A novel and efficient (NHC)CuI (NHC = N-heterocyclic carbene) catalyst for the oxidative carbonylation of amino compounds

Article abstract of DOI:10.1016/j.tetlet.2007.06.049

Oxidative carbonylation of amino compounds to prepare corresponding 2-oxazolidinones, ureas, and carbamates selectively in the presence of (NHC)Cu^I without any additives was firstly achieved in good yields and selectivities.

Full text of DOI:10.1016/j.tetlet.2007.06.049

Tetrahedron Letters 48 (2007) 5883–5886
A novel and efficient (NHC)CuI (NHC = N-heterocyclic carbene)
catalyst for the oxidative carbonylation of amino compounds
Shuzhan Zheng,a,b Fuwei Li,c Jianming Liua,b and Chungu Xiaa,*
aState Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics,
Chinese Academic of Sciences, Lanzhou 730000, PR China
bGraduate School of the Chinese Academy of Sciences, Beijing 100039, PR China
cDepartment of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore
Received 16 May 2007; revised 7 June 2007; accepted 11 June 2007
Available online 14 June 2007
Abstract—Oxidative carbonylation of amino compounds to prepare corresponding 2-oxazolidinones, ureas, and carbamates selec-
tively in the presence of (NHC)CuI without any additives was firstly achieved in good yields and selectivities.
Ó 2007 Elsevier Ltd. All rights reserved.
2-Oxazolidinone, urea, and carbamate have often
successfully been employed as intermediates for fine
chemicals, pharmaceuticals, cosmetics, and pesticides.1
Classical synthesis of these three N-containing carbonyl
compounds are mainly conducted by phosgenation of
the corresponding amino compounds with toxic phos-
gene or its derivatives,2 which may result in serious envi-
ronmental pollution and equipment corrosion. With the
increasing environmental concerns, there is a great
demand for finding some efficient and environmentally
benign methods in place of such toxic and dangerous
reagents. Recently, production of 2-oxazolidinone, urea,
and carbamate by oxidative carbonylation of corre-
sponding amino compounds as effective alternative have
been considerably studied. Many transition metals
including Rh,3 Ru,4 and especially Pd5 have been
reported to catalyze this process. However, the reported
literatures are mostly focus on the costly noble metal
catalysts, and additives such as KI and I2 need to be
used. Therefore, it is necessary to develop a non-noble
metal catalytic system to mediate this reaction with an
efficient performance.
compared to tertiary phosphine, thereby enhancing the
stability of NHC complexes toward heat and moisture.
This characteristic property is very important for a cat-
alyst to keep efficient activity in the carbonylation be-
cause they always proceed under rigorous conditions.
Many catalytic applications for NHC complexes have
been involved recently,7 including several Pd or Rh
NHC complexes mediated carbonylations.8 However,
very few non-expensive metal NHC complex systems
were reported for all the carbonylation. So in this hand
it is also necessary to develop a cheap metal NHC com-
plex system to catalyze oxidative carbonylation reaction.
Actually, we have developed some good processes for
the syntheses of 2-oxazolidinone, urea, and carbamate
with good yields and selectivities.9 As a part of our
ongoing interest in the construction of N-containing
carbonyl compounds with oxidative carbonylation
method, we herein report a (NHC)CuI catalytic system
without any additives for the oxidative carbonylation
of amino compounds to synthesize 2-oxazolidinone,
urea, and carbamate selectively under different reaction
conditions (Eqs. 1–3). To the best of our knowledge, this
is the first catalyst example for the oxidative carbonyl-
ation of amino compounds to selectively prepare three
different N-containing carbonyl compounds.
Starting with the isolation of the first stable N-hetero-
cyclic carbene (NHC) by Arduengo et al. in 1991,6
NHCs have emerged as efficient ligands in metal-medi-
ated reactions due to their strong r-donor properties
O
R1
H2N
R2
OH
IPrCuI
-H2O
HN
R1
O
ð1Þ
+
CO/O2
*
Corresponding author. Tel.: +86 9318 276531; e-mail: cgxia@
R2
lzb.ac.cn
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.06.049

