Acrylate formation from CO2 and ethylene: catalysis with palladium and mechanistic insight

Article abstract of DOI:10.1039/c5cc01932j

We report the first catalyst based on palladium for the reaction of CO₂, alkene and a base to form sodium acrylate and derivatives. A mechanism similar to a previously reported Ni(0)-catalyst is proposed based on stoichiometric in situ NMR experiments, isolated intermediates and a parent palladalactone. Our palladium catalyst was applied to the coupling of CO₂ with conjugated alkenes.

Full text of DOI:10.1039/c5cc01932j

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Acrylate formation from CO2 and ethylene: Catalysis
with palladium and mechanistic insight
Cite this: DOI: 10.1039/x0xx00000x
S. Chantal E. Stieber,a Núria Huguet,a Takeharu Kageyama,a Ivana Jevtovikj,a
Piyal Ariyananda,a Alvaro Gordillo,a,b Stephan A. Schunk,b Frank Rominger,c
Peter Hofmann,a,c and Michael Limbacha,d
*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
acid.11 A series of cationic palladium methylacrylate complexes
release methylacrylate upon addition of ethylene or propylene as
demonstrated by Aresta et al..12 Heating the corresponding neutral
compounds under CO2 and ethylene pressure resulted in ethyl- and
methylester formation with a molar ratio of 1.6 relative to Pd.
Despite this initial work, no improvements with regards to TON or
mechanistic insight have been reported.
We report the first catalyst based on palladium for the
reaction of CO2, ethylene and a base to form sodium acrylate
and derivatives. A mechanism similar to a previously
reported Ni(0)-catalyst is proposed based on stoichiometric in
situ NMR experiments, isolated intermediates and a parent
palladalactone. Our palladium catalyst was applied to the
coupling of CO2 with conjugated alkenes.
Our study commenced with a preliminary screening of a one-pot
procedure under ethylene and CO2 pressure, analogous to our
recently reported conditions for nickel (cf. Table 1).6 A variety of
palladium precursors were examined and (COD)PdCl2 and (η5-
Cp)Pd(η3-allyl) resulted in the highest catalytic activity. When
(COD)PdCl2 was used as a precursor, a reducing agent such as Zn(0)
was necessary to form a Pd(0) species to enter the catalytic cycle.
However, significant TON was demonstrated without metallic
The use of CO2 as a cheap and abundant C1 building block is of
high interest and a hot topic of current academic and industrial
research.1 An attractive reaction with industrial relevance is the
synthesis of sodium acrylate from CO2 and ethylene, which allows
for access to a different raw material base compared to the state of
the art propylene route.2 Initial work in the carboxylation of
unsaturated hydrocarbons with CO2 was done by Hoberg et al. in the
1980s. Several nickel complexes were identified to couple CO2 and
alkenes in a stoichiometric manner to form nickelalactones.3,4 While
the stoichiometric cleavage of those metallalactones has been
rationalized and achieved over the years,5 we recently reported the
first one-pot catalytic reaction of sodium acrylate from CO2,
ethylene, and a base using a nickel catalyst with TON > 100.6
Extensive screening of bases identified sodium 2-fluorophenolate as
the best base for the reported reaction conditions: The Brønsted
basicity is sufficiently high,7 and the nucleophilicity sufficiently low
to deprotonate the nickelalactone, but extensive reactivity with CO2
is avoided. This demonstrates the feasibility of the one-pot process,
but opens questions regarding the fate of the catalyst and ways to
improve turnover.
Since only nickel is known to be catalytically active in a one-pot
system for the formation of sodium- or lithium acrylate8 from CO2,
ethylene, and a base, there is considerable interest in expanding the
scope of metals used in catalysis. In the nickel system,
nickelalactones have been identified as key intermediates.9 This
suggests that metals which form metallalactones may be candidates
for the catalytic coupling of CO2 and alkenes. A few examples of
substituted palladalactones are known,10 but none has been generated
by the direct reaction of an alkene and CO2.
reductant
for
(η5-Cp)Pd(η3-allyl)
and
1,2-
bis(dicyclohexylphosphino)ethane (dcpe) as the most promising
ligand (TON ~19, cf. supporting information). Given the relative
stability and commercial availability of (COD)PdCl2, the reaction
conditions were optimized with this precursor and dcpe.
Table 1 Pd-catalyzed formation of sodium acrylate from CO2, ethylene and a
base under different conditions.
Entry (COD)PdCl2
[mmol]
dcpe
[mmol]
0.22
Base
Temp.
