Photocarboxylation of Benzylic C–H Bonds

Abstract

The carboxylation of sp 3 -hybridized C–H bonds with CO 2 is a challenging transformation. Herein, we report a visible-light-mediated carboxylation of benzylic C–H bonds with CO 2 into 2-arylpropionic acids under metal-free conditions. Photo-oxidized triisopropylsilanethiol was used as the hydrogen atom transfer catalyst to afford a benzylic radical that accepts an electron from the reduced form of 2,3,4,6-tetra(9H-carbazol-9-yl)-5-(1-phenylethyl)benzonitrile generated in situ . The resulting benzylic carbanion reacts with CO 2 to generate the corresponding carboxylic acid after protonation. The reaction proceeded without the addition of any sacrificial electron donor, electron acceptor or stoichiometric additives. Moderate to good yields of the desired products were obtained in a broad substrate scope. Several drugs were successfully synthesized using the novel strategy.

Reductive radical-polar crossover: traditional electrophiles in modern radical reactions

“…The group of König has since reported on an elegant carboxylation of benzylic C-H bonds by merging HAT catalysis with RRPCO (Scheme 8). 13 First, a thiyl radical is generated via oxidation and subsequent deprotonation of a thiol HAT catalyst. The thiyl radical then abstracts a benzylic H atom to generate a benzylic radical and reform the thiol catalyst.…”

Polythioethers with Controlled α,ω‐End Groups Prepared by Visible Light Induced Thiol–Ene Click Polymerization of Dithiol and Divinyl Ether with 4‐(N,N‐diphenylamino)benzaldehyde as Organocatalyst

“…After adding TEMPO at the initial stage of the reaction, no polymer was formed within 6 h. In the another run, TEMPO was added to the reactant after the reaction was carried out for 6 h. Then, the reaction was continued for another 6 h. The result showed that the M n of the product at 12 h was basically consistent with the M n of the product at the first 6 h (Figure S4, Supporting Information), which further proved that the reaction is free radical mediated. Taking the above results, the study of Meng et al [ 53 ] and the mechanism of thiol–ene radical reaction by Hoyle et al [ 5 ] into account, we speculated the reaction mechanism of the reaction ( Figure ). DPAB is converted to an excited state (DPAB*) under visible light, (DPAB*) and reacts with SH to generate (DPAB) − and (SH) + , and then (SH) + generates a S and H + .…”

Surface Engineering of Organic Polymers by Photo‐induced Free Radical Coupling with p‐Dimethylaminophenyl Group as A Synthesis Block

“…We tentatively proposed a photoinduced free radical coupling reaction (Scheme ). This mechanism combines features proposed by Yang, Meng and Seo for abstraction of H atom and photoredox‐catalyzed α‐amino radical couplings respectively. During this photo‐introduced process, the triplet state of the excited BP would abstract hydrogen from the C–H bonds on a polymer surface and form carbon‐centered radicals .…”