102132-69-4Relevant articles and documents
Palladated azobenzenes and regiospecific aromatic metaloxylation
Mahapatra, Amiya K.,Bandyopadhyay, Debkumar,Bandyopadhyay, Pinaki,Chakravorty, Animesh
, p. 2214 - 2221 (2008/10/08)
Three groups of ortho-palladated azobenzenes (4, 8, and 9) and their high-yield regiospecific oxidation (called aromatic metaoxylation, Ar-Pd → Ar-OPd) by m-chloroperbenzoic acid (m-CPBA) in acetonitrile solution are described. Schematically, the metaloxylation reaction is (Chemical Equation Presented) the product being an azophenol chelate (5, 6, 10,11). Several modes of binding-bidentate N,O and C,N and tridentate C,N,O-of o-hydroxyazobenzenes to palladium(II) are characterized. The C,N,O mode occurs in 8. The Pd-O bond in 8 can be reversibly cleaved by acids, resulting in sharp color changes that are useful for indicator action in acid-base titrations in acetonitrile. The tridentate binding of the organic ligands in 8 and 9 (C,N,S mode) is unequivocally revealed by their 1H NMR spectra. The oxidation of 4 by m-CPBA is complicated by the formation of two products: 5 and 6. The oxidation of 8b furnishes 11 while 9 yields 10. A notable feature is that the metaloxylation reaction is very much faster than the possible oxidation of other centers (e.g. pyridine in 8b or SR″ in 9). In all cases the identity of the metaloxylated product has been established by its independent synthesis from preformed (by nonoxidative routes) hydroxyazobenzenes and palladium(II) salts. The reaction 9c → 10c has been kinetically characterized (295.8 K), affording the rate law d[10c]/dt = k[9c]2[m-CPBA] with k = 3.02 × 104 M-2 s-1. It is proposed that an adduct (13) consisting of two 9c molecules bridged (at palladium(II) centers) by a peroxo oxygen of m-CPBA is formed as a reactive intermediate, which decomposes in the rate-determining step to afford 10c. The key step is the heterolytic dissociation of the O-O bond, allowing insertion of oxygen into the Pd-C bond of one 9c molecule.
