a reexamination of the syntheses of representative
examples of the reported precatalysts and their application to the
catalytic reaction. A reported PNP−Fe pincer complex proved
to be a mixture of starting materials; a mono-Schiff base cobalt complex
in fact the bis-ligated adduct and a monomeric copper­(II) PNP–pincer
complex a di- or oligomeric copper­(I) species. In our hands, neither
these complexes nor any other of the selected precatalysts investigated
showed any activity in a Suzuki cross-coupling reaction of an electronically
activated aryl bromide with phenyl boronic acid. Meanwhile, switching
the nucleophile to the BuLi-activated phenyl boronic pinacol ester
gave some promising activity with cobalt precatalysts.