Catalytic Hydrogenation of Urea Derivatives and Polyureas

Article abstract of DOI:10.1002/ejoc.202100775

We present herein the catalytic hydrogenation of various urea derivatives to amines and methanol. The reaction is catalyzed by a ruthenium or an iridium Macho pincer complex and produces amine and methanol in very good to excellent yields. Moreover, we also expand this concept to demonstrate the first example of the hydrogenative depolymerization of polyureas to produce diamines and methanol in moderate yields.

Full text of DOI:10.1002/ejoc.202100775

Communications
doi.org/10.1002/ejoc.202100775
Very Important Paper
Catalytic Hydrogenation of Urea Derivatives and Polyureas
[a]
[a]
Amit Kumar* and James Luk
We present herein the catalytic hydrogenation of various urea
derivatives to amines and methanol. The reaction is catalyzed
by a ruthenium or an iridium Macho pincer complex and
produces amine and methanol in very good to excellent yields.
Moreover, we also expand this concept to demonstrate the first
example of the hydrogenative depolymerization of polyureas to
produce diamines and methanol in moderate yields.
The development of a cost-effective and sustainable integrated
approach for the capture of CO and its conversion to methanol
2
lies at the heart of the methanol economy as proposed by Olah
[1]
and Prakash. The dominant CO capture technology involves
2
the reaction of CO with amines to form carbamate or urea
2
derivatives. Efficient hydrogenation of such carbamate or urea
derivatives to methanol with the regeneration of amines can
present alternative technology for the transformation of CO to
2
methanol. However, the hydrogenation of urea derivatives is
the most challenging of all carbonyl bonds due to their low
Scheme 1. Previously reported catalysts for the hydrogenation of urea
derivatives and the work described herein on the catalytic hydrogenation of
urea derivatives and polyureas.
[2]
polarizability. As a matter of fact, urea derivatives have been
[3,4]
used as solvents in hydrogenation reactions.
Till date, only
two catalysts (1, 2 Scheme 1), both from the Milstein group
have been reported for hydrogenation of urea derivatives with
[
5–7]
[8]
broad substrate scopes.
Prakash,
Klankermayer have also utilized ruthenium-based catalysts (3,
Scheme 1) for the hydrogenation of urea derivatives. How-
and Leitner and
scarce. For example, Deng has reported degradation of
polyureas by reacting them with urea and alcohol in the
[9]
[
13]
4
presence of a CuOÀ ZnO catalyst to form dicarbamates.
Matsumoto has utilized supercritical CO to hydrolyse polyureas
ever, only a single substrate (diphenyl urea) has been hydro-
genated using catalysts 3, and 4. Considering the significance
and difficulty level associated with this reaction, it is important
to explore new catalysts for the efficient hydrogenation of urea
derivatives to methanol and amines.
2
[14]
to form amines. Although the approach of catalytic hydro-
genation has been utilized for the depolymerisation of various
[
15–17]
[15,16,18–20]
plastics such as polyesters,
polycarbonates,
there has been no report
[21,22]
[21–23]
nylons,
and polyurethanes,
Additionally, the expansion of this concept to the hydro-
genative depolymerisation of polyureas can present a new
on the hydrogenative depolymerisation of polyureas despite a
significant output in the area of homogeneous catalytic
hydrogenation.
