Methyl N-phenyl carbamate synthesis from aniline and methyl formate: Carbon recycling to chemical products

Article abstract of DOI:10.1002/cssc.201402827

Methyl N-phenyl carbamate was synthesized from aniline by using methyl formate as a green and efficient carbonylating agent. High yields were obtained at milder reaction conditions compared to the conventional CO/CH₃OH route. Studies on the reaction sequence led to suggest an alternative and more efficient route to the carbamate via formanilide as intermediate.

Full text of DOI:10.1002/cssc.201402827

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DOI: 10.1002/cssc.201402827
Methyl N-Phenyl Carbamate Synthesis from Aniline and
Methyl Formate: Carbon Recycling to Chemical Products
Mohammad S. Yalfani,[a] Giulio Lolli,*[b] Thomas E. Mꢀller,[a] Aurel Wolf,[b] and
Leslaw Mleczko[b]
Methyl N-phenyl carbamate was synthesized from aniline by
using methyl formate as a green and efficient carbonylating
agent. High yields were obtained at milder reaction conditions
compared to the conventional CO/CH3OH route. Studies on
the reaction sequence led to suggest an alternative and more
efficient route to the carbamate via formanilide as intermedi-
ate.
tion reactions can also represent an example of recycling
carbon into important industrial chemicals.
MF can be activated by a strong base such as alkali methox-
ide (MOCH3, M: Na or K), which results in its decarbonylation.[9]
MF acts as weak acid and is deprotonated and rapidly dissoci-
ated to CO and methanol, creating an equilibrium [Eqs. (1) and
(2)].[10] As we discussed in a previous study,[11] for catalytic
amounts of methoxide (0.1 mol% in this case), the deprotonat-
ed MF is limited by the base availability. This results in a low
pressure of CO, the term on the right side of the equation,
which thus depends directly on the methoxide amount
[Eq. (1)]. When the reaction reaches to the equilibrium, obvi-
ously the decarbonylation of MF would be independent from
the base concentration (see S1 in the Supporting Information).
This had been confirmed also by other authors who observed
that the MF decarbonylation is dependent on MOCH3 nature
and amount.[10,12f] Such a pseudo-equilibrated system can be
used in the carbonylation reaction along with an appropriate
catalyst.
Carbamate synthesis currently receives particular attention be-
cause carbamates provide a safer access compared to the clas-
sic phosgene route to isocyanates, the main component for
polyurethanes manufacturing.[1] The catalytic carbonylation of
amines or nitro compounds and the reaction of amines with
dimethyl carbonate are the most promising routes for the syn-
thesis of carbamates.[2,3] The carbonylation of amines requires
CO,[4] which, despite good reactivity as carbonylating agent, is
toxic and is prone to side reactions. On the other hand, di-
methyl carbonate can be produced by the oxidative carbonyla-
tion of methanol or the ring opening of epoxides with CO2 to
cyclic carbonates followed by transesterification with metha-
nol.[5] The disadvantages of these routes are a typically low
yield and the stoichiometric formation of glycols as byproduct,
respectively. Therefore, a safe, highly efficient, green, and eco-
nomically viable approach is required for the production of
carbamates from amines or nitro precursors.
CH3Oꢀ þ HCOOCH3 Ð CH3OH þ ꢀCOOH3
ꢀCOOH3 Ð CO þ CH3Oꢀ
ð1Þ
ð2Þ
Recently, we successfully applied MF in the carbonylation of
phenol to methyl phenyl carbonate.[11] The results of this study
were interesting from a mechanistic point of view, but the
yield was too low to be successfully applied industrially. Here,
we present the application of MF as carbonylating agent for
aniline to produce methyl N-phenyl carbamate (MPC).
Formic acid and its derivatives such as methyl formate (MF)
have shown diverse applications as C1 building blocks in the
synthesis of base chemicals.[6] The use of MF in carbonylation
reactions introduces it as safe and green carbonylating agent.
