Cobalt/rhodium heterobimetallic nanoparticle-catalyzed oxidative carbonylation of amines in the presence of carbon monoxide and molecular oxygen to ureas

Article abstract of DOI:10.1002/adsc.200900106

An environmentally friendly oxidative carbonylation of aliphatic and aromatic primary amines to ureas has been successfully achieved in the presence of a catalytic amount of cobalt/rhodium heterobimetallic nanoparticles without any promoters. The catalyst system could be reused with only a slight loss of catalytic activity.

Full text of DOI:10.1002/adsc.200900106

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DOI: 10.1002/adsc.200900106
Cobalt/Rhodium Heterobimetallic Nanoparticle-Catalyzed
Oxidative Carbonylation of Amines in the Presence of Carbon
Monoxide and Molecular Oxygen to Ureas
Ji Hoon Park,a Jae Chun Yoon,a and Young Keun Chunga,*
a
Intelligent Textile System Research Center, and Department of Chemistry, College of Natural Sciences, Seoul National
University, Seoul 151-747, Korea
Fax : (+82)-2-889-0310; phone: (+82)-2-880-6662; e-mail: ykchung@snu.ac.kr
Received: February 17, 2009; Published online: June 2, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900106.
Abstract: An environmentally friendly oxidative
carbonylation of aliphatic and aromatic primary
amines to ureas has been successfully achieved in
the presence of a catalytic amount of cobalt/rhodi-
um heterobimetallic nanoparticles without any pro-
moters. The catalyst system could be reused with
only a slight loss of catalytic activity.
volume ratio, but their small size also results in a
higher surface area that makes their surface atoms
very active. To sustain their high activity, it is impor-
tant to stabilize catalytic metallic nanoparticles. Re-
cently, the use of heterobimetallic nanoparticles as
catalysts has attracted much attention because their
catalytic performance is generally superior to that of
a single nanometal by itself and there is potential to
create new types of catalysts for reactions which may
not be achieved by monometallic catalysts.[10] We re-
cently reported that cobalt/rhodium nanoparticles[11]
[Co2Rh2, derived from Co2Rh2(CO)12] immobilized on
charcoal (Co2Rh2/C) were quite useful catalysts in car-
bonylation reactions, including the Pauson–Khand re-
Keywords: aerobic oxidation; amines; bimetallic
catalysts; carbonylation; nanoparticles; ureas
Recently, the development of new synthetic strategies action,[12] silylcarbocyclization,[13] cyclohydrocarbony-
that take green chemistry into consideration has lation,[14] and aminocarbonylation.[15] In the explora-
become a very important topic.[1] Catalysis can be tion of the use of Co2Rh2/C as a catalyst, we recently
considered as one of the central themes in the green studied a Co2Rh2/C-catalyzed oxidative carbonylation
chemistry. Among the catalytic reactions, a transition of amines in the presence of carbon monoxide. Here,
metal-catalyzed oxidative transformations of amines we report on our recent intriguing findings in the
is one of the most important functional transforma- Co2Rh2/C-catalyzed oxidative carbonylation of amines
tions in organic synthesis, because they produce to ureas.
useful nitrogen compounds that are versatile building
Our catalytic system is quite effective for the oxida-
blocks and intermediates for the synthesis of biologi- tive carbonylation of amines to ureas in the presence
cally active nitrogen compounds.[2,3] In the context of of carbon monoxide and can be reused several times
the green chemistry, an aerobic oxidation is recog- with only a slight loss of catalytic activity. This is the
nized to be very important. Thus, many methods for first example of the use of a nanoparticle catalyst in
transition metal-catalyzed aerobic oxidative transfor- the oxidative carbonylation of amino compounds to
mation of amines to the corresponding imines,[4] car- ureas.
bamates,[5] nitrones,[6] and ureas[7] have been explored.
The initial investigation of oxidative carbonylation
Particularly, most Pd-based catalysts showed high ac- was carried out using benzylamine (1a) as a probe
tivity, but the reactions are mostly promoted by an substrate to optimize the reaction conditions, and the
iodine-containing promoter,[8] which is very corrosive results are summarized in Table 1. As Table 1 shows,
in the reaction system.
the catalytic reaction went to completion in the ab-
The emergence of transition metal nanoparticles sence of any promoters and the yields obtained
has led to an explosive growth in catalysis.[9] Transi- ranged from 45% to 74%. Thus, the reaction was not
tion metal naoparticles are potentially attractive cata- highly sensitive to the pressures of CO and O2, the
lysts due to their small size and high surface-to- ratio of CO:O2, the reaction time, and the solvent.
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Ji Hoon Park et al.
