Oxy-Borylenes as Photoreductants: Synthesis and Application in Dehalogenation and Detosylation Reactions

Abstract

While the range of accessible borylenes has significantly broadened over the last decade, applications remain limited. Herein, we present tricoordinate oxy-borylenes as potent photoreductants that can be readily activated by visible light. Facile oxidation of CAAC stabilized oxy-borylenes (CAAC)(IPr₂ Me₂ )BOR (R=TMS, CH₂ CH₂ C₆ H₅ , CH₂ CH₂ (4-F)C₆ H₄ ) to their corresponding radical cations is achieved with mildly oxidizing ferrocenium ion. Cyclovoltammetric studies reveal ground-state redox potentials of up to -1.90 V vs. Fc^+/0 for such oxy-borylenes placing them among the strongest organic super electron donors. Their ability as photoreductants is further supported by theoretical studies and showcased by the application as stoichiometric reagents for the photochemical hydrodehalogenation of aryl chlorides, aryl bromides and unactivated alkyl bromides as well as the detosylation of anilines.

Keywords: Boron Compounds; EPR Spectroscopy; Photochemistry; Radical Reactions; SET Reduction.

Scheme 1

a) One electron oxidation of borylene 1 to radical cation 2. b) Organic electron donors 3–5. Dipp=2,6‐diisopropylphenyl.

Scheme 2

Synthesis and isomerization of tricoordinate siloxyborylenes Z‐7 and E‐ 7.

Figure 1

Molecular structures of a) Z‐7 (only the independent molecule A is depicted), b) E‐7, c) Z‐10^Cy , d) Z‐11^Cy . Hydrogen atoms are omitted for clarity and thermal ellipsoids are set at 50 % probability. Selected bond lengths [Å] and angles for Z‐7 (molecule A): B1−O1 1.457(5), B1−C1 1.444(6), B1−C24 1.601(6), O1−B1−C24−N3 ca. −71°. For E‐7: B1−O1 1.440(3), B1−C1 1.446(4), B1−C24 1.614(3), O1−B1−C24−N3 ca. −80°. For Z‐10^Cy : B1−O1 1.448(2), B1−C1 1.453(3), B1−C24 1.615(3), O1−B1−C24−N3 ca. −65°. For Z‐11^Cy : B1−O1 1.445(2), B1−C1 1.451(2), B1−C24 1.609(2), O1−B1−C24−N2 ca. 62°.

Scheme 3

Synthesis of phenethyl‐ and 4‐fluorophenethyl‐substituted borylenes 10 and 11.

Figure 2

a) UV/Vis absorption spectra of E‐7, Z‐7, 10 ^Et and 11 ^Et in THF and b) pictorial drawings of HOMO and LUMO+2 for 10 ^Et calculated at the CAM‐B3LYP/6‐31G(d) level of theory.

Scheme 4

Synthesis of boryl radical cations 12–14 by one electron oxidation.

Figure 3

a) Molecular structures of 12^Cy (left) and 14^Cy (right). Hydrogen atoms are omitted for clarity and thermal ellipsoids are set at 50 % probability. Selected bond lengths [Å] and angles for 12^Cy : B1−O1 1.385(3), B1−C1 1.518(4), B1−C24 1.612(4), O1−B1−C24−N3 ca. −80°. For 14^Cy : B1−O1 1.383(7), B1−C1 1.515(8), B1−C24 1.603(8), O1−B1−C24−N2 ca. −77°. b) Spin density isosurface plots ((ρ_α−ρ_β), 5×10⁻³ a.u.), obtained with PW6B95/def2‐TZVP c) Liquid‐state CW EPR spectra of the radical 12^Cy (left) and 14^Cy (right) recorded in ACN at X‐band and at rt. The black curve represents the experimental spectrum and the red curve is the best‐fit simulation.

Scheme 5

Photoreduction of various aryl and alkyl halides and tosylated amines with borylene 7. Reactions were performed at a 0.1 mmol scale. Yields were determined by GC‐FID measurements using mesitylene as internal standard. ^a Byproduct which partially decomposed during the reaction. ^b Yield determined by ¹H NMR measurements using CH₂Br₂ as internal standard. ^c vs. SCE.