Furthermore, recent studies focused on carbon recycling have
achieved the conversion of CO2 into formic acid and its deriva-
tives.[7] This hydrogenation reaction is favorable when it is per-
formed under basic conditions, resulting in formate salts,
which, in the presence of methanol, are separable as MF.[8]
Thus, the application of MF produced from CO2 in carbonyla-
We have chosen heterogeneous supported Pd catalysts be-
cause Pd-based catalysts are well known in the oxidative car-
bonylation reactions with both CO and MF.[3,12] Pd is the metal
which is known to catalyze these reactions due to (i) its ability
to be oxidized–reduced easily during the reaction and (ii) high
tendency to form complexes with carbamoyl and methoxide
groups.[13] The oxidative carbonylation of aniline with MF cata-
lyzed by commercially available Pd-based catalysts was carried
out in two steps as illustrated in Scheme 1 (see also reactor de-
tails in S2 in the Supporting Information).
[a] Dr. M. S. Yalfani, Dr. T. E. Mꢀller
CAT Catalytic Center
Step 1: Activation of MF catalyzed by NaOCH3. During this
step, the activation of MF establishes the equilibrium between
MF and CO/CH3OH, as soon as the reaction temperature is sta-
bilized. The formation of CO during the activation of MF (de-
pending on the amount of NaOCH3) caused an increase of the
internal pressure in the reactor to 10–70 bar. Thereafter, this in-
ternal pressure is called PCO. It should be noted here that no
additional CO gas was added into the reactor.
RWTH Aachen University
Worringerweg 2, 52074 Aachen (Germany)
[b] Dr. G. Lolli, Dr. A. Wolf, Prof. Dr. L. Mleczko
Bayer Technology Services GmbHd-51368 Leverkusen (Germany)
Fax: (+49)214-3081118
E-mail: giulio.lolli@bayer.com
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201402827.
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of its initial activity. This loss could be explained by the forma-
tion of inactive colloidal Pd0 (known as Pd Black).[23] This is
a well-known problem and is mainly tackled by metal recovery
and regeneration.[24] Nevertheless, the catalyst recyclability
might be improved by doing the reaction in ionic liquid
medium.[25]
To unravel the reaction pathways, a series of reactions was
carried out at 1408C. The reactions were quenched after differ-
ent periods of time (0.25 to 4 h) and the product mixtures
were analyzed (Figure 1). Apart from MPC, diphenylurea (DPU),
Scheme 1. Overview on the two-step oxidative carbonylation of aniline by
MF using a supported Pd catalyst.
Step 2: Oxidative carbonylation of aniline over Pd catalyst.
After the equilibrium had been stabilized (constant tempera-
ture and PCO), the reactor was charged with aniline and air to
start the oxidative carbonylation reaction in the presence of Pd
catalyst and NaI promoter.
Using Pd supported on SiO2, Al2O3 and C, very similar results
were obtained at 1408C and PCO 67–70 bar (Table 1), whereby
conversions of aniline higher than 95% and selectivities to
MPC higher than 70% were achieved. Regarding the small dif-
ferences in activities of Pd catalysts, we selected Pd/C, which is
Table 1. Results of the oxidative carbonylation of aniline with MF using
supported Pd catalysts.
Figure 1. Product distribution during the oxidative carbonylation of aniline
with MF at 1408C (MAN N-methylaniline, for other abbreviations, see
Scheme 2). Reaction conditions: MF (30 mL, 0.49 mmol), NaOCH3 (0.5 mmol),
supported-Pd catalyst (0.011 mmol atom Pd), NaI (90 mg, 0.6 mmol), aniline
(0.93 mg, 10 mmol) in methanol (5 mL), CO/air (2:1).
Entry
Catalyst
Conv.[a] [%]
Select.[b] [%]
TOF[c] [hꢀ1
]
formanilide (FAN) and N-methylaniline (MAN) were identified in
the reaction mixture. Aniline was converted very fast and 85%
conversion was achieved within of 0.25 h. The concentration of
DPU and FAN reached a maximum after 0.25 h and decreased
upon prolonged reaction times. In contrast, the concentration
of MPC continuously increased. After 4 h, MPC was obtained in
high selectivity (>98% selectivity). Obviously, in two parallel
reaction pathways aniline was converted via DPU and FAN to
MPC as the only product (Scheme 2).
