Oxygen-atom insertion of NHCecopper complex: The source of oxygen from N, N-dimethylformamide

Article abstract of DOI:10.1016/j.jorganchem.2013.06.017

An unprecedented protocol for oxygen-atom insertion reaction of NHCecopper complexes has been developed by employing N,N-dimethylformamide as the oxygen source, which allows the preparation of imidazolinones from carbene complexes and decodes one of the m

Full text of DOI:10.1016/j.jorganchem.2013.06.017

Journal of Organometallic Chemistry 743 (2013) 44e48
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Communication
Oxygen-atom insertion of NHCecopper complex: The source
of oxygen from N,N-dimethylformamide
*
Wei Zeng, Enyu Wang, Rui Qiu, Muhammad Sohail, Shaoxiang Wu, Fu-Xue Chen
Department of Applied Chemistry, School of Chemical Engineering & the Environment, Beijing Institute of Technology, No. 5 South Zhongguancun Street,
Haidian District, Beijing 100081, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
An unprecedented protocol for oxygen-atom insertion reaction of NHCecopper complexes has been
developed by employing N,N-dimethylformamide as the oxygen source, which allows the preparation of
imidazolinones from carbene complexes and decodes one of the mysterious transformations of NHC
emetal complexes as well.
Received 29 March 2013
Received in revised form
13 June 2013
Accepted 18 June 2013
Ó 2013 Elsevier B.V. All rights reserved.
Keywords:
Carbene complex
Copper
Oxygen insertion
N,N-Dimethylformamide
Trifluoromethyl
1. Introduction
from N,N-dimethylformamide (DMF) (Eq. (4)). DMF can participate
in many reactions by serving as a multipurpose building block for
Carbenes are among the most active and useful intermediates in
organic reactions. Metalecarbene complexes in which carbenes can
be stabilized have been widely applied as catalysts to organic
synthesis [1]. Transition metal-catalyzed insertion of diazo com-
pounds into the XeH (X ¼ O, N, S, etc) bond is a useful carbene
transfer reaction (Eq. (1)) and has become an efficient tool for the
construction of CeX bonds under mild reaction conditions [2].
Rhodium and copper complexes have been extensively studied in
such asymmetric insertion reactions [3e9]. Due to the unique
properties of Nitrogen-Heterocyclic Carbenes (NHCs) (Fig. 1),
transitionemetal complexes of NHCs are playing an increasingly
important role in the fields of materials, medicine, and especially in
catalysis [10]. However, much less is known about the transfer
reactivity of NHCs complexes themselves. Severin has reported that
NHCs are oxidized to imidazolinones by N2O which thermally de-
composes and liberates dinitrogen (Eq. (2)) [11]. Zhang and co-
workers have reported the ultrafine gold nano-particles catalyzed
oxidation of NHCs affording complicated imidazolinones (Eq. (3))
[12]. Herein, we report the first example of oxygen-atom insertion
reaction of NHCecopper complexes, in which the oxygen-atom is
various units [13], while its primary use is as an effective polar
solvent for various chemical reactions.
Previous work
(1)
(2)
(3)
This work
(4)
* Corresponding author. Fax: þ86 10 68918296.
E-mail address: fuxue.chen@bit.edu.cn (F.-X. Chen).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.06.017

W. Zeng et al. / Journal of Organometallic Chemistry 743 (2013) 44e48
45
entries 3e5 vs entry 1). However, when Ruppert’s reagent was
employed as the CF3 source, no product was observed (Table 1,
entry 6). Instead of carbeneeCu complex, its precursor free carbene
1a gave no product (Table 1, entry 7) indicating that dissociation of
carbeneecopper complex was not involved in this transformation.
However, other late transition metalecarbene complexes exhibited
no conversion under same reaction conditions (Table 1, entries 8e
10). It is noteworthy that the similar outcome was gained in N,N-
dimethylacetamide (DMAc) instead of DMF (Table 1, entry 13),
while in other solvents such as THF and MeCN the reaction was
absolutely stopped (Table 1, entries 11e12), suggesting the neces-
sity of the amide which has an important clue for the mechanistic
details. When PhCONMe2 was employed in THF, less amount of
compound 3a was obtained in 44% yield (Table 1, entry 14).
