Nitrogen-containing organobases as promoters in the cobalt(II)-Schiff base catalyzed oxidative carbonylation of amines

Article abstract of DOI:10.1016/j.tetlet.2012.05.015

The use of organic bases as promoters in the cobalt(II)-Schiff base complex catalyzed oxidative carbonylation of amines was investigated. The generality of the reaction was also studied by submitting different amines to the same procedure and by changing the reaction conditions. Very good yields in the corresponding ureas were achieved in toluene with a catalyst loading of 0.5 mol % and using TBD (1,5,7-triazabicyclo[4.4.0]dec-5-ene) as promoter. Methyl carbamates were obtained in methanol.

Full text of DOI:10.1016/j.tetlet.2012.05.015

Tetrahedron Letters 53 (2012) 3590–3593
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Nitrogen-containing organobases as promoters in the cobalt(II)–Schiff base
catalyzed oxidative carbonylation of amines
⇑
Francesco Saliu , Benedetto Putomatti, Bruno Rindone
Department of Environmental Science, University of Milano-Bicocca, Piazza della Scienza 1, 20126 Milano, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
The use of organic bases as promoters in the cobalt(II)–Schiff base complex catalyzed oxidative carbon-
ylation of amines was investigated. The generality of the reaction was also studied by submitting differ-
ent amines to the same procedure and by changing the reaction conditions. Very good yields in the
corresponding ureas were achieved in toluene with a catalyst loading of 0.5 mol % and using TBD
(1,5,7-triazabicyclo[4.4.0]dec-5-ene) as promoter. Methyl carbamates were obtained in methanol.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 30 March 2012
Revised 28 April 2012
Accepted 2 May 2012
Available online 11 May 2012
Keywords:
Cobalt
Oxidative carbonylation
Amines
Organic bases
Carbamates and ureas are very important carbonyl compounds
deriving from amines. They are present as functional groups in
various pharmaceuticals, agrochemicals, dyes, and additives. The
conventional methods for their formation employ phosgene.1 The
environmental risk connected to the use of this hazardous reagent
encouraged efforts to find new methods with low environmental
impact. The main alternatives proposed in recent years are the
use of substitutes such as organic carbonates, CO2, and CO itself
as the source of the carbonyl moiety.2
The metal-catalyzed oxidative carbonylation of amines to form
ureas or carbamates has been extensively reviewed.3–5 It is consid-
ered attractive from the standpoint of atom economy because it
employs very cheap starting materials and the only side product
is water. Compared with carboxylation reactions, the use of CO
raises some concern about its toxicity, but it has the advantage of
requiring milder conditions and less energy consumption. More-
over, it is possible to use it as a carbonyl source for formates6,7 or
aldehydes.8 Very good catalytic efficiencies for this kind of reaction
are reported by the use of palladium.9–11 Particularly, mixed ureas12
and oxazolidinones13 were formed in good to excellent yields.
Since some analysts14 forecast shortages of noble metals and
rising costs could restrict their industrial application, the use of a
catalytic system based on cheaper metal such as cobalt represents
an important alternative.
results are reported in the synthesis of ureas from primary aromatic
amines15 and of oxazolidinones from b-amino alcohols.16 In partic-
ular, the preparation of N,N0-diphenyl urea from aniline was investi-
gated by testing various catalytic systems comprising Co(II)–Schiff
base complex and the addition of pyridine-base17 or NaI18 as pro-
moters was demonstrated to be beneficial. Good conversions of
aniline (85%) and interesting yields of the corresponding N,N0-diphe-
nyl urea (79%) were achieved using Co(II)salophen(OH)2 as the
catalyst and 4-picoline as the promoter under a mixture of CO/
O2 = 3:1 at 150 °C for 7 h.17 Methods for heterogenization of this
homogeneous catalyst, such as encapsulation in confined cages of
molecular sieves19 and immobilization on silica via sol-gel pro-
cess,20 were also proposed for the preparation of N,N0-diphenyl urea
and methyl-N-phenyl carbamate from aniline. Relevant amounts of
iodine-containing promoters were used also in these cases, which
allowed to operate with CO/O2 = 9:1 mixtures.
