Ruthenium-Catalyzed Synthesis of Dialkoxymethane Ethers Utilizing Carbon Dioxide and Molecular Hydrogen

Article abstract of DOI:10.1002/anie.201606427

The synthesis of dimethoxymethane (DMM) by a multistep reaction of methanol with carbon dioxide and molecular hydrogen is reported. Using the molecular catalyst [Ru(triphos)(tmm)] in combination with the Lewis acid Al(OTf)₃resulted in a versatile catalytic system for the synthesis of various dialkoxymethane ethers. This new catalytic reaction provides the first synthetic example for the selective conversion of carbon dioxide and hydrogen into a formaldehyde oxidation level, thus opening access to new molecular structures using this important C₁source.

Full text of DOI:10.1002/anie.201606427

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201606427
German Edition: DOI: 10.1002/ange.201606427
CO Utilization
2
Ruthenium-Catalyzed Synthesis of Dialkoxymethane Ethers Utilizing
Carbon Dioxide and Molecular Hydrogen
Katharina Thenert, Kassem Beydoun, Jan Wiesenthal, Walter Leitner, and
Jꢀrgen Klankermayer*
Abstract: The synthesis of dimethoxymethane (DMM) by
a multistep reaction of methanol with carbon dioxide and
molecular hydrogen is reported. Using the molecular catalyst
[
Ru(triphos)(tmm)] in combination with the Lewis acid
Al(OTf)3 resulted in a versatile catalytic system for the
synthesis of various dialkoxymethane ethers. This new catalytic
reaction provides the first synthetic example for the selective
conversion of carbon dioxide and hydrogen into a formalde-
hyde oxidation level, thus opening access to new molecular
structures using this important C source.
1
Scheme 1. a) Industrial production of DMM starting from CH OH
3
O
xygenated compounds such as methanol, dimethyl ether
DME), and oxymethylene ethers (OMEs) are known to be
and/or formaldehyde. b) The new approach for synthesis of DMM
starting from methanol using CO /H .
(
2
2
attractive candidates for fuels or fuel additives because of
their ability to reduce soot formation during the combustion
[
1]
in diesel engines. Especially, oxymethylene dimethyl ethers
such as dimethoxymethane (DMM or methylal) and poly-
(
bifunctional heterogeneous catalysts to control the sequential
in situ formation of formaldehyde and its subsequent
condensation in gas-phase continuous-flow processes.
[
6a–i]
oxymethylene) dimethyl ethers (OMEx) have been gaining
increasing interest in recent years because of their advanta-
Recently, the group of Deng and Wang described a successful
solution-phase batch reaction for the synthesis of DMM using
[
1,2]
geous properties as fuel additives.
DMM is the first
member in this homologous series and therefore sometimes
RuCl as a molecular catalyst for the oxidation/condensation
of methanol into DMM.
3
[
6j]
referred to as OME . Moreover, this molecule represents an
1
important building block and can thus be applied as a source
However, all current syntheses of DMM use methanol as
a starting material and involve an oxidative step to attain the
[
1a,2,3]
for the synthesis of higher order OMEs.
In addition,
DMM has an attractive profile as a green solvent, and has
been successfully used in pharmaceutical and perfume
industries.
formaldehyde-level of the central CH -unit. To circumvent
2
this redox-inefficient pathway, a reductive approach to
generate DMM directly from MeOH and CO /H would
2
2
Industrially, DMM is produced by a two-step process
comprising oxidation of methanol into formaldehyde over
either silver or modified iron-molybdenum catalysts (ex.
provide an attractive alternative route (Scheme 1b). Herein,
we describe the first catalytic process to achieve this goal,
opening the possibility to harness renewable energy via the
[
4]
Formox Process), and the condensation of formaldehyde
with methanol in the presence of acid catalysts to afford the
combination of CO and H2 directly into potential fuel
2
[7]
candidates.
[
5]
corresponding DMM product (Scheme 1a).
