A novel ZrO2-SO42- supported palladium catalyst for syntheses of disubstituted ureas from amines by oxidative carbonylation

Article abstract of DOI:10.1016/S0040-4039(01)00124-1

The syntheses of disubstituted ureas by carbonylation of a series of amines in the presence of a sulfate modified zirconia supported palladium catalyst was investigated at an initial total pressure of 4.0 MPa and 135°C. High conversions and yields were achieved for the synthesis of symmetric dialkylureas. This supported catalyst could also be easily separated and recovered after reaction.

Full text of DOI:10.1016/S0040-4039(01)00124-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 2161–2163
2−
A novel ZrO2–SO4 supported palladium catalyst for syntheses
of disubstituted ureas from amines by oxidative carbonylation
Feng Shi, Youquan Deng,* Tianlong SiMa and Hongzhou Yang
State Key Laboratory for Oxo Synthesis and Selective Oxidation and The Laboratory of Environmental and Applied Catalysis,
Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
Received 27 November 2000; revised 17 January 2001; accepted 19 January 2001
Abstract—The syntheses of disubstituted ureas by carbonylation of a series of amines in the presence of a sulfate modified zirconia
supported palladium catalyst was investigated at an initial total pressure of 4.0 MPa and 135°C. High conversions and yields were
achieved for the synthesis of symmetric dialkylureas. This supported catalyst could also be easily separated and recovered after
reaction. © 2001 Elsevier Science Ltd. All rights reserved.
The disubstituted ureas are very useful intermediates
for the production of pharmaceuticals and agricultural
chemicals.1–3 The traditional syntheses for symmetric
dialkylureas include reactions of amines with carbamyl
2−
Pd/ZrO2–SO4 was prepared as follows: PdCl2 (0.18 g)
halides, isocyanates or phosgene.4,5 Highly toxic and
was mixed with zirconyl chloride solution (12 ml, 0.62
corrosive phosgene or hydrogen chloride were
M), and under stirring, a 25% NH3–H2O solution was
inevitably involved in these processes, which caused
added dropwise into the solution until the pH reached
serious problems of environmental pollution and equip-
8–9. The resulting precipitate was filtered, washed sev-
ment corrosion. Recently, a process for preparing 1,3-
eral times with distilled water, and dried at 120°C for
disubstituted ureas through reacting a cyclic carbonic
10 h, and then crushed and screened. The resulting
acid ester with an amine over a proper catalyst was
powder was further impregnated with H2SO4 (15 ml, 1
disclosed.6 This method, however, was expensive to use
M) for 25–30 min, and then it was calcined in air at
on a large scale. The transformation of amines through
600°C for 2 h. The XPS analysis (VG ESC ALAB 210)
catalytic carbonylation to produce disubstituted ureas
showed that S6+ and Pd2+ were detected and the Pd and
provides an alternative environmental benign method
sulfate contents on the catalyst surface were ca. 6.3 and
and has been investigated over many years. Homoge-
4.6 wt%, respectively. For the purpose of comparison,
neous Pd,7–9 Ru,10 Co,11 Mn,12 Se,13–15 and more
Ru/ZrO2–SO42− was also prepared with the same proce-
recently, W16,17catalysts were used for the oxidative
2−
dure as Pd/ZrO2–SO4
.
carbonylation of amines into the ureas, and the yields
were usually good to excellent. One of the problems
associated with the previously reported catalyst system
was the isolation of catalysts from the products and
their reusability, and a stoichiometric oxidizing reagent
such as I2 may be used.
The results of catalytic carbonylation of n-butylamine
for the synthesis of the corresponding symmetric
dialkylureas showed that only low and moderate activi-
2−
ties were observed when using ZrO2–SO4 or PdCl2 as
the catalysts, respectively (Table 1, entries 1 and 2).
Much better results, however, were obtained if the
In this work, the syntheses of dialkylureas by oxidative
carbonylation of a series of amines catalyzed by a
sulfate modified dioxide zirconium supported palladium
catalyst were investigated. It was found that excellent
performance for the synthesis of symmetric aliphatic
dialkylureas was achieved:
2−
Pd/ZrO2–SO4
catalyst was employed and almost
100% conversion and 98% selectivity were achieved
(entry 3). This indicated that there might exist syner-
gism between Pd2+ and ZrO2–SO42−, which could sig-
nificantly enhance the catalyst performance. Since the
resulting dialkylureas were not dissolved in CH3CN,
crystallized product was formed and the catalyst and
the desired product could be separated by simple filtra-
tion. The pure product could be easily obtained by
* Corresponding author. Fax: +86-931-8277088; e-mail: ydeng@
ns.lzb.ac.cn
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00124-1

