British chemists have discovered that iron complexes with N-heterocyclic carbenes can catalyze Suzuki cross-coupling. The reactants used were aromatic boronic acid esters and aryl chlorides, and the reaction resulted in the formation of substituted biaryls. The study was published in the journal Nature Synthesis.
The Suzuki reaction creates a carbon-carbon bond between two organic molecules, a boronic acid and a halide. The cross-coupling is catalyzed by various palladium(0) complexes with phosphines, N-heterocyclic carbenes, and other ligands. In 2010, Japanese chemist Akira Suzuki received the Nobel Prize for his discovery of this reaction.
A few years ago, chemists led by Robin B. Bedford at the University of Bristol wanted to see if the Suzuki reaction could be carried out with cheaper iron-based catalysts. In 2018, they found that it could be done: in the presence of an iron carbene complex, aryl halides reacted with boronic acid esters to form a cross-coupling product. But only aryl halides containing a directing group adjacent to the halogen reacted.
And recently, the same scientists discovered that if the reaction conditions are carefully chosen, the directing group will be unnecessary. The chemists carried out the reaction as follows: they mixed an aryl chloride, a boronic acid ester, magnesium bromide, methyl magnesium bromide, iron bromide (+3), and a source of carbene ligand in a mixture of dioxane and 2-methyltetrahydrofuran. The scientists then heated the mixture for several hours at 100 degrees Celsius - and obtained a cross-coupling product at the output. As the chemists note, in most experiments, a homocoupling product of boronic ester molecules was formed as a by-product, which had to be separated by chromatography.
The scientists then showed that their reaction worked with a variety of boron derivatives and aryl chlorides, and the conversion yield in some cases reached 90 percent. But if one of the starting materials contained a carbonyl group or an unsubstituted amino group, the desired product was not formed.
Thus, chemists have discovered a new method for conducting the Suzuki reaction using an iron catalyst. The researchers have not yet determined which complex acts as the catalytically active particle. But they assume that in the first stage of the reaction, iron is reduced to oxidation state +1, and then oxidatively adds aryl chloride.
Previously, we talked about how, using a modification of the Suzuki reaction, chemists learned to obtain diarylamines.
