CO2 activation by electrogenerated divalent samarium for aryl halide carboxylation

Article abstract of DOI:10.1039/c9ob01752f

The reductive carboxylation of aryl halides has been investigated using a samarium electrode as a sacrificial anode to yield the corresponding benzoic acids, providing a smooth strategy for CO₂ activation. Carboxylation occurred after an efficient reduction of carbon dioxide mediated by an electrogenerated Sm(ii)-complex acting as a strong monoelectronic reductive reagent.

Full text of DOI:10.1039/c9ob01752f

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DOI: 10.1039/C9OB01752F
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CO2 Activation by Electrogenerated Divalent
Samarium for Aryl Halides Carboxylation
Sakna Bazzi,a Gaëtan Le Duc,b Emmanuelle Schulz,a Corinne Gosminib and Mohamed Mellah*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The reductive carboxylation of aryl halides has been investigated requirement to achieve it. Further optimization led to the
using a samarium electrode as a sacrificial anode to yield development of catalytic carboxylation conditions based on
corresponding benzoic acids, providing a smooth strategy for CO2 Pd,6 Ni,7 or Cu8 complexes. Yet, the major drawback of these
activation. Carboxylation occurred after an efficient reduction of transformations remains the required use of sub-
carbon dioxide mediated by an electrogenerated Sm(II)-complex stoichiometric quantities of a reducing additive like the
as a strong monoelectronic reductive reagent.
pyrophoric Et2Zn, Zn or Mn powder.5-8 In this paper, we wish to
propose an original strategy based on the formation of a [CO2]
radical anion. Only a few examples are known in the
literature.9 To the best of our knowledge, no methods
combining CO2 reduction and carboxylation of aryl halides
have been reported.
Electrochemistry represents a much more favorable and eco-
friendly option to activate CO2 under relatively mild and safe
conditions.10,11 Under electrochemical conditions, a large
variety of organic halides were investigated towards the
synthesis of the corresponding carboxylic acids.10 Interestingly,
only a few reports described the electrocarboxylation of
aromatic halides.12 The first electrocarboxylation of aryl halides
was realized under CO2 atmosphere using Al or Mg sacrificial
anodes which prevented the undesired oxidation of substrates
or products (condition A, Eq.(1) in Scheme 1).13 The use of
Pd,12b,14 Ni15 and Co13b complexes as catalysts allowed for a
lower applied potential, which consequently improved the
efficiency and selectivity of the reaction (condition B, Eq.(1) in
Scheme 1).
CO2 represents an interesting renewable carbon source due to
its abundance, readily availability, recyclability and non-
toxicity.1 Recently, valorization of CO2 as a C1 building block
received a considerable and ever-increasing attention by the
scientific community.2 CO2 fixation onto organic compounds to
produce the corresponding carboxylic acids is considered a
promising way to access these valuable compounds.3 Benzoic
acids are high-valued compounds due to their agrochemical
and widespread pharmaceutical applications.4 Therefore there
has been much interest in studying their synthesis from cheap
and commercially available aromatic chlorides. Furthermore,
aryl chlorides are known to be industrial hazards : the
infamous polychlorobiphenyls (PCBs)5; their transformation
with carbon dioxide will provide a win-win situation where
toxic waste is converted to high value compounds.
Recent developments in organometallic chemistry led to the
discovery of several CO2-based transformations.5 However, the
CO2 insertion in organic backbones remains a significant
challenge due to the high thermodynamic stability and kinetic
inertia of this gas. Thus, the use of strong organometallic
nucleophiles such as Grignard and organolithium reagents is a
Previous electrochemical approaches
Condition A
O
Mg or Al anode
e-
R
O
C
O
R
R
M
X
C
e-
OH
Eq. (1)
a. Sakna Bazzi, Dr. Gaëtan Le Duc, Dr. Emmanuelle Schulz, Dr. Mohamed Mellah
Institut de Chimie Moléculaire et des Matériaux d’Orsay (UMR 8182), Equipe
Catalyse Moléculaire
Condition B
Pd or Ni catalyst
Univ. Paris Sud, CNRS, Université Paris-Saclay
115, rue Georges Clémenceau, 91405 Orsay, Cedex, France
E-mail: mohamed.mellah@u-psud.fr.
