Carboxylation of benzylic and aliphatic C-H bonds with CO2 induced by light/ketone/nickel

Article abstract of DOI:10.1021/jacs.9b12529

A photoinduced carboxylation reaction of benzylic and aliphatic C-H bonds with CO₂ is developed. Toluene derivatives capture gaseous CO₂ at the benzylic position to produce phenylacetic acid derivatives when irradiated with UV light in the presence of an aromatic ketone, a nickel complex, and potassium tert-butoxide. Cyclohexane reacts with CO₂ to furnish cyclohexanecar-boxylic acid under analogous reaction conditions. The present photoinduced carboxylation reaction provides a direct access from readily available hydrocarbons to the corresponding carboxylic acids with one carbon extension.

Full text of DOI:10.1021/jacs.9b12529

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Communication
Carboxylation of Benzylic and Aliphatic C–
H Bonds with CO2 Induced by Light/Ketone/Nickel
Naoki Ishida, Yusuke Masuda, Yuuya Imamura, Katsushi Yamazaki, and Masahiro Murakami
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b12529 • Publication Date (Web): 28 Nov 2019
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Carboxylation of Benzylic and Aliphatic C–H Bonds with CO2
Induced by Light/Ketone/Nickel
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Naoki Ishida,* Yusuke Masuda, Yuuya Imamura, Katsushi Yamazaki, and Masahiro Murakami*
Department of Synthetic Chemistry and Biological Chemistry, Kyoto University, Katsura, Kyoto 615-8510, Japan
Supporting Information Placeholder
cohol, which generates a nucleophilic allylcopper spe-
cies through copper-induced retro-allylation extruding
the benzophenone derivative. The allylcopper interme-
diate adds onto CO2 to furnish a carboxylic acid. We
have tried to expand the scope to include substrates that
potentially generate benzylic radicals, and thus, exam-
ined a carboxylation reaction of p-methoxytoluene (1).
Initial attempts to apply the previous conditions for al-
lylic carboxylation failed to prompt carboxylation of 1.
Detailed examination of other metals9 disclosed that
nickel(II) complexes, which have been employed for
various carboxylation reactions,10 were effective. Rep-
resentative conditions are shown in Scheme 1. A solu-
tion containing p-methoxytoluene (1, 0.40 mmol), xan-
thone (25 mol %), NiCl2·6H2O (5 mol %), di(2-
pyridyl)methane (10 mol %), and t-BuOK (1.3 equiv) in
benzene (4 mL) was irradiated with UV LED lamps
(lmax = 365 nm) under an atmospheric pressure of CO2
at an ambient temperature. After 4 h, 2N aq. HCl was
added to quench the reaction. Carboxylic acid 2 was
formed together with a trace amount of 1,2-di(p-
anisyl)ethane. Purification by acid/base extraction fol-
lowed by column chromatography on silica gel (ethyl
acetate/methanol = 10/1) to give carboxylic acid 2 in
84% yield. Thus, the carboxylation reaction occurred
site-selectively at the benzylic C–H bond with the meth-
oxy C–H bonds remaining unreacted.
ABSTRACT: A photoinduced carboxylation reaction of
benzylic and aliphatic C–H bonds with CO2 is developed.
Toluene derivatives capture gaseous CO2 at the benzylic
position to produce phenylacetic acid derivatives when
irradiated with UV light in the presence of an aromatic
ketone, a nickel complex, and potassium t-butoxide.
Cyclohexane reacts with CO2 to furnish cyclohex-
anecarboxylic acid under analogous reaction conditions.
The present photoinduced carboxylation reaction pro-
vides a direct access from readily available hydrocar-
bons to the corresponding carboxylic acids with one-
carbon extension.
