A general method for the preparation of N-heterocyclic carbene-silver(I) complexes in water

Article abstract of DOI:10.1007/s11243-012-9667-3

A new synthetic method leading to N-heterocyclic carbene-silver(I) complexes [(R₂-NHC)₂Ag]⁺ [AgX₂] ^- is developed by using benzimidazolium compounds, NaOH (as a base), silver salts and water (as the reaction medium). Single-crystal X-ray structure revealed that compound 1 comprises a linear [Ag-(Et₂-Bimy) ₂]⁺ cation and a linear [AgBr₂]^- anion. These two ions are linked through an Ag^I-Ag^I association and staggered at an angle of 90.3.

Full text of DOI:10.1007/s11243-012-9667-3

Transition Met Chem (2013) 38:113–118
DOI 10.1007/s11243-012-9667-3
A general method for the preparation of N-heterocyclic
carbene–silver(I) complexes in water
Muhammad Jawwad Saif Kevin R. Flower
•
Received: 24 August 2012 / Accepted: 22 October 2012 / Published online: 6 November 2012
Ó Springer Science+Business Media Dordrecht 2012
Abstract A new synthetic method leading to N-heterocyclic
ligands can produce stable metal–NHCs with strong metal–
?
carbene–silver(I) complexes [(R -NHC) Ag] [AgX2] is
-
carbon bonds [19]. Transmetallation with NHC–Ag com-
plexes is now a well-established technique for the prepara-
tion of different metal–NHC complexes. Transmetallation
with Ag(I)–NHC has been explored for the preparation of
various transition metal–NHC complexes, for example,
Au(I), Rh(I), Rh(III), Pd(II), Cu(I), Cu(II), Ru(II), Ru(III),
Ru(IV), Ni(II) and Pt(II) complexes [19].
2
2
developed by using benzimidazolium compounds, NaOH (as a
base), silver salts and water (as the reaction medium). Single-
crystal X-ray structure revealed that compound 1 comprises a
?
-
linear [Ag-(Et -Bimy) ] cation and a linear [AgBr ] anion.
2
2
2
I
I
These two ions are linked through an Ag –Ag association and
staggered at an angle of 90.3°.
The commonly employed Arduengo method for the
preparation of NHCs involves the deprotonation of azolium
salts with potassium hydride, sodium hydride or potassium
tert-butoxide [8]. The most common synthetic methodol-
ogy for the preparation of metal–NHC complexes involves
the reaction of pre-formed NHCs [20]. A drawback of such
methods is the use of a strong base under strictly anhydrous
and inert conditions. Another commonly employed method
in the syntheses of NHC complexes of silver is the use of a
silver base as the deprotonating agent. A variety of silver
bases (Ag O, AgOAc and Ag CO ) and solvents (dichlo-
Introduction
In recent years, N-heterocyclic carbenes (NHCs) have
attracted much attention, and this could be attributed to the
isolation of a stable carbene by Arduengo in 1991 [1]. NHC
ligands are now considered as surrogates to phosphines in
organometallic chemistry [2–4]. NHCs are widely known as
versatile ligands for a large variety of coordination com-
pounds [5]. NHC–metal complexes have been effectively
used as catalysts for organic transformations like hydrosi-
lylation [6, 7], alkene metathesis [8], hydroformylation [9],
copolymerization [10, 11], C–C and C–N coupling reactions
2
2
3
romethane, 1,2-dichloroethane, DMSO, acetone, methanol,
acetonitrile and DMF) have been used in this regard [19,
21, 22]. However, interestingly, there are very few reports
on the preparation of NHC–metal complexes in water.
Probably NHC–carbenes have always been considered too
reactive to be prepared in water. Cazin and co-workers
have reported a method for the synthesis of silver–NHC
complexes in water by refluxing imidazolium salts with
silver oxide in water for 24 h and obtained neutral [(R2-
NHC)-Ag–X]-type Ag–NHC complex [23]. However, this
method suffers from the drawbacks of low yields and long
reaction times.
