Visible-light-driven external-photocatalyst-free alkylative carboxylation of alkenes with CO2

Article abstract of DOI:10.1007/s11426-021-1004-y

Herein, we report a novel protocol for visible-light-driven alkylative carboxylation of alkenes with CO₂ in the absence of external photocatalyst. Under the irradiation of visible light, a variety of 4-alkyl-1,4-dihydropyridines (alkyl-DHPs) serve as not only alkyl radical precursors but also photoexcited reductants probably with the potential to reduce benzyl radicals. Several styrenes and acrylates are applicable in this reaction to give structurally diverse carboxylic acids in good to excellent yields. These reactions feature mild reaction conditions (1 atm of CO₂, room temperature, visible light, photocatalyst- and transition metal-free), good functional group tolerance, easy scalability, as well as high regio-, and chemo-selectivity. Mechanistic investigations provide evidence that alkyl radical, benzyl radical and carbanion might be involved in this reaction, providing a novel strategy for CO₂ utilization.[Figure not available: see fulltext.]

Full text of DOI:10.1007/s11426-021-1004-y

SCIENCE CHINA
Chemistry
•ARTICLES•
https://doi.org/10.1007/s11426-021-1004-y
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Visible-light-driven external-photocatalyst-free alkylative
carboxylation of alkenes with CO2
Ya-Nan Niu1†, Xing-Hao Jin2†, Li-Li Liao2†, He Huang2, Bo Yu2,
Yu-Ming Yu1* & Da-Gang Yu2*
1Urumqi Key Laboratory of Green Catalysis and Synthesis Technology; Key Laboratory of Energy Materials Chemistry, Ministry of Education;
Key Laboratory of Advanced Functional Materials, Autonomous Region; College of Chemistry, Xinjiang University, Urumqi 830046, China;
2Key Laboratory of Green Chemistry & Technology of Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064,
China
Received March 5, 2021; accepted April 6, 2021; published online June 10, 2021
Herein, we report a novel protocol for visible-light-driven alkylative carboxylation of alkenes with CO2 in the absence of external
photocatalyst. Under the irradiation of visible light, a variety of 4-alkyl-1,4-dihydropyridines (alkyl-DHPs) serve as not only
alkyl radical precursors but also photoexcited reductants probably with the potential to reduce benzyl radicals. Several styrenes
and acrylates are applicable in this reaction to give structurally diverse carboxylic acids in good to excellent yields. These
reactions feature mild reaction conditions (1 atm of CO2, room temperature, visible light, photocatalyst- and transition metal-
free), good functional group tolerance, easy scalability, as well as high regio-, and chemo-selectivity. Mechanistic investigations
provide evidence that alkyl radical, benzyl radical and carbanion might be involved in this reaction, providing a novel strategy for
CO2 utilization.
carbon dioxide, external-photocatalyst-free, visible light, alkylative carboxylation, alkene
Citation:
Niu YN, Jin XH, Liao LL, Huang H, Yu B, Yu YM, Yu DG. Visible-light-driven external-photocatalyst-free alkylative carboxylation of alkenes with
CO2. Sci China Chem, 2021, 64, https://doi.org/10.1007/s11426-021-1004-y
1 Introduction
carboxylation reactions have been well developed via tradi-
tional transition-metal catalysis [7–11] and electrochemical
methods [12,13]. Recently, with the development of photo-
chemistry [20–22], photocatalysis has become a novel tool
for transformations with CO2 [14–19]. Usually, two patterns
of selectivity exist in the photo-induced hydrocarboxylation
reactions, one being the Markovnikov regioselectivity to
give the α-carboxylative products while the other being anti-
Markovnikov regioselectivity to give the β-carboxylative
products. For example, Iwasawa group [23,24] pioneeringly
reported the hydrocarboxylation of styrenes with Markov-
nikov regioselectivity via visible-light photoredox/Rh dual
catalysis. Later, Jamison group achieved a β-selective hy-
drocarboxylation using the photocatalyst p-terphenyl in a
continuous flow set-up under ultraviolet irradiation [25].
Carbon dioxide (CO2), which is an inexpensive, nontoxic and
recyclable one-carbon building block, has been widely used
for the synthesis of structurally diverse chemicals [1–5].
Among these transformations, the preparation of carboxylic
acids has particularly received substantial attention, which
are highly valuable motifs in many bioactive molecules and
useful synthons [6].
Notably, the carboxylation of alkenes with CO2 is one of
the most prevalent methods for the synthesis of carboxylic
acids [7–19]. With great efforts from many groups, hydro-
†These authors contributed equally to this work.
*Corresponding authors (email: yym2009@iccas.ac.cn; dgyu@scu.edu.cn)
© Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021
chem.scichina.com link.springer.com

