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

Abstract

A straightforward and scalable approach to a previously unreported class of cyclic hypervalent Br(III) species capitalizes on the anodic oxidation of aryl bromide to dimeric benzbromoxole that serves as a versatile platform to access a range of structurally diverse Br(III) congeners such as acetoxy-, alkoxy-, and ethynyl-λ³-bromanes as well as diaryl-λ³-bromanes. The synthetic utility of dimeric λ³-bromane is exemplified by photoinduced Minisci-type heteroarylation reactions and benzylic oxidation.

Figure 1

Electrochemical approach to benzbromoxoles 8–13.

Figure 2

Synthesis of λ³-bromanes 8–13. Stability was measured in a 0.02 M solution at room temperature. Ellipsoids are shown at 50% probability, with hydrogen atoms omitted for clarity. Selected bond distances (Å) and angles (deg) for λ³-bromane 9: Br1–O2, 2.034(2); Br1–O1, 1.9240(16); Br1–C7A, 1.931(3); O1–Br1–O2, 173.63(8); Br1–O1–Br1′, 114.32(14); O2–Br1–C7A, 83.06(10); C7A–Br1–O1, 90.73(9); and Br1–O2–C3–C3A, 1.4(3). Selected bond distances (Å) and angles (deg) for diaryl-λ³-bromane 11: Br1–O2, 2.365(2); Br1–C6, 1.933(2); Br1–C5, 1.959(2); C6–Br1–O2, 176.7(1); C6–Br1–C5, 98.76(10); O2–Br1–C5, 78.1(1); and Br1–O2–C3–C4, −8.8(2). For a discussion of crystalline state structures of dimeric λ³-bromane 9 and diaryl-λ³-bromane 11, see the Supporting Information.

Figure 3

(A) Background and iR drop-corrected linear sweep voltammograms (LSV) of λ³-bromanes 8–13 and λ³-iodane 9-I (c = 1 mM) were recorded at 100 mV·s^–1 in MeCN. (B) Oxidation half-peak potentials E_P/2 are for bromide 7 and iodide 7-I in HFIP (c = 5 mM).

Figure 4

Reactivity of λ³-bromane 9. ^aThe yield of 15 was determined by ¹H NMR using mesitylene as an internal standard and was calculated based on λ³-bromane 9. ^bReaction conditions: heterocycle (0.5 mmol), 9 (0.5 mmol), TFA (2.5 mmol), and THF (3 mL); ^c0.6 mmol of 9; ^d0.8 mmol of 9; ^e0.6 mmol of 9, 1:1 cyclohexane/EtOAc (3.0 mL); ^fheterocycle (0.5 mmol), 9 (0.6 mmol), TFA (0.5 mmol), 1:1 formamide/EtOAc (3.0 mL); ^g7 h; and ^h2.5 mmol of TFA, 3 h.