[
12–14], hydrogenation [15, 16] and asymmetric transfor-
mations [17]. Metal–NHCs have also been employed in
medical sciences [18]. Being excellent r-donors, these
M. J. Saif (&)
Department of Applied Chemistry, Government College
University Faisalabad, Faisalabad, Pakistan
e-mail: jawwadsaif@gmail.com
Herein, we report a facile method to prepare Ag(I)–
NHC complexes by using benzimidazolium compounds,
NaOH as a base, and silver salts in water as the reaction
medium.
K. R. Flower
School of Chemistry, University of the Manchester,
Manchester M13 9PL, UK
123

1
14
Transition Met Chem (2013) 38:113–118
Results and discussion
Table 1 Crystal data and structure refinement for 1
Empirical formula
22 28 2 2 2
C H N Ag Br
In the present work, a new method has been developed and
studied for the preparation of NHC–Ag complexes. Benzim-
idazolium salts reacted with silver acetate, in the presence of
NaOH as a base, in aqueous medium to produce silver–N-
heterocyclic carbene complexes (Scheme 1). Apparently,
there is no need to use a phase-transfer catalyst in cases where
imidazolium/benzimidazolium salts are water soluble.
The methods for N-alkylation of benzimidazole are well
established. A reported method [24] was modified for the
synthesis of benzimidazolium salts where N-alkylation of
benzimidazole is performed in the first step with alkyl
halides in the presence of potassium hydroxide (KOH)
acting as base. Subsequent quaternization with a second
mole of alkyl halide afforded the desired benzimidazolium
compounds.
Formula weight
Temperature
724.04
293(2) K
0.71073
˚
Wavelength (A)
Crystal system
Space group
Orthorhombic
Pcab
Unit cell dimensions
˚
a (A)
16.1137 (7)
17.2552 (9)
18.2271 (9)
90
˚
b (A)
c ( A˚ )
a (°)
b (°)
90
c (°)
90
3
V (A )
˚
5,067.96(4)
1
Z
In a typical experiment to prepare NHC–Ag(I) complex,
silver acetate was added to an aqueous solution of benz-
imidazolium bromide in the presence of NaOH as a base.
Formation of Ag(I)–NHC complex started after few min-
utes. Sodium hydroxide extracts the N–CH–N proton, and a
carbene is generated that immediately reacts with silver.
Ag–NHC complex is insoluble in water and can be sepa-
rated easily by filtration. The product was fully character-
ized by NMR, mass and elemental analysis techniques
-3
Dc(g cm
F (000)
)
1.90
2,815.6
Crystal size (mm)
0.30 9 0.10 9 0.10
2.1–26.8
5,351
h range (°)
Reflections collected
R
1
0.0937
wR
2
0.2514
2
Goodness of fit on F
-3
1.062
˚
rqmax (e A
)
2.586
(
see experimental) and single-crystal X-ray diffraction
-3
˚
rqmin (e A
)
-2.396
studies; see Table 1 for crystallographic data and Fig. 1 for
ORTEP representations of the molecular structures.
The most informative feature in the proton NMR spec-
trum of compound [1,3-diethylbenzimidazol-2-ylidene
1
3
complex is an indication of carbene formation. C NMR
showed the signal for carbene carbons at d 188.67, which is a
recognized carbene signal. In the mass spectrum of [1,3-
silver(I)]AgBr (1) is the absence of the C-1 proton signal
2
that was observed for 1,3-diethylbenzimidazolium bromide
diethylbenzimidazol-2-ylidene silver(I)]AgBr (1), two peaks
2
at d 11.45. The absence of this signal for the silver–carbene
appeared at 457 and 455. Since there exist two isotopes for
Scheme 1 Preparation of
silver–carbene complex in water
1
1
2
2
1
1
2
1
2
-
+
3
-
2
1
1
1
1
2
2
2
2
1
23

Transition Met Chem (2013) 38:113–118
115
˚
and 2.079 A for Ag(1)–C(1) and Ag(1)–C(12), respectively.