[10]
[24–30]
technology for the chemical recycling of polyureas. We have
recently discovered a new methodology for the synthesis of a
broad range of polyureas from the ruthenium catalysed
We started our investigation by optimizing catalytic con-
ditions for the hydrogenation of diphenylurea. The hydro-
genation reaction was studied using catalysts 5–10 (2 mol%) in
[11]
dehydrogenative coupling of diamines and methanol. Devel-
opment of the reverse reaction i.e. hydrogenative depolymer-
isation of polyureas to methanol and diamines will therefore
enable the circular economy of recycling of polyureas. Although
several methods (e.g. pyrolysis, solvolysis, aminolysis, and
t
the presence of KO Bu (4 mol%) at 50 bar of H and 130°C in
2
1
THF. The analysis of the reaction outcome by the H NMR
spectroscopy, and GC-MS revealed that the best yield (85%
yield of aniline, and 80% yield of methanol) was observed using
the iridium-Macho pincer catalyst 9 (Table 1, entry 5). N-meth-
ylaniline was obtained as a side-product in a ~15% yield. Ru-
Macho pincer complex 5 also resulted in a similar yield of
aniline (80%, entry 1), and methanol (72%). No reaction was
observed using Gusev’s Ru-SNS catalyst 6 under this catalytic
condition (entry 2). Interestingly, hydrogenation of diphenyl
urea (1 mmol) using the non-pincer catalyst 7 resulted in a high
yield of aniline (70%, 1.4 mmol), however, poor selectivity
towards methanol (~40%, 0.4 mmol) was observed with the
[12]
glycolysis) have been investigated for the chemical recycling
of plastics, reports on chemical recycling of polyureas are
[
a] Dr. A. Kumar, J. Luk
School of Chemistry, University of St. Andrews,
North Haugh, St. Andrews, KY169ST, UK
E-mail: ak336@st-andrews.ac.uk
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejoc.202100775
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concomitant formation of N-methyl aniline in ~60% yield
~0.6 mmol, entry 3). Remarkably, the Milstein’s RuPNN catalyst
a metal-catalyst, entry 14) or just in the presence of the
(
complex 5 (in the absence of a base, entry 15) suggesting that
t
8
resulted in the formation of aniline and methanol in 80%, and
both a metal complex (e.g. 5, 7–9) and a base (e.g. KO Bu) are
7
5% yields respectively (entry 4). No reaction was obtained in
essential for the catalysis.
the case of Ru(PPh ) (H)(Cl)(CO) (10, entry 6) suggestive of the
Upon optimization of catalytic conditions for the hydro-
genation of diphenyl urea, we employed this methodology for
the hydrogenation of other urea derivatives. Under the catalytic
combination of 2 mol% of complex 9, and 4 mol% KO Bu, a
wide range of urea derivatives were hydrogenated under 50 bar
3
3
important role of metal-ligand cooperation. Changing the
solvent from THF to toluene, using complex 5 resulted in a
lower yield presumably because of the lower solubility of
diphenylurea in toluene (entry 7). Using 1,4-dioxane or anisole
as solvents resulted in relatively lower yields of methanol and
aniline in comparison to that of THF (Table 1, entry 8, 9). A
much lower selectivity of methanol was obtained when KOH
was used as a base with the concomitant formation of the by-
product N-methylaniline (entry 10), whereas good yields of
t
of H2 (130°C, 24 h) in THF to produce methanol and the
corresponding amine in very good to excellent yields (Table 2,
entries 1–8). N-methyl or N-formyl amines were also detected
1
by the GC-MS and the H NMR spectroscopy as minor side-
products. This explains the slightly lower yield of methanol in
comparison to the corresponding amines (Scheme 2). No
conversion of cyclic ureas – N,N’-trimethyleneurea (entry 9), and
1,3-dimethyl-2-imidazolidinone (entry 10) were observed under
the reaction conditions.
We then attempted to utilize this method for the hydro-
genative depolymerisation of polyureas. Gratifyingly, under the
analogous conditions used for the hydrogenation of urea
aniline and methanol were obtained in the case of K PO4
3
(entry 11). Interestingly, lowering the base loading to 2 mol%
while keeping the remaining conditions the same, resulted in a
lower yield of methanol and aniline (entry 12). This is suggestive
of a dual role of base: (a) to generate the coordinatively
unsaturated ruthenium complex (catalytically active species) by
the NÀ H deprotonation and concomitant abstraction of chloride
ligand from the precatalyst 5, and (b) to assist in the hydro-
genation process by enabling facile decomposition of a hemi-
aminal intermediate as suggested earlier for the hydrogenation
derivatives (Table 2), polyurea PU1 (M =5500) was depolymer-
n
ized to produce 40% yield of 4,7,10-trioxa-1,13-tridecanedi-
amine, and 27% yield of methanol (Table 3, entry 1). The use of
anisole as a solvent resulted in a lower yield of diamine, and
methanol whereas a higher yield was obtained in the case of
DMSO. Moreover, increasing the reaction time to 72 h in THF
also increased the yield of diamine and methanol to 60%, and
41% respectively (Table 3, entry 1). Utilizing the ruthenium
analogue catalyst 5, resulted in a slightly lower yield of diamine
[22]
of amides.
