Room-Temperature Hydration of Alkynes Catalyzed by Different Carbene Gold Complexes and their Precursors

Article abstract of DOI:10.1002/cctc.201501065

The room-temperature hydration of alkynes catalyzed by NHC-gold(I) (NHC=N-heterocyclic carbene), NAC-gold(I) (NAC=nitrogen acyclic carbene), and isocyanide gold(I) complexes was investigated carefully in the presence of different weakly coordinating anion

Full text of DOI:10.1002/cctc.201501065

DOI: 10.1002/cctc.201501065
Full Papers
Room-Temperature Hydration of Alkynes Catalyzed by
Different Carbene Gold Complexes and their Precursors
Yun Xu,[a, b] Xingbang Hu,*[a] Shufeng Zhang,[a] Xiuxing Xi,[a] and Youting Wu*[a]
The room-temperature hydration of alkynes catalyzed by NHC-
gold(I) (NHC=N-heterocyclic carbene), NAC-gold(I) (NAC=ni-
trogen acyclic carbene), and isocyanide gold(I) complexes was
investigated carefully in the presence of different weakly coor-
dinating anions. NHC(IPr)-AuCl/KB(C6F5)4 (NHC(IPr)=1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene) was found to be the
most active catalyst at room temperature, and the room-tem-
perature hydration of different alkynes could be completed in
7 h using only 0.5 mol% NHC(IPr)-AuCl/KB(C6F5)4. It was further
demonstrated that the catalyst system could be simply reused
at least six times without a noticeable loss of catalytic activity.
Introduction
In recent years, gold catalysts have been paid more and more
research attention because of their unique catalytic properties
in many useful organic reactions.[1] Among various synthetic re-
actions, active gold species have a special affinity for alkyne
hydration, which is an effective way to prepare ketones with
100% atom economy.[2–7] The hydration of alkynes has also
been used as a benchmark to investigate the catalytic ability
of gold catalysts.[4e] Although many effective catalysts have
been reported for the hydration of alkynes, the implementa-
tion of this reaction at room temperature is still a challenge.[2]
Researchers in this specialized field have a persistent interest
in the development of alternative catalysts for the hydration of
alkynes. Among the catalysts reported, gold-based com-
pounds,[3] especially compounds of gold and N-heterocyclic
carbenes (NHC-AuX, X=Cl, SbF6, BF4, OTf, or Br3), have taken
a prominent place.[2f,n,4–6]
the hydration reaction could be performed at room tempera-
ture. However, it requires 72 h to achieve 35% yield with
1 mol% catalyst.[4e]
Recently, we reported that the hydration of alkynes cata-
lyzed by isocyanide gold compounds (NCR-Au) could be per-
formed quite well at room temperature.[7] Usually, NCR-Au is
used as the starting material for the synthesis of NHC-Au.[3c,8]
NCR-Au was proven to be an effective catalyst for the hydra-
tion of alkynes at room temperature,[7] whereas NHC-Au itself
served as an effective catalyst for this reaction only at high
temperature.[2f,3e,4–6] Does this mean that the precursor NCR-Au
is more active than NHC-Au for the hydration reaction? How
can we further improve the catalytic ability of NHC-Au? Herein,
we will aim to answer these two questions and try to obtain
a more effective NHC-Au-based catalyst system for the room-
temperature hydration of alkynes.
