New Route for Substituted Aromatics Emerges

Palladium catalyst enables synthesis with water as only byproduct.

By Chemical Processing Staff

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A research team from the University of Wisconsin-Madison has discovered a new palladium catalyst that promises to improve production of substituted aromatic molecules. The catalyst allows hydrogen to be peeled off cyclic molecules and replaced with desired substitutes. The hydrogen then combines with oxygen to form water.

This provides a versatile and efficient strategy for synthesizing substituted aromatic molecules with fundamentally different selectivity constraints from existing methods, say the researchers.

Traditional methods rely on modifying an existing aromatic molecule, but the stability of these molecules can make such approaches difficult, says Shannon Stahl, a chemistry professor who led the research. These methods have limitations in the types and patterns of chemical groups that can be attached, he adds.

The palladium catalyst incorporates an unconventional ortho-dimethylaminopyridine ligand, report Stahl and colleagues Yusuke Izawa and Doris Pun in a recent article in Science. They used the catalyst to convert substituted cyclohexanones to phenols. The reaction involves successive dehydrogenation of two saturated carbon-carbon bonds of the six-membered ring and uses molecular oxygen as the hydrogen acceptor.

While the new method can be used to make a broad spectrum of aromatic molecules for science and industry, pharmaceutical companies notably have expressed interest in the development. 

This is mostly due to the higher catalyst cost that drug companies can tolerate relative to other higher volume, lower cost chemicals, Stahl explains. "As we (and others) develop improved catalysts, it's reasonable to expect that fine chemical and other higher-volume industries will start to consider these methods."

However, Stahl emphasizes that while the catalyst is already composed of commercially available components, the catalyst system requires more development before it is suitable for large-scale industrial production.

Currently, the team is collaborating with a pharmaceutical company on other aerobic oxidation reactions — alcohol oxidation is the main focus — to develop reactors that will enable such reactions to be carried out safely on a process scale.

"The catalysts need to be more robust than they are now. We've been able to reduce the catalyst loading as low as 1 mol % Pd, but we'd like to get this lower, and preliminary studies suggest that catalyst stability can be more of a challenge on larger scales," notes Stahl.

Additional research could focus on catalytic dehydrogenation of saturated carbon-carbon bonds. "[This] is quite undeveloped as a method to prepare molecules of moderate complexity, such as those present in pharmaceuticals. For the most part, such methods have been limited to bulk chemicals, and generally simple hydrocarbons. So we'll be focusing on expanding the scope of the method — for example, targeting other classes of molecules such as heterocycles," says Stahl.

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