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 λ³ -bromanes as well as to their challenging preparation from the highly toxic and corrosive BrF₃ precursor. In this context, we present a straightforward and scalable approach to chelation-stabilized λ³ -bromanes by anodic oxidation of parent aryl bromides possessing two coordinating hexafluoro-2-hydroxypropanyl substituents. A series of para-substituted λ³ -bromanes with remarkably high redox potentials spanning a range from 1.86 V to 2.60 V vs. Ag/AgNO₃ 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 λ³ -bromane activation is exemplified by oxidative C-C, C-N, and C-O bond forming reactions.

Keywords: anodic oxidation; cyclic voltammetry; electrochemistry; hypervalent bromine; oxidative coupling.

Figure 1

New approach to aryl‐λ ³‐bromanes.

Figure 2

Synthesis of bromides 2 a–g. [a] Average yield of two runs on 0.3 mmol scale. [b] Yield on 10.0 mmol scale.

Figure 3

Top: Background and iR drop corrected linear sweep voltammograms (LSV) of aryl bromides 2 a–g (c=5 mM) recorded at 10 mV s⁻¹. Bottom: Plot of the half‐peak potentials E _P/2 (extracted from the LSVs) vs. the σ _p ⁺ substituent constants.

Figure 4

Synthetic applications of electrogenerated bromane reagent 3 a.