Carbon-Electrode-Mediated Electrochemical Synthesis of Hypervalent Iodine Reagents Using Water as the O-Atom Source

Abstract

Photoelectrocatalytic water splitting is an important goal of modern chemistry that is hindered by the slow kinetics of anodic water oxidation. Additionally, the O2 byproduct of water oxidation catalysis is of little economic value. One way to increase the energy efficiency as well as economic feasibility of photoelectrocatalytic water splitting as an industrial process is to couple the oxidation of water to the formation of value-added, oxygenated products such as olefin epoxidation or other selective oxygenation or oxidation reactionsrather than the coupling of two O-atom equivalents to O2. However, attempts at direct olefin epoxidation at the same anode where water oxidation catalysis is happening suffer from competing one-electron oxidation or overoxidation of the olefin substrate. Hence, an O-atom-transfer mediator is needed. Hypervalent iodine reagents are known to perform olefin epoxidations via O-atom transfer, oxidation reactions, as well as a myriad of other group-transfer reactions. A drawback addressed in the following work is that the synthesis of hypervalent iodine reagents typically requires the use of a powerful oxidant such as IO4 – or KHSO5 with multiple steps and potentially explosive intermediates or side products. Herein, we report proof-of-principle electrochemical syntheses of a series of hypervalent iodine reagents, including: λ3-o-iodosobenzoic acid (IBA) in up to 58% isolated yield with a Faradaic efficiency 89%; the previously inaccessible λ3-ethyl 2-iodosobenzoate (IBA-ester) with ∼78% Faradaic efficiency; the well-known λ5-2-iodoxyphenyl tert-butyl sulfone (IBX-sulfone) with ∼51% Faradaic efficiency; and a mixture of λ3-2-iodosobenzene sulfonic acid (IBA-sulfonic acid) with λ5-2-iodoxybenzene sulfonic acid (IBX-sulfonic acid) with an overall Faradaic efficiency of >78%. Mechanistic studies using H2 18O in the electrochemical synthesis demonstrate that the O-atom in the product is initially transferred from the oxidized, O-atom-containing surface of the glassy carbon electrode, with 18O from H2 18O then replacing the O-atom vacancy on the carbon electrode surface. Overall, the present contribution demonstrates the electrochemical, green synthesis of novel and well-known hypervalent iodine reagents using the O-atom from H2O. The reported studies also provide insights into the electrochemical mechanism needed to optimize the synthesis yields as well as to exploit those syntheses using O-atom equivalents generated via (photo)­electrochemical water splitting.

Selective multi-electron aggregation at a hypervalent iodine center by sequential disproportionation

“…This mechanism enables selective I( v ) synthesis at >100 mV lower potential than previous reports. 21,24 These results demonstrate selective aggregation of two versus four redox equivalents at a main group element and provide a strategy for multi-electron oxidation by sequential disproportionation steps.…”

“…[18][19][20] Accordingly, hypervalent I(III) and I(V) electrosyntheses are plagued by high potentials and/or mixtures of I(III) and I(V) products and application of iodoxybenzene mediators in catalysis is extremely limited. [21][22][23][24][25][26][27][28][29][30] For example, Bystron et al described the electrosynthesis of 2-iodoxybenzoic acid (IBX), a common I(V)-based reagent for alcohol and amine oxidation, by sequential anodic oxidation of 2-iodobenzoic acid (IBA) to 2-iodosylbenzoic acid (IsBA) (1.45 V vs. Fc + /Fc) followed by oxidation to IBX (1.65 V vs. Fc + /Fc) in 0.2 M H 2 SO 4 . 21 We view the development of strategies to stabilize and control the reactivity of iodanyl radicals as critical to unlocking sustainable hypervalent iodine redox chemistry and catalysis.…”

Recent Updates on Electrogenerated Hypervalent Iodine Derivatives and Their Applications as Mediators in Organic Electrosynthesis

“…Also in 2021, Folkman and others have demonstrated a novel electrochemical synthetic route to HIRs using water as an abundant, green chemistry O-atom source ( Folkman et al, 2021 ). The bulk electrolysis was conducted in a U-cell equipped with a glassy carbon electrode at 1.8 V vs. Ag/AgNO 3 , using 0.1 M LiClO 4 as the supporting electrolyte and 1% H 2 O in MeCN as the O-atom source ( Schemes 1F,G ).…”

“…Apart from iodine (III) compounds, the aforementioned method by Folkman and others ( Folkman et al, 2021 ) also enables the electrochemical synthesis of iodine (V) species ( Schemes 1I,J ). In the same paper, the widely used IBX-sulfone 21 and a mixture of IBA-sulfonic acid 22 with IBX-sulfonic acid 23 were obtained via bulk anodic oxidation electrolysis of PhI-sulfone and PhI-sulfonic acid using water as the O-atom source.…”

Electrochemical Formation and Activation of Hydrogen Peroxide from Water on Fluorinated Tin Oxide for Baeyer–Villiger Oxidation Reactions

“…Interestingly, the formation of H 2 O 2 by 2e-WOR and in situ oxygenation of organic substrates have apparently not been reported. In this context, in the emerging field of electrochemical organic synthesis, , water has been used as an oxygen source for various transformations, although H 2 O 2 was not reported to be involved in these reactions. − …”