1
2
3
4
5
6
7
Pd(5%)/SiO2
Pd(5%)/Al2O3
Pd(5%)/C
Pd(10%)/C
Pd(10%)/C[d]
Pd(1%)/C
Pd(1%)/C[e]
95.2
97.4
97.7
96.2
99.7
94.8
87.8
72.8
82.4
81.9
85.4
98.5
79.6
62.3
300
368
364
372
223
343
249
[a] Conversion of aniline. [b] Selectivity to MPC. [c] molMPC molPdꢀ1 hꢀ1
[d] Reaction time 4 h. [e] Catalyst recovered from entry 6. Reaction condi-
tions: 1408C, 2 h.
.
Similar reactions were also performed using CO/CH3OH in-
stead of MF to supplement the above findings. In order to
have a fair comparison, the amount of CO used and the total
pressure is comparable to the amount of MF previously em-
ployed and both are in large excess compared to aniline (see
S3 in the Supporting Information). The product distribution
after 0.25 h reaction time (Figure 2) clearly showed the forma-
tion of DPU as the main product of oxidative carbonylation
with CO with as high as 80% selectivity. At longer reaction
times, the concentration of DPU decreased and consequently
MPC increased. In this system, FAN was detected only in negli-
gible amounts. These observations are in line with the pro-
posed reaction sequence (Scheme 2–Route A) reported in pre-
vious studies, in which DPU was the main intermediate for
MPC formation.[14–19] As a mechanism, it was assumed that
MPC is formed through the formation of DPU as intermediate
via a Pd-carbamoyl complex.[3,15–18,26,27]
often preferred in this type of catalysis and is comparable with
the previous reports, for a further optimization.[14–16] Varying
the Pd content in the carbon-supported catalyst, highest selec-
tivity to MPC was observed for Pd(10%)/C (entry 4). Prolonging
this reaction to 4 h, almost full conversion of aniline to MPC
(1.48 g MPC produced from 0.93 g aniline) was achieved
(entry 5). The results obtained with MF were compared to
those reported using the CO/CH3OH system,[3,14–22] in which the
reactions were performed in the temperature range of 150–
2008C. This showed advantages of the MF route: higher yields
were achieved at lower reaction temperature as well as a com-
petitive catalytic activity in terms of turnover frequency (TOF)
(reported TOF were between 30–460 hꢀ1 for similar catalytic
systems).[14–21] The supported Pd catalyst was recycled and
used in the second run (entry 7), maintaining more than 70%
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Scheme 2. Reaction sequence of the oxidative carbonylation of aniline.
Accordingly, we could conclude that the MPC formation
from aniline using MF follows two different parallel reaction se-
quences: i) DPU formation by the reaction of aniline with CO/
CH3OH released during the decarbonylation of MF followed by
methanolysis of DPU (Scheme 2, Route A) and ii) FAN formation
by the reaction of aniline with MF followed by the oxidative re-
action of FAN with methanol (Scheme 2, Route B). The forma-
tion of FAN as an intermediate to MPC avoids the internal recy-
cling of aniline in the second step of Route A enabling to ach-
ieve higher yields to MPC.
The gas consumption during the reaction was followed by
analyzing the pressure profile during the oxidative carbonyla-
tion of aniline (Figure 3). During the reactions using MF and
CO/CH3OH, a pressure drop of 3 and 9.5 bar was observed, re-
Figure 2. Product distribution during the oxidative carbonylation of aniline
with CO/CH3OH at 1408C. Reaction conditions: methanol (25 mL,
0.62 mmol), NaOCH3 (0.5 mmol), supported-Pd catalyst (0.011 mmol atom
Pd), NaI (90 mg, 0.6 mmol), aniline (0.93 mg, 10 mmol), CO/air (2:1).