The interplaner angle between the benzimidazole (C1) ring
and the C(1)–Ag(1)–Ag(2) plane is 92.58° and that between
benzimidazole (C12) ring and the C(12)–Ag(1)–Ag(2) plane
is 84.50°. The angle C(1)–Ag(1)–C(12) is 175.8° and pro-
I
I
poses a linear geometry. The separation of Ag –Ag in
˚
compound 1 is 2.925 A and is at the low end of the known
˚
ligand-unsupported Ag–Ag interaction (range 2.80–3.30 A)
[
25]. The crystal structure of compound 1 is similar to the
0
reported N,N -diethylbenzimidazolylidene silver bromide
prepared in organic solvents [26, 27].
Conventional silver oxide was also investigated as a
silver source under identical experimental conditions.
However, the yield of the Ag–NHC complex was poor as
compared to that when silver acetate was used. This fact
might be interpreted in terms of solubility difference of
silver salts in aqueous medium. Suitability of KOH as base
was also investigated, but the best yields could be obtained
using NaOH as the base. KOH produced 81 % isolated
yield of the product, whereas NaOH produced up to 93 %
yield of NHC–Ag complex 1.
Fig. 1 ORTEP diagram of Ag–NHC complex (1) giving 50 %
probability ellipsoids. H-atoms are excluded for clarity
1
07
109
and Ag , so two peaks appeared that are
silver, Ag
?
assigned to [M–AgBr ] with 100 % relative intensity. In
-
ESI scan, two peaks for AgBr appeared at 267 and 269 with
2
-
2
1
07
109
equal intensity for both silver isotopes (Ag and Ag ).
According to the available literature, mono-azolium
halides can produce Ag(I)–NHC complexes of various
ionic and neutral structures (Scheme 2) [19].
Cazin and co-workers have reported the synthesis of Ag–
Experiments were extended to prepare a series of four
NHC–Ag complexes 1–4. Good to excellent yields were
obtained in all cases. Experimental conditions and results
are summarized in Table 2.
NHC complex in H O from silver oxide obtaining a neutral
2
complex of type 2 (Scheme 2) [23]. In our experiments,
using silver acetate as the starting material in the presence of
NaOH in water, the only product was of type 1. Single-
crystal X-ray structure showed that compound 1 comprises
In conclusion, N-heterocyclic carbenes are produced in
water by in situ deprotonation of benzimidazolium com-
pounds by NaOH and are further reacted with silver salts to
produce NHC–Ag(I) complexes. This work describes the
preparation of NHC–silver complexes, but also suggests the
potential of preparing other metal–NHC complexes in water.
?
-
linear [Ag-(Et -Bimy) ] cation and a linear [AgBr ] anion
2
2
2
I
Fig. 1). These two ions are linked through an Ag –Ag
I
(
association and staggered in an angle of 90.33°. Benzimid-
azole-2-yl ligands are characterized by a small difference
˚
(
0.02–0.03 A) in N–C bond lengths to the metal-bonded
carbon atom, and the cationic Ag–C bond distance is 2.086
Experimental
+
-
(
R -NHC)-Ag-X
Chemicals whose syntheses are not explicitly mentioned
were purchased from chemical suppliers and used without
further purification. A reported method [24] was modified
for the synthesis of benzimidazolium salts where N-alkyl-
ation of benzimidazole is performed in the first step with
[
(R -NHC) Ag] [AgX ]
2
2
2
2
Type - 1
Type - 2
X
X
(
R -NHC)
Ag
Ag
(NHC-R2)
2
Table 2 Percentage yield
NHC–
Ag(I)
Isolated
yields (%)
Base
a
Type - 3
1
1
2
2
3
3
4
4
92.5
81
92
85
93
80
93
76
NaOH
KOH
X
X
(
R -NHC) Ag
Ag
Ag Ag (NHC-R2)2
X
NaOH
KOH
2
2
X
NaOH
KOH
Type - 4
a
After stirring the reaction
mixture for 40 min at room
temperature
NaOH
KOH
Scheme 2 Examples of ionic and neutral Ag(I)–NHC complexes
19]
[
123

1
16
Transition Met Chem (2013) 38:113–118
alkyl halides in the presence of potassium hydroxide (KOH)
acting as base. Subsequent quaternization with a second
mole of alkyl halide afforded the desired benzimidazolium
NMR:d:140.2(N–CH–N), 131.7(Ar), 129.6(Ar),127.0(Ar),
120.8 (CH), 113.5 (Ar), 49.2 (N–CH ), 42.5 (N–CH ), 13.5
2
2
?
