Oxidative carbonylation of aniline with new cobalt catalytic systems

Article abstract of DOI:10.1139/v05-080

New neutral cobalt(II) complexes containing 2,9-bis(2-hydroxyphenyl)-1,10- phenanthroline and 2,9-bis(2-hydroxyphenyl)-2,2′-bipyridine ligands have been synthesized. These complexes are active catalysts in the oxidative carbonylation of aniline in 1-butanol and NaI as a promoter. The principal products obtained are N,N′-diphenylurea and 1-butyl-N-phenylcarbamate.

Full text of DOI:10.1139/v05-080

764
Oxidative carbonylation of aniline with new cobalt
catalytic systems1
Aránzazu Orejón, Aída Castellanos, Pilar Salagre, Sergio Castillón, and
Carmen Claver
Abstract: New neutral cobalt(II) complexes containing 2,9-bis(2-hydroxyphenyl)-1,10-phenanthroline and 2,9-bis(2-
hydroxyphenyl)-2,2′-bipyridine ligands have been synthesized. These complexes are active catalysts in the oxidative
carbonylation of aniline in 1-butanol and NaI as a promoter. The principal products obtained are N,N′-diphenylurea and
1-butyl-N-phenylcarbamate.
Key words: cobalt, oxidative carbonylation, aniline, N,N′-diphenylurea, 1-butyl-N-phenylcarbamate.
Résumé : Des complexes du cobalt(II) contenant les ligands 2,9-bis(2-hydroxyphenyl)-1,10-phénanthroline et 2,9-bis(2-
hydroxyphényl)-2,2′-bipyridine ont été synthétisés. Ces complexes sont des catalyseurs actifs pour la réaction de carbo-
nylation oxydante de l’aniline dans le 1-butanol, en utilisant le NaI comme promoteur. Les produits principaux de la
réaction sont le N,N′-diphénylurée et le N-phenylcarbamate de 1-butyle.
Mots clés : cobalt, carbonylation oxydante, aniline, N,N′-diphénylurée, N-phenylcarbamate de 1-butyle.
[Traduit par la Rédaction] Orejón et al. 768
Introduction
ally developed, mainly because the use of a precious metal
such as palladium is necessary and also because the process
involves three CO molecules and, unless CO is available at a
negligible cost, the process is uneconomical.
Industrially important materials, such as high-performance
plastics, polyurethane elastomers, and adhesives, are usually
obtained from isocyanates. The reaction of phosgene with
amines is one of the most important industrial methods for
the formation of isocyanates, carbamates, and ureas (1). As
there are restrictions on the use of very toxic materials such
as phosgene, and since large amounts of corrosive HCl are
produced as a side product in the conversion of aromatic
amine to isocyanate, interest has been increasing in the de-
velopment of alternative methods for isocyanate production
(1).
One of the proposed approaches is to first synthesize the
carbamate, either by the oxidative carbonylation of amines
or by the reductive carbonylation of nitro compounds in the
presence of an alcohol, followed by thermal elimination of
alcohol to form isocyanates (2) (Scheme 1).
With regard to oxidative carbonylation, several processes
based on metal catalysts promoting the formation of carba-
mates from organic hydroxyl compounds, carbon monoxide,
amines, and an oxygen-containing oxidizing agent have also
been reported (4, 5). It is known that cobalt complexes are
active catalysts in the oxidative carbonylation of amines.
The most widely used cobalt complexes are N,N′-
bis(salicylidene)ethylenediaminocobalt(II) ([Co(salen)], 1),
[Co(tetrakis-(4-methoxyphenyl)porphine)], [Co(phthalocy-
anine)], and [CoTTP] (TTP = mesotetraphenylporphinato
dianion) (4b, 5). In this context, [Co(salen)] (1) catalyzes the
oxidative carbonylation of ortho-, meta-, and para-substituted
aromatic primary amines in methanol to give ureas, iso-
cyanates, carbamates, and azo derivatives (5).
Concerning the mechanism of this reaction, it has been
proposed that the oxidative reaction takes place through the
formation of a superoxo complex of cobalt(III), known to be
formed by coordination of the dioxygen on the [Co(salen)]-
type complexes. These complexes undergo carbonylation to
give the carbamoyl complexes that could be the origin of the
free isocyanate or the carbamate (5).
The interest in developing the reductive carbonylation of
nitro compounds is demonstrated by the numerous patents
and papers in the literature (3). However, the commercial
production of isocyanates by this route has not been industri-
Received 27 October 2004. Published on the NRC Research
Press Web site at http://canjchem.nrc.ca on 23 June 2005.
