An unexpected barrier to the palladium-catalysed formation of carbon-nitrogen bonds has been uncovered by UK chemists. 

Palladium-catalysed cross-coupling has become an indispensable tool to organic chemists, say Geoffrey Cloke at the University of Sussex and Stephen Caddick at University College London. It is widely used to make carbon-carbon and carbon-nitrogen bonds in all kinds of molecules, from pharmaceuticals to electronic materials. Cloke and Caddick have been looking at ways to extend this reaction to include the previously unexplored alkyl-amine cross-coupling, as a general way to make carbon-nitrogen bonds. 

The researchers say that, by studying the mechanism at each step in the catalytic cycle, any competing side reactions leading away from the desired product can be suppressed. 'Once the limitations are highlighted we can begin to design catalytic protocols to circumvent these issues,' said Cloke.

In the first steps of the cycle, the alkyl component attaches to the palladium, followed by the amine. In the key step, the two components then join together and leave the metal behind, a process called reductive elimination. The chemists found that they could successfully make amine-metal-alkyl complexes using an electron-rich palladium centre. Surprisingly, however, these complexes were so stable they wouldn't undergo the final reductive elimination step to form the carbon-nitrogen bond, said Cloke. 

'This is an interesting study that may facilitate the development of mild and effective palladium-based catalysts,' said Gregory Fu, an expert in cross-coupling reactions from the Massachusetts Institute of Technology, US. 'It points to potential difficulty in the reductive-elimination step, which might not have been anticipated prior to the Caddick and Cloke investigation,' he said. 

James Mitchell Crow