Transfer hydrogenation of cyclic carbonates and polycarbonate to methanol and diols by iron pincer catalysts

Article abstract of DOI:10.1039/c9gc02052g

Herein, we report the first example on the use of an earth-abundant metal complex as the catalyst for the transfer hydrogenation of cyclic carbonates to methanol and diols. The advantage of this method is the use of isopropanol as the hydrogen source, thus avoiding the handling of flammable hydrogen under high pressure. The reaction offers an indirect route for the reduction of CO₂ to methanol. In addition, poly(propylene carbonate) was converted to methanol and propylene glycol. This methodology can be considered as an attractive opportunity for the chemical recycling of polycarbonates.

Full text of DOI:10.1039/c9gc02052g

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DOI: 10.1039/C9GC02052G
ARTICLE
Transfer Hydrogenation of Cyclic Carbonates and Polycarbonate
to Methanol and Diols by Iron Pincer Catalysts
Received 00th January 20xx,
Accepted 00th January 20xx
Xin Liu, Johannes G. de Vries and Thomas Werner*
DOI: 10.1039/x0xx00000x
Herein, we report the first example on the use of an earth-abundant metal complex as catalyst for the transfer
hydrogenation of cyclic carbonates to methanol and diols. The advantage of this method is the use of isopropanol as
hydrogen source, thus avoiding the handling of flammable hydrogen under high pressure. The reaction offers an indirect
route for the reduction of CO2 to methanol. In addition, poly(propylene carbonate) was converted to methanol and
propylene glycol. This methodology can be considered as an attractive opportunity for the chemical recycling of
polycarbonates.
with seminal contributions from the groups of Sanford,17
Prakash and Olah18, 19 Klankermayer and Leitner20, 21 employing
noble metal catalysts based on ruthenium.
Introduction
Carbon dioxide (CO2) is an economic, renewable and safe C1-
building block for the production of organic chemicals and
materials.1-5 The conversion of this thermodynamically stable
molecule into value-added products is challenging but at the
same time creates the opportunity to develop new concepts
and catalysts. Methanol is one of the most versatile and
largest chemical commodities with a global demand of >70
million metric tons in 2015.6 Currently, methanol is produced
from syngas which is obtained mainly from natural gas and
other fossil resources.7 Methanol is an indispensable precursor
for various industrial products and bulk chemicals such as
acetic acid, formaldehyde, gasoline (via methanol-to-gasoline)
and olefins which are produced by methanol-to-olefins (MTO)
a) Indirect method for the conversion of CO2 to methanol
O
This work:
OH OH
[Fe]
CO2
CH3OH
+
+
O
O
n
Br
R1
R2
H
N
iPrOH
n
P(iPr)2
CO
R1
R2
Fe
H
(iPr)2P
b) Waste polycarbonate as a C1 resource to produce methanol
O
HO
R
OH
[Fe]
CO2
CH3OH
O
O
iPrOH
n
R
Scheme 1. Iron-based catalysts for transfer hydrogenation of organic carbonates.
So far, only a few examples on the use of non-noble metal
catalysts were reported, e.g. Beller and coworker employed a
cobalt catalyst22 for the direct reduction of CO2 while the
group of Prakash reported a manganese-based23 system. The
latter described a sequential process for the reduction of CO2
to methanol via pre-formed formamide reaching TONs up to
36. Notably, Milstein and co-workers were the first introduced
the concept of indirect CO2-reduction as an alternative route
to methanol.24 They used Ru pincer-type complexes of the
general structure [(PNN)Ru(CO)(H)] for the catalytic
hydrogenation, e.g. of dimethyl carbonate to methanol under
mild conditions. The group of Ding reported the use of the Ru-
MACHO pincer catalyst for the reduction of cyclic carbonates,
e.g. ethylene carbonate.25 This would in principle allow the
selective production of ethylene glycol and methanol from CO2
and ethylene oxide via ethylene carbonate in a modified Shell
“OMEGA process”.26 The replacement of noble-metal catalysts
by earth-abundant substitutes is also of particular interest for
the indirect CO2-reduction. Most recently three independent
parallel studies by the groups of Milstein,27 Leitner28 and
Rueping29 on the hydrogenation of organic carbonates to
methanol using manganese pincer-type complexes as catalysts
and methanol-to-propene (MTP).8, Furthermore, methanol
9
has attracted significant attention as a liquid fuel, energy
carrier in methanol fuel cells as well as hydrogen-storage
material (12.5 wt% H2). Olah recently introduced the concept
of the “methanol economy” where methanol is the main
carbon feedstock and energy carrier.10, 11
For these reasons, the reduction of CO2 to methanol is of
particular interest. Heterogeneous catalysts for the direct
reduction of CO2 with hydrogen were extensively studied and
numerous active systems have been reported.12, 13 However,
most of them require high reaction temperatures (>200°C) and
high hydrogen pressure. Homogeneous catalysts are
potentially more active and energy efficient but often require
the use of additives.14-16 Recently, great advances have been
made in the field of direct hydrogenation of CO2 to methanol
a. Addres Address here. Leibniz-Institut für Katalyse an der Universität Rostock,
Albert-Einstein-Straße 29a, 18059, Rostock, Germany.
E-mail: Thomas.Werner@catalysis.de
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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were reported. However, high pressures of highly flammable even lower yields were achieved (Entry 7). If no catalyst is
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hydrogen gas and/or high temperatures were required in the employed some background reaction isDoObI:s1e0r.1v0e3d9/uCn9GdCer02b0a5s2iGc
above mentioned protocols.
conditions leading to 16% 8a and 11% diisopropyl carbonate
In general, transfer hydrogenations using hydrogen donors (see SI, Scheme S1). However, in this case the formation of
such as readily available and non-toxic isopropanol are MeOH is not observed. It has to be mentioned that iron
attractive alternatives to classical hydrogenation reactions.30-32 complex 3 had only low catalytic activity for the reduction of
These protocols are usually operational simple, avoiding the diisopropyl carbonate, indicating that the latter is not likely an
use of molecular hydrogen and expensive high pressure intermediate in the catalytic cycle (see SI, Scheme S2).
reactors. Notably, so far the only example of a transfer Typically, diisopropyl carbonate was observed as by-product
hydrogenation of cyclic carbonates to methanol and diols was which is formed from the cyclic carbonate 7a by
reported by Hong and co-workers using a ruthenium catalyst.33 transesterification with iPrOH in the above mentioned
Although the transfer hydrogenation of esters is known,34-36 to reactions. This explains the reduced methanol yield, which is in
the best of our knowledge there is no report on the transfer some cases significantly lower than expected when only iPrOH
hydrogenation of organic carbonates by homogeneous was used as the solvent (Table 1, entries 1–6).
catalysts based on earth abundant metals. The synthesis of
Table 1 Transfer hydrogenation of cyclic carbonate 7a in the presence of base metal
cyclic carbonates from CO2 and epoxides is a 100% atom
PNP pincer-type complexes (1-6).a
economical reaction and can be performed even at room
38
temperature and 1 atm CO2.37,
Previously, we described
O
efficient catalytic systems based on organo- and earth-
HO
Et
OH
15 mol% 16, 05 mol% KOtBu
140 °C, 624 h, sol/iPrOH
O
O
+
MeOH
40
abundant metal catalysts for this reaction.39,
Herein, we
Et
report the first example of the transfer hydrogenation of cyclic
carbonates as well as poly carbonates to methanol and diols
using iron pincer-type catalysts (Scheme 1).
