A structure/catalytic activity study of gold(i)-NHC complexes, as well as their recyclability and reusability, in the hydration of alkynes in aqueous medium

Article abstract of DOI:10.1039/c5cy01278c

We conducted a structure/catalytic activity study of water-soluble gold(i) complexes-supporting sulfonated NHC ligands-in the hydration of alkynes in pure water or water nsp;:nsp;methanol (1nsp;:nsp;1), as well as their recyclability. Comparative studies were carried out with the addition of different silver salts. Our results indicate that the bulkier complex is the most effective and that the addition of methanol as co-solvent not only shortens reaction times but also stabilizes the less bulky complexes.

Full text of DOI:10.1039/c5cy01278c

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DOI: 10.1039/C5CY01278C
Structure/Catalytic Activity Study of Gold(I)ꢀNHC
Complexes, Recyclable and Reusable, in the Hydration
of Alkynes in Aqueous Medium
Gabriela A. Fernández, Alicia B. Chopa,Φ and Gustavo F. Silbestri*,§
Instituto de Química del Sur (INQUISUR), Departamento de Química, Universidad Nacional del Sur, Av. Alem
1253, B8000CPB Bahía Blanca, Argentina.
ABSTRACT
We conducted a structure/catalytic activity study of waterꢀsoluble gold(I) complexes ꢀsupporting
sulfonated NHC ligandsꢀ in the hydration of alkynes in pure water or water:methanol (1:1), as well as
their recyclability. Comparative studies were carried out with the addition of different silver salts. Our
results indicate that the bulkier complex is the most effective and that the addition of methanol as coꢀ
solvent not only shortens reaction times but also stabilizes the less bulky complexes.
KEYWORDS: waterꢀsoluble Nꢀheterocyclic Carbene; NHCꢀGold(I); recyclability
INTRODUCTION
Since the first discovery of Nꢀheterocyclic carbenes (NHCs) by Arduengo, work on the synthesis of
metalꢀNHC compounds has revolutionized the field of catalysis, gradually displacing the typical
phosphine and amineꢀtype ligands in view of their higher stability and reactivity [1]. Besides, aqueous
catalysis has become an important topic in Green Chemistry due to the positive implications of the use
of water as a reaction medium. Also, it is important to mention that water insolubility of many organic

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products facilitates separation process and, in the case of waterꢀsoluble catalysts, opens the way for their
recycling and reuse. Although most of the transition metals has been coordinated with NHC ligands and
most of the resulting complexes has been used in organometallic catalysis [2], only a few of them,
predominantly with Ru [3] and Pd [4] and, in a few examples with Ag [5], Cu [6], Rh [7], Pt [5, 8] and
Ir [9] have been investigated in aqueous phase catalysis [10]. It is noteworthy that, although the
chemistry of waterꢀsoluble NHCꢀgold(I) complexes is still a relatively young field, it has yielded
successful results in various areas of application such as medicine, luminescence and catalysis [11, 12].
It is known that hydration of alkynes is an appropriate reaction for the synthesis of aldehydes and
ketones. In order to replace the initial Hgꢀcatalysts, efficient methodologies using catalysts containing
Rh [13], Pd(II) [14], Ru(III) [15], Cu(II) [16], or Pt [5] have been reported. Since 1991, when Fukuda
and Utimoto reported the hydration of alkynes catalyzed by Na[AuCl4] [17], a development of research
on the use of gold catalysts has been initiated [11]. The synthesis of new water soluble NHCꢀgold
complexes, the study of their physicochemical properties as well as their application to various chemical
transformations ꢀincluding their potential recoveryꢀ is a very significant contribution to organometallic
catalysis and to sustainable organic chemistry in general. Continuing with our research on waterꢀsoluble
NHCꢀgold(I) complexes [11c], herein we report the structure/catalytic activity study of five complexes
(C1ꢀC5, Figure 1) in the hydration of alkynes in aqueous medium, as well as their recyclability.
SO3Na
iPr
iPr
iPr
iPr
H
N
N
N
N
N
N
N
N
N
Au Cl
iPr
Au Cl
Au Cl
Au Cl
Au Cl
N
iPr
NaO3S
NaO3S
NaO3S
NaO3S
SO3Na
C1(w) 111 g/L
C2 (w) 80 g/L
C3 (w) 180 g/L C4 (w) 645 g/L
C5
Figure 1. Structure and water solubility of sulphonated NHCꢀgold(I) complexes.
RESULTS AND DISCUSSION

