Visible-Light-Induced C(sp²)–C(sp³) Coupling Reaction for the Regioselective Synthesis of 3-Functionalized Coumarins

Abstract

A photocatalysis strategy for the regioselective synthesis of 3-functionalized coumarins is reported. With visible light irradiation, a direct and regioselective C­(sp2)–C­(sp3) coupling reaction of 3-(2-hydroxyphenyl)­acrylates with ethers or thioethers occurs by using Ru­(bpy)3Cl2·6H2O as a photocatalyst and TBHP as an oxidant. The cascade process involves alkenylation of the C­(sp3)–H bond of ethers and lactonization, furnishing 3-alkylated coumarins as the final products. This approach is characterized by a broad substrate scope, mild reaction conditions, and simplified operation. The synthesis of 3-alkylated coumarins could be realized by a one-pot procedure, starting from commercially available salicylaldehyde.

Photocatalytic Strategy to Realize N-Debenzylation via Aerobic Oxidation Catalyzed by 4CzIPN

“…The catalytic effects of tetraphenylporphyrin (H 2 TPP), methylene blue, and methyl violet were poor (Table , entries 4 and 5). It also surprised us that Ru­(bpy) 3 Cl 2 ·6H 2 O, exhibiting excellence performance in oxidative cross-coupling reactions related to the radical mechanistic process, − had no catalytic effect on this debenzylation system (Table , entry 6). Both increasing 4CzIPN from 2 to 4 mol % and decreasing 4CzIPN from 2 to 1 mol % resulted in reduced yields (Table , entries 7 and 8).…”

Multiple-cycle photochemical cascade reactions

“…77 Under the irradiation of blue LEDs, the group of Wang and Lu also reported a mild and effective cascade reaction of photocatalytic sequence isomerization, cyclization and C–H functionalization of 3-(2-hydroxyphenyl)acrylates 84 with ethers or thioethers 85 (Scheme 21b). 78 The protocol features good functional group tolerance and a broad substrate scope with respect to each reaction component, giving the desired 3-functionalized coumarins 86 in good yields in the presence of Ru(bpy) 3 Cl 2 ·6H 2 O as the photocatalyst and TBHP as the oxidant. Mechanistically, after the energy transfer from the excited state Ru( ii )* to 84 , the formed ( Z )-isomer 84-A would undergo an intramolecular cyclization to offer intermediate 84-B , which then undergoes photocatalytic radical addition with a carbon radical generated from ethers or thioethers and sequential oxidative deprotonation to form the desired product 86 .…”

Photo/Electrochemical‐Mediated C(sp3)−H Bond Functionalization of (Thio)Ethers

“…Notably, the scheme uses readily available starting materials and inexpensive copper salts to form photocatalysts in situ and can provide useful designs for the addition of various carbon/heteroatom-centered radicals to alkynes in an economical and environmentally friendly manner In 2021, Lu and Wang's group reported a direct, regioselective C(sp 2 )À C(sp 3 ) coupling reaction of 3-(2hydroxyphenyl)acrylate with ether or thioether under visible light irradiation using Ru(bpy) 3 Cl 2 -6H 2 O as the photocatalyst and TBHP as the oxidant, and achieved the synthesis of 3alkylated coumarins (Scheme 24). [33] The method has the advantages of a wide range of substrates, mild reaction conditions, and simple operation. Excitingly, the synthesis of 3alkylated coumarins can be achieved using a one-pot method starting with commercially available salicylaldehyde.…”

“…In 2021, Lu and Wang's group reported a direct, regioselective C(sp 2 )−C(sp 3 ) coupling reaction of 3‐(2‐hydroxyphenyl)acrylate with ether or thioether under visible light irradiation using Ru(bpy) 3 Cl 2 ‐6H 2 O as the photocatalyst and TBHP as the oxidant, and achieved the synthesis of 3‐alkylated coumarins (Scheme 24). [33] The method has the advantages of a wide range of substrates, mild reaction conditions, and simple operation. Excitingly, the synthesis of 3‐alkylated coumarins can be achieved using a one‐pot method starting with commercially available salicylaldehyde.…”