Hydration of alkynes at room temperature catalyzed by gold(I) isocyanide compounds

Article abstract of DOI:10.1039/c4gc01322k

An effective method using gold(I) isocyanide complexes as catalysts for the transformation of various alkynes to the corresponding ketones is successfully developed. The hydration process proceeds smoothly at room temperature with quite high yield (up to 99%). The catalytic center is the isocyanide-Au(I)⁺ cation. Further theoretical research reveals a direct hydration mechanism by H₂O, and the rate-determining step has an energy barrier of 23.7 kcal mol^?1. These results show a good example to reduce unnecessary steps and achieve milder reaction conditions at the same time for the hydration of alkynes.

Full text of DOI:10.1039/c4gc01322k

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Hydration of alkynes at room temperature
catalyzed by gold(I) isocyanide compounds†
Cite this: DOI: 10.1039/c4gc01322k
a,b
a
a
a
a
Yun Xu, Xingbang Hu,* Jing Shao, Guoqiang Yang, Youting Wu* and
a
Zhibing Zhang
An effective method using gold(I) isocyanide complexes as catalysts for the transformation of various
alkynes to the corresponding ketones is successfully developed. The hydration process proceeds
+
Received 14th July 2014,
Accepted 17th September 2014
smoothly at room temperature with quite high yield (up to 99%). The catalytic center is the isocyanide-Au(I)
2
cation. Further theoretical research reveals a direct hydration mechanism by H O, and the rate-determin-
DOI: 10.1039/c4gc01322k
ing step has an energy barrier of 23.7 kcal mol−1. These results show a good example to reduce unnecessary
www.rsc.org/greenchem
steps and achieve milder reaction conditions at the same time for the hydration of alkynes.
8
SO H-PMO(Et), 70 °C for (Ph P)AuCH in the presence of
3
3
3
Introduction
1
8
19
H
2
SO
4
,
65 °C for abnormal Au-NHC,
and reflux
Obviously, it is still a challenge to develop
2
0–24
The addition of water to alkynes is an environmentally friendly temperature.
method to prepare ketones with 100% atomic economy. more effective gold catalysts for alkyne hydration at room
However, the traditional alkyne hydration was realized using temperature.
1
mercury(II) salts, a highly toxic compound, as the catalyst.
In the last decade, an increasing number of studies have
Even though different Brønsted acids were also found to have focused on the preparation of gold isocyanide complexes
2
–4
catalytic activity for the hydration of alkynes,
require, in most cases, stoichiometric or excess amounts of properties.
acids and only electron-rich alkynes as the substrates can lead used as the starting materials for the synthesis of other novel
the processes (Au-RNC) and the investigation of their structure and
2
5–27
But so far, Au-RNC complexes were mainly
2
8
to high conversions.
catalysts.
Among the complexes obtained via Au-RNC,
To overcome the disadvantages of mercury(II) salts and Au-NHC represents a kind of the most extensively studied
Brønsted acids, a variety of organometallic catalysts were compound in organometallic chemistry and catalysis
5
6,7
8,9
29–39
explored, such as compounds containing Pt, Ag, Ru, Au, (Scheme 1).
To realize the reported catalytic ability of Au-
1
0
and even biomimetic catalysts. Among them, the unique cat- NHC for the hydration of alkynes, high reaction temperatures
alytic ability of gold(I) and gold (III) has attracted much atten- are needed to obtain satisfactory conversions.
tion, because gold centers show high affinity to activate CuC far as we know, there is almost no report that the Au-RNC
triple bonds by nucleophilic attack. Though gold compounds complex can be directly used as a catalyst in not only alkyne
were thought to be potential and excellent catalysts for alkyne hydration, but also other reactions.
