Visible-Light-Driven Carboxylation of Aryl Halides by the Combined Use of Palladium and Photoredox Catalysts

Article abstract of DOI:10.1021/jacs.7b04838

A highly useful, visible-light-driven carboxylation of aryl bromides and chlorides with CO₂ was realized using a combination of Pd(OAc)₂ as a carboxylation catalyst and Ir(ppy)₂(dtbpy)(PF₆) as a photoredox catalyst. This carboxylation reaction proceeded in high yields under 1 atm of CO₂ with a variety of functionalized aryl bromides and chlorides without the necessity of using stoichiometric metallic reductants.

Full text of DOI:10.1021/jacs.7b04838

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Communication
Visible Light-Driven Carboxylation of Aryl Halides by the
Combined Use of Palladium and Photoredox Catalysts
Katsuya Shimomaki, Kei Murata, Ruben Martin, and Nobuharu Iwasawa
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 28 Jun 2017
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Visible Light-Driven Carboxylation of Aryl Halides by the Combined
Use of Palladium and Photoredox Catalysts
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Katsuya Shimomaki,† Kei Murata,†,# Rubén Martín‡,§ and Nobuharu Iwasawa†,*
† Department of Chemistry, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8551, Japan
‡ Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, Av. Països
Catalans 16, 43007 Tarragona, Spain
§ Catalan Institution for Research and Advanced Studies (ICREA), Passeig Lluïs Companys, 23, 08010, Barcelona,
Spain
Supporting Information Placeholder
Scheme 1. Transition metal-catalyzed carboxylation
of aryl halides
ABSTRACT: A highly useful, visible light-driven car-
boxylation of aryl bromides and chlorides with CO2 was
realized using a combination of Pd(OAc)2 as a carboxy-
lation catalyst and Ir(ppy)2(dtbpy)(PF6) as a photoredox
catalyst. This carboxylation reaction proceeded in high
yields under 1 atm of CO2 with a variety of functional-
ized aryl bromides and chlorides without the necessity of
using stoichiometric metallic reductants.
Previous report
cat. Pd, Ni, Cu
excess Metallic Reductant
(ZnEt2, Mn)
X
CO2H
CO2
R
R
This work
Pd/Photoredox
X = Cl, Br, I
Dual Catalyst System
Non-Metallic Reductant (amine)
As a method to utilize CO2 as a one-carbon source, the
carboxylation reaction of organic halides via the Gri-
gnard reagents is one of the most basic reactions in or-
ganic synthesis. More recently, the transition metal-
catalyzed carboxylation of organic halides such as aryl,
alkenyl and alkyl halides with CO2 has attracted consid-
erable attention and several reactions have been devel-
oped using Pd, Ni and Cu catalysts.1,2 However, these
catalysts under visible light irradiation in the presence of
an excess amount of iPr2NEt as an electron-donor under
a CO2 atmosphere at room temperature (Table 1, entry
1~3). Ru(bpy)3(PF6)2 4a, which was employed in the
Rh-catalyzed hydrocarboxylation,9 was found to give the
desired carboxylic acid 2a albeit in low yield, and the
hydrogenated product 3a was also produced as a by-
product (entry 1). The conversion of substrate 1a and the
yield of carboxylic acid 2a were improved with
Ir(ppy)2(dtbpy)(PF6) 4c having a higher reduction poten-
tial (entry 3). In this carboxylation reaction, palladium
catalyst, photoredox catalyst, amine, light irradiation and
CO2 gas were all necessary, and lack of one component
suppressed the reaction (Table S1). To further improve
the yield of the carboxylation product, it was necessary
to suppress the formation of the hydrogenated product
3a. As the ammonium salts of hydrogen bromide and
carboxylic acid were thought to be produced with the
progress of the reaction, addition of base was exam-
ined.10 And it was found that a better result was obtained
by carrying out the reaction in the presence of Cs2CO3 as
a base (entry 3 vs 4). The efficiency of the reaction was
further improved by using 2-diphenylphosphino-2',4',6'-
triisopropylbiphenyl (PhXphos) as a less electron-
donating ligand (entry 4~6), and the carboxylic acid 2a
reactions necessitate the use of an excess amount of me-
1a,1d,2e
tallic reductants such as ZnEt2
and Zn1c,2c or
Mn1b,e-i,2a-d,f,g powder, which is not desirable from the
standpoint of the development of environmentally-
benign processes (Scheme 1). Thus, it is highly desirable
to develop a useful carboxylation reaction of organic
halides, which does not necessitate the use of a stoichi-
ometric metallic reductant. In this paper, we report a
useful and general method for Pd-catalyzed carboxyla-
tion1a,g,2e,3,4 of aryl bromides and chlorides just by using
Pd/photoredox dual catalysts5,6,7 in the presence of an
amine as an electron donor instead of an excess amount
of metallic reductants under visible light irradiation
(Scheme 1).8,9
We first examined the carboxylation of 3,4-
(methylenedioxy)bromobenzene 1a using catalytic
amounts of Pd(OAc)2, 2-dicyclohexylphosphino-2',4',6'-
triisopropylbiphenyl (Xphos)1a and various photoredox
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Page 2 of 5
was produced in 88% yield with 2.5 mol% each of moderate yields and the hydrogenated products were
1
2
3
4
5
6
7
8
Pd(OAc)2 and Ir(ppy)2(dtbpy)(PF6) 4c (entry 7).