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Niu et al. Sci China Chem
With incorporation of photoredox catalysis with nickel
complex, König group [26] demonstrated a ligand-controlled
regio-divergent hydrocarboxylation of styrenes with CO2, in
which both α- and β- selectivities were achieved. Very re-
cently, our group [27] have reported a charge-transfer com-
plex (CTC) strategy that enabled the first visible-light-driven
anti-Markovnikov hydrocarboxylation of alkenes with CO2
by using 4-alkyl-1,4-dihydropyridine (alkyl-DHP) as an
ideal reductant (Figure 1b), demonstrating the great appli-
cation potential of this photocatalyst-free and transition-
metal-free photochemistry.
In contrast to hydrocarboxylation, difunctionalizing car-
boxylation of alkenes with CO2 is more challenging but
could provide more complex carboxylic acids in a straight-
forward fashion [14–19]. It is worth noting that several
groups have developed visible-light photoredox catalysis to
provide α-carboxylative products from alkenes, CO2 and
various radical precursors (Figure 1a, left) [28–36]. For ex-
ample, Martin [28], Wu [29], Xi [30] and our group [31]
independently reported the alkylative carboxylation of acti-
vated alkenes with CO2 under redox-neutral conditions. Wu
group [29] also realized a silacarboxylation via photoredox/
hydrogen atom transfer (HAT) dual catalysis. Besides, pho-
sphonocarboxylation was also developed by our group [32].
What’s more, Li group [33] developed an intermolecular
multi-component arylative carboxylation of styrenes and our
group [34] reported the dearomative arylcarboxylation of
indoles, as well as our group [37] reported a dicarboxylation
reaction, all with CO2 under reductive conditions. Inspired
by the fact that iron-sulfur proteins can serve as interesting
redox catalysts for CO2 fixation [38,39], our group [40] de-
veloped a visible-light-driven thiocarboxylation of styrenes
and acrylates with CO2 for the generation of β-carboxylated
products (Figure 1a, right). However, these reactions mainly
rely on the use of external photocatalysts, including ex-
pensive iridium complexes and pre-synthesized fluorescent
dyes, therefore devoid of cost-efficiency from a perspective
of economy and efficiency. So, it is highly desirable to de-
velop more economical process in the absence of external
photocatalysts, which also might imply a unique mechanism
or inspire new reactions. Herein we report a novel strategy
for visible-light-driven external-photocatalyst-free alkyla-
tive carboxylation of alkenes with CO2 (Figure 1d), which
efficiently applies 4-alkyl DHPs as both radical precursors
and photoexcited reductants under mild conditions.
Figure 1 Visible-light-driven carboxylation of alkenes with CO2. EWG is
electron-withdrawing group (color online).
photocatalyst and following C–C bond cleavage at C-4 po-
sition to give an alkyl radical [45]. Recently, Melchiorre
group [46–49] pioneered and systematically demonstrated
that 4-substituted-DHPs could also represent the extra-
ordinary excited-state reactivity as H-DHP and serve as re-
ductants and radical-type alkylating/acylation reagents upon
visible light irradiation (Figure 1c). Based on this finding,
novel transformations, such as Giese-type addition, Minisci-
type reaction and asymmetrical acyl cross-coupling reaction,
were developed. Although this strategy features economical
characteristics, to the best of our knowledge, this excited-
state reactivity of alkyl-DHPs has never been discovered in
photochemical carboxylation reactions with CO2. Interest-
ingly, when we applied Cy-DHP in our previously reported
anti-Markovnikov hydrocarboxylation reaction (Figure 1b)
[27], the cyclohexyl-carboxylative product was obtained as
the main product in 46% yield with α-carboxylation
(Eq. (1)), while the formation of trace amount of desired β-
carboxylative product was observed. Considering the sig-
nificance of preparing important alkylative carboxylation
products from alkenes under more economical conditions,
we wondered the viability to realize a novel and efficient
visible-light-driven external-photocatalyst-free alkyl-car-
boxylation of alkenes with CO2.
1,4-Dihydropyridine derivatives (H-DHP) are traditionally
understood as hydride (H−) sources in their ground state and
creatively developed as proper reductants in photochemistry
to trigger single electron transfer (SET) process in excited
state [41–44]. 4-Alkyl-1,4-dihydropyridines (alkyl-DHPs),
possessing a similar structure to H-DHP, have been applied
as alkylating reagents in photoredox catalysis and related
dual catalysis, which undergo a SET with excited external
2 Experimental
To an oven-dried Schlenk tube (25 mL) containing a stirring

