A direct and practical approach for the synthesis of N-heterocyclic carbene coinage metal complexes

Article abstract of DOI:10.1016/j.tet.2012.07.009

A novel direct and practical synthetic route leading to N-heterocyclic carbene coinage metal complexes has been developed by using air stable, commercial available Au(III) salt [MAuCl₄·2H₂O], CuCl_n (n=1,2) or AgCl, and imidazolium salts as starting materials. The reaction proceeded without sacrificing carbene transfer agent (Ag ₂O) or using highly sensitive free NHC.

Full text of DOI:10.1016/j.tet.2012.07.009

Tetrahedron 68 (2012) 7949e7955
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A direct and practical approach for the synthesis of N-heterocyclic carbene
coinage metal complexes
,
b
,
a
a
Shifa Zhu a , Renxiao Liang , Huanfeng Jiang
*
a School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, Guangdong, China
b Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 May 2012
Received in revised form 22 June 2012
Accepted 3 July 2012
Available online 16 July 2012
A novel direct and practical synthetic route leading to N-heterocyclic carbene coinage metal complexes
has been developed by using air stable, commercial available Au(III) salt [MAuCl4$2H2O], CuCln (n¼1,2) or
AgCl, and imidazolium salts as starting materials. The reaction proceeded without sacrificing carbene
transfer agent (Ag2O) or using highly sensitive free NHC.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Coinage metal
N-Heterocyclic carbene
Gold
Silver
Copper
1. Introduction
disadvantages: (a) three-step reaction required for each method to
form Au(I)eNHC from the imidazolium salts and gold(III) salts; (b)
Group 11 elements (Cu, Ag, Au), conventionally called as coinage
metals, have aroused intense interest in the past decades.1 Gold,
unlike its homolog Cu and Ag, known as an inert coinage metal, has
attracted more and more interests due to its unexpected catalytic
capabilities in the past decade.1 Bothinorganic gold salts and organic
gold coordination complexes have been used extensively in various
organic transformations.2 Especially in recent years, it has been
found that gold could form stable coordination complexes with
N-heterocyclic carbene (NHC) ligands.3 In particular, Au(I)eNHC,
which has been successfully used as catalysts in many important
reactions, such as nucleophilic additions, FriedeleCrafts reactions,
CeH activations, hydrogenations, cross-coupling reactions, and
oxidations.3bee In the literature,4 there are two common ways to
synthesize these Au(I)eNHC complexes: (1) using the silver oxide
(Ag2O) route developed by Lin4a to generate the appropriate
[Ag(NHC)Cl] complexes4b that are then used as NHC transfer agents
to react with the [Au(DMS)Cl] (DMS¼dimethylsulfide),4c or (2) free
NHC react with the [Au(DMS)Cl]4c (Scheme 1). [Au(DMS)Cl], which
is slightly unstable, was often prepared freshly from gold(III) salts
[MAuCl4$2H2O] and used immediately. Therefore, both approaches
are actually started from the same starting materials: imidazolium
salts and [MAuCl4$2H2O]. These approaches hold the following
1 equiv of expensive Ag2O is sacrificed as the carbene transfer agent
in Eq. 1, which is not desirable in terms of both green chemistry and
atom efficiency; (c) isolation of the highlysensitive and unstable free
NHC is required for Eq. 2, which is operationally troublesome.
Ag2O
N
N
N
N
N
N
N
N
(1)
(2)
R
R
R
R
R
R
R
R
R
Me2S-AuCl
Me2S
Au
Cl
Cl
H
Ag
Cl
MAuCl4*2H2O
Me2S
Base
Me2S-AuCl
R
N
N
Free NHC
N
N
R
R
Cl
Au
Cl
H
Scheme 1. Conventional ways to synthesize of Au(I)eNHC.
In this context, developing a direct and practical approach
without sacrificing carbene transfer agent (Ag2O) or using highly
sensitive starting material (free NHC) to synthesize the important
and useful Au(I)eNHC complexes would be highly desirable.
Herein, we would like to report a direct and practical one-step
* Corresponding author. E-mail address: zhusf@scut.edu.cn (S. Zhu).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.07.009

7950
S. Zhu et al. / Tetrahedron 68 (2012) 7949e7955
procedure to synthesize Au(I)eNHC complex, this method can also
be extended to prepare other coinage metal (Ag and Cu) complexes.
At the outset of the project, we targeted to synthesize Au(I)e
NHC from the imidazolium salts and gold(III) salts. Gold(III) is well
known for its strong oxidizing character,5 and in fact, could be
easily reduced into gold(I) complexes in the presence of sulfide,
phosphine or other organic and inorganic reductants.5,6 Therefore,
we then speculated that it is possible to use the stable and com-
mercially available gold(III) salts [MAuCl4$2H2O], instead of the
slightly unstable [Au(DMS)Cl], as the gold source to react with the
imidazolium salts directly to prepare the Au(I)eNHC complexes in
one step (Scheme 2).
