Carbon dioxide as a C1 building block for the formation of carboxylic acids by formal catalytic hydrocarboxylation

Article abstract of DOI:10.1002/anie.201304529

A happy marriage of two processes: An effective catalytic system was identified for the direct synthesis of carboxylic acids from non-activated olefins or alcohols, CO₂, and H₂. Detailed analysis together with labeling studies indicated that the overall hydrocarboxylation of simple olefins results from a combination of the reverse water-gas shift (rWGS) reaction and a hydroxycarbonylation step, each promoted by a rhodium catalyst (see scheme). Copyright

Full text of DOI:10.1002/anie.201304529

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Chemie
DOI: 10.1002/anie.201304529
CO2 Utilization
Carbon Dioxide as a C1 Building Block for the Formation of Carboxylic
Acids by Formal Catalytic Hydrocarboxylation**
Thomas G. Ostapowicz, Marc Schmitz, Monika Krystof, Jꢀrgen Klankermayer, and
Walter Leitner*
Carbon dioxide is an attractive carbon resource for exploi-
tation in chemical transformations.[1] In particular, its use as
C1 building block in carbon–carbon bond-forming reactions
would open new routes for the direct synthesis of carboxylic
acids and their derivatives. However, the chemical inertness
caused by the thermodynamic and kinetic stability of the CO2
molecule pose major challenges for selective carbon dioxide
transformation. Whereas there are well-established synthetic
protocols for stoichiometric reactions with Grignard and
other organometallic reagents, the direct catalytic synthesis of
carboxylic acids from CO2 and readily available substrates
remains currently largely elusive.
from ethene and CO2 was reported to occur under very drastic
conditions (p = 700 bar, T= 1808C) in the presence of [RhCl-
(PPh3)3] and HBr,[14] but details of this transformation remain
unknown.
Herein we disclose a catalytic protocol for the synthesis of
free carboxylic acids directly from CO2, H2, and simple olefins
(Scheme 1). This transformation—formally referred to as
Arene carboxylic acids can be prepared catalytically via
Scheme 1. Catalytic hydrocarboxylation of olefins with CO2.
[2]
their salts from substrates comprising C Zn, C B,[3] or even
À
À
À
[4]
À
C Br bonds. Arene C H bonds were reported to be
carboxylated by the use of substoichiometric amounts of
hydrocarboxylation[15]—is thermodynamically feasible and
provides an attractive direct route to free carboxylic acids
through the use of CO2 as a C1 building block.
Al2Cl6/Al.[5] Most recently, the catalytic carboxylation of C
À
H-acidic molecules was described for heterocycles,[6] poly-
halogenated arenes,[6] and terminal alkynes.[7] The synthesis of
acrylic acid from ethene and CO2 in a stepwise manner was
described;[8] this approach enabled repeated use of the Ni
reagent corresponding to approximately 10 turnovers.[9] Fur-
thermore, some examples were reported in which coupling
reactions between CO2 and alkenes,[10] dienes,[11] allenes,[12] or
alkynes[13] in the presence of Pd, Ni, or Fe catalysts and
superstoichiometric amounts of organometallic reducing
agents, such as ZnR2, AlR3, Grignard reagents, and silanes,
gave carboxylic acids after aqueous work up. Most notably, as
early as 1978, the formation of propionic acid (38% yield)
Our study was inspired by a transformation described by
Simonato et al., who used formic acid as a hydrocarboxylation
reagent.[16] As HCOOH is well-known to be accessible by the
catalytic hydrogenation of carbon dioxide,[17] we envisaged
the possibility of directly converting H2, CO2, and olefins into
carboxylic acids. Cyclohexene (1) was used as the substrate
for an initial screening of various metal complexes for the
production of cyclohexanecarboxylic acid (2) or the undesired
hydrogenation product cyclohexane (3) as two possible
products. From a range of metal precursors and complexes,
a combination of [{RhCl(CO)2}2] and PPh3 proved to be the
most efficient catalyst system for the formation of 2 when
CH3I was added as a promoter. Hydrocarboxylation occurred
selectively to give 2 in 69% yield at 96% conversion[18] along
with the hydrogenation product 3 in 10% yield and a trace
amount of cyclohexyl iodide (4) as a by-product (Table 1,
entry 2).
