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 nickelbased catalysts in the Suzuki biaryl cross-coupling of simple arylboronic acids with aryl halides, we undertook a reexamination of the syntheses of representative examples of the reported pre-catalysts and their application to the catalytic reaction. A reported PNP-Fe pincer complex, in our hands, 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 pre-catalysts 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 pre-catalysts