Indirect reduction of CO2and recycling of polymers by manganese-catalyzed transfer hydrogenation of amides, carbamates, urea derivatives, and polyurethanes

Article abstract of DOI:10.1039/d1sc02663a

The reduction of polar bonds, in particular carbonyl groups, is of fundamental importance in organic chemistry and biology. Herein, we report a manganese pincer complex as a versatile catalyst for the transfer hydrogenation of amides, carbamates, urea derivatives, and even polyurethanes leading to the corresponding alcohols, amines, and methanol as products. Since these compound classes can be prepared using CO₂as a C1 building block the reported reaction represents an approach to the indirect reduction of CO₂. Notably, these are the first examples on the reduction of carbamates and urea derivatives as well as on the C-N bond cleavage in amides by transfer hydrogenation. The general applicability of this methodology is highlighted by the successful reduction of 12 urea derivatives, 26 carbamates and 11 amides. The corresponding amines, alcohols and methanol were obtained in good to excellent yields up to 97%. Furthermore, polyurethanes were successfully converted which represents a viable strategy towards a circular economy. Based on control experiments and the observed intermediates a feasible mechanism is proposed.

Full text of DOI:10.1039/d1sc02663a

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Indirect reduction of CO and recycling of
2
polymers by manganese-catalyzed transfer
Cite this: Chem. Sci., 2021, 12, 10590
hydrogenation of amides, carbamates, urea
derivatives, and polyurethanes†
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Xin Liu and Thomas Werner *ab
a
The reduction of polar bonds, in particular carbonyl groups, is of fundamental importance in organic
chemistry and biology. Herein, we report a manganese pincer complex as a versatile catalyst for the
transfer hydrogenation of amides, carbamates, urea derivatives, and even polyurethanes leading to the
corresponding alcohols, amines, and methanol as products. Since these compound classes can be
2
prepared using CO as a C1 building block the reported reaction represents an approach to the indirect
reduction of CO . Notably, these are the first examples on the reduction of carbamates and urea
2
derivatives as well as on the C–N bond cleavage in amides by transfer hydrogenation. The general
applicability of this methodology is highlighted by the successful reduction of 12 urea derivatives, 26
carbamates and 11 amides. The corresponding amines, alcohols and methanol were obtained in good to
excellent yields up to 97%. Furthermore, polyurethanes were successfully converted which represents
a viable strategy towards a circular economy. Based on control experiments and the observed
intermediates a feasible mechanism is proposed.
Received 14th May 2021
Accepted 28th June 2021
DOI: 10.1039/d1sc02663a
rsc.li/chemical-science
2
(
(
Fig. 1a). This can be ascribed to resonance stabilization
Fig. 1b). Amide resonance is leading to the delocalization of the
Introduction
The hydrogenation and transfer hydrogenation of polar bonds, nitrogen electronic lone pair which lowers the reactivity of the
37
in particular carbonyl groups, has attracted great attention carbonyl group (Fig. 1b, I). Also, intermolecular hydrogen
1–7
during the past four decades. These reactions are of synthetic bonding between amide groups can increase their stability
38
signicance as they represent an environmentally benign additionally (Fig. 1b, II). In comparison with amides, carba-
approach to fundamental synthetic building blocks such as mates and urea derivatives are even less reactive, mainly due to
8
–10
alcohols and amines.
the (transfer) hydrogenation of ketones
as well as of more challenging substrates such as esters
Signicant progress has been made in the additional resonance stabilization by the second oxygen or
11–14
15–18
39
and aldehydes
nitrogen atom, respectively (Fig. 1b, III and IV).
Numerous procedures have been developed for the reduc-
1
9–21
and
22–24
40
amides.
However, the (transfer) hydrogenation of organic tion of amide-related substrates, e.g. hydrogenation, hydro-
urea derivatives and carbamates as well as the C–N bond silylation
41–43 44,45
and hydroborylation.
