Palladium-catalyzed reductive carbonylation of nitrobenzene for producing phenyl isocyanate

Article abstract of DOI:10.1016/j.tetlet.2019.151310

Direct reductive carbonylation of nitrobenzene to phenyl isocyanate with carbon monoxide was performed using various types of palladium catalysts together with many types of N-donor ligands. The effect of reaction time, pressure, temperature, ligand amount, and molar ratio to establish the optimized conditions was also investigated. With this, we were able to achieve up to 100% conversion and 63.5% yield with PdCl₂ and alkylimidazole system (1:3) within 2 h at 220 °C and 1400 psi of CO in toluene.

Full text of DOI:10.1016/j.tetlet.2019.151310

Journal Pre-proofs
Palladium-catalyzed reductive carbonylation of nitrobenzene for producing
phenyl isocyanate
Thanh Tung Nguyen, Anh Vy Tran, Hye Jin Lee, Jayeon Baek, Yong Jin Kim
PII:
S0040-4039(19)31094-9
DOI:
https://doi.org/10.1016/j.tetlet.2019.151310
TETL 151310
Reference:
Tetrahedron Letters
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Received Date:
Revised Date:
Accepted Date:
27 August 2019
12 October 2019
21 October 2019
Please cite this article as: Nguyen, T.T., Tran, A.V., Lee, H.J., Baek, J., Kim, Y.J., Palladium-catalyzed reductive
carbonylation of nitrobenzene for producing phenyl isocyanate, Tetrahedron Letters (2019), doi: https://doi.org/
10.1016/j.tetlet.2019.151310
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Tetrahedron Letters
Palladium-catalyzed reductive carbonylation of nitrobenzene for producing phenyl
isocyanate
Thanh Tung Nguyena,b, Anh Vy Trana,b, Hye Jin Leea,b, Jayeon Baekb *, Yong Jin Kima,b *
aGraduate School, Korea University of Science and Technology (UST), 217 Gajeong-ro, Yuseong-gu, Daejeon-si 34113, Republic of Korea
bGreen Chemistry & Material Group, Korea Institute of Industrial Technology (KITECH), 89 Yangdaegiro-gil, Ipjang myeon, Cheonan-si 31056, Republic of
Korea.
*Corresponding author. E-mail address: jbaek@kitech.re.kr (J. Baek), yjkim@kitech.re.kr (Y. J. Kim).
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Direct reductive carbonylation of nitrobenzene to phenyl isocyanate with carbon monoxide was
performed using various types of palladium catalysts together with many types of N-donor
ligands. The effect of reaction time, pressure, temperature, ligand amount, and molar ratio to
establish the optimized conditions was also investigated. With this, we were able to achieve up
to 100% conversion and 63.5% yield with PdCl2 and alkylimidazole system (1:3) within 2 h at
220 oC and 1400 psi of CO in toluene.
Available online
©2019 Elsevier Ltd. All rights reserved.
Keywords:
Palladium catalysts
Reductive Carbonylation
Phenyl isocyanate
N-alkylimidazole
1. Introduction
[25], which was reported to be very efficient. However, these
derivatives seem to be far too toxic to be applied in industry [27-
Nowadays, organic isocyanates (R–NCO) have become more
and more industrially and commercially important as chemical
intermediates in the manufacture of thermoplastic foams,
elastomers, adhesives, and agrochemicals [1]. They can be
converted to carbamates and ureas through the addition of
alcohol and amine, respectively; and those carbamates and ureas
can also be transformed back to isocyanates via thermal
decomposition (Scheme 1a and 1b). Because of this conversion,
mono- and diisocyanates have established their importance in the
synthesis of a variety of polyurethane (flame-retarding) foams [2-
6], (bio-degradable) plastics [7-10], pesticides [11, 12], adhesives
[13-15], and coatings [16-19].
