Please cite this article in press as: Schmalzbauer et al., Redox-Neutral Photocatalytic CÀH Carboxylation of Arenes and Styrenes with CO2, Chem
ll
Article
Redox-Neutral Photocatalytic CÀH
Carboxylation of Arenes and Styrenes with CO2
Matthias Schmalzbauer,1,4 Thomas D. Svejstrup,2,4 Florian Fricke,1 Peter Brandt,2
Magnus J. Johansson,2,3 Giulia Bergonzini,2, and Burkhard Konig
*
¨
SUMMARY
The Bigger Picture
CO2 is a highly abundant and
sustainable carbon source that
serves as a feedstock for the
biosynthesis of organisms.
However, it is also considered a
greenhouse gas, therefore, the
fixation of CO2 in synthetic
processes is of great
Carbon dioxide (CO2) is an attractive one-carbon (C1) building block
in terms of sustainability and abundance. However, its low reactivity
limits applications in organic synthesis as typically high-energy re-
agents are required to drive transformations. Here, we present a
redox-neutral C─H carboxylation of arenes and styrenes using a
photocatalytic approach. Upon blue-light excitation, the anthrolate
anion photocatalyst is able to reduce many aromatic compounds to
their corresponding radical anions, which react with CO2 to afford
carboxylic acids. High-throughput screening and computational
analysis suggest that a correct balance between electron affinity
and nucleophilicity of substrates is essential. This novel methodol-
ogy enables the carboxylation of numerous aromatic compounds,
including many that are not tolerated in classical carboxylation
chemistry. Over 50 examples of C─H functionalizations using CO2
or ketones illustrate a broad applicability. The method opens new
opportunities for the valorization of common arenes and may find
application in late-stage C─H carboxylation.
environmental value. In the field of
renewable energy storage and
conversion of CO2 into solar fuels,
large strides have been made into
effective catalytic reductions.
However, the need for a
stoichiometric reductant is a
significant disadvantage for the
use of CO2 as a C1 synthon in
synthesis, limiting its use. A mild,
direct, redox-neutral, and
INTRODUCTION
transition-metal-free insertion of
CO2 into a C─H bond, as reported
here, accomplishes highest
energy and atom economy,
avoiding pre-functionalization.
Using a new mechanistic
Photosynthesis, the most important photobiological process on our planet, allows
photoautotrophs to store energy in the form of chemical bonds by absorbing sun-
light. Driven by that energy, carbon dioxide (CO2) is captured from the atmosphere
and serves as a carbon feedstock for the organisms to build up sugars and biomass in
manifold, the methodology
presents a straightforward,
sustainable, and atom-efficient
alternative to current approaches
and paves the way to develop
novel applications of CO2 in
chemical synthesis.
The electrochemical and catalytic dihydrogen reductions of CO2 have been devel-
oped in the field of renewable energy storage.2–4 However, the use of CO2 as a
C1 building block in organic synthesis has received far less attention, despite
resembling the principle of biological carbon fixing process the most.5 The high
thermodynamic stability and kinetic inertness of CO2 require the use of stoichio-
metric amounts of reactive reaction partners, such as Grignard reagents or organo-
lithium compounds for chemical conversion.6 Aiming for a better efficiency and an
increased atom economy, a variety of catalytic carboxylation methods have been
developed. These processes make use of the readily available aryl bromides, which
undergo carboxylation with CO2 in the presence of catalytic Pd(OAc)2, as reported
by Mart´ın and Correa.7 Daugulis showed that Cu(I) catalyzes the carboxylation of aryl
iodides.8 Tsuji and co-workers applied NiCl2(PPh3)2 to carboxylate more inert aryl
and vinyl chlorides under 1 atm CO2 at room temperature.9 The reaction scope
was extended to sulfonates,10 ester derivatives,11,12 allylic alcohols,13 benzylic
ammonium salts,14 arylsulfonium salts,15 and unsaturated hydrocarbons.16–20
Chem 6, 1–15, October 8, 2020 ª 2020 Elsevier Inc.
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