Among them,
cleavage in amides remain a major challenge. Indeed, to the hydrogenation methods stand out as a green approach. During
best of our knowledge, catalytic transfer hydrogenations of the past decade, hydrogenation of amides through either C–N
these important compound classes have not been reported so or C–O bond cleavage were widely studied. Surprisingly, only
2
5–27
28–30
31–33
far. Notably, amides,
carbamates,
urea derivatives
as a few catalytic systems based on ruthenium have been reported
34,35
36,46,47
well as polyurethanes
hydrogenation offers a facile approach to the indirect reduction tives,
can be produced from CO , and their for the hydrogenation for carbamates,
and urea deriva-
respectively. Most recently, the group of Milstein re-
2
48–51
3
6
of CO2 to methanol. However, compared to other carbonyl ported a catalytic system based on earth abundant manganese
52
derivatives these compounds are less reactive towards hydro- for the hydrogenation for these compound classes. Direct
genation and nucleophilic attack to the carbonyl group hydrogenation and transfer hydrogenation are two parallel
53,54
strategies for the reduction of carbonyl compounds.
a
Complementary to the direct hydrogenation, transfer hydroge-
nation allows reductions to proceed without the need of pres-
Leibniz-Institute for Catalysis, Albert-Einstein-Str. 29a, 18059 Rostock, Germany.
E-mail: thomas.werner@catalysis.de
b
2
Department of Chemistry, Paderborn University, Warburger Str. 100, 33098 surized H and special experimental settings. However, this
5
5–57
Paderborn, Germany
strategy is mainly limited to the reduction of ketones
esters.
and
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d1sc02663a
21,58–60
Most recently, signicant contributions were made
1
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Table 1 Catalyst and solvent screening for the manganese-catalyzed
a
transfer hydrogenation of 1,3-diphenylurea (1a)
Entry
[Mn]
Co-solvent
2a/%
3a/%
4a/%
1
2
3
4
5
6
7
8
Mn–I
—
—
—
—
—
2 2
CH Cl
THF
Toluene
1,4-Dioxane
54
47
52
71
46
39
78
91
73
46
19
7
—
45
—
5
64
89
60
9
63
77
85
54
90
35
19
—
14
73
Mn–II
Mn–III
Mn–IV
—
Mn–IV
Mn–IV
Mn–IV
Mn–IV
Mn–IV
b
b
b
b
9
c
b
10
Toluene
a
Reaction conditions: 1a (0.5 mmol), catalyst [Mn] (2 mol%), KOtBu
ꢀ
(6 mol%), iPrOH (2 mL), 120 C, 16 h. Yields were determined by GC
b
using mesitylene as the internal standard. iPrOH (1 mL) and
cosolvent (1 mL). 100 C.
Fig. 1 (a) Trends in carbonyl reactivity. (b) Resonance stabilization. (c)
Manganese-catalyzed transfer hydrogenation of amides, carbamates,
urea derivatives, and polyurethanes reported in this work.
c
ꢀ
manganese complex Mn–IV was found to be the most efficient
catalyst (entry 4). In this case the amine 2a was obtained in 71%
yield along with carbamate 4a in 54% and methanol in 45%
yield, respectively. The formation of methanol clearly indicates
that a transfer hydrogenation takes place. In the absence of
catalyst Mn–IV the formation of methanol was not observed and
the products 2a and 4a were obtained in 46% and 90% yield,
respectively (entry 5). Other catalysts based on earth-abundant
Co and Fe proved to be less efficient (for details, see ESI, Table
S1†). Our group recently reported that cosolvents can increase
6
1–63
in the transfer hydrogenation of amides.
Klankermayer and
co-workers reported an efficient Ru-catalyzed transfer hydroge-
nation of cyclic amides to cyclic tertiary amines, while the
groups of Xu, Fan and Xiao reported the Ru- and B(C F ) -
61
6
5 3
62,63
catalyzed reduction of aliphatic amides to amines.
Notably,
in all cases a selective C–O cleavage was observed over the
possible C–N bond cleavage. Despite these advancements,
methodologies to reduce carbamates and urea derivatives by
transfer hydrogenation have not been reported. Based on our
interest in the transfer hydrogenation
hydrogen reactions, we envisioned to overcome this limitation
by using earth-abundant metal catalysts. Herein, we report the
64
the efficiency and selectivity of a hydrogen transfer reaction.
64,65
and borrowing
Therefore, different solvents were evaluated as cosolvent for the
reduction of 1a in the presence of catalyst Mn–IV (entries 6–9).
An excellent yield of 2a (91%) and methanol 3a (89%) were
obtained and no side product 4a was detected when toluene was
used. Lower yields of both products were detected when the
66

rst examples of the transfer hydrogenation of carbamates, urea
derivatives and amides under C–N bonds cleavage (Fig. 1c).