Excited by this unusual oxygen-atom insertion of NHCecopper
complex, we set out to further survey the process although the
yield was only modest. To improve the yield, the loading of
Kþ[CF3B(OMe)3]ꢀ has been increased since 3a was formed only in
14% yield when 1.0 equivalent of Kþ[CF3B(OMe)3]ꢀ were employed
(Table 2, entry 2 vs 1). In situ generated (IPr)CuCl also enabled this
transformation smoothly (Table 2, entry 3 vs 2). Consequently, the
tedious preparation of NHCecopper complex can be avoided.
Interestingly, those NHCecopper complexes with more basic an-
ions showed good reactivity giving 3a in up to 99% yield (Table 2,
entries 4e5). With basic tert-butyloxide copper complex Me3SiCF3
promoted this reaction in 56% yield while no product in the pres-
ence of (IPr)CuCl (Table 2, entry 6 vs Table 1, entry 6). Further
screening the reaction conditions revealed that either higher or
lower reaction temperature destroyed the yield (Table 2, entries 7e
8 vs entry 4). It should be mentioned that the existence of oxygen
couldn’t make any improvements available, suggesting that the
oxygen-atom in the product should not from the oxygen in air
(Table 2, entry 9), nor the oxygen comes from water (Table 2, entry
10). Unpredictably, 2.0 equivalent of DMF in THF produced 3a in
moderate yield (Table 2, entry 11) while the reaction furnished no
Fig. 1. Structure and acronyms of NHCs.
2. Results and discussion
During the course of our previous studies on the NHCecopper
complex catalyzed trifluoromethylation of aromatic aldehydes [14],
we were interested in the employment of storage-stable
Kþ[CF3B(OMe)3]ꢀ as the CF3 source rather than Ruppert’s reagent
(Me3SiCF3) which is air and moisture sensitive [15]. Using 2-
naphthaldehyde (2a) as the model substrate, (IPr)CuCl catalyzed tri-
fluoromethylation was initially tried using Kþ[CF3B(OMe)3]ꢀ in dry
DMF. Quite surprisingly, instead of the anticipated 1,2-adduct 2,2,2-
trifluoro-1-(naphthalen-2-yl)ethanol (4a), the oxygen-atom inser-
tion product of (IPr)CuCl 3a was isolated in 56% yield under the
standard reaction conditions (Table 1, entry 1). The structure of 3a was
confirmed bythe X-raycrystallographyof its singlecrystal(Fig. 2) [16].
As a matter of fact, the aldehyde (2a) did not take part in the
reaction (Table 1, entry 2). Both copper and trifluoromethyl reagent
are required for the oxygen-atom insertion reaction (Table 1,
Table 1
Effects of the reaction parameters in the oxygen-atom insertion of NHCecopper complex.a
Entry
Change from the “standard conditions”
Yield of
3ab (%)
1
2
3
4
None
No 2a
56
55
0
No (IPr)CuCl
No Kþ[CF3B(OMe)3]ꢀ
0
5
6
7
8
B(OMe)3 instead of Kþ[CF3B(OMe)3]ꢀ
Me3SiCF3 instead of Kþ[CF3B(OMe)3]ꢀ
1a instead of (IPr)CuCl
(IPr)Rh(cod)Cl instead of (IPr)CuCl
(IPr)AgCl instead of (IPr)CuCl
(IPr)PdCl instead of (IPr)CuCl
THF instead of DMF
MeCN instead of DMF
DMAc instead of DMF
PhCONMe2 (2.0 equiv) instead of DMF in THF
0
0
0
0
0
0
0
0
9
10c
11
12
13d
14
56
44
a
Reaction conditions: 2a (0.2 mmol, 1.0 equiv), Kþ[CF3B(OMe)3]ꢀ (0.4 mmol, 2.0 equiv), (IPr)CuCl (0.03 mmol, 15 mol%) in DMF (1.0 mL, 0.2 M), 30 ꢁC, stirring 10 h under Ar
atmosphere.
b
c
Isolated yield.