29] and it appears difficult to separate the catalyst from the final
product [32]. Alternatively, group 8–10 metal compounds can be
applied, and in 1967, Hardy and Bennett were the first to report
the direct conversion of nitro compounds to isocyanates using
rhodium, palladium or other noble metal salts as catalyst with a
Lewis acid promoter at high temperature and high pressure of
carbon monoxide [21, 33]. Regarding the direct carbonylation of
mono- or dinitroaromatic compounds, it has been reported that
heterogeneous catalyst such as Pd/C or Rh/C [31] as well as
inorganic polymeric precursors like PdCl2 and RhCl3 give poor
results [34]. N-containing ligands are also often used in Pd-
catalyzed carbonylation reactions for enhancing activity and
stability of Pd catalyst since they can have coordination with the
palladium atom and prevent aggregation of Pd black. Such
soluble palladium complexes consisting of N-heterocyclic ligands
such as Pd(Py)2Cl2 has been employed to generate phenyl
isocyanate from nitrobenzene at pressures up to 1600 psi in the
temperature range of 170-230 °C, though the turnover rates of
isocyanates is not very high [35]. Also the addition of a Lewis
acid promoter (such as MoCl5, VCl4, FeCl3, etc.) may strongly
increase both the rate and selectivity of the reaction towards
isocyanates [36-40], but the catalyst is quite rapidly deactivated.
The conventional route to make methylene diphenyl
diisocyanate (MDI) and toluene diisocyanate (TDI), as well as a
variety of other isocyanates is often referred to as the ‘phosgene
route’ (Scheme 1c) [20]. Despite the high yield and good
selectivity obtained with the phosgene route, there are major
drawbacks including toxicity of phosgene, formation of corrosive
hydrochloric acid, and limited concentration of reactant.
Carbonylation with carbon monoxide is an attractive
alternative, because it can provide a one-step conversion of nitro
aromatics to the isocyanate without the use of environmentally
unfriendly phosgene [21-24]. In the direct method (Scheme 1d), a
catalyst activates the nitro group and carbon monoxide to form
the isocyanate with liberation of carbon dioxide. Initially, a
variety of elements were investigated as catalyst for the
conversion of nitroaromatic compounds to isocyanates [24-26],
such as sulfur [27-29], copper [30, 31], and especially selenium
In this research we present the direct reductive carbonylation of
nitrobenzene to phenyl isocyanate using the catalytic system
consisting of PdCl2 together with N-donor ligands (monodentate
and bidentate, phosphine and amine) and promoters. The
optimized reaction conditions using the PdCl2 and 1-
butylimidazole system, which showed best activity, is also

2
Tetrahedron Letters
achieved by variation of reaction time, temperature, and
Using 1-butylimidazole as the ligand together with PdCl2, the
amount of ligand was investigated, and the results are
summarized in Table 2. The results show that the best
catalyst/ligand ratio is 1/3. More or less amount of ligand
resulted in the decrease in phenyl isocyanate yield. Less amount
of ligand might not be enough to both create the active species
and provide the necessary basicity for the reaction, while more
amount of ligand might react with nitrobenzene to form the
undesirable but stable intermediate (Fig. S1) which will use up
the substrate and hinder the carbonylation process.
pressure. Moreover, the complex of PdCl2 and 1-butylimidazole,
which is believed to be the active species, was also prepared and
tested as catalyst for the reductive carbonylation reaction.
2. Results and Discussion
2.1. Reductive carbonylation of nitrobenzene to phenyl
isocyanate using PdCl2
To begin with, the reductive carbonylation of nitrobenzene to
phenyl isocyanate was carried out using palladium chloride as
main catalyst and various types of N-donor ligands (Fig. 1) in
It is known that addition of metal oxides of V, Mo, W, Nb, and
other groups of V and VI transition metals promote the catalytic
activity of PdCl2 in the carbonylation of nitrobenzene [23]. The
promoting effect can be explained in various ways. Firstly, the
compounds mentioned above are Lewis acids, so they could be
used to reoxidize Pd(0) to Pd(II), thus preventing the formation
of inactive Pd black. They could also activate the nitro group by
coordination, favoring the following deoxygenation.
