AROMATIC HYDROXYLATION - OXYGEN INSERTION INTO THE METAL-CARBON BOND OF THIOAZOBENZENE PALLADACYCLES

High-yield regiospecific oxidation, termed aromatic metaloxylation, (A r-Pd-->Ar-OPd) of cyclopalladated thioazobenzenes (2) by peracids (YCO 3H) in CHCl3 solution is described. The oxidized pink violet product ( 3) has been characterized by spectroscopy. The free hydroxyazo ligands (4) are liberated from (3) by reductive (hydrazine) elimination of th e metal. The kinetics of metaloxylation have been examined using diffe rent peracids and yield the rate law: d[(3)]/dt = k[(2)]2[YCO3H]. The net DELTAH(not-equal) (20-50 kJ) is small and DELTAS(not-equal) (-115 to -130 J deg-1 mol-1) is large. This suggests an associative mechanis m. Comparatively less negative DELTAS(not-equal) (-30 to -47 J deg-1 m ol-1) for (2d) and (2e) may be due to the steric retardation of molecu lar association in the rate determining step provided by the benzyl gr oup. It is proposed that in the transition state YCO3H binds to two me tal centres bridging via peroxo oxygen, followed by heterolytic cleava ge of the O-O bond and oxygen insertion into the Pd-C bond.