It presents a challenge of primary importance to ex-
ploit carbon dioxide (CO2) as the C1 resource. In the
field of organic synthesis, it is highly desired to develop
reactions in which CO2 is captured into organic skele-
tons through C–C bond formation.1 Whereas various
functional groups such as carbon–halogen and C–C
double bonds have been successfully subjected to reac-
tions fixing CO2, examples of reactions directly fixing
CO2 into hydrocarbons remained limited to those of al-
lylic,2 aromatic,3 and vinylic C–H bonds.4,5 There have
been no report for direct carboxylation of benzylic me-
thyl C–H bonds of methylbenzenes and C(sp3)–H bonds
of saturated hydrocarbons promoted by homogeneous
catalysts.6 We now report a new photoinduced reaction
fixing CO2 into benzylic and aliphatic C(sp3)–H bonds,
which presents a direct access from readily available
hydrocarbon molecules to carboxylic acids with one-
carbon extension.
Scheme 1. Carboxylation of p-Methoxytoluene (1)
MeO
UV light (365 nm)
xanthone (25 mol %)
NiCl2·6H2O (5 mol %)
(2-Py)2CH2 (10 mol %)
t-BuOK (1.3 equiv)
H
1 (0.40 mmol)
MeO
+
It is known that, when excited by a photon, the oxygen
of a carbonyl group abstracts a hydrogen atom from a
C(sp3)–H bond to furnish a pair of ketyl radical and al-
kyl radical.7 Such an activation process has been ex-
ploited for direct functionalization of C(sp3)–H
bonds.2a,8 We previously reported a carboxylation reac-
tion of allylic C–H bonds with CO2, in which light, a
benzophenone derivative, and a copper(I) complex co-
operate.2a Mechanistically, the resulting pair of ketyl
radical and allyl radical couples to give a homoallyl al-
CO2H
benzene, rt, 4 h
CO2
(1 atm)
2 84%
O
O
N
N
(2-Py)2CH2
xanthone
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Scheme 2. Possible Mechanistic Pathway
Page 2 of 6
1
2
3
MeO
e
Light
CO2
4
5
6
*
O
MeO
Ar
Ar
MeO
H
O
Ni0
7
CO2NiI
Ar
Ar
8
MeO
9
H
H
10
11
12
13
14
15
16
17
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22
23
24
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27
28
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49
50
51
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53
54
55
56
57
58
59
60
OH
e
O
MeO
Ar Ar
NiI
Ar Ar
CO2
t-BuO
t-BuOH
A possible mechanistic pathway for the production of
the carboxylic acid 2 from CO2 and p-methoxytoluene
(1) is depicted in Scheme 2. Initially, xanthone absorbs
UV light to get excited. The carbonyl oxygen of the
excited ketone abstracts hydrogen from the benzylic C–
H bond of 1 to generate a pair of ketyl radical and
benzylic radical, both of which are electronically neutral.
The ketyl radical is deprotonated by t-BuOK11 to give a
ketyl radical anion, which transfers a single electron to
the nickel(II) salt that is employed as the catalyst
precursor. Ultimately, a nickel(0) species results12 to
initiate the catalytic cycle of nickel (right cycle). It
combines with the benzylic radical (vide supra), giving
rise to benzylnickel(I) species. Carbon dioxide inserts
into the C–Ni(I) bond to give a nickel(I) carboxylate,10g,h
which accepts a single electron from the ketyl radical
anion to release the carboxylate anion of 2 with
The results of carboxylation reactions of other
methylbenzene derivatives with the catalysts shown in
Scheme 1 were summarized in Table 1.
m-
Methoxytoluene (5) was carboxylated site-selectively at
the benzylic C–H bond with the methoxy C–H bonds
remaining unreacted (entry 1), as with the case of p-
methoxytoluene (1). p-Xylene (7) selectively underwent
mono-carboxylation to afford the arylacetic acid 8 in
75% yield (entry 2). No dicarboxylated product was
formed, probably because of the low solubility of the
resulting carboxylate salt in benzene. m-Xylene (9, en-
try 3), o-xylene (11, entry 4), and mesitylene (13, entry
5) also afforded the corresponding mono-carboxylated
products 10, 12, and 14, respectively.
