Visible-light Photocatalytic Alkylsulfonylation of Aroylhydrazides with Alkylsulfonyl Radicals

In view of the importance of sulfonyl groups in organic molecules, the introduction of sulfonyl groups and the synthesis of sulfonyl compounds have been widely reported. Among them, the synthesis of sulfonylhydrazides has attracted much attention because of their biological activities in antitumor and antibacterial activities. Here, we developed a photoinduced reaction of potassium alkyltrifluoroborates, DABCO center dot(SO2)(2) (1,4-Diazabicyclo[2.2.2]octane, DABCO), and aroylhydrazides under visible light irradiation, which obtained N'-acyl-N-alkylsulfonylhydrazides in moderate to good yields in one-pot. This reaction works in a green and mild way with a broad substrate scope. Mechanistic study shows that after giving rise to N-acyldiazenes via the oxidation of aroylhydrazides, the transformation is initiated by alkyl radicals generated in situ from potassium alkyltrifluoroborates in the presence of photocatalyst. The subsequent insertion of sulfur dioxide and alkylsulfonylation of N-acyldiazenes with alkylsulfonyl radical intermediates afford the corresponding N'-acyl-N-alkylsulfonyl-hydrazides. General procedure for visible-light photocatalytic alkylsulfonylation of aroylhydrazides with alkylsulfonyl radicals: to a solution of aroylhydrazides 1 (0.2 mmol) and K2CO3 (0.3 mmol) in MeCN (2.0 mL) was added I-2 (0.12 mmol). The reaction was stirred for 3 h at room temperature, then quenched with sat. aq. Na2S2O3 (15 mL). The layers were separated and the aqueous layer extracted with dichloromethane (DCM) (15 mLx3). The combined organic layers were then washed with brine (15 mL), and dried over Na2SO4. Evaporation of the solvent under reduced pressure afforded the crude N-acyldiazenes, which was used in the subsequent transformation without further purification. The crude N-acyldiazenes were combined with potassium alkyltrifluoroborates 2 (0.24 mmol), DABCO center dot(SO2)(2) (0.2 mmol) and 9-mesityl-10-methylacridinium perchlorate ([Mes-Acr]ClO4) (0.004 mmol) in a tube. The tube was evacuated and backfilled with N-2 three times before the addition of MeCN (2.0 mL). The mixture was then placed around blue light emitting diodes (LEDs) (30 W) with a distance of 10 cm, and was stirred under blue light irradiation for overnight at room temperature. After completion of reaction as indicated by thin layer chromatography (TLC), the solvent was evaporated and the residue was purified directly by flash column chromatography (V(n-hexane)/V(ethyl acetate)=5:1-2:1) to give the N'-acyl-N-sulfonylhydrazides 3.