8
16
S. Zhu et al. / Tetrahedron Letters 53 (2012) 815–818
other combinations of aurate salts and bases were tested as well
(entries 8–10). Interestingly, both KAuCl O/Na CO and
Á2H
NaAuCl O/K CO gave the mixtures of 2a and 3a (entries 8
Á2H
and 9). It is worthy to mention that IMes-AuCl (3a) can be formed
in 60% yield when the former combination was applied (entries 8).
The results here provided an alternative and straightforward way
to prepare the Au(III)-NHC complexes, which were often indirectly
1
step
??
N
N
N
N
+
MAuCl *2H O
4
2
2
3
R
R
R
R
4
2
?
4
2
2
3
Au
Cl
H Cl
3
2
1
Scheme 2. The retrosynthetic analysis of Au(I)-NHC.
obtained by oxidation of the corresponding Au(I)-NHC with X
2
or
could furnish the
IMes-AuCl (2a) selectively as well, albeit in relatively lower yield
65%) (entry 10). Different bases were also investigated. Cs CO
gave only moderate yield of product 2a (entry 11). K PO produced
a in 72% yield, however, accompanied with 3a in 12% yield (entry
2). NaOH was inefficient for this reaction at all (entry 13). In addi-
7
PhICl
2
4 2 2 3
. The combination of KAuCl Á2H O/K CO
and MAuCl
with NaAuCl
ferent solvents were screened for the reactions. When toluene was
used as the solvent, a mixture of IMes-AuCl (2a) and IMes-AuCl
3a) were obtained in 53% total yield, with 3a (40%) being domi-
4
Á2H
2
O (M = K, Na) as the metal source (Table 1). Started
(
2
3
4
Á2H
2
O as the gold source and Na CO as the base, dif-
2
3
3
4
2
1
3
(
tion, the reactions can not happen without addition of the base
nant (entry 1, Table 1). These results indicated both Au(III)-NHC
and Au(I)-NHC could be formed directly from the imidazolium
salts and chloroaurate salts, which encouraged us to examine the
reaction conditions carefully to enhance the selectivity of
IMes-AuCl (2a). When DCE was used instead, the yield of IMes-
AuCl was increased to 36%, however, the reaction conversion
52%) is still low (entry 2). Trace Au(I)-NHC 2a was observed when
the reaction was performed in THF (entry 3). When pyridine was
used as the solvent, the ratio of IMes-AuCl (2a) with IMes-AuCl
3a) equals to 1:1, but the overall yield is excellent (up to 88%) (en-
(
entry 14).
3
The structures of IMes-AuCl (2a) and IMes-AuCl (3a) were ver-
ified with the single-crystal X-ray diffraction analysis (Scheme 3),
4
c
which were consistent with the reported data.
With the optimized reaction conditions (Table 1, entry 5) on
hand, we then explored its potential substrates scope. Four most
commonly used imidazolium salts 1a–1d were tested under the
standard conditions. All of them could give satisfied results (Table
(
3
2
). In order to obtain better results, small modifications were made
for each substrate. For example, NaAuCl O was better than
Á2H
KAuCl O only in the case of imidazolium salt 1a (Table 1). For
Á2H
salts 1b–1d, KAuCl O has been proven better than NaAuCl
Á2H
O. Moreover, increasing the reaction temperature from 80 °C
(
try 4). It is surprising that when 3-Cl-pyridine was used as the sol-
vent instead, IMes-AuCl (2a) could be formed solely in 93% isolated
4
2
4
2
4
Á
yield. No IMes-AuCl
mentioning that the reaction was not sensitive to moisture at all,
because two crystal water existed in each MAuCl O molecule.
Á2H
Even the mixed co-solvent of 3-Cl-pyridine and H O (volume
ratio = 5:1) could furnish the product 2a as well, the yield is up
to 81% (entry 6). When net H O served as the solvent, however,
3
(3a) was detected (entry 5). It is worthy
4
2
2H
2
to 110 °C could enhance the yield further (2d in Table 2). Under
the reaction conditions list in Table 2, no Au(III)-NHC complexes
were detected or isolated for the substrates 1b,1c and 1d.