o
toluene at 220 C and 1400 psi of carbon monoxide for 4 h, and
the results are summarized in Table 1. The result shows that
pyrazole (L1), 4-dimethylaminopyride (DMAP) (L2), pyridine
(L3), and benzimidazole (L4) had some catalytic activity while
1,2,4-triazole (L5), pyrazine (L6), and 1,3,5-triazine (L7) almost
had no activity (entries 1-7). It also shows that among the
imidazoles chosen for this reaction, only the ones with the
electron–donating groups (such as alkyls; L9-L11) exhibited
fairly good catalytic activity (Table 1, entries 4 and 8-16).
Scheme 1. (a, b) The indirect, (c) the conventional, and (d) the direct method for the reductive carbonylation of nitro compounds.
Fig. 1. The structure of the N-donor ligands.

Table 2
With this information, some types of metal oxides were tested
as promoters and the results are summarized in Table 3 (entries
1-6). The results show that among the oxides chosen for this
reaction, the best promoter for this reaction was TiO2, still it was
not as good as when not using the promoter. Complex of
moblydenum and 1-methylimidazole, i.e. [l-Hmim]4[Mo8O26(1-
mim)2]·2H2O, could be made using MoO3 and 1-methylimidazole
(1 to 1 ratio) under reflux condition in 12 h [41]. Similarly to
molybdenum, 1-methylimidazole could also be able to form
complexes with zinc [42], titanium, Ti2S6O(MeIm)2 [43],
vanadium, and [VOL(1-methimz)2] [44] while tungsten can form
bidentate complex with 2-(1H-Imidazol-2-yl)pyridine [45]. So it
is believed that all of the promoter used in this experiment can
form complex with 1-butylimidazole, creating a competition
between main catalyst (PdCl2) and promoter in making complex
with 1-butylimidazole, thus reduces the amount of ligand needed
for the reaction, and decreases the phenyl isocyanate yield.
Effect of catalyst/ligand on the reductive carbonylation of nitrobenzene to
phenyl isocyanate.a
Entry
1
Substrate/Catalyst/Ligand
25/1/1
Conversion (%)
95.1
Yield (%)
40.5
2
3
4
5
6
7
25/1/2
25/1/3
25/1/4
25/1/5
25/1/6
25/1/10
92.6
100
99.4
100
100
96.7
37.7
52
50.8
48.3
38.2
15
Table 1
a
The reaction was carried out with nitrobenzene (7.5 mmol), PdCl2 (0.3
Screening of the N-donor ligands for the reductive carbonylation of
mmol), and 1-butylimidazole in 25 mL of toluene at 220 oC.
nitrobenzene to phenyl isocyanate.a
Table 3
Effect of promoter on the reductive carbonylation of nitrobenzene to phenyl
isocyanate.a
Entry
1
N-donor ligand
Conversion (%)
100
Yield (%)
28.8
L1
2
3
L2
L3
100
96.6
62.3
34.6
14.2
25.9
17.8
100
25.4
14.8
24.2
1.9
0.6
0.7
1.2
38.4
38.2
34.9
8.6
0
Conversion
(%)
Yield
(%)
Entry
Promoter
Catalyst/Promoter
4
L4
1
2
3
4
5
6
-
-
100
100
100
100
100
100
52
5
L5
TiO2
V2O5
MoO3
WO3
ZnO
1/1
1/1
1/1
1/1
1/1
48.8
35.9
43.5
44.3
47.1
5
L6
6
L7
8
L8
9
L9
10
11
12
13
14
15
16
L10
L11
L12
L13
L14
L15
L16
100
a
The reaction was carried out with nitrobenzene (7.5 mmol), PdCl2 (0.3
mmol), 1-butylimidazole (0.9 mmol), and promoter in 25 mL of toluene at
97
220 oC.