produced more than other bromobenzenes under the
conditions using PhXphos (Table S14). This result was
attributed to the direct reduction of bromobenzene de-
rivatives by a photoredox catalyst,12 and in these cases,
the desired carboxylic acids were obtained in good yield
by using tBuXphos as an electron-rich phosphine ligand
(Table S13). These results also showed that carbonyl
and cyano groups were compatible in this reaction. Thus,
by the appropriate choice of the Xphos type ligands, the
carboxylation of a variety of bromobenzenes proceeded
just by using catalytic amounts of the Pd catalyst and the
photoredox catalyst under atmospheric pressure of CO2
using amine as an electron donor. It is also noted that the
yields were improved compared to the reaction using
ZnEt2 in most cases probably because the side reaction
of arylpalladium bromide intermediate with Et2Zn was
avoided in this reaction.1a
Table 1. Screening of reaction conditions
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entry
Ligand
Catalyst Additive Conv.
/%a
Yield
/%a
2a 3a
1b
2b
3b
4
Xphos
Xphos
none
none
12
48
3
9
4a
4b
4c
4c
4c
4c
4c
12 25
48 45
58 28
65 13
75 10
Xphos
none
100
93
Xphos
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Table 2. Substrate scope of aryl bromides
5
tBuXphos
PhXphos
PhXphos
100
93
6
7c
100
88
6
a Determined by 1H-NMR spectra. b Using iPr2NEt (6.0
c
equiv.). Using Ir catalyst 4c (2.5 mol%).
PF6
tBu
R
N
R
(PF6)2
N
PR2
N
N
N
N
N
R'
R
iPr
iPr
Ru
N
Ir
N
N
tBu
iPr
: R = Cy
tBuXphos : = tBu
R
R'
Xphos
R = F, R' = CF3
Ir(dF(CF3)ppy)2(dtbpy)(PF6) 4b
R = R' = H
Ru(bpy)3(PF6)2 4a
PhXphos : = Ph
Ir(ppy)2(dtbpy)(PF6) 4c
Generality of the reaction was examined using various
functionalized aryl bromides (Table 2). A wide range of
substrates bearing alkyl 1c, alkoxy 1d, halide 1e~g, in-
ternal alkyne 1k and alkene 1l at 4-position gave the
corresponding methyl ester derivatives in good yield
after the methyl esterification with trimethylsilyldiazo-
methane (TMSCHN2).11
In particular, with 4-
chlorobromobenzene 1g as a substrate, 4-chlorobenzoate
5g was selectively obtained. Cobalt- and nickel-
catalyzed carboxylations of sterically hindered aryl tri-
flates was reported by the Tsuji’s group,2d but 2,6-
diisopropylphenyl triflate did not give the carboxylic
acid under their conditions. On the contrary, the carbox-
a
b
Reaction time is 4 hour. Using tBuXphos instead of
PhXphos. c Reaction time is 8 hour.
