A green route to polyurethanes: Oxidative carbonylation of industrially relevant aromatic diamines by CO2-based methyl formate

Article abstract of DOI:10.1039/d0gc02412k

The oxidative carbonylation of toluene-2,4-diamine (TDA) with methyl formate (MF), which can be produced from CO2, provides a possible route for the non-phosgene production of isocyanate precursors and enables a valuable utilization of the greenhouse gas. Extensive analysis of the product spectrum has provided detailed insight into the reaction network leading to the target product toluene-2,4-dicarbamate (TDC) and the most important side products. The most prominent one has been identified as methylene-bridged tetracarbamate 5, which is also an interesting precursor for applications in polyurethane chemistry. The side products are caused by three different reaction paths: N-formylation by MF, condensation with in situ formed formaldehyde, and N-methylation by in situ formed dimethyl carbonate (DMC). The influence of the catalyst on product distribution was evaluated for PdCl2/CuCl2 and a large number of heterogeneous Pd-catalysts. The oxidic support materials ZrO2, CeO2 and SiO2 were found to partially suppress the undesired side reactions leading to higher yields of TDC and tetracarbamate 5. The ratio of TDC to 5 was demonstrated to be affected significantly by the choice of the support. The synthetic protocol was extended to the synthesis of dicarbamates from 4,4′-methylenedianiline (MDA) and 2,4-diaminomesitylene (17). These results encourage further investigations into the design of selective catalysts for the production of isocyanate precursors from CO2 as a C1 source.

Full text of DOI:10.1039/d0gc02412k

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A green route to polyurethanes: oxidative
carbonylation of industrially relevant aromatic
Cite this: DOI: 10.1039/d0gc02412k
diamines by CO2-based methyl formate†
a,c
Christine Hussong,a Jens Langanke*a,b and Walter Leitner
*
The oxidative carbonylation of toluene-2,4-diamine (TDA) with methyl formate (MF), which can be pro-
duced from CO2, provides a possible route for the non-phosgene production of isocyanate precursors
and enables a valuable utilization of the greenhouse gas. Extensive analysis of the product spectrum has
provided detailed insight into the reaction network leading to the target product toluene-2,4-dicarbamate
(TDC) and the most important side products. The most prominent one has been identified as methylene-
bridged tetracarbamate 5, which is also an interesting precursor for applications in polyurethane chem-
istry. The side products are caused by three different reaction paths: N-formylation by MF, condensation
with in situ formed formaldehyde, and N-methylation by in situ formed dimethyl carbonate (DMC). The
influence of the catalyst on product distribution was evaluated for PdCl2/CuCl2 and a large number of
heterogeneous Pd-catalysts. The oxidic support materials ZrO2, CeO2 and SiO2 were found to partially
suppress the undesired side reactions leading to higher yields of TDC and tetracarbamate 5. The ratio of
TDC to 5 was demonstrated to be affected significantly by the choice of the support. The synthetic proto-
col was extended to the synthesis of dicarbamates from 4,4’-methylenedianiline (MDA) and 2,4-diamino-
mesitylene (17). These results encourage further investigations into the design of selective catalysts for
the production of isocyanate precursors from CO2 as a C1 source.
Received 15th July 2020,
Accepted 20th August 2020
DOI: 10.1039/d0gc02412k
rsc.li/greenchem
carbamate group as required in the production of poly-
urethane materials.