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Table 3. Co2Rh2-catalyzed oxidative carbonylation of various
Table 1. Optimization of Co2Rh2 catalyzed oxidative carbon-
aliphatic amines.[a]
ylation of benzylamine.[a]
Entry Amine
Urea
Yield
[%][b]
1
2
3
4
2b 84
Entry Solvent
CO/O2 [atm] Time [h] Yield [%][b]
1
2
3
4
5
6
7
8
Toluene
Toluene
Methanol 4/1
Toluene
Toluene
THF
Toluene
toluene
3/0.5
4/1
15
12
12
3
45
67
51
64
71
68
74
0[c]
3b 73
4b 71
5b 59
6b 44
4/1
4/0.5
4/0.5
4/0.1
5/0
3
15
3
12
[a]
Reaction conditions: benzylamine (1.0 mmol), Co2Rh2
catalyst (3 mol%, 50 mg), CO, and O2 in 2 mL solvent at
1008C.
Isolated yield.
N-Benzylformamide was obtained in 34% yield.
5
[b]
[c]
[a]
Reaction conditions: aliphatic amine (1.0 mmol), CO (5
atm), O2 (0.1 atm), and Co2Rh2 catalyst (3 mol%, 50 mg)
in 2 mL toluene at 1008C for 3 h.
[b]
Isolated yield.
However, each affected the yield of the reaction to a
certain extent. As the pressure of oxygen decreased,
the yield gradually increased. Interestingly, without spectively. Thus, the leaching was negligible. When
any oxygen, N-benzylformamide instead of urea was the recovered catalyst after the first run was treated
obtained in 34% yield. The best yield (74%) was ob- with 1 atm of CO for 20 min, the yield of the reaction
tained when the reaction was carried out in toluene slightly increased to 65%. However, when the same
under CO and O2 (4 atm/0.1 atm) for 3 h.
catalyst was treated with hydrogen, the yield of the
To check the recyclability, Co2Rh2/C was separated reaction was 54%.
and reused 4 times (Table 2, entries 1, 2, 5, and 6).
After optimization, oxidative carbonylations of
Unfortunately, the activity of the catalytic system other aliphatic primary amines were studied
gradually decreased from 74% to 58%, 35%, and (Table 3). As seen in Table 3, all the reaction went to
31%, respectively. An ICP-AES study shows that the completion within 3 h under relatively mild reaction
percentages of cobalt and rhodium bled from Co2Rh2/ conditions and the yield was highly dependent upon
C after the first catalytic run was ca. 0.24% of the the substrate. When butylamine (entry 1) was the sub-
original cobalt and 0.2% of the original rhodium, re- strate, the yield was 84%. The sterically bulky tert-bu-
tylamine (entry 4) gave the corresponding urea 5b in
59% yield, and allylamine (entry 5) gave 6b in 44%
Table 2. Reuse of Co2Rh2 catalyst for oxidative carbonyla-
yield. Studies on the transition metal-catalyzed oxida-
tion reaction of benzylamine.[a]
tive carbonylation of allylamine are quite rare.[16]
Entry
Catalyst
Yield [%][b]
Thus it is not easy to draw any conclusions. It seems
that the presence of a double bond is not helpful for
the yield of the reaction.
1
2
1st run
74
58
65
54
35
31
2nd run
We also investigated the Co2Rh2/C-catalyzed oxida-
tive carbonylation of aromatic primary amines
(Table 4). In this case, due to their low solubility in
toluene, tetrahydrofuran (THF) was used and the re-
action time was lengthened to 15 h. The presence of
an electron-donating or an electron-withdrawing sub-
stituent at the para- or meta-position of aniline
strongly affected the yield of the reaction. High yields
were obtained for anilines bearing an electron-donat-
ing group (entries 2–4). However, with an electron-
withdrawing group such as p-Cl and m-F, the yields
abruptly dropped to 12% and 26%. Steric crowded-
3[c]
4[d]
5
2nd run (after treatment with CO)
2nd run (after treatment with H2)
3rd run
4th run
6
[a]
Reaction conditions: benzylamine (1.0 mmol), CO
(5 atm), O2 (0.1 atm), and Co2Rh2 catalyst in 2 mL tolu-
ene at 1008C for 3 h.
[b]
[c]
Isolated yield.
The recovered catalyst after the first run was treated
with CO bubbling for 20 min in toluene.
The recovered catalyst after the first run was treated
with H2 bubbling for 20 min in toluene.
[d]
1234
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Table 4. Co2Rh2-catalyzed oxidative carbonylation of various aromatic amines.[a]
Entry
Aniline
Urea
Yield [%][b]
1
7b
8b
81
89
2
3
4
5
9b
80
85
63
10b
11b
6
7
12b
28
13b
14b
12
26
8
[a]
Reaction conditions: aromatic amine (1.0 mmol), CO (5 atm), O2 (0.1 atm), and Co2Rh2 catalyst (3 mol%, 50 mg) in 2 mL
THF at 1008C for 15 h.
Isolated yield.