Due to the fact that iodine derivatives are very corrosive in the
reaction system, alternative procedures are needed. Here we
evaluated the use of nitrogen containing bases. In particular we
tested some amidines and guanidines, known as environmentally
friendly base catalysts. These organic superbases have recently
attracted much attention in organic synthesis due to their wide
applicability.21 To the best of our knowledge, this is the first
reported application of bicyclic organic bases as promoters in the
cobalt oxidative carbonylation of amines.
Although the application of Co(II)salen complexes in catalytic
oxidative reactions is very well documented, there are few literature
converging on their use in the oxidative carbonylation. Promising
In order to check the generality of the reaction, the amines sub-
mitted to the oxidative carbonylation were both aliphatic and aro-
matic (Fig. 1). The solvent was varied from polar protic (methanol)
to non-polar aprotic (toluene). The N,N-bis(salicylidene)ethylenedi-
⇑
Corresponding author.
E-mail address: francesco.saliu@unimib.it (F. Saliu).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.05.015

F. Saliu et al. / Tetrahedron Letters 53 (2012) 3590–3593
3591
O
X
C
N
NH2
NHCOOMe
X
O
H
N
H
N
O
Co
N
C
NH2
NHCOOMe
O
N
N
N
N
O
X
X
X
X
X
X
X
4
a
1
: X = Br
2
3
5
6
8
7
9
b
: X = Cl
: X = H
c
CHO
CHO
NH
H
N
H
N
HN
d
: X = F
H
N
e
: X = Me
O
N
f
: X = OMe
O
: X= SO3- Na+
h
N
H
N
O
NH2
12
H
X
X
14
10
13
11
15
O
N
O
N
N
N
H
N
H
N
N
CHO
N
N
H
O
N
COOMe
O
N
COOMe
COOMe
19
17
20
21
16
18
24
22
23
H
N
H
N
NH2
OH
OH
NH
O
NH
NH2
27
NH2
O
O
O
O
25
26
28
29
30
Figure 1. Substrates and products in the cobalt catalyzed oxidative carbonylation of amines (27 and 29 were used as racemic mixtures).
aminocobalt(II) 1 was used as a model catalyst of Co–Schiff base
complexes.
replacing methanol with toluene (Table 1 entries 7 and 15). Inter-
estingly, a significant amount of urea was isolated also from a reac-
tion carried out in water (Table 1 entry 10). Control experiments
showed that no reaction occurred when the substrates were trea-
ted under the same reaction conditions but using no catalyst or
an uncomplexed cobalt(II) ion (CoCl2).
By submitting substituted-primary aromatic to the oxidative
carbonylation, the higher conversions and yields were achieved
with the amines bearing an electron-donating substituent on the
phenyl ring (Table 1 entries 1, 4, 6, 11 and 13).
In the first set of experiments the catalytic ratio (r = [substrate]/
[catalyst]) was 25. No promoters were added. The results are shown
in Table 1. By operating at 100 °C, conversion and selectivity in
methyl carbamate 4 were high in all but one case. Diarylurea 5
and azoderivative 6 were also occasionally present (Table 1 entries
6 and 8).