A. Orejón, A. Castellanos, P. Salagre, and C. Claver.2
Departament de Química Física i Inorgànica, Universitat
Rovira i Virgili, Marcel·lí Domingo s/n, 43007 Tarragona,
Spain.
S. Castillón. Departament de Química Analítica i Orgànica,
Universitat Rovira i Virgili, Marcel·lí Domingo s/n, 43007
Tarragona, Spain.
Modification of [Co(salen)] complexes has enabled the
improvement of the efficiency of many catalytic processes,
but as far as we know cobalt complexes with modified salen
ligands have never been studied before in the oxidative
carbonylation of amines. In this paper, we study the reaction
of oxidative carbonylation with salen-modified complexes
such as the already known complexes 2–4 (6) to determine
the effects in the catalytic activity of electron-donating 2 and
electron-withdrawing groups 3 and 4 in the aromatic rings.
We also prepared the new complexes 5 and 6 in which the
1This article is part of a Special Issue dedicated to Professor
Howard Alper.
2Corresponding author (e-mail: claver@quimica.urv.es).
Can. J. Chem. 83: 764–768 (2005)
doi: 10.1139/V05-080
© 2005 NRC Canada

Orejón et al.
765
Scheme 1. Methods for isocyanate production.
Oxidative carbonylation
RNH2 + CO + 1/2O2 + R'OH
H2O
O
RN
C
O + R'OH
R N C OR'
H
Carbamates
Isocyanates
RNO2 + 3CO + R'OH
2CO2
Reductive carbonylation
Scheme 2. Salen-modified complexes.
N
O
N
O
N
O
N
Co
Co
But
O
tBu
tBu
But
1
2
NO2
N
O
N
O
N
O
N
O
Co
Co
O2N
NO2
3
4
N
O
N
O
N
O
N
O
Co
Co
5
6
1
imine groups have been substituted by the heteroaromatic
rings 2,2′-bipyridine and 1,10-phenathroline (Scheme 2).
recorded on a Varian Gemini spectrometer with a H reso-
nance frequency of 400 MHz.
Synthesis of the ligands
Experimental
Synthesis of 2,9-bis(2-hydroxyphenyl)-1,10-phenanthroline
(7)
General procedures
All syntheses were carried out in a nitrogen atmosphere at
room temperature using standard Schlenk techniques. Sol-
vents were distilled and deoxygenated prior to use unless
otherwise stated. Aniline and the catalyst N,N-bis(salicyli-
dene)ethylenediaminocobalt(II) are commercial products.
Cobalt–salen derivative complexes were synthesized accord-
ing to literature procedures (6). Aniline was distilled with
Na prior to use. Elemental analyses were carried out in a
n-Butyllithium (16.2 mL, 25.8 mmol of a 1.6 mol/L solu-
tion) was added to a stirred solution of 2-bromophenol
(1.2 mL, 11.3 mmol) in 40 mL of anhydrous diethyl ether.
After 24 h at room temperature, the white suspension ob-
tained was slowly added to a warm solution (50 °C) of 1,10-
phenanthroline (0.388 g, 2.15 mmol) in 25 mL of anhydrous
toluene. Immediately, a red solution was formed, which was
maintained at 50 °C for 48 h. The resulting brown solution
was then cooled to 0 °C and slowly hydrolyzed with H2O
1
Carlo Erba microanalyser EA 1108. H NMR spectra were
© 2005 NRC Canada

766
Can. J. Chem. Vol. 83, 2005
Scheme 3. Method for the preparation of 9-bis(2-hydroxy-
phenyl)-1,10-phenantroline (7) and 2,9-bis(2-hydroxyphenyl)-2,2′-
bipyridine (8) ligands.
(20 mL). The organic phase was separated and the aqueous
phase was extracted with ethyl acetate. The organic phases
were combined, 2 g of MnO2 (Fluka No. 63548) was added,
and the reaction mixture was stirred at room temperature for
24 h. After this time, the solution was dried over MgSO4 for
2 h and then filtered through Celite®. The solution was
evaporated to dryness with a rotatory evaporator and puri-
fied using flash chromatography with a mixture of hexane –
ethyl acetate (2:1) to yield 0.253 g (33% yield) of 7 as a
+
2BuLi
OH
Br
1
yellow solid. H NMR (400 MHz, DMSO, 298 K) δ: 7.03
(td, J = 6.6, 1.5 Hz, 2H, arom), 7.09 (dd, J = 8.1, 0.9 Hz,
2H, arom), 7.42 (td, J = 6.9, 1.5 Hz, 2H, arom), 8.08 (s, 2H,
phenanthroline), 8.28 (dd, J = 7.8, 1.4 Hz, 2H, arom), 8.61
(d, J = 8.7 Hz, 2H, phenanthroline), 8.70 (d, J = 8.7 Hz, 2H,
phenanthroline), 13.95 (s, 2H, -OH). 13C NMR (300 MHz,
DMSO, 298 K) δ: 117.86, 119.16, 120.36, 121.31, 126.29,
127.38, 128.61, 132.0, 138.29, 141.57, 157.12, 159.11.