37.5
80.8
20.1
86.3
20.6
Effect of temperature on the reductive carbonylation was
carried out by varying the reaction temperature from 160 to 240
oC. Table 4 (entries 2 and 7-10) shows that when increasing the
reaction temperature, the nitrobenzene conversion and the phenyl
isocyanate yield increased and achieved maximum value at T =
0
8.6
0
o
a
220 C (100% conversion and 63.5% yield). However, when
The reaction was carried out with nitrobenzene (7.5 mmol), PdCl2 (0.3
o
mmol), and ligand (1.8 mmol) in 25 mL of toluene at 220 oC.
increasing the temperature to higher than 220 C, the phenyl
isocyanate yield decreased because of the thermolysis–
decarboxylation that led to the formation of N,N’-
diphenylcarbodiimide as explained above [46-50].
After surveying the N-donor ligands and promoter effects, we
turned our attention to other reaction conditions such as time,
temperature, pressure, and substrate/catalyst molar ratio; and the
results are summarized in Table 4.
As the CO pressure increases from 1000 to 1400 psi, the
nitrobenzene conversion and the phenyl isocyanate yield
increased and achieved maximum value at P = 1400 psi (Table 4,
entry 2 and 11-12). But when continuing to increase the pressure
to higher than 1400 psi, more byproducts were formed, and the
phenyl isocyanate yield decreased (entries 13-14). This may be
due to the deactivation of catalyst caused by the formation of the
inactive PdCl2(CO) or Pd2Cl4(CO)2 complexes at high CO
pressure [51].
When the reductive carbonylation was conducted by varying
the reaction time from 1 to 8 hours, the maximum yield of 63.5%
was achieved at 2-hour reaction time (Table 4, entries 1-6). This
can be attributed to the formation of phenyl isocyanate during the
2-hour reaction then converted to the undesired by-product of
C6H5-N=C=N-C6H5 when prolonging the reaction time. Although
it is stated that N,N’-diphenylcarbodiimide, or C6H5-N=C=N-
C6H5, could be formed at 250 oC with or without catalyst [46-49]
or higher temperature [50], it still has a chance to be formed in
our reaction with prolonged reaction time.

4
Tetrahedron Letters
Table 4
Effect of reaction conditions on the reductive carbonylation of nitrobenzene to phenyl isocyanate.a
Entry
Substrate/Catalyst
Time (h)
Pressure (psi)
Temperature (oC)
Conversion (%)
Yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
25/1
25/1
25/1
25/1
25/1
25/1
25/1
25/1
25/1
25/1
25/1
25/1
25/1
25/1
100/1
50/1
20/1
1
2
3
4
5
8
2
2
2
2
2
2
2
2
2
2
2
1400
1400
1400
1400
1400
1400
1400
1400
1400
1400
1000
1200
1600
1800
1400
1400
1400
220
220
220
220
220
220
160
180
200
240
220
220
220
220
220
220
220
58.4
100
100
100
100
100
34.5
84.5
94.5
95.9
59.7
81.7
94.5
97.4
41
29.9
63.5
50.1
52
39
29
7.9
34.8
40.8
14.8
25.1
38.4
40.8
47.5
11.2
19.4
63.6
80.6
97.4
a The reaction was carried out with nitrobenzene (7.5 mmol)
2.2. Effect of the two-component system of PdCl2 and 1-
butylimidazole
We also attempt to optimize reaction condition by decreasing
the substrate/catalyst ratio from 100 to 25. The nitrobenzene
conversion and the phenyl isocyanate yield reached maximum
value at the ratio of 25, then it did not show any increase when
further increasing the amount of catalyst (Table 4, entries 2 and
15-17).
Under the optimized reaction conditions, we tried to find the
active species of our catalytic system. Table 6 (entries 1-3) shows
that the reaction only happened when using the two-component
system, which means that first palladium chloride and 1-
butylimidazole might have reacted with each other to create some
active species, then that active species would act as catalyst for
the reaction. For this reason, we would try to make that catalyst
in acetone and the synthesized Pd(1-butylimidazole)2Cl2 [Pd(1-
Other metal halide compounds were employed as main
catalyst to find alternative for expensive PdCl2, and the results
are listed in Table 5. Although PtCl2 and RuCl3 showed some
catalytic activity but not as good as PdCl2, NbCl5, FeCl2, and
CuCl2 showed some little activity while other metal halides
showed no activity.