Generally, these NHC-AuX catalysts are quite effective be-
cause of the special electron-donating and -withdrawing ability
of the NHC.[1b,5] Although NHC-AuX catalysts modified by dif-
ferent functional groups on the nitrogen heterocycle[2f,4d] and
coordinated with different anions (NHC-AuX, X=Cl,[2f,3e,4b,d]
SbF6,[4f,g,5,6] BF4,[2n] OTf,[4c] or Br3)[4a] have been used for the hy-
dration of alkynes, most of these reactions work at high tem-
peratures (e.g., 1208C for NHC-AuSbF6,[6a] 110 8C for NHC-
AuCl,[2f] 708C for NHC-AuOTf[4c] and MeOH/H2O, under reflux
for NHC-AuBr3[4a]). If NHC-Au-phenolate was used as a catalyst,
Results and Discussion
Six different catalysts were examined in this study: two NHC-
gold(I) complexes [NHC(IPr)-AuCl (NHC(IPr)=1,3-bis(2,6-diiso-
propylphenyl)imidazol-2-ylidene)
and
NHC(IMes)-AuCl
(NHC(IMes)=1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene)],
two nitrogen acyclic carbene gold(I) complexes [NAC(Cy)-AuCl
(NAC(Cy)=(cyclohexylamino)(diethylamino)methylidene) and
NAC(Ph)-AuCl
(NAC(Ph)=(diethylamino)(2,6-dimethylphenyl-
amino)methylidene)], and two simple isocyanide gold(I) com-
plexes [NC(Cy)-AuCl (NC(Cy)=cyclohexylisocyanide) and
[a] Dr. Y. Xu, Assoc. Prof. X. Hu, S. Zhang, X. Xi, Prof. Y. Wu
School of Chemistry and Chemical Engineering
Nanjing University
NC(Ph)-AuCl
(NC(Ph)=2,6-dimethylphenylisocyano)]
(Scheme 1). These catalysts were synthesized according to
known procedures.[3c,6b,7,8e,9] The hydration of phenylacetylene
was selected as the probe reaction.
Nanjing 211198 (P.R. China)
E-mail: huxb@nju.edu.cn
ytwu@nju.edu.cn
Usually, gold chloride complexes of NHC and NAC (NHC-
AuCl and NAC-AuCl) are used as precursors to synthesize NHC-
AuX and NAC-AuX (X=SbF6, BF4, OTf, PF6).[2f,3e,4b–g,5,6] It has
also been reported that NHC-AuCl itself can serve as a catalyst
for the hydration of phenylacetylene at high temperature.[2f]
[b] Dr. Y. Xu
China Pharmaceutical University
Nanjing 211198 (P.R. China)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201501065.
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equilibrium concentration of the gold cation in solution should
be an effective way to influence the catalytic activity of these
catalysts. For example, the addition of NaCl can inhibit the hy-
dration of phenylacetylene catalyzed by NHC-AuCl because the
increased ClÀ concentration makes the equilibrium shift from
NHC-Au+ towards NHC-AuCl.[2f] The use of weakly coordinating
anions is an effective method to release gold cations.[4e,7] Al-
À
though many weakly coordinating anions (such as SbF6À, BF4
,
PF6À, and OTfÀ) have been used in the hydration reaction cata-
lyzed by NHC-AuCl, the reaction catalyzed by NHC-AuX (X=
SbF6, BF4, PF6, and OTf) still requires
a high temper-
ature.[2b,n,4c,f,g,6a] Herein, we have also tested the hydration of
phenylacetylene using NHC-AuBF4 and NHC-AuSbF6 as cata-
lysts at room temperature. To avoid the influence of the Ag
ion, which is thought to be a catalyst for the hydration of phe-
nylacetylene,[11] KBF4 and KSbF6 were used instead of AgBF4
and AgSbF6. NHC(IPr)-AuSbF6 is known as a very active catalyst
for the alkyne hydration reaction at 1208C as only 50 ppm cat-
alyst was required for the hydration of phenylacetylene.[6a]
However, if the temperature was lowered to 258C, the reaction
catalyzed by NHC(IPr)-AuSbF6 proceeds quite slowly (conver-
sion=28% in 5 h using 5 mol% catalyst) and other catalytic
compounds have nearly no function in the reaction (Figure 1b
Scheme 1. Catalysts used for the hydration of alkynes.
However, if we lowered the reaction temperature to 258C,
both the reactions catalyzed by NHC-AuCl and NAC-AuCl (Fig-
ure 1a) proceed very poorly. Although 99% yield can be ob-
tained in 24 h in the room-temperature hydration of phenyl-
acetylene catalyzed by NC(Cy)-AuCl in the presence of
KB(C6F5)4,[7] this reaction almost does not work if only NC(Cy)-
AuCl or NC(Ph)-AuCl was used as the catalyst (Figure 1a).
The gold cation is thought to be the active center in the hy-
dration of phenylacetylene.[2f,4e,7,10] Hence, the control of the
À
À
and c). This suggests that BF4 and SbF6 are not good addi-
tives to release gold cations at room temperature.
À
Tetrakis(pentafluorophenyl)borate [B(C6F5)4 ], known as
a noncoordinating anion, has been used as an excellent chlo-
À
ride scavenger.[12,13] Recently, B(C6F5)4 has been used success-
fully as a counterion in the gold-catalyzed room-temperature
Figure 1. Hydration of phenylacetylene using 5 mol% catalyst in the presence of a) no additive, b) KBF4, c) KSbF6, and d) KB(C6F5)4 at 258C.