FAB-MS m/z: 364.4 [M]. Elemental anal. calcd.: C 79.1, H
4.42, N 7.69; found: C 78.4, H 4.44, N 7.29.
OLi
Li
N
N
N
N
H
H
H
H
N
N
N
N
Li
Li Li
OLi LiO
Li
Synthesis of 2,9-bis(2-hydroxyphenyl)-2,2′-bipyridine (8)
n-Butyllithium (16.2 mL, 25.8 mmol of a 1.6 mol/L solu-
tion) was added to a stirred solution of 2-bromophenol
(1.2 mL, 11.3 mmol) in 40 mL of anhydrous diethyl ether.
After 24 h at room temperature, the white suspension ob-
tained was slowly added to a warm solution (50 °C) of 2,2′-
bipyridine (0.35 g, 2.3 mmol) in 25 mL of anhydrous tolu-
ene. Immediately, a red solution was obtained, which was
maintained at 50 °C for 48 h. The brown solution obtained
was then cooled to 0 °C and hydrolyzed slowly with H2O
(20 mL). The organic phase was separated and the aqueous
phase was extracted with ethyl acetate. The organic phases
were combined, 2 g of MnO2 (Fluka No. 63548) was added,
and the reaction mixture was stirred at room temperature for
24 h. After this time, the solution was dried over MgSO4 for
2 h and then filtered through Celite®. The solution was evap-
orated to dryness with a rotatory evaporator and purified us-
ing flash chromatography with a mixture of CH2Cl2–hexane
OLi LiO
MnO2/H2O
MnO2/H2O
N
N
N
N
OH HO
OH HO
7
8
1 h of heating, the solution was immersed in a cold bath.
The solid was filtered and washed with water and absolute
ethanol. The red precipitate was dried under vacuum to yield
0.192 g (94% yield) of 5. Elemental anal. calcd.: C 68.42, H
3.35, N 6.65; found: C 69.00, H 3.75, N 6.69.
1
(5:1) to yield 0.11 g (14% yield) of 8 as a yellow solid. H
Synthesis of 2,9-bis(2-hydroxyphenyl)-2,2′-bipyridine
cobalt (6)
NMR (400 MHz, DMSO, 298 K) δ: 6.98 (t, J = 7.2 Hz, 2H,
arom), 7.08 (d, J = 8.4 Hz, 2H, arom), 7.37 (td, J = 7.2,
0.8 Hz, 2H, arom), 7.87 (dd, J = 8.0, 0.8 Hz, 2H, arom),
8.02 (m, 4H, bipyridine), 8.13 (dd, J = 7.2, 1.6 Hz, 2H,
bipyridine), 14.20 (s, 2H, -OH). 13C NMR (300 MHz,
DMSO, 298 K) δ: 117.86, 119.16, 120.36, 121.31, 126.29,
127.38, 128.61, 132.0, 138.29, 141.57, 157.12, 159.11. Ele-
mental anal. calcd.: C 77.63, H 4.74, N 8.23; found: C
77.00, H 4.78, N 7.98.
In a three-necked round flask equipped with a pressure-
equalizing funnel and a water condenser, 2,9-bis(2-hydroxy-
phenyl)-2,2′-bipyridine (8) (0.04 g, 0.12 mmol) was dis-
solved in 95% ethanol (3 mL) under a nitrogen atmosphere.
A solution of Co(OAc)2·4H2O (0.03 g, 0.11 mmol) in
0.6 mL of water was introduced into the pressure-equalizing
funnel and added dropwise to the refluxing solution of the
ligand. Initially, the solution became orange and a brown
precipitate started to appear. After 1 h of heating, the solu-
tion was immersed in a cold bath. The solid was filtered,
then washed with water and absolute ethanol. The brown
precipitate was dried in a vacuum desiccator to yield 0.046 g
(90% yield) of 6. Elemental anal. calcd.: C 66.50, H 3.55, N
7.05; found: C 66.12, H 3.76, N 6.90.
