Chemists in China couple aromatic molecules with an organocatalyst and no transition metal
Chinese chemists have successfully coupled aromatic molecules without the use of a transition metal catalyst - something that people have been trying to do for years with little success. Such cross-coupling reactions are crucial to organic synthesis and typically require expensive metals such as palladium. Efforts to find cheaper and more widely available alternatives have proved challenging.
Now, Wei Liu, from Wuhan University, and colleagues appear to have succeeded by using an organic catalyst, DMEDA (N,N’-dimethylethane-1,2-diamine) in the presence of the base potassium tert-butoxide. The team coupled unactivated benzene with a range of aryl iodides in the presence of the organic catalyst and the base.
The researchers suggest that the reaction proceeds via the formation of a radical, with the potassium salt initiating radical formation in the presence of DMEDA. ’In radical trap experiments the coupling was inhibited by a classical radical scavenger which suggested that radical species are involved,’ says team member Aiwen Lei.
Could trace amounts of transition metal have contaminated the experiment? ’Obviously this is one of the most important factors,’ says Lei. ’We have checked the contamination of trace amounts of transition metals by ICP [inductively coupled plasma atomic emission spectroscopy] and excluded the involvement of small amounts of transition metals in this transformation.’ In addition, the potassium tert-butoxide used in the work was purified by sublimation to remove any contaminants.
Lei believes that the work could herald a new direction in organic synthesis. ’This is the first report of organocatalysis in carbon-carbon coupling or direct arylation between aryl halides and arenes, which could be considered as a conceptually different approach towards biaryl syntheses.’
Commenting on the work, Carsten Bolm, an organic synthesis expert from Aachen University in Germany, says, ’To be able to prepare cross-coupling products without the use of transition metals is an important scientific advance. Although at the present stage the substrate scope is by far too limited to make the process synthetically attractive, the findings illustrate that new reaction paths in direct C-H arylations are still to be discovered, and as such this work will be highly stimulating to the community.’
W Liu et al, J. Am. Chem. Soc., 2010, DOI: 10.1021/ja103050x