Even polyurethanes can be reduced to the corresponding diols,
amines and methanol, realizing a transfer hydrogenative
degradation of commercial polyurethane materials into valu-
able products.
ꢀ
reaction temperature was reduced to 100 C (entry 10). Other
hydrogen donors, different bases, variation of the catalyst
amount and reaction temperatures were also studied but did
not lead to an improvement (for details, see ESI, Tables S2–S5†).
Results and discussion
Substrate scope of urea derivatives
Investigation of reaction conditions
Having the optimized reaction conditions in hand, we explored
We began our studies by investigating the catalytic activities of the scope of the manganese-catalyzed transfer hydrogenation of
67–73
a series of well-known manganese complexes
in the transfer different urea derivatives (Table 2). Under standard reaction
hydrogenation of 1,3-diphenylurea (1a) (Table 1). These reac- conditions symmetrically substituted urea derivatives 1a–1e
tions were performed in the presence of 2 mol% manganese were converted to desired amines 2a–2e in up to 91% and
complexes Mn–I–IV and 6 mol% of KOtBu (entries 1–4). We methanol in up to 89% yield. The conversion of 1a was also
chose isopropanol (iPrOH) which is oen used in transfer performed on a 50 mmol scale leading to desired products,
hydrogenation reactions as solvent and hydrogen donor. The aniline (2a) and methanol (3a) in 84% and 76%, respectively.
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Table 2 Substrate scope of urea derivatives 1 in the manganese-
a
catalyzed transfer hydrogenation
Fig. 2 Kinetic profile for the transfer hydrogenation of 1a. Reaction
conditions: 1a (0.5 mmol), Mn–IV (2 mol%), KOtBu (6 mol%), toluene
ꢀ
(
1 mL), iPrOH (1 mL), 120 C, 16 h. The yield was determined by GC
using mesitylene as the internal standard.
Subsequently, a series of control experiments were per-
formed (Fig. 3). In the absence of catalysts Mn–IV substrate 1a
was fully converted in the presence of catalytic amounts of
KOtBu (6 mol%) yielding the carbamate 4a in 90% along with
a
Reaction conditions: substrates 1a–1l (0.5 mmol), Mn–IV (2 mol%),
ꢀ
KOtBu (6 mol%), toluene (1 mL), iPrOH (1 mL), 120 C, 16 h. The the amine 2a in an expectable yield of 46% (Fig. 3a). Notably, the
yield was determined by GC using mesitylene as the internal
synthesis of carbamates from urea derivatives and alcohols
b
standard. Isolated yields are given in brackets. 50 mmol 1a, 30 h.
c
typically requires high reaction temperatures and metal cata-
3
6 h.
74–76
lysts.
To the best of our knowledge, a base-promoted
conversion of urea derivatives to carbamates has not been re-
ported so far. Other bases gave only poor or no conversion (for
Unsymmetric urea derivatives 1f and 1g were converted to the details, see ESI, Scheme S1†). In the absence of KOtBu no
respective amines 2a, 2b and 2f, in up to 97% yield. Notably, conversion of 1a was observed which indicates again that
also trisubstituted urea derivatives 1h–1j as well as sterically carbamate 4a is a reaction intermediate. If hydrogen (5 bar) is
hindered tetrasubstituted substrates 1k and 1l were selectively used instead of the hydrogen donor isopropanol no conversion
converted to methanol and the corresponding amines 2a, 2g–2j
in good yields. Eventhough for the full conversion of 1k and 1l
the reaction time had to be extended to 36 h.
Mechanism study
Several control experiments were performed to shed light on the
mechanism. Firstly, we monitored the conversion of the model
substrate 1a under the standard reaction conditions (Fig. 2).
The kinetic prole of the Mn–IV catalyzed transfer hydro-
genation of 1a indicates that 1a is converted to 4a. Simulta-
neously, the formation of the amine 2a and methanol (3a) is
observed. The yield of carbamate 4a reaches a maximum of 71%
aer 2.5 h. Subsequently, 4a is reduced to the amine 2a and
methanol (3a). These results indicate that carbamate 4a is most
likely a reaction intermediate in the transfer hydrogenation of
urea derivatives. If carbamate 4a is converted under the stan-
dard reaction conditions full conversion is achieved aer 12 h
(
for details, see ESI, Fig. S2†). The kinetic prole of this reaction
Fig. 3 Control experiments for the conversion of 1a and possible
a
b
also shows the simultaneous formation of amine 2a and
methanol (3a).
intermediates 4a and 5a. Result from Table 1, entry 8.