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1. Synthesis of Gold(I) Complexes. Complexes C1ꢀC5 were synthesized from D[OAI:u10C.1l0(3t9h/Ct)5]CY0(t1h27t8C=
tetrahydrothiophene) [18] and the corresponding imidazolium compounds L1ꢀL5 through the silver
oxide route developed by Lin and coꢀworkers [19], based on [AgꢀNHCꢀCl] complexes as NHC transfer
agents [20] (Scheme 1). Previously, we have determined that this synthetic route turned out to be the
most effective (90 to 95% of isolated complex), avoiding the formation of the respective bisꢀcarbene
complexes [Au(I)ꢀ(NHC)2]+ [11c].
N
N
N
N
N
N
Ag2O
Ar
Ar/R
Au(tht)Cl
Ar
Ar/R
Ar
Ar/R
Ag
Cl
Au
L1-5
C1-5
Cl
90ꢀ95% isolated yield
Scheme 1. General procedure for the synthesis of NHCꢀgold(I) complexes.
As far as we now, [(3ꢀsulfonatepropyl)imidazolꢀ2ꢀylidene] gold(I) chloride (C5) has not been
previously reported. Once the complex is isolated as a solid by removal of the solvent (methanol) under
vacuum it should be stored under a moistureꢀfree atmosphere. Otherwise, the immediate appearance of
the characteristic violet coloration, indicating the presence of Au nanoparticles in the medium, is
detected. The complex is stable in a methanol solution.
2. Catalytic Experiments. Our initial exploration started with the hydration of phenylacetylene, using
C1 as catalyst, as a model system. The reactions were carried out in pure water. The results obtained are
summarized in Table 1.

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Table 1. Hydration of phenylacetylene catalyzed by complex C1. a
AgX salt
(mol%)
Temp
(°C)
Time
(h)
C1
(mol%)
Entry
Solvent
Yield (%)b
1
2
1
1
1
2
2
1
1
1
1
2
1
1
2
1
1
1
1
ꢀ
H2O
H2O
80
80
5
5
100
100
100
98
SCN (1)
3
SCN (2)
H2O
80
4
4
SCN (1)
H2O
80
3
5
ꢀ
H2O
80
3
70
6
ꢀ
H2O
90
3
100
100
55
7
ꢀ
H2O
100
100
100
100
100
80
1.17
0.5
0.5
0.5
0.5
0.75
0.25
0.66
0.33
0.13
0.5
8
SCN (1)
H2O
9
SbF6 (1)
H2O
68
10
11
12
13
14
15
16
17c
ꢀ
H2O
100
100
100
100
100
100
100
83e
ꢀ
H2O:MeOH
H2O:MeOH
H2O:MeOH
H2O:MeOH
H2O:MeOH
H2O:MeOH
MeOHd
ꢀ
ꢀ
100
100
100
100
100
SCN (1)
OTs (1)
SbF6 (1)
ꢀ
aExperiments were carried out with 0.5 mmol of phenylacetylene, in 3.0 mL water or
b
c
1.5 mL:1.5 mL water:methanol. Isolated yield. Quantitative yield was obtained after
40 min. dStoichiometric amount of pure water. eTogether with 4% of (1ꢀ
methoxyvinyl)benzene.
We decided to initiate the study working at 80°C with addition of 1 mol % of C1. Under these reaction
conditions, acetophenone was obtained in a quantitative yield after 5 h (entry 1). Next, with the main