A reference reaction without Pd catalyst and promoter (NaI)
allowed us to rationalize these results (see S4 in the Support-
ing Information). When CO/CH3OH was used, aniline was not
converted within 2 h. In contrast, the reaction with MF in the
presence of base (NaOCH3) provided 45% conversion of aniline
and >97% selectivity to FAN. Without the base, the yield of
FAN was improved to 85%, suggesting an electrophilic attack
of aniline to MF mechanism (see S5 in the Supporting Informa-
tion). In other words, FAN was produced only, when MF was
employed as carbonyl source. Munshi et al. has also shown the
solvent and additive-free formation of FAN from aniline and
formic acid.[28] This is in agreement with previous studies on
the use of CO as carbonylating agent which showed that tem-
peratures higher than 1808C are required for the carbonylation
of aniline to FAN.[29]
Figure 3. Pressure profiles of the oxidative carbonylation of aniline using MF
and CO/CH3OH at 1408C. For reaction conditions, see experimental section.
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100–1208C.[17–19] At 1208C, the MPC formation was more pro-
nounced and MPC became the main product. By performing
the reaction at 1408C, the highest selectivity to MPC was ach-
ieved (>80%), while no DPU was detected after 2 h. Under
such conditions, FAN seems to have higher persistency than
DPU. In comparison, the conventional route using CO/CH3OH
in the presence of methanol required temperatures up to
1708C to achieve such a yield for MPC.[14–22] In the MF system,
further increase in temperature to higher than 1408C (up to
1608C) did not significantly improve the selectivity to MPC.
In summary, MPC was obtained from aniline and MF as car-
bonylating agent in high yield and under relatively mild condi-
tions. Even though the C1-building block (MF) is the same as in
a previous report for the production of carbonates,[11] for car-
bamates we found a different reaction pathway and obtained
a high yield that is compatible to industrial application. Nota-
bly, the reaction does not require any external CO source. The
product distribution and the analysis of the reaction profile
suggest FAN as a key intermediate, which opens a new route
to MPC formation. This provides new insight into an alternative
route to polyurethane precursors[30] using MF as carbonylating
agent. MF, which is readily obtained from CO2, gives an oppor-
tunity to recycle carbon into the value chain. The method is
expected to be applicable to other important amines and dia-
mines used in polyurethane production.[31] To demonstrate its
industrial applicability, we applied the procedure to toluene di-
amine (an important amine for polyurethane production)
which resulted in 80% isolated yield for toluene dicarbamate,
a precursor for toluene diisocyanate (TDI).
Table 2. Oxidative carbonylation of aniline with MF whereby PCO was
varied by adjusting the amount of NaOCH3.
[b]
[c]
[d]
[e]
Entry NaOCH3
[mmol]
PCO
MF/
XAN
SMPC
[%]
SFAN
[%]
SMAN
[%]
[a]
[bar] NaOCH3
[%]
1
2
3
4
0.15
0.2
0.3
12
38
47
67
3266
2450
1633
980
50.4
96.4
97.2
96.2
6.7
74.8
75.2
85.4
66.7
22.2
22.0
13.4
26.5
3.0
2.8
0.5
1.2
[a] mol/mol. [b] Conversion of aniline. [c] Selectivity to MPC. [d] Selectivity
to FAN. [e] Selectivity to MAN. Reaction conditions: 1408C, 2 h.
spectively. This means that the carbonylation of aniline is con-
ducted by both MF and CO. As we showed above, up to 45%
of aniline is converted to FAN through the direct reaction with
MF.
Further, the efficiency of the oxidative carbonylation of ani-
line with MF was studied under different PCO adjusted by the
amount of NaOCH3 (Table 2). High conversions of aniline were
obtained, when the concentration of NaOCH3 was sufficiently
high to establish a PCO above 30 bar. With increasing NaOCH3
concentration and consequentially PCO pressure, also the selec-
tivity to MPC increased. This indicates that the presence of
NaOCH3 as strong base may be essential to deprotonate FAN
as well as CH3OH and, thus, acts as co-catalyst in the Pd-cata-
lyzed MPC formation.
To investigate the temperature dependence, the oxidative
carbonylation of aniline with MF was performed in the temper-
ature range of 80–1608C (Figure 4). At 808C, less than 40% of
Experimental Section
Oxidative carbonylation of aniline with MF: A 160 mL Parr high
pressure reactor was charged with MF (30 mL, 0.49 mol), NaOCH3
solution (25 wt% in methanol, 0.115 mL, 0.5 mmol), supported Pd
catalyst (0.011 mmol Pd) and NaI (90 mg, 0.6 mmol). The reaction
mixture was stirred at 700 rpm and heated to the desired tempera-
ture (80–1608C). After the temperature had stabilized and the in-
ternal pressure (PCO) had become almost constant, the reactor was
pressurized with artificial air (50% of PCO). Immediately thereafter,
aniline (0.93 g, 10 mmol) dissolved in methanol (5 mL) was injected
into the reactor (4 mLminꢀ1) using a high pressure HPLC pump.
The injection of aniline was considered as start of the reaction (t=
0). After 2 h, the reaction was quenched by cooling the reactor
with ice-water. The autoclave was opened and the reaction prod-
ucts were analyzed by gas chromatography (GC) and gas chroma-
tography–mass spectrometry (GC–MS).
Methods: The conditions of GC analysis were as follows: Instru-
ment: Thermo SCIENTIFIC TRACE GC Ultra, Column: OPTIMA 5
Amine (30 mꢂ0.25 mmꢂ0.4 mm); 80–2808C, 5 min isothermal,
128Cminꢀ1, 30 min isothermal; FID 2508C; methyl benzoate as ex-
ternal standard.
Figure 4. Product distribution obtained in the oxidative carbonylation of ani-
line after 2 h reaction time in the temperature range 80-1608C. Reaction
conditions: MF (30 mL, 0.49 mmol), NaOCH3 (0.5 mmol), supported-Pd cata-
lyst (0.011 mmol atom Pd) and NaI (90 mg, 0.6 mmol), aniline (0.93 mg,
10 mmol) in methanol (5 mL).
The conditions of GC–MS analysis were as follows: Instrument:
VARIAN CP3800 Gas Chromatograph–VARIAN 1200 L Quadrupole
MS/MS; Column: SE34 (30 mꢂ0.32 mm); 80–2808C, 5 min isother-
mal, 128Cminꢀ1, 30 min isothermal; FID 2508C.
aniline was converted and FAN was the main product. On in-
creasing the temperature to 1008C, the conversion of aniline
increased to higher than 80% and the main product was DPU.
DPU was also shown to be the main product of the oxidative
carbonylation of aniline by CO in the temperature range of
Oxidative carbonylation of aniline with CO/CH3OH: A 160 mL
Parr autoclave reactor was charged with MeOH (25 mL,
0.62 mmol), NaOCH3 solution (25 wt% in methanol, 0.115 mL,
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Acknowledgements
The authors would like to acknowledge H. Eschmann, E. Biener
and J. Wurlitzer for developing the GC analysis.
Keywords: aniline · carbamate · carbon recycling · methyl
formate · oxidative carbonylation
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Received: August 12, 2014
Revised: October 24, 2014
Published online on && &&, 0000
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 5
&
5
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These are not the final page numbers!

COMMUNICATIONS
M. S. Yalfani, G. Lolli,* T. E. Mꢀller, A. Wolf,
L. Mleczko
Polyurethane precursor: Methyl N-
phenyl carbamate was synthesized from
aniline by using methyl formate as
a green and efficient carbonylating
agent. Compared to the conventional
CO/CH3OH route, high yields were ob-
tained with milder reaction conditions.
Studies on the reaction sequence sug-
gest an alternative and more efficient
route to the carbamate via formanilide
as intermediate.
&& – &&
Methyl N-Phenyl Carbamate Synthesis
from Aniline and Methyl Formate:
Carbon Recycling to Chemical
Products
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 5
&
6
&
ÞÞ
These are not the final page numbers!