This work
b. Dr. Corinne Gosmini, Dr. Gaëtan Le Duc
LCM
R
X
e-
Sm anode
O
C
O
X2SmO
R
CNRS, Ecole polytechnique, Institut Polytechnique de Paris
91228 Palaiseau, France.
C
O
OH
C
O
SmII
reductant
Eq. (2)
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Scheme 1. electrochemical fixation of CO2.
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Despite the vast amount of reports on this subject, both in
organometallic catalysis and electrochemistry, these methods
are often limited to a CO2 insertion pathway. Therefore, an
efficient and sustainable carboxylation reaction, mediated by
CO2 activation, would be a welcome development.
View Article Online
DOI: 10.1039/C9OB01752F
We sought to address this problem with divalent samarium
complexes that are widely known for their high reducing
power, capable of transforming selectively various functional
groups.16 Recently, our group developed a novel tool based on
using electrochemistry to generate in situ Sm(II) species acting
as reducing reagents, resulting directly from a samarium
“soluble anode”.17 This procedure was found to be highly
effective for many C-C bond formation reactions.18 During our
investigations, we also proved the effect of electrolytic
medium containing ammonium salts (nBu4NI and nBu4NPF6) on
the reactivity of the SmIIX2 reagent.19
Figure 1. Samarium anode oxidation: Sm electrode surface 20
As far as we know, the CO2 reduction was evaluated with mm2, scanning potential between -1.5 and 0.9 V vs SCE in DMF
Sm(II) complexes but only to provide Sm(III)-oxalate or Sm(III)- with nBu4NPF6 [0.1 M]. Scan rate: 100 mV.s-1. a) blue line:
carbonate as isolated species.20 DFT calculations also without additive; b) green line: in presence of chlorobenzene
supported the formation of a thermodynamically stable 0.025 M; c) Red line: after bubbling CO2 in the solution for 5
dinuclear Sm(III)-oxalate structure.21 The unique example min.
where the samarium mediated a CO2 reduction to form C-C
bonds was reported by Ogawa and coworkers via The electrochemical behavior of the electrogenerated Sm(II)
photoinduced reductive carboxylation of alkyl halides with CO2 species was then evaluated by cyclic voltammetry (Fig. 2).
using a SmI2/Sm mixed reduction system.22
Electrogenerated solution of SmCl2 exhibited a redox couple
Herein we report our results for the activation of CO2 by with a formal potential of -1.5 V/SCE in DMF (green). This
electrogenerated low-valent samarium species to access latter is in the typical potential window of the SmIII/SmII redox
various benzoic acids directly from their corresponding aryl couple. Upon the introduction of CO2 (red), the CV revealed a
bromides and chlorides under mild conditions (Eq. (2), Scheme total loss of the SmII to SmIII oxidation wave while the
1).
Electrolysis reactions were conducted in an undivided cell, change in the electrochemical behavior of samarium
equipped with two electrodes: a cylindrical rod of samarium as evidenced
chemical reaction between CO2 and SmII
reduction signal remained at -1.4 V/SCE. This remarkable
a
the anode with a stainless-steel grid as the cathode. The electrogenerated in solution, to form a new SmIII-[CO2].-
transformation was performed by adding 5 mmol of 4- species in the medium.
methoxybromobenzene 1a in DMF with nBu4NBF4 [0.01 M] as
a supporting electrolyte along with continuous bubbling of
CO2. The pressure was stabilized at 1 atm. After reaching a
quantity of charge equal to 2.2 F/mol (i. e. 110 % of the
theoretical amount), the reaction was stopped. The carboxylic
acid was extracted and identified by 1H NMR. Gladly, the
expected 4-methoxy benzoic acid 2a was isolated with 71 %
yield without homocoupling product traces.