dichloroethane (DCE) was used instead, the yield of IMeseAuCl was
increased to 36%, however, the reaction conversion was still low
(52%) (entry 2). Trace IMeseAuCl 2a was observed when the re-
action was performed in THF (entry 3). When pyridine was used as
the solvent, the ratio of IMeseAuCl (2a) to IMeseAuCl3 (3a) equals
to 1:1, but the overall yield is up to 88% (entry 4). It is surprising that
when 3-Clepyridine was used as the solvent, IMeseAuCl (2a) could
be formed solely in 93% yield. No IMeseAuCl3 (3a) was detected
(entry 5). It is worthy to note that the reaction was not sensitive to
water because there are two crystal water in each [MAuCl4$2H2O]
molecule. To further figure out the effect of the water, a mixed
solvent (3-Clepyridine/H2O¼5:1) was employed, the yield of 2a
was still up to 81% (entry 6). When H2O was served as the sole
solvent, however, the yield of 2a drop to 14% (entry 7). Therefore, 3-
Clepyridine has proved to be the solvent of choice. Besides the
combination of [NaAuCl4$2H2O] and Na2CO3, other combinations of
gold(III) salts and bases were also investigated (entries 8e10). It is
interesting that both [KAuCl4$2H2O]/Na2CO3 and [NaAuCl4$2H2O]/
K2CO3 gave the mixtures of 2a and 3a (entries 8 and 9). It is worthy
to mention that IMeseAuCl3 (3a) could be formed in 60% yield
when the former combination was applied. The results here pro-
vided an alternative and straightforward way to prepare Au(III)e
NHC complexes, which were often indirectly obtained by oxidation
of the corresponding Au(I)eNHC with Cl2 or Br2.7 The combination
of [KAuCl4$2H2O] with K2CO3 could furnish the IMeseAuCl (2a)
selectively as well, albeit in relatively low yield (65%) (entry 10).
Different bases were also investigated: Cs2CO3 gave only moderate
yield of product 2a (entry 11). K3PO4 produced 2a in 72% yield,
however, accompanying with 3a in 12% yield (entry 12). NaOH was
inefficient for this reaction at all (entry 13). In addition, the re-
actions did not occur in the absence of the base (entry 14). The
structures of IMeseAuCl (2a) and IMeseAuCl3 (3a) were verified by
the single-crystal X-ray diffraction analysis (Scheme 3). Selected
bond distances and bond angles are given in the figure caption. The
gold atom in 2a is two-coordinated, as is usual for gold(I) com-
plexes, and exhibits a linear geometry with a C(1)eAu(1)eCl(1)
1 step
N
N
N
N
+
R
MAuCl4*2H2O
R
R
R
???
Cl
Au
Cl
H
1
2
Scheme 2. The retrosynthetic analysis of Au(I)eNHC.
2. Results and discussion
2.1. Synthesis of NHCeAuCl and NHCeAuCl3 complexes
Initial efforts were made to systematically investigate various
metalation conditions, using IMes$HCl (1a) as the model substrate
and [NaAuCl4$2H2O] or [KAuCl4$2H2O] as gold(III) salt (Table 1).
Firstly, different solvents were tested for the reactions when
[NaAuCl4$2H2O] was used as the gold source and Na2CO3 as the
base. When toluene was used as the solvent, a mixture of
IMeseAuCl (2a) and IMeseAuCl3 (3a) was obtained in 53% total
yield, with 3a (40%) being dominated (entry 1, Table 1). When
ꢀ
ꢀ
bond angle value of 180.0 . The Au(1)eC(1) bond length (1.916 A) in
structure 2a is in good agreement with those reported NHCegold(I)
Table 1
One step to synthesize Au(I)eNHC complexes from imidazolium salts and com-
mercially available Au(III) saltsa
MAuCl4*2H2O
N
N
N
N
N
N
Mes
Mes
+
Mes
Mes
Mes
Mes
Base
Cl
H
Au
Cl Au Cl
Cl
2a
Cl
3a
1a
Entry
Gold source
Base
Sol
Yieldb
2a (%)
3a (%)
1
2
3
4
NaAuCl4$2H2O
NaAuCl4$2H2O
NaAuCl4$2H2O
NaAuCl4$2H2O
NaAuCl4$2H2O
NaAuCl4$2H2O
NaAuCl4$2H2O
KAuCl4$2H2O
NaAuCl4$2H2O
KAuCl4$2H2O
NaAuCl4$2H2O
NaAuCl4$2H2O
NaAuCl4$2H2O
NaAuCl4$2H2O
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
K2CO3
K2CO3
Cs2CO3
K3PO4
NaOH
Toluene
DCEc
THF
13
36
Trace
44
93
81
14e
28
59
65
50
72
d
40
16
d
44
d
d
d
60
24
d
d
12
d
d
Pyridine
5
3-Clepyridine
3-Clepyridine
H2O
3-Clepyridine
3-Clepyridine
3-Clepyridine
3-Clepyridine
3-Clepyridine
3-Clepyridine
3-Clepyridine
6d
7
8
9
10
11
12
13
14
d
d
a
Reactions were carried out in
a
Schlenk tube using 1 (0.15 mmol),
[MAuCl4$2H2O] (0.14 mmol), and 0.5 ml 3-Clepyridine in a Schlenk tube, heating at
Scheme 3. ORTEP diagꢀram (a) IMeseAuCl 2a and (b) IMeseAuCl3 3a. Selected bond
80 ꢀC under the atmosphere of N2 for 24 h.
ꢀ
distances (A)andangles( )in2a:Au(1)eC(1),1.916(19); Au(1)eCl(1), 2.285(5); C(1)eAu(1)e
b
isolated yield, the ratio of 2a to 3a was determined by 1H NMR.
DCE (Dichloroethane).
3-Clepyridine/H2O (5:1) as the solvent.
ꢀ
ꢀ
Cl(1), 180.000(2). Selected bond distances (A) and angles ( ) in 3a: Au(1)eC(1), 2.016(7);
Au(1)eCl(1), 2.299(2); Au(1)eCl(2), 2.281(3); Au(1)eCl(3), 2.262(3)); C(1)eAu(1)eCl(1),
178.4(2); C(1)eAu(1)eCl(2), 88.25(19); C(1)eAu(1)eCl(3), 91.29(19); Cl(2)eAu(1)eCl(3)
178.63(10).
c
d
e
Determined by 1H NMR.

