The halogenation of aromatic rings under mild conditions is hard to perform and the synthetic methods currently available are not very selective.

Jan Reedijk and co-workers, Leiden University, the Netherlands, have performed an oxidative halogenation which occurs with high selectivity for the 5-position on the aromatic ring. The reaction of bis(2-hydroxybenzyl)-1,3-diaminopropane (H₂bhbd) with CuCl₂ in acetonitrile generated linear trinuclear [Cu₃(bhcbd)₂Cl₂](CH₃CN) (see picture). The ligands of this cluster contained phenol moieties which had been chlorinated. Bromination could also be achieved by using CuBr₂ instead of CuCl₂.
The reactions can be performed under exceptionally mild conditions, i.e., at room temperature, using air as the sole oxidant, and with cheap and non-toxic CuX₂ as the halide source.

^{Image: © Wiley-VCH}

• Selective Copper-Mediated Halogenation of Aromatic Rings Under Mild Conditions
  I. Gamba, P. Gamez, E. Monzani, L. Casella, I. Mutikainen, J. Reedijk,
  Eur. J. Inorg. Chem. 2011.
  DOI: 10.1002/ejic.201100489