Current Developments in Electrosynthesis and Electrocatalysis

Organofluorine compounds are ubiquitous in the pharmaceutical and agrochemical industries, as they impart several biochemical advantages. These include potency, biocompatibility, pharmacoki-netics, and lipophilicity, all of which are key to developing valuable new products. Hence, the need to develop new methods to prepare fluorinated compounds is continually strong, and in particular more sustainable methods. In this presentation I will describe in detail our efforts towards these goals. We are developing several strategies for the synthesis of fluorinated building blocks that are driven by electrochemical redox reactions. Electrochemistry is emerging as a useful tool to conduct sustainable and selective redox reactions. Our research efforts have centred on both an oxidative alkene fluorofunctionalisa-tion strategy and a reductive defluorination strategy. Cu is in the spotlight as it represents the only metal capable of catalyzing CO 2 reduction to multicar-bon products. However, its catalytic performance is determined collectively by a number of param-eters including its composition and structure, electrolyte, and cell configuration. It remains a chal-lenge to disentangle and understand the individual effect of these parameters. In this work, we study the effect of the electrode–electrolyte interface on CO 2 reduction in water by coating CuO electrodes with polymers of varying hydrophilicities/phobicities. Hydrophilic polymers such as poly(vinyl alco-hol) and poly(vinylpyrrolidone) exert negligible influence, while hydrophobic polymers such as poly(vinylidene fluoride) and polyethylene significantly enhance the activity, selectivity, and stability of CuO-derived electrodes toward C 2 H 4 production. From ex situ characterizations, electrolysis in deuterated water, and molecular dynamics simulations, we propose that the improved catalytic performance triggered by hydrophobic polymers originates from restricted water diffusion and a higher local pH near the electrode surface. These observations shed light on interfacial manipulation for promoted CO 2 -to-C 2 H 4 conversion. 1 The chemistry of hypervalent halogen species has experienced remarkable advancement in the recent decades. In comparison to hypervalent iodine(III) compounds, little research has been done on the isoelectronic bromine(III) counterparts. 1, 2 This is mainly due to the difficult-to-control reactivity of λ 3 -bromanes as well as to the challenges associated with the conventional protocol for their prep-aration from the highly toxic and corrosive BrF 3 precursor. 3, 4 In this context, we present a straight-forward and scalable approach to λ 3 -bromanes by anodic oxidation of parent aryl bromides. A series of para-substituted λ 3 -bromanes with remarkably high redox potentials spanning a range from 1.86 V to 2.60 V vs. Ag/AgNO 3 was synthesized by the electrochemical method. We demonstrate that the bench-stable bromine(III) species can be activated by addition of a Lewis or a Brønsted acid. A synthetic example of the λ 3 -bromane activation is oxidative arene-arene homocoupling. 2 The developed electrochemical approach to λ 3 -bromanes offers considerable advantages compared to previously established methods since stoichiometric reagents are replaced by electric current and the use of hazardous precursors is omitted. Therefore, our approach may open the door to the development of unprecedented synthetic transformations that would benefit from the unique properties of hypervalent bromine(III) species. Mechanistic studies on formation and activation of the bromanes are underway. 2 Owing to its many distinct characteristics, electrochemistry represents an attractive approach to dis-covering new reactions and meeting the prevailing trends in organic synthesis. In particular, electro-catalysis—a process that integrates electrochemistry and small-molecule catalysis—has the potential to substantially improve the scope of synthetic electrochemistry and provide a wide range of useful transformations. Despite its attractive attributes and extensive applications in energy-related fields, electrocatalysis has been used only sparingly in synthetic organic chemistry. Thus, there exists a clear impetus for inventing new catalytic strategies to improve the scope of synthetic electrochemistry and provide new platforms for reaction discovery and synthetic innovations. Toward this end, we developed a new catalytic approach that combines electrochemistry and redox-metal catalysis for the functionalization of alkenes to access a diverse array of vicinally functionalized structures. This talk details our design principle underpinning the development of electrocatalytic alkene difunctionali-zation and hydrofunctionalization with a particular emphasis on enantioselective electrocatalysis. In addition, our recent forays into electroreductive chemistry will be discussed, in which we harness the power of deeply reducing potentials to achieve previous challenging organic transformations. Redox mediators are often used in electro- and photochemistry to enable reactions that are other-wise not feasible or very slow. 1 For a long time, researchers in both fields have independently developed their own systems based on the individual requirements. Herein we present a new organocat-alyst platform based on the phenanthro[9,10-d]imidazole structure (MED, see figure below), which can be used both for electro- and photochemical applications. 2,3 Moreover, the mediator properties can be flexibly tuned by variation of R 1 and R 2 . photochemical Homogeneous catalysts (“mediators”) are useful for tuning selectivity in organic electrosynthesis but can have a negative impact on the overall mass and energy balance if used only once or recycled inefficiently. 1 In this context, we have developed a polymer-based approach which allows for simul-taneous separation and recycling of mediators in a single step (see figure, right) using membrane filtration. For this purpose, 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) was attached to a polymeth-acrylate backbone, resulting in well-soluble and task-specific homopolymer 1 (see figure, left). 2,3 Re-versible redox behaviour was observed for the polymediator using cyclic voltammetry. The successful conversion of several test substrates using catalytic amounts of 1 demonstrated that such polymers can be applied for oxidation of alcohols under mild conditions. Furthermore, recyclability was demonstrated in preparative-scale electrolysis. 2

[1]  K. Miyamoto Chemistry of Hypervalent Bromine , 2018, PATAI'S Chemistry of Functional Groups.