7a
8a
Ent
ry
1
2
3
4
5
6
7
Cat.
KOtBu/
mol%
5.0
5.0
5.0
5.0
5.0
5.0
-
5.0
-
10
2.5
1.0
5.0
5.0
Solvent
Conv.
/ %
18
66
81
34
57
72
52
99
37
99
85
56
52
99
Yield
Yield
8a/ %b
11
MeOH/ %b
1
2
3
4
5
6
6
3
3
3
3
3
3
3
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
14
37
60
-
Results and discussion
We started our investigations with three PNP pincer-type
51
80
-
complexes 1–3 based on Fe, Mn and Co (Scheme 2).
55
63
51
29
41
22
91
-
H
N
H
N
H
N
iPrOH
iPrOH
(iPr)2P Mn P(iPr)2
(iPr)2P Fe P(iPr)2
(iPr)2P Co P(iPr)2
Br
Br
CO
Br
H
8
9
THF/iPrOH
THF/iPrOH
THF/iPrOH
THF/iPrOH
THF/iPrOH
THF/iPrOH
THF/EtOH
95
Br
CO
CO
c
-
2
1
3
10
11d
12e
13f
14
92
81
52
47
91
34
76
36
35
59
H
N
H
N
H
N
(Cy)2P Fe P(Cy)2
(iPr)2P Fe P(iPr)2
(iPr)2P Fe P(iPr)2
H
H
Br
Br
Br
BH4
CO
CO
CO
5
6
4
a
Scheme 2. Base metal PNP pincer-type complexes 1–6 used in this study.
Reaction conditions: Catalyst 1–6 (5 mol%), KOtBu (5 mol%), 7a (1.0 mmol),
b
THF (0 or 1.0 mL), iPrOH (4 mL), 140 °C, 6 h. Determined by GC using
mesitylene as the internal standard. c 2-hydroxybutyl isopropyl carbonate was
observed as the product. 2.5 mol% of catalyst 3 was used. 1.0 mol% of
catalyst 3 was used. K2CO3 was used as base instead of KOtBu, 24 h reaction
In preliminary experiments, 2-butylene carbonate (7a) was
d
e
selected as
a benchmark substrate for the transfer
f
hydrogenation in the presence of catalysts 1–3 (Table 1). The
reaction was conducted during 6 h with 5 mol% of catalyst in
the presence of KOtBu (5 mol%) using iPrOH as hydrogen
source at 140 °C. Compared to the Mn- and Co-PNP pincer-
type complexes 1 and 2 the Fe-complex 3 was found to be the
most efficient catalyst for the transfer hydrogenation of
carbonate 7a (entries 1 and 2 vs. 3). Notably, this is the first
example of the use of iron pincer catalysts for the reduction of
cyclic carbonates. Next, complexes 4–6 were also prepared
and tested. In the presence of catalyst 4 the formation of 8a
and MeOH was not observed; instead 2-hydroxybutyl isopropyl
carbonate and diisopropyl carbonate were obtained (Entry 4).
In contrast, use of catalysts 5 and 6 gave 8a and MeOH
although in disappointing yields under the same reaction
conditions (Entries 5 and 6). Catalyst 6 is a well-known base-
free pincer-type complex.41 However, in the absence of base
time.
To increase the solubility of the catalyst precursor and
facilitate complete deprotonation several solvents were tested
(see SI, Table S1). In the presence of THF as co-solvent the
transfer hydrogenation of 7a using catalyst 3 led to diol 8a and
MeOH in excellent yields of 95% and 91% respectively (Entry
8). In the absence of base only the formation of 2-
hydroxybutyl isopropyl carbonate was observed as the only
product at a moderate conversion (37%) of 7a (Entry 9). In
contrast, an increase of the base loading to 10 mol% led to full
conversion, but a low MeOH yield (34%) was obtained (Entry
10). In this case we observed the formation of diisopropyl
carbonate as byproduct. When the catalyst loading was
reduced to 2.5 mol% of 3 a reaction time of 24 h was
2 | J. Name., 2012, 00, 1-3
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necessary to achieve 85% conversion of 7a (Entry 11). Even at optimized conditions using iron complex
ARTICLE
3
Viaews ArtticrlaenOsnfliener
DOI: 10.1039/C9GC02052G
1 mol% catalyst loading moderate yields of 8a (52%) and hydrogenation catalyst (Table 2). Ethylene carbonate (7b) as
MeOH (36%) were still achieved (Entry 12). A study on the well as various substituted carbonates were efficiently reduced
impact of the reaction temperature revealed that 140°C was to MeOH and diols (Entries 1–10). Yields up to 97% were
the optimal temperature (see SI, Table S2). Hong and obtained, even though in some cases the reaction time had to
coworkers reported the use of Ru-PNP complexes in be extended to achieve conversions >90%. Notably, the
combination with K2CO3 as base.33 In our hands K2CO3 proved amount of MeOH obtained in all reactions is as expected in the
to be less effective in combination with catalyst 3 compared to range of the corresponding amounts of 8. This indicates that
KOtBu (Entry 13). Recently, we reported ethanol as an efficient reduction of the carbonate is indeed in operation rather than
renewable hydrogen source for the transfer hydrogenation formation of 8 by transesterification. Significantly, the reaction
reaction of esters.36 However, the conversion of cyclic can be scaled up to multi-gram quantities; e.g. ethylene
carbonate 7a with ethanol as hydrogen donor gave only carbonate (7a, 10 mmol) was converted to afford 8a in 87%
moderate yields of the desired products (Entry 14).
and methanol in 83% yield (Entry 2). We also investigated the
synthesis of diol 8a and MeOH in a one pot two step reaction
sequence from butylene oxide and CO2 (see SI for details). In
Table 2. Substrate scope for the transfer hydrogenation of cyclic carbonates 7 in the
presence of iron pincer catalyst 3.a
the first step we used a bifunctional phosphonium salt
43
previously developed in our group to prepare 7a.42,
The
O
OH OH
5 mol% 3, 5 mol% KOtBu
O
O
carbonate was subsequently converted in the presence of
catalyst 3 to yield diol 8a (59%) and MeOH in 59% and 47 yield,
respectively. Additionally, we investigated the possibility of
catalyst recycling in the conversion of 7a to 8a and MeOH (see
SI, Figure S1). Even though the catalyst could be separated
from the reaction products and reused, the yields on 8a and
MeOH dropped already significantly in the second run.