3,4-
Dimethylanisole (15) underwent carboxylation site-
selectively at the 4-methyl group (entry 6). Since the
excited carbonyl oxygen is electrophilic rather than nu-
cleophilic, it abstracted hydrogen from the most elec-
tron-rich C–H bond. 4-Methylveratrole (17) underwent
an efficient carboxylation reaction to produce homovera-
tric acid 18 in 81% yield (entry 7). 4-Fluoro- (19, entry
8), 4-chloro- (21, entry 9), and 4-methoxy-3-
cyanotoluene (23, entry 10) were carboxylated with the
functional groups being retained. On the other hand, no
carboxylic acid products were produced in case of 4-
dimethylamino-, 4-methylthio-, 4-cyano-, and 3-
benzoyltoluenes.
regeneration of the Ni(0) species.
anisyl)ethane byproduct would be generated by homo-
dimerization of the benzylic radical intermediate.
The 1,2-di(p-
The result of the carboxylation reaction of cyclo-
propylmethylanisole 3 (Scheme 3) is consistent with the
proposed mechanism shown in Scheme 2. When 3 was
subjected to the reaction conditions, the cyclopropane
ring was opened and the acyclic carboxylic acid 4 was
produced as the major product (30% yield, E:Z = 9:1). A
carboxylated product keeping the cyclopropyl ring was
not formed.13,14 This result indicates that the formation
of a benzylic radical species is involved in the mechanis-
tic pathway.
The carboxylation reaction of ethylbenzene (25) af-
forded carboxylic acid 26 in 15% yield along with the
formation of 2,3-diphenylbutane (17% yield, 1:1 dr,
Scheme 3. Carboxylation of Cyclopropylmethylben-
zene 3
Scheme 4. Carboxylation of Ethylbenzene (23)
UV light (365 nm)
xanthone (25 mol %)
NiCl2·6H2O (5 mol %)
(2-Py)2CH2 (10 mol %)
t-BuOK (1.3 equiv)
MeO
CO2H
UV light (365 nm)
MeO
xanthone (25 mol %)
NiCl2·6H2O (5 mol %)
H
Me
CO2
(1 atm)
26 15%
+
(2-Py)2CH2 (10 mol %)
H
benzene, rt, 4 h
t-BuOK (1.3 equiv)
3 (0.40 mmol)
benzene, rt, 4 h
Me
Me
+
CO2H
25
(0.40 mmol)
Me
4
30% (E:Z = 9:1)
CO2
(1 atm)
17%, 1:1 dr
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Journal of the American Chemical Society
Table 1. Carboxylation of Methylbenzene Derivativesa
failed to incorporate CO2, under the present reaction
1
2
3
4
5
6
7
8
conditions. The C–C double bond might coordinate to
nickel, retarding coupling of the cyclohexenyl radical
species with nickel.
UV light (365 nm)
R
xanthone (25 mol %)
NiCl2·6H2O (5 mol %)
(2-Py)2CH2 (10 mol %)
t-BuOK (1.3 equiv)
H
(0.40 mmol)
+
Next, we tackled a challenge to carboxylate saturated
hydrocarbons. When reaction conditions similar to
those shown in Scheme 1 were applied to a solvent
amount of cyclohexane (27), only a trace amount of cy-
clohexanecarboxylic acid (28) was formed. After exten-
sive screening of the reaction parameters,9 we found
slightly modified reaction conditions that were suitable
for carboxylation of 27 (Scheme 5). When a mixture of
27 (1 mL), ketone 29 (0.10 mmol), Ni(NO3)2·6H2O
(0.01 mmol), bipyridine 30 (0.02 mmol), and t-BuOK
(0.40 mmol) in t-BuOH (4 mL) was irradiated with UV
LED lamps, 0.32 mmol of carboxylic acid 28 was pro-
duced. It is 32 times equivalent of the employed nickel.