S. Zhu et al. / Tetrahedron 68 (2012) 7949e7955
7951
complexes.7b,8 The bond distance of Au(1)eC(1) in 3a is 2.016 A,
3-Cl-Pyridine
Na2CO3, 80oC, 12 h
ꢀ
IMes-AuCl
+
IMes-AuCl3
IMes-AuCl + IMes-AuCl3
which is also in close agreement with those reported organo-
golde(III) complexes.7b,9 The C(1)eAu(1)eCl(1) and Cl(2)eAu(1)e
Cl(3) bonds are nearly linear, with angles between 178.4ꢀ and
178.6ꢀ.
2a
3a
3a
2a
100%
50%
50%
100%
0%
Scheme 5. The control reaction.
With the optimized reaction conditions on hand, we then ex-
plored its potential substrates scope. Four most common imida-
zolium salts 1aed were tested under the standard conditions. All of
them gave very satisfied results (Table 2). In order to obtain better
results, small modifications were made for each substrate. For ex-
ample, [NaAuCl4$2H2O] was better than [KAuCl4$2H2O] in the case
of imidazolium salt 1a (Table 1). For salts 1bed, [KAuCl4$2H2O] has
proven better than [NaAuCl4$2H2O]. Moreover, increasing the re-
action temperature from 80 to 110 ꢀC could enhance the yield
further (2d in Table 2). Under the reaction conditions list in Table 2,
no Au(III)eNHC complexes were detected or isolated for the sub-
strates 1b,c and 1d.
reduced product IMeseAuCl 2a (7%)(Table 3, entry 2). Shorter or
longer time resulted in more starting material 1a or more
IMeseAuCl 2a (entries 1, 3e4). When the reaction time prolongs to
48 h, all of the starting materials were converted into IMeseAuCl 2a
(entry 5). The time-dependent results undoubtedly supported the
hypothesis that Au(III)eNHCs were formed initially, which were
then reduced further to Au(I)eNHCs under the reaction conditions.
Table 3
Time-dependent of the formation of IMeseAuCl3 (3a)
Table 2
One step to synthesize Au(I)eNHC complexes from imidazolium salts and com-
mercially available Au(III) saltsa
MAuCl4*2H2O
N
N
N
N
R
R
R
R
K2CO3, 3-Cl-Pyridine
80 oC, 24 h
Cl
H
Au
Cl
(1a)
(2a)
IMes
IMes-AuCl
HCl
SIMes HCl (1b)
SIMes-AuCl (2b)
IPr
IPr-AuCl
HCl
(1c)
(1d)
(2c)
(2d)
SIPr HCl
SIPr-AuCl
IMeseAuCl (2a)
SIMeseAuCl (2b)
Yield¼93%b
Yield¼72%b
Yield¼91%c
Yield¼83%b
Yield¼89%c
Yield¼52%b
Yield¼60%c
Yield¼74%d
IPreAuCl (2c)
SIPreAuCl (2d)
a
Reactions were carried out using 1 (0.15 mmol), [MAuCl4$2H2O] (0.14 mmol),
and 0.5 ml 3-Clepyridine in a Schlenk tube, heating at 80 ꢀC under the atmosphere
of N2 for 24 h, the yield refers to isolated yield.
Entry
Reaction time (h)
Product distributiona
b
[NaAuCl4$2H2O] as the gold source, and Na2CO3 as the base.
[KAuCl4$2H2O] as the gold source, and Na2CO3 as the base.
The reaction was set at 110 ꢀC for 24 h using [KAuCl4$2H2O] as the gold source,
c
1a
2a
3a
d
1
2
3
4
5
1
3
12
24
48
23%
4%
d
d
77%
89%
73%
29%
d
and Na2CO3 as the base.
7%
27%
71%
100%
d
Based on the results listed in Table 1, the reaction could go
through the following pathway: Au(III)eNHC formed initially,
which was then reduced to Au(I)eNHC under the reaction condi-
tions (Scheme 4).
d
a
Product ratios were determined by 1H NMR. And the reaction was set under the
atmosphere of N2.
Interestingly, when Au(DMS)Cl (DMS¼dimethylsulfide) was
served as the metalation reagent to react with IMes$HCl, a linear
[Au-(IMes)2]þ cation gold complex (2a0) was obtained as the major
product (60%) instead (Scheme 6), the desired product IMeseAuCl
(2a) was formed only in 14% yield.
Imidazole Salt
Au(I)-NHC
Au(III)-NHC
MAuCl4*2H2O
Scheme 4. Possible pathway for the formation of Au(I)eNHC.
To verify the above hypothesis, a control reaction was designed
(Scheme 5). In the presence of 3-chloropyridine and Na2CO3, equal
molar mixture of IMeseAuCl (2a) and IMeseAuCl3 (3a) was stirred
for 12 h at room temperature and it was found that IMeseAuCl3
(3a) was converted completely into IMeseAuCl (2a). This obser-
vation indicated that gold(III)eNHC could be easily converted into
gold(I)eNHC under the reaction conditions.
Based on the above results, it comes to us to consider the pos-
sibility to selectively prepare gold(III)eNHC by varying the reaction
conditions. By careful controlling the reaction conditions,
IMeseAuCl3 (3a) could be formed in 89% selectivity, accompanying
with small amount of unreacted imidazoloium salt 1a (4%) and
2.2. Synthesis of NHCeCuCl
After developing the unique procedure for the synthesis of
Au(I)eNHC complex, we were then wondering if this method could
be extended to synthesize NHCeCuCl or NHCeAgCl.
Me2S-AuCl
+ (IMes)2Au+ Cl-
IMes-AuCl
IMes*HCl
K2CO3, 3-Cl-Pyridine
110oC, 16 h
1a
2a
2a'
14%
60%
Scheme 6. The reaction of IMes$HCl with Me2SeAuCl.

