Reductive activation of arenes by potassium metal with potassium salts

Benzene is routinely dried by refluxing over potassium or other alkali metals and is inert to the metal. However, dramatic chemistry occurs when potassium salts are added. At 150 °C, dimerisation occurs to afford biphenyl as the product. In the absence of salt, no reaction occurs. We propose that the process is initiated by activation of the arene by the salt followed by electron transfer from potassium. In support of this, within the added salt, systematic alteration of (i) the anion and (ii) the cation shows that the cation is the important component; thus, K+ is effective but Na+ and Li+ are not. Studies with a mixture of benzene and benzene-d6 show facile transfer of H−/D− ions between molecules during the reaction. Extension of the study to other arene hydrocarbons shows the generality of the chemistry.