- ?
(CH ) MS: (ESI , m/z) 187.0 [(M–Br )] , 100 %.
3
1
13
compounds. H NMR (400 MHz) and C NMR (100.57
1
MHz) were recorded on a Bruker DPX400 spectrometer. H
1
3
1-Methyl-3-allylbenzimidazolium bromide
and C{1H} NMR spectra were referenced to CHCl (d =
3
7
.26 ppm) and CHCl (d = 77.0 ppm), respectively. Work-
3
Quaternization of 1-methylbenzimidazole was performed
1
with allyl bromide as the alkylating agent. Yield 78 %, H
ups were carried out in the open air.
Crystals suitable for X-ray crystallographic analysis were
grown by layering of n-hexane onto a dichloromethane solu-
tion of the complex. Data collections were carried out using u
and x scans on a Nonius Kappa CCD diffractometer using
NMR: (400 MHz, D O) d: 9.12 (s, 1H C1), 7.72–7.69 (m,
2
2H, N–CH–N), 7.55–7.51 (m, 2H, Ar), 6.02–5.92 (m, 1H,
CH), 5.33–5.22 (m, 2H, =CH ), 4.96 (d, 2H, CH ,
2
2
1
3
J = 7.13 Hz), 3.95 (s, 3H, CH3) C NMR: d: 132.0 (Ar),
31.2 (Ar), 129.6 (CH), 126.7 (Ar), 120.7 (=CH ), 112.9
˚
graphite monochromated Mo-Ka radiation (k = 0.71073 A).
1
2
Experimental details are summarized in Table 1. The crystal
structurewassolvedbydirectmethodsusingSHELXS-97and
refined by full matrix least squares with SHELXL-97 [28].
?
Ar), 49.0 (CH ), 42.6 (CH ) 32.8 (CH ) MS: (ESI , m/z)
(
2 2 3
-
74.1 [(M–Br )] , 100 %.
?
1
1
-Methylbenzimidazole
1-Methyl-3-ethylbenzimidazolium bromide
KOH (2.24 g, 0.04 mol) was added to a solution of benz-
imidazole (2.36 g, 0.02 mol) in 20 mL of DMF. The mixture
wasstirredfor15 minat 18–20 °C, andiodomethane (2.84 g,
Quaternization of 1-methylbenzimidazole was performed
1
with bromoethane. Colourless powder, yield 72 %. H NMR:
(400 MHz, D
Ar), 4.76 (q, 2H, N–CH
N–CH ), 1.63 (t, 3H, CH
142.3 (N–CH –N), 133.4 (Ar), 126.1 (Ar), 113.6 (Ar), 42.2
(N–CH ), 35.3 (N–CH ), 15.0 (CH ) MS: (ESI , m/z) 162.1
3 3
O) d:10.5 (s, 1H, N–CH –N), 7.65–7.52 (m, 4H,
2 2
0
.02 mol) was added dropwise under vigorous stirring. After
2
, J = 7.32, 7.36 Hz), 4.18 (s, 3H,
13
3
h, the mixture was diluted with 50 mL of water and
3
, J = 7.12, 7.06 Hz) C NMR: d:
3
extracted with CHCl (6 9 25 mL). The combined extracts
3
2
?
were washed with brine and dried over MgSO . The solvent
4
2
-
?
was rotary evaporated to yield the product in 94 % yield.