At a lower temperature (60 °C for 24 h), diarylurea 5 was in-
stead the major reaction product (Table 1 entries 2, 5, 9, 12 and
14). The change in selectivity between carbamate and urea prod-
ucts with reaction temperature has been already observed by
Dombek et al.22 Urea was also the main product observed by
A nearly quantitative conversion was obtained with the primary
aliphatic amine 1-methylbenzylamine 7 (Table 1 entry 16). The
main reaction product was the corresponding carbamate 9, but
Table 1
Conversion and selectivity in the oxidative carbonylation of primary amines catalyzed by N,N-bis(salicylidene)ethylenediaminocobalt(II) and without the use of promoter
Entrya
Substrateb
T (°C)
Solvent
Reaction time (h)
Conversion (%)
Carbamate (%)
Urea (%)
Azoderivative (%)
1
2
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
4-Bromoaniline 2a
‘‘
4-Chloroaniline 2b
‘‘
Aniline 2c
Aniline 2c
4-Fluoroaniline 2d
‘‘
100
60
100
60
100
100
100
60
100
100
60
100
60
100
100
100
100
MeOH
‘‘
MeOH
‘‘
MeOH
Toluene
MeOH
‘‘
H20
MeOH
‘‘
MeOH
‘‘
Toulene
MeOH
MeOH
MeOH
12
24
12
24
12
7
12
24
12
12
24
12
24
7
37
16
70
24
82
69
74
56
32
100
86
100
95
74
98
80
1
84
1
87
—
88
3
—
85
9
70
2
—
—
82
—
85
1
53
—
80
57
—
64
—
63
62
28
—
—
1
—
1
1
—
1
1
‘‘
24
—
2
—
1
—
—
—
—
4-Methylaniline 2e
‘‘
4-Methoxyaniline 2f
‘‘
‘‘
Benzylamine 7
Piperidine 16
Pyrrolidine 17
12
12
12
68
53
61
100
100
2
a
Reaction conditions: 10 mmol of substrate, 0.4 mmol of Co(II)salen, 25 ml of solvent , PCO = 10 atm, PO2 = 2 atm; CAUTION (see the experimental section).
Catalyst: Co(sulfosalen) 1h.
b

3592
F. Saliu et al. / Tetrahedron Letters 53 (2012) 3590–3593
CH3
H3C
H3C
H3C
CH3
CH3
CH3
N
N
N
N
N
N
N
N
Promoter:
N
N
N
H
N
N
N
CH3
MTBD
4-phenyl-
4-picoline
DMAP
DMAN
DBU
TBD
pyridine
Figure 2. Organic bases tested as promoters in the cobalt catalyzed oxidative carbonylation of amines.
Table 2
Table 3
Yields in the oxidative carbonylation of fluoro-aniline catalyzed by Co(II)salen
catalyst obtained by using different nitrogen-containing organic bases
TBD (1,5,7 triazabicyclo[4.4.0]dec-5-ene) as
ene)ethylenediaminocobalt(II)oxidative carbonylation of amines
a promoter in the N,N-bis(salicylid-
Entrya
Promoter
pKa
Yieldc (%)
Entrya
Substrate
Yield (%)
b
1
2
3
4
5
6
7
8
9
None
52
66
68
78
84
65
74
81
68
1
2b
3c
4d
5
4-Methoxyaniline 2f
4-Methoxyaniline 2f
4-Methoxyaniline 2f
4-Methoxyaniline 2f
4-Fluoroaniline 2d
86
63
-
Pyridine
4-Phenyl-pyridine
4-Picoline
DMAP
DMAN
DBU
5.17 (12.33)
5.25
6.00 (13.21)
9.56
12.10 (19.9)
(24.13)
17.86 (25.96)
17.33 (24.33)
81
81
78
82
87
84
61
89f
6e
7e
8
4-Fluoroaniline 2d
4-Methoxyaniline 2f
1-Methylbenzylamine 7
Cyclohexylamine
Piperidine 16
2-Amino-3-phenyl-1-propanol 29
TBD
MTBD
9
10
11
a
Reaction conditions: 0.05 mmol of Cosalen 0.025 mmol of organic base pro-
moter, 10 mmol of fluoroaniline , 10 mL of toluene, PO2 = 2 bar PCO = 22 bar; 140 °C,
a
b
c
d
e
f
7 h.
For the condition see the experimental section.23
170 °C.
Without Cosalen.