Next, we examined the reaction of aryl chlorides. The
carboxylation of aryl chlorides was reported using a Ni
catalyst,1b but the reaction using a Pd catalyst has not
been reported yet. Under the reaction conditions of aryl
ylation
of
equally
congested
2,4,6-
triisopropylbromobenzene 1n proceeded in 76% yield by
the combined use of Pd and photoredox catalysts proba-
bly because the electron-transfer step was not affected
by steric congestion so much. The bromides of electron-
rich heteroarenes such as thiophene 1o and indole 1p,q
also gave the methyl esters 5o~q. On the other hand, the
reaction of bromobenzenes containing ester 1i, ketone 1r
and nitrile 1j gave the carboxylic acids 2i,j,r in low to
bromides
using
PhXphos,
4-
trifluoromethylchlorobenzene 6e gave the corresponding
carboxylic acid 2e in 31% yield, and the unreacted 6e
was partially recovered. However, by using tBuXphos as
an electron-rich ligand, the carboxylic acid 2e was ob-
tained in 96% yield probably due to the acceleration of
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Journal of the American Chemical Society
the oxidative addition step (Table S15). Various aryl In our previous Rh/photoredox dual catalysis for hy-
1
2
3
4
5
6
7
8
chlorides gave the corresponding methyl esters in good
yield after the methyl esterification with TMSCHN2
(Table 3). In the Pd-catalyzed carboxylation reaction
using ZnEt2, the aryl chlorides did not give the carbox-
ylic acids.1a On the contrary, the present reaction pro-
ceeded with a wide variety of substrates, and the car-
boxylation of various functionalized aryl chlorides pro-
ceeded in good yield by using the photoredox catalyst
and iPr2NEt.
drocarboxylation reaction, the key Rh(I) hydride com-
plex was generated by two-electron and two-proton
transfers to Rh(I) carboxylate complex to give dihydride
Rh(III) carboxylate complex, which eliminated the car-
boxylic acid salt with the assistance of a base.9 On the
contrary, in the present system, it is likely that the direct
reduction of Pd(II) species to Pd(0) through successive
two-electron transfer proceeded by the photoredox ca-
talysis.
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Table 3. Substrate scope of aryl chlorides
Scheme 2. Proposed mechanism
path b
CO2
Ar
Ar
X
L
Pd(II)
CO2
X = Br, Cl
X
B
L
Pd(II) Ar
path a
CO2
X
E
e–
X–
OCOAr
2e–
Pd(OAc)2
+
LPd(0)
L
Pd(II)
X
Ligand (Xphos)
A
C
e–
ArCOO–
X–
CO2
L
Pd(I) Ar
e–
F
LPd(I) OCOAr
D
In conclusion, we have developed a novel direct car-
boxylation of aryl halides by the combined use of
Pd(OAc)2
as
a
carboxylation
catalyst
and
Ir(ppy)2(dtbpy)(PF6) 4c as a photoredox catalyst using
iPr2NEt as an easily availlable, non-metallic electron-
donor. This carboxylation of aryl bromides and chlorides
does not necessitate the use of a stoichiometric metallic
reductant, and proceeded in good yield with various
functionalized aryl bromides and chlorides by using ap-
propriate ligands, PhXphos or tBuXphos. In particular,
the transition metal-catalyzed carboxylation of a very
sterically hindered 2,4,6-triisopropylbromobenzene 1n
and the Pd-catalyzed carboxylation of aryl chlorides
were realized for the first time. Application of this car-
boxylation reaction to other transition metal catalysts
and expansion of the substrate scope such as alkenyl and
alkyl halides are now in progress.
Concerning the mechanism of this reaction, the follow-
ing results have been obtained. As the resting state of
this reaction, PhPdBr(Xphos) was observed by a 31P-
NMR spectrum under the catalytic reaction conditions
(Figure S1).13 It was also found that the first reduction
potential of the isolated PhPdBr(Xphos) was -2.28 V (vs
Fc/Fc+), which was much lower than the reduction po-
tential of the reductant Ir(II) (Figure S4).14,15 Interesting-
ly, CV measurement of this complex under CO2 atmos-
phere showed a new peak at about -1.4 V.16 From these
data, two major possibilities are thought to exist for the
actual palladium species reduced during the reaction
(Scheme 2). The first one is (ArCOO)PdX(Xphos) C
generated in a small amount in equilibrium with
ArPdX(Xphos) B under CO2 (path a).13 Photoredox-
catalyzed one-electron reduction of C would give
(ArCOO)Pd(I)(Xphos) D and X–, and the generated D
would undergo further one-electron reduction to give
Ar(0)(Xphos) and ArCOO–. In this case, the reduction of
the carboxylate complex C would shift the equilibrium
and the carboxylation reaction would proceed catalyti-
cally.17 The other possibility of the reducible species is a
CO2-coordinated ArPd(II)X(Xphos) E, again generated
in a small amount, and this species would be able to un-
dergo one-electron reduction to give a CO2-coordinated
ArPd(I)(Xphos) species F, which would have higher
ability to undergo addition to CO2 to give
(ArCOO)Pd(I)(Xphos) D (path b). This would then un-
dergo the similar process as described above.18
ASSOCIATED CONTENT
Supporting Information. Detailed experimental proce-
dures, spectral data, analytical data. This material is availa-
ble free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
niwasawa@chem.titech.ac.jp
Present Address
#
Institute of Industrial Science, the University of Tokyo, Ko-
maba, Meguro-ku, Tokyo 153-8505, Japan
ACKNOWLEDGMENT
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CO2, see: (a) Tazuke, S.; Kazama, S.; Kitamura, N J. Org. Chem.