1
C4Im)2Cl2] was analyzed using H NMR (Fig. S3), FTIR (Fig.
S4), and EA (Table S1).
In 1H NMR spectra of 1-butylimidazole and Pd(1-
butylimidazole)2Cl2, the protons of the 1-butyl group are similar,
while the protons of the imidazole ring experienced a downfield
shift (8.10 ppm compared to 7.60 ppm, 7.25 ppm compared to
7.15, and 7.12 compared to 6.93), suggesting that there was a
coordination between palladium and 1-butylimidazole. Also,
there are very few impurity peaks in the NMR diagram of Pd(1-
C4Im)2Cl2 complex, suggesting that the complex was well-made.
Table 5
Effect of metal halides on the reductive carbonylation of nitrobenzene
to phenyl isocyanate.a
FTIR spectra of PdCl2, 1-butylimidazole and Pd(1-C4Im)2Cl2
are shown in Fig. S4. The peaks in the 3200 – 2800 cm-1 region
belong to C–H stretching region of 1-butylimidazole [52], and
the peaks in the 1700–1000 cm-1 region belong to C=C, C–C,
C=N, C–N stretching region in the imidazole ring of 1-
butylimidazole. All of these peaks appeared in both Fig. S4b and
S4c but didn’t appear in Fig. S4a, suggesting that the complex of
PdCl2 and 1-butylimidazole was successfully made.
Entry
1
Metal Halides
PdCl2
Conversion (%)
100
Yield (%)
63.5
2
3
FeCl2
CoCl2
NiCl2
CuCl2
ZnCl2
NbCl5
RuCl3
InCl3
49.7
17.3
25.7
61.7
16.9
94.8
87
15
0
4
0
5
15
0
As indicated in Table S1, although being synthesized in
acetone, EA results of the synthesized Pd(1-C4Im)2Cl2 complex
does not correspond to the EA of acetone solvate [Pd(1-
butylimidazole)2Cl2(acetone)x], but matches with that of Pd(1-
C4Im)2Cl2 (calculated and published research) [53], also
indicating that the desired complex was successfully made.
6
7
12.2
39.7
0
8
9
26.7
30.4
97.1
10
11
SnCl2
PtCl2
0
44.1
a
The reaction was carried out with nitrobenzene (7.5 mmol), metal halide
(0.3 mmol), and 1-butylimidazole (0.9 mmol) in 25 mL of toluene at 220 oC.

Table 6
resulted in a drop in catalytic activity of Pd(1-C4Im)2Cl2, which
can be explained similarly to the above sections.
Effect of the two component systems of PdCl2 and 1-butylimidazole on the reductive
carbonylation of nitrobenzene.a
2.3. Interaction between substrates (nitrobenzene and CO) and
Pd(1-butylimidazole)2Cl2
For further understanding the interaction between
nitrobenzene and Pd(1-C4Im)2Cl2, more characterizations were
done via cyclic voltammetry (CV).
Ligands
Conversion
(%)
Yield
(%)
Entry
Catalysts
To measure the oxidation–reduction potential of nitrobenzene
with and without Pd(1-C4Im)2Cl2 as catalyst, CV analysis of
nitrobenzene together with the complex was performed and the
diagrams are described in Fig. 2. It is indicated that there is one
(Catalyst/Ligand)
1
2
3
PdCl2
-
-
24.6
26.4
100
0
0
1-butylimidazole
PdCl2
1-butylimidazole
63.5 oxidation peak at around -1.04 V and one reduction peak at
around -1.22 V (Fig. 2a), which corresponds to the one-electron
(1/3)
participating oxidation–reduction between nitrobenzene and
4
Pd(OAc)2
1-butylimidazole
7.5
0.9
(1/3)
radical (Scheme S1) [54].