(IPr)PdCl was in situ generated.
DMAc ¼ N,N-dimethylacetamide.
d

46
W. Zeng et al. / Journal of Organometallic Chemistry 743 (2013) 44e48
3aeS perspicuously reveals that the oxygen-atom is transferred
from DMF, although the use of thio-DMF leads to low yield.
(5)
Based on above experiment results, a plausible mechanism for
such an oxygen-atom insertion process was outlined in Scheme 2.
Initially, the trifluoromethyl borate salt would react with DMF
under formation of a negative oxygen ion intermediate I [17]. The
oxygen-atom insertion of NHCecopper complex II started with
intermediate I to generate less stable complex III. After the disso-
ciation of copper chloride, carbanionic intermediate IV was pro-
duced in a way similar to Zhou’s mechanism [18]. Finally, the
desired product was generated through CeO bond cleavage, in
which the trifluoromethyl played a promoting role; meanwhile, the
critical byproduct alkylecopper species was released which was
difficult to identify the structure. In order to stabilize any copper
species or byproducts, (R)-binap, a well-known bisphosphine
ligand, was used to coordinate with copper in the 13C isotope-
labeled DMF experiment. 31P NMR analysis further confirmed
that the alkylecopper species has been formed (see Supporting
information).
Fig. 2. X-ray structure of product 3a.
product in the absence of DMF (referring to Table 1, entries 11e12),
indicating involvement of DMF in the reaction mechanism.
With this newly established oxygen-atom insertion reaction in
hand, different NHCecopper complexes have been examined.
Other three NHCs (Fig. 1) have been applied to this reaction with
their NHCeCu complexes generated in situ in THF, and the results
are summarized in Scheme 1. Encouragingly, the NHCs 1bed al-
ways afforded the desired corresponding products 3bed in good to
excellent yields. For comparison, better results were obtained by
the NHCs containing unsaturated cyclic backbone, possibly due to
the stability of NHCecopper complexes. To the best of our knowl-
edge, it is a new protocol for complicated imidazolinones through
oxygen-atom insertion reaction.
Preliminary mechanism observations indicating that the
oxygen-atom in the product is from DMF get more supports from
the thio-DMF alternative experiment (Eq. (5)). Under the optimized
reaction conditions, instead of DMF, thio-DMF was treated with
(IPr)CuOtBu and Kþ[CF3B(OMe)3]ꢀ. The corresponding thio-product
3aeS was obtained in 45% yield of which the structure is confirmed
by ESI-MS, 13C NMR and IR analysis. Close comparison of 3a and
3. Conclusions
We have discovered a new oxygen-atom insertion reaction of
NHCecopper complex using N,N-dimethylformamide as the oxy-
gen source. Thio-DMF alternative experiment confirms that the
oxygen-atom of the insertion product is originated from the
carbonyl moiety of DMF. The present oxygen-atom insertion reac-
tion shows an excellent applicability for other NHCecopper com-
plexes allowing the preparation of complicated imidazolinones for
the first time. Further efforts should be made to elucidate the
detailed reaction path way of oxygen-atom insertion and the role of
trifluoromethyl reagent.
Table 2
Effects of structure and reaction conditions on the oxygen-atom insertion reaction of
NHCecopper complex.a
Entry
(IPr)CuX
Kþ[CF3B(OMe)3]ꢀ
Temp. (ꢁC)
Yieldb (%)
(x equiv)
1
(IPr)CuCl
(IPr)CuCl
1.0
6.0
6.0
6.0
6.0
0
6.0
6.0
6.0
6.0
6.0
30
30
30
30
30
30
80
0
14
56
44
99
66
44
16
22
85
56
72
2
3c
4
(IPr)CuCl
(IPr)CuOtBu
(IPr)CuF
5
6d
7
(IPr)CuOtBu
(IPr)CuOtBu
(IPr)CuOtBu
(IPr)CuOtBu
(IPr)CuOtBu
(IPr)CuOtBu
8
9e
10f
11g
30
30
30
a
Reaction conditions: (IPr)CuX (0.045 mmol, 1.0 equiv), Kþ[CF3B(OMe)3]ꢀ, DMF
(1.0 mL), 30 ꢁC, stirring 10 h under Ar atmosphere.