The yield of 28 based on t-BuOK (0.40 mmol) is 80%.
A similar mechanism is expected to operate for the car-
boxylation of 27.
R
CO2H
benzene, rt, 4 h
CO2
(1 atm)
products
entry
substrates
9
1b
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
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52
53
54
55
56
57
58
59
60
H
H
CO2H
CO2H
MeO
Me
MeO
Me
5
6 30%
8 75%
2
7
Me
Me
3
4
H
CO2H
10 69%
9
Me
Me
H
CO2H
Scheme 5. Carboxylation of Cyclohexane (27)
12 55%
11
UV light (365 nm)
Me
Me
ketone 29 (0.10 mmol)
Ni(NO3)2·6H2O (0.01 mmol)
bpy 30 (0.02 mmol)
5
H
CO2H
t-BuOK (0.40 mmol)
Me
Me
CO2
+
(1 atm)
14 43%
16 58%
18 81%
13
15
17
CO2H
t-BuOH (4 mL), rt, 3 h
H
MeO
MeO
28 0.32 mmol
(32 equiv to Ni)
27 (1 mL)
H
H
H
6b
CO2H
MeO
OMe
O
MeO
MeO
F
MeO
MeO
F
7
8
9
N
N
H
H
H
H
CO2H
CO2H
CO2H
CO2H
t-Bu
t-Bu
29
30
The photoinduced carboxylation reaction was exam-
ined using other cyclic and acyclic saturated hydrocar-
bons under the same reaction conditions (Scheme 6).
Cyclopentane (31) also underwent the carboxylation
reaction to give cyclopentanecarboxylic acid (32), alt-
hough the yield was lower than that of cyclohexane
(0.10 mmol, 10 equiv to Ni).15 An isomeric mixture of
carboxylic acids (0.08 mmol, 8 equiv to Ni) was pro-
duced when pentane (33) was used as the substrate. The
ratio of 34:35:36 was 1:8:3, which reflected the stability
of the alkyl radical intermediates as well as the numbers
of the parent C–H bonds.
20 34%
22 33%
19
21
Cl
Cl
MeO
NC
MeO
NC
10
24 33%
23
a Reaction conditions: methylbenzenes (0.40 mmol), xan-
thone (0.10 mmol, 25 mol %), NiCl2·6H2O (0.02 mmol, 5
mol %), di(2-pyridyl)methane (0.04 mmol, 10 mol %), t-
BuOK (0.50 mmol, 1.3 equiv), benzene (4 mL), CO2 (1
atm), UV LED lamps (lmax = 365 nm), ambient tempera-
ture, 4 h. b Xanthone (50 mol %), NiCl2·6H2O (10 mol %),
di(2-pyridyl)methane (20 mol %).
In conclusion, we have developed a carboxylation
reaction of C(sp3)–H bonds with CO2 which is induced
by light/ketone/nickel. Substituted benzene derivatives
were effectively carboxylated at the benzylic position
under an atmospheric pressure of CO2 at ambient tem-
perature. The carboxylation reaction was successfully
extended to saturated hydrocarbons such as cyclohexane
and pentane. The present carboxylation reaction pro-
vides a direct access from readily available hydrocar-
bons to the corresponding carboxylic acids with one-
carbon extension.