To clarify the interaction between the electrogenerated SmII(II)
species and carbon dioxide, we initiated our studies by running
electrochemical measurements. First, the effect of CO2
addition on the oxidation of the Sm anode was evaluated by
use of an electrode of 20 mm2 surface (Figure 1). In a solution
of 0.1 M of nBu4NPF6 in DMF, the Sm electrode oxidation
begins, as expected, at -0.5 V/SCE (blue curve). While the
addition of aryl chloride does not induce a drastic change (see
the green trace), a significant potential shift towards the
negative values from -0.5 V to -0.8 V was immediately
observed after CO2 bubbling was initiated (red curve). This
observation indicates that the CO2 facilitates the oxidation of
the anode and its solubilization by releasing samarium salts.
Figure 2. Cyclic voltammetry: scan rate =100 mV.s-1; nBu4NPF6
(0.1 M in MeCN) blank (blue); SmCl2 (green); SmCl2 + CO2 (red).
According to these findings, the interaction between
electrogenerated divalent samarium and CO2 proves the
unique reductive character of this species compared to other
2 | J. Name., 2012, 00, 1-3
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electrogenerated reagents (e.g., Mg2+, Al3+, Li+) that remain the absence of reaction suggests that the carboxVyileiwc AmrtiocleieOtnylinies
inert towards this CO2 activation.
an inhibitor of the reaction. When the DbOroI:m10i.n10e39a/tCo9mOBs0w17e5r2eF
placed in ortho-position to each other (see 1m), the targeted
The scope of this new reductive electrochemical carboxylation product 2m was only isolated in low yield, and benzoic acid
was thus studied with various aryl halides. Electrolysis was was obtained as main product (43 % NMR yield), meaning
performed efficiently at a fixed potential of -0.5 V/ SCE but a debromination can prove to be a non-negligible side reaction
galvanostatic mode was selected and a current value of 100 in this case.
mA was applied between both electrodes. The results are The reaction also proved successful with readily available aryl
gathered in Schemes 2 and 3.
chloride derivatives, albeit targeted products 2a-2h were
Aryl bromide derivatives functionalized with donating groups isolated in slightly lower yield, compared to their brominated
in ortho-, meta- or para-positions were isolated in high yields, analogs (see Scheme 3). Use of dichlorinated halides 5p, 5q
up to 80 % (see 2a to 2e). Benzoic acid was also smoothly and 5r remarkably led to the preparation of functionalized
prepared from phenyl bromide (2f). Apparently, electronic benzoic acids in higher yields than the corresponding
effects did not dramatically impact this reaction since 2g, 2h, bromine–containing ones, specifically for the ortho-
and 2i bearing electron-withdrawing groups in the para- disubstituted compound 2q, indicating the low dehalogenation
position were also prepared in satisfying yields under the same rate in this case. Starting from 1-bromo-2- chlorobenzene,
conditions.
benzoic acid 2f was isolated with 41% yield, and compound 2q
Very different behavior was nevertheless observed for 4- could not be observed by NMR spectroscopy.
bromophenol 1j. The phenolic derivative inhibited completely This reactivity is comparable to the mixture of products
the reaction and the starting material was totally recovered. observed from 1,2-dibromobenzene (1m) with two halides in
This absence of reaction is attributed to coordination between ortho-position reacting at approximately the same rate (see
the hydroxy group and the samarium species. Also, the Scheme 2).
presence of a nitro group in para-position (see 1k) was not Considering all the tested dihalogenated compounds,
compatible with the reaction conditions, resulting in the monocarboxylation was generally observed as the main
production of a complex product mixture. In this case, and pathway, and only in the case of 1,4-dibromobenzene (1l), 20%
according to our previous report for the chemoselective terephthalic acid could be isolated. The first carboxylation thus
reduction of nitroarenes,18b we assume an initial reduction of clearly deactivated the second halogenated position and
the nitro group, leading to potential numerous radical inhibited any further insertion of CO2. Nevertheless,
couplings.