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hydrohydrazinations of alkynes.[14] A high concentration of free
gold cations should be generated upon the addition of
B(C6F5)4À. The chloride abstraction can be monitored by NMR
spectroscopy. A 1:1 mixture of NHC(IPr)-AuCl and KB(C6F5)4 was
stirred in THF for 1 h. After the removal of KCl and THF,
be obtained in 7 h for all the substrates investigated here. If
the catalyst loading was further reduced to 0.05 mol%, al-
though the reaction also works at room temperature, only
23.0% acetophenone was obtained in 24 h.
Previously, we reported a rare example of the room-temper-
ature hydration of alkynes catalyzed by NC(Cy)-AuCl.[7] Herein,
it is found that for the hydration of various alkynes, the reac-
tions catalyzed by 0.5 mol% NHC(IPr)-AuCl/KB(C6F5)4 are always
faster than those catalyzed by 0.5 mol% NC(Cy)-AuCl/KB(C6F5)4
(Figure 2).
a
13C NMR spectrum shows that the d value of the carbene
carbon atom shifts from 175.51 to 173.68 ppm (the NMR spec-
tra of NHC(IPr)-AuCl and NHC(IPr)-Au[B(C6F5)4] are presented in
the Supporting Information). As expected, the hydration reac-
tion catalyzed by NHC(IPr)-AuCl in the presence of KB(C6F5)4
works perfectly at room temperature (conversion=99% in 2 h;
Figure 1d). This result is far better than our previous finding
that 99% conversion could be obtained in 24 h for the same
reaction catalyzed by NC(Cy)-AuCl and KB(C6F5)4.[7] Notably,
there are almost no examples of alkyne hydration catalyzed by
NHC(IPr)-AuCl under such mild conditions.[2f,4–6,15] Previous re-
search has shown that NHC(IMes)-AuCl is not active for the hy-
dration reaction.[6a] Herein, it is further demonstrated that even
if KB(C6F5)4 is used, NHC(IMes)-AuCl is still a very poor catalyst
for this reaction. However, in the presence of KB(C6F5)4, the
NAC-AuCl catalytic system becomes more effective than that
shown previously, and the reaction time was shortened to 7 h
rather than 5 days.[3c] Notably, NC(Cy)-AuCl showed a similar
catalytic ability to NAC-AuCl in the presence of KB(C6F5)4 al-
though NC(Cy)-AuCl was the precursor for the synthesis of
NAC-AuCl.[3c,8]
Usually, the hydration of internal alkynes is more difficult be-
cause of the low reactivity of these compounds.[15] Herein, we
also tested the catalytic ability of NHC(IPr)-AuCl/KB(C6F5)4 for
the hydration of some internal alkynes (1-phenylpropyne and
1,2-diphenylacetylene) at room temperature (Figures S1 and
S2). Satisfactorily, these reactions proceed quite well at room
temperature. The hydration of phenylacetylene can be com-
pleted in 3 h, and two products were obtained with a prefer-
ence to phenylacetone.
As NHC-AuX compounds are expensive, the recycling of
NHC-AuX catalysts is of great significance and has attracted
wide attention.[10] However, the poor stability of NHC-AuCl
compounds makes recycling a big challenge at high tempera-
ture. NHC(IPr)-AuCl benefits from the mild hydration conditions
reported here and is quite stable during the whole reaction
process. It makes the reuse of the catalyst system quite simple.
When the reaction finishes, there are two phases in the reac-
tion system. After phase separation, the water phase that con-
tains the catalyst was reused directly without any treatment.
The catalyst was recycled and reused six times without a no-
ticeable loss of catalytic activity (Figure 3). This is quite mean-
ingful because no example of catalyst recycling has been re-
ported to date for the high-temperature hydration of alkynes
catalyzed by other NHC-gold compounds.[2f,4–6] The results ob-
tained here suggest that the NHC(IPr)-AuCl/KB(C6F5)4 system is
robust and recyclable under mild conditions.