5884
S. Zheng et al. / Tetrahedron Letters 48 (2007) 5883–5886
O
and its catalytic activity was enhanced. It is also worth
IPrCuI
ð2Þ
ð3Þ
+
CO/O2
2RNH2
to note here that there is no need to add any other
iodide-containing promoters in this catalyst system to
obtain good yield. On the other hand, solvents also play
a key role in the activity and selectivity. The effects of
different solvents on the reaction were investigated with
IPrCuI as catalyst, it is seen that acetonitrile, 1,4-diox-
ane and THF have proved to be a suitable reaction med-
ium (Table 1, entries 1, 5, and 7), whereas toluene
afforded lower yields (Table 1, entry 6). Poor results
were observed when the reaction was carried out in
1,2-dimethoxyethane (DME) and DMF (Table 1, entries
8 and 9).
RHN NHR
-H2O
O
IPrCuI
+
CO/O2
RNH2
CH3OH
+
RHN OCH3
-H2O
The air- and moisture-stable NHC–CuI complexes were
readily prepared by deprotonation of 1,3-bis(2,6-di-iso-
propylphenyl)-imidazolium chloride with KOt-Bu in
the presence of different CuX with good yields (Scheme
1).10 And initial investigation of oxidative carbonylation
was carried out using 1-amino-2-propanol (1a) as a
probe substrate to optimize the reaction conditions,
and the results are listed in Table 1. The influence of hal-
ogen anions on the NHC–CuI catalyst was first exam-
ined in the preparation of 5-methyloxazolin-2-one (2a).
IPrCuI shows excellent reactivity (100% conv. and
99% sel.) (Table 1, entry 1), surprisingly IPrCuBr gives
poor results and IPrCuCl has no reactivity at all. The re-
sults indicate that the anions on the metal center have a
remarkable influence on the activity of the NHC–CuI,
this phenomenon is similar to that of the Pd based sys-
tem.5e,9a A comparison experiment was also done using
CuI as the catalyst, and only 53% conversion of 1a was
observed (Table 1, entry 4). We considered that the
bulky NHC ligand pushed a positive effect on Cu(I),
After optimization of the reaction conditions, oxidative
carbonylation of other b-aminoalcohols were further
tested. As seen in Table 2, IPrCuI catalyst shows excel-
lent catalytic activity to almost all the employed b-
aminoalcohols under mild conditions, providing the
corresponding 2-oxazolidinones in high isolated yields.
The diethanolamine (Table 2, entry 4) could also be con-
verted to the corresponding 2-oxazolidinones through
the oxidative carbonylation reaction. To our delight,
optical activity of 2-oxazolidinones could be efficiently
Table 2. Synthesis of 2-oxazolidinones catalyzed by IPrCuIa
Entry Substrate
Product
Yieldb
(%)
O
i-Pr
i-Pr
1
96
86
HN
HN
O
H2N
OH
OH
i-Pr
Cl
i-Pr
N
N
i-Pr
CuX
N
N
KOt-Bu, CuX
2a
2b
1a
THF
O
i-Pr
H2N
2
3
4
5
i-Pr
i-Pr
O
1b
O
IPrCuCl: X=Cl; IPrCuBr: X=Br; IPrCuI: X=I
91
HN
O
H2N
HO
OH
1c
Scheme 1. Synthesis of copper N-heterocyclic carbene.
2c
O
HO
N
OH
1d
90
N
O
Table 1. Oxidative cyclocarbonylation of 1-amino-2-propanol
catalyzed by copper carbenea
H
O
2d
O
IPrCuX
100 oC, 3h
O
H2N
OH
1e
HN
(2a)
Conversion (%) Selectivity (%)
CO/O2
+
HN
O
H2N
OH
93c
2e
(1a)
O
Entry Catalyst Solvent
H2N
OH
86d
IPrCuI
Dioxane
IPrCuBr Dioxane
IPrCuCl Dioxane
99 (96b)
O
HN
1
2
3
4
5
6
7
8
9
100
8
—
53
100
100
6
7
87
—
96
99
37
99
6
1f
2f
CuI
Dioxane
THF
Toluene
Acetonitrile 100
DMF
DME
IPrCuI
IPrCuI
IPrCuI
IPrCuI
IPrCuI
NH2
OH
N
O
NH2
2g
90
1g
O
99
99
a Reaction condition: 0.01 mmol of IPrCuI, 1 mmol of aminoalcohol,
5
a Reaction condition: 0.01 mmol of catalyst, 1 mmol of 1-amino-2-
dioxane (4 ml), PCO = 4.8 MPa, PO ¼ 0:2 MPa, 100 °C for 3 h.
2
b Isolated yield.
propanol, solvent (4 ml), PCO = 4.8 MPa, PO ¼ 0:2 MPa, 100 °C for
2
20
c
½aꢀD ꢁ15 (c 2, ethanol).
3 h.
20
b Isolated yield.
d
½aꢀD ꢁ62 (c 1, chloroform).

S. Zheng et al. / Tetrahedron Letters 48 (2007) 5883–5886
5885
O
N-containing carbonyl compounds. The understanding
of the reaction mechanism is ongoing in our laboratory.
IPrCuI
-H2O
+
CO/O2
2RNH2
RHN NHR
(1h-1l)
(2h-2l)
Acknowledgements
R = n-C3H7 2h, 86%; R = n-C4H9 2i, 96%; R = t-C4H9 2j, 93%;
R = cyclohexyl 2k, 19%; R = C6H5 2l, <5%
We are grateful to the Chinese National Sciences Foun-
dation (20533080) and The National Science Fund for
Distinguished Young Scholars (20625308) for financial
support.
Reaction condition : 0.01mmol of IPrCuI, 1mmol of amine,
dioxane (4ml), PCO =4.8MPa, PO2 =0.2MPa, 100 oC for 3h.
O
IPrCuI
RNH2
+
CO/O2
CH3OH
+
RHN OCH3
-H2O
(1h-1l)
(3h-3l)
Supplementary data
R = n-C3H7 3h, 98%; R = n-C4H9 3i, 98%; R = t-C4H9 3j, 93%;
R = cyclohexyl 3k, 89%; R = C6H5 3l, <10%
Experimental procedures and spectral data for the com-
pounds are given in the Supplementary data. Supple-
mentary data associated with this article can be found,
in the online version, at doi:10.1016/j.tetlet.2007.06.049.
Reaction condition : 0.01mmol of IPrCuI, 1mmol of amine,
CH3OH (4ml), PCO =4.8MPa, PO2 =0.2MPa, 100 oC for 3h.
Scheme 2. Selective synthesis of urea and carbamate by IPrCuI
catalyzed oxidative carbonylation of primary amines under optimized
conditions.
References and notes
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