Synthesis of the neutral cobalt(II) complexes
Synthesis of 2,9-bis(2-hydroxyphenyl)-1,10-phenanthroline
cobalt (5)
In a three-necked round flask equipped with a pressure-
equalizing funnel and
a water condenser, 2,9-bis(2-
hydroxyphenyl)-1,10-phenanthroline (7) (0.189 g, 0.52 mmol)
was dissolved in 95% ethanol (12 mL) under a nitrogen at-
mosphere. A solution of Co(OAc)2·4H2O (0.121 g, 0.486 mmol)
in 3 mL of water was introduced into the pressure-equalizing
funnel and added dropwise over the reluxing solution of the
ligand under a nitrogen atmosphere. Initially, the solution
became orange and a red precipitate started to appear. After
General procedure for the oxidative carbonylation of
amines
A mixture of the substrate (13.2 mmol), cobalt complex
(0.25 mmol), NaI (2.72 mmol for sust/NaI = 5 or 1.89 mmol
for sust/NaI = 7), and 1-butanol (20 mL) was charged in a
100 mL autoclave. The autoclave was flushed with carbon
© 2005 NRC Canada

Orejón et al.
767
Scheme 4. Method for the preparation of neutral(II) cobalt complexes.
N
N
N
O
N
O
EtOH 95%
Co
.
+ Co(OAc)2 4H2O
OH HO
7
1,10-Phenanthroline
2,2-Bipyridine
5
6
1,10-Phenanthroline
2,2-Bipyridine
=
=
N
N
N
N
8
Scheme 5. Oxidative carbonylation of aniline.
[1]
O
[cat]
2
NH2 + CO + 1/2O2
N
H
C
+ H2O
N
H
9
O
O
C OR
[2]
N
H
C
N
H
+ ROH
N
H
NH2
+
10
monoxide and pressurized to 50.1 bar (1 bar = 100 kPa),
then carefully pressurized with oxygen to a total pressure of
52.7 bar (4.93% O2–CO) (1 bar = 100 kPa). (Caution! Re-
verse addition, i.e., O2 before CO, is dangerous). The auto-
clave was heated electrically to the desire temperature and
stirred for 3 h. The reaction mixture was cooled to room
temperature, then concentrated by rotatory evaporation. The
the 9-bis(2-hydroxyphenyl)-1,10-phenanthrolinecobalt(II) (5)
and 2,9-bis(2-hydroxyphenyl)-2,2′-bipyridinecobalt(II) (6) in
good yields (Scheme 4). Elemental analysis confirmed the
formation of the cobalt complexes.
Oxidative carbonylation of aniline
To avoid the use of phosgene, carbamates were synthe-
sized from the oxidative carbonylation of aniline according
to the following reaction (Scheme 5), where aniline reacted
catalytically with carbon monoxide and oxygen to form
N,N′-diphenylurea, which further reacted to form the
carbamate.
1
crude material was analyzed by H NMR in deuterated ace-
tone.
Results and discussion
Reaction [1] (Scheme 5) is a catalytic reaction, while rxn.
[2] (Scheme 5) is essentially thermal. However, the second
reaction can be catalysed by Lewis acids. Although the two
steps are independent, the alcoholisis of the urea to form the
carbamate releases aniline, which is the starting material in
the first reaction. Both reactions therefore become dependent
on one another, and full conversion is achieved by an itera-
tive process.
Therefore, at a given temperature, the reaction of the con-
version of ureas into carbamates will be similar and the ratio
of carbamate to urea will be an indicator of the activity of
the catalysts in the oxidative carbonylation. Iodide as the ad-
ditive has a beneficial effect on the reaction rate (8).
Results for the cobalt precursors 1–6 are summarized in
Table 1. To compare the activity with other cobalt catalytic
systems, the [Co(salen)] complex 1 was first used as the cat-
alyst precursor in the oxidative carbonylation of aniline. Ini-
tially, the reaction was performed in 1-butanol at 130 °C
(internal temperature of 110 °C) at 60 bar (1 bar = 100 kPa)
and with NaI as the additive. Carbamate (44%) and 56% of
urea were obtained with a total conversion (Table 1, entry 1).
When tert-butyl groups were introduced into the aromatic
rings of the salen ligand (precursor 2) only the urea deriva-
tive was obtained (Table 1, entry 3). As expected, when the
Synthesis of the ligands 9-bis(2-hydroxyphenyl)-1,10-
phenanthroline (7) and 2,9-bis(2-hydroxyphenyl)-2,2′-
bipyridine (8)
In this paper we prepared the ligands 9-bis(2-hydroxy-
phenyl)-1,10-phenanthroline (7) and 2,9-bis(2-hydroxy-
phenyl)-2,2′-bipyridine (8) (Scheme 3) following the method
published by Burger and Baumeister (7).