dibrominated aryl substrates are substantially more prone to
dehalogenation than dichlorinated ones.
anode : Sm
100 mA,
cathode: stainless steel
O
Br
OH
anode : Sm
O
O
O
100 mA,
Cl
R
R
cathode: stainless steel
OH
(1 atm)
DMF, nBu4NBF4
2a to 2n
(isolated yield)
1a to 1n
O
O
R.T., 4h
R
R
(1 atm)
DMF, nBu4NBF4
R.T., 4h
2a 2r
to
5a 5r
to
O
O
O
O
O
(isolated yield)
OH
OMe
(69 %)
OH
OH
OH
Me
OH
O
MeO
Me
O
O
O
O
Me
OH
OH
OH
OH
OH
2c
2d
2e
2a (71 %)
2b
(80 %)
O
(71 %)
O
(78 %)
Me
F
MeO
F3C
O
O
O
2f
2g
2h
(56 %)
2a (63 %)
2c
(60 %)
O
(51 %)
(65 %)
OH
MeO2C
OH
OH
OH
HO
OH
O
O
O
F3
C
F
OH
OH
OH
2i
2f
(61 %)
O
2j
(-)
(76 %)
2g (69 %)
2h
(78 %)
Cl
Cl
2q (61 %)
O
O
O
Cl
2r
OH
OH
2p
(66 %)
(59 %)
OH
OH
OH
Scheme 3. Samarium-promoted carboxylation of aryl chlorides
5a – 5r.
S
Br
S
Br
O2N
2o (45 %)
2n (42 %)
2l
2k
(66 %)
(-)
2m
(8 %)
(with 43% 2f)
Scheme 2. Samarium promoted carboxylation of aryl bromides
1a - 1o.
More complex aryl substrates were also investigated for
testing the tolerance towards heteroaromatic or polycyclic
compounds. Under the same conditions, 3-bromothiophene
reacted easily delivering the corresponding carboxylic acid in
42 % yield (2n, Scheme 2). The double reactivity of 2,8-
dibromo-dibenzothiophene was also tested, but only
dibenzothiophene-2-carboxylic acid (2o, Scheme 2) could be
Dibromodisubstituted substrates were then investigated, and
4-bromobenzoic acid 2l was isolated in 66 % yield, together
with 20
% terephthalic acid (resulting from double
carboxylation) and traces of benzoic acid (which could result
from monocarboxylation and debromination). Furthermore,
attempts to transform isolated 2l into terephthalic acid failed:
isolated with
a good yield of 45 %, instead of 8-
bromodibenzothiophene-2-carboxylic acid. Heteroaromatic
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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compounds bearing nitrogen atoms (4-bromo-1H-indole or 3- samarium species was greatly enhanced in theVipewreAsrteicnlecOenlionef
bromopyridine) were also engaged in the carboxylation amino-based derivatives.25 Under our conditions, the
DOI: 10.1039/C9OB01752F
reaction, but they hampered the reactivity, even after ammonium cations are transformed to tertiary amines via
prolonged reaction time, probably due to the high azophilicity cathodic reduction;17 thus based on this argument, we assume
of samarium species.
that a very active low-valent species was formed that cannot
The transformation nevertheless performed well starting from be prepared by the addition of nBu4NPF6 to a chemically
benzylbromide (6a), whereas benzylchloride (6b) proved by far prepared solution of SmI2. In an attempt to prove CO2 radical
less reactive (respectively 72% and 11% isolated yields) formation, one equivalent of the nitroxyl radical (2,2,6,6-
following the strength of C-Cl and C-Br bonds (see Scheme 4).