7952
S. Zhu et al. / Tetrahedron 68 (2012) 7949e7955
As summarized in Table 4, CuCl could furnish the desired
products NHCeCuCl in excellent yields, the yields are typically
Under the similar conditions, silver chloride was chosen as the
metal source, imidazolium salts 1aed were tested as the substrates
for the metallization. As summarized in Table 5, the corresponding
Table 4
Preparation of Cu(I)eNHC complexesa
Table 5
Preparation of (NHC)2AgþClꢁ complexesa
CuCl or CuCl2*2H2O
N
N
N
N
R
R
R
R
Base, 3-Cl-Pyridine
N
N
N
R
R
R
Cl
H
Cu
Cl
4a-4d
AgCl
Base, 3-Cl-Pyridine
N
N
Ag+
R
R
Cl-
R
1a-1d
Cl
H
N
Product
Copper source
Base
Temp (ꢀC)
Yieldb (%)
1a-1d
5a-5d
IMeseCuCl (4a)
SIMeseCuCl (4b)
IPreCuCl (4c)
CuCl
CuCl2$2H2O
CuCl
CuCl2$2H2O
CuCl
CuCl2$2H2O
CuCl
Na2CO3
Na2CO3
K2CO3
Na2CO3
K2CO3
K2CO3
K2CO3
K2CO3
80
80
90
83
70
62
99
89
91
70
Product
Base
Temp (ꢀC)
Yieldb (%)
110
110
110
110
110
110
(IMes)2AgþClꢁ (5a)
(SIMes)2AgþClꢁ (5b)
(IPr)2AgþClꢁ (5c)
Na2CO3
K2CO3
K2CO3
K2CO3
80
110
110
110
83
60
94
99
(SIPr)2AgþClꢁ (5d)
SIPreCuCl (4d)
a
The reaction was set under the atmosphere of N2 for 24 h.
Isolated yield.
CuCl2$2H2O
b
a
The reaction was set under the atmosphere of N2 for 24 h.
Isolated yield.
b
AgeNHC complexes could be formed efficiently (83e99%) under
the standard reaction conditions, except in the case of 5b (60%).
However, the X-ray diffraction analysis of 5a revealed that the
structures of 5 was different with the corresponding gold and
copper analogues 2 and 4, it consists of a linear [Ag-(IMes)2]þ cation
and a chloride anion, with one silver ligated to two NHC ligands
(Scheme 8). Selected bond distances and bond angles are given in
ranging from 70 to 99%. Furthermore, CuCl2$2H2O, with two crystal
water each molecule, could give the same NHCeCuCl as well. As
Au(III) being reduced into Au(I), the Cu(II) was also reduced into
Cu(I).10 Different with the gold analogous, no NHCeCuCl2 was
isolated or detected. In general, CuCl gave higher yields than
CuCl2$2H2O. The products from CuCl2$2H2O was also confirmed by
X-ray diffraction analysis (Scheme 7). Selected bond distances and
bond angles are given in the figure caption. The copper atom is
ꢀ
two-coordinated with Cu(1)eC(11) bond distance being 1.898 A.
C(11)eCu(1)eCl(1) is also in linear geometry with a bond angle
value of 180.0ꢀ.
ꢀ
ꢀ
Scheme 7. ORTEP diagram of IMeseCuCl 4a. Selected bond distances (A) and angles ( )
in 4a: Cu(1)eC(11), 1.898(6); Cu(1)eCl(1), 2.0914(17); C(1)eCu(1)eCl(1), 180.000(1).
Scheme 8. ORTEP diagram of (IMes)2AgþClꢁ 5a. Selected bond distances (A) and an-
ꢀ
gles (ꢀ) in 5a: Ag(1)eC(1), 2.087(4); C(1)eAg(1)eC (1_4), 178.2(3).
2.3. Synthesis of NHCeAgCl
After the success of applying this system to the synthesis of
CueNHC, this unique process was further investigated for the
AgeNHC complexes. It is well known that silver transmetalation is
a well-established method for the preparation of [(NHC)M] com-
plexes. Although AgeNHC complexes are typically synthesized
from the reaction of imidazolium salts and Ag2O,4a however, the
system usually required shielding from the light. Furthermore,
silver chloride, AgCl, was often a side-product when scavenging the
halide with silver salts in the reactions. Rare applications of AgCl
were found in organic synthesis because of its lower stability and
insolubility in most organic solvent. Therefore, it is challenging to
make use of the AgCl directly in organic chemistry. However, with
the success in preparing the corresponding NHC gold and copper
complex, we are highly curious about the possibility that if AgCl
could be directly used as the silver source to prepare AgeNHC.
the figure caption. Among which, Ag(1)eC(1) bond distance is
ꢀ
2.087 A. The C(1)eAg(1)eC(1_4) is also nearly linear, with angle of
178.2ꢀ. Two planes of the imidazolium ring are staggered in an
angle of 121.36ꢀ.
2.4. Transmetalation from (NHC)2AgDClL complexes
Silver transmetalation is a well-established method for the
preparation
of
[(NHC)M]
complexes.
Therefore,
with
Ag(NHC)2]þClꢁ 5 on hand, we further explored the carbene transfer
reactions. Using dichloromethane as the solvent, CuCl, DMSeAuCl,
and PdCl2 could be transmetallated with 5a smoothly. The corre-
sponding metal complexes 2a, 4a, and 6 could be formed in ex-
cellent yields (Scheme 9).