Introduction
The realization of the Green Chemistry principles depends pri-
The synthesis of aromatic dicarbamates is in the focus of
marily on the design criteria we are applying for the com- current Green Chemistry research because this compound
pounds and processes of the future: are they depleting or class provides a non-phosgene access to the corresponding di-
renewable, toxic or benign, persistent or readily degradable?1 isocyanates, which are utilized in large quantities as industrially
Through strategic decisions in line with the goals of sustain- important building blocks in polyurethane chemistry.2 N-Aryl
ability, benefits can be created which are multiplied in cas- carbamates can be synthesized, for example, by the reaction of
cades. For example, the utilization of CO2 as a feedstock lowers aromatic amines with urea and alcohol3 or by the conversion of
waste whereby additionally it results in the avoidance of toxic aromatic urea derivatives by means of alcohols or organic car-
phosgene, in a decrease of CO2 emissions, in a slowdown of bonates in the presence of PbO or sodium alcoholate etc.4
the rise of CO2 levels, and finally in a mitigation of the global Aromatic amines can also be converted directly into the corres-
climate change.1 The research reported in this paper was ponding carbamates using organic carbonates, whereby organic
motivated by this systems thinking, investigating the possi- lead and zinc salts or sodium alcoholates are capable of catalys-
bility to open a CO2-based pathway for the construction of the ing this reaction.5 In this context promising results were
achieved utilizing Zn4O(OAc)6 in the transformation of toluene-
2,4-diamine (TDA) with dimethyl carbonate (DMC).6 Another
approach to N-aryl carbamates is the catalytic reductive
aRWTH Aachen University, CAT Catalytic Center, Worringerweg 2, 52074 Aachen,
Germany. E-mail: leitner@itmc.rwth-aachen.de
bCovestro Deutschland AG, Catalysis and Technology Incubation, Kaiser-Wilhelm-
Allee 60, 51368 Leverkusen, Germany. E-mail: jens.langanke@covestro.com
cMax Planck Institute for Chemical Energy Conversion, Stiftstraße 34-36, 45470
Mülheim a. d. Ruhr, Germany
carbonylation of nitro aromatics, whereupon the activity of the
palladium/phenanthroline catalytic system is enhanced by
addition of phosphoric acid.7 As a very atom efficient route com-
prising two C1 building blocks several research groups have
investigated the oxidative carbonylation of aromatic amines by
reacting them with carbon monoxide/methanol mixtures in the
presence of various transition metal complexes.8
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0gc02412k
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The use of the greenhouse gas carbon dioxide as a C1 build- reactor to generate the desired CO/methanol mixture.11–15
ing block is of particular interest in synthetic chemistry.9 In After heating to 140 °C for one hour, a constant CO pressure
the last three years, several research groups have published of 74 bar was formed and the reactor was pressurized with 29
their results on the synthesis of N-aryl carbamates through the bar of synthetic air. Using a HPLC pump the oxidative
direct reaction of aromatic amines with carbon dioxide using a carbonylation was started by adding a methanolic TDA solution.
wide variety of catalysts and reagents.10 However, due to the The solvent and reagent MF was present in large excess
high thermodynamic stability of CO2 and the weak nucleophi- (MF : TDA ca. 100 : 1) with low catalyst loading corresponding to
licity of aromatic amines either stoichiometric quantities of 0.2 mol% total amount of Pd relative to TDA. After 4 h, the reac-
organotin compounds or titanium alkoxides or extended reac- tion was terminated by cooling the reactor to room temperature
tion times are required.10 An approach to overcome this and the reaction mixture was analyzed by HPLC using an exter-
problem is the utilization of methyl formate (MF) in the oxi- nal calibration for the quantification of TDC. TDC was then iso-
dative carbonylation of aromatic amines for the synthesis of lated by column chromatography. While fair yields up to 39% of
N-aryl carbamates.11–13 MF can be efficiently produced by CO2 TDC could be obtained with this procedure, the performance of
hydrogenation integrated with the subsequent esterification of the catalyst system was found to be unstable under these con-
formic acid with methanol.14 Furthermore, MF can be catalyti- ditions and a robust protocol could not be achieved.11,16a The
cally decomposed to CO/MeOH in a controlled manner. The complexity of the reaction network was identified as a major
catalytic interconversion of CO2/formate/CO has been coined challenge, leading to a range of side products in varying compo-
as the “formic acid triangle”.14a Thus, CO2-derived MF can be sition. Hence, a crucial step in the development of the approach
regarded as liquid, easily transportable and safe source for CO/ was the detailed analysis of the corresponding species and their
methanol mixtures.11–15 The idea of using MF for the synthesis reversible or irreversible formation in consecutive or parallel
of aromatic carbamates was demonstrated previously only for reaction pathways.
the monofunctional model substrate aniline.11–13 Here, we
present the first detailed investigations on the oxidative
Unravelling the reaction network of oxidative carbonylation of
carbonylation of actual polyurethane precursors TDA, 4,4′-
TDA with MF
methylenedianiline (MDA) and 2,4-diaminomesitylene employ-
ing MF as CO2-based CO/MeOH source in combination with
several Pd based catalysts.
