Complexes of copper(I) with aromatic compounds facilitate selective electrophilic aromatic substitution

We report on the selective formation of ortho-nitrophenol, ortho-nitrotoluene (1-Methyl-2-nitrobenzene) and ortho-phenolsulfonic acid (2-Hydroxybenzenesulfonic acid) through reaction of the Cu(I)-aromatic ring complex of phenol or toluene. In the case of nitration of phenol, the reaction could be formulated as: (Figure presented.) The copper(I)-aromatic ring complex is formed in situ from Cu(II) and metallic copper in deaerated aqueous solutions containing the aromatic compounds. Phenol or toluene shift the comproportionation reaction of copper(0) and copper(II) towards the formation of Cu(I): (Formula presented.) The stability constant of Cu(I)-phenol in aqueous solutions was reported previously as 900 ± 100 M⁻¹. The surprisingly stable d⟶π* Cu(I)-phenol complex causes distortion of the ring structure which facilitates selective substitutions. Similar results are reported for sulfonation of phenol (in aqueous solutions) and for nitration and sulfonation of toluene in a two phase process.