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F. Shi et al. / Tetrahedron Letters 42 (2001) 2161–2163
In order to examine the scope and the limitation of this
catalyst system, iso-butyl amine and two secondary
amines as substrates were also tested. As for iso-butyl
amine (entry 8), the selectivity for desired di-isobutyl
urea was moderately reduced in comparison with that
of n-butylamine. Only 80% conversion and 37% selec-
tivity were achieved when using dibutylamine as sub-
strate (entry 9). Even poor results were obtained for the
benzyl methylamine (entry 10). The main by-products
were corresponding formamides for the secondary
amines. The results suggested that the catalytic perfor-
mance could be remarkably inhibited by the steric
hindrance of the substrate molecules.
sieving the resulting solid mixture, for example, 79% of
the directly isolated yield was obtained for the dibutyl-
urea (Table 1, entry 3), since the catalyst particles and
the crystallized product did not adhere to each other
and the size of the catalyst particles was much smaller
than that of the crystallized product.
Other primary aliphatic amines, including hexylamine,
cyclohexylamine, and dodecylamine were also tested
2−
with Pd/ZrO2–SO4 catalyst (entries 4–6). The results
suggested that the directly isolated yield would be
increased remarkably with the increasing of the C num-
ber or the chain length of the attached alkyl, e.g. almost
99% of directly isolated yield could be achieved when
using dodecylamine as the substrate. This may be par-
tially due to even further lower solubility of those
amines due to the higher C number or the longer chain
length of the attached alkyl. These results also sug-
gested that better yield could be obtained with proper
choice of solvent.
2−
Under same reaction conditions, Ru/ZrO2–SO4 was
used as catalyst (entries 11–14) and it can be seen that
2−
Ru/ZrO2–SO4 was relatively less effective for the car-
bonylation of amines to afford the corresponding ureas
in comparison with Pd/ZrO2–SO42−. The fact that
higher conversion and selectivity for aliphatic amines
but poor conversion and selectivity for aromatic amine
could also be achieved over the Ru/ZrO2–SO42− catalyst
indicated that the reaction mechanism of carbonylation
over these two catalysts were similar.
Only a moderate conversion and poor selectivity were
obtained when aniline was used as substrate (entry 7).
The main by-products were quinazoline (32%), benzene
isocyamato (25%), and N-phenyl formamide (2%).
Since the conversion and selectivity for the desired
product was so low, no crystallized product could be
separated out directly. Such low reactivity of aniline
towards the oxidative carbonylation may be related to
the weak nucleophilicity of aniline.
2−
Using Pd/ZrO2–SO4 as catalyst, an attempt for the
syntheses of asymmetric disubstituted ureas by oxida-
tive carbonylation of an aliphatic amine with aniline
(Table 2) was also made, in which aniline was also
worked as solvent. Since aniline was excessive, the
conversion of other aliphatic amine was higher than
Table 1. Results of syntheses of symmetric urea by oxidative carbonylation of aminesa
a The reactions were conducted in a stainless steel autoclave with a glass tube inside. Cat.: 35 mg; substrate: 2 ml (liquid substrate) or 1.5 g (solid
substrate); solvent: CH3CN, 15 ml; P(CO)=3.5 MPa; P(O2)=0.5 MPa; T=135°C; t=1 h. After reaction, the reacted mixture was directly
analyzed with a HP 6890/5793 GC–MS instrument equipped with a HP-5MS column (30 cm in length). The qualitative and quantitative data
of the substrate and products were directly given by the system of GC–MS chemstation according to the area of each chromatograph peak.

F. Shi et al. / Tetrahedron Letters 42 (2001) 2161–2163
2163
Table 2. Results of synthesis of asymmetric ureas by oxidative carbonylation of aminesa
a Same conditions as in Table 1 but the solvent was 15 ml aniline.
90%, and two kinds of ureas were produced, i.e. sym-
metric and unsymmetric disubstituted ureas. This sug-
gested that competition reactions exist between similar
and different amines. Since the resulting ureas were well
soluble in the aniline, no crystallized product could be
separated out directly. It could be seen that the best
result was obtained in the synthesis of 1-phenyl, 3-hexyl
urea, with conversion and selectivity of 99 and 88%,
respectively (Table 2, entry 2). As to n-butylamine and
cyclohexylamine, the formation of symmetric disubsti-
tuted ureas was more favourable.
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In summary, the syntheses of disubstituted ureas by
oxidative carbonylation of a series of aliphatic amines
in the presence of a sulfate modified zirconia supported
palladium catalyst could be performed with excellent
conversion and selectivity. This supported catalyst
could also be easily separated and recovered from the
reaction medium after carbonylation. The reaction con-
ditions could still be further optimized. To the best of
our knowledge this is the first reported study of a
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