[(M–Br )] , 100 %.
1-Ethylbenzimidazole was prepared in a similar way by
using bromoethane as the alkylating agent.
[1,3-Diethylbenzimidazol-2-ylidene silver(I)]AgBr (1)
2
1
1
,3-Diethylbenzimidazolium bromide
1
,3-Diethylbenzimidazolium bromide (255.1 mg, 1 mmol)
was dissolved in 20 mL of water. NaOH (160 mg, 4.0 mmol)
was added, and the solution was stirred for 5 min. Silver(I)
acetate (166.9 mg, 1 mmol) was added, and the mixture was
stirred for 1–2 h. During this time, water-insoluble ben-
zimidazol-2-ylidene silver complex started to gather at the
bottom. The reaction mixture was filtered and washed with
plenty of water and dried on the bench. Analytically pure
complex was obtained by dissolving the crude material in
-Ethylbenzimidazole was reacted with an equimolar
quantity of 1-bromoethane in THF (25 mL) for 12–24 h.
Quaternary benzimidazolium salt precipitated out of the
reaction mixture. The solvent was evaporated, and the crude
product was washed with diethyl ether (10 mL). Colourless
1
powder, yield 86 %. H NMR: (400 MHz, D O) d: 11.45
2
(
s, 1H, N–CH–N), 7.75 (m, 2H, Ar), 7.68 (m, 2H, Ar), 4.72
(
q, J = 7.70, 7.38 Hz, 4H, N–CH ), 1.75 (t, J = 7.38 Hz,
2
10 mL of dichloromethane, filtering over Celite and evapo-
1
3
6
H, CH3) C NMR: d: 142.3 (N–CH–N), 131.2 (Ar), 127.1
rating the solvent to yield white-colouredproduct, which was
further recrystallized from dichloromethane/n-hexane mix-
(
Ar), 113.0 (Ar), 42.9 (N–CH ), 14.9 (CH ).
2 3
? - ?
MS: (ESI , m/z) 174.2 [(M–Br ) , 100 %].
ture to yield white crystals. White crystals, yield 94.5 %,
1
melting point: 130 °C (dec) H NMR: (400 MHz, CDCl ) d:
3
1
-Ethyl-3-allylbenzimidazolium bromide
7.45–7.41 (m, 4H, Ar), 7.37, 7.33 (m, 4H, Ar), 4.42 (q,
J = 7.6, 7.3 Hz, 4H, CH ), 1.46 (t, J = 7.3, 7.3 Hz, 6H,
2
1
3
Allyl bromide was used as the alkylating agent. Colourless
1
CH3) C NMR: d: 188.6 (carbene), 133.3 (Ar), 124.0 (Ar),
111.4 (Ar), 44.5 (CH ), 15.1 (CH ) MS: (ESI , m/z) 457
?
powder, yield 89 % H NMR: (400 MHz, D O)d: 9.17(s, 1H,
2
2
3
-
?
?
- ?
N–CH–N), 7.77–7.70 (m, 2H, Ar), 7.55–7.52 (m, 2H, Ar),
[(M–AgBr )] , 100 %, (ESI , m/z) 455 [(M–AgBr )] ,
2 2
-
-
77 % (ESI , m/z) 269 [AgBr ] , 100 %, (ESI , m/z) 267
-
6
.03–5.93 (m, 1H, CH), 5.33–5.23 (m, 2H, N-CH ), 4.96
2
2
-
[AgBr ] , 100 % Anal: Calculated: C = 36.5, H = 3.9,
(
t, J = 5.7, 5.6 Hz, 2H, =CH ), 4.41–4.35 (q, 2H, J = 7.50,
2
2
1
3
7
.49, 7.49 Hz, N–CH ), 1.48 (t, J = 7.0 Hz, 3H, CH )
2
C
N = 7.7 Found: C = 36.9, H = 3.90, N = 7.4.