[*] Dr. T. G. Ostapowicz, M. Schmitz, M. Krystof,
Prof. Dr. J. Klankermayer, Prof. Dr. W. Leitner
Institut fꢀr Technische und Makromolekulare Chemie
RWTH Aachen University
Worringerweg 1, 52074 Aachen (Germany)
E-mail: leitner@itmc.rwth-aachen.de
Significant further enhancement of the transformation
towards the desired carboxylic acid product was observed
when acidic additives were added to the reaction mixture. The
most pronounced effect was observed with p-TsOH·H2O, in
which case an additive/Rh ratio of 4:1 led to an 88% yield of
2, as determined by GC analysis. After aqueous workup, 2 was
isolated in 86% yield as a yellowish oil with higher than 97%
purity. Further purification by crystallization from pentane
gave colorless crystals (see the Supporting Information for
details). Optimization of the amount of the additive led to
a remarkable maximum yield of 2 of 92% at a 7:1 ratio of p-
TsOH·H2O to rhodium (Table 1, entry 8). The reaction could
be carried out even in neat cyclohexene to avoid the use of
acetic acid as a solvent, although under these conditions the
Prof. Dr. W. Leitner
Max-Planck-Institut fꢀr Kohlenforschung
45470 Mꢀlheim an der Ruhr (Germany)
[**] This research was supported by a Kekulꢁ grant (FCI) to T.G.O and
the DFG (IRTG 1628 “SeleCa”). Additional support was received
from the Cluster of Excellence “Tailor-Made Fuels from Biomass”
(TMFB), which is funded by the Excellence Initiative of the German
federal and state governments to promote science and research at
German universities. We thank C. Merkens for collecting and Dr. M.
Hçlscher for solving the X-ray crystallographic data of [Ph3MePI]
[Rh(CO)(PPh3)I3] as well as C. Poßberg for HRMS measurements.
Fruitful discussion with Prof. Dr. Willi Keim is gratefully acknowl-
edged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201304529.
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Table 1: Influence of different promotor compounds and acidic additives
Table 2: Investigation of the scope of the hydrocarboxylation with CO2
on the hydrocarboxylation of cyclohexene with CO2 and H2.[a]
and H2.[a]
Entry
Substrate
Product
Yield [%][b]
88 (86)
Conv. [%][b]
1
2
98
95
91 (81)
Entry
Promotor
Acidic additive[b]
Conv.[c]
[%]
Yield of
Yield of
2 [%][c]
3 [%][c]
exo: 50
endo: 12
3[c]
93
1
2
3
4
5
6
7
8
9
–
–
–
20
96
97
85
96
99
99
99
87
90
98
95
<1
69
77
41
65
75
88
92
47
46
73
71
5
10
6
21
8
4
2
5
9
CH3I
CH3I
CH3I
CH3I
CH3I
CH3I
CH3I
I2
HBTA
TFA
MSA
p-TsOH
p-TsOH·H2O
p-TsOH·H2O[d]
p-TsOH·H2O
p-TsOH·H2O
p-TsOH·H2O
p-TsOH·H2O
4
5
6
73[d]
98
97
99
74[e]
10
11
12
LiI
15
<1
11
77[f] (75)
4
4 + LiI[e]
[a] Reactions were carried out with 1 (1.88 mmol), [{RhCl(CO)2}2]
(46 mmol), the promotor (925 mmol), Ph3P (460 mmol), and the acidic
additive (330 mmol) in acetic acid (0.65 mL) in a 10 mL autoclave (see
the Supporting Information for details). [b] HBTA: bis(trifluorometha-
ne)sulfonimide; TFA: trifluoroacetic acid; MSA: methanesulfonic acid;
p-TsOH: para-toluenesulfonic acid. [c] Yields and conversion were
determined by GC analysis. [d] The reaction was carried out with
TsOH·H2O (650 mmol). [e] The reaction was carried out with a 4/LiI ratio
of 2:8.