Recently, our group described the first organometallic
Many efforts have been dedicated to developing a one-
step direct oxidation of methanol into DMM and a number of
selective oxidation catalysts have been reported, thus yielding
catalytic system for the hydrogenation of CO into methanol
2
by employing the highly versatile triphos-based ruthenium
catalyst, [Ru(triphos)(tmm)] [triphos = 1,1,1-tris(diphenyl-
phosphinomethyl)ethane, tmm = trimethylene methane], to
catalyze this transformation through a series of hydride
transfer and protonolysis steps promoted by an acid addi-
[
6]
DMM in up to 100% selectivity. Most of these systems use
[*] K. Thenert, Dr. K. Beydoun, J. Wiesenthal, Prof. Dr. W. Leitner,
[8]
Prof. Dr. J. Klankermayer
Institut fꢀr Technische und Makromolekulare Chemie
RWTH Aachen University
Worringerweg 2, 52074 Aachen (Germany)
E-mail: jklankermayer@itmc.rwth-aachen.de
tive. This system, which is very generally applicable to the
[
9]
hydrogenation of carboxylic and carbonic acid derivatives,
[10]
was also employed for the N-methylation of ammonia and
amines using CO /H as a C synthon. Consequently, in the
[11]
2
2
1
first set of experiments this complex was used in combination
with selected acidic co-catalysts using methanol, CO , and H
Prof. Dr. W. Leitner
Max-Planck-Institut fꢀr Kohlenforschung
2
2
as substrates. Reactions were carried out with a 1:3 ratio of
the two gases under a total pressure of 80 bar (room
temperature) and reaction temperatures between 80–1208C.
Using the [Ru(triphos)(tmm)] catalyst in the absence of acid
4
5470 Mꢀlheim an der Ruhr (Germany)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201606427.
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1
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did not result in formation of any detectable CO hydro-
Table 2: Optimization of the ruthenium catalysts for the synthesis of
2
[
a]
DMM with CO and molecular hydrogen.
genation products. However, using the ruthenium precursor
in combination with trifluoromethanesulfonylimide (HNTf2)
at 1208C, resulted in the formation of DMM with a low
turnover number (TON) of four, together with a similar
amount of methylformate (MF; Table 1, entry 1). Interest-
2
[b]
[b]
Entry Catalyst/ligand
x [mmol] TON
MF)
TON
(DMM)
(
Table 1: Ruthenium-catalyzed synthesis of MF and DMM using CO and
molecular hydrogen.
1
2
3
4
[Ru(cod)(methylallyl)
[Ru(triphos)(CO)(H)(Cl)]
[Ru(triphos)(tmm)]
2
]/triphos 12.5
33
–
20
71
–
90
214
2
[
a]
12.5
25
6.0
[Ru(triphos)(tmm)]
104
[a] Reaction conditions: Ruthenium catalyst (x mmol), Al(OTf)3
(
25 mmol), methanol (2 mL), CO /H (20/60 bar), 18 h, 808C. [b] TON
2 2
[
b]
[b]
Entry
Acid
x [mmol]
T [8C]
TON
(
TON
was determined by NMR spectroscopy using mesitylene as an internal
standard. cod=1,5-cyclooctadiene.
MF)
(DMM)
1
2
3
4
5
6
7
HNTf2
HNTf2
12.5
12.5
12.5
12.5
12.5
12.5
25
120
100
120
100
80
7
4
61
32
94
98
125
136
27
16
32
77
53
54
Al(OTf)3
dependence became clearly evident when a lower loading of
Al(OTf)3
Al(OTf)3
Al(OTf) /p-TsOH
Al(OTf)3
[
Ru(triphos)(tmm)] was used (6 mmol), thus resulting in
a significant increase of the TONs for MF and DMM
formation to 104 and 214, respectively (Table 1, entry 7 and
80
80
3
Table 2, entry 4). Introducing different CO pressures to the
2
[a] Reaction conditions: Catalyst=[Ru(triphos)(tmm)] (12.5 mmol), acid
reaction revealed that 20 bar of CO was the optimal pressure
2
(x mmol), methanol (2 mL), CO /H (20/60 bar), 18 h. [b] Turnover
2 2
for DMM formation with a favorable CO /H pressure ratio
number (TON) was determined by NMR spectroscopy using mesitylene
as an internal standard. Tf=trifluoromethanesulfonyl, Ts=4-toluene-
sulfonyl.
2
2
of 1:3 (see the Supporting Information).