S. Zhu et al. / Tetrahedron 68 (2012) 7949e7955
7953
3
ꢀ
ꢀ
ꢀ
b¼15.0634(7), c¼16.4470(7) A,
a¼90,
b¼90,
g
¼90 , V¼3808.6(3) A ,
CuCl
IMes-CuCl
Z¼4, Dc¼1.312 g/cm3, T¼293(2) K. 3352 unique reflections
2a (83%)
N
N
N
[R(int)¼0.0960]. F(000)¼1568 Final R1 [with I>2 (I)]¼0.0461, wR2
s
Mes
Mes
Mes
Cl-
Mes
(all data)¼0.1262. The structures were solved by direct methods and
refined on F2 using full matrix least-squares methods using SHELXTL-
97. Anisotropic thermal parameters were refined for non-hydrogen
atoms within the main backbone of the molecules. Hydrogen atoms
were localized in their calculated positions and refined using a riding
model.
DMS-AuCl
PdCl2
Ag+
IMes-AuCl
4a (94%)
N
(IMes)2PdCl2
6 (96%)
5a
Scheme 9. Transmetalation from (IMes)2AgCl 5a.
4.3. Synthesis of imidazol(idin)ium salts
4.3.1. IMes$HCl (1a) and SIMes$HCl (1b).11
3. Conclusion
4.3.1.1. Preparation of N,N0-(ethane-1,2-diylidene)bis(2,4,6-
trimethylaniline). In
a flask, 2,4,6-trimethylaniline (6.075 g,
In summary, we described a direct and practical approach for
the synthesis of Au(I)eNHC complexes from imidazolium salts
and commercially available Au(III) salts. This process proceeded
without sacrificing carbene transfer agent (Ag2O) or using highly
sensitive free NHC. The control reaction proved that the Au(III)e
NHC complexes were formed initially, which were then reduced
to Au(I)eNHC under the reaction conditions. Furthermore, this
system could also be extended to copper and silver as well. In the
case of copper, both CuCl and CuCl2$2H2O furnished the same
products NHCeCuCl in good to excellent yields. And for the silver,
AgCl could be used as the metal source. Different with gold and
copper partners, AgCl gave a linear and ion complex [Ag-
(NHC)2]þClꢁ, with one silver ligated to two NHC ligands. The re-
actions described in this paper are not sensitive to the air,
moisture, and light. We believed that, owing to the above ad-
vantages, these metalation methods will be the choice for the
synthesis of gold, silver, and copper NHC complexes, especially for
the goldeNHC. Work to expand the scope and applying the
method to other transitional metals is currently underway in our
laboratory.
45 mmol) and 40% glyoxylaldehyde (3.250 g, 22.5 mmol) were
added to ethyl alcohol (100 ml), the mixture was stirred at room
temperature for 12 h and then yellow solid was separated out, the
insoluble material was filtered and the filtrate was washed with
cold ethyl alcohol, after dried in vacuum, gave 4.7 g yellow solid of
imine in 71% yield.
4.3.1.2. Preparation of IMes$HCl (1a). In a flask, the imine (3.0 g,
10 mmol) was dissolved in tetrahydrofuran (25 ml), followed by
dropwise addition of chloromethyl ethyl ether (1.04 g, 11 mmol),
the mixture was stirred under N2 at 40 ꢀC for 18 h, and then ethyl
ether (25 ml) was added to separate white solid, the solid was
filtered and washed with ethyl ether, the white solid was dried
under vacuum affording 2.2 g IMes$HCl in 65% yield. 1H NMR
(CDCl3, 400 MHz):
d 2.15 (s, 12H), 2.33 (s, 6H), 7.00 (s, 4H), 7.70 (s,
2H), 10.73 (s, 1H); 13C NMR (CDCl3, 100 MHz):
129.8, 130.7, 134.1, 139.2, 141.1.
d 17.6, 21.1, 124.8,
4.3.1.3. Preparation of SIMes$HCl (1b). In a flask, the imine (1.3 g,
4.45 mmol) was suspended in ethyl alcohol (65 ml), then cooled to
0 ꢀC and sodium borohydride (3.36 g, 89 mmol) was added slowly,
after stirring for 30 min, the mixture was heated to reflux until the
color of the solution turn into colorless. Cool to room temperature,
then aqueous saturated sodium chloride (12 ml) was added to stir
for 30 min, after that, 50 ml water and 40 ml chloroform was added,
then organic phase was separated, washed with small amount of
water, dried under anhydrous sodium sulfate, and then volatiles
were removed under vacuum, yielding light yellow oil. The oil was
purged to next step. In the same flask, ammonium chlorite
(261.9 mg, 4.9 mmol) and triethyl orthoformate (4 ml) was added,
the mixture was stirred under N2 at 110 ꢀC overnight, then ethyl
ether was added to separated yellow solid, filtered and then
recrystallized by dichloromethane and ethyl ether, affording 1.3 g
4. Experimental section
4.1. General considerations
All glassware was oven-dried prior to use. All reagents were
purchased from commercial sources and used without further
purification. Imidazolium salts were prepared according to litera-
ture. 1H and 13C spectra were recorded at 400 MHz and 100 MHz,
respectively, on a Bruker Spectrospin 400 MHz spectrometer. Pro-
ton and carbon chemical shifts were referenced to the residual
proton resonance in CDCl3 (d (ppm) 7.26 and 77.16, respectively).
yellow SIMes$HCl in 85% yield. 1H NMR (CDCl3, 400 MHz):
d 2.30 (s,
4.2. Crystal data of 2a, 3a, 4a, and 5a
6H), 2.40 (s, 12H), 4.61 (s, 4H), 6.97 (s, 4H), 7.27 (s, 2H), 9.16 (s, 1H);
13C NMR (CDCl3, 100 MHz):
140.3, 160.1.
d 17.9, 21.0, 51.9, 129.9, 130.3, 135.0,
Crystal data of 2a: CCDC No. 824114, C21H24AuClN2, Mr¼536.84,
ꢀ
colorless block, a¼14.766(3), b¼29.218(6), c¼9.7341(19) A,
a¼90,
3
3
ꢀ
ꢀ
b
¼90,
2373 unique reflections [R(int)¼0.0575]. F(000)¼2080 Final R1 [with
I>2
(I)]¼0.0342, wR2 (all data)¼0.0874. Crystal data of 3a: CCDC No.
g
¼90 , V¼4199.6(14) A , Z¼8, Dc¼1.698 g/cm , T¼293(2) K.