Identification and calibration of the target product TDC.
Fig. 1 shows a typical HPLC chromatogram of a reaction
mixture obtained as described above. Several distinct groups
of signals can be distinguished and assigned to classes of com-
pounds as outlined below. In the developed HPLC analysis,
the target product TDC appeared with a retention time of Rt =
8.3 min. Relevant side products were detectable at Rt = 3.3, 3.7,
5.1, 5.3 and 14.2 min. In addition, there were some partially
Results and discussion
Oxidative carbonylation of toluene-2,4-diamine with methyl
formate
Our research focused on the comprehensive study of the Pd- smaller peaks with Rt ≈ 10 min together with many overlap-
catalysed synthesis of TDC by the oxidative carbonylation of ping peaks in the area Rt > 15 min. By measuring external stan-
TDA using liquid MF and synthetic air (see Scheme 1).
dard solutions, TDC was calibrated, whereby in a first experi-
The previously reported catalytic system Pd(10%)/C-NaI8a,12,13 ment with the Pd(10%)/C catalyst the yield of TDC was 35%
was used as starting point for our investigations.11,16a MF and after four hours reaction time. This value was confirmed by
the methanolic sodium methanolate solution together with the yield of isolated TDC (34%) determined after column
the catalyst system were pre-equilibrated in a stainless-steel chromatography of the reaction mixture.
Scheme 1 Synthesis of TDC by oxidative carbonylation of TDA with MF.
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Fig. 1 HPLC analysis of a typical reaction mixture of the oxidative carbonylation of TDA with MF: assignment of TDC and the identified side pro-
ducts 1–5 by UV detection at 214 nm.
Formamides from N-formylation with MF
and formic acid.18b The selective formation of the regioisomer
1 can be explained by the different reactivity of the two amino
groups in TDA.19
As already described in the literature, the non-catalytic
N-formylation of aromatic amines with MF in a SN2t reaction
is competing with the oxidative carbonylation, whereby form-
anilide (FAN) is formed in case of aniline as model
substrate.11–13 It has not yet been completely clarified whether
FAN is an intermediate which in the presence of oxygen
further reacts palladium-catalysed with methanol forming the
carbamate or whether FAN is only an unproductive side
product.11,12 In the case of the industrially relevant substrate
TDA, there are five different products possible from this reac-
tion, whereby in addition to the formamide function, also an
amine or carbamate function can be present.
As part of the analysis of the product spectrum, we exam-
ined the reaction of TDA with MF in the presence of sodium
methanolate, but in the absence of the Pd catalyst. In this way
formation of formamide 1 (Rt = 3.7 min) and diformamide 2
(Rt = 3.3 min) was verified as shown in Fig. 2.17,18 For compari-
son, 2 was also synthesized according to literature from TDI
In order to investigate the role of N-formylation with regard
to carbamate production, the oxidative carbonylation with MF
was carried out using the mono-formamide 1 instead of TDA.
From this reaction 2 (14%) and 3 (Rt = 5.1 min, 18%) were iso-
lated by column chromatography together with 14% of TDC.
The formation of TDC demonstrates that the formamide group
has been converted into a carbamate unit under the con-
ditions of the oxidative carbonylation. Nevertheless, the
majority of the original formamide function remained
unreacted. Furthermore, compounds 1–4 were detected by
HPLC analysis in the reaction mixture upon injection of TDC
under the conditions of the oxidative carbonylation instead of
TDA, showing the possibility of exchanging the carbamate
function for formamide groups by MF. These results indicate
that formamide groups are less reactive than free amine
groups regarding carbamate formation, constituting undesired
side products for TDC production via oxidative carbonylation
rather than intermediates as proposed in previous
literature.11,12 However, the utilization of diformamide 2 for
the direct non-phosgene TDI synthesis has been patented via a
different route, making the formamides potentially useful side
products in the context of carbamate production in general.20
Additionally, formamides are also key intermediates in the
production of fragrances, pharmaceuticals, dyes and agro-
chemicals.18a Thus, the observed formation of 1 and 2 from
TDA and MF under basic conditions suggests that the reaction
of amines with “green” methyl formate additionally enables a
sustainable route for the synthesis of industrially important
formamides.18c
Fig. 2 Structures and retention times (Rt) of formamide products
resulting from the competing N-formylation during the oxidative
carbonylation of TDA with MF.
