Single-Electron-Transfer Oxidation of Trifluoroborates and Silicates with Organic Reagents: A Comparative Study

Abstract

In this report, the single-electron-transfer oxidation of alkyl trifluoroborates and silicates has been studied. Different types of oxidation reagents have been examined, focusing on organic oxidants and particularly the use of dyes in photocatalytic oxidations. Both trifluoroborates and silicates could provide C-centered radicals when using a tritylium salt or the Ledwith–Weitz aminium salt. Photocatalysis with the Fukuzumi reagent suggested that trifluoroborates are more easily oxidized than biscatecholato silicates under these conditions.

Key words

radicals - dyes - photocatalysis - trifluoroborates - silicates - single-electron transfer - oxidation

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• 18 To a Schlenk flask were added the organotrifluoroborate 1 or organosilicate 3 (0.3 mmol, 1 equiv), 9-mesityl-10-methylacridinium perchlorate as photocatalyst (0.03 mmol, 10 mol%), and TEMPO (0.66 mmol, 2.2 equiv.). The Schlenk flask was sealed with a rubber septum and evacuated–purged with vacuum–argon three times. Degassed DMF (3 mL) was introduced followed by two freeze–pump–thaw cycles. The reaction mixture was stirred under blue LEDs irradiation at room temperature for 24 h under an argon atmosphere. The reaction mixture was diluted with Et₂O (50 mL), washed with sat. NaHCO₃ (2×), brine (2×), dried over MgSO₄, and evaporated under reduced pressure. The reaction residue was purified by flash column chromatography on silica gel.
• 19 The generated TEMPO N-oxide anion could be silylated or borylated. The resulting anionic products would be eliminated during the aqueous workup. We thank one of the referees for this suggestion.