5
6
Pd(1-butylimidazole)2Cl2
-
82.5
79.5
57.1
53.2
From Fig. 2, it can be clearly seen that upon adding Pd(1-
C4Im)2Cl2 the reduction peak of nitrobenzene started at a more
positive position than when not using any catalyst or when using
Pd(OAc)2 (-0.95 V compared to -1.02 V and -1.05 V,
respectively). It suggests that the presence of Pd(1-C4Im)2Cl2 as
catalyst would take less energy for nitrobenzene to undergo
reduction, thus the reductive carbonylation of nitrobenzene
would take place more easily than Pd(OAc)2 catalyst (Table 6,
entries 3-5). Moreover, the increased current for nitrobenzene
reduction after addition of Pd(1-C4Im)2Cl2 can be related to the
increased turnover frequency which corresponds to higher phenyl
isocyanate yield at constant reaction time (Tables 5-7).
Pd(1-butylimidazole)2Cl2 1-butylimidazole
(1/1)
a
The reaction was carried out with nitrobenzene (7.5 mmol), catalyst (0.3
mmol), and 1-butylimidazole in 25 mL of toluene at 220 oC.
Table 6 shows that when applied as catalysts, Pd(1-C4Im)2Cl2
prepared in acetone gave as nearly good result as our best
catalytic system consisting of PdCl2 and 1-butylimidazole (1/3)
(entries 3 and 5), but the activity decreased when adding more 1-
butylimidazole (entry 6), suggesting that when more 1-
butylimidazole is added the active species is affected and its
catalytic activity decreased.
Contrary to PdCl2, Pd(OAc)2 didn’t show any catalytic
activity when using together with 1-butylimidazole, which may
attributed to inappropriate formation of active site (entry 4). It is
also noteworthy that the type of Pd precursor greatly influence
the catalytic activity.
Table 7
Activities of Pd(1-butylimidazole)2Cl2 in the reductive carbonylation of
nitrobenzene to phenyl isocyanate.a
Temperature
(oC)
Pressure
(psi)
Yield
(%)
Entry
Conversion (%)
Fig. 2. Cyclic voltammetry (CV) profiles of (a) nitrobenzene, and its
1
2
3
4
5
6
7
220
220
220
220
180
200
240
1400
1000
1200
1725
1400
1400
1400
82.5
39.3
98.1
85.8
36.8
69.7
99.1
57.1
18.6
26.9
37.7
21.5
39.3
27.1
mixture with (b) Pd(OAc)2 and (c) Pd(1-C4Im)2Cl2 in acetonitrile.
Finally, the proposed mechanism of palladium-catalyzed
reductive carbonylation of nitrobenzene into phenyl isocyanate
(as shown in Scheme S2) could be explained similarly to the
previously proposed mechanism [54], where a series of stepwise
deoxygenation/carbonylation processes involving metallacyclic
intermediates take place. These intermediates containing the
metallacyclic carbonyl group could be detected by FTIR. To
prove this proposition, Pd(1-C4Im)2Cl2 complex was also
introduced with CO during 24 h at room temperature and its
FTIR spectrum was measured (Fig. 3). It exhibits two
characteristic ν(CO) bands appeared at 2167 and 2123 cm-1 in the
solid state probably due to solid-state splitting [55] in the metal
carbonyl complexes. This strongly proves that there are some
interactions between palladium and CO but, it was limited by
detection of intermediates during the reaction.
a
The reaction was carried out with nitrobenzene (7.5 mmol) and Pd(1-
C4Im)2Cl2 (0.3 mmol) in 25 mL of toluene.
Once again, to determine the optimized conditions for the
reaction, Pd(1-C4Im)2Cl2 was also tested as catalyst in different
variations of pressure and temperature, which is shown in Table
7. Similar to the PdCl2/1-butylimidazole (1/3) system, 220 oC and
1400 psi seem to be the best reaction condition, where any
changes in pressure (entries 2-4) and temperature (entries 5-7)

6
Tetrahedron
rate was 20 mV/s and the response was observed under a
potential from 0 to -2 V, using a chronoamperometric method.