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goal of reducing reaction time, we studied the effect of different parameters such aDsOtI:h1e0.1a03d9d/Ci5tiCoYn012o78fCa
silver salt [21] and the increment of either the catalyst loading or the temperature. Results resumed in
Table 1 show that, although the addition of 1 mol% of AgSCN was ineffective, it was possible to reduce
reaction time to 4 h by doubling the load of salt (entries 2 and 3); on the other hand, reaction time was
further reduced by an increment of catalyst loading (2 mol%), with the addition of 1 mol% of AgSCN,
(entry 4) as well as by an increment of the bath temperature (90 ºC, 3 h; 100 °C, 70 min) (entries 6 and
7). Meanwhile the addition of 1 mol% of either AgSCN or AgSbF6 caused no benefits to the reaction
carried out at 100ºC, showing a decline in the catalytic activity giving 55% and 68% yield respectively
(compare entry 7 with entries 8 and 9), the use of 2 mol% of catalyst reduced the reaction time to 30
min (entry 10).
Although these reactions ꢀcarried out in pure waterꢀ gave excellent results we considered interesting to
study the effect of methanol as coꢀsolvent, specially taking into account the results reported by Laguna
[22]. So, we performed a series of reactions in a water:methanol (1:1) mixture where we analyzed the
effect of temperature as well as the catalyst loading and the addition of different silver salts. As
expected, a decrease of the temperature from 100 ºC to 80 °C increased the reaction time from 30 to 45
min (Table 1, entries 11 and 12); on the other hand, an increase in the load of the catalyst (2 mol%)
reduced the reaction time to 15 minutes (entry 13). A comparative analysis on the addition of different
silver salts (1 mol%) shows that AgSbF6 was the most effective: the desired product was obtained in
100% yield in only 8 minutes in the presence of 1 mol% of C1! (entries 14ꢀ16). Taking into account that
the reactions in pure water are slower than in a water:methanol mixture, we carried out a reaction in
methanol adding a stoichiometric amount of pure water. This reaction was slower than in
water:methanol, giving 83% of ketone after 30 min. (compare entries 11 and 17) together with a small
amount of (1ꢀmethoxyvinyl)benzene (4%). The results obtained in two experiments carried out at
shortest reaction times (5 to 15 min) enable us to propose the generation of the ketone through the
hydrolysis of a ketal intermediate (Scheme 2). Similar results have been reported by other authors. [21,
23]

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Scheme 2. Reaction carried out in methanol.
In order to obtain some kinetic information about coꢀsolvent effect we monitored the conversion of
phenylacetylene over time. The kinetic plot, Figure 2, shows a rapid activation in the reactions carried
out in water:methanol.
Figure 2. Hydration of phenylacetylene with C1 (1 mol%). Reactions were run at 100 °C.
Taking into account that the effect of weakly coordinating anions, introduced as silver salts additives, is
an issue of growing concern in gold catalysis [23, 24], we considered interesting to obtain a kinetic plot
in order to compare experiments 11 and 14ꢀ16 (Table 1). For this purpose, the active specie, prepared in
methanol from C1 and the corresponding silver salt (see Supporting Information), was added to a
solution of phenylacetylene in water. Scheme 3 shows that, under this condition, the lower activity
shown by C1/SCNꢀ is due to a longer induction period.

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Scheme 3. Anion effect with catalyst C1. Reactions were run in water:methanol at 100 °C.
2.2. Catalyst loading. A series of experiments was performed in water as well as in water:methanol, at
100 °C, gradually reducing catalyst loading from 1 to 0.05 mol%. The results are summarized in Table
2. It is important to mention that complex activation was performed working at 100 ° C and not by the
addition of silver salts. As can be seen, the catalyst turned out to be active in all cases. The drawback
was that halving the catalyst loading leads to a doubling of the reaction time to obtain similar results. In
all cases the reactions carried out in water were slower (from 1.17 h to 13 h) (entries 1ꢀ5) than in
water:methanol (from 30 min to 7 h) (entries 6ꢀ10). Furthermore, it is noteworthy that the catalyst
besides being active at 0.05 mol% was recycled and reused five times without losing catalytic activity.