4.3.2. IPr$HCl (1c) and SIPr$HCl (1d).11
s
4.3.2.1. The preparation of IPr$HCl and SIPr$HCl followed the
procedures for the synthesis of IMes$HCl and SIMes$HCl. IPr$HCl (1c,
824115, C21H24AuCl3N2, Mr¼607.74, colorless block, a¼10.596(2),
ꢀ
b¼13.932(3), c¼15.456(3) A,
a
¼90,
b¼90,
g
¼90ꢀ, V¼2281.6(8), Z¼4,
61%): 1H NMR (CDCl3, 400 MHz):
d
1.22 (d, J¼6.4 Hz, 12H), 1.27 (d,
Dc¼1.769 g/cm3, T¼293(2) K. 5214 unique reflections [R(int)¼0.0951].
J¼6.4 Hz, 12H), 2.39e2.46 (m, 4H), 7.34 (d, J¼8 Hz, 4H), 7.56 (t,
F(000)¼1176, Final R1 [with I>2 (I)]¼0.0448, wR2 (all data)¼0.1127.
s
J¼8 Hz, 2H), 8.12 (s, 2H), 10.03 (s, 1H). 13C NMR (CDCl3, 100 MHz):
Crystal data of 4a: C21H24ClCuN2, Mr¼403.41, colorless block,
d
23.7, 24.7, 29.1, 124.7, 126.8, 129.9, 132.1, 138.5, 145.0.
ꢀ
a¼14.743(3), b¼29.395(6), c¼9.5209(19) A,
a¼90,
b
¼90,
g
¼90ꢀ,
SIPr$HCl (1d, 69%): 1H NMR (CDCl3, 400 MHz):
d 1.26 (d,
3
ꢀ
3
V¼4126.0(14) A , Z¼8, Dc¼1.299 g/cm , T¼293(2) K. 2366 unique
reflections [R(int)¼0.0354]. F(000)¼1680 Final R1 [with
J¼6.8 Hz, 12H), 1.41 (d, J¼6.8 Hz, 12H), 3.00e3.07 (m, 4H), 4.86 (s,
4H), 7.29 (d, J¼8 Hz, 4H), 7.48 (t, J¼8 Hz, 2H), 8.30 (s, 1H). 13C NMR
I>2
s
(I)]¼0.0436, wR2 (all data)¼0.1274. Crystal data of 5a:
(CDCl3, 100 MHz):
158.7.
d 23.7, 25.4, 29.2, 55.2, 124.9, 129.3, 131.5, 146.0,
C42H48AgClN4,
Mr¼752.16,
colorless
block,
a¼15.3730(6),

7954
S. Zhu et al. / Tetrahedron 68 (2012) 7949e7955
4.4. Synthesis of [(NHC)AuCl] (2aed)4c
for 3 h, the Schlenk tube was allowed to cool to room temperature,
1.0 ml dichloromethane was added to dissolve the product, then
the mixture passed through a short pad of silica gel eluting with
dichloromethane until the product was all fully obtained.
Dichloromethane was removed by rotary evaporator; the residual
solution was added to n-pentane to precipitate solid powders, after
filtered and dried, affording the mixture of [(IMes)AuCl] (2a) in 7%
yield and [(IMes)AuCl3] (3a) in 89% yield (determined by NMR
yield). Pure [(IMes)AuCl3] (3a) could be obtained by simply diffu-
sion of pentane into the DCM solution of 3a. 1H NMR (CDCl3,
Based on the conditions listed in Table 4, equal molar of po-
tassium chloroaurate dihydrate (58 mg, 0.14 mmol) or sodium
chloroaurate dehydrate (56 mg, 0.14 mmol), the imidazol(idin)
ium chloride (0.15 mmol) and potassium carbonate (94 mg,
0.68 mmol) or sodium carbonate (72 mg, 0.68 mmol) were in-
troduced in a Schlenk tube equipped with a magnetic stirring bar.
The Schlenk tube was then added 3-chloropyridine (0.5 ml), the
reaction mixture was stirred at 80 ꢀCor 110 ꢀC for the time in-
dicated in Table 4. The Schlenk tube was allowed to cool to room
temperature, 1.0 ml dichloromethane was added to dissolve the
product, then the mixture passed through a short pad of silica gel,
eluting with dichloromethane until all the product coming comes
out. Dichloromethane was then removed by rotary evaporator;
the residual solution was added to a stirring n-pentane to pre-
cipitate solid powders, filtered to afford the corresponding [(NHC)
AuCl].
400 MHz):
d C
2.25 (s, 12H), 2.36 (s, 6H), 7.03 (s, 4H), 7.30 (s, 2H). 13
NMR (CDCl3, 100 MHz):
144.8.
d 18.6, 21.2, 125.6, 130.0, 132.4, 135.3, 141.1,
4.7. Synthesis of [(NHC)CuCl] (4aed)13
Cuprous chloride (14.0 mg, 0.14 mmol) or cupric dichloride
dihydrate (24.0 mg, 0.14 mmol) and, the imidazol(idin)ium chloride
(0.15 mmol) and potassium carbonate (93.8 mg, 0.68 mmol) or
sodium carbonate (for [(SIMes)CuCl], 72.1 mg, 0.68 mmol) were
introduced in a Schlenk tube equipped with a magnetic stirring bar.