Scheme 4). The additional methyl group at the benzylic
radical center makes the radical species more stable, i.e.,
less reactive, and also, sterically encumbers its coupling
with a nickel(0) species. Thus, the direct homodimeriza-
tion of the radical species is allowed. Cyclohexene
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Page 4 of 6
mation: Carbon–Carbon Bond Forming Reactions by Transi-
Scheme 6. Carboxylation of Hydrocarbons 31 and 33
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1
2
3
4
5
6
7
8
UV light (365 nm)
ketone 29 (0.10 mmol)
Ni(NO3)2·6H2O (0.01 mmol)
bpy 30 (0.02 mmol)
t-BuOK (0.40 mmol)
CO2
+
(1 atm)
CO2H
t-BuOH (4 mL), rt, 3 h
H
32 0.10 mmol
(10 equiv to Ni)
31 (1 mL)
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
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43
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45
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47
48
49
50
51
52
53
54
55
56
57
58
59
60
CH3
CH3
UV light (365 nm)
CO2H
ketone 29 (0.10 mmol)
Ni(NO3)2·6H2O (0.01 mmol)
bpy 30 (0.02 mmol)
34
35
CH3
CH3
33 (1 mL)
CH3
CO2H
t-BuOK (0.40 mmol)
+
t-BuOH (4 mL), rt, 3 h
CO2
(1 atm)
CO2H
CH3
CH3
（2） For catalytic reactions: (a) Ishida, N.; Masuda, Y.; Uemoto,
S.; Murakami, M. A Light/Ketone/Copper System for Car-
boxylation of Allylic C–H Bonds of Alkenes with CO2.
Chem. Eur. J. 2016, 22, 6524-6527. (b) Michigami, K.; Mita,
T.; Sato, Y. Cobalt-Catalyzed Allylic C(sp3)–H Carboxyla-
tion with CO2. J. Am. Chem. Soc. 2017, 139, 6094-6097.
For an example using a stoichiometric amount of an alumi-
num reagent: (c) Tanaka, S.; Watanabe, K.; Tanaka, Y.; Hat-
tori, T. EtAlCl2/2,6-Disubstituted Pyridine-Mediated Car-
boxylation of Alkenes with Carbon Dioxide. Org. Lett. 2016,
18, 2576-2579.
（3） For a catalytic reaction: (a) Suga, T.; Mizuno, H.; Takaya, J.;
Iwasawa, N. Direct Carboxylation of Simple Arenes with
CO2 through a Rhodium Catalyzed C–H Bond Activation.
Chem. Commun. 2014, 50, 14360-14363. For examples us-
ing a stoichiometric amount of Lewis acids: (b) Suzuki, Y.;
Hattori, T.; Okuzawa, T.; Miyano, S. Lewis Acid-Mediated
Carboxylation of Fused Aromatic Compounds with Carbon
Dioxide. Chem. Lett. 2002, 31, 102-103. (c) Olah, G. A.;
Török, B.; Joschek, J. P.; Bucsi, I.; Esteves, P. M.; Rasul, G.;
Prakash, G. K. S. Efficient Chemoselective Carboxylation of
Aromatics to Arylcarboxylic Acids with a Superelectrophil-
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Chem. Soc. 2002, 124, 11379-11391. (d) Schäfer, A.; Saak,
W.; Haase, D.; Müller T. Silyl Cation Mediated Conversion
of CO2 into Benzoic Acid, Formic Acid, and Methanol. An-
gew. Chem. Int. Ed. 2012, 51, 2981-2984.
36
total 0.08 mmol
(8 equiv to Ni)
34:35:36 = 1:8:3
ASSOCIATED CONTENT
The Supporting Information is available free of charge on
the ACS Publications website.
Results of optimization and details of experimental proce-
dures including spectroscopic data of new compounds
(PDF)
AUTHOR INFORMATION
Corresponding Authors
*naisida@sbchem.kyoto-u.ac.jp
*murakami@sbchem.kyoto-u.ac.jp
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
（4） For catalytic reactions: (a) Lejkowski, M. L.; Lindner, R.;
́
Kageyama, T.; Bodizs, G. É.; Plessow, P. N.; Müller, I. B.;
This work was supported by JSPS KAKENHI Grant Num-
bers 15H05756 (M.M.), 18H04648 (N.I.) (Hybrid Cataly-
sis), 19K15562 (Y.M.), JST ACT-C Grant Number
JPMJCR12Z9 (M.M.), Yazaki Memorial Foundation for
Science and Technology (N.I.), and Naohiko Fukuoka Me-
morial Foundation (N.I.).