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Table 2. Effectiveness of C1. Catalyst loading.a
iPr
iPr
N
N
iPr
NaO3S
iPr
SO3Na
Au
O
Cl
C1
100 °C
Time Yield (%)b
(h)
C1
(mol%)
Entry
Solvent
1
2
1.0
0.5
H2O
H2O
1.17
2
100
100
100
100
100
100
100
100
100
100
3
0.25
0.10
0.05
1.0
H2O
4
4
H2O
8
5
H2O
13
0.5
1
6
H2O:MeOH
H2O:MeOH
H2O:MeOH
H2O:MeOH
H2O:MeOH
7
0.5
8
0.25
0.10
0.05
2
9
4
10
7
aConditions for a typical experiment: 0.5 mmol of
phenylacetylene; water (3 mL); water:methanol (1.5
mL:1.5 mL); heating (oil bath). bIsolated yield.
2.3. Steric Hindrance Effect. Next, in order to evaluate the steric hindrance effect over the catalytic
activity, caused by the substituents attached to the nitrogen atoms, we studied the hydration of
phenylacetylene in the presence of 1 mol% of NHCꢀgold(I) less bulky complexes (C2ꢀC5). All
reactions were performed in water and water:methanol, with and without the addition of a silver salt, at
the appropriate temperature considering the thermal stability of the complexes. Table 3 summarizes the
principal results obtained.

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Table 3. Hydration of phenylacetylene catalyzed by complexes C2ꢀC5.a
Entry
NHCꢀAu
[Au] mol% AgX salt (mol%)
Solvent
Temp (°C) Time (h)
Yieldb
1
1
1
5
5
1
1
1
1
1
5
1
1
1
1
1
1
1
1
1
5
5
1
1
1
1
1
5
1
1
1
ꢀ
H2O
H2O
100
100
100
100
30
24
24
24
24
24
13
6
0
0
1
SCN (1)
2
SbF6 (1)
H2O
0
3
iPr
SO3Na
N
N
ꢀ
H2O
0
4
iPr
Au
SbF6 (10)
H2O
0
5
Cl
ꢀ
H2O:MeOH
H2O:MeOH
H2O:MeOH
TMS
100
100
100
80
100
100
100
0
6
OTs (1)
7
C2
SbF6 (1)
7.5
24
3
8
ꢀ
9
SbF6 (1)
H2O
80
0
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
SbF6 (10)
H2O
30
3
0
SO3N
N
N
ꢀ
H2O:MeOH
H2O:MeOH
H2O:MeOH
H2O:MeOH
H2O:MeOH
TMS
80
48
48
48
48
24
48
24
24
24
24
24
60
60
60
24
24
0.5
60
60
24
95
35
80
56
44
0
Au
SCN (1)
80
Cl
OTS (1)
80
SbF6(1)
80
C3
ꢀ
100
80
ꢀ
ꢀ
H2O
80
0
SCN (1)
H2O
80
0
SbF6 (1)
H2O
80
0
ꢀ
H2O
80
0
SO3Na
N
N
SbF6 (10)
H2O
30
0
Au
Cl
ꢀ
H2O:MeOH
H2O:MeOH
H2O:MeOH
H2O:MeOH
TMS
80
62
52
48
14
0
C4
OTs (1)
80
SbF6 (1)
80
ꢀ
100
80
ꢀ
SbF6 (10)
H2O
30
0
SO3Na
N
N
H
ꢀ
H2O:MeOH
H2O:MeOH
H2O:MeOH
80
20
35
19
Au
Cl
SbF6 (1)
ꢀ
80
C5
100