To the Schlenk tube was then added 3-chloropyridine (0.5 ml), the
reaction mixture was stirred at 110 ꢀC for 16 h. Cool to room tem-
perature, 1.0 ml dichloromethane was added to dissolve the prod-
uct, then the mixture passed through a short pad of silica gel
covered with a pad of Celite eluting with dichloromethane until the
product was completely washed out. Dichloromethane was then
removed by rotary evaporator; the residual solution was added to
n-pentane to precipitate solid powders, filtered and dried, affording
the corresponding [(NHC)CuCl].
4.4.1. [(IMes)AuCl] (2a) 1H NMR (CDCl3, 400 MHz):
2.35 (s, 6H), 6.99 (s, 4H), 7.10 (s, 2H); 13C NMR (CDCl3, 100 MHz):
17.8, 21.1, 122.2, 129.5, 134.6, 134.7, 139.8, 173.4.
d 2.10 (s, 12H),
d
4.4.2. [(SIMes)AuCl] (2b) 1H NMR (CDCl3, 400 MHz):
2.32 (s, 12H), 3.99 (s, 4H), 6.94 (s, 4H); 13C NMR (CDCl3, 100 MHz):
18.0, 21.1, 50.7, 129.8, 134.6, 135.5, 139.0, 195.1.
d 2.30 (s, 6H),
d
4.4.3. [(IPr)AuCl] (2c) 1H NMR (CDCl3, 400 MHz):
d 1.21 (d,
J¼6.8 Hz, 12H), 1.34 (d, J¼6.8 Hz, 12H), 2.50e2.61 (m, 4H), 7.17 (s,
2H), 7.28 (d, J¼8H, 4 Hz), 7.50 (t, J¼8 Hz, 2H); 13C NMR (CDCl3,
100 MHz):
d 24.0, 24.5, 28.8, 123.1, 124.3, 130.7, 134.0, 145.6, 175.3.
4.7.1. [(IMes)CuCl] (4a) 1H NMR (CDCl3, 400 MHz):
2.35 (s, 6H), 7.00 (s, 4H), 7.05 (s, 2H); 13C NMR (CDCl3, 100 MHz):
17.8, 21.1, 122.3, 129.5, 134.6, 135.1, 139.5.
d 2.10 (s, 12H),
4.4.4. [(SIPr)AuCl] (2d) 1H NMR (CDCl3, 400 MHz):
d 1.33 (d,
J¼6.8 Hz, 12H), 1.41 (d, J¼6.8 Hz, 12H), 3.00e3.10 (m, 4H), 4.04 (s,
d
4H), 7.22 (d, J¼7.6 Hz, 4H), 7.41 (t, J¼7.6 Hz, 2H); 13C NMR (CDCl3,
100 MHz):
d 24.1, 25.1, 29.0, 53.5, 124.6, 130.0, 134.0, 146.5, 196.0.
4.7.2. [(SIMes)CuCl] (4b) 1H NMR (CDCl3, 400 MHz):
2.32 (s, 12H), 3.95 (s, 4H), 6.95 (s, 4H); 13C NMR (CDCl3, 100 MHz):
18.0, 21.0, 51.0, 129.8, 134.9, 135.4, 138.7.
d 2.30 (s, 6H),
4.5. Synthesis of [(IMes)2AuDClL] (2a0)
d
(Me2S)AuCl (84 mg, 0.28 mmol) and the IMes$HCl (200 mg,
0.60 mmol) and potassium carbonate (187.7 mg, 1.36 mmol) were
introduced in a Schlenk tube equipped with a magnetic stirring bar.
To the Schlenk tube was then added 3-chloropyridine (1.0 ml), the
reaction mixture was stirred at 110 ꢀC for 16 h, then cool to room
temperature, 1.0 ml dichloromethane was added to dissolve the
product, then the mixture passed through a short pad of silica gel
covered with a pad of Celite eluting with dichloromethane until the
product was all obtained. Dichloromethane was removed by rotary
evaporator; the residual solution was added to n-pentane to pre-
cipitate solid powders, filtered and dried, affording the mixture of
[(IMes)AuCl] (2a) and [(IMes)2Au]þClꢁ (2a0), the two products were
then isolated through column chromatography on silica gel to give
21.0 mg [(IMes)AuCl] (2a) in 14% yield and 141 mg [(IMes)2Au]þClꢁ
(2a0) in 60% yield.
4.7.3. [(IPr)CuCl] (4c) 1H NMR (CDCl3, 400 MHz):
d 1.23 (d,
J¼6.8 Hz, 12H), 1.30 (d, J¼6.8 Hz, 12H), 2.51e2.62 (m, 4H), 7.13 (s,
2H), 7.29 (d, J¼8 Hz, 4H), 7.49 (t, J¼8 Hz, 2H); 13C NMR (CDCl3,
100 MHz):
d 23.9, 24.8, 28.7, 123.2, 124.2, 130.6, 134.4, 145.6, 180.5.
4.7.4. [(SIPr)CuCl] (4d) 1H NMR (CDCl3, 400 MHz):
d 1.34 (d,
J¼7.2 Hz, 12H), 1.36 (d, J¼7.2 Hz, 12H), 3.01e3.11 (m, 4H), 4.01 (s,
4H), 7.23 (d, J¼5.2 Hz, 4H), 7.39 (t, J¼8.0 Hz, 2H); 13C NMR (CDCl3,
100 MHz):
d 23.9, 25.5, 28.9, 53.7, 124.5, 130.0, 134.4, 146.6, 202.9.