[°C]
100
TONa
1
2
3
4
5
0.20
0.20
0.20
0.20
0.10
2-F-PhONa
2-F-PhONa
2-F-PhONa
NaHMDS
9
0.22
120
27
24
4
0.22
145
0.22
145
0.11
2-F-PhONa
145
29
a TON reported as (mmol product)/(mmol [Pd]) after 20 h using sodium 3-
(trimethylsilyl)-2,2,3,3-d4-propionate as a internal standard. Reactions were
performed using Zn (10 mmol), base (20 mmol), ethylene (10 bar), CO2 (20
bar) and THF (30 mL), 20 h reaction time.
Yamamoto et al. formed substituted palladalactones upon
oxidative addition of substituted cyclic acid anhydrides to
(Ph3P)2Pd(styrene). Those lactones could be cleaved with CO or
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A TON of 9 for sodium acrylate was achieved by stirring 0.20 characterization (Figure 1). The Pd(1)-O(2) and Pd(1)-C(3) bond
mmol (COD)PdCl2, 0.22 mmol dcpe, 20 mmol sodium 2- lengths compare well with in Yamamoto’s methyl-substituted
fluorophenolate, 10 mmol Zn under 10 bar ethylene, and 20 bar CO2 palladalactone, (PPh3)2Pd(-CH2CHCH3CO2-) (2.059(4) and 2.068(5)
at 100 °C and 300 rpm for 20 h in an autoclave. Increasing the Å vs. 2.064(7) and 2.070(9) Å, respectively).10a
temperature to 145 °C resulted in a TON of 24. Decreasing the
catalyst loading to 0.10 resulted in a TON of 29 (see Table 1). The
use of sodium 2-fluoro-phenolate gave a significant higher TON as
compared to NaHMDS, similar to the reported nickel system.6
Demonstrating the feasibility of palladium in the catalytic
formation of sodium acrylate from CO2 and ethylene raised the
question whether similar species are involved in the mechanism as in
the
butylphosphino)ethane (dtbpe), the parent ethylene-complex,
nickelalactone, acrylic acid and acrylate- -complexes were isolated.
nickel
system:
For
nickel
and
1,2-bis(di-tert-
π
The reaction of dtbpe with (η5-Cp)Pd(η3-allyl) and ethylene
furnished (dtbpe)Pd(η2-CH2=CH2) (1), which was immediately
reacted with acrylic acid to form (dtbpe)Pd(η2-CH2CHCO2H) (2) in
76% yield (Scheme 1). The solid state infrared spectrum of this
compound shows a C=O stretch at 1634 cm-1, comparable to that
reported for (dtbpe)Ni(η2-CH2CHCO2H) at 1627 cm-1.9 While the
nickel analogue decomposed to CO2 and ethylene at 100 °C,
(dtbpe)Pd(η2-CH2CHCO2) turned out to be stable at 100 °C for at
least 17 h.
Figure
1
Solid state structures of (dtbpe)Pd(CH2CHCO2H)
) (right, one molecule
of THF omitted) at 50% probability. H atoms omitted for clarity.
(2) (left, second
molecule in unit cell omitted) and (dcpe)Pd(-CH2CH2CO2-) (
5
In situ prepared Pd(0)-complex 1 afforded under catalytic
conditions (10 bar CO2, 5 bar ethylene, 20 equiv. NaOPh, THF,
120 °C, 15 h) indeed sodium acrylate in slightly over-stoichiometric
amounts (TON 1.4).
C2H4 (30 bar),
CH2CHCO2H,
dtbpe, THF,
17 oC, 2 h
tBu2
tBu2
THF, 0 oC, 16 h
P
P
OH
Pd
P
P
(COD)PdCl2 + dcpe + Zn0
Pd
Pd
tBu2
tBu2
76%
O
1
2
PhONa (20.0 equiv),
C2H4 (5 bar), CO2 (10 bar),
THF, 120 °C, 15 h
ONa
O
Cy2
P
NaO
CO2
O
TON = 1.4
Pd
P
Cy2
CH2CHCO2H,
THF-d8,
70 oC, 2 h
6
A
C
Cy2
Cy2
Cy2
P
Pd
P
P
OH
O
+
Pd
Pd
P
P
P
Cy2
Cy2
P
Cy2
Cy2
Cy2
P
O
ONa
O
Pd
O
P
Pd
P
Cy2
Cy2
O
3
4
5
O
Scheme
(dcpe)Pd(CH2CHCO2H) (
1
Synthesis of (dtbpe)Pd(CH2=CH2)
(
) and (dcpe)Pd(-CH2CH2CO2-) (
1
), (dtbpe)Pd(CH2CHCO2H)
).
(2),
B
4
5
OH
ONa
ONa O
In the solid state the C(1)-C(2) bond length in (dtbpe)Pd(η2-
CH2CHCO2H) (2) is comparable to its nickel analogue (1.424(8) vs.