Schäfer, A.; Rominger, F.; Hofmann, P.; Futter, C.; Schunk,
S. A.; Limbach, M. The First Catalytic Synthesis of an Acry-
late from CO2 and an Alkene -- A Rational Approach. Chem.
Eur. J. 2012, 18, 14017−14025. (b) Hendriksen, C.; Pidko, E.
A.; Yang, G.; Schäffner, B.; Vogt, D. Catalytic Formation of
Acrylate from Carbon Dioxide and Ethene. Chem. Eur. J.
2014, 20, 12037−12040. (c) Manzini, S.; Huguet, N.; Trapp,
O.; Schaub, T. Palladium- and Nickel-Catalyzed Synthesis
of Sodium Acrylate from Ethylene, CO2, and Phenolate Ba-
ses: Optimization of the Catalytic System for a Potential
Process. Eur. J. Org. Chem. 2015, 7122−7130. (d) Hopkins,
M. N.; Shimmei, K.; Uttley, K. B.; Bernskoetter, W. H. Syn-
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Journal of the American Chemical Society
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Y.; Yu, D.-G. Catalytic Lactonization of Unactivated Aryl
C–H Bonds with CO2: Experimental and Computational In-
vestigation. Org. Lett. 2018, 20, 3776-3779. (b) Shigeno, M.;
Hanasaka, K.; Sasaki, K.; Nozawa-Kumada, K.; Kondo, Y.
Direct Carboxylation of Electron-Rich Heteroarenes Promo-
ted by LiOt-Bu with CsF and [18]Crown-6. Chem. Eur. J.
2019, 25, 3235-3239.
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（12） Ishida, N.; Kamae, Y.; Murakami, M. Preparation of
Ni(cod)2 Using Light as the Source of Energy. Organometal-
lics 2019, 38, 1413-1416.
（7） Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Modern Molec-
ular Photochemistry of Organic Molecules; University Sci-
ence Books: Sausalito, CA, 2010.
9
（13） 1H NMR analysis of the crude reaction mixture showed that
25% of 26 remained intact and a complex mixture of other
unidentified non-carboxylated products was produced.
（14） Nonhebel, D. C. The Chemistry of Cyclopropylmethyl and
Related Radicals. Chem. Soc. Rev. 1993, 22, 347-359.
（15） The lower yield with cyclopentane can be ascribed to the
stabilities of the cycloalkylnickel(I) intermediate. The nick-
el(I) moiety with the ligand is sterically bulky. A cyclohex-
ylnickel(I) would accommodate the bulky nickel substituent
in its staggered conformation. A cyclopentylnickel(I) would
be less stable due to torsional strain. Acyclic pent-2-
ylnickel(I) and pent-3-ylnickel(I) would be less reactive than
cyclohexylnickel(I) because they are more sterically con-
gested owing to the lack of the cyclic constraint.
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（8） (a) Sammes, P. G. Photoenolization. Tetrahedron 1976, 32,
405-422. (b) Kamijo, S.; Hoshikawa, T.; Inoue, M. Photo-
chemically Induced Radical Transformation of C(sp3)–H
Bonds to C(sp3)–CN Bonds. Org. Lett. 2011, 13, 5928-5931.
(c) Xia, J.-B.; Zhu, C.; Chen, C. Visible Light-Promoted
Metal-Free C–H Activation Diaryl Ketone-Catalyzed Selec-
tive Benzylic Mono-and Difluorination. J. Am. Chem. Soc.
2013, 135, 17494-17500. (d) Masuda, Y.; Ishida, N.; Mura-
kami, M. Light-Driven Carboxylation of o-Alkylphenyl Ke-
tones with CO2. J. Am. Chem. Soc. 2015, 137, 14063-14066.
(e) Cuadros, S.; Dell’amico, L.; Melchiorre, P. Forging Fluo-
rine-Containing Quaternary Stereocenters by a Light-Driven
Organocatalytic Aldol Desymmetrization Process. Angew.