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Niu et al. Sci China Chem
Table 1 Optimization of reaction conditions
a)
bar was charged with 4-alkyl DHP 1 (0.2 mmol, 1.0 equiv.),
alkene 2 (0.3 mmol, 1.5 equiv., if solid) and transferred to
glovebox to add LiOtBu (64.0 mg, 0.8 mmol, 4.0 equiv.).
The tube was then evacuated and back-filled with CO2 for
three times. Subsequently, 2 (if liquid) and dimethyl sulf-
oxide (DMSO) (3.0 mL) were added under CO2 atmosphere.
The reaction was stirred in water bath and irradiated with a
30 W blue light-emitting diode (LED) lamp (3 cm away,
with a cooling fan to keep the reaction temperature at
25–30 °C) for 12 h. The resulting mixture was quenched by
2 mL of 2N HCl (aq.), 4 mL of H2O and diluted with 4 mL of
EtOAc and then stirred for 10 min and extracted by EtOAc
for 4 times and the combined organic phases were con-
centrated in vacuo. The residue was purified by silica gel
flash column chromatography (CH2Cl2 as eluent) to give the
pure desired alkylative carboxylation product 3.
b)
c)
Entry
1
Alteration from standard conditions
Yield
56%
NaOtBu (2.5 equiv.) instead of LiOtBu,
NMP instead of DMSO
d)
2
3
No change
KOtBu instead of LiOtBu
NaOtBu instead of LiOtBu
CsF instead of LiOtBu
88% (96%)
79%
4
70%
5
82%
6
0.5 mol% of fac-Ir(ppy)3 as additive
0.5 mol% of 4CzIPN as additive
LiOtBu (3 equiv.) instead of (4 equiv.)
LiOtBu (2 equiv.) instead of (4 equiv.)
LiOtBu (1 equiv.) instead of (4 equiv.)
No light
89%
7
92%
d)
8
(90%)
d)
9
(89%)
d)
10
11
12
13
(55%)
3 Results and discussion
e)
n.d.
No LiOtBu
n.d.
As the cyclohexyl-carboxylative product was isolated along
with the formation of hydrocyclohexylation product in 33%
NMR yield (Eq. (1)), we tried to optimize the reaction effi-
ciency. We initiated the study by employing the reaction with
4-Cy-DHP 1a and 4-vinyl-1,1′-biphenyl 2a as model sub-
strates under atmospheric pressure of CO2 by using NaOtBu
as base. An isolated yield of 56% was achieved (Table 1,
entry 1). After systematic investigation (Please see more
details in Supporting Information onilne), we were delighted
to get the desired product 3aa in 88% yield in the presence of
4 equiv. of LiOtBu as the base and DMSO as solvent (entry
2). LiOtBu might not only serve as a base but also a reagent
for CO2 enrichment in reaction mixture [50,51]. The for-
mation of hydrocyclohexylation product 3aa′ (7% isolated
yield) was obviously suppressed. Other bases, such as KO
N2 instead of CO2
n.d.
a) Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), LiOtBu
(0.8 mmol), DMSO (3.0 mL), 1 atm of CO2, 30 W blue LEDs, room
temperature (rt), 12 h. b) DMSO=dimethyl sulfoxide, NMP=1-methylpyr-
rolidin-2-one, ppy=2-phenylpyridine. c) Yields were isolated yields. d)
Ultra performance liquid chromatography (UPLC) yields were given in
parentheses with 100 μL anisole as an internal standard. e) n.d.=not de-
tected. Diethyl 2,6-dimethylpyridine-3,5-dicarboxylate 4 was observed as
byproduct.
could also be subject to this reaction system, affording the
corresponding products (3ea, 3fa) in excellent yields. To our
delight, when testing tertiary alkyl-DHPs, we successfully
isolated the difunctionalizing products (3ga–3ha) in good
yields. Importantly, in spite of the instability of primary alkyl
radical, 32% yield of product 3ia could also be isolated when
tBu, NaOtBu and CsF, were also examined to give lower
n
testing Pr-DHPs as radical precursor. These interesting re-
yields (Table 1, entries 3–5). External photocatalysts, such as
sults are also a supplement for our recently reported alky-
lative carboxylation of alkenes via CO2 recycling [31], in
which unstablized secondary alkyl radical precursors per-
formed in a mediocre manner.
fac-Ir(ppy)3
and
2,4,5,6-tetra(9H-carbazol-9-yl)iso-
phthalonitrile (4CzIPN) slightly promoted this reaction
(Table 1, entries 6 and 7). Besides, when we decreased the
amount of LiOtBu, a slightly lower yields for product 3aa
were obtained. Therefore, although we used 4 equiv. of
LiOtBu to achieve the best yields, it is also fine to use less
LiOtBu in applications (Table 1, entries 8–10). A series of
control experiments revealed that CO2, visible light, and base
all were essential for this transformation (Table 1, entries 11–
13).
With the optimized conditions in hand, we investigated the
scope of alkyl-DHPs (Scheme 1). As illustrated in Scheme 1,
alkyl-DHPs with secondary or tertiary alkyl groups were
compatible in this reaction system. Several secondary alkyl-
DHPs gave the corresponding products in excellent yields
(3aa–3da). N-Cbz protected α-amino-DHP and α-oxo-DHP
To extend the scope of this reaction, we further evaluated
various alkenes for the reaction with Cy-DHPs under 1 atm
of CO2 (Scheme 2). As illustrated in Scheme 2, styrenes
bearing EWGs (e.g., 3ab–3ae) at para position showed
better reactivity than those with electron-neutral (e.g., 3af–
3ag) or electron-donating groups (EDGs, e.g., 3ah–3aj).
This phenomenon might arise from the balance of stereo-
electronic effect at the benzylic position, allowing facile al-
kyl radical addition to alkenes and SET reduction of benzyl
radicals to the benzylic carbanions. Notably, styrenes bearing
C–Cl bond (3af, 3al) were also amenable to this reaction for
the generation of chlorinated products, which was of po-
tential for application in coupling reactions. Steric hindrance