1
1–14,17,19–23
As
4
0
1
1,12
hydration,
most of the reactions catalyzed by gold com-
Both experimental and theoretical research studies in this
pounds require high reaction temperatures, such as 120 °C for paper show, to our surprise, that Au-RNC has excellent activity
the hydration catalyzed by gold N-heterocyclic carbenes (Au-
1
2,13
11
NHC)/AgSbF
6
,
Au-NHC/HSbF
6
,
and Au-NHC@porous
1
4
15
organic polymers, 100–150 °C for bis(phosphane) gold(I),
00 °C for NaAuCl4,
1
6
17
1
80 °C for Au-NHC
and Au-SH/
a
School of Chemistry and Chemical Engineering, Nanjing University,
Nanjing 210093, PR China. E-mail: huxb@nju.edu.cn, ytwu@nju.edu.cn;
Fax: +86 2583 336599; Tel: +86 2583 596665
b
China Pharmaceutical University, Nanjing, 211198, PR China
†
Electronic supplementary information (ESI) available: Experimental details and
1
13
characterization results ( H-NMR,
C-NMR, elemental analysis, and MS);
hydration of various alkynes catalyzed by Cat-2; computational methods and
Scheme 1 The relationship between Au–RNC and Au–NHC and their
Cartesians coordinates of the optimized structures. See DOI: 10.1039/c4gc01322k
catalytic ability for the hydration of alkynes.
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in catalyzing the hydration of alkynes at room temperature. It reaction with only KB(C
6
F
5
)
4
was also performed and no
is a good example that avoids the use of complicated catalysts product was observed (entry 6). All the results mentioned
and eliminates harsh reaction conditions in the hydration of above indicated that the active center in this reaction system
+
+
alkynes (Scheme 1).
should be the (CyNC)Au cation (Cat-1 ).
Using an environmentally preferable solvent is quite impor-
tant for a green process. The solvents screening experiments
on methanol, ethanol, hexane, dioxane, acetone and THF
show that the reaction performed in methanol gives the best
result (entries 5, 12–16 in Table 1). Using methanol as a
solvent is quite green according to the framework for the
Results and discussion
Hydration of phenylacetylene catalyzed by gold isocyanide
Two simple gold isocyanide complexes (RNC)AuCl (R = Cy, 2,6-
Me C H ) were synthesized according to the literature pro-
42
2
6
3
environmental assessment of solvents, Sanofi’s solvent selec-
4
3
cedures (see ESI† for detailed synthetic procedures and charac-
tion guide, and the EHS assessment for alternative sol-
3
1,34,38
terization).
The hydration of phenylacetylene (1a)
44
vents. The amount of solvent has an obvious influence on
catalyzed by (CyNC)AuCl (Cat-1) was selected as a probe to
investigate the activity of the catalyst (Table 1). No aceto-
phenone (1b) was observed when Cat-1 was used as the catalyst
alone (entry 1). As we know, the active center of gold(I) com-
the conversion of phenylacetylene. Comparing with the stan-
dard conditions, increasing the amount of methanol results in
low conversion of the reactants (entries 5, 7–9).
It has been found that some alkynes can be converted into
the vinyl ether intermediate by the addition of methanol to the
4
1
plexes in many reactions should be the gold(I) cation. To
release the gold cation, some weakly coordinating anions
CuC bond, and vinyl ether can then be hydrolyzed into
−
−
−
−
(
such as SbF6 , BF4 , B(C H ) , and B(C F ) ) were added to
11,12
6
5 4
6
5 4
ketone.
The catalysis reported here should not proceed in
6
the reaction system (entries 2–5). Though AgSbF itself can be
such a pathway, because no vinyl ethers were detected by
GC-MS during the reactions of all the substrates investigated
7
used as the catalyst for the hydration of alkynes, almost no
ketone was produced in our research when using Cat-1/AgSbF6
here (Tables 1 and 2). Furthermore, when only H O was used
2
as the catalyst because the reaction temperature is far lower
as both the solvent and the reactant, the reaction conversion
still can reach 57.0% in 24 h (entry 17 in Table 1).
than that in ref. 7. Among these anions (SbF , BF4−
−
,
6
−
−
−
B(C H ) , and B(C F ) ), only B(C F ) which has the weakest
6
5 4
6
5 4
6 5 4
Substrate scope of the gold isocyanide catalysts
coordination ability gives satisfactory results (entry 5). To
exclude the possibility of KB(C F ) acting as a catalyst, a
6
5 4
To explore the substrate scope of this catalytic system, the
hydration of different alkynes was investigated under the stan-
dard conditions (Table 2). Cat-1 shows excellent catalytic reac-
tivity at room temperature for a wide range of substrates,
including electron-rich and electron-deficient alkynes, aro-
matic and non-aromatic alkynes. In total seventeen alkynes
were investigated, and the isolated or GC yields of fifteen
alkynes are larger than 95%. For relatively electron-deficient
alkynes, such as 6a, only 63% yield was obtained (entry 6).