+
MeOH
R1
R2
140 °C, 624 h, THF/iPrOH
n
R1
R2
n
7
8
Entry
1
2d
Substrate
t/ h
6
18
Yield 8/ %b
95c
72, 87c
Yield MeOH/ %c
O
91
83
7a
O
O
Et
O
3
4e
6
12
97c
53c
92
35
Other simple carbonates 7b–7f could be converted to 8b–8f in
good to excellent yields (Entries 3–8). Interestingly, the double
bonds in substrates 7g and 7h are unaffected under the
reaction conditions and the unsaturated diols 8g and 8h were
obtained in yields of 85% and 71% respectively. The
disubstituted carbonate 7i as well as the internal disubstituted
substrate 7j were converted and excellent yields of the diols
and MeOH were achieved (Entries 11 and 12). The six-
membered carbonate 7k was also reduced but only a
moderate conversion was observed after 24 h (Entry 13).
Subsequently, we identified and monitored the reaction
intermediates produced in the transfer hydrogenation of 2-
butylene carbonate (7a) to elucidate the possible reaction
pathways using the PNP pincer-type Fe (II) catalyst 3 (See SI,
for details). We only observed the presence of isopropyl
formate (11) as intermediate. Notably, as mentioned above, in
the absence of base the formation of 2-hydroxybutyl isopropyl
carbonate was observed (Table 1, Entry 9). This is in
agreement with the excellent study of Hong and coworkers
who observed the formation of 2-hydroxyethyl isopropyl
carbonate and 11 in the Ru-catalyzed transfer hydrogenation
of 7b.33
7b
O
O
O
O
O
5
6
91
90
7c
O
O
O
O
O
Me
6
7
8
12
24
24
81
84
83
73
77
71
7d
7e
7f
O
nBu
O
Hex
O
O
Ph
O
9
12
24
85
71
82
62
7g
7h
O
O
O
10
O
O
2
O
11
24
86
78
7i
7j
O
O
Me
Me
O
12
13
12
24
92
86
39
O
O
Me
Me
O
51c
7k
O
O
a
Reaction conditions: Catalyst 3 (5 mol%), KOtBu (5 mol%), 7 (1.0 mmol),
b
c
THF (1 mL), iPrOH (4 mL), 140 °C, 6–24 h. Yield of the isolated product.
Determined by GC using mesitylene as the internal standard. d 10 mmol of 7b
was used. e 100 °C.
To demonstrate the potential of this new catalytic system, a
variety of cyclic carbonates 7a-7k were tested under the
This journal is © The Royal Society of Chemistry 20xx
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a) Proposed catalytic cycle
O
View Article Online
DOI: 10.1039/C9GC02052G
H
N
H
H
O
H
O
E
Fe
E
H
CO
TS-1
H
N
H
HN
H
N
Br
Fe
KOtBu
E
E
E
Fe
Fe
E
E
E
CO
H
CO
H
CO
3b
3a
3
Y
O
X
H
O
H
H
H
N
O
E
X
Y
Fe
E
X
Y
H
CO
TS-2
b) Putative reaction pathways
HO
OH
8b
O
iPrOH
O
O
O
Figure 1. Reaction profile of the transfer hydrogenation of cyclic carbonate 7a. Yields
were determined by GC using mesitylene as the internal standard.
7b
iPrO
11
H
[Fe]
Pathway I
[Fe]
Based on these results a putative catalytic cycle and reaction
pathway shown in Scheme 3 are proposed. Initially, complex 3
is deprotonated to afford the 16e complex 3a. Subsequently,
the dehydrogenation of iPrOH leads to the 18e complex 3b
and acetone via TS-1 (Scheme 3a). Finally, hydrogen transfer
from complex 3b to the C=O group of a carbonyl substrate e.g.
a carbonate via TS-2 regenerates 3a under liberation of the
reduced carbonyl species. The reduction of 7b to MeOH can
proceed via two possible pathways (Scheme 3b). The initial
reduction of 7b leads to intermediate 9 which rearranges to
formate 10. The formation of formates like 10 was proposed
by Leitner and coworkers as an intermediate in the
hydrogenation reactions even though it was not observed by
us.28 However, the presence of 11 indicates that 10 might be
formed in low concentrations and is directly transesterified to
11 under liberation of diol 8b. In turn 11 is reduced to
formaldehyde 12 which is a frequently observed intermediate
in this type of reactions33 which is than further reduced to
MeOH. A second possible pathway which cannot be excluded
and is often proposed for the hydrogenation of cyclic
carbonates using pincer catalysts27-29 is the reduction of 7b to
12 via 10.
H
O
HO
O
O
O
[Fe]
[Fe]
HO
CH3OH
O
H
H
H
10
12
OH
9
HO
Pathway II
8b
c) Control experiments
5 mol% 3
5 mol% KOtBu
140 °C, 3 h, THF/iPrOH
H
O
O
HO
OH
+
Eq.1
HO
+
MeOH
62%
O
iPrO
H
10
8b, 85%
11, 18%
5 mol% 3
5 mol% KOtBu
140 °C, 3 h, THF/iPrOH
O
Eq.2
Eq.3
MeOH
97%
iPrO
H
11
5 mol% 3
5 mol% KOtBu
140 °C, 3 h, THF/iPrOH
CH2O
MeOH
98%
n
Scheme 3. Putative reaction pathway and control experiments.
To probe this proposal we studied the conversion of 10 under
our transfer hydrogenation conditions using catalyst
3
(Scheme 3c, Eq. 1). Notably, beside the final products 8b and
MeOH which were obtained in 85% and 62% yield,
respectively, the reaction intermediate 11 was observed in
18% yield. Additionally, we could prove that 11 can be
completely converted under our standard reaction conditions
giving MeOH in 97% yield (Eq. 2). Also formaldehyde (12),
obtained from the depolymerization of paraformaldehyde at
high temperatures, was reduced in the presence of 3, as
proposed, leading to MeOH in an excellent yield of 98% (Eq. 3).
