Photoredox-Catalyzed Radical–Radical Coupling of Potassium Trifluoroborates with Acyl Azoliums

Michael J. Rourke; Matthew J. McGill; Daniel Yang; Emelia J. Farnam; Joshua L. Zhu; Karl A. Scheidt

doi:10.1055/s-0041-1738448

Synlett, Table of Contents

Synlett 2023; 34(18): 2175-2180
DOI: 10.1055/s-0041-1738448

cluster

Modern Boron Chemistry: 60 Years of the Matteson Reaction

Photoredox-Catalyzed Radical–Radical Coupling of Potassium Trifluoroborates with Acyl Azoliums

Michael J. Rourke‡

,

Matthew J. McGill‡

,

Daniel Yang

,

Emelia J. Farnam

,

Joshua L. Zhu

,

Karl A. Scheidt ∗

Abstract

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Abstract

Potassium trifluoroborates have gained significant utility as coupling partners in organic synthesis, particularly in the Suzuki–Miyaura coupling reaction. Recently, they have also been used as radical precursors under oxidative conditions to generate carbon-centered radicals. These versatile reagents have found new applications in photoredox catalysis, including radical substitution, conjugate-addition reactions, and transition-metal dual catalysis. In addition, this photomediated redox-neutral process has enabled radical–radical coupling with persistent radicals in the absence of a metal, and this process remains to be fully explored. In this study, we report the radical–radical coupling of potassium benzylic trifluoroborate salts with isolated acyl azolium triflates, which are persistent-radical precursors. The reaction is catalyzed by an organic photocatalyst and forms isolable tertiary alcohol species. These products can be transformed into a range of substituted ketone products by simple treatment with a mild base.

Key words

potassium trifluoroborate - photoredox catalysis - organophotocatalysis - acyl azolium compounds - radical–radical coupling - ketones

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References

References and Notes

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24 Coupling of Potassium Trifluoroborates and Acyl Azolium Triflates; General ProcedureIn a glovebox with an inert N₂ atmosphere, a flame-dried 2-dram vial equipped with a magnetic stirrer bar was sequentially charged with 4CzIPN (2 mol%), the appropriate acyl azolium 2 (1 equiv, 0.5 mmol), and RBF₃K salt 1 (1.1 equiv), which were suspended in anhyd freeze–pump–thaw degassed MeCN (2.5 mL, 0.2 M). TFA (1.0 equiv) was then added to the mixture, and the vial was sealed and irradiated, with stirring, by Kessil PhotoReaction PR 160L (λ = 456 nm) LEDs at 100% intensity, arranged radially around the vial. After 18–24 h of irradiation, the vial was removed from the light source, and the reaction was monitored by LC-MS for the disappearance of the acyl azolium and the formation of the resulting tetrahedral intermediate analogous to 4a. DBU (1.0 equiv) was added, resulting in darkening of the reaction mixture under stirring. The disappearance of the tetrahedral intermediate was monitored by LC-MS (typically 10 min). The mixture was then diluted with EtOAc (12.5 mL) then washed once with an equivalent volume of sat. aq NH₄Cl. The organic layer was dried via a brine wash and the aqueous layers were back-extracted with EtOAc. The combined organic layers were then dried (Na₂SO₄), concentrated in vacuo, and purified by flash column chromatography [silica gel, hexanes–EtOAc (20:1)]
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26 1-Cyclohexyl-2-phenylethanone (3g)Clear oil; yield: 76.8 mg (76%). ¹H NMR (500 MHz, CDCl₃): δ = 7.34–7.29 (m, 2 H), 7.28–7.23 (m, 1 H), 7.20–7.14 (m, 2 H), 3.73 (s, 2 H), 2.46 (tt, J = 11.5, 3.5 Hz, 1 H), 1.86–1.72 (m, 4 H), 1.69–1.62 (m, 1 H), 1.41–1.31 (m, 2 H), 1.31–1.13 (m, 3 H). ¹³C NMR (126 MHz, CDCl₃): δ = 211.4, 134.6, 129.6, 128.7, 127.0, 50.3, 48.0, 28.7, 26.0, 25.8
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