Abstract
The Stille coupling reaction is a versatile method to mainly form aromatic C—C bonds. However, up to now, the use of palladium catalysts is necessary. Here, a palladium‐free and photocatalytic Stille‐type coupling reaction of aryl iodides and aryl stannanes catalyzing a conjugated microporous polymer‐based phototcatalyst under visible light irradiation at room temperature is reported. The novel coupling reaction mechanism occurs between the photogenerated aryl radical under oxidative destannylation of the aryl stannane, and the electron‐activated aryl iodide, resulting into the aromatic C—C bond formation reaction. The visible light‐promoted Stille‐type coupling reaction using the polymer‐based pure organic photocatalyst offers a simple, sustainable, and more economic synthetic pathway toward palladium‐free aromatic C—C bond formation.
Highlights
A palladium-free and photocatalytic Stille-type coupling reaction of aryl iodides and aryl stannanes catalyzing a conjugated microporous polymerbased phototcatalyst under visible light irradiation at room temperature is reported
Metal-free photocatalytic systems[8,9] offer a more sustainable and environmentally friendly alternative compared to the traditional transition metal complexes such as
The search for pure organic and palladium- have been employed as efficient photocatalyts for visible lightfree alternative catalysts still remains a huge challenge for the promoted photocatalytic reactions
Summary
Tracec) a)Reaction conditions: halide (0.2 mmol), stannane (0.2 mmol), 5 mg P-Az-B (9 mol%), 5 mL THF, white LED (1.2 W cm−2), room temperature, 24 h; b)Isolated yield via chromatogrphy; c)Determined by gas chromatography mass spectroscopy (GCMS). It could be observed that only the Stille-type coupling of electron withdrawing group-substituted aryl iodides with activated energy lower than the LUMO of P-Az-B (−1.10 V vs SCE) with aryl stannanes was successful (entries 1–12). The reaction of aryl bromide and chloride with aryl stannane did not lead to any product This could be explained by their high reduction potentials as shown in Figure S12 (Supporting Information). To further study the mechanistic insight of the photocatalytic Stille-type coupling reaction, we preformed time-resolved photoluminescence spectroscopy using L-Az-B as photocatalyst and furanyl stannane and iodobenzene as substrates to mimic the catalytic system. By adding both coupling partners, the photoluminescence lifetime was reduced to 0.9 ns This demonstrated a photoinduced electron transfer between the photocatalyst and the substrates during the catalytic cycle. To extend the scope of substrates such as vinyl-containing compounds, alkyl halides or stannannes, further design of the conjugated organic photocatalysts with more sufficient redox potentials is needed
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