b
Isolated yield.
c
(IPr)CuCl was in situ generated.
d
Me3SiCF3 (6.0 equiv) was used instead of Kþ[CF3B(OMe)3]ꢀ.
e
The reaction was conducted under an air atmosphere.
Commercial wet DMF was used instead of dry DMF.
DMF (0.09 mmol, 2.0 equiv) in THF (1.0 mL).
Scheme 1. Oxygen-atom insertion reaction of different NHCs. Reaction conditions: 1
(0.045 mmol), CuCl (0.045 mmol), THF (0.5 mL), Kþ[CF3B(OMe)3]ꢀ (0.27 mmol,
6.0 equiv), DMF (0.5 mL), 30 ꢁC, 10 h.
f
g

W. Zeng et al. / Journal of Organometallic Chemistry 743 (2013) 44e48
47
(56.8 mg, 0.27 mmol, 6.0 equiv) in DMF (0.5 mL) was added
dropwise. Then the mixture was kept stirring at 30 ꢁC for 10 h. After
that, the reaction was quenched with water. Aqueous layer was
extracted with EtOAc (15 mL ꢂ 3). The combined organic layer was
washed with brine, dried over Na2SO4 and concentrated under
reduced pressure. Then the crude product was purified by column
chromatography on silica gel to give the imidazolinone 3aed.
4.3. Analytical data of imidazolinones
4.3.1. 1,3-Bis(2,6-diisopropylphenyl)-1H-imidazol-2(3H)-one (3a)
18 mg, 99% yield. 1H NMR (400 MHz, CDCl3):
d
7.45 (t, J ¼ 7.6 Hz,
2H), 7.29 (d, J ¼ 8.0 Hz, 4H), 6.83 (s, 2H), 2.71e2.78 (m, 4H), 1.31 (d,
J ¼ 6.8 Hz,12H),1.21 (d, J ¼ 6.8 Hz,12H); 13C NMR (100 MHz, CDCl3):
d
151.1,146.3,131.2,128.4,122.8,112.1, 27.8, 22.8, 22.6. IR (KBr) 3086,
2960, 2926, 2868, 1678, 1471, 1421, 1131, 923, 810, 750 cmꢀ1. ESI-MS
calcd. for C27H36N2O: 405.3 [M þ H]þ; Found: 405.3.
4.3.2. 1,3-Dimesityl-1H-imidazol-2(3H)-one (3b)
13.5 mg, 94% yield. 1H NMR (400 MHz, CDCl3):
d
6.95 (s, 4H),
Scheme 2. Proposed mechanism of the oxygen-atom insertion reaction of NHCe
copper complex.
6.32 (s, 2H), 2.30 (s, 6H), 2.20 (s, 12H); 13C NMR (100 MHz, CDCl3):
d
150.6, 138.4, 136.4, 132.2, 129.1, 112.3, 21.1, 18.0. IR (KBr) 3128,
4. Experimental section
2967, 2918, 1670, 1491, 1413, 1261, 1237, 908, 855, 789 cmꢀ1. ESI-MS
calcd. for C21H26N2O: 321.1 [M þ H]þ; Found: 321.4; calcd. for
4.1. General procedures and materials
C
21H24N2O: 343.1 [M þ Na]þ; Found: 343.3.
All reactions were carried out under argon atmosphere using
typical vacuum-line techniques unless otherwise noted. Solvents
were transferred via syringe and were introduced into the re-
action vessels through a rubber septum. CuCl, NaOtBu, (R)-Binap
and N,N-dimethylmethanethioamide (DMF-S) were purchased
from Alfa Aesar and stored in an argon-filled glovebox.