Pair = 1 bar, PCO = 9 bar.
Solvent = methanol, product = methylcarbamate.
Product: 4-benzyl-1,3-oxazolidin-2-one (30).
Numbers in brackets are the pKa values reported in acetonitrile.21,40,41
b
c
Isolated yield based on fluoroaniline.
also urea 10 was formed in significant yields. Traces of formamide
11 were also detected.
after chromatographic separation (Table 3 entry 11). No urea was
detected, indicating that intramolecular cyclization is the most
favoured pathway. As a confirmation, 4-ethyl-1,3-oxazolidin-2-
one 28 was also recovered as the main product (57% isolated yield)
by operating in methanol and without the use of promoter.
The improvement in the yields of ureas obtained using TBD
additive as the promoter, can be explained under two aspects,
and in a similar manner as already reported for pyridine type
additives.16,27,28
First, TBD could act as a two-electron axial ligand coordinated to
Co(II) center ion and lead to the formation of a metal superoxo com-
plex. It is well known that four-coordinate square-planar Co(II)
Schiff base complexes do not bind oxygen by themselves, but can
give reversible adducts in the presence of Lewis bases.29,30 In the
reaction carried out without promoter, the amine substrate itself
acts as the basic ligand necessary for the coordination of oxygen to
cobalt.14 Thus, all these metal superoxo species, whichare both basic
and oxidant, convert the substrates into products by acting as hydro-
gen abstractor.31–35 Under this view, TBD could add stability to the
corresponding superoxo complex. Basicity and steric hinderance
play here a crucial role as in the case of pyridine-type promoters.
At present we have no kinetic evidence for a reaction between
the superoxo complex coordinated to TBD and a second complex
molecule having a coordinated primary or secondary amine. For-
mation of hydroperoxocobalt(III) species arising from hydrogen
transfer reactions and regeneration of five coordinate cobalt com-
plexes have been suggested in the oxidation of phenols25,36 and in
the oxidation of anilines with tert-butyl hydroperoxide.37 A similar
mechanism could be involved here in the formation of an interme-
diate amino radical.
Noteworthy, the secondary aliphatic amines piperidine 16 and
pyrrolidine 17 gave the corresponding carbamates 18–19 in meth-
anol (Table 1 entries 17 and 18) indicating that a free isocyanate
could not be the intermediate in these reactions, since no isocya-
nate can be formed from a secondary amine. The poor recovery
observed was due to the formation of a sticky tar by-product, that
was not possible to recover after silica gel chromatography.
With the aim of decreasing the catalyst loading, we tested the
use of various organic bases as promoters (Fig. 2). Fluoroaniline
was chosen as the substrate, the temperature was fixed at 140 C,
the substrate/catalyst ratio was raised to r = 200 and reaction time
lowered at 7 h.23 We also decreased the amount of oxygen in order
to operate under safer conditions, outside the range of flammabil-
ity reported for CO/O2 mixture.24–26 Under these conditions ureas
were obtained in very good selectivity. The highest yield (Table 2
entry 5) was obtained using 4-dimethylamino-pyridine (DMAP).
A good result (Table 2 entry 8) was obtained also with the bicyclic
guanidine 1,5,7 triazabicyclo[4.4.0]dec-5-ene (TBD) which is, com-
pared with DMAP, more environmentally acceptable.
Therefore, the oxidative carbonylation procedure based on the
use of Co(II)salen with the promotion of TBD was tested on differ-
ent amines as substrates and by changing the reaction conditions.
Very good yields in the corresponding ureas were obtained with
primary amines, both aromatic and aliphatic (Table 3 entries 1, 5, 8
and 9). The increase of temperature had a negative effect on the
overall yield (Table 3 entry 2) probably due to the catalyst decompo-
sition. A good yield was also observed by replacing air with pure oxy-
gen. (Table 3 entry 4). In methanol, methyl carbamates were instead
the main products (Table 3 entries 6 and 7). Urea was obtained in
minor amounts by using piperidine. In this case the formation of a
polymeric material was also observed (Table 3 entry 10).
