112945-76-3Relevant articles and documents
Aromatic hydroxylation via cyclometalation. Metaloxylation of palladated 2-(alkylsulfinyl) azobenzenes
Sinha, Chittaranjan,Bandyopadhyay, Debkumar,Chakravorty, Animesh
, p. 1173 - 1178 (2008/10/08)
Several azo sulfoxide ligands belonging to the class 2-(sulfinyl)azobenzenes (3) have been synthesized: C6H4(R′)-N=N-C6H4X (R′ = H, 4′-Me, 5′-Me; X = 2-S(O)R, R = Me, CH2Ph). These react with Na2PdCl4 in ethanol, affording brown-red cyclopalladated species having the coordination sphere Pd(C,N,S)Cl (4). The mode of ligand binding is established with the help of high-resolution 1H NMR and IR data. In chloroform solution 4 reacts with m-chloroperbenzoic acid (mCPBA), resulting in high-yield regiospecific oxidation (metaloxylation) of the C-Pd moiety into C-O-Pd. The resulting pink-violet Pd(O,N,S)Cl complexes (5) have been characterized spectroscopically and in one case by independent synthesis via a nonoxidative route. From 5 the free hydroxyazo ligand (6) can be liberated via reductive (hydrazine) elimination of the metal. The metalation-metaloxylation-demetalation sequence thus affords a high-yield route for regiospecific aromatic hydroxylation of the azobenzenes under consideration. The kinetics of the metaloxylation reaction has been examined in two cases. The rate law is d[5]/dt = k[4]· [mCPBA]. The enthalpy of activation is small (～9 kcal). The large negative entropy of activation (～30 eu) suggests an associative mechanism. It is proposed that in the transition state mCPBA binds to the metal center via peroxy oxygen followed by heterolytic cleavage of the O-O bond and oxygen insertion into the Pd-C bond.