7
8
74[g]
54[h]
99
>99
9
10
11
90[i] (76)
78[j]
93
97
93
78[k]
yield of 2 decreased to 59%. Notably, the optimized
conditions rule out the formation of free formic acid as the
actual reactant owing to the unfavorable thermodynamics of
the formation of HCOOH from H2 and CO2 under acidic
conditions.
In attempts to replace methyl iodide as the promotor,
lithium iodide and I2 also promoted reasonable conversion
into 2 with yields around 47%, but their use resulted in
a higher degree of hydrogenation to 3 (Table 1, entries 9 and
10). The application of other iodide salts, such as NaI and KI,
proved inefficient and caused a drop in the yield of 2 below
10%.
Variation of the steric and electronic parameters of the
phopshine showed that monodentate phosphines were gen-
erally suitable as ligands, whereas bi- and tridenate ligands led
to complete suppression of the formation of 2. A maximum
yield of 2 was observed with a P/Rh ratio in the range of 5:1 to
8:1 with PPh3, with rapid decay to values below 5% above and
below these limits (see the Supporting Information for
details).
Having established a standard set of reaction conditions,
we surveyed the scope of the reaction (Table 2). With the
substrate cyclopentene, a very good yield of 91% was
observed for the desired hydrocarboxylation product
(Table 2, entry 2; pentanecarboxylic acid was isolated in
82% yield). Norbornene was hydrocarboxylated in 62% yield
with high exo selectivity (Table 2, entry 3). In the case of
cyclohexenes with methyl substituents at different positions,
isomeric mixtures of the carboxylation products were
obtained in high yields, whereby the distribution of product
12
13
74 (73)
74[l]
>99
99
14
15
62[m] (55)
64[n]
>99
>99
[a] Reactions were carried out with the substrate (1.88 mmol),
[{RhCl(CO)2}2] (46 mmol), Ph3P (460 mmol), p-TsOH·H2O (330 mmol),
and CH3I (925 mmol) in acetic acid (0.65 mL) in a 10 mL autoclave.
[b] Yields and conversion were determined by GC analysis. Yields in
parentheses are for the isolated product. Workup procedures are
provided in the Supporting Information. [c] The exo/endo selectivity was
1
determined on the basis of H NMR spectroscopy. [d] 1-CO2H: 44%,
2/3-CO2H: 20%, 5-CO2H: 9%. [e] 1-CO2H: 46%, 2/3-CO2H: 20%, 5-
CO2H: 8%. [f] 1-CO2H: 52%, 2/3-CO2H: 18%, 5-CO2H: 7%. [g] 1-CO2H:
42%, 2-CO2H: 22%, 3-CO2H: 10%. [h] 1-CO2H: 31%, 2-CO2H: 17%, 3-
CO2H: 6%. [i] 1-CO2H: 53%, 2-CO2H: 26%, 3-CO2H: 11%. [j] 1-CO2H:
43%, 2-CO2H: 24%, 3-CO2H: 11%. [k] 1-CO2H: 45%, 2-CO2H: 23%, 3-
CO2H: 10%. [l] 1-CO2H: 48%, 2/3-CO2H: 19%, 5-CO2H: 7%. [m] 1-
CO2H: 41%, 2-CO2H: 15%, 3-CO2H: 6%. [n] 1-CO2H: 37%, 2-CO2H:
19%, 3-CO2H: 8%.
isomers was largely independent of the original substitution
pattern (Table 2, entries 4–6). Similar observations were
made for the hydrocarboxylation of linear n-alkenes bearing
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the double bond at the 1-, 2-, or 3-position (Table 2, entries 7–
11). These product distributions are reminiscent of hydro-
formylation under isomerizing conditions[19] and indicate
insertion into an rhodium–alkyl intermediate as a plausible
could be increased to 71% by the addition of LiI (Table 1,
entry 12).
Insight into the reaction mechanism was obtained from
isotope-labeling studies (Scheme 2). The use of D2 instead of
hydrogen resulted in deuterium incorporation with a large
À
step in the formation of the C C bond.
The conversion/time profile for the conversion of cyclo-
hexene (1) under standard conditions is depicted in Figure 1.
