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.
We report a photocatalytic version of the Barbier type reaction using readily available allyl or benzyl bromides and aromatic aldehydes or ketones as starting materials to generate allylic or benzylic alcohols.
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