1986, 51, 4548. (b) Ito, Y.; Uozu, Y.; Matsuura, T J. Chem. Soc.,
This research was supported by JST ACT-C Grant Number
JPMJCR12Y3 and JSPS KAKENHI Grant Number
24245019, 15H05800 and 17H06143.
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Chem. Commun. 1988, 562. (c) Tagaya, H.; Onuki, M.; Tomioka,
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(9) We recently reported the Rh-catalyzed hydrocarboxylation of
alkenes by using a combination of Rh and photoredox catalysts
with N,N-diisopropylethylamine as a sacrificial electron donor
instead of metallic reductants under visible light irradiation. See:
Murata, K.; Numasawa, N.; Shimomaki, K.; Takaya, J.; Iwasawa,
N. Chem. Commun. 2017, 53, 3098.
(10) Reduction of aryl halides could proceed by the photoredox
catalyst and an amine under photo-irradiation conditions in the
absence of Pd catalyst. However, there would also be a Pd-
catalyzed reduction process because the amount of the hydrogen-
ated product was increased under the same conditions in the pres-
ence of Pd(OAc)2 amd Xphos. For more discussions on the for-
mation of the reduction product, see SI.
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(13) No detectable formation of (PhCOO)PdBr(Xphos) was ob-
served when PhPdBr(Xphos) was subjected to a CO2 atmosphere,
and it is possible that there is an equilibrium between
PhPdBr(Xphos) and (PhCOO)PdBr(Xphos), the former of which
is thermodynamically more stable. The decarboxylation of palla-
dium carboxylates is a well-known process, although usually
higher temperature is required. See: (a) Myers, A. G.; Tanaka, D.;
Mannion, M. R. J. Am. Chem. Soc. 2002, 124, 11250. (b) Tanaka,
D.; Romeril, S. P.; Myers, A. G. J. Am. Chem. Soc. 2005, 127,
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Lett. 2010, 12, 1000. See SI for more detailed discussions on the
mechanism of this reaction.
(14) The reduction potential of [PhPd(Xphos)]+ was also lower
than the Ir catalyst.
(15) For examples of reductive quenching cycle of
[Ir(ppy)2(dtbpy)]+, see: (a) Condie, A. G.; Gonzalez-Gomez, J. C.;
Stephenson, C. R. J. J. Am. Chem. Soc. 2010, 132, 1464. (b)
Tucker, J. W.; Nguyen, J. D.; Narayanam, J. M. R.; Krabbe, S.
W.; Stephenson, C. R. J. Chem. Commun. 2010, 46, 4985.
(c)Larraufie, M. H.; Pellet, R.; Fensterbank, L.; Goddard, J. P.;
Lacote, E.; Malacria, M.; Ollivier, C. Angew. Chem. Int. Ed. 2011,
50, 4463. (d) Miyake, Y.; Nakajima, K.; Nishibayashi, Y. J. Am.
Chem. Soc. 2012, 134, 3338.
(16) This new peak was rather weak, but repeated experiments
confirmed its presence. See SI for details.
(17) The counter cations of the halide and carboxylate anions are
both protons generated from the oxidized amine, and these two
acids are neutralized with the excess Cs2CO3 and/or amine.
(18) Prior additional one-electron reduction of CO2-coordinated
ArPd(I)(Xphos) to [ArPd(0)(Xphos)]– to work as the actual car-
boxylation species is also conceivable.
(7) For examples of the reaction using Pd/photoredox dual cata-
lysts except the reduction or oxidation of Pd, see: (a) Osawa, M.;
Nagai, H.; Akita, M. Dalton Trans. 2007, 827. (b) Zhang, H.;
Huang, X. Adv. Synth. Catal. 2016, 358, 3736.
(8) For examples of UV light-driven carboxylation reactions with
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Pd/Ir Dual Catalyst System
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CO2
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• Without excess metallic reductant
• Wide substrate scope
• High yields
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