As a secondary consideration, TBD itself is a very strong base,
able to deprotonate a coordinated amine at any stage of the cata-
lytic cycle to form active intermediates such as in the case of the
formation of a carbamoyl metal complex (RNHCO–Co(III)L4).
By starting from the racemic mixture of -amino alcohol 2-Ami-
no-3-phenyl-1-propanol (29) the corresponding 4-benzyl-1,3-
oxazolidin-2-one (30) was isolated as a racemate in very good yield

F. Saliu et al. / Tetrahedron Letters 53 (2012) 3590–3593
22. Dombek, D. B.; Leung, T. W. Chem. Commun. 1992, 205–206.
3593
Finally, the results showed in Table 2 suggest that thermody-
namic basicity is not the sole criterion for predicting the promoting
activity. In fact, it is well reported that TBD could act as a bifunc-
tional nucleophilic catalyst38,39 in the activation of carbonyl
groups. This could be the reason of the minor reactivity observed
by using other bicyclic organic bases.
The use of TBD as a promoter in the oxidative carbonylation of
amine under bis(salicylaldehyde)ethylenediimine-cobalt(II) cataly-
sis was demonstrated to be beneficial and could represent an inter-
esting development in the field of the Co–Schiff base complexes
catalyzed oxidative carbonylation, because bicyclic guanidine bases
are less toxic and more environmentally friendly than pyridine-
bases.
23. Experimental section: Amines were commercial grade reagents purchased
from Aldrich. Solvents were solvent grade from Fluka. The catalyst N,N-
bis(salicylidene)ethylenediaminocobalt(II)
commercial product (Aldrich Inc.). The reactions were carried out in a 100-
mL stainless steel autoclave equipped with mechanical stirrer and an
hydrate
(Cosalen)
was
a
a
automatic temperature controller. Carbon monoxide and dioxygen were high
purity grade. GC–MS analyses were performed with a Hewlett Packard 5890A
instrument, (split/splitless injector, capillary column SPB-5, 30 m, 0.32 mm
I.D.) equipped with a 5971A Mass Selective Detector. 1H NMR spectra were
recorded on a Varian Mercury 400 instrument in CDCl3 . Chemical shifts were
reported in parts per million (d), relative to the internal standard of
tetramethylsilane (TMS). Infrared (IR) spectra in solutions were recorded on
a Nicolet Avatar 360 Fourier transform (FT)-IR spectrometer, using calcium
fluoride cells. CAUTION: Some reactions were carried out under oxygen
pressure and in the presence of carbon monoxide. While we experienced no
difficulties in performing these reactions, appropriate precautions should
always be used when combining carbon monoxide, oxygen and organic
materials under pressure. In particular, read the related refererences.22–24
Representative reaction procedure: 10 mmol of substrate, 0.05 mmol of
Cosalen, 0.03 mmol TBD, 20 ml of toluene were put in a glass liner, fitted in
an autoclave, charged with oxygen (2 bar) and carbon monoxide (22 bar) and
heated in a thermoregulated oil bath at 140 °C for 7 h. The autoclave was then
allowed to cool to room temperature and degassed. Direct recovery of the
ureas from the reaction medium was obtained by cooling the reaction mixture
to 0 °C. The resulting precipitate was filtered and re-crystallized from
methanol. The other products were recovered after evaporation of the
solvent under reduced pressure and silica gel chromatography (Merck 0.05-
0.2 mm (R = 50), using dichloromethane-ethyl acetate 1:1 as the eluting
mixture. Isolated products were characterized by MS, IR, and 1H NMR and
the related spectra were compared with the literature data.
Acknowledgements
We thank our students Andrea Agnelli and Andrea Bosisio for
technical support. This work was financial supported by the
University of Milan-Bicocca FAR 2008.
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