Several amino acids were tested to catalyze the transition-metal-free direct C–H arylation of
unactivated benzene derivatives. Among them, proline was found to be an excellent catalyst
for the cross-coupling between aryl halides and unactivated arenes. The reaction
presumably involves an aryl radical anion as the intermediate based on several
experiments. The reaction using this catalyst system offers an option toward establishing an
environmentally benign and cost-effective route to biaryls.