3
1
23

Transition Met Chem (2013) 38:113–118
117
[
(
1-Ethyl-3-allylbenzimidazol-2-ylidene silver(I)]AgBr2
2)
free ofchargevia http://www.ccdc.cam.ac.uk/conts/retrieving.
html or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: (?44) 1223-336-
033; or e-mail: deposit@ccdc.cam.ac.uk.
The same procedure was used except that 1-ethyl-3-allyl-
benzimidazolium bromide was used to prepare the carbene.
1
White crystals, yield 88 % H NMR: (400 MHz, CDCl ) d:
3
7
.45–7.38 (m, 4H, Ar), 7.36–7.31 (m, 4H, Ar), 6.01–5.87
(
m, 2H, CH), 5.28–5.14 (m, 4H, =CH ), 5.01–4.97 (m, 4H,
2
References
N–CH ), 4.43 (q, J = 7.30, 4H, CH ), 1.47 (t, J = 6.9, 6H,
2
2
1
3
1
. Arduengo AJ, Harlow RL, Kline M (1991) A stable crystalline car-
bene. J Am Chem Soc 113(1):361–363. doi:10.1021/ja00001a054
. Huang J, Grasa G, Nolan SP (1999) General and efficient cata-
lytic amination of aryl chlorides using a palladium/bulky nucle-
ophilic carbene system. Org Lett 1(8):1307–1309. doi:10.1021/
ol990987d
CH3) C NMR: d: 189.1 (carbene), 133.8 (Ar), 131.8
CH), 124.1 (Ar), 119.3 (=CH ), 111.9 (Ar), 51.9 (CH ,
(
2
2
2
?
allyl), 44.6 (CH , ethyl), 15.9 (CH ) MS: (ESI , m/z) 481.2
2
3
-
?
?
- ?
[
(M–AgBr )] , 100 %, (ESI , m/z) 479.2 [(M–AgBr )] ,
2 2
-
-
4 % (ESI , m/z) 268.8 [AgBr ] , 94 %, (ESI , m/z)
-
9
2
2
-
66.8 [AgBr ] , 100 % Anal: Calculated: C = 38.5, H = 3.7,
3. Nair V, Bindu S, Sreekumar V (2004) N-heterocyclic carbenes:
reagents, not just ligands! Angew Chem Int Ed 43(39):5130–5135.
doi:10.1002/anie.200301714
2
N = 7.5 Found: C = 38.6, H = 3.3, N = 7.9.
4
5
6
. Nolan SP (2006) N-heterocyclic carbenes in synthesis. Wiley,
Weinheim
. Bourissou D, Guerret O, Gabbai FP, Bertrand G (2000) Stable
carbenes. Chem Rev 100(1):39–92
. Berthon-Gelloz G, Mark o´ IE (2006) Efficient and selective hydro-
silylation of alkenes and alkynes catalyzed by novel N-heterocyclic
carbene Pt0 complexes. In: N-heterocyclic carbenes in synthesis.
Wiley, New York, pp 119–161. doi:10.1002/9783527609451.ch6
. Gade LH, C e´ sar V, Bellemin-Laponnaz S (2004) A modular
assembly of chiral oxazolinylcarbene–rhodium complexes: effi-
cient phosphane-free catalysts for the asymmetric hydrosilylation
of dialkyl ketones. Angew Chem Int Ed 43(8):1014–1017. doi:
[
1-Methyl-3-allylbenzimidazol-2-ylidene
silver(I)]AgBr (3)
2
The same experimental procedure was employed except
that 1-methyl-3-allylbenzimidazolium bromide was used to
1
prepare the carbene. White crystals, yield 87 % H NMR:
7
(
400 MHz, CDCl ) d: 7.49–7.33 (m, 8H, Ar), 6.10–5.84
3
(
m, 2H, CH), 5.35–5.19 (m, 4H, =CH ), 5.06 (m, 4H, CH ),
2
2
1
3
4
1
1
.10 (s, 6H, CH3) C NMR: d: 190.6 (carbene), 134.5,
33.6 (Ar), 131.9 (Ar), 124.1 (Ar), 119.2 (Ar), 111.7 and
1
0.1002/anie.200353133
8
9
. Bantreil X, Nolan SP (2011) Synthesis of N-heterocyclic carbene
ligands and derived ruthenium olefin metathesis catalysts. Nat
Protoc 6(1):69–77
. Chen AC, Ren L, Decken A, Crudden CM (2000) Rhodium
carbene complexes: highly selective catalysts for the hydrofor-
mylation of styrene derivatives. Organometallics 19(18):3459–
?