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While the oxidative carbonylation of 1, as expected, resulted direct reaction of TDA with formaldehyde.25,26 To verify the
only in the formation of regioisomer 3, the reaction of TDA hypothesis, the methylene-bridged TDA dimer 13 was
always led to 3 and 4 as demonstrated by isolating 4% of the employed as substrate instead of TDA under the very same oxi-
pure regioisomer 4 (Rt = 5.3 min) by column chromatography. dative carbonylation conditions and led to formation of carba-
The NMR spectra of 1–4 were fully consistent with the cis/trans mate 5 (15%).
1
rotamers expected in solution.21–23 In the H NMR spectrum,
The methylene-bridged TDC dimer 5 is a very interesting
the –NHC(O)H function shows a characteristic signal pattern compound with high application potential. It can serve as a
due to the presence of cis- and trans-rotamers. The more stable precursor for the synthesis of the tetraisocyanate 5′, which can
3
cis-rotamer has a JH,H coupling constant of about 2 Hz, be used as a highly reactive crosslinking agent in polyurethane
3
whereas the trans-rotamer appears with a JH,H coupling con- chemistry.11,27 Contrary to the previously patented synthesis of
stant of about 11 Hz.21,22 Additionally, in the case of the cis- 5′,27 our findings provide a non-phosgene, one pot route to
rotamer the protons ortho to the –NHC(O)H group are this linker, making 5 a valuable co-product in this context
deshielded in comparison to the corresponding ortho-protons encouraging further efforts to optimize its yield in addition to
of the trans-rotamer.23
TDC.
Methylene-bridged dimer from condensation with in situ
Methylated products from N-methylation with in situ formed
formed formaldehyde
dimethyl carbonate
The side product with Rt = 14.2 min has been detected as the The side products with Rt ≈ 10 and Rt > 15 min (see Fig. 1)
major side product in the HPLC analysis and is therefore of were identified as N-methylated products. Their origin can be
paramount importance. We succeeded in isolating this new associated with the presence of dimethyl carbonate (DMC)
compound in its pure form from the reaction mixture by that is known to be formed under the conditions of oxidative
column chromatography as yellow orange crystals. 1H NMR, carbonylation with MF.15 The presence of DMC in the reaction
13C NMR and HR-MS analysis revealed the surprising result mixture was confirmed together with dimethyl oxalate (DMO)
that this isolated compound can be characterized as the by GC-MS. Reaction of DMO with sodium methanolate can
methylene-bridged tetracarbamate 5 (see Scheme 2). In other also lead to DMC formation.28 Furthermore, TDC is sensitive
words, the most important side product of the oxidative to methanolysis under thermal conditions leading to regener-
carbonylation of TDA is “TDC-CH2-TDC” (5). This finding ation of the primary amine functions together also with DMC
raised the question how the C1 bridge has been incorporated. as coproduct.29
A plausible proposal invokes partial Pd-catalysed oxidation of
As a reagent with dual reactivity, it is well described that
methanol to formaldehyde,24 which then reacts, possibly DMC can not only convert amines into carbamates, but it can
in situ, via electrophilic aromatic substitution and subsequent also cause N-methylations.30,31 In order to verify its potential
condensation with two molecules of TDA to give the tetra- for generating N-methylated side products, we examined the
amine 13 as intermediate.25 Analogous transformations for the direct reactions of TDA, TDC, 1 and 2 with DMC. Thus, com-
synthesis of 13 have been reported for example by conversion pounds 6–12 were isolated or at least identified in binary mix-
1
of TDA and methanol with a bio-catalyst (rat-liver S9) or by the tures by H NMR, 13C NMR and HR-MS. Fig. 3 illustrates the
Scheme 2 Synthesis, retention time (Rt) and application potential of the side product 5.11,27
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formamide unit in compounds 8 and 10 reveal that the carba-
mate and formamide functions are more prone to
N-methylation by DMC than the free amine.
