Summary
High-yield regiospecific oxidation, termed aromatic metaloxylation, (Ar-Pd → Ar-OPd) of cyclopalladated thioazobenzenes (2) by peracids (YCO3H) 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 the metal. The kinetics of metaloxylation have been examined using different peracids and yield the rate law: d[(3)]/dt = k[(2)]2[YCO3H]. The net ΔH ≠ (20–50 kJ) is small and ΔS ≠ (-115 to -130 J deg−1 mol−1) is large. This suggests an associative mechanism. Comparatively less negative ΔS ≠ (-30 to -47 J deg−1 mol−1) for (2d) and (2e) may be due to the steric retardation of molecular association in the rate determining step provided by the benzyl group. It is proposed that in the transition state YCO3H binds to two metal centres bridging via peroxo oxygen, followed by heterolytic cleavage of the O-O bond and oxygen insertion into the Pd-C bond.
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The referee has recommended the generalized rate law \(d[(3)]dt = \frac{{K_1 K_2 k'[(2)]^2 [YCO_3 H]}}{{1 + K_1 [YCO_3 H] + K_1 K_2 [(2)][YCO_3 H]}}\) Under the experimental conditions {K 1[YCO3H] + K 1 K 2 [(2)][YCO3H]} ≪ 1 and the rate law is reduced to Equation 7.
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Sinha, C. Aromatic hydroxylation: oxygen insertion into the metal-carbon bond of thioazobenzene palladacycles. Transition Met Chem 19, 41–48 (1994). https://doi.org/10.1007/BF00166265
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DOI: https://doi.org/10.1007/BF00166265