11.2 (Ar), 51.8 (CH ), 35.9 (CH ) MS: (ESI , m/z) 453.2
2
3
-
?
?
- ?
[
(M–AgBr )] , 100 %, (ESI , m/z) 451.2 [(M–AgBr )] ,
2 2
-
6 % (ESI , m/z) 268.8 [AgBr ] , 94 %, (ESI , m/z)
-
-
7
2
2
-
66.8 [AgBr ] , 100 % Anal: Calculated: C = 36.7, H = 3.3,
2
N = 7.7 Found: C = 36.3, H = 2.9, N = 7.20.
3
461. doi:10.1021/om0003739
1
1
1
0. Okuyama K-i, Sugiyama J-i, Nagahata R, Asai M, Ueda M,
Takeuchi K (2003) Direct synthesis of polycarbonate from carbon
monoxide and bisphenol a catalyzed by Pd–carbene complex.
Macromolecules 36(19):6953–6955. doi:10.1021/ma034442h
1. Vehlow K, Wang D, Buchmeiser MR, Blechert S (2008) Alter-
nating copolymerizations using a Grubbs-type initiator with an
unsymmetrical, chiral N-heterocyclic carbene ligand. Angew
Chem Int Ed 47(14):2615–2618. doi:10.1002/anie.200704822
2. Lee S, Hartwig JF (2001) Improved catalysts for the palladium-
[
1-Methyl-3-ethylbenzimidazol-2-ylidene
silver(I)]AgBr (4)
2
The same procedure was used except that 1-methyl-3-
ethylbenzimidazolium bromide was used to prepare the
1
carbene. White crystals, yield 87 % H NMR: (400 MHz,
CDCl ) d: 7.46–7.33 (m, 8H, Ar), 4.42–4.32 (q, J = 7.4 Hz,
ˆ
catalyzed synthesis of oxindoles by amide I ± -arylation. Rate
3
4
H, CH ), 4.08 (s, 6H, CH ), 1.52 (t, J = 7.2 Hz, 6H, CH )
2
acceleration, use of aryl chloride substrates, and a new carbene
ligand for asymmetric transformations. J Org Chem 66(10):
3402–3415. doi:10.1021/jo005761z
3
3
1
3
C NMR: d: 189.8 (carbene), 133.1 (Ar), 124.3 (Ar), 111.4
?
(
Ar), 44.6 (CH ), 36.0 (CH ), 16.2 (CH ) MS: (ESI , m/z)
2 3 3
¨
3. Ozdemir I, Demir S, C¸ etinkaya B, Gourlaouen C, Maseras F,
1
-
2
?
29.2 [(M–AgBr )] , 96 %, (ESI , m/z) 427.2 [(M–AgBr )] ,
?
- ?