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aExperiments were carried out with 0.5 mmol of phenylacetylene, in 3.0 mL water or 1.5 mL:1.5
mL water:methanol. bIsolated yield.
Notably, there was no reaction in water with any of these complexes. First, the reactions carried out with
C2 were negative although reaction times were extended to 24 h, even using either Au loadings of 5
mol% or adding different silver salts (entries 1ꢀ4). Similar results were obtained with C4 (entries 18–
21). It should be mentioned that the substrate was completely recovered and the generation of Au
nanoparticles (violet coloration of the solution) was not detected. We supposed that these unexpected
results could be due to a probable thermally sensitive intermediate, so, we carried out two reactions at
30 ºC. In order to activate these reactions we used loadings of 5 mol% and 10 mol% of catalyst and
AgSbF6, respectively. Once more, the reactions were negative (entries 5 and 22). Up to now we cannot
explain these results.
On the other hand, taking into account that C3 and C5 are unstable in water, we carried out an
experiment with C3 adding the complex over a mixture of the silver salt (1 mol%) and the substrate in
water, considering that the active species could be generated prior to the decomposition of the complex.
Gradually, the solution turned to violet and, after 3 h, the complex was totally decomposed. Similar
results were obtained at 30 ºC with loadings of 5 mol% and 10 mol% of catalyst and AgSbF6,
respectively (entries 10 and 11). Experiment 28 shows that, under similar conditions, C5 was
completely decomposed in just 30 min. It is noteworthy that these reactions were negative, indicating
that gold nanoparticles did not act as catalyst.
In contrast to the results obtained in pure water, all the reactions carried out in water:methanol were
positive. The results obtained showed that a decrease of steric hindrance caused a decrease of catalytic
effect. For example, using 1 mol% of C2 a quantitative yield was obtained, although after a prolonged
reaction time (13 h) (entry 6). As shown in experiments 7 and 8, the addition of 1 mol% of AgOTs and
AgSbF6 reduced the reaction time to 6 h and 7.5 h, respectively. At this point is appropriate to cite that
similar conclusions about counterion effects has been previously reported about gold(I) catalysts [25].

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The reactions catalyzed with C3 gave an excellent yield but after 48 h (entry 12). In this case the
addition of different silver salts [AgSCN, AgOTs and AgSbF6] caused no beneficial effects, providing
conversions of 35%, 80% and 56% respectively, in a similar reaction time (entries 13ꢀ15). On the other
hand, complex C4 gave moderate conversions even after 60 h [26] showing that, once more, the
addition of silver salts negatively affects the reaction (entries 23 to 25). Finally, complex C5 showed
lower activity giving 20% yield after 60 h; in this case the addition of AgSbF6 produced a positive effect
(35%, 60 h) (entries 29 and 30). In order to determine whether the unexpected negative effect produced
by the addition of salts is due to different induction periods or, the catalytic rate is really different and
anionꢀdependant, we considered interesting to obtain a kinetic plot in order to compare experiments 12ꢀ
15 (Table 3). Figure 3 shows that the reaction rates are anionꢀdependent.
Figure 3. Anion effect with complex C3. Reactions were run in water:methanol.
Experiments 16, 26 and 31, carried out at 100 °C, show that an increase in temperature caused no
significant benefits in reducing reaction times or increasing the yields. It is worth mentioning that, in
none of the experiments carried out with complexes C3 and C5 the characteristic violet coloration was
observed.
It is tempting to speculate that the positive influence of the addition of methanol as coꢀsolvent may be
due to the generation of a more effective precatalyst. Two methanol coordinated cationic complexes, C6

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and C7, were prepared from C1 and C3 respectively, according to reported procedures [27]. Next, we
carried out a series of reactions in order to compare the catalytic activity of C1 and C6 as well as of C3
and C7, either in water or in water:methanol. The results obtained, sketched in Scheme 4, support our
hypothesis. Thus, C6 was found to be more active than C1 working in water (85% vs. 42% yield) while
their catalytic activity were similar in water:methanol (90% vs. 88% yield). On the other hand, C7 was
more active than C3 under both conditions. Even more, the reaction carried out in water was positive,
yielding 15% of the desired product after 24 h. It should be mentioned that no violet coloration was
detected working with C7 in water.
13
The CꢀNMR spectra confirm the presence of a more acidic gold center; thus, a slight upfield shift of
the signal of carbenic carbons was observed: 173.3 ppm (C1) vs 160.2 ppm (C6); 169.7 ppm (C3) vs
159.7 ppm (C7). Stability and catalytic activity of different precatalysts in water are currently being
examined in our laboratory. The results will be published in due course.
Scheme 4. A comparison of the catalytic activity of C1 and C3 with the corresponding methanol
coordinated cationic complexes C6 and C7.
2.4. Kinetic Information. Thereafter, in order to obtain some kinetic information about the influence of
the steric bulk around the metal center we monitored the reaction over time, in the absence of silver salts