Moreover, increasing the base loading to 10 mol% resulted
in a poor selectivity of methanol (20%), with the remaining
product observed as the N-methyl aniline (entry 13). No
conversion of diphenyl urea was observed when the catalysis
t
was performed just in the presence of KO Bu (in the absence of
Amit Kumar completed his Integrated M.Sc.
Chemistry degree (2007–2012) at the Indian
Institute of Technology (IIT), Roorkee where
he received several research fellowships and
awards (Indian Academy of Science, DAAD –
Germany, IIT-ParisTech, KVPY & INSPIRE from
the Govt of India) along with the Institute
Silver Medal. He then won the Rhodes Schol-
arship and pursued his DPhil (2012–2016)
under the supervision of Prof. Andrew Weller
at the University of Oxford, UK. Upon com-
pletion of his DPhil, Amit received the PBC
fellowship (Planning & Budgeting Committee,
Israel) to carry his postdoctoral research with
Prof. David Milstein at the Weizmann Institute
of Science, Israel where he was promoted to
be a Senior Postdoctoral Fellow in 2019. Amit
was awarded the FGS (Feinberg Graduate
School) Prize for the outstanding achieve-
ments in postdoctoral research 2018 by the
Weizmann Institute of Science, Israel. In Jan
James Luk is a 4th-year MChem student at the
University of St Andrews. He has achieved the
Dean’s list in 2018/19, 2019/20, and 2020/21
academic years. His research interests are
homogeneous catalysis and green chemistry.
James plays football for the University of St
Andrews Men’s 5th team. He is also grade 8 at
saxophone and guitar and has led and
participated in many bands and ensembles,
including the Hertfordshire County Wind
Sinfonia.
2
020, Amit started his independent academic
career as a Leverhulme Trust Early Career
Researcher at the School of Chemistry, Univer-
sity of St. Andrews. His research interests are
organometallic catalysis, energy storage, and
circular chemistry.
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[a]
Table 1. Optimization of catalytic conditions for the hydrogenation of
diphenylurea.
Table 2. Substrate scope for the hydrogenation of urea derivatives.
[
a]
[
a] Catalytic conditions: diphenylurea (1 mmol), complex 5–10
[b]
(0.02 mmol), base (0.04 mmol), solvent (2 mL), H
2
(50 bar), 130°C, 24 h.
t
t
t
0
.02 mmol KO Bu was used.[c] 0.1 mmol KO Bu was used. [d] No KO Bu
t
[
a] Catalytic conditions: urea derivative (1 mmol), 9 (0.02 mmol), KO Bu
was used. [e] No metal-complex was used. Products were detected by the
°
(
0.04 mmol), THF (2 mL), H
2
(50 bar), 130 C, 24 h. [b] Combined yield of
1
GC-MS and their yields were calculated by the H NMR spectroscopy using
both the amine. Products were detected by the GC-MS and their yields
were calculated by the H NMR spectroscopy using 1,1’-diphenylethene as
1
,1’-diphenylethene as the internal standard. The conversion was
1
determined by the GC-MS.
the internal standard. The conversion was determined using the GC-MS.
and methanol under the same catalytic conditions (entry 2).
obtained when a control experiment was conducted without
adding a catalyst (e.g. 9 and KO Bu) while keeping the
t
Hydrogenation of polyurea PU2 (M =2925) was not successful
n
using either catalyst 9 or 5 and no methanol or p-xylenedi-
amine was detected after the reaction time (entries 3,4, Table 3).