The excellent catalytic ability of NHC(IPr)-AuCl/KB(C6F5)4
urged us to explore the hydration of various alkynes. If
5 mol% NHC(IPr)-AuCl/KB(C6F5)4 was used, the hydration of 1-
octyne, cyclohexylacetylene, 2-propynylbenzene, and 4-me-
thoxyphenylacetylene can be finished in 20 min with a good
yield of ketone (Table 1). Even though the catalyst loading was
reduced to 0.5 mol%, 99% conversion of alkynes also can also
Table 1. Hydration of various alkynes under standard conditions[a]
.
Entry Reactant
Catalyst Time
[mol%][b]
Yield
[%][c]
Conclusions
5
0.5
0.05
3 h
7 h
24 h
>99.9 (91)
99.4
23.0
To investigate the catalytic ability of gold(I) for the hydration
of alkynes, three ligand types (N-heterocyclic carbene (NHC),
nitrogen acyclic carbene, and isocyanide) and four different
1
2
3
5
0.5
20 min >99.9 (98)
2.3 h 98.7
counterions (ClÀ, BF4À, SbF6À, and B(C6F5)4 ) were used to form
À
complexes with gold(I). Both the ligand and counterion play
vital roles in the catalysis for the following reasons:
(1) NHC(IMes)-AuX (NHC(IMes)=1,3-bis(2,4,6-trimethylphenyl)-
imidazol-2-ylidene) is not active no matter what counterion
was used; (2) Ligand-AuCl is not active no matter what the
ligand is; (3) The addition of KB(C6F5)4 can improve the catalytic
ability of these gold(I) compounds. As far as we know, al-
though many NHC-Au catalysts have been investigated for the
hydration of alkynes,[2f,n,4–6] NHC(IPr)-AuCl/KB(C6F5)4 (NHC(IPr)=
1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) is the most ef-
fective at room temperature. Carbene gold(I) compounds have
been used widely as catalysts in many reactions.[1a,b,16] The ad-
dition of KB(C6F5)4 to increase the concentration of free gold
5
0.5
20 min >99.9 (93)
4 h 98.9
5
0.5
20 min >99.9 (95)
4 h 99.8
4
5
5
0.5
20 min >99.9 (99)
2.3 h 98.5
[a] Reaction conditions: substrate (1 mmol), catalyst, methanol (1 mL),
H2O (1 mL), 258C. [b] Loading of NHC(IPr)-AuCl/KB(C6F5)4. [c] GC yield with
biphenyl as the internal standard (isolated yield presented in parenthe-
ses).
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Figure 2. Hydration of a) cyclohexylacetylene, b) 4-methoxyphenylacetylene, c) 2-propynylbenzene, and d) 1-octyne catalyzed by 0.5 mol% NHC(IPr)-AuCl/
KB(C6F5)4 or NC(Cy)-AuCl/KB(C6F5)4 at 258C.
column (50 m, 0.25 mm) and a hydrogen flame detector. The struc-
ture and concentration of the product and byproducts were fur-
ther identified by using an HP6890 GC–MS spectrometer by com-
paring retention times and fragmentation patterns with authentic
samples.
Synthesis of isocyanide gold(I) complexes
Preparation of (tht)AuCl: HAuCl4·4H2O (1 g) was dissolved in H2O/
C2H5OH (20 mL, 4:1) and excess tetrahydrothiophene was added
slowly dropwise under stirring. The yellow solution became turbid,
and a white solid appeared. (tht)AuCl was collected by filtration
and dried in a thermostatic vacuum drier. (tht)AuCl was obtained
Figure 3. Recycling of the NHC(IPr)-AuCl/KB(C6F5)4 system in the hydration of
as a colorless powder (Yield: 65%).
phenylacetylene.
Preparation of NC(Cy)-AuCl and NC(Ph)-AuCl: (tht)AuCl (0.5 g)
was dissolved in THF (20 mL), and a solution of cyclohexylisonitrile
(0.2 g) or 2,6-dimethyphenyl isocyanide (0.2 g) in THF was added.
The reaction mixture was stirred for 12 h at RT. The solvent was re-
moved, and the resulting precipitate was recrystallized from THF/
cations should be a useful method to improve the catalytic
ability of carbene gold(I) compounds in these reactions.
light petroleum (1:10) (Yield: 55%, 67%).