Base on the results observed in Table 1, we proposed that the
reaction could go through the following pathway: initially, the imi-
dazolium reacted with aurate salt giving gold(III)-NHC, which was
then reduced to the desired gold(I)-NHC under the reaction condi-
tions (Scheme 4).
4
2
2
2
the yield of 2a decreased to 14% (entry 7), presumably because
of the low solubility of the imidazolium salts in water. Therefore,
-Cl-pyridine has proved to be the solvent of choice for this
reaction. Besides the combination of NaAuCl
3
4
Á2H
2 2 3
O and Na CO ,
Table 1
One step to synthesize Au(I)-NHC complexes from imidazolium salts and commer-
cially available Au(III) saltsa
MAuCl4*2H2O
N
N
N
N
N
N
Mes
Mes
Mes
Mes + Mes
Mes
Base
H Cl
Au
Cl Au Cl
Cl
Cl
1a
2a
3a
Entry
Gold source
Base
Sol.
Yieldb (%)
2
a 3a
1
2
3
4
5
NaAuCl
NaAuCl
NaAuCl
NaAuCl
4
4
4
4
Á2H
Á2H
Á2H
Á2H
2
2
2
2
O
Na
Na
Na
Na
2
2
2
2
CO
CO
CO
CO
3
3
3
3
Toluene
DCE
THF
Pyridine
3-Cl-pyridine
3-Cl-pyridine
13
36
Trace
44
93
40
16
—
44
—
—
—
60
24
—
O
O
O
NaAuCl
4
Á2H
2
O
O
O
Na
2
CO
3
c
6
NaAuCl
NaAuCl
4
Á2H
Á2H
2
Na
Na
Na
2
CO
2
CO
2
CO
3
3
3
81
14
d
7
8
9
4
2
H
2
O
KAuCl
NaAuCl
KAuCl
4
Á2H
2
O
3-Cl-pyridine
3-Cl-pyridine
3-Cl-pyridine
3-Cl-pyridine
3-Cl-pyridine
3-Cl-pyridine
3-Cl-pyridine
28
59
65
50
72
—
4
Á2H
2
O
K
K
2
CO
CO
3
10
11
12
13
14
4
Á2H
2
O
2
3
NaAuCl
NaAuCl
NaAuCl
NaAuCl
4
4
4
4
Á2H
Á2H
Á2H
Á2H
2
O
2
O
2
O
2
O
Cs
2
CO
PO
3
—
12
—
K
3
4
NaOH
—
—
—
3
Scheme 3. ORTEP diagram (a) IMes-AuCl 2a and (b) IMes-AuCl 3a. Selected bond
a
Reactions were set in schlenk tube with 1a (0.15 mmol), MAuCl
4
Á2H
2
O
distances (Å) and angles (deg) in 2a: Au(1)-C(1), 1.916(19); Au(1)-Cl(1), 2.285(5);
C(1)-Au(1)-Cl(1), 180.000(2). Selected bond distances (Å) and angles (deg) in 3a:
Au(1)-C(1), 2.016(7); Au(1)-Cl(1), 2.299(2); Au(1)-Cl(2), 2.281(3); Au(1)-Cl(3),
2.262(3)); C(1)-Au(1)-Cl(1), 178.4(2); C(1)-Au(1)-Cl(2), 88.25(19); C(1)-Au(1)-
Cl(3), 91.29(19); Cl(2)-Au(1)-Cl(3), 178.63(10).
(
0.14 mmol), and 0.5 ml 3-Cl-pyridine at 80 °C for 24 h.
b
Isolated yield, the ratio of 2a to 3a was determined by 1H NMR.
c
3
2
-Cl-Pyridine/H O (5:1) as the solvent.
d
1
Determined by H NMR.