The reduction peak current value for nitrobenzene inserted in Fig.
2 is acquired when the starting current is zero. Elemental analysis
(EA) was conducted on a FlashSmart CHNS/O Elemental
Analyzer (Thermo Scientific, MA, USA).
4.2. Preparation of PdCl2 and 1-butylimidazole complexes
Pd(1-butylimidazole)2Cl2 [Pd(1-C4Im)2Cl2] was synthesized
by the following method: PdCl2 (0.5 mmol) and 1-butylimidazole
(1 mmol) in 50 mL of acetone were stirred at reflux condition for
24 h. After the solvent was evaporated, the complex was
precipitated in acetone/hexane, filtered, and washed with hexane,
then dried under vacuum overnight. Light yellow powder,
1
94.18% yield. H NMR (DMSO-d6, 300 MHz): δ 8.10 (s, 1H),
7.26 (t, J = 1.5 Hz, 1H), 7.19–7.09 (m, 1H), 4.01 (t, J = 7.1 Hz,
2H), 1.78–1.53 (m, 2H), 1.19 (dq, J = 14.5, 7.3 Hz, 2H), 0.86 (t, J
= 7.3 Hz, 3H). FTIR (acetone, cm-1): νmax 3160, 3151, 3120 (–CH
stretching, imidazole); 2960, 2933, 2923, 2875, 2860 (–CH
stretching, –CH3 and –CH2); 1625, 1536, 1517 (–C=N stretching,
imidazole), 1469, 1461 (–C=C–); 1373, 1346 (C–C); 1261, 1240
(C–N stretching, imidazole); 1114, 1097 (–C–H bending,
imidazole). Elemental Analysis: Calculated for Pd(C7H12N2)2Cl2
= PdC14H24N4Cl2, C, 39.50%; H, 5.68%; N 13.16%. Found C,
39.50%; H, 5.84%; N, 13.11%.
Fig. 3. FTIR spectra of (a) neat Pd(1-C4Im)2Cl2 and (b) 24 h after
exposed to CO at room temperature.
3. Conclusions
In this research, after optimizing the reaction conditions by
testing various catalytic systems consisting of different types of
ligands, promoters with different reaction time, temperature, and
pressure, we developed a catalyst system comprising PdCl2 and
alkylimidazole (1:3) for producing phenyl isocyanate from
reductive carbonylation of nitrobenzene. Best result was 100%
conversion and 63.5% yield (63.5% selectivity) using 1-
butylimidazole as ligand within 2 h at 220 oC and 1400 psi of CO
in toluene with substrate/catalyst molar ratio of 25. We were also
successful in synthesizing the Pd(1-C4Im)2Cl2 complex, which is
believed to be the most probable catalyst of the reaction and
achieved comparable result of 82.5% conversion and 57.1% yield
(69.2% selectivity).
4.3. Catalytic testing
All the palladium-catalyzed reductive carbonylation reactions
were conducted in a 100 mL stainless steel autoclave equipped
with a magnetic stirrer and electrical heater. Nitrobenzene (7.5
mmol), different type of palladium catalyst (0.25 mmol,
substrate/catalyst = 25), ligand and promoter (if necessary), and
toluene (15 mmol) were added to the autoclave, respectively. The
reactor was sealed and flushed three times with carbon
monoxide. Then carbon monoxide was introduced into the
autoclave (1400 psi), and the reactor was heated up to required
temperature with stirring. Upon completion, the reactor was
cooled to room temperature, and the residual gas was released.
The reaction mixture was analyzed by gas chromatography (GC).
Quantitative analyses were made on an Agilent 6890N gas
chromatograph equipped with a flame ionization detector (FID)
equipped with an DB-5MS column (30 m x 0.25 mm x 0.25 μm)
and qualitative analyses were achieved on an Agilent 6890N-
5975 mass spectrometer-gas chromatograph (MSD-GC)
equipped with an HP-5 column (30 m × 0.32 mm × 0.25 μm).
