Electrochemical Generation of Hypervalent Bromine(III) Compounds

Abstract

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.

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

“…To better understand the mechanism of bromane formation, we intended to combine detailed electroanalytical studies with quantum chemical modeling. In our previous work, [15] we have already obtained some information about bromoarene oxidation by performing preliminary electroanalytical experiments. Thus, voltammetric analysis of 1 at 10 mV s À 1 between 0 and 2.7 V versus Ag/0.01 M AgNO 3 (E ref = À 87 mV vs. Fc/Fc + couple) [16] using an electrolyte consisting of 0.1 M NBu 4 BF 4 in HFIP showed that each of the bromoarenes exhibits a single irreversible anodic feature.…”

“…Inspired by the successful in situ generation of reactive hypervalent iodine species via anodic oxidation of iodoarene precursors, [13,14] we presented a convenient electrochemical approach toward synthesis of 2. [15] The method is based on the anodic oxidation of parent aryl bromides 1 containing two coordinating hexafluoro-2-hydroxy-propanyl substituents under galvanostatic conditions in an undivided cell (Eq. (1) in Scheme 1).…”

Benchtop-Stable Hypervalent Bromine(III) Compounds: Versatile Strategy and Platform for Air- and Moisture-Stable λ3-Bromanes

“…The use of the "Martin ligand" is an excellent alternative strategy for the preparation of stable λ 3 -bromane (4; Figure 1B). 10,11 These approaches, however, are less than ideal in terms of versatility. In 1984, Frohn and co-workers revolutionized this field by the synthesis of difluoro-λ 3 -bromane 6 (Frohn's reagent): 12 the use of the substitution reaction of BrF 3 with arylsilanes 5 circumvents the unfavorable oxidation process.…”

“…They elegantly overcame the energetic impasse by leveraging intramolecular λ 3 -bromane formation with the pyrolysis reaction of the diazonium salt of 2-amino-2′-bromobiphenyl ( 1 ), obtaining cyclic o , o ′-diphenylene-λ 3 -bromane ( 2 ; Figure A). The use of the “Martin ligand” is an excellent alternative strategy for the preparation of stable λ 3 -bromane ( 4 ; Figure B). , These approaches, however, are less than ideal in terms of versatility. In 1984, Frohn and co-workers revolutionized this field by the synthesis of difluoro-λ 3 -bromane 6 (Frohn’s reagent): the use of the substitution reaction of BrF 3 with arylsilanes 5 circumvents the unfavorable oxidation process.…”

Electrochemical Generation of Hypervalent Bromine(III) Compounds

“…wherein the slope provides a measure of the influence of the substituents upon the observed potential, while the intercept refers to the E P/2 of the unsubstituted compound of the series. [29] The stabilization of the characteristic pseudo-trigonal bipyramidal geometry of l 3 -bromanes 3 by 2-benzobromaoxole rings (for X-ray structure of 3 a, see Table 1 graphics) [30] endows Martins bromine(III) species with remarkable stability. Given the high oxidizing power and strong electrophilicity of l 3 -bromanes, [13a] the observed stability is striking.…”