Additionally, the tetraamine 13 was also reacted with DMC
resulting in a complex mixture of compounds. The NMR-ana-
lysis clearly revealed both N-methylated amine and
N-methylated carbamate functions. The corresponding reten-
tion times (Rt) were in the range between 16.5 and 18.7 min.
Based on these results, we assign the areas in the HPLC ana-
lysis with Rt > 15 min mainly to N-methylated side products of
the methylene bridged dimers.
Description of the full reaction network
In summary, the oxidative carbonylation of TDA with MF is
characterised by a complex reaction network (see Fig. 4). The
formation of side products can be divided into three main cat-
egories: first, N-formylation by the reaction with MF; second,
the electrophilic aromatic substitution/condensation with
formaldehyde resulting in methylene-bridged side products;
and third the N-methylation by the reaction with DMC.
Fig. 3 Structures and retention times (Rt) of compounds 6–12.
corresponding structures and retention times (Rt) obtained by
HPLC analysis. The presence of a free amino group in combi-
nation with an N-methylated carbamate or N-methylated
Fig. 4 Oxidative carbonylation of TDA with MF: complex reaction network.
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Evaluating the side products with regard to potential benefits, the oxidic supports and maintained clearly below 10 area% in
they are of different significance. As described above, the for- the HPLC analysis of the total product mixture for ZrO2, CeO2
mamide function can at least partly be transformed into a car- and SiO2. This can be explained by the known activity of TiO2
bamate function under the conditions of the oxidative to catalyse methylation reactions using DMC.31a In contrast,
carbonylation, albeit at lower rates than the starting amine. the support material CeO2 is known to suppress N-methylation
Additionally, formamides are interesting industrial key inter- when using Pd/CeO2 in the formation of carbamates from
mediates and the diformamide 2 can be used as starting DMC.31 This is probably due to selective cleavage of DMC
material for the non-phosgene TDI production.18,20 The dimeric which is necessary for carbamate formation.32a The different
side product 5 has an interesting application potential as pre- behaviour of the two oxidic supports may be related to the
cursor for a highly reactive crosslinking agent in polyurethane high Lewis acidity of TiO2 as compared to the strong surface
chemistry.27 Only the N-methylation is irreversible and leads to basicity of CeO2.32 Although N-methylation cannot be sup-
unproductive and undesired side products. Thus, the reaction pressed completely,31b this reactivity of Pd/CeO2 allows the
of the starting materials and intermediates with DMC resulting catalyst to convert eventually formed DMC productively into
in N-methylated compounds must be suppressed to improve carbamates also under the conditions of oxidative carbonyla-
the value generation of the process. This however, the formation tion. Therefore, the catalyst Pd/CeO2 was chosen for a detailed
of DMC can hardly be eliminated because, as mentioned above, analysis of the reaction profile for the oxidative carbonylation
it can be formed in three different ways. Consequently, the dual of TDA (Fig. 5).
reactivity of DMC should be controlled by shifting it towards
Using Pd/CeO2 under standard reaction conditions, the
carbamate production, while simultaneously suppressing unde- yield of TDC increases within the first two hours already up to
sired N-methylations. This defines the central challenge for 40%, reaching a plateau in the range of 39–45%. The mono-
catalyst development in this field.
formylated TDA 1 is produced initially in very small amounts
only and reacts further to disappear almost completely after
2 h. The concentration of the double formylated species 2 is
Catalyst control of the reaction network
In order to evaluate to what extend the nature of the catalyst never outside the detection limit. The concentration of the two
can be exploited to control the reaction network, eight hetero- regioisomers 3 and 4 containing both the formyl and carba-
geneous palladium catalysts with 5% metal loadings on mate function showed some variation, but a clear downward
different supports were tested and compared to the homo- trend with reaction time can be noted. Generally, the concen-
geneous catalytic system PdCl2/CuCl2.8b In all experiments NaI tration/time-profiles for species 1–4 are consistent with their
was employed as promoter.8 Calibration of the HPLC method formation as less productive intermediates for TDC formation.