4.8. Synthesis of [(NHC)2Ag]DClL (5aed)14
Silver chloride (20.0 mg, 0.14 mmol), the imidazol(idin)ium
chloride (0.15 mmol) and potassium carbonate (93.8 mg,
0.68 mmol) or sodium carbonate (for [(SIMes)CuCl], 72.1 mg,
0.68 mmol) were introduced in a Schlenk tube equipped with
a magnetic stirring bar. To the Schlenk tube was then added 3-
chloropyridine (0.5 ml), the reaction mixture was stirred at
110 ꢀC for 16 h. Cool to room temperature, 1.0 ml dichloro-
methane was added to dissolve the product, then the mixture
passed through a short pad of silica gel covered with a pad of
Celite eluting with dichloromethane until the product was
completely washed out. Dichloromethane was removed by rotary
evaporator; the residual solution was added to n-pentane to
precipitate solid powders, filtered and dried, affording the cor-
responding [(IMes)2Ag]﹢Clꢁ.
4.5.1. [(IMes)2Au]þClꢁ (2a0) 1H NMR (CDCl3, 400 MHz):
12H), 2.44 (s, 6H), 6.89 (s, 4H), 7.17 (s, 2H); 13C NMR (CDCl3,
100 MHz): 17.1, 21.3, 123.1, 129.1, 134.1, 134.5, 139.4, 185.1.
d 1.70 (s,
d
4.6. Synthesis of [(IMes)AuCl3] (3a)12
Sodium chloroaurate dihydrate (56 mg, 0.14 mmol) and,
IMes$HCl (50 mg, 0.15 mmol) and potassium carbonate (94 mg,
0.68 mmol) were introduced into a Schlenk tube equipped with
a magnetic stirring bar. To the Schlenk tube was then added
3-chloropyridine (0.5 ml), the reaction mixture was stirred at 80 ꢀC

S. Zhu et al. / Tetrahedron 68 (2012) 7949e7955
7955
4.8.1. [(IMes)2Ag]﹢Clꢁ (5a) 1H NMR (CDCl3, 400 MHz):
(s, 12H), 2.35 (s, 6H), 7.00 (s, 4H), 7.14 (s, 2H); 13C NMR (CDCl3,
100 MHz): 17.8, 21.2, 122.8, 122.8, 129.7, 134.7, 135.3, 139.9.
d
2.07
Supplementary data
d
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tet.2012.07.009.
4.8.2. [(SIMes)2Ag]﹢Clꢁ (5b) 1H NMR (CDCl3, 400 MHz):
(s, 6H), 2.30 (s, 12H), 4.00 (s, 4H), 6.95 (s, 4H); 13C NMR (CDCl3,
100 MHz): 18.0, 21.0, 51.1, 51.2, 129.9, 135.1, 135.5, 138.9.
d 2.29
References and notes
d
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4.8.3. [(IPr)2Ag]﹢Clꢁ (5c) 1H NMR (CDCl3, 400 MHz):
d 1.22
(d, J¼7.2 Hz, 12H), 1.28 (d, J¼6.8 Hz, 12H), 2.49e2.59 (m, 4H), 7.21
(d, J¼1.6 Hz, 2H), 7.30 (d, J¼8 Hz, 4H), 7.50 (t, J¼8 Hz, 2H); 13C NMR
(CDCl3, 100 MHz):
145.6.
d 24.0, 24.7, 28.7, 123.6, 123.7, 124.4, 130.8, 134.5,
4.8.4. [(SIPr)2Ag]﹢Clꢁ (5d) 1H NMR (CDCl3, 400 MHz):
d 1.33
(d, J¼1.2 Hz, 12H), 1.35 (d, J¼1.2 Hz, 12H), 3.00e3.10 (m, 4H), 4.07
(s, 4H), 7.24 (d, J¼8.0 Hz, 4H), 7.41 (t, J¼8.0 Hz, 2H); 13C NMR (CDCl3,
100 MHz):
d 24.0, 25.4, 28.9, 53.9, 53.9, 124.7, 130.1, 134.5, 146.6.
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4.9. Transmetalation from (NHC)2AgDClL complexes
To a Schlenk tube with a stirring bar, adding [(IMes)2Ag]﹢Clꢁ
(0.021 mmol, 15.7 mg) and metal chloride (CuCl, DMSeAuCl or
PdCl2) (0.046 mmol), the Schlenk tube system was then added
1.0 ml dichloromethane, the reaction was then stirred at room
temperature overnight. After that, the reaction mixture was fil-
tered, and the volatile material was removed by rotary evaporator
to obtain products of 2a, 4a, and 6.
Met. Chem. 2002, 27, 177.
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4.9.1. [(IMes)2PdCl2] (6).15 1H NMR (CDCl3, 400 MHz):
12H), 2.47 (s, 6H), 6.77 (s, 2H), 6.93 (s, 4H); 13C NMR (CDCl3,
100 MHz): 18.9, 21.3, 122.5, 122.8, 135.8, 136.2, 137.5, 170.9.
d
1.95 (s,
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We thank the National Natural Science Foundation of China (Nos.
20902028 and 21172077), the Program for New Century Excellent
Talents in University (NCET-10-0403), Guangdong Natural Science
Foundation (Nos. 9451064101002851 and 10351064101000000),
and The National Basic Research Program of China (973) (No.
2011CB808600) for financial support. This work was also supported
by ‘the Fundamental Research Funds for the Central Universities,
SCUT (No. 2012ZZ0038)’ and ‘the opening Foundation of Zhejiang
Provincial Top Key Discipline’. And the authors appreciate Prof. Tong
Chun Kuang (Analytical and testing center of SCUT) for the collec-
tion of crystal data.
14. Sentman, A. C.; Csihony, S.; Waymouth, R. M.; Hedrick, J. L. J. Org. Chem. 2005,
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