The coordination-catalyzed ortho-halogenation of azobenzene

Azobenzene is halogenated by chlorine and bromine selectively ortho to theazo group when its solutions are treated with the respective halogen in the presence of a palladium(II) catalyst. All possible ortho-chlorinated products are obtained, and 2,6,2′,6′-tetrachloroazobenzene is obtained as the major product upon exhaustive chlorination. The specificity for ortho-halogenation is derived from the interaction of the halogen with a carbon-metal σ-bonded intermediate, di-μ-halobis[2-(phenylazo)phenyl]dipalladium(II) (I). Intermediates such as (I) were isolated from reaction mixtures and characterized by their far-IR spectra and their reactions with chlorine and triphenylphosphine.