with standard solutions enabled quantification of TDC The yield of the methylene-bridged TDC dimer 5 increases
together with the potentially valuable side products 1–5 in the over time reaching a nearly constant value of 13% after
reaction mixture. The amount of N-methylated compounds 3.5 hours. Comparing the concentration/time profiles of TDC
was estimated based on the area percentage of the side pro- and 5 indicates that they are formed in parallel pathways
ducts with Rt ≈ 10 min and Rt > 15 min in the chromatogram. rather than 5 being a secondary product from TDC. The total
Reactions were carried out for different time intervals to yield of all potentially valuable compounds which could be
obtain insight into the development of conversion and selecti- quantified in the reaction mixture increases within the first
vity to the individual products. All reactions were conducted at three hours and then ranges between about 60 to 70%. At this
least twice to evaluate the robustness of the procedure and stage, the product mixture is dominated by the two most valu-
reproducibility of the catalyst performance. As proxy for cata- able products TDC and 5 with 53% cumulated yield.
lyst efficiency, based on the total amount of Pd atoms the time
A notable correlation between the relative yields of TDC and
dependence of an overall turnover number (TON) was used to 5 is observed for the different catalysts comparing the results
define the turnover frequency (TOF). Preliminary results of a with the highest total yield of these two target products
screening to identify suitable conditions were published (Fig. 6). The combined yields of TDC and 5 range between
previously.16
48% and 53% in all these cases. As can be seen, higher yields
The homogeneous system PdCl2/CuCl2 resulted in TDC of TDC correlate with significantly lower yields of 5, ranging
yields reaching up to 38% and total yields of 1–5 + TDC of 53% from a TDC : 5-ratio of nearly 9 : 1 for Pd/ZrO2 to 1.5 : 1 for
in some experiments. However, the system exhibited inconsist- Pd/C. As described before, the formation of TDC-CH2-TDC (5)
ent performance and large variations were observed between can be explained as a result of the formation of formaldehyde
individual experiments. Consistent selectivities and TDC as reactive C1 building block under the conditions of the oxi-
yields of 40–50% over independent experiments were achieved, dative carbonylation.24–26 The experimental result, that
however, using the oxidic supports ZrO2 and CeO2 exhibiting Pd(5%)/C gives the highest yield of 5 is in accordance with litera-
the highest TONs in the range of 190–200 with standard devi- ture, since it has been suggested previously that during the Pd-
ations of only 2–4%. For comparison, the averaged TONs of catalysed decomposition of methanol carbonaceous overlayers,
the other heterogeneous catalysts range between 143 and 184 most likely elemental carbon, which are present during the
with standard deviations of 9–16%. With the exception of reaction favour CH2O formation by hindering its dehydrogena-
TiO2, the formation of N-methylated products was lowest with tion to CO.24a,f Furthermore, the equally high yields of 5 in the
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Fig. 5 Yields of TDC, 1, 2, 3 + 4 and 5 in dependence of the reaction time together which the total yield of these products [50–83 bar CO; 3.7–5.9
vol% O2; 140 °C; MF (485 mmol); NaOMe (0.5 mmol); Pd(5%)/CeO2 (0.011 mmol Pd); NaI (0.6 mmol); TDA (5 mmol) in MeOH (5 mL)].