The production of polycarbonate is one the most successful
processes for the conversion of CO2 into value-added products,
which is even performed on industrial scale.1 Due to
environmental concerns, the chemical recycling of waste
polymers e.g. polycarbonate into valuable chemicals is
becoming a hot topic in the field of catalysis.44 To the best of
our knowledge, there are no examples known of
depolymerisation
of
polycarbonates
by
transfer
hydrogenation. We envisioned 3 to be a suitable catalyst for
this reaction. Thus, we converted commercially available
poly(propylene carbonate) (Mn 50·103 g·mol–1) in the
4 | J. Name., 2012, 00, 1-3
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presence of catalyst 3 (Scheme 4). Under the optimized
reaction conditions we obtained propylene glycol and MeOH in
65% and 43% yield, respectively. Notably, the formation of
MeOH and diols was not detected when the well-known
complex 6 was used under base-free conditions (see SI,
Scheme S4). We investigated if the base (KOtBu) had an
additional role beside the deprotonation of 3 to form the
catalytically active species 3a (Scheme 3a). Notably, in the
absences of 3 we observed the depolymerisation of the
poly(propylene carbonate) under our reaction conditions in
the presence of catalytic amounts (5 mol%) KOtBu. The
respective cyclic carbonate 7c and diol 8c were identified as
Experimental
View Article Online
DOI: 10.1039/C9GC02052G
The complexes 1–6 were prepared using standard Schlenk
techniques or a glove box. Complexes 1, 2, 3, 4, 5 and 6 were
prepared according to literature procedures. The analytical
data of the iron complexes are consistent with those
previously reported in the literature.45-48 Carbonate 7b was
obtained from abcr GmbH and used without further
purification. Carbonate 7k was obtained from Sigma-Aldrich
and used without further purification. Carbonates 7a, 7c, 7d,
7e, 7f, 7g, 7h, 7i and 7j were prepared according to our
previously reported protocol. The analytical data were
consistent with those previously reported in the literature. 49
1
products in the H NMR of the reaction mixture (see SI). The
latter might be formed by transesterification e.g. of 7c with
iPrOH. Thus, it can not be ruled out that also under the
reaction conditions free base affects the polymers structure,
namely leads to partial depolymerisation, which in turn
facilitates the transfer hydrogenation of lower molecular
weight fragments.
General procedure for the preparation of carbonates (GP1)
A 45 cm3 autoclave was charged with 5 mol% CaI2, 5 mol%
poly(ethylene glycol) dimethyl ether (PEG DME, Mn ~500
g·mol-1) and the respective epoxide. The reactor was sealed
and charged with 1–10 bar CO2. The reaction mixture was
stirred for 3–34 h at 25–45 °C. Subsequently the reactor was
cooled <20°C with an ice bath and the CO2 was released
slowly. The reaction mixture was filtered over silica gel (SiO2)
with cyclohexane/ethyl acetate. After removal of all volatiles in
vaccuo the respective carbonate 7 was obtained.
O
HO
OH
5 mol% 3, 5 mol% KOtBu
140 °C, 30 h, THF/iPrOH
MeOH
43 %
O
O
+
n
Me
Me
8c, 65 %
1.35 g
Scheme 4. First example on the transfer hydrogenation of polycarbonate.
General procedure for the transfer hydrogenation of cyclic
carbonates 7a–7k using catalyst 3 (GP2)
With respect to green and sustainable chemistry this catalytic
protocol contributes in several ways. It represents an example
of the utilization of CO2 as a C1 building block which may lead
to the production of commodities and fuels (MeOH) from CO2.
Compared to other transfer hydrogenation systems for cyclic
carbonates the present protocol uses a catalyst based on iron
which is an earth-abundant metal. In this protocol, isopropanol
or ethanol (which can be obtained from renewable resources)
is employed as hydrogen sources for the reduction, thus
avoiding the handling of flammable hydrogen under high
pressure. Notably, the oxidized products, namely acetone and
acetaldehyde, respectively, are also valuable products. Thus,
overall no waste is produced in this reaction. The chemical
recycling of plastics become more and more important. In this
respect it is of interest that the reported catalytic system is
also capable of depolymerising commercially available
poly(propylene carbonate) to propylene glycol and methanol.
In a Schlenk vessel, complex 3 (24 mg, 0.050 mmol) and KOtBu
(5.6 mg, 0.050 mmol) were dissolved in THF (1.0 mL) under
argon. The mixture was stirred for
5 min at 23 °C.
Subsequently, the cyclic carbonate 7 (1.0 mmol) and iPrOH
(4.0 mL) were added in one portion. The Schlenk vessel was
placed in a preheated oil bath (140 °C) and the mixture was
stirred for 6–24 h. The reaction mixture was cooled to 23°C
and subsequently cooled to 0°C in an ice bath for 1 h. The
residual H2 was released carefully and the mixture was
analyzed by GC with mesitylene as the internal standard to
determine the MeOH yield. After removal of all volatiles in
vacuo the crude mixture was purified by column
chromatography on silica gel (SiO2) with cyclohexane/ EtOAc as
eluent. After removal of all volatiles in vacuo the respective
diols (8a–8j) were obtained.
General procedure for the transfer hydrogenation of
poly(propylene carbonate) (PPC) (GP3)
Conclusions
In a Schlenk vessel, Complex 3 (24 mg, 0.050 mmol) and KOtBu
(5.6 mg, 0.050 mmol) were dissolved in THF (1.0 mL) under
In conclusion, we have demonstrated the use of an earth-
abundant metal-based catalyst for the transfer hydrogenation
of cyclic carbonates using a PNP pincer-type iron complex.
Various cyclic carbonates were converted into the respective
diols and MeOH. Notably, the reduction of ethylene carbonate
which is an intermediate in Shell’s OMEGA process led to
MeOH in 90% yield. Moreover, this catalytic system proved to
be suitable for the depolymersation of poly(propylene
carbonate) by transfer hydrogenation which is currently under
further investigations in our group.
argon. The mixture was stirred for
5 min at 23 °C.
Subsequently poly(propylene carbonate) (average Mn of
50·103 g·mol–1 by GPC, 1.0 mmol) and iPrOH (4.0 mL) were
added in one portion. The mixture was placed in a preheated
oil bath (140 °C) and stirred for 30 h. The reaction mixture was
cooled to 23°C and subsequently cooled to 0°C in an ice bath
for 1 h. The residual H2 was released carefully and the mixture
was analyzed by GC with mesitylene as the internal standard
to determine the MeOH and diol 8c yield.