Chem. Int. Ed. 2017, 56, 11875-11879. (f) Shen, Y.; Gu, Y.;
Martin, R. sp3 C–H Arylation and Alkylation Enabled by the
Synergy of Triplet Excited Ketones and Nickel Catalysts, J.
Am. Chem. Soc. 2018, 140, 12200-12209. (g) Masuda, Y.;
Ishida, N.; Murakami, M. Light-Promoted Benzylic C–H
Acylation Using a Benzoyl Group as the Photo-Directing
Group. Chem. Asian J. 2019, 14, 403. (h) Zhang, L.; Si, X.;
Yang, Y.; Zimmer, M.; Witzel, S.; Sekine, K.; Rudolph, M.;
Hashmi, A. S. K. The Combination of Benzaldehyde and
Nickel-Catalyzed Photoredox C(sp3)–H Alkylation/Arylation.
Angew. Chem. Int. Ed. 2019, 58, 1823-1827. (i) Dewanji, A.;
Krach, P. E.; Rueping, M. The Dual Role of Benzophenone
in Visible-Light/Nickel Photoredox-Catalyzed C–H Aryla-
tions: Hydrogen-Atom Transfer and Energy Transfer. Angew.
Chem. Int. Ed. 2019, 58, 3566-3570.
（9） See SI for details.
（10） Selected examples: (a) Takimoto, M.; Mori, M. Novel Cata-
lytic CO2 Incorporation Reaction: Nickel-Catalyzed Regio-
and Stereoselective Ring-Closing Carboxylation of Bis-1,3-
dienes. J. Am. Chem. Soc. 2002, 124, 10008-10009. (b)
Yeung, C. S.; Dong, V. M. Beyond Aresta’s Complex: Ni-
and Pd-Catalyzed Organozinc Coupling to CO2. J. Am.
Chem. Soc. 2008, 130, 7826-7827. (c) Williams, C. M.;
Johnson, J. B.; Rovis, T. Nickel-Catalyzed Reductive Car-
boxylation of Styrenes Using CO2. J. Am. Chem. Soc. 2008,
130, 14936-14937. (d) Fujihara, T.; Nogi, K.; Xu, T.; Terao,
J.; Tsuji, Y. Nickel-Catalyzed Carboxylation of Aryl and Vi-
nyl Chlorides Employing Carbon Dioxide. J. Am. Chem. Soc.
2012, 134, 9106-9109. (e) León, T.; Correa, A.; Martin, R.
Ni-Catalyzed Direct Carboxylation of Benzyl Halides with
CO2. J. Am. Chem. Soc. 2013, 135, 1221-1224. (f) Meng, Q.-
Y.; Wang, S.; Huff, G. S.; König, B. Ligand-Controlled Re-
gioselective Hydrocarboxylation of Styrenes with CO2 by
Combining Visible Light and Nickel Catalysis. J. Am. Chem.
Soc. 2018, 140, 3198-3201. (g) Charboneau, D. J.; Brudvig,
G. W.; Hazari, N. ; Lant, H. M. C.; Saydjari, A. K. Devel-
opment of an Improved System for the Carboxylation of Ar-
yl Halides through Mechanistic Studies. ACS Catal. 2019, 9,
3228-3241. (h) Diccianni, J.; Hu, C. T.; Diao, T.
(Xantphos)Ni(I)-Alkyl Mediated Insertion of CO2. Angew.
Chem. Int. Ed. 2019, 58, 13865-13868.
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（11） Since there is an equilibrium between t-BuO- and t-BuOCO2
-
under a CO2 atmosphere, it might be t-BuOCO2 that depro-
tonates the neutral ketyl radical. For generation of t-
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UV light
Ketone cat.
Ni cat.
CO2
(1 atm)
C(sp3)
C(sp3)
CO2H
+
H
t-BuOK
benzylic
and
aliphatic
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