case of Pd(5%)/TiO2 and Pd(5%)/Al2O3 are also in line with lit-
Subjecting MDA to the standard conditions of oxidative
erature, since both metal oxides have previously been used as carbonylation revealed the formation of the desired dicarba-
support materials in the Pd-catalysed oxidation of methanol, mate MDC together with products from N-formylation in solu-
whereby the formation of formaldehyde was observed both as tion, as determined by HPLC analysis (see Scheme 3). Both
an intermediate and as a product.24
MDC and 16 were isolated directly from the reaction mixture,
whereas the two formamides 14 and 15 were identified by
independent synthesis from MDA with MF in the presence of
sodium methanolate. As illustrated in Fig. 7, the reactivity
Oxidative carbonylation of 4,4′-methylenedianiline and 2,4-
diaminomesitylene
So far, the product analysis of the oxidative carbonylation of pattern of MDA is very similar to that of TDA. The dicarbamate
TDA with MF had provided detailed insights into the reaction MDC is formed rapidly in the beginning, the main side
network and based on these results the yield of the potentially product 16 converts also to MDC more slowly over time. The
valuable products could be significantly increased by choosing total yield of both products reaches up to 40% with a
suitable catalysts minimizing some N-methylation as the most maximum of 24% for MDC after 3.5 hours. A major limitation
detrimental side reaction (compare Fig. 6). The method was to reach higher yields is at least partly the fact that the ureas
next tested with industrially important 4,4′-methylenedianiline assumed as intermediates in carbamate formation12,31b are
(MDA) and 2,4-diaminomesitylene. As catalyst, Pd(5%)/Al2O3 very poorly soluble in the methanol solvent leading to signifi-
was applied because it is the cheapest material among those cant amounts of solid precipitates. Filtrating the reaction
giving robust results with low methylation activity and because mixture resulted in 300 mg of solid material that consisted
formation of the methylene bridged dimer is not relevant with mainly of 1,3-bis(4-benzylphenyl)urea derivatives according to
both substrates.
IR and NMR analysis.33
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Fig. 6 Influence of the support material on the palladium-catalysed oxidative carbonylation of TDA with MF. Reaction conditions: 485 mmol MF;
0.5 mmol NaOMe; 0.011 mmol Pd; 0.6 mmol NaI; 5 mmol TDA in 5 mL MeOH; 140 °C; reaction time 4 h. Quantified by HPLC-analysis using external
standard solutions.
The oxidative carbonylation of 2,4-diaminomesitylene (17)
employing MF was studied using Pd(5%)/Al2O3 as well as
Pd(5%)/SiO2 as catalysts (see Scheme 4). Both catalysts per-
formed almost identical, and the yield of the dicarbamate 18
reached up to 30% after 5 h reaction time (Fig. 8). Again sig-
nificant amounts of solid urea products were observed.12,31b,34
The mono-carbamate 19 was detected as intermediate. The for-
mation of N-formamides was less pronounced, which may be
reflecting steric hindrance. On the other hand, the presence of
dimethylated amine 20 in the reaction mixture shows that
Scheme 3 Oxidative carbonylation of MDA with MF.
N-methylation is not fully suppressed in this case.
Fig. 7 Oxidative carbonylation of MDA with MF: dependence of reac-
tion time on the yield of MDC and 16 [69–72 bar CO; 3.6–4.5 vol% O2;
MF (485 mmol); NaOMe (0.5 mmol); Pd(5%)/Al2O3 (0.011 mmol Pd); NaI
(0.6 mmol); MDA (2.5 mmol) in MeOH (5 mL)].
Scheme 4 Oxidative carbonylation of 2,4,6-trimethyl-1,3-benzenedia-
mine (17) with MF.
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product structures therefore is expected to contribute to
unlocking the potential of CO2 as building block for poly-
urethane production following the Green Chemistry
principles.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Fig. 8 Oxidative carbonylation of 17 with MF: dependence of reaction
time on the yield of 18 [65-81 bar CO; 4.2-5.2 vol% O2; 485 mmol MF;
0.5 mmol NaOMe; 0.011 mmol Pd; 0.6 mmol NaI; 5 mmol 17 in 5 mL
MeOH].
The authors gratefully acknowledge funding for the Carbon2Chem
project by the German Federal Ministry of Education and Research
(BMBF) under the grant number 03EK3042C. Open Access
funding provided by the Max Planck Society.
Conclusions
Notes and references
The results of this study point out that the oxidative carbonyla-
tion of industrially relevant diamines utilizing CO2-based
methyl formate (MF) as reagent indeed enables a green non-
phosgene route to polyurethanes. In this regard the Pd-cata-
lysed oxidative carbonylation of toluene-2,4-diamine (TDA)
with MF has been studied in detail. Three different pathways
causing side products have been found: Firstly, the nucleophi-
lic substitution at the carbonyl group of MF (SN2t mechanism)
without the participation of the Pd-catalyst or oxygen leading
to formamides. Secondly, the electrophilic aromatic substi-
tution/condensation with in situ formed formaldehyde result-
ing in the methylene-bridged tetracarbamate 5. Due to its own
potential the dimer 5 is of particular interest for industrial
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