tetramethylpiperidin-1-yl)oxy (TEMPO) was added to the
reaction mixture in the presence of 1a. Under these
conditions, 4-methoxybenzoic acid 2a could not be isolated
and the starting material 1a was totally recovered (path 2 in
Scheme 5). Noteworthy, a trapping product between the
TEMPO and the aryl halide was not detected under these
conditions. The TEMPO may interact with the CO2 radical
O
X
anode : Sm
OH
100 mA,
cathode: stainless steel
O
O
(1 atm)
DMF, nBu4NBF4
R.T., 4h
2s
6a
6b
X= Br,
X=Cl,
X= Br, 41%
X=Cl, 11%
Scheme 4. Samarium-promoted carboxylation of benzyl- anion, yielding a highly unstable, non isolable, intermediate
bromide and chloride.
that decomposes further. Moreover, an electrochemical
measurement was conducted targeting the Sm anode
The increased reactivity of benzyl bromides compared to oxidation in the presence of TEMPO and CO2. The obtained
benzyl chlorides suggests a different mechanism than that curve was similar to the red line (curve c) from Figure 1 (see
described for nickel catalysis.23
SI), indicating that the addition of TEMPO did not affect the
reduction of CO2. These control experiments confirm that the
electrogenerated Sm(II)X2 initially reduces the CO2 via a single
electron transfer (SET) to generate the active radical anion (I in
Scheme 5). This latter is sufficiently powerful to attack the aryl
halide by a radical substitution mechanism, finally producing,
after treatment, the corresponding carboxylic acid (see path
3).
O
From
sacrificial anode
H+
OSmX2
O
(path 1)
(path 2)
oxalic acid
C
C
Sm0
X2SmO
without Ar-X
- e-
SmIIX2
O
C
TEMPO
O
O
SmX2
no reaction
O
with Ar-X
(I)
X
SmX2
O
O
HO
O
R
C
R
H+
(path 3)
+
X
with Ar-X
R
Conclusions
(II)
In summary, a new procedure for the carboxylation of aryl and
benzyl halides is here described affording functionalized
carboxylic acids in good yields. It relies on an unprecedented
CO2-activation by an electrochemical process featuring the use
of a “soluble” samarium anode. Electrolysis generates a
powerful reductive species in situ which directly reduces
carbon dioxide. Work is now in progress to broaden this
process to the introduction of CO2 to a wider range of
functional substrates and to develop a catalytic samarium
version.
Scheme 5. CO2 activation using Sm(II)X2 : (path 1) without Ar-X
; (path 2) in the presence of TEMPO (3 equiv) ; (path 3) with
ArX.
To gain insight into the mechanistic pathway of this
transformation, we performed some control experiments (see
Scheme 5). In the absence of aryl halides, CO2 reduction was
effective, and oxalic acid was detected by 13C NMR as the
unique product (path 1). Indeed, several previous studies
concerning the reactions between Sm(II) salts and CO2
described the formation of a samarium oxalate complex.20, 21
Subsequently, the electrolysis was carried out without CO2. In
this case, the aryl chloride was completely recovered. These
results confirm that the starting material is completely
unreactive in our conditions thus reflecting a mechanism
initiated by the formation of the CO2 radical anion.
In contrast, a solution of SmI2 prepared in THF following the
original Kagan’s method,24 did not furnish oxalic acid, even
after 16h. Furthermore, after the addition of phenyl bromide,
only the homocoupling product was found in the medium.
These results thus evidenced that the in situ electrogeneration
of Sm(II)X2 allowed for the formation of a very active, low-
valent species supporting our previous report for the reduction
of aliphatic ketones17,18a and alkyl halides.19 Furthermore,
Procter’s group also proved that the reducing ability of
Conflicts of interest
There are no conflicts to declare.
Notes and references
‡ This work was financially supported by the CNRS, the Ministère
de l’Enseignement Supérieur et de la Recherche and the French
National Research Agency (Charmmmat ANR-11-LABX-0039
fellowship to G.L.D.).
1
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Organic & Biomolecular Chemistry
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DOI: 10.1039/C9OB01752F
COMMUNICATION
TOC
First combination of samarium and electrochemistry
towards the effective reduction of CO2 for the synthesis
of benzoic acids from aryl halides
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