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Niu et al. Sci China Chem
photoredox-catalysis [52], we found that acrylates 3aw and
3ax also underwent selective external-photocatalyst-free al-
kylative carboxylation of alkenes in this system. This reac-
tion system was also suitable for non-terminal alkenes with
phenyl or methyl group at β-position, providing corre-
sponding products 3ya and 3za in moderate yields. To our
delight, a gram-scale reaction also worked smoothly to give
3aa in 79% yield, highlighting the potential utility of this
method.
To gain more insight into this reaction, we further con-
ducted control experiments (Figure 2). The addition of ra-
dical
scavenger
2,2,6,6-tetramethylpiperding-1-oxyl
Scheme 1 Substrate scope of 4-alkyl-DHPs. Reaction conditions: 1 (0.2
mmol), 2a (0.3 mmol), LiOtBu (0.8 mmol) in DMSO (3.0 mL) under 1 atm
of CO2, 30 W blue LEDs, rt, 12 h. Isolated yields were given.
(TEMPO) under the standard conditions resulted in no for-
mation of product 3aa (Figure 2a). Instead, both the Cy-
TEMPO adduct 5 and benzyl-TEMPO adduct 6 were de-
tected by HRMS. Moreover, we further isolated the Cy-
TEMPO adduct 5 in 11% yield in the absence of 2a under the
standard conditions (Figure 2b). These results indicated that
cyclohexyl and benzylic radicals might be involved in this
reaction. When 1 equiv. of D2O was added in the standard
conditions, 39% deuterium incorporation at the benzylic
position of 3aa′ was observed (Figure 2c). Nonetheless,
when we increased the amount of D2O to 3 equiv., this re-
action was inhibited and no conversion of 1a was observed,
which might be due to the deactivation of LiOtBu by D2O.
Even so, we could also reasonably infer that benzyl carba-
nion might be formed in this reaction [28–37].
To shed light on the mechanism of this external-photo-
catalyst-free reaction, we further conducted ultraviolet-visi-
ble spectroscopy (UV-Vis) spectroscopic experiment (Figure
3), from which the external-photocatalyst-free reaction sys-
tem was found to show obvious absorption in visible light
region. Then we tested the single or the mixture of reaction
components by UV-Vis absorption spectroscopy. Substrate
2a had almost no absorption in the visible region. Although
substrate 1a had an absorption peak at 339 nm, clear red-shift
absorption (up to 500 nm) displayed by adding stoichio-
Scheme 2 Substrate scope of activated alkenes. Reaction conditions: 1a
(0.2 mmol), 2 (0.3 mmol), LiOtBu (0.8 mmol), DMSO (3.0 mL), 1 atm of
CO2, 30 W blue LEDs, rt, 12 h. Isolated yields were given.
has little effect on the reaction, which is found when com-
paring the yields of products 3am and 3ap with products
3ag. Moreover, styrenes with α-position substituted with
methyl or ethyl groups (3an, 3ao) could also be applied in
this system. What’s more, the 1,1-diarylethylenes bearing a
C–S bond could also react smoothly in our conditions. Im-
portantly, thiophene 3av also did not deter the carboxylation.
As for other activated alkenes such as acrylates, which have
been widely investigated as radical acceptors in visible-light
Figure 2 Control experiments (color online).