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at 100 °C. As seen in Figure 4, the 100% conversion was reached after a given period. The kinetic plot
shows that all gold complexes reveal similar active catalyst systems after drastically different induction
periods. Thus, the induction is less favored with decreasing size of the Nꢀsubstituents.
Figure 4. Comparison of reaction progress: (•) C1 (0.5 h); (♦) C2 (13 h); (■) C3 (48 h); (▲) C4 (72 h)
and (ꢀ) C5 (100 h). Reactions were run in water:methanol at 100 °C.
Our results confirm that the steric bulk is an important factor in both the stability and the catalytic
activity of gold(I) complexes in aqueous medium, being the bulkier NHC complex the most effective
catalyst in the studied reaction. These results are consistent with those reported by other authors on
different reactions carried out in conventional organic solvents [28].
Probably, the lower activity exhibited by complexes C2 to C5 (pH between 6.4 and 7.2) could be due to
the sulfonated moiety which is flexible enough to coordinate the metal or to interfere with the reaction
center. Such coordination could be eliminated working at an acidic pH. The graph presented in Figure 5
shows that the induction time is drastically reduced in the reaction carried out with the less active
catalyst (C5) at pH = 2, improving the catalytic effect. This result supports the existence of coordination
between the metal and the sulfonate group.

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Figure 5. Comparison of reaction progress: (■) C5; (♦) C5 in acid medium. Reactions were run in
water:methanol at 80 ºC.
A preliminary Density Functional Theory (DFT) computational study supports the experimental results
showing that the alkyl chain folds generate a strong steric hindrance around the metal center. In
addition, the strong interaction between metal and oxygen atom of sulfonate affected the positive charge
distribution on the metal center. We continue working on the topic and the results will be published in
due course.
2.5. Catalyst Recycling. This study was carried out in water:methanol taking into account that all the
catalysts were stable in this medium. Each recycling experiment was repeated until the catalyst was
almost inactive. The number of cycles that attained this conversion can be found in Figure 6a. Recovery
experiments showed that the smaller gold(I)ꢀNHC complexes (C4 and C5) suffered from significant
activity losses after the initial cycles (4 and 3 cycles, respectively) whereas the bulky complexes (C1-
C3) have attained high levels of recyclability (9, 6 and 6 cycles, respectively) [29]. The decline in
catalytic activity could be due either to degradation or losing of the catalyst in each process of extraction
and reloading of reagent [11b], which would be noted when the remaining amount of catalyst is not
sufficient to reach the 100% conversion. For example, a recycling experiment using 0.5 mol% of C1,
showed a loss of catalytic activity in the first recycling loop (Figure 6b).

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Figure 6. (a) Recycling experiment: cycle numbers vs percentage conversion of (a) different [Au]
complexes (1 mol%); (b) C1 (0.5 mol%).
Thermomorphic multicomponent solvent (TMS) systems, which consist of a mixture of three solvents of
high, intermediate and low polarity, forms two phases at low temperature but becomes homogeneous at
high temperature [30]. The catalytic reaction is then run homogeneously at high temperature and phase
separation is performed at low temperature. Considering our experience in the use of TMS systems as
an alternative of catalyst recycling [8], experiments in water:toluene:DMF (TMS system at 80 °C) were
performed with complexes C2-C4. Unfortunately, in all cases, the desired product was not obtained
even heating for 24 h, recovering the starting material (Table 3, entries 9, 16 and 25).
2.6. Scope and Limitation. Next, taking into account the higher catalytic activity shown by C1, we
studied the reactivity of selected terminal alkynes catalyzed by C1 in water at 100 °C in absence of
silver salts. Table 4 shows that these terminal alkynes were oxidized to the corresponding ketone [31] in
quantitative yields, according to Markovnikov’s rule, in acceptable reaction times (entries 1ꢀ6).
Moreover, entries 5 and 6 show that the catalytic system can be extended to enines yielding the
corresponding α,βꢀunsaturated ketone. As expected, the reaction carried out with an internal enyne was
not regioselective and a mixture of ketones was obtained (entry 7).