PU2 was completely recovered after completion of the reaction
remaining condition the same as described in Table 3 (reaction
time 24 h) confirming the necessity of external catalyst.
Having demonstrated the hydrogenation of urea derivatives
and polyureas, we carried some mechanistic investigations to
understand the reaction pathway for the hydrogenation
reaction. Analysis of the reaction mixture after hydrogenation of
urea derivatives from Table 2 showed the presence of N-
time. Under similar catalytic conditions, polyurea PU3 (M =
n
4
498) was also hydrogenated to produce 4,4’-methylenebis
cyclohexylamine) and methanol in up to 51%, and 43% yields
respectively (Table 3, entry 5). No conversion of PU1 was
(
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to form another equivalent of amine and formaldehyde.
Subsequent hydrogenation of formaldehyde produces metha-
nol. The side-product N-methylamine could result from the
dehydration of hemiaminal intermediate to form an imine
followed by subsequent hydrogenation to produce N-meth-
ylamine. Hong has recently demonstrated that hydrogenation
of N-benzyl formamide in the presence of the Ru-MachoBH
catalyst (3) could form both benzylamine or N-meth-
ylbenzylamine as major products depending on the catalytic
conditions (e.g. reaction temperature and the amount of
Scheme 2. Proposed pathway for the catalytic hydrogenation of urea
derivatives.
[31]
methanol used as a solvent). The presence of methanol was
found to facilitate the dehydration reaction possibly via a
hydrogen-bonding interaction between methanol and the
hydroxy group of the hemiaminal intermediate. We suspect
that in our case, formation of N-methyl amine could result from
a similar pathway.
[a]
Table 3. Hydrogenative depolymerisation of Polyureas.
In conclusion, hydrogenation of urea derivatives and
polyureas to produce (di)amines, and methanol in moderate to
excellent yields using a ruthenium (5) or an iridium pincer
complex (9) has been accomplished. Although, high yields for
the hydrogenation of polyureas could not be achieved, this is
the first demonstration of hydrogenative depolymerisation of
polyureas, which we believe presents attractive opportunities
for the closed-loop production/recycling of polyureas. Future
work to improve the catalytic activity, and expansion of
substrate scope, in particular using commercial polyurea waste
is in progress.
Acknowledgements
AK thanks the Leverhulme Trust for an early career fellowship. JL
thanks the University of St. Andrews for the Undergraduate
Research Assistantship. We thank the research group of Prof. Matt
Clarke (School of Chemistry, University of St. Andrews) for assisting
with lab equipment/facilities. We also thank Prof. Eli Zysman-
Colman and Dr. Subeesh Madayanad Suresh (School of Chemistry,
University of St. Andrews) for their help with the GC-MS.
t
[
(
7
a] Catalytic conditions: Polyurea (1 mmol), 9 or 5 (0.02 mmol), KO Bu
0.04 mmol), solvent (2 mL), H
2
(50 bar), 140°C, 24 h. [b] Reaction time
2 h. [c] Reaction time 36 h. Products were detected by the GC-MS and
1
their yields were calculated by the H NMR spectroscopy using 1,1’-
diphenylethene as the internal standard.
Conflict of Interest
1
formamide and N-methylated amines by the H NMR spectro-
scopy or the GC-MS (e.g. Figure S26, and Figure S28).
Interestingly, hydrogenation of formanilide under the catalytic
conditions used for Table 2, entry 1, resulted in the formation of
aniline and methanol in more than 90% yields. Formation of N-
methyl aniline was also observed in ~5% yield. This suggests
that the hydrogenation of urea derivatives to methanol and
amines in the presence of complex 9 proceeds via a formamide
intermediate. Based on the previous study, a proposed path-
way for the catalytic hydrogenation of urea derivatives has
been outlined in Scheme 2. Addition of the first equivalent of
The authors declare no conflict of interest.
Keywords: Catalysis · Hydrogenation · Pincer · Polyurea · Urea
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