1.417(3) Å, cf. Figure 1).9 The palladium-ligand bond lengths of
Pd(1)-C(2) and Pd(1)-C(1) are longer than those for nickel (2.089(6)
and 2.144(5) Å vs. 1.942(2) and 1.988(2) Å, respectively) due to the
palladium centre. Efforts to synthesize a dtbpe-ligated palladalactone
either directly by addition of CO2 and ethylene or indirectly by
addition of β-propiolactone to (η5-Cp)Pd(η2-allyl) failed. Reductive
routes from palladium(II)dihalides were not met with success either.
Alternative routes towards a palladalactone were explored with
various palladium precursors and ligands: (Dialkyl)Pd(II)-complexes
are prone to undergo reductive elimination, either thermally or acid-
induced.14 Indeed, (dcpe)Pd(CH2CH3)2 (3),15 which was synthesized
starting from Pd(OAc)2 and Et3Al in 50% yield, formed
(dcpe)Pd(H2C=CH2)15,16 upon heating (70 °C, THF-d8, 2 h).
+ CO2
- CO2
F
F
F
O
Figure 2 Proposed catalytic cycle for Pd catalyzed coupling of CO2 and ethylene
to form sodium acrylate.
We have not been able to isolate a palladalactone or other
potential catalytic intermediates such as 1 or 4 from a crude reaction
mixture, these findings lead us to the assumption of the catalytic
cycle proposed in Figure 2. However, in the crude reaction mixture
of a catalytic experiment, both sodium 2-fluorophenolate and 2-
fluorophenol were identified by 1H NMR spectroscopy, and the
palladium complex (dcpe)Pd(-OPhFCO2-) (6) was crystallized after
aqueous work-up as a minor side product (<1% of phosphine species
by 31P NMR, see supporting information). The formation of Pd(II)-
complex 6 can be rationalized if one assumes that in the presence of
CO2 sodium 2-fluorophenolate is susceptible to a Kolbe-Schmitt
reaction and forms salicylates, which are capable to trap Pd(0). Thus,
Zn(0) has a twofold function: Generation of catalytically active
Synthesis of the elusive palladalactone 5 was finally achieved
from acrylic acid and (dcpe)Pd(CH2CH3)2 (3) upon heating in THF-
d8 (70 °C, 2 h, cf. Scheme 1). In solution, a mixture of palladalactone
5 and acrylic acid-complex 4 formed (ratio 4:5 ca. 1:1).
Recrystallization at −35 °C yielded crystals of palladalactone
(dcpe)Pd(-CH2CH2CO2-) (5) suitable for crystallographic
Pd(0) species from Pd(II) precursors and the reduction of any Pd(II
species such as 6 formed during catalysis.
)
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Once the coupling of CO2 and ethylene with palladium was
optimized, preliminary investigations with conjugated alkenes were
conducted. Reaction of 1,3-butadiene with (COD)PdCl2, dcpe,
sodium 2-fluorophenolate and Zn under CO2 (20 bar, THF, 145 °C,
20 h) resulted in a TON of 24 relative to Pd for the single CO2
insertion product sodium (E)-penta-2,4-dionate (Scheme 2). The
conformation was based on its characteristic 1H NMR coupling
constant of J = 15.4 Hz.4 Under similar conditions the reaction of
(E)-piperylene gave a TON of 50 for sodium sorbate relative to Pd.
5
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Scheme 2 Catalytic formation of acrylates from dienes.
In conclusion, we have found a palladium catalyst for the catalytic
synthesis of sodium acrylates or
α,β,γ,δ-unsaturated carboxylates
from CO2, alkenes or 1,3-dienes and a base characterized by a TON
> 20 relative to Pd and TOF > 24/day. Efforts are currently
underway to further elucidate the catalytic cycle and optimize the
system, which in its present form is still less efficient than the best
reported Nickel catalyst. However, this work represents, to our
knowledge, the first example of a catalytic coupling of CO2 with
ethylene or other alkenes at a palladium catalyst, and the first
crystallographically characterized parent palladalactone.
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6
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S.C.E.S, N.H., I.J., T.K., P.A., A.G., P.H. and M.L. work at
CaRLa of the University of Heidelberg, which is co-financed by the
University of Heidelberg, the State of Baden-Württemberg, and
BASF SE. Financial support from these institutions and the German
Federal Ministry for Education and Research (BMBF, grant #
01RC1015A; Chemische Prozesse und stoffliche Nutzung von CO2:
Technologien für Nachhaltigkeit und Klimaschutz).
Limbach, Chem.
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The first palladium catalyst for the reaction of CO2, alkenes and a
base to acrylates is reported (TON up to 50).
This journal is © The Royal Society of Chemistry 2012
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