Further increase in the electron withdrawing ability of the sub-
a
Table 1 Optimization of the hydration conditions
b
c
Entry
Catalyst (mol)
Solvent
Conv. (%) stituent by CF results in a quite slow reaction rate. The GC
3
yield of 7b is only 5% in 24 hours and 8% in 48 hours (entry
1
2
3
4
5
6
5% Cat-1
5% Cat-1 + 5% AgSbF
5% Cat-1 + 5% KBF
5% Cat-1 + 5% KB(C
5% Cat-1 + 5% KB(C
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Ethanol
Hexane
NR
0.6
3.7
1.5
7). For electron-donating alkynes, 5a and 8a, satisfactory iso-
6
lated yields (>97%) were obtained, especially when the meth-
oxyl is located at the ortho-position (entries 5 and 8). No
matter the CuC triple bond binds with the aromatic ring or
alkane, the reactions perform quite well. Besides alkynes, the
hydration of diynes, such as 16a and 17a, can also be well cata-
lyzed by gold isocyanide with high yields at room temperature
(entries 16 and 17).
4
6
H
F
5
)
4
6
5
)
4
99.9
NR
99.9
32.2
5.0
20.2
<1.0
27.2
25.7
10.1
3.4
6 5 4
5% KB(C F )
d
7
8
5% Cat-1 + 5% KB(C
5% Cat-1 + 5% KB(C
5% Cat-1 + 5% KB(C
1% Cat-1 + 1% KB(C
1% Cat-1 + 1% KB(C
5% Cat-1 + 5% KB(C
5% Cat-1 + 5% KB(C
5% Cat-1 + 5% KB(C
5% Cat-1 + 5% KB(C
5% Cat-1 + 5% KB(C
5% Cat-1 + 5% KB(C
6
6
6
6
6
6
6
6
6
6
6
F
F
F
F
F
F
F
F
F
F
F
5
5
5
5
5
5
5
5
5
5
5
)
4
)
4
)
4
)
4
)
4
)
4
)
4
)
4
)
4
)
4
)
4
e
f
9
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
f
2 6 3
In addition to Cat-1, another gold isocyanide (2,6-Me C H -
NC)AuCl (Cat-2) was also investigated and its catalytic ability
was compared with that of Cat-1 (Fig. 1). Though the activity of
Cat-2 is a little weaker than that of Cat-1, the reaction catalyzed
by Cat-2 can still proceed quite well at room temperature
Dioxane
Acetone
THF
0.8
57.0
None
a
Reaction conditions: phenylacetylene (0.5 mmol), solvent (0.5 ml), (Table S1 in ESI†).
b
2
H O (0.5 ml), room temperature, 24 h. The catalyst amount is based
c
Another route to synthesize these ketones is the catalytic
on the molar percent to 1a. Conversion of 1a determined by GC.
4
5–53
d
e
aerobic oxidation of alcohols.
It should be noticed that
Methanol (0.5 ml) and H
2
O (1.0 ml). Methanol (1.0 ml) and H
2
O
f
(0.5 ml). Methanol (2.0 ml) and H O (0.5 ml).
2
the only by-product is water for an ideal oxidation process, and
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a
Table 2 Hydration of various alkynes catalyzed by Cat-1
b
Entry
1
Reactant
Product
Yield (%)
>99
2
3
>99 (96)
>99
4
5
>99 (95)
>99 (97)
63
Fig. 1 Kinetic plot of conversion of 5a with respect to time under stan-
dard conditions.
c
6
some of these oxidations can be performed at room
4
5–49
48,49
temperature.
bases
However, NaNO2
or excess amounts of
4
5–47,50,51
are usually indispensable in these aerobic oxi-
c
d
7
8 [5]
dations. Though some of the alcohols used in the oxidation
routes are cheaper than the alkynes in the hydration routes,
the following characteristics of the route reported here make
itself a more green choice for the synthesis of ketones: 100%
atomic economy, mild reaction conditions, an environmentally
preferable solvent, no need of acids or bases.