2
H (5 bar) was
used instead of iPrOH.
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was achieved which supports that indeed a transfer hydroge- Table 3 Scope of carbamates 4 for the manganese-catalyzed transfer
a
hydrogenation
nation takes place under the standard conditions. In addition,
the reaction of the proposed intermediate 4a under the stan-
dard reaction conditions, yields the corresponding amine 2a
and methanol (3a) in excellent yields of 92% and 84%, respec-
tively (Fig. 3b). formanilide (5a) was proposed by Milstein as an
intermediate in the hydrogenation of carbamates and urea
52
derivatives. The Formamide 5a was not observed under the
standard conditions, but it was detected at lower base loading
ꢀ
(
(
4 mol% KOtBu), temperature (110 C) and short reaction time
3 h) by GC and GCMS (for details, see ESI, Fig. S6†). Interme-
diate 5a was also successfully converted to 2a and methanol (3a)
under our standard conditions (Fig. 3b and ESI, Fig. S3†).
36,48,52
Considering these results and previous reports,
we
propose the catalytic cycle shown in Fig. 4. Initially, the pre-
catalyst Mn–IV is deprotonated by KOtBu to afford the
complex Mn–IV-1. Subsequently, dehydrogenation of iPrOH by
Mn–IV-1 leads to complex Mn–IV-2. Hydrogen transfer from
complex Mn–IV-2 to the C]O group of the carbamate inter-
mediate 4a furnishes Mn–IV-3. The formation of 4a from 1a and
iPrOH was demonstrated to be a catalytic process, and KOtBu
has been proven as an efficient catalyst for this reaction (see
above). Elimination of iPrOH, from complex Mn–IV-3 leads to
Mn–IV-4 by metal–ligand cooperation. The release of form-
amide 5a regenerates catalyst Mn–IV-1. The reaction of 5a with
Mn–IV-2 gives complex Mn–IV-5, which eliminates the nal
product, amine 2 and Mn–IV-6. Finally, formaldehyde (6) can be
reduced to methanol (3a).
Substrate scope of carbamates
Encouraged by these results, we sought to extend the substrate
scope to carbamates 4 (Table 3). Thus, carbamates bearing
heteroaromatic substituents 4b–4d were successfully reduced to
amines and methanol in excellent yields of up to 94% and 88%
a
Reaction conditions: substrates 4b–4q (0.5 mmol), Mn–IV (2 mol%),
ꢀ
KOtBu (6 mol%), toluene (1 mL), iPrOH (1 mL), 120 C, 16 h. Yield
was determined by GC using mesitylene as the internal standard.
b
c
d
ꢀ
5
0 mmol 4b, 30 h. Isolated yield. 140 C, 16 h.
respectively. In an upscaling experiment 50 mmol of 4b were
converted to 2a and methanol in 84% and 76%, respectively.
Additionally, carbamates with different functional groups have
been tested under standard conditions (for details, see ESI,
Table S12†). Carbamates bearing aliphatic and benzylic
substituents 4e–4h were also converted to the desired products
in yields up to 80%. The conversion of sterically more
demanding disubstituted carbamates 4i–4n also proceeded
smoothly leading to the corresponding amines and methanol in
yields up to 83% and 76%, respectively. Finally, we studied the
reaction of carbamates with different alkoxy groups (4o–4q).
The respective amines, alcohols and methanol were obtained in
good yield even though in some cases the reaction temperature
ꢀ
had to be increased to 140 C.
Fig. 4 Proposed mechanism for the transfer hydrogenation of urea
derivatives by catalyst Mn–IV.
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Table 4 Scope of the N-Boc deprotection by manganese-catalyzed Table 5 Scope of amides 5 for the manganese-catalyzed transfer
a
a
transfer hydrogenation
hydrogenation
a
Reaction conditions: substrates 4r–4v (0.5 mmol), Mn–IV (2 mol%),
ꢀ
KOtBu (6 mol%), toluene (1 mL), iPrOH (1 mL), 140 C, 16 h. The
yield was determined by GC using mesitylene as the internal
b
1
standard. The yield was determined by H-NMR using mesitylene as
c
a
the internal standard. Isolated yield.
