Oxidation of Ferric Xylenol Orange Chelates by Hydrogen Peroxide in Aqueous Solution: Conception of Oxygen Singlet Atoms Generation From Hydrogen Peroxide

Abstract

We observed for the first time the reaction of oxidation of ferric xylenol orange chelates by hydrogen peroxide in aqueous solution. The reaction is accompanied with decoloration of the violet aqueous solution. Based on generally accepted conception, there is a process of free radical chain oxidation of indicator molecule in the solution. However, after investigating the final colorless solution by 1H NMR-spectroscopy we found the modified but not broken structure in which the initial hydrocarbon core remained mainly unchanged. We concluded that kind of reaction was an oxyfunctionalization by hydrogen peroxide versus free radical chain destruction. We argued steps of the reaction such as N-oxidation, Cope’s elimination, and certain rearrangements with possible products oligomerization. There was a need to explain the mechanism of interaction between the ferric iron ion and the hydrogen peroxide molecule and to argue the nature of intermediate reactive oxygen species. There is similarity between the ferric-catalyzed hydroperoxide xylenol orange oxidation and the peroxygenase-catalyzed biochemical oxyfunctionalization reactions. However, based on literature data and molecular orbital modeling, we proposed another mechanism of interaction between the ferric iron ion and the hydrogen peroxide molecule instead the tetravalent iron generation. Concretely, we proposed the hydrogen peroxide zwitter-ionization (isomerization to oxywater molecule) and subsequent intramolecular disproportionation with generation of a water molecule and a singlet oxygen atom as a reactive oxygen species. In this view, the iron ion oxidation state is unchanged during the reaction and remains ferric. An oxyfunctionalization of any organic substrate by hydrogen peroxide in the presence of ferric iron ions is promising approach in organic synthesis. However, the usage of organic ligands for ferric iron ions as components of catalysts is limited and requires only non-oxidizable compounds. On the other hand, one can choose an oxidation substrate as a ligand for ferric iron ions that is the formation of chelate complex of ferric catalyst with an organic substrate.Forcitation:Chumakov A.A., Kotelnikov O.A., Slizhov Yu.G. Oxidation of ferric xylenol orange chelates by hydrogen peroxide in aqueous solution: conception of oxygen singlet atoms generation from hydrogen peroxide. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 2018. V. 61. N 2. P. 15-22

Substantiation for Oxywater Zwitterions and Singlet Oxygen Atoms Generation from Hydrogen Peroxide Molecules in Aqueous Solutions

“…For Fenton reaction systems, the zwitterionization of hydrogen peroxide in Lewis acid-base complex with Me n+ (Equation (16)) is also followed by intramolecular disproportionation of oxywater (Equation (17) . In contrast to our previous argumentation for the free radical chain mechanism of hydrogen peroxide decomposition [9], we now substantiate a 1 D-oxene-mediated pathway of hydrogen peroxide disproportionation in aqueous solutions. A singlet oxygen atom interacts with another hydrogen peroxide molecule through targeting the unshared electron pair of oxygen atom by the vacant atomic orbital:…”

“…Oxidation of ferric xylenol orange chelates. The NMR-spectroscopy data are available in our previous paper [11] where the N-oxidation of tertiary amine fragments is argued:…”

“…The scheme of proposed subsequent N-oxide Cope elimination and some rearrangements with possible oligomerization of intermediates is in the article [11].…”

“…For ferric chelate oxidation, the direct oxidant is also a singlet 1 D-oxygen atom generated by the chelated Fe 3+ ions. The N-oxidation of xylenol orange molecule (Equation (10)) occurs via targeting of the nitrogen atom unshared electron pair by 1 D-oxene vacant atomic orbital [11]. The oxidation of primary alcohol group of pyridoxine (Fig.…”

“…Earlier, Chumakov A.A. et al [9] used Equation 7as the probable first step of non-catalytic (thermal or photochemical) hydrogen peroxide decomposition. It was shown that one hydrogen atom transfer led to increase in Gibbs free energy +39.9 kJ/mol in the gas phase.…”