13C labeled DMF (CARBONYL-13C, 99%) was purchased from
Cambridge Isotope Laboratories, Inc. All the reagents were used
without further purification. 1,3-Bis(2,6-di-i-propylphenyl) imi-
dazolium chloride, Kþ[CF3B(OMe)3]ꢀ, (IPr)Rh(cod)Cl, (IPr)AgCl,
glyoxal-bis(2,4,6-tri-methylphenyl)imine and glyoxal-bis(2,6-di-
(i-propyl)phenyl)imine were prepared according to literature
methods. The 1H NMR (400 MHz) spectra for solution in CDCl3
were recorded on Bruker Avance 400 and Varian Mercury 400.
Chemical shifts were reported downfield in ppm from tetrame-
4.3.3. 1,3-Dimesitylimidazolidin-2-one (3c)
7.5 mg, 52% yield. 1H NMR (400 MHz, CDCl3):
d 6.92 (s, 4H), 3.77
(s, 4H), 2.30 (s, 12H), 2.27 (s, 6H); 13C NMR (100 MHz, CDCl3):
d
157.3, 137.5, 137.1, 133.5, 129.3, 44.3, 21.0, 17.9. IR (KBr) 2917, 2851,
2360, 2342, 1701, 1487, 1413, 1271, 858, 750 cmꢀ1. ESI-MS calcd. for
C
C
21H26N2O: 323.2 [M þ H]þ; Found: 323.4. ESI-MS calcd. for
21H26N2O: 345.2 [M þ Na]þ; Found: 345.4. Elemental analysis:
calcd.: C 78.22%, H 8.13%, N 8.69%; Found: C 78.30%, H 8.29%, N
8.48%.
4.3.4. 1,3-Bis(2,6-diisopropylphenyl)imidazolidin-2-one (3d)
12.1 mg, 66% yield. 1H NMR (400 MHz, CDCl3):
d 7.32 (t,
J ¼ 8.0 Hz, 2H), 7.20 (d, J ¼ 7.6 Hz, 4H), 3.81 (s, 4H), 3.13e3.20 (m,
4H), 1.32 (d, J ¼ 6.8 Hz, 12H), 1.24 (d, J ¼ 6.8 Hz, 12H); 13C NMR
(100 MHz, CDCl3): d 158.9, 148.2, 133.6, 128.7, 124.0, 46.7, 29.0, 24.7,
thylsilane (CDCl3,
chemical shift (
t ¼ triplet, q ¼ quartet, m ¼ multiplet), coupling constants (Hz),
integration and assignment. 13C NMR spectra were collected on
Bruker Avance 400 and a Varian Mercury 400 (100 MHz) with
complete proton decoupling. Chemical shifts were reported in
d
¼ 7.26). Spectra were reported as follows:
23.9. IR (KBr) 2961, 2928, 2868, 1688, 1660, 1458, 1411, 1276, 1262,
803, 766, 749 cmꢀ1. HRMS calcd. for C27H38N2O: 407.30 [M þ H]þ;
Found 407.30.
d
ppm), multiplicity (s ¼ singlet, d ¼ doublet,
Acknowledgments
ppm from the tetramethylsilane (CDCl3,
d
¼ 77.0). The IR spectra
We thank National Natural Science Foundation of China
(21172018) and Beijing Institute of Technology for the financial
support.
were recorded on Thermo Scientific Nicolet iS10 with KBr pellets.
Elemental analyses were performed on an Elementar Vario MI-
CRO CUBE instrument. HRMS was recorded on Bruker Apex IV
FTMS. All melting points were determined on a XT4A melting
point apparatus without correction. Analytical thin layer chro-
matography (TLC) was performed using F254 pre-coated silica
gel plate. Column chromatography was performed with silica gel
(200e300 mesh). Petroleum ether (PE) used had a boiling point
range of 60e90 ꢁC.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2013.06.017.
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