2
4
ˆ
Bruneau C, Dixneuf PH (2008) Direct arylation of arene C aˆ ’H
ˆ
bonds by cooperative action of NHCarbene aˆ ’ruthenium(II) cata-
lyst and carbonate via proton abstraction mechanism. J Am Chem
Soc 130(4):1156–1157. doi:10.1021/ja710276x
-
00 % (ESI , m/z) 268.8 [AgBr ] , 97 %, (ESI , m/z) 266.8
-
-
1
2
-
AgBr ] , 100 % Anal: Calculated: C = 34.5, H = 3.4, N =
[
2
8.0 Found: C = 34.0, H = 3.1, N = 7.6
1
1
4. Stauffer SR, Lee S, Stambuli JP, Hauck SI, Hartwig JF (2000)
High turnover number and rapid, room-temperature amination of
chloroarenes using saturated carbene ligands. Org Lett
2
(10):1423–1426. doi:10.1021/ol005751k
Supplementary materials
5. Powell MT, Hou D-R, Perry MC, Cui X, Burgess K (2001) Chiral
imidazolylidine ligands for asymmetric hydrogenation of aryl
alkenes. J Am Chem Soc 123(36):8878–8879. doi:10.1021/ja01
6011p
CCDC 889528 contains the supplementary crystallographic
data for Ag(I)–NHC complex 1. These data can be obtained
123

1
1
18
Transition Met Chem (2013) 38:113–118
6. Perry MC, Cui X, Powell MT, Hou D-R, Reibenspies JH, Burgess K
(2002) Optically active iridium imidazol-2-ylidene-oxazoline com-
23. Citadelle CA, Nouy EL, Bisaro F, Slawin AMZ, Cazin CSJ
(2010) Simple and versatile synthesis of copper and silver
N-heterocyclic carbene complexes in water or organic solvents.
Dalton Trans 39(19):4489–4491
24. Pilarski B (1983) A new method for N-alkylation of imidazoles
and benzimidazoles. Liebigs Ann Chem 6:1078–1080. doi:
10.1002/jlac.198319830619
plexes: preparation and use in asymmetric hydrogenation of aryl-
alkenes. J Am Chem Soc 125(1):113–123. doi:10.1021/ja028142b
7. Gade LH, Bellemin-Laponnaz Sp (2007) Mixed oxazoline-carb-
enes as stereodirecting ligands for asymmetric catalysis. Coord
Chem Rev 251(5–6):718–725
1
1
8. Barnard PJ, Baker MV, Berners-Price SJ, Day DA (2004) J Inorg
Biochem 98:1642
9. Lin IJB, Vasam CS (2007) Preparation and application of
N-heterocyclic carbene complexes of Ag(I). Coord Chem Rev
25. Singh K, Long JR, Stavropoulos P (1997) Ligand-unsupported
metal-metal (M=Cu, Ag) interactions between closed-shell d10
trinuclear systems. J Am Chem Soc 119(12):2942–2943. doi:10.
1021/ja963664a
1
2
51(5–6):642–670
26. Lee KM, Wang HMJ, Lin IJB (2002) Structural diversity of
N-heterocyclic carbene complexes of silver(i). J Chem Soc Dal-
ton Trans 14:2852–2856
2
2
0. Hermann WA, Koher C, GooDen LJ, Artus GRJ (1996) Chem
Eur J 2:1627
1. Guerret O, Sol e´ S, Gornitzka H, Teichert M, Trinquier G, Ber-
trand G (1997) 1,2,4-triazole-3,5-diylidene: a building block for
organometallic polymer synthesis. J Am Chem Soc 119(28):
27. Huang W, Zhang R, Zou G, Tang J, Sun J (2007) An iodide/anion
exchange route to benzimidazolylidene silver complexes from
benzimidazolium iodide: crystal structures of N, N’-dibu-
tylbenzimidazolylidene silver chloride, bromide, cyanide and
nitrate. J Organomet Chem 692(17):3804–3809
6
668–6669. doi:10.1021/ja964191a
2. Guerret O, Sol e´ S, Gornitzka H, Trinquier G, Bertrand G (2000)
,2,4-triazolium-5-ylidene and 1,2,4-triazol-3,5-diylidene as new
2
1
28. Sheldrick GM (1997) SHELX-97, program for crystal structure
solution. University of Goettingen, Germany
ligands for transition metals. J Organomet Chem 600(1–2):112–117
1
23