Further research on this matter will try to find more effective
ligands and more customized reaction conditions throughout the
studying the mechanism of the reaction, as well as the recovery
of phenyl isocyanate from the reaction mixture.
4. Experimental
4.1. Materials
All chemicals and reagents (salts, ligands, and oxides) were
purchased from Sigma-Aldrich Chemical Co., U.S.A., and used
immediately without any further purification. Toluene was of
analytic grades and was distilled with appropriate drying agents
under a nitrogen atmosphere prior to use. An over 99% purity
carbon monoxide cylinder was purchased from Gong-Dan
Industrial Gas Co, Korea.
The conversion of nitrobenzene and the yield and selectivity
for phenyl isocyanate were calculated based on the calibration
curves of pure samples of substrates and products using the
following equations:
(
)
Conversion of nitrobenzene %
(
)
푁ꢀ푡푟표푏푒푛푧푒푛푒 푟푒푚푎ꢀ푛푒푑 (푏푦 퐺퐶 − 퐹퐼퐷) 푚푚표푙
=
× 100%
The proton nuclear magnetic resonance (1H NMR) spectra
were recorded on a Bruker Spectrospin 300 MHz spectrometer at
푁ꢀ푡푟표푏푒푛푧푒푛푒 푐ℎ푎푟푔푒푑 (푚푚표푙)
(
)
Yield of phenyl isocyanate %
o
25 C with tetramethylsilane (TMS) as an internal standard. The
(
)
푃ℎ푒푛푦푙 ꢀ푠표푐푦푎푛푎푡푒 푝푟표푑푢푐푒푑 (푏푦 퐺퐶 − 퐹퐼퐷) 푚푚표푙
Fourier-transform infrared spectroscopy (FTIR) spectra were
recorded using Nicolet FTIR spectrometer (Nicolet 6700,
Thermoelectron Co., MA, USA). The electrochemical properties
were studied using an electrochemical workstation (VSP
Potentiostat, Bio-Logic Science Instruments SAS, France).
Cyclic voltammetry (CV) analysis was conducted using 32 mM
nitrobenzene with 32 mM of Pd catalysts in acetonitrile at 25 °C
and 0.1 M tetrabutylammonium perchlorate was used as
electrolyte. Glassy carbon, Pt wire, and Ag/AgCl were used as a
working, counter, and reference electrode, respectively. The scan
=
푁ꢀ푡푟표푏푒푛푧푒푛푒 푐ℎ푎푟푔푒푑 (푚푚표푙)
× 100%
Selectivity of phenyl isocyanate %
푌ꢀ푒푙푑 표푓 푝ℎ푒푛푦푙 ꢀ푠표푐푦푎푛푎푡푒 (푏푦 퐺퐶 − 퐹퐼퐷) %
(
)
(
)
=
× 100%
(
)
퐶표푛푣푒푟푠ꢀ표푛 표푓 푛ꢀ푡푟표푏푒푛푧푒푛푒 (푏푦 퐺퐶 − 퐹퐼퐷) %
Acknowledgments

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Supplementary Material
Supplementary material that may be helpful in the review
process should be prepared and provided as a separate electronic
file. That file can then be transformed into PDF format and
submitted along with the manuscript and graphic files to the
appropriate editorial office.

8
Tetrahedron
Highlights
 PdCl2 with 1-butylimidazole (1:3) is
active for direct reductive carbonylation

Active species Pd(1-C4Im)2Cl2 was
successfully synthesized for the
carbonylation


Interaction between substrates and
Pd(1-C4Im)2Cl2 was detected
Mechanism involving metallacyclic
carbonyl compounds of palladium is
proposed

Declaration of interests
☒ The authors declare that they have no known
competing financial interests or personal
relationships that could have appeared to influence
the work reported in this paper.
☒The authors declare the following financial
interests/personal relationships which may be
considered as potential competing interests: