A breakthrough in ligand chemistry has led to the development of a new catalyst to synthesize the aromatic amines that are central building blocks in many drugs, pesticides and fine chemicals, report scientists at Ruhr-Universität Bochum (RUB), Bochum, Germany.
Currently, the manufacture of aromatic amines involves reacting primary or secondary amines with ring-shaped aryl halides in the presence of a palladium catalyst. These reactions often rely on electron-rich phosphine ligands to stabilize the palladium. So, for example, commercially important phosphines such as cataCXium and DalPhos contain different electron-donating groups within them.
Unfortunately, the finite electron-donating capacity of such groups has hampered development of more potent catalysts that could also operate under milder conditions.
Now, two RUB research teams, led by professors Viktoria Däschlein-Gessner and Lukas Goossen, successfully have attached the newly created ligands to the palladium catalyst in a way that significantly increases the efficiency of the reaction.
The teams previously studied a novel class of electron-rich phosphine ligands bearing an ylide (neutral, dipolar molecule) substituent at the phosphorus atom. They now have developed a ylide-functionalized phosphine ligand (YPhos) that stabilizes the palladium catalyst using different chemical bonds than those employed by existing ligands.
The new YPhos ligands also boast an easy and scalable synthesis process that uses inexpensive and widely available starting materials, claim the researchers.
“We have increased the activity of the palladium catalysts to such an extent that the reaction is faster and more efficient than with existing systems optimized [by industry] over many years,” says Däschlein-Gessner.
For example, the YPhos ligand can couple chlorine-containing aromatic compounds with many different amines at room temperature within one hour. With existing catalysts, this often takes several hours and temperatures of 100°C or higher, she explains.
The European Research Council considers the work so important that it provided the maximum €1.5 million ($1.73 million) in funding available under its Starting Grant scheme for ground-breaking research.
Däschlein-Gessner and Goossen now are working to optimize the catalyst structure and test whether it can be transferred to other types of reactions (Figure 1).
Meanwhile, Umicore, Brussels, one of whose specialties is emissions control catalysts, is collaborating with the RUB groups to bring the catalyst to market.