Organic chemists develop new catalyst to selectively activate carbon-hydrogen bonds

Substituted aromatics are one of the most important building blocks of organic compounds such as pharmaceuticals, crop protection agents and many materials. The function of a molecule depends on the spatial arrangement of the different structural units, the substitution pattern.

A research group at the Otto Diels Institute of Organic Chemistry at Kiel University has now presented a method in the journal Chemical Compounds with particularly attractive but often inaccessible alternative modes are produced more efficiently than ever before. To achieve the desired activation of carbon-hydrogen (CH) bonds, they developed a special palladium catalyst that, for the first time, allows selective access to previously impossible positions within the molecule.

Closing long-standing research gaps

With their new method, the scientists have narrowed a long-standing research gap.

“In principle, substituted aromatic compounds have three positions where catalysts can be attached to initiate the reaction — called the ortho, meta and para positions. Depending on the position, you end up forming compounds with fundamentally different properties. different chemical products,” says Manuel van Gemmeren, professor of organic chemistry at Kiel University.

For the ortho and para positions, it is known how to make the catalyst attack there specifically. Now, for the first time, Manuel van Gemmeren and his team can selectively target meta locations directly. This allowed them to produce the meta-substituted benzyl ammonium species, a versatile compound that can be used for further research in organic chemistry.

Usually, these compounds occur only in small amounts mixed with other products. “Until now, they had to be separated from each other with great effort. Alternatively, you needed tedious synthetic routes to produce them in a targeted manner. In both cases, unnecessary waste products are generated,” van Gemmeren explains.

Using the new method, meta-substituted benzyl ammonium compounds can now be produced more efficiently. The research team around van Gemmeren used a principle that had not been described in the literature before: They designed a palladium catalyst that could interact with electrical charges in the molecule.

This dramatically changes the composition of the resulting product, favoring alternative modes that were previously difficult to produce. Calculations by colleagues at the Catalan Institute of Chemistry (ICIQ) in Spain show that charge interactions are indeed responsible for this.

Methods that are also of interest to pharmaceutical or agricultural companies

These fundamental research findings may also be of interest to pharmaceutical or agricultural companies that build large libraries of structurally related molecules to study their biological activity. “As long as as many compounds as possible are systematically examined, our method could be a useful tool for bridging gaps in prior knowledge,” says van Gemmeren.

The development of the new method is the result of many years of preliminary work starting at the University of Münster. Before coming to Kiel University in 2022, van Gemmeren founded his own CH bond activation research group here through the Emmy Noether program of the German Research Foundation (DFG). In Kiel, he will also implement his ERC Starting Grant project “DULICAT”, from which the concept of the new approach emerged.