Full conversion of 1 was observed after 16 h, at which time the
Scheme 2. Isotopic labeling pattern observed by NMR spectroscopy
and MS for the rhodium-catalyzed reaction of cyclohexene (1) with
CO2 and H2 under the standard conditions (see Table 2). H218O/D2O
(16.7 mmol) was added in addition to the standard reagents in the
experiments with labeled water (see the Supporting Information for
details).
Figure 1. Conversion and yield of the major intermediates and prod-
ucts with respect to the reaction time for the hydrocarboxylation of
degree of scrambling around the cyclohexyl ring. Similar
isotopic patterns were observed when D2O was added to the
reaction mixture. The use of a 13CO2-enriched gas mixture
proved unambiguously that CO2 was the source of carbon in
the carboxylic acid functionality by a corresponding increase
in the 13C NMR signal. Most significantly, however, incorpo-
ration of 18O in the carboxylic acid moiety was observed by
mass spectrometry when the reaction was carried out in the
presence of H218O. No 18O was incorporated in the carboxylic
acid group when 2 was exposed to H218O under the reaction
conditions in control experiments. This result clearly indicates
that the carboxylic acid group is not constructed by the
incorporation of an intact CO2 molecule, but rather suggests
a hydroxycarbonylation pathway involving carbon monoxide
(CO) and water as reactive species.
The formation of CO under the reaction conditions by
a reverse water–gas shift reaction (rWGSR) was corroborated
by GC analysis of the gas phase with [{RhCl(CO)2}2] or RhI3
as the catalyst precursor in the absence of cyclohexene (see
the Supporting Information for details). Control experiments
in which CO (5 bar) and H2 gas (10 bar) were used under the
standard conditions with p-TsOH·H2O as a water source gave
2 in 79% yield. An increase in the partial pressure of CO
above 30 bar, however, resulted in a sharp decrease in the
yield of 2, which indicated catalyst poisoning.
*
cyclohexene (1) under standard conditions (see Table 1, entry 7): 2,
~
!
4, 5.
main product cyclohexanecarboxylic acid (2) was present in
84% yield. However, the formation of 2 started only after an
induction period of about 1 h. During this period, the rapid
formation of cyclohexyl iodide (4) was observed. The yield of
4 reached a maximum of about 16% before this compound
was consumed, and cyclohexyl acetate (5) and 2 were formed.
At the highest rate of formation of the product 2, the amount
of 5 present in the solution reached a maximum of about 3%.
Cyclohexane (3) is only a minor by-product, the amount of
which never exceeded 5% during the course of the reaction.
The observed conversion/time profile is most consistent
with the initial formation of 4 from cyclohexene (1) in the
presence of the iodide source. However, 4 does not appear to
be the most reactive intermediate, but is converted by
reaction with the solvent acetic acid into the acetate 5,
which is readily consumed in the catalytic transformation.
Indeed, the use of 5 as the substrate also gave 2 in 71% yield
at 95% conversion under the standard conditions. Conse-
quently, alcohols can also be used efficiently as substrates: the
conversion and product distribution were almost identical to
those observed with comparably substituted alkenes (Table 2,
entries 12–15). This extension to alcohol substrates widens the
potential synthetic scope of the transformations significantly.
Intrigued by the observation of initially formed cyclohexyl
iodide (4) in the conversion/time profile, we applied this
compound instead of CH3I as a promotor for the catalytic
system. With a Rh/4 ratio of 1:10, full conversion of cyclo-
hexene (1) was reached to give 2 in 73% yield if both 1 and 4
are considered as the alkyl source (Table 1, entry 11). An Rh/
4 ratio of 1:2 still resulted in a 54% yield of 2, and this yield
The importance of the coordination ability of CO, PPh3,
and IÀ at rhodium as the catalytically active center is
illustrated by the isolation of an unprecedented monomeric
anionic RhIII complex containing all three components in its
ligand sphere (Figure 2). When left to stand in air after
complete conversion of the substrate, the red reaction
solution turned dark brown within minutes, and brown
crystals suitable for X-ray crystallographic analysis could be
obtained by slow evaporation of the solvent. The molecular
structure revealed a slightly distorted octahedral arrangement
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agreement with this hypothesis, almost no production of 2 was
observed when PPh3 was omitted from the optimized reaction
conditions. Similarly, the positive effect of IÀ and H+ can be
rationalized qualitatively by the need to balance the stabili-
zation of the intermediates in a dynamic equilibrium between
the two cycles.
