Water - not just a solvent

Scientists in Germany have used water to improve the catalytic activity of coupling reactions making them greener and faster. 

The Suzuki-Miyaura coupling reaction is a powerful tool in organic synthesis, commonly used to make pharmaceutically active compounds. There has been recent interest in making the reaction greener by using water to eliminate the need for environmentally unfriendly organic solvents; however, until now this has not led to improvements in the coupling reaction.  

Herbert Plenio and Christoph Fleckenstein at the Technical University of Darmstadt have successfully used water not only to make the reaction more environmentally friendly but also to improve the coupling rate. 

'Water is primarily viewed as a green solvent,' said Plenio, 'but we wanted to show that water can be the solvent in which carbon-carbon bond forming reactions work much better than in organic solvents.' 

'Previous catalysts employed were not optimised for use in water,' explained Plenio. By synthesising highly water soluble ligands for the Pd catalyst, the team were able to perform the coupling reactions in pure water with much smaller amounts of catalyst. The catalysts were found to be 10-100 times more active than other systems. 

Plenio and Fleckenstein aim to develop this method for other reactions in the future. 'Improvements in catalysis will result in better synthesis - from an ecological as well as from an economic point of view,' Plenio said. 'The particular challenge is to design chemical transformations and chemical processes in such a manner that they produce a minimum amount of waste and require as little energy as possible. Catalysis is the tool to achieve just that.' 

Sarah Dixon 

Link to journal article

Aqueous cross-coupling: highly efficient Suzuki–Miyaura coupling of N-heteroaryl halides and N-heteroarylboronic acids
Christoph A. Fleckenstein and Herbert Plenio, Green Chem., 2007
DOI: 10.1039/b711965h

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Christoph A. Fleckenstein and Herbert Plenio

The palladium complex of the new disulfonated 9-(3-phenylpropyl)-9-PCy₂-fluorene ligand is a highly active catalyst for aqueous Suzuki coupling reactions of N-heterocyclic chlorides and N-heterocyclic boronic acids; catalyst loadings of 0.02–0.1 mol% of Pd and two equiv. of phosphine result in the near quantitative formation of the respective coupling products at 100 °C.