Electrochemistry and Reactivity of Chelation‐stabilized Hypervalent Bromine(III) Compounds

Abstract

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.

Iodine–Iodine Cooperation Enables Metal-Free C–N Bond-Forming Electrocatalysis via Isolable Iodanyl Radicals

“…, 1 g + ) or the ortho -iodobromo compounds ( i.e. , 1i + ), which suggests that in the context of organohalides, the large atomic radius and high polarizability of iodine may be critical to the observed X–X interactions; potential translation of X–X interaction to the smaller halogens such as in hypervalent bromine chemistry remains an area for development. , In contrast, a BCP is observed for the iodanyl radical cation derived from 1j (the BCP is observed both for 1j + and the corresponding deprotonated analogue, see Figure S18). This result is consistent with previous reports that an ortho -carboxylate substituent can stabilize iodanyl radicals generated during the reduction of I­(III) compounds. , …”

Electrochemical Synthesis of Dimeric λ3-Bromane: Platform for Hypervalent Bromine(III) Compounds

“…In a quest to solve the challenge of λ 3 -bromane synthesis, we recently disclosed an inherently safe, inexpensive, and straightforward approach to chelation-stabilized hypervalent bromine(III) reagents 6 (Martin’s bromanes) by anodic two-electron oxidation of parent aryl bromides 5 under constant current conditions in an undivided cell (eq 2, Figure 1 ). 14 The suitability of hypervalent Br(III) reagents 6 for the oxidative amination of anilines and the homocoupling of electron-rich arenes was also demonstrated. However, the remarkably stable doubly chelated structure of Martin’s bromanes 6 limits their application in reactions involving ligand exchange at the hypervalent Br(III) center, e.g., functional group transfer reactions.…”

“…Electrochemical properties of the 7/8 redox couple and reduction potentials of λ 3 -bromanes 9 – 13 were studied with cyclic voltammetry (CV) in MeCN or HFIP and 0.1 M NBu 4 BF 4 as an electrolyte, a glassy carbon working electrode, and a Ag/0.01 M AgNO 3 reference ( E 0 = −87 mV vs Fc/Fc + couple) . Bromide 7 exhibits a single irreversible anodic feature with a half-peak potential ( E P/2 = 2.62 V; see Figure B and the SI), which is higher than that of aryl bromide 5 (RCF 3 ; E P/2 = 2.54 V), a precursor of Martin’s bromane . As anticipated, the corresponding iodide 7-I is oxidized at a considerably lower potential ( E P/2 = 2.01 V; see Figure B) .…”

“…The published conditions for the electrochemical generation of bromine(III) species 6 ( 14 ) (glassy carbon as the working electrode and platinum foil as the counter electrode in a TBA-BF 4 /HFIP electrolyte; current density j = 10 mA cm –2 , passed charge equivalents 2F) were used as the starting point for the anodic oxidation of aryl bromide 7 (Table S1, SI ). NMR spectra of an electrolyzed solution of 7 in HFIP (with an added equal volume of CHCl 3 - d ) revealed the presence of starting aryl bromide 7 (77% conversion) together with a new product (70% NMR yield) that could be isolated from the electrolysis mixture by aqueous workup and separated from the unreacted 7 by reversed-phase chromatography.…”

Trendbericht Anorganische Chemie 2023: Hauptgruppen

“…synthetisieren. Während der Chelatligand das hochreaktive Brom(III)-Zentrum stabilisiert, bleibt dessen Reaktivität gleichzeitig erhalten und lässt sich durch eine Säure wie Trifluormethansulfonsäure reaktivieren 44). Fluorierteorganische Verbindungen sind wichtige Bausteine für diverse Materialien sowie pharmazeutische und landwirtschaftliche Produkte.…”