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Table 4. Hydration reaction of terminal and internal alkynes
catalyzed by C1 in water.a
Entry
1
R
R’
H
Time (h)
1.17
Product
Yield (%)b,c
100 (96)
ꢀC5H11
1
2
3
4
ꢀC3H7
H
H
H
1.17
1.17
2
100 (97)
100 (99)
100 (96)
2
3
H3C
4
H2C
C
5
6
H
H
2
2
100 (98)
100 (97)
CH3
5
6
23
40
H2C
7
8
C
7
C2H5
6
2
CH3
8d
H
100 (99)
9

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9d
H
2
100 (99)
10
aConditions for a typical experiment: 0.5 mmol of alkyne; 1 mol% of
b
C1; water (3.0 mL); heating (oil bath). Quantified by GC, using the
external standard method. cIsolated yields between brackets. dHeating
at 60 ºC with the addition of AgSbF6 (1 mol%).
It is important to notice that although C1 is highly soluble in water, some of alkynes tested in Table 4,
do not. If surface effects are really important, higher alkyne amounts per volume of water could lead to
different results. So, we carried out a series of reactions with the double and triple amount of
phenylacetylene, pꢀmethylphenyl acetylene and 1ꢀethynylcyclohexꢀ1ꢀene, in the same volume of water.
In all cases, the increase in the concentration led to similar results as those resumed in Table 4.
It should be mentioned that the reactions carried out with propꢀ2ꢀynꢀ1ꢀol and butꢀ3ꢀynꢀ1ꢀol under the
general conditions (100 ºC) gave a mixture of products in which none of the expected ketones were
detected. In order to obtain the corresponding hydroxy ketones, the reactions must be carried out at 60
ºC, activating the catalyst by the addition of AgSbF6 (entries 8 and 9).
CONCLUSIONS
In summary, we have shown that waterꢀsoluble Au(I) complexes, supporting sulfonated NHC ligands,
are active and recyclable catalysts in the alkyne hydration in water:methanol (1:1), even in the absence
of silver salts. Moreover, the bulkiness of the ligands influences the catalytic properties; thus, higher
yields, in shortest times, are obtained with bulkier NHC ligands. In addition, we have shown that the
bulkier catalyst C1 is highly effective in water. Thus, the hydration reaction carried out with different
alkynes gave a quantitative yield of ketone in acceptable reaction times (1.17 to 2 h). The use of pure
water as reaction medium facilitates the catalyst recycling. Further work, in progress in our laboratory,
is aimed to assess the influence of different coꢀsolvents in both the stability and the effectiveness of the
catalysts.

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Finally, it should be mentioned that the effect of the less effective catalyst C5 could be improved
working at acidic pH. A preliminary DFT study shows that the sulfonated chain generates a strong steric
hindrance around the metal center. A complete DFT study is in progress in our laboratory.
Supporting Information. Experimental results and 1H and 13C NMR spectra for compounds C5ꢀC7.
Corresponding Author
* gsilbestri@uns.edu.ar
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the
final version of the manuscript.
Notes
§ Member of CONICET
Φ Member of CIC
Acknowledgments
This work was partially supported by CONICET, CIC, ANPCYT (PICT 2012ꢀ1449), and the
Universidad Nacional del Sur, Bahía Blanca, Argentina. CONICET is thanked for a research fellowship
to G.A.F.. We thank the Empresa Neuquina de Servicios de Ingeniería S.E., Argentine, for the provision
of D2O.
References and Footnotes

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its activity.

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commercially available.

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Graphical Abstract
249x124mm (150 x 150 DPI)