Further insight into the formation of DMM was obtained
from a labeling experiment using the catalyst system [Ru-
1
3
(
triphos)(tmm)]/Al(OTf)3 in the presence of CD OD in
3
ingly, decreasing the reaction temperature to 1008C caused
combination with nonlabelled CO and H . The NMR analysis
2 2
1
2
a significant increase in DMM formation with a TON of 61
revealed the expected formation of MF with the C at the
[
10]
13
(
entry 2). Applying the recently developed catalyst system
formyl group and C in the ester functionality (Scheme 2).
comprised of [Ru(triphos)(tmm)] and Al(OTf) resulted in
3
a significant increase in reactivity, thus giving a TON of 32 at
1
208C, and 94 at 1008C (entries 3 and 4). An additional slight
increase to a TON of 98 could be observed after 18 hours at
a reaction temperature of 808C, but the formation of MF was
also more pronounced (entry 5). Gratifyingly, when Al(OTf)3
was used together with the Brønsted acid p-toluenesulfonic
acid (p-TsOH), the TON of DMM increased to 125 at the
expense of the reduction of formate (entry 6). A similar effect
could be observed at higher loading of the Lewis-acidic co-
catalyst, and with 25 mmol of Al(OTf) a TON of 136 could be
3
Scheme 2. Isotopic labeling observed by NMR spectroscopy for the
obtained (entry 7). Notably, the formation of either DMM or
MF could not be detected in the reaction solution in the
absence of CO2.
[
Ru(triphos)(tmm)]/Al(OTf) -catalyzed synthesis of DMM from
3
13
3
CD OD.
In the subsequent experiments, the influence of the
catalyst precursor and the catalyst concentration were inves-
Moreover, the formation of methoxymethanol (MM) could
be observed with the anticipated labeling resulting from the
sequential reduction of labeled MF (Scheme 2). The final
tigated. Using an in situ system of [Ru(cod)(methylallyl) ] in
2
combination with the triphos ligand resulted in the formation
of DMM with a TON of 71 (Table 2, entry 1), thus corre-
sponding to only around 50% of the performance of the
isolated catalyst. Using the complex [Ru(triphos)(CO)(H)-
1
3
product DMM was found to have the C label incorporated
only at the methyl groups, thus confirming their origin from
methanol and the generation of the CH O unit from CO and
2
2
(
Cl)] did not yield any DMM (entry 2). Interestingly, a higher
H . These results clearly indicate the construction of the
2
loading of [Ru(triphos)(tmm)] (25 mmol) resulted in a lower
TON for DMM formation and decreased formation of MF
with a low TON of 20 (entry 3). This result indicates that the
performance of the catalytic system is dependent on the
additive to catalyst ratio and additionally on the catalyst
corresponding carboxyl group of MF and the methylene
groups of MM and DMM from CO as the C source.
2
1
Thus, these experimental observations lead us to propose
a sequential pathway of DMM formation starting from
methanol, CO , and H . First, MF is formed by the well-
2
2
[8b,12]
deactivation, by dimerization, at high loadings.
This
known hydrogenation to the formic acid oxidation level
2
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decrease of the TONs, and 1-octanol (entry 3), 1-nonanol
(
(
entry 4), and 1-decanol (entry 5) afforded the products
DOM, DNM, and DDM) with TONs of 78, 60, and 51,
respectively. Similarly, the use of the secondary alcohol,
isopropanol, resulted in the formation of diisopropoxyme-
thane diether (DiPrM) with a low TON of 29 (entry 6).
Benzyl alcohol was also tested for this reaction, and gave
dibenzyloxymethane diether (DBnM) with a TON of 99
Scheme 3. Possible reaction pathway for the [Ru(triphos)(tmm)]/
Al(OTf) -catalyzed synthesis of DMM using methanol and CO /H
a C synthon.
3
2
2
as
1
(entry 7).
coupled with esterification (Scheme 3). The Ru-Triphos
system is able to induce further hydrogenation to MM
corresponding to the formaldehyde oxidation level. Trans-
acetalization with the solvent methanol leads to the DMM
product. Whereas the reduction sequence is resulting from
the hydrogenation activity, the esterification and acetalization
steps are expected to be largely facilitated by the Brønsted
and/or Lewis acidity of the multifunctional catalytic system. It
is noteworthy that this is the first example for a catalytic
In conclusion, we have described a novel catalytic syn-
thesis towards dimethoxymethane (DMM or methylal) start-
ing from methanol, CO , and H . This reaction pathway
2
2
provides the first direct reductive access to DMM, and
constitutes the starting member of the oxymethylene-ether
series (OME ), currently discussed as potential fuel candi-
1
dates based on renewable hydrogen. The catalytic system was
based on the Ru-Triphos unit in combination with Lewis and/
or Brønsted cocatalysts. The multifunctionality of the catalyst
system was crucial for the complex reaction sequence,
comprising various hydrogenation and esterification/acetali-
zation steps. Moreover, the reaction was found to be general
for the synthesis of dialkoxymethane ethers from CO , H ,
hydrogenation of CO which terminates selectively at the
2
[7c]
formaldehyde level.
