Dieter Lenoir

Oxidation of Orange II ([4-[(2-hydroxynaphtyl)azo]benzenesulfonic acid], sodium salt) by hydrogen peroxide catalyzed by iron(III) complexed to tetra amido macrocyclic ligands (Fe^III–TAML activators) in aqueous solutions at pH 9–11 leads to CO₂, CO, phthalic acid and smaller aliphatic carboxylic acids as major mineralization products. The products are non-toxic according to the Daphnia magna test. Several organic intermediates have been identified by HPLC and GC-MS that allowed the detailed description of Orange II degradation. The catalytic oxidation can also be performed by organic oxidants such as benzoyl peroxide, tert-butyl and cumyl hydroperoxides. Kinetic studies of the catalyzed oxidation indicated that Fe^III–TAML activators react first with ROOR′ to form an oxidized catalyst (k_I), which then oxidizes Orange II (k_II). Neglecting the reversibility of the first step, the rate equation is rate = k_Ik_II[Fe^III][ROOR′][Dye]/(k_I[ROOR′] + k_II[Dye]); here Fe^III and ROOR′ represent the catalyst and peroxide, respectively. The rate constant k_I equals (74 ± 3) × 10³, (1.4 ± 0.1) × 10³, 24 ± 2, and 11 ± 1 M⁻¹ s⁻¹ for benzoyl peroxide, H₂O₂, t-BuOOH, and cumyl hydroperoxide at pH 9 and 25 °C, respectively. An average value of k_II equals (3.1 ± 0.9) × 10⁴ M⁻¹ s⁻¹ under the same conditions. The unraveling of the kinetic mechanism allows the comprehension of the robust reactivity, and this is discussed in detail using the representative results of DFT calculations.

Mixtures of sugars, sugar alcohols or citric acid with urea and inorganic salts form stable melts if heated to 70 °C. The polarity of the melts was estimated using solvatochromic dyes and found to be in the range of DMF or water, depending on the sensitivity of the dye for hydrogen bonds. To explore the use of the melts as solvents in catalysis, Rh-catalyzed hydrogenations and Pd-catalyzed Suzuki reactions were performed. The hydrogenation of a double bond is sensitive to the composition of the melt, and no effect of the chiral melts on the stereochemical outcome of the reaction was observed. The Suzuki coupling proceeds rapidly, cleanly and quantitatively in all examined melts. The results recommend the non-toxic sugar–urea–salt melts as more sustainable reaction media for chemical transformations.