Reaction conditions: substrate 5a–5i (0.5 mmol), Mn–IV (2 mol%),
ꢀ
KOtBu (6 mol%), toluene (1 mL), iPrOH (1 mL), 100 C, 12 h. The
yield was determined by GC using mesitylene as the internal
b
c
ꢀ
standard. Isolated yield. 120 C, 12 h.
The tert-butyloxycarbonyl protecting group (Boc) is
frequently used in organic synthesis to protect amines as their
corresponding carbamates. The Boc group is stable to nucleo-
philic reagents, hydrogenolysis and base hydrolysis. Traditional
approaches for N-Boc deprotection are based on Brønsted
formamide derivatives 5c–5e were successfully converted to the
respective amines yielding the desired products in up to 73%,
while the disubstituted formamides 5f and 5g gave the corre-
sponding products in 81% and 72% yield. To further extend the
substrate scope, we evaluated the reaction of benzanilide (5h)
and N-methylphenyl benzamide (5i). Under our standard reac-
tion conditions C–N bond cleavage also occurred, leading to the
corresponding amines and benzyl alcohol (3f) in moderate
yields up to 51%.
77
acids. We envisioned that our method might be suitable for
the deprotection of N-Boc protected amines under transfer
hydrogenation conditions. However, the reduction of tert-butyl
phenylcarbamate (4r) gives low yields of the corresponding
products under our standard conditions (for details, see ESI,
Table S7†). To promote the reaction, we then evaluated the
ꢀ
effect of temperature. By increasing the temperature to 140 C
good yields of aniline (2a), methanol (3a) and tert-butyl alcohol
(3e) were observed (78%, 70% and 71%, respectively). Next, the
Recycling of polyurethanes to valuable products
scope for the conversion of N-Boc protected amines 4s–4v was
explored (Table 4). The desired amines were obtained in yields
up to 81% indicating the feasibility of this alternative depro-
tection approach.
Polyurethanes are the 6th most used class of polymers world-
wide with an annual production of 18 million tons per year.
78
The application of polyurethanes ranges from foams, varnishes
and adhesives to insulation materials. Notably, polyurethanes
34
can also be prepared from CO
2
.
Substrate scope of amides
In order to establish a circular economy, the development of
To the best of our knowledge, so far there is no precedence in new recycling methods is of particular interest. Most recently,
the literature on C–N bond cleavage in amides via transfer Milstein et al. and Schaub et al. demonstrated that the hydro-
79,80
hydrogenation. Encouraged by the above-mentioned observa- genation of polyurethane is possible,
and valuable chem-
tion of formamide 5a as an intermediate of the reduction of icals such as diols, diamines and methanol can be obtained.
urea derivative 1a as well as the possibility of efficiently Thus, this new strategy was extended to the transfer hydroge-
reducing 5a to the corresponding amine 2a and methanol under nation of polyurethanes (Fig. 5). Under the optimized reaction
our standard conditions (Fig. 3b), the general potential of this conditions (for details, see ESI, Table S8†), commercially avail-
reaction was evaluated. Various amides 5 in the presence of able polyurethane 7a was successfully reduced to the corre-
catalyst Mn–IV (2 mol%) and KOtBu (6 mol%) were converted sponding amine 8a, diol 9a and methanol in moderate yields of
(
1
Table 5). Notably, in these cases a reaction temperature of 65%, 52% and 47%, respectively. Furthermore, polyurethanes
00 C was sufficient (for details, see ESI, Table S6†). Aromatic 7b and 7c were also depolymerized smoothly. In the case of the
ꢀ
formamides 5a and 5b gave excellent yields of 91% and 86%, depolymerisation of 7b the diamine 8b and diol 9b were ob-
respectively on the corresponding amine. The (cyclo-)aliphatic tained in isolated yields of 41 and 34%, respectively.
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Fig. 5 First example on the transfer hydrogenation of polyurethanes 7 (0.5 mmol of substrate was used according to the repeating unit of
a
79
b
polyurethanes). Molecular weight could not be determined. Isolated yield.
Research. We also thank Libo Yao, Dr Lars Longwitz, Dr Wu Li
and Dr Duo Wei as well as Jan T ¨o njes for helpful discussions.
Conclusions
The reported method is a highly efficient transfer hydrogena-
tion protocol for the reduction of amides, carbamates, urea
derivatives and even polyurethane using a pincer catalyst based Notes and references
on earth-abundant manganese and catalytic amounts of KOtBu.
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