In summary, an effective catalytic system was identified
for the direct formation of carboxylic acids from non-
activated olefins, CO2, and H2 with high conversion and
selectivity under optimized reaction conditions. Internal and
terminal olefins were converted with regioselectivities similar
to those observed in hydroformylation. Other substrates that
are able to form rhodium–alkyl intermediates under the
reaction conditions may also undergo this transformation, as
shown in particular for primary and secondary alcohols.
Mechanistic studies suggest that the formal hydrocarboxyla-
Figure 2. Molecular structure of [CH3PPh3][RhI4(CO)(PPh3)] as deter-
mined by single-crystal X-ray diffraction. Hydrogen substituents are
omitted for clarity. Thermal ellipsoids are at the 50% probability level.
See the Supporting Information for structural parameters.
at the RhIII center with four iodide ligands spanning two
triangular faces and the PPh3 and CO ligands arranged cis to
one another at the remaining coordination sites. The phos-
phonium cation [CH3PPh3]+, which acts as a counterion, is
most likely formed by the reaction of PPh3 with CH3I.
In agreement with all experimental findings, a combina-
tion results from a combination of rhodium-catalyzed
rWGSR and hydroxycarbonylation cycles. Thus, the present
findings provide a new approach for the catalytic synthesis of
carboxylic acids by using CO2 as a C1 building block.
tion of two rhodium catalytic cycles is proposed to facilitate Experimental Section
General procedure: Under an argon atmosphere, [{RhCl(CO)2}2]
the synthesis of carboxylic acids from olefins or alcohols, CO2,
and H2 in the present system (Scheme 3). In one cycle, the
(46.0 mmol), an olefin (1.88 mmol), and CH3I (925 mmol) were placed
in a Schlenk tube along with acetic acid (0.65 mL). The red-brown
solution was transferred with a cannula to a stainless-steel autoclave
containing PPh3 (460 mmol) and p-TsOH·H2O (330 mmol). The
autoclave was pressurized with CO2 (4.1 g), and then H2 (10 bar)
was added to give a total pressure of about 70 bar at room
temperature. The reaction mixture was stirred and heated to 1808C
in an aluminum cylinder. After 16 h, the autoclave was cooled to 08C
and then carefully vented. The resulting mixture was analyzed by gas
chromatography with 1-phenylethanol and n-dodecane as internal
standards. Product yields (Y) were found to be reproducible within
DY=Æ 2% in two independent reactions for selected experiments.
Received: May 26, 2013
Revised: July 29, 2013
Published online: September 3, 2013
Scheme 3. Proposed mechanism of the overall hydrocarboxylation as
coupled catalytic cycles of the WGS equilibrium and hydroxycarbonyla-
tion.
Keywords: carbon dioxide · carboxylic acids ·
homogeneous catalysis · hydrocarboxylation · rhodium
.
rhodium-catalyzed rWGSR sets up an equilibrium between
CO2/H2 and CO/H2O. In the second cycle, the substrates
undergo hydroxycarbonylation through the reversible forma-
tion of a rhodium–alkyl intermediate. A combination of
rWGSR and carbonylation was described earlier for ruthe-
nium catalysts under basic conditions but resulted in reduc-
tive formation of the C1-elongated alcohols.[20] The WGS
equilibrium is typically adjusted under basic conditions;
however, activity of Rh catalysts under acidic conditions has
been noted previously.[21] Although the exact details of the
two cycles remain the subject of further studies, these catalytic
cycles bear resemblance to well-established rhodium and
palladium catalysis in the Monsanto process,[22] hydroformy-
lation,[19] and other carbonylation[23] processes. Hence, the
rWGSR cycle would be expected to be favored by species
with a lower P/Rh ratio, whereas the carbonylation cycle
would benefit from PPh3 in the coordination sphere. In good
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