In sharp contrast to hypervalent iodine(III) compounds, the isoelectronic bromine(III) counterparts have been little studied to date. This knowledge gap is mainly attributed to the difficult-to-control reactivity of l 3 -bromanes as well as to their challenging preparation from the highly toxic and corrosive BrF 3 precursor. In this context, we present a straightforward and scalable approach to chelation-stabilized l 3bromanes by anodic oxidation of parent aryl bromides possessing two coordinating hexafluoro-2-hydroxypropanyl substituents. A series of para-substituted l 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 intrinsic reactivity of the bench-stable bromine(III) species can be unlocked by addition of a Lewis or a Brønsted acid. The synthetic utility of the l 3 -bromane activation is exemplified by oxidative C À C, C À N, and C À O bond forming reactions.
Homogeneous catalysts (“mediators”) are useful for tuning selectivity in organic electrosynthesis. However, they can have a negative impact on the overall mass and energy balance if used only once or recycled inefficiently. In a previous work, we introduced the polymediator concept, in which soluble redox‐active polymers catalyze the electrochemical reaction, allowing for recovery by dialysis or pressure‐driven membrane filtration. Using anodic alcohol oxidation as a test case, it was shown that TEMPO‐modified polymethacrylates (TPMA) can serve as efficient and reusable mediators. In the present study, the properties of a TPMA sample with well‐defined molecular weight distribution were studied using cyclic voltammetry and compared to low‐molecular TEMPO species. The non‐catalytic profiles of TPMA are shaped by diffusive and adsorptive processes, whereby the latter only become pronounced at low mediator concentrations and high scan rates. Electrocatalytic studies suggest that under the applied conditions, TPMA‐catalyzed alcohol oxidation is a predominantly homogeneous process. The homogeneous kinetics are determined rather by the mediator potential than by steric influences of the polymer backbone.
Hypervalent bromine(III) reagents possess a higher electrophilicity and a stronger oxidizing power compared to their iodine(III) counterparts. Despite the superior reactivity, bromine(III) reagents have a reputation of hard-to-control and difficult-to-synthesize compounds. This is partly due to their low stability, and partly because their synthesis typically relies on the use of the toxic and highly reactive BrF 3 as a precursor. Recently, we proposed chelation-stabilized hypervalent bromine(III) compounds as a possible solution to both problems. First, they can be conveniently prepared by electro-oxidation of the corresponding bromoarenes. Second, the chelation endows bromine(III) species with increased stability while retaining sufficient reactivity, comparable to that of iodine(III) counterparts. Finally, their intrinsic reactivity can be unlocked in the presence of acids. Herein, an in-depth mechanistic study of both the electrochemical generation and the reactivity of the bromine(III) compounds is disclosed, with implications for known applications and future developments in the field.
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