[b]
ness also affected the yield of the reaction. Compared et al.[8c] obtained the same compound in 2% yield
[6b]
to that (63%) of 3,5-dimethylaniline (entry 5), the using Pd
(OAc)2
as the catalyst, a base (K2CO3),
yield of 2,6-dimethylaniline (entry 6) was quite poor and a promoter (I2). Deng et al. reported[17b] 12% in
(28%).
the presence of ZrO2-SO42À-supported palladium cata-
Compared to the carbonylation of aliphatic and ar- lyst. It has been known[18] that the palladium-cata-
omatic primary amines to 1,3-disubstituted ureas, the lyzed oxidative carbonylation proceeded through an
catalytic carbonylation of secondary amines to tetra- isocyanate intermediate. For the secondary amine, an
substituted ureas is less well explored.[8c,17] Thus we in- isocyanate intermediate could not be formed. Thus
vestigated the Co2Rh2/C-catalyzed oxidative carbony- we envisioned that our reaction mechanism might be
lation of dibutylamine under the same reaction condi- different from that of the palladium-catalyzed reac-
tions [Eq. (1)]. After work-up, the corresponding tion.
urea was obtained 38% yield. Interestingly, Alper
According to the experimental results, primary aro-
matic amines were less reactive than aliphatic ones in
the Co2Rh2-catalyzed oxidative carbonylation. This
means that nitrogen nucleophilicity plays an impor-
tant role in the reactivity. Moreover, this was con-
firmed by the observation that the presence of an
electron-withdrawing or electron-releasing substituent
at the para position of the ring strongly affected sub-
strate reactivity.
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Ji Hoon Park et al.
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General Procedure for CoRh-Catalyzed Oxidative
Carbonylation of Alkylamines to 1,3-Dialkylureas
Reactions were performed in a 100-mL stainless steel auto-
clave equipped with a stirring bar and the following were
placed in the autoclave in the order, alkylamine (1.0 mmol),
toluene (2 mL), and 3 mol% Co2Rh2 (50 mg of the immobi-
lized Co2Rh2). The reactor was purged with O2 gas, and
charged with 5 atm of CO and heated at 1008C for 3 h.
After the reactor had been cooled to room temperature, the
solution was filtered and concentrated, and the product was
isolated by chromatography on a silica gel column eluting
with hexane and ethyl acetate (v/v, 1:3).
General Procedure for CoRh-Catalyzed Oxidative
Carbonylation of Arylamines to 1,3-Diarylurea
Reactions were performed in a 100-mL stainless steel auto-
clave equipped with a stirring bar and the following were
placed in the autoclave in the order, arylamine (1.0 mmol),
THF (2 mL), and 3 mol% Co2Rh2 (50 mg of the immobi-
lized Co2Rh2). The reactor was purged with O2 gas, and
charged with 5 atm of CO and heated at 1008C for 15 h.
After the reactor had been cooled to room temperature, the
solution was filtered and concentrated. Additionally, 5 mL
DMF were added to the mixture and the resulting solution
was stirred for 5 min. Then, excess H2O was poured into the
solution to precipitate the crude product, which was purified
by washing with various solvent.
Scheme 1. Possible mechanism for the Co2Rh2 catalyzed oxi-
dative carbonylation of primary/secondary amines to ureas.
Considering the results obtained above, we postu-
late a reaction mechanism for this newly developed
oxidative carbonylation of amines, as shown in
Scheme 1. Under the pressure of carbon monoxide,
carbon monoxide first adsorbs on the nanoparticle
surface. This makes the carbonyl susceptible to nucle-
ophilic attack by the amine. The resulting intermedi-
ate (a CO-amine adduct) reacts with O2 to give the
amide and the hydroxy species. Attack of another
amine and carbon monoxide gives the product, urea
and water, and regeneration of the initial state.
Recycling Experiment
In order to recycle the catalyst, it was separated by centrifu-
gation from the reaction solution and washed twice with
DMF/acetone and dried under vacuum. The recovered cata-
lyst was then used with/without regeneration under same re-
action conditions
In summary, a by-product-free catalytic method for
the synthesis of symmetrically N,N’-substituted ureas
from amine, CO, and O2 has been developed. The cat-
alytic system used in this study can be applied to both
aliphatic and aromatic amines to give ureas.
Regeneration methods: The recovered catalyst after the
first run was treated with CO or H2 gas bubbling for 20 min
in toluene.
Acknowledgements
This work was supported by the Korean Government
(MOEHRD) (KRF-2008-341-C00022), and the Korea Sci-
ence and Engineering Foundation (KOSEF) grant by the
Korea Government (MEST) (R11-2005-065). JHP and JCY
thank the Brain Korea 21 for fellowships.
Experimental Section
Immobilization of Co/Rh Nanoparticles on Charcoal
To a two-neck flask were added o-dichlorobenzene (10 mL),
oleic acid (0.2 mL), and trioctylphosphine oxide (0.4 g).
While the solution was heated at 1808C, a solution of the
metal carbonyl Co2Rh2(CO)12 (0.8 g) in 25 mL o-dichloro-
benzene was injected into the flask. The resulting solution References
was heated to 1808C for 2 h and then concentrated to a
volume of 5 mL. The concentrated solution was cooled to
room temperature. To the cooled solution was added 30 mL
of THF. After the solution had been well stirred for 10 min,
flame-dried charcoal (1.6 g) was added to the solution. After
the resulting solution had been refluxed for 12 h, the precip-
itates were filtered and washed with diethyl ether (20 mL),
dichloromethane (20 mL), acetone (20 mL), and methanol
(20 mL). Vacuum drying gave a black solid.
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