To study the versatility of this new approach towards the
synthesis of dialkoxymethanes, the synthetic scope of this
transformation with selected alcohols was investigated
2
2
and the respective alcohols. Furthermore, the formation of
methoxymethanol (MM) and methylformate (MF) could be
identified as possible intermediates on the pathway to the
construction of the methylene group from the CO /H .
(
Table 3). Under the presented reaction conditions and in
presence of ethanol, the reaction afforded the target product
diethoxymethane (DEM) with a TON of 118 (entry 1). The
use of 1-butanol in the reaction yielded dibutoxymethane
diether (DBM) with a TON of 110 (entry 2). However, the
linear alcohols with a longer carbon chain resulted in a slight
2
2
Consequently, this new catalytic reaction provides the first
synthetic example for the selective conversion of CO and H
2
2
into a compound having the formaldehyde oxidation level,
thus opening access to new fields on the catalytic chess board
[
7e]
of CO hydrogenation. Future work in our laboratories will
2
Table 3: Ruthenium-catalyzed synthesis of dialkoxymethanes (DAM)
be directed towards the synthesis of cyclic and poly(oxy-
methylene) ethers using this newly established catalytic
pathway.
[
a]
using variable alcohols with CO and molecular hydrogen.
2
Experimental Section
[
b]
Entry
ROH
TON
(
General procedure for the synthesis of dimethoxymethane DMM
from methanol, CO , and H : A 2.0 mL solution of [Ru(triphos)-
DAM)
2
2
(tmm)] (0.009 g, 12.5 mmol) and Al(OTf)3 (0.012 g, 25 mmol) in
1
2
3
4
5
6
methanol was prepared under an argon atmosphere in a Schlenk
tube containing a stirring bar. After stirring for 5 minutes, the solution
was transferred to a carefully degassed and dried 20 mL stainless-steel
autoclave. The autoclave was pressurized at room temperature with
2
0 bar CO and then H was added up to a total pressure of 80 bar.
2
2
The reaction mixture was stirred with a magnetic stir bar and heated
to 808C using a preheated aluminum cone. After 18 h the autoclave
was cooled in an ice bath and then carefully vented. The turnover
1
number (TON) of DMM in solution was analyzed by H NMR
spectroscopy using mesitylene as an internal standard.
Acknowledgments
This work was supported by the Cluster of Excellence “Tailor-
Made Fuels from Biomass”, which is funded by the Excel-
lence Initiative by the German Federal and State Govern-
ments to promote science and research at German univer-
sities. We also thank Umicore AG for a generous gift of
ruthenium precursor. Additional support through the Proj-
7
[
(
a] Reaction conditions: [Ru(triphos)(tmm)] (6 mmol), Al(OTf)3
25 mmol), ROH (2 mL), CO /H (20/60 bar), 18 h, 808C. [b] TON was
2 2
determined by NMR spectroscopy using mesitylene as an internal
standard.
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4
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Communications
CO Utilization
2
K. Thenert, K. Beydoun, J. Wiesenthal,
W. Leitner,
J. Klankermayer*
&&&& — &&&&
Ruthenium-Catalyzed Synthesis of
Dialkoxymethane Ethers Utilizing Carbon
Dioxide and Molecular Hydrogen
Come together! The molecular catalyst
the first synthetic example for the selec-
[
Ru(triphos)(tmm)] in combination with
tive conversion of carbon dioxide and
hydrogen into the formaldehyde oxidation
level, thus opening access to new
molecular structures using this important
C1 source.
the Lewis acid Al(OTf) enables the syn-
3
thesis of dimethoxymethane from carbon
dioxide, molecular hydrogen and metha-
nol. This new catalytic reaction provides
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5
These are not the final page numbers!