NC(Cy)-AuCl: 1H NMR (CDCl3, 300 MHz): d=3.94–3.88 (m, 1H),
Experimental Section
2.06–1.98 (m, 2H), 1.87–1.72 (m, 4H), 1.57–1.41 ppm (m, 4H);
13C NMR (CDCl3, 300 MHz): d=54.97, 31.54, 24.52, 22.62 ppm; ele-
mental analysis calcd for C7H11AuClN (calculated mass=341.02): C
24.61, H 3.25, N 4.10; found C 24.99, H 3.41, N 4.10.
Materials and methods
All solvents and chemicals were commercially available and used
without further purification unless otherwise stated. NMR spectra
were determined by using a Bruker DPX 300 MHz spectrometer
using CDCl3 as the solvent with TMS as the internal standard. Ele-
mental analysis was performed by using an Elementarvario EL II.
The purity of the raw materials and the reaction solution was ana-
lyzed by using a GC5890C gas chromatograph fitted with a SE-30
1
NC(Ph)-AuCl: H NMR (CDCl3, 300 MHz): d=7.39–7.33 (t, 1H), 7.20–
7.17 (d, 2H), 2.46 ppm (s, 6H); 13C NMR (CDCl3, 300 MHz): d=
144.90, 136.25, 131.08, 128.51, 18.44 ppm; elemental analysis calcd
for C9H9AuClN (calculated mass=363.01): C 29.73, H 2.49, N 3.85;
found C 29.84, H 2.71, N 3.74.
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Synthesis of NHC-gold(I) complexes
Acknowledgements
A 50 mL Schlenk flask was charged with N,N’-bis(2,6-diisopropyl-
phenyl)imidazol-2-ylidene (186 mg) and THF (30 mL), and then
(tht)AuCl (153 mg) was added. The resulting dark green solution
was protected from light and stirred at RT for at least 24 h. The re-
maining steps were then performed in air. THF (5 mL) was re-
moved under reduced pressure, and light petroleum (20 mL) was
added, which resulted in immediate precipitation. After it was col-
lected by filtration, the precipitate was recrystallized from dichloro-
methane/pentane (1:5) (Yield: 75%).
This work was supported by the National Natural Science Foun-
dation of China (No. 21176110 and 21376115)
Keywords: alkynes · carbene ligands · gold · isocyanide
ligands · ligand effects
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NHC(IPr)-AuCl: H NMR (CDCl3, 300 MHz) d=7.53–7.48 (t, 2H), 7.31
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NHC(IMes)-AuCl was prepared by a similar method (Yield=80%).
1
NHC(IMes)-AuCl: H NMR (CDCl3, 300 MHz) d=7.11 (s, 2H), 6.89 (s,
4H), 2.44 (s, 6H), 1.70 ppm (s, 12H); 13C NMR (CDCl3, 300 MHz): d=
185.14, 139.42, 134.48, 134.01, 128.81, 122.96, 21.28, 17.16 ppm; el-
emental analysis calcd for C21H24AuClN2 (calculated mass=536.13):
C 46.98, H 4.51, N 5.22; found C 46.74, H 4.56, N 5.11.
Synthesis of NAC-gold(I) complexes
Diethylamine (3 equiv.) was added to a solution of an isocyanide
gold(I) complex (NC(Cy)-AuCl or NC(Ph)-AuCl) (100 mg) in dichloro-
methane (10 mL). The mixture was stirred at RT and protected
from light for 3 days. The addition of saturated NH4Cl solution, ex-
traction with dichloromethane, and filtration furnished NAC(Cy)-
AuCl or NAC(Ph)-AuCl as a colorless solid.
NAC(Cy)-AuCl: 1H NMR (CDCl3, 300 MHz): d=5.47–5.44 (br, 1H),
4.27–4.18 (m, 1H), 3.96–3.89 (q, 2H), 3.31–3.24 (q, 2H), 2.10–2.06
(m, 1H), 1.76–1.70 (m, 2H), 1.63–1.58 (m, 2H), 1.44–1.33 (m, 4H),
1.30–1.26 (t, 3H), 1.23–1.18 ppm (t, 3H); 13C NMR (CDCl3, 300 MHz):
d=188.40, 59.22, 53.71, 41.04, 34.28, 25.10, 24.76, 14.71,
11.97 ppm; elemental analysis calcd for C11H23AuClN2 (calculated
mass=414.11): C 31.78, H 5.58, N 6.74; found C 31.86, H 5.39, N
6.65.
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Published online on December 3, 2015
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim