Electrochemical Synthesis of Hypervalent Iodine Oxidants

Abstract

Hypervalent iodine compounds are well established, environmentally benign and highly selective oxidants used in organic synthesis. Their chemical synthesis is based on oxidation of iodobenzenes and it usually involves handling toxic or at least potentially unstable oxidants (e.g. bromate, chlorine, persulfate or percarboxylic acids) making the procedure unsuitable for a larger scale production. Obvious alternative in this case represents electrochemical synthesis where the need for application of oxidants is obviated by oxidation of iodobenzenes at the electrode surface. Indeed, this field has developed quickly within last few years. Majority of published electrochemical works focuses on synthesis of l3-iodanes (iodosylbenzenes and diaryliodonium salts). In addition to that, the use of environmentally unfriendly solvents such as fluorinated alcohols is very common. A number of publications describing electrochemical synthesis of l5-iodanes (iodyl compounds) is very limited. In the present contribution, a short summary of the hypervalent iodine oxidants electrochemical synthesis development achieved in our group will be given. At the beginning we have investigated anodic oxidation of 2-iodobenzoic acid in aqueous environment using boron-doped diamond anode. It was shown that both 2-iodosylbenzoic acid as well as 2-iodylbenzoic acid (IBX) can be obtained with high yields especially in acidic environment [1, 2]. Though, the practical applicability of this very reaction in aqueous environment is limited by solubility of the involved compounds being approximately 1 mM, this “key finding” provided solid base for further development of the electrochemical process of iodanes synthesis.We applied two different strategies to solve the problem of limited solubility. The first was modification of the iodobenzene-based precursor structure by appropriate solubilising groups in order to increase its solubility in aqueous systems. This allowed synthesising various l3- and l5-iodanes in practically relevant concentrations. The second approach was to work in non-aqueous environment such as anhydrous acetic acid, in which solubility of majority of iodobenzenes is sufficiently high. Morover, it represents cheap and environmentally friendly solvent allowing efficient anodic oxidation of iodobenzenes to corresponding l3-iodanes, in particular iodosylbenzene (di)acetates. The electrosynthetic process was operated with high yields and fair current efficiencies in batch and flow electrolysers. In some cases, the products could be separated, others exist only in the solution and decompose upon any attempt for isolation.Beside electrosynthesis, we also investigated kinetics and mechanism of iodobenzenes oxidation be combining experiments with advanced density functional theory calculations. This approach allowed, for example, relating oxidation peak potentials of iodobenzene and iodobenzoic acids to their structure. Unexpectedly low peak potential of 2-iodobenzoic acid was explained by interaction of carboxylic group and iodine atom leading to stabilisation of the formed radical cation. [3] In summary, electrochemical synthesis of hypervalent iodine oxidants in environmentally friendly solvents was shown to interesting and in numerous occasions beneficial what compared to classical chemical route.[1] T. Bystron, A. Horbenko, K. Syslova, K.K. Hii, K. Hellgardt, G. Kelsall, 2-Iodoxybenzoic Acid Synthesis by Oxidation of 2-Iodobenzoic Acid at a Boron-Doped Diamond Anode, ChemElectroChem, 5 (2018) 1002-1005.[2] B. Devadas, J. Svoboda, M. Krupička, T. Bystron, Electrochemical and spectroscopic study of 2-iodobenzoic acid and 2-iodosobenzoic acid anodic oxidation in aqueous environment, Electrochimica Acta, 342 (2020) 136080.[3] T. Bystron, B. Devadas, K. Bouzek, J. Svoboda, V. Kolarikova, J. Kvicala, Anodic Oxidation of Iodobenzene and Iodobenzoic Acids in Acetic Acid Environment – Electrochemical Investigation and Density Functional Theory Study, ChemElectroChem, 8 (2021) 3755-3761.The work was supported from European Regional development Fund-Project "Organic redox couple based batteries for energetics of traditional and renewable resources (ORGBAT)" No. CZ.02.1.01/0.0/0.0/16_025/0007445. Figure 1