This method is shown in Scheme 3. First, a solution of n-
butyllitium (1.6 mol/L) in hexanes was added to a solution
of 2-bromophenol in diethyl ether. After 12 h at room tem-
perature, the lithium salt was obtained as a white suspen-
sion. This salt was added dropwise to a solution of 1,10-
phenathroline or 2,2′-bipyridine in toluene, which was
heated at 50 °C. The solution became red and after 48 h at
50 °C, hydrolysis and treatment with MnO2 led to the isola-
tion of the ligands. The yields were generally low, but at-
tempts to increase them failed.
Synthesis of the neutral cobalt(II) complexes 9-bis(2-
hydroxyphenyl)-1,10-phenanthrolinecobalt(II) (5) and
2,9-bis(2-hydroxyphenyl)-2,2′-bipyridinecobalt(II) (6)
Reaction of the ligands 9-bis(2-hydroxyphenyl)-1,10-
phenanthroline (7) and 2,9-bis(2-hydroxyphenyl)-2,2′-bipyri-
dine (8) with Co(OAc)2·4H2O in an ethanolic solution gave
© 2005 NRC Canada

768
Can. J. Chem. Vol. 83, 2005
Table 1. Oxidative carbonylation of aniline.a,b
Complexes 5 and 6, together with the already known 1–4,
were active as catalyst precursors in the oxidative carbonyl-
ation of aniline to give N,N′-diphenylurea and carbamates.
The use of [Co(salen)] derivatives with withdrawing
groups in the ligand, particularly in the case of complex 4,
led to a higher conversion into the carbamate than with com-
plex 1. This shows that the modification of salen complexes
can be an efficient way to improve conversions in the oxida-
tive carbonylation of amines.
Entry
Precursor
T/Tint (°C)
9 (%)
10 (%)
56
38
1
2
3
4
5
6
7
8
1
1
2
3
4
5
5
6
130/110
160/130
130/110
160/130
160/130
130/110
160/130
160/130
44
62
—
63
83
6
100
37
17
94
72
50
28
50
References
a0.26 mmol catalyst; 3%–5% O2/CO; 13.2 mmol aniline; 20 mL 1-
BuOH; 2.72 mmol NaI; P = 60 bar (1 bar = 100 kPa); (sust/NaI = 7,
sust/cat = 50).
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reaction with the [Co(salen)] precursor was performed at
160 °C (internal temperature of 130 °C), the ratio of
carbamate to urea increased to 62:38 (Table 1, entry 2).
To study the effect of withdrawing substituents in the
ligand, we also examined the activity of complexes 3 and 4
with nitro groups in the phenyl rings of the ligand. Intro-
ducing a nitro group in the para position to the oxygen atom
led to an active catalyst that provided
a
similar
carbamate:urea ratio to the one obtained using [Co(salen)] as
the catalyst precursor (Table 1, entries 2 and 4). When the
nitro group was present in the ring bearing the amino groups
(precursor 4), the results were better, and 83% of the
carbamate was obtained with a carbamate:urea ratio of 83:17
(Table 1, entry 5).
The cobalt precursor 5 at 130 °C (internal temperature of
110 °C) provided urea in an almost exclusive way (Table 1,
entry 6), while at 160 °C the ratio of carbamate to urea in-
creased to 28:72 (Table 1, entry 7), but this was smaller than
when [Co(salen)] (1) was used (Table 1, entry 2), so the ac-
tivity of this complex was low. Therefore, when the tempera-
ture increased, the overall yield of the process also
increased, mainly because the conversion rate of urea into
carbamate increased.
The cobalt compound with the bipyridine ligand 6 was
also active at 160 °C (internal temperature of 130 °C) in the
oxidative carbonylation of aniline. Complete conversion was
obtained and the carbamate:urea ratio was 50:50 (Table 1,
entry 8), i.e., the activity was higher than with complex 5,
which had a phenathroline ligand (Table 1, entry 7).
7. P. Burger and J.M. Baumeister. J. Organomet. Chem. 575, 214
(1999).
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catalyzed reactions, see: K. Fagnou and M. Lautens. Angew.
Chem. Int. Ed. 41, 26 (2002).
Conclusions
New cobalt catalytic systems 5 and 6 have been prepared
and characterized.
© 2005 NRC Canada