4-Ethyl-1, 3-dioxolan-2-one (7a)49
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
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Following GP-1, 1,2-epoxybutane (41.6 g, 28.0 mmol), CaI2 1H NMR (400 MHz, CDCl3) δ= 6.12–5.77 (m, 1H), 5.51 (dq, J=
View Article Online
DOI: 10.1039/C9GC02052G
(1.70 mg, 5.78 mmol), PEG DME (2.88 mg, 5.76 mmol) and CO2 17.1, 0.9 Hz, 1H), 5.44 (dt, J= 10.4, 0.9 Hz, 1H), 5.23–5.05 (m,
(10 bar) were reacted at 25 °C for 24 h. The product 7a (65.5 g, 1H), 4.61 (dd, J= 8.6, 8.1 Hz, 1H), 4.16 (ddd, J= 8.2, 7.4, 0.7 Hz,
564 mmol, 98%) was obtained as a colourless liquid.
1H) ppm. 13C NMR (75 MHz, CDCl3, 25 °C) δ= 154.80, 132.17,
1H NMR (400 MHz, CDCl3) δ= 4.75–4.55 (m, 1H), 4.47 (dd, J= 121.25, 77.37, 69.10 ppm.
8.4, 7.9 Hz, 1H), 4.03 (dd, J= 8.4, 7.0 Hz, 1H), 1.86–1.57 (m,
4-(3-Butenyl)-1,3-dioxolan-2-one (7h)49
2H), 0.96 (t, J= 7.4 Hz, 3H) ppm. 13C NMR (75 MHz, CDCl3, 25
°C) δ= 155.17, 78.06, 69.05, 26.91, 8.47 ppm.
Following GP-1, 1,2-epoxy-5-hexene (1.98 g, 20.2 mmol), CaI2
(300 mg, 1.02 mmol), PEG DME (510 mg, 1.02 mmol) and CO2
(10 bar) were reacted at 25 °C for 24 h. The product 7h (2.70 g,
4-Methyl-1, 3-dioxolan-2-one (7c)49
Following GP-1, 1,2-epoxypropane (1.96 g, 33.7 mmol), CaI2 19.0 mmol, 94%) was obtained as a colourless liquid.
(5.06 mg, 1.72 mmol), PEG DME (861 mg, 1.72 mmol) and CO2 1H NMR (400 MHz, CDCl3) δ= 5.80 (ddt, J= 17.0, 10.2, 6.7 Hz,
(10 bar) were reacted at 25 °C for 24 h. The product 7c (3.24 g, 1H), 5.20–5.00 (m, 2H), 4.82–4.68 (m, 1H), 4.55 (dd, J= 8.5, 7.9
31.7 mmol, 94%) was obtained as a colourless liquid.
Hz, 1H), 4.10 (dd, J= 8.5, 7.2 Hz, 1H), 2.40–2.10 (m, 2H), 2.07–
1H NMR (400 MHz, CDCl3) δ= 4.93–4.73 (m, 1H), 4.53 (ddt, J= 1.87 (m, 1H), 1.79 (dddd, J= 14.0, 8.7, 7.0, 5.2 Hz, 1H) ppm. 13
C
8.4, 7.7, 0.7 Hz, 1H), 3.99 (ddt, J= 8.3, 7.2, 0.7 Hz, 1H), 1.52– NMR (75 MHz, CDCl3, 25 °C) δ= 154.96, 136.07, 116.45, 76.33,
1.32 (m, 3H) ppm. 13C NMR (75 MHz, CDCl3, 25 °C) δ= 155.14, 69.33, 33.07, 28.67 ppm.
73.69, 70.71, 19.32 ppm.
4, 5-Dimethyl-1, 3-dioxolan-2-one (7i)49
4-Butyl-1, 3-dioxolan-2-one (7d)49
Following GP-1, trans-2,3-dimethyloxirane (2.05 g, 28.4 mmol),
Following GP-1, 1,2-epoxyhexane (2.04 g, 20.1 mmol), CaI2 CaI2 (408 mg, 1.3 mmol), PEG DME (694 mg, 1.39 mmol) and
(300 mg, 1.02 mmol), PEG DME (510 mg, 1.02 mmol) and CO2 CO2 (20 bar) were reacted at 70 °C for 24 h. The product 7i
(10 bar) were reacted at 25 °C for 24 h. The product 7d (2.73 g, (2.78 g, 23.9 mmol, 84%, cis/trans = 1.38/1) was obtained as a
19.1 mmol, 94%) was obtained as a colourless liquid.
colourless liquid.
1H NMR (400 MHz, CDCl3) δ= 4.79–4.64 (m, 1H), 4.53 (dd, J= 1H NMR (300 MHz, CDCl3) δ= 4.87-4.77 (m, 2H, cis), 4.36–4.27
8.4, 7.8 Hz, 1H), 4.07 (dd, J= 8.4, 7.2 Hz, 1H), 1.87–1.74 (m, (m, 2H, trans), 1.44–1.38 (m, 6H, trans), 1.35–1.29 (m, 6H, cis)
1H), 1.74–1.61 (m, 1H), 1.50–1.30 (m, 4H), 0.97–0.89 (m, 3H) ppm. 13C NMR (75 MHz, CDCl3, 25 °C) δ= 14.31 (CH3, cis), 18.29
ppm. 13C NMR (75 MHz, CDCl3, 25 °C) δ= 155.15, 77.12, 69.43, (CH3, trans), 76.09 (CH, cis), 79.95 (CH, trans), 154.56 (C=O,
33.53, 26.42, 22.24, 13.79 ppm.
trans), 154.67 (C=O, cis) ppm.
4-Hexyl-1, 3-dioxolan-2-one (7e)49
4, 4-Dimethyl-1, 3-dioxolan-2-one (7j)49
Following GP-1, 1,2-epoxyoctane (2.04 g, 15.9 mmol), CaI2 (230 Following GP-1, 2,2-dimethyloxirane (2.03 g, 28.2 mmol), CaI2
mg, 0.783 mmol), PEG DME (390 mg, 0.780 mmol) and CO2 (10 (408 mg, 1.3 mmol), PEG DME (694 mg, 1.39 mmol) and CO2
bar) were reacted at 25 °C for 24 h. The product 7e (2.71 g, (10 bar) were reacted at 25 °C for 24 h. The product 7j (3.02 g,
15.7 mmol, 98%) was obtained as a colourless liquid.
26.0 mmol, 92%) was obtained as a colourless liquid.
1H NMR (400 MHz, CDCl3) δ= 4.71 (qd, J= 7.5, 5.4 Hz, 1H), 4.60– 1H NMR (300 MHz, CDCl3) δ= 4.17 (s, 2H, CH2), 1.55 (s, 6H, CH3)
4.46 (m, 1H), 4.08 (dd, J= 8.4, 7.2 Hz, 1H), 1.81 (dddd, J= 14.4, ppm. 13C NMR (75 MHz, CDCl3, 25 °C) δ= 154.60, 81.72, 75.39,
9.4, 5.9, 3.7 Hz, 1H), 1.69 (ddt, J= 14.0, 10.7, 5.1 Hz, 1H), 1.55– 53.49, 26.04 ppm.
1.43 (m, 1H), 1.41–1.27 (m, 7H), 0.93–0.86 (m, 3H) ppm. 13C
NMR (75 MHz, CDCl3, 25 °C) δ= 155.12, 77.09, 69.42, 33.88,
1, 2-Butanediol (8a) (Table 2, entry 2)33
31.51, 28.80, 24.33, 22.46, 14.00 ppm.
Following GP-2, complex 3 (24 mg, 0.50 mmol), KOtBu (56mg,
0.50 mmol), THF (5.0 mL), 4-ethyl-1, 3-dioxolan-2-one (7a, 1.16
g, 10.0 mmol) and iPrOH (25 mL) were reacted. After column
4-Phenyl-1, 3-dioxolan-2-one (7f)49
Following GP-1, styrene oxide (1.98 g, 16.5 mmol), CaI2 (245 chromatography (SiO2, cyclohexane/EtOAc = 1:2) the title
mg, 0.834 mmol), PEG DME (414 mg, 0.838 mmol) and CO2 (10 compound 8a (646 mg, 7.17 mmol, 72%) was obtained as a
bar) were reacted at 25 °C for 24 h. The product 7f (2.51 g, colorless oil.
15.3 mmol, 93%) was obtained as a pale yellow liquid.
1H NMR (300 MHz, CDCl3) δ= 4.08 (br s, 2H), 3.64–3.52 (m, 2H),
1H NMR (400 MHz, CDCl3) δ= 7.57–7.33 (m, 5H), 5.76–5.64 (m, 3.48–3.34 (m, 1H), 1.47 (m, 2H), 0.97 (t, 3H) ppm. 13C NMR (75
1H), 4.83 (dd, J= 8.6, 8.2 Hz, 1H), 4.37 (dd, J= 8.6, 7.9 Hz, 1H) MHz, CDCl3, 25 °C) δ= 73.73, 66.31, 25.95, 9.98 ppm.
ppm. 13C NMR (75 MHz, CDCl3, 25 °C) δ= 154.79, 135.78,
1, 2-Propanediol (8c) (Table 2, entry 5)33
129.76, 129.26, 125.86, 77.99, 71.17 ppm.
Following GP-2, complex 3 (23.5 mg, 0.05 mmol), KOtBu (5.6
mg, 0.05 mmol), THF (1.0 mL), 4-Methyl-1, 3-dioxolan-2-one
4-Vinyl-1,3-dioxolan-2-one (7g)49
Following GP-1, butadiene monoxide (2.00 g, 28.5 mmol), CaI2 (7c, 102 mg, 1.0 mmol) and iPrOH (4 mL) were reacted. After
(420 mg, 1.42 mmol), PEG DME (714 mg, 1.42 mmol) and CO2 column chromatography (SiO2, cyclohexane/EtOAc = 1:2) the
(10 bar) were reacted at 25 °C for 24 h. The product 7g (3.08 g, title compound 8c (68.7mg, 0.904 mmol, 91%) was obtained as
27.0 mmol, 95%) was obtained as a colourless liquid.
a colourless oil.
6 | J. Name., 2012, 00, 1-3
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1H NMR (300 MHz, CDCl3) δ= 4.38 (br s, 2H), 3.88–3.79 (m, 1H), After column chromatography (SiO2, cyclohexane/EtOAc = 1:2)
View Article Online
3.56–3.51 (m, 1H), 3.36–3.30 (m, 1H), 1.08 (t, 3H) ppm. 13C the title compound 8h (82.0 mg, 0.707 mmol, 71%) was
DOI: 10.1039/C9GC02052G
NMR (75 MHz, CDCl3, 25 °C) δ= 68.35, 67.97, 18.73 ppm.
obtained as a colourless oil.
1H NMR (300 MHz, CDCl3) δ= 5.75 (ddt, J= 16.8, 10.2, 6.6 Hz,
1H), 5.06–4.84 (m, 2H), 4.11–3.10 (m, 5H), 2.31–1.88 (m, 2H),
1, 2-Hexanediol (8d) (Table 2, entry 6)33
Following GP-2, complex 3 (23.5 mg, 0.05 mmol), KOtBu (5.6 1.43 (t, J= 6.5 Hz, 2H) ppm. 13C NMR (75 MHz, CDCl3, 25 °C) δ=
mg, 0.05 mmol), THF (1.0 mL), 4-butyl-1, 3-dioxolan-2-one (7d, 138.08, 115.00, 71.73, 66.62, 32.14, 29.97 ppm.
144 mg, 1.0 mmol) and iPrOH (4 mL) were reacted. After
2, 3-Butanediol (8i) (Table 2, entry 11)25
column chromatography (SiO2, cyclohexane/EtOAc = 1:2) the
title compound 8d (95.2 mg, 0.807 mmol, 81%) was obtained Following GP-2, complex 3 (23.5 mg, 0.05 mmol), KOtBu (5.6
as a colourless oil.
mg, 0.050 mmol), THF (1.0 mL), 4, 5-dimethyl-1, 3-dioxolan-2-
1H NMR (300 MHz, CDCl3) δ= 3.78–3.27 (m, 5H), 1.47–1.24 (m, one (7i, 142 mg, 1.0 mmol) and iPrOH (4 mL) were reacted.
6H), 0.94–0.83 (m, 3H) ppm. 13C NMR (75 MHz, CDCl3, 25 °C) After column chromatography (SiO2, cyclohexane/EtOAc= 1:3)
δ= 77.37, 66.76, 32.78, 27.75, 22.71, 14.00 ppm
the title compound 8i (77.1 mg, 0.857 mmol, 86%) was
obtained as a colourless oil.
1H NMR (300 MHz, CDCl3) δ= 3.80–3.74 (m, 2H, cis isomer),
1, 2-Octanediol (8e) (Table 2, entry 7)29
Following GP-2, complex 3 (23.5 mg, 0.05 mmol), KOtBu (5.6 3.53–3.48 (m, 2H, trans isomer), 2.57–2.02 (m, 2H), 1.17–1.11
mg, 0.050 mmol), THF (1.0 mL), 4-hexyl-1, 3-dioxolan-2-one (m, 6H) ppm. 13C NMR (75 MHz, CDCl3, 25 °C) δ= 72.2, 70.7,
(7e, 172 mg, 1.0 mmol) and iPrOH (4 mL) were reacted. After 19.0 16.7 ppm.
column chromatography (SiO2, cyclohexane/EtOAc= 1:2) the
title compound 8e (122.3 mg, 0.838 mmol, 84%) was obtained
2-Methyl-1, 2-propanediol (8j) (Table 2, entry 12)25
as a colourless oil.
Following GP-2, complex 3 (23.5 mg, 0.05 mmol), KOtBu (5.6
1H NMR (300 MHz, CDCl3) δ= 3.73 (m, 1H), 3.68 (dd, 1H), 3.46 mg, 0.050 mmol), THF (1.0 mL), 4, 4-dimethyl-1, 3-dioxolan-2-
(dd, 1H), 2.52 (br s, 2H), 1.26–1.48 (m, 10H), 0.87 (t, 1H) ppm. one (7j, 142 mg, 1.0 mmol) and iPrOH (4 mL) were reacted.
13C NMR (75 MHz, CDCl3, 25 °C) δ= 72.37, 66.82, 33.16, 31.76, After column chromatography (SiO2, cyclohexane/EtOAc = 1:2)
29.32, 25.53, 22.60, 14.08 ppm.
the title compound 8j (82.4 mg, 0.915 mmol, 92%) was
obtained as a colourless oil.
1H NMR (300 MHz, CDCl3) δ= 4.18 (br s, 2H), 1.52 (s, 6H) ppm.
1-Phenyl-1, 2-ethanediol (8f) (Table 2, entry 8)33
Following GP-2, complex 3 (23.5 mg, 0.05 mmol), KOtBu (5.6 13C NMR (75 MHz, CDCl3, 25 °C) δ= 154.60, 81.72, 75.39, 53.49,
mg, 0.050 mmol), THF (1.0 mL), 4-Phenyl-1, 3-dioxolan-2-one 26.04 ppm.
(7f, 164 mg, 1.0 mmol) and iPrOH (4 mL) were reacted. After
column chromatography (SiO2, cyclohexane/EtOAc = 1:1) the
title compound 8f (114 mg, 0.827 mmol, 83%) was obtained as
Conflicts of interest
a colourless oil.
There are no conflicts of interest to declare.
1H NMR (300 MHz, CDCl3) δ= 7.38-7.26 (m, 4H), 4.86–4.80 (m,
1H), 3.81–3.73 (m, 1H), 3.70–3.63 (m, 1H), 3.70–3.63 (m, 1H),
2.74–2.58 (m, 1H), 2.30–2.12 (m, 1H) ppm. 13C NMR (75 MHz,
CDCl3, 25 °C) δ= 140.82, 128.56, 128.03, 126.07, 74.70, 68.09
ppm.
Acknowledgements
X. Liu is grateful for the financial support of the Chinese
Scholarship Council (CSC). The authors thank Dr. T. Xia for
providing some of the iron catalysts. The authors gratefully
acknowledge the support from the Leibniz ScienceCampus
But-3-ene-1,2-diol (8g) (Table 2, entry 9)50
Following GP-2, complex 3 (23.5 mg, 0.05 mmol), KOtBu (5.6
mg, 0.050 mmol), THF (1.0 mL), 4-vinyl-1,3-dioxolan-2-one (7g,
114 mg, 1.0 mmol) and iPrOH (4 mL) were reacted. After
column chromatography (SiO2, cyclohexane/EtOAc = 1:2) the
title compound 8g (74.2 mg, 0.843 mmol, 85%) was obtained
as a colourless oil.
1H NMR (300 MHz, CDCl3) δ= 5.78 (ddd, J= 17.3, 10.6, 5.6 Hz,
1H), 5.29 (dt, J= 17.3, 1.5 Hz, 1H), 5.15 (dt, J= 10.6, 1.4 Hz, 1H),
4.29–4.07 (m, 1H), 3.61 (dd, J= 11.4, 3.3 Hz, 1H), 3.43 (dd, J=
11.3, 7.5 Hz, 1H), 2.93 (s, 2H) ppm. 13C NMR (75 MHz, CDCl3, 25
°C) δ= 136.69, 116.75, 73.31, 66.23 ppm.
Phosphorus
rostock.de).
Research
Rostock
(www.sciencecampus-
Notes and references
1. Q. Liu, L. Wu, R. Jackstell and M. Beller, Nature Commun.,
2015, 6, 5933.
2. M. Aresta, A. Dibenedetto and A. Angelini, Chem. Rev., 2014,
114, 1709–1742.
3. J. Artz, T. E. Müller, K. Thenert, J. Kleinekorte, R. Meys, A.
Sternberg, A. Bardow and W. Leitner, Chem. Rev., 2018, 118,
434–504.
4. A. J. Kamphuis, F. Picchioni and P. P. Pescarmona, Green
Chem., 2019, 21, 406–448.
5. M. Aresta, Carbon Dioxide as Chemical Feedstock, Wiley-
VCH, Weinheim, 2010.
6. https://www.methanol.org/
5-Hexene-1,2-diol (8h) (Table 2, entry 10)51
Following GP-2, complex 3 (23.5 mg, 0.05 mmol), KOtBu (5.6
mg, 0.050 mmol), THF (1.0 mL), 4-(3-butenyl)-1,3-dioxolan-2-
one (7h, 142 mg, 1.0 mmol) and iPrOH (4 mL) were reacted.
This journal is © The Royal Society of Chemistry 20xx
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Journal Name
7. K. Weissermel and H. J. Arpe, Industrial Organic Chemistry, 41. S. Werkmeister, K. Junge, B. Wendt, E. Alberico, H. Jiao, W.
View Article Online
4th edn., Wiley-VCH Verlag GmbH & Co. KGaA, 2008.
8. A. Goeppert, M. Czaun, J. P. Jones, G. K. S. Prakash and G. A.
Olah, Chem. Soc. Rev., 2014, 43, 7995–8048.
9. M. Beller, G. Centi and L. Sun, ChemSusChem, 2017, 10, 6–
13.
10. G. A. Olah, Angew. Chem. Int. Ed., 2005, 44, 2636–2639.
Baumann, H. Junge, F. Gallou and MD. OBIe: l1l0e.r1,03A9n/Cge9GwC.0C2h0e5m2G.
Int. Ed., 2014, 53, 8722–8726.
42. T. Werner and H. Büttner, ChemSusChem, 2014, 7, 3268–
3271.
43. H. Büttner, J. Steinbauer and T. Werner, ChemSusChem,
2015, 8, 2655–2669.
11. G. A. Olah, A. Goeppert and G. K. S. Prakash, Beyond Oil and 44. S. Westhues, J. Idel and J. Klankermayer, Sci. Adv., 2018, 4,
Gas: The Methanol Economy, Wiley-VCH Verlag GmbH & Co.
KGaA, 2009.
12. F. L. Liao, Y. Q. Huang, J. W. Ge, W. R. Zheng, K. Tedsree, P.
eaat9669.
45. M. Pena-Lopez, P. Piehl, S. Elangovan, H. Neumann and M.
Beller, Angew. Chem. Int. Ed., 2016, 55, 14967–14971.
Collier, X. L. Hong and S. C. Tsang, Angew. Chem. Int. Ed., 46. K. Junge, B. Wendt, A. Cingolani, A. Spannenberg, Z. H. Wei,
2011, 50, 2162–2165.
H. J. Jiao and M. Beller, Chem. Eur. J., 2018, 24, 1046–1052.
13. W. Wang, S. P. Wang, X. B. Ma and J. L. Gong, Chem. Soc. 47. S. Chakraborty, H. G. Dai, P. Bhattacharya, N. T. Fairweather,
Rev., 2011, 40, 3703–3727.
14. K. Dong, R. Razzaq, Y. Hu and K. Ding, Top. Curr. Chem., 2017,
375, 23.
15. J. Klankermayer, S. Wesselbaum, K. Beydoun and W. Leitner,
Angew. Chem. Int. Ed., 2016, 55, 7296–7343.
M. S. Gibson, J. A. Krause and H. R. Guan, J. Am. Chem. Soc.,
2014, 136, 7869–7872.
48. I. Koehne, T. J. Schmeier, E. A. Bielinski, C. J. Pan, P. O.
Lagaditis, W. H. Bernskoetter, M. K. Takase, C. Wurtele, N.
Hazari and S. Schneider, Inorg. Chem., 2014, 53, 2133–2143.
16. L. Wu, Q. Liu, R. Jackstell and M. Beller, in Carbon Dioxide 49. J. Steinbauer and T. Werner, ChemSusChem, 2017, 10, 3025–
and Organometallics, ed. X.-B. Lu, Springer International
Publishing, Cham, 2016, pp. 279–304.
17. N. M. Rezayee, C. A. Huff and M. S. Sanford, J. Am. Chem.
Soc., 2015, 137, 1028–1031.
18. J. Kothandaraman, A. Goeppert, M. Czaun, G. A. Olah and G.
K. S. Prakash, J. Am. Chem. Soc., 2016, 138, 778–781.
19. S. Kar, R. Sen, A. Goeppert and G. K. S. Prakash, J. Am. Chem.
Soc., 2018, 140, 1580–1583.
3029.
50. S. P. Chavan, K. P. Pawar, C. Praveen and N. B. Patil,
Tetrahedron, 2015, 71, 4213–4218.
51. K. Matsumoto, K. Oohana, M. Hashimoto, K. Usuda, T.
Shimoda, H. Ohshima, Y. Suzuki and T. Togawa, Tetrahedron,
2018, 74, 3981–3988.
20. S. Wesselbaum, V. Moha, M. Meuresch, S. Brosinski, K. M.
Thenert, J. Kothe, T. V. Stein, U. Englert, M. Holscher, J.
Klankermayer and W. Leitner, Chem. Sci., 2015, 6, 693–704.
21. S. Wesselbaum, T. vom Stein, J. Klankermayer and W.
Leitner, Angew. Chem. Int. Ed., 2012, 51, 7499–7502.
22. J. Schneidewind, R. Adam, W. Baumann, R. Jackstell and M.
Beller, Angew. Chem. Int. Ed., 2017, 56, 1890–1893.
23. K. Sayan, A. Goeppert, J. Kothandaraman and G. K. S.
Prakash, ACS Catal., 2017, 7, 6347–6351.
24. E. Balaraman, C. Gunanathan, J. Zhang, L. J. W. Shimon and
D. Milstein, Nature Chem., 2011, 3, 609–614.
25. Z. B. Han, L. C. Rong, J. Wu, L. Zhang, Z. Wang and K. L. Ding,
Angew. Chem. Int. Ed., 2012, 51, 13041–13045.
26. W. G. Reman and E. M. Van Kruchten, Google Patents, 1996.
27. A. Kumar, T. Janes, N. A. Espinosa-Jalapa and D. Milstein,
Angew. Chem. Int. Ed., 2018, 57, 12076–12080.
28. A. Kaithal, M. Holscher and W. Leitner, Angew. Chem. Int.
Ed., 2018, 57, 13449–13453.
29. V. Zubar, Y. Lebedev, L. M. Azofra, L. Cavallo, O. El-Sepelgy
and M. Rueping, Angew. Chem. Int. Ed., 2018, 57, 13439–
13443.
30. S. Werkmeister, J. Neumann, K. Junge and M. Beller, Chem.
Eur. J., 2015, 21, 12226–12250.
31. D. Wang and D. Astruc, Chem. Rev., 2015, 115, 6621–6686.
32. G. A. Filonenko, R. van Putten, E. J. M. Hensen and E. A.
Pidko, Chem. Soc. Rev., 2018, 47, 1459–1483.
33. S. H. Kim and S. H. Hong, ACS Catal., 2014, 4, 3630–3636.
34. S. H. Lee and G. I. Nikonov, Chemcatchem, 2015, 7, 107–113.
35. A. Dubey and E. Khaskin, Acs Catalysis, 2016, 6, 3998–4002.
36. R. A. Farrar-Tobar, B. Wozniak, A. Savini, S. Hinze, S. Tin and
J. G. de Vries, Angew. Chem. Int. Ed., 2019, 58, 1129–1133.
37. H. Büttner, L. Longwitz, J. Steinbauer, C. Wulf and T. Werner,
Top. Curr. Chem., 2017, 375.
38. R. R. Shaikh, S. Pornpraprom and V. D'Elia, Acs Catalysis,
2018, 8, 419–450.
39. H. Büttner, J. Steinbauer, C. Wulf, M. Dindaroglu, H. G.
Schmalz and T. Werner, ChemSusChem, 2017, 10, 1076-1079.
40. J. Steinbauer, A. Spannenberg and T. Werner, Green Chem.,
2017, 19, 3769–3779.
8 | J. Name., 2012, 00, 1-3
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