8
9
>99 (99)
>99 (95)
>99 (98)
Hydration mechanism of the reaction catalyzed by gold
isocyanide
1
0
It has been found in the literature that alkynes with methanol
as a solvent can be hydrated to the expected ketones via vinyl
1
1,12
ether intermediates.
However, for the hydration catalyzed
1
1
1
1
1
2
3
4
>99
by the gold isocyanide complexes (Cat-1 and Cat-2), no signal
of vinyl ether intermediates was detected by GC-MS analysis.
Besides, the reaction still can perform well without the
addition of methanol (entries 13 and 14, Table 1). These two
>99 (99)
>99 (98)
97
evidences mentioned above imply a direct hydration of alkynes
7
by H
2
O. To further understand the hydration mechanism
catalyzed by the gold isocyanide, a DFT based theoretical
investigation was performed (see ESI† for computational
details) (Fig. 2). According to the known knowledge of other
4
1
Au(I) catalysts and results listed in Table 1, the active center
+
+
in the reaction should be the (Cy–NuC)Au (Cat-1 ) cation.
1
5
6
>99 (99)
97 (95)
+
The binding between Cat-1 and 1a is found to be highly
exothermic (−47.3 kcal mol− ). With the assistance of Cat-1 ,
1
+
c
c
1
H O can attack 1a via a transition state TS1 (Fig. 2). The
2
barrier of TS1 is 23.7 kcal mol− . The CuC triple bond of 1a
1
becomes almost a double bond in TS1 (length increasing from
1
7
98 (97)
1.212 to 1.342 Å). An enol form of the intermediate (Int1 or
Int1′) can be produced via TS1 (Fig. 2 and 3). After that, enol–
keto isomerisation via a proton transfer process (TS2 or TS2′)
takes place to generate the final product with a total energy
a
Reaction conditions: alkyne (1.0 mmol), Cat-1 (0.05 mmol), KB(C
O (1.0 ml), methanol (1.0 ml), room temperature,
4 h. Yield determined by GC with biphenyl as the internal standard.
6 5 4
F )
(0.05 mmol), H
2
−1
change of −45.0 kcal mol . However, the barrier of TS2 or
b
2
TS2′ is too high (36.5 or 37.6 kcal mol− ) to support the room
1
c
Isolated yields are shown in parenthesis. 10% mol catalyst, 48 h.
d
Reaction time: 48 h [24 h].
temperature hydration of 1a. Considering that the microenvir-
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Fig. 3 Water-assisted enol–keto isomerisation in the hydration of
phenylacetylene. The values in the reaction pathway are in kcal mol−
1
.
Fig. 2 Proposed mechanism of the hydration of phenylacetylene. The
values in the reaction pathway are in kcal mol . The other values are
bond lengths in Å.
The other values are bond lengths in Å.
−1
onment of the reaction is full of water, a water assisting enol–
5
4–58
keto isomerisation process should be more feasible.
In
this process, the proton of C–O–H transfers to an explicit H O,
2
2
and meanwhile, one proton of this explicit H O transfers to
the terminal carbon of the CvC bond. In this case, the tran-
sition from the Int1′ hydrate (Int1′-W) to the final product only
−
1
needs to overcome a quite low barrier (22.5 kcal mol of TS2′-
W). Of course, if there are two or more explicit water molecules
to join the process and assist the proton transfer, the barrier
should be further reduced to smaller than 22.5 kcal
−
1 54–58
mol
.
Thus, the rate-determining step of this hydration
process should be H O attacking the CuC triple bond with a
2
−1
barrier of 23.7 kcal mol (Fig. 2).
Another interesting pathway for the reaction of the terminal
alkyne using the gold catalyst as gold(I) may react with the
terminal alkyne to produce a gold vinylidene (LAuvCvCR )
2
intermediate and this intermediate can further react with
other functional groups, such as the cycloisomerization of
5
9,60
alkyne.
reacting with H
found that the addition of H O to Rea″ is not easy because of a
The possibility of similar gold vinylidene (Rea″)
2
O was further investigated (Fig. 4). It was
2
−1
quite high barrier of TS1″ (51.4 kcal mol ). This energy
barrier is 27.7 kcal mol higher than that of TS1. Further-
more, the energy barrier for the proton transfer from C–O–H reaction pathway are in kcal mol . The other values are bond lengths in Å.