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Niu et al. Sci China Chem
Figure 4 Proposed mechanism (color online).
catalyst-free reaction (Figure 4). Taking the reaction of 1a for
an example, the reaction is initiated from the generation of
cyclohexyl radical A. Then this radical could add to alkene to
produce a more stable benzylic radical B, which undergoes
SET reduction with the photoexcited 1a to give the benzylic
carbanion C and regenerate cyclohexyl radical A. Benzylic
carbanion C subsequently undergoes nucleophilic attack on
CO2, which is followed by protonation during workup to
afford the desired product 3aa. Though we preferred this
pathway, we could not rule out other possibilities. Besides,
several side reactions might also occur in the above steps,
generating the by-products 3aa′ and cyclohexane.
Figure 3 UV-Vis absorption spectra of different components in DMSO
(path length=1 cm) for each species and picture of solutions in quartz
cuvettes (color online).
metric LiOtBu regardless of the presence of 2a, which also
showed a yellow color. We speculated that the interactions
between 1a and base were responsible for the absorption of
visible light thus triggering the reaction. Furthermore, since
alkene 2a exhibited no effect on the absorption of 1a in this
UV-Vis measurement, we did not prefer the possible ex-
istence of electron-donor-acceptor (EDA) complex or CTC
between 1a and 2a at this stage [53–55].
After understanding the visible-light absorption properties
of 1a in the presence of LiOtBu, we further suspected whe-
ther photoexcited 1a could undergo C–C bond cleavage in
the absence of 2a under 1 atm of CO2. As shown in Eq. (2),
30% of the pyridine byproduct 4 was isolated with the re-
covery of 1a in 59% yield, indirectly suggesting that pho-
toexcited 1a might generate cyclohexyl radical via a SET
process with CO2 or other reagents, albeit with moderate
efficiency.
4 Conclusions
In summary, we disclosed the visible-light-driven external-
photocatalyst-free alkylative carboxylation of alkenes with
CO2. The 4-alkyl DHPs act as both radical precursors and
photoexcited reductants. By using this strategy, we manage
to prepare a variety of structurally diverse functionalized
carboxylic acids under mild reaction conditions (1 atm, rt).
Mechanistic studies indicate alkyl radical, benzyl radical and
benzyl carbanion might be generated in this transformation
as key intermediates. Further mechanistic studies and ap-
plication of this photochemistry are underway in our lab.
Acknowledgements This work was supported by the National Natural
Science Foundation of China (21822108, 21772129), the Fok Ying Tung
Education Foundation (161013), Sichuan Science and Technology Program
(20CXTD0112), and Fundamental Research Funds for the Central Uni-
versities.
Conflict of interest The authors declare no conflict of interest.
Supporting information The supporting information is available online at
http://chem.scichina.com and http://link.springer.com/journal/11426. The
supporting materials are published as submitted, without typesetting or
editing. The responsibility for scientific accuracy and content remains en-
tirely with the authors.
Based on mechanistic studies and previous work, we
proposed a possible mechanism for this external-photo-

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