−
1
2
Fig. 4 Reaction between gold vinylidenes and H O. The values in the
−1
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−
1
to the terminal carbon is 45.9 kcal mol (TS2″), which is
9
8 F. X. Zhu, W. Wang and H. X. Li, J. Am. Chem. Soc., 2011,
133, 11632–11640.
.4 kcal mol− higher than that of TS2. Hence, for the
1
hydration of alkynes catalyzed by gold(I) isocyanide com-
pounds, the reaction mechanism is more likely to be the
9 F. Chevallier and B. Breit, Angew. Chem., Int. Ed., 2006, 45,
1599–1602.
pathway presented in Fig. 2 and 3. The barrier height of the 10 T. Tachinami, T. Nishimura, R. Ushimaru, R. Noyori and
rate-determining step for the pathway in Fig. 2 and 3 is only
H. Naka, J. Am. Chem. Soc., 2013, 135, 50–53.
3.7 kcal mol− , and this barrier height is reasonable for the 11 P. Nun, R. S. Ramon, S. Gaillard and S. P. Nolan, J. Organo-
1
2
6
1
reaction to proceed readily at room temperature.
met. Chem., 2011, 696, 7–11.
12 N. Marion, R. S. Ramon and S. P. Nolan, J. Am. Chem. Soc.,
2
009, 131, 448–449.
13 N. Marion, R. Gealageas and S. P. Nolan, Org. Lett., 2008,
0, 1037–1037.
Conclusions
1
In summary, the hydration of different alkynes was success-
fully performed at room temperature in the presence of gold
isocyanide complexes. The substrates investigated here include
electron-rich and electron-deficient alkynes, aromatic and
14 W. L. Wang, A. M. Zheng, P. Q. Zhao, C. Xia and F. W. Li,
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and A. Presa-Soto, Eur. J. Inorg. Chem., 2011, 9, 1442–
+
non-aromatic alkynes. The isocyanide Au(I) cation is extra-
1
447.
6 N. A. Romero, B. M. Klepser and C. E. Anderson, Org. Lett.,
012, 14, 874–877.
7 G. A. Fernandez, A. S. Picco, M. R. Ceolin, A. B. Chopa and
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8 E. Mizushima, K. Sato, T. Hayashi and M. Tanaka, Angew.
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9 X. Y. Xu, S. H. Kim, X. Zhang, A. K. Das, H. Hirao and
S. H. Hong, Organometallics, 2013, 32, 164–171.
0 C. E. Czegeni, G. Papp, A. Katho and F. Joo, J. Mol. Catal. A:
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1 P. de Fremont, R. Singh, E. D. Stevens, J. L. Petersen and
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2 A. Almassy, C. E. Nagy, A. C. Benyei and F. Joo, Organo-
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3 R. Casado, M. Contel, M. Laguna, P. Romero and S. Sanz, J.
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5 S. Coco, C. Cordovilla, P. Espinet, J. Martin-Alvarez and
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6 M. Benouazzane, S. Coco, P. Espinet, J. M. Martin-Alvarez
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polated to be the active center that shows quite high catalytic
ability for most of the substrates. Theoretical research further
reveals the hydration mechanism which includes two steps:
H O attacking the CuC triple bond and explicit H O assisting
1
1
1
1
2
2
2
2
2
2
2
2
2
2
2
2
the proton transfer to the CvC double bond.
2
9–39
Au-RNC, as a precursor for the synthesis of Au-NHC,
shows comparable catalytic ability to Au-NHC in the hydration
of alkynes. This is a rare example that Au-RNC is used as the
catalyst directly to obtain satisfactory hydration results. Con-
sidering the simple structure and high reactivity of gold iso-
cyanide, it deserves to receive more attention in other reactions.
Because gold isocyanide is a homogeneous catalyst, developing
a method to recycle it will bring a more green process. Sup-
6
2
63–65
porting the gold isocyanide on zeolite or ionic liquids
will be an effective method to reuse this compound.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (no. 21176110 and 21376115) and
Jiangsu Province Natural Science Foundation (BK20141311).
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