Hydration of terminal alkynes catalyzed by a water-soluble salen-Co(III) complex

Article abstract of DOI:10.1016/S1872-2067(14)60105-4

A water-soluble salen-Co(III) complex was studied as catalyst for hydration of terminal alkynes to methyl ketones in the presence of H₂SO₄ as a co-catalyst. The products were obtained with excellent yields using relatively low catalyst loadings and a simple protocol. Notably, the products were easily separated from the catalyst after reaction by extraction, and the catalyst could be recovered and reused with only a slight loss of activity.

Full text of DOI:10.1016/S1872-2067(14)60105-4

Chinese Journal of Catalysis 35 (2014) 1695–1700
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Hydration of terminal alkynes catalyzed by a water‐soluble
salen‐Co(III) complex
Shoufeng Wanga,b, Chengxia Miaob, Wenfang Wangb, Ziqiang Leia,*, Wei Sunb,#
a Laboratory of Eco‐Environment‐Related Polymer Materials of Ministry of Education, Key Laboratory of Polymer Materials of Gansu Province, College of
Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, Gansu, China
b State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000,
Gansu, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A water‐soluble salen‐Co(III) complex was studied as catalyst for hydration of terminal alkynes to
methyl ketones in the presence of H2SO4 as a co‐catalyst. The products were obtained with excellent
yields using relatively low catalyst loadings and a simple protocol. Notably, the products were easily
separated from the catalyst after reaction by extraction, and the catalyst could be recovered and
reused with only a slight loss of activity.
Received 20 March 2014
Accepted 11 April 2014
Published 20 October 2014
© 2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
Keywords:
Hydration
Alkynes
Water‐solubility
Cobalt
Salen complex
1. Introduction
tion are based on expensive transition metals. Separation of
products from these catalytic systems requires tedious proce‐
Addition of water to alkynes is very important in organic
synthesis because, in principle, addition reactions can be per‐
formed with 100% atom efficiency. This means there is no
waste formation and the reactions fulfill the requirements of
green chemistry. The traditional method for alkyne hydration
involves catalysis with a Hg(II) salt, and selectivity follows Mar‐
kovnikov’s rule yield methyl ketones [1–7]. This method is still in
use today because it is reliable. However, over the past 30 years,
metal catalysts with low toxicity and high activity, such as Pt [8],
Fe [9,10], Pd [11], Ir [12], Ag [13–15], Au [16–18], Ru [19–24],
have been investigated to replace Hg(II). In general, the more
active and selective of the available catalysts for alkyne hydra‐
dures. Very recently, Naka group [25] developed a novel method
for hydration of terminal alkynes catalyzed by a recyclable, wa‐
ter‐soluble porphyrin‐Co(III) complex. Compared with hetero‐
geneous analogues, soluble homogeneous supports provide in‐
creased solvent compatibility and accelerated reaction rates.
Inspired by pioneering research in this area and the contin‐
ued interest in catalysis with salen metal complexes [26–30],
we investigated a water‐soluble salen‐Co(III) complex as a cat‐
alyst for hydration of terminal alkynes. This complex conven‐
iently catalyzed hydration of terminal alkynes in good yields.
After the reaction, the products were easily separated by ex‐
traction with an organic solvent, and the catalyst could be re‐
* Corresponding author. E‐mail: leiziqiang@hotmail.com
# Corresponding author. Fax: +86‐931‐8277088; E‐mail: wsun@licp.cas.cn
This work was supported by the National Natural Science Foundation of China (21103207, 21133011).
DOI: 10.1016/S1872‐2067(14)60105‐4 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 35, No. 10, October 2014

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Shoufeng Wang et al. / Chinese Journal of Catalysis 35 (2014) 1695–1700
used three times.
2. Experimental
2.1. General
Table 1
Screening of reaction conditions for the hydration of phenylacetylene.
O
H2O (4.4 eq)
80 oC, 20 h
2a
1a
Phenylacetylene derivatives and other commercially availa‐
ble chemicals were obtained from Alfa Aesar, Sigma‐Aldrich,
J&K Scientific Ltd, or Tokyo Chemical Industry Co., Ltd and used
as received without pretreatment unless otherwise stated.
Salen ligands and salen‐Co(III)‐OAc complexes C1 and C2 were
prepared according to reported methods [31]. The substituted
sulfonatosalen‐Co(III) complex C3 was synthesized by con‐
densation of the 1,2‐ethylenediamine with 5‐sulfonatosa‐
licylaldehyde and sequential addition of cobalt acetate in good
yield (see Scheme 1) [31,32]. 4‐Ethynylbiphenyl, 4‐ethynyl‐
1,2‐dimethoxybenzene, and 1‐ethynyl‐naphthalene were pre‐
pared according to literature procedures [33].
Entry
Catalyst
C1
C2
C3
C3
C3
C3
C3
C3
Solvent
Acid (%)
H2SO4 (2.0)
H2SO4 (2.0)
H2SO4 (2.0)
HCl (4.0)
CF3SO3H (4.0)
HNTf2 (4.0)
H2SO4 (2.0)
H2SO4 (2.0)
H2SO4 (2.0)
H2SO4 (2.0)
Yield a (%)
1
2
3
4
5
6
7
8
9
10
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
H2O
99
99
99
94
75
45
78b
5
91c, 89d, 83e
96f
C3
C3
CH3OH
CH3OH
Reaction conditions: phenylacetylene 0.5 mmol, catalyst 10 μmol
(2.0%), H2O 4.4 eq. (40 μL), solvent 0.625 mL, [Alkyne]0 = 0.8 mol/L, 80
°C, 20 h.
a The yield was determined by GC with nitrobenzene as an internal
standard.
b The reaction was carried out under an argon atmosphere.
c First run, isolated yield.
2.2. General procedure for hydration of terminal alkynes
The 10 mL schlenk tube was charged with phenylacetylene
(0.5 mmol, 51 mg), methanol (0.625 mL), catalyst (10 μmol,
2.0%), and then H2SO4 (10 μmol, 2.0%) dissolved in H2O (2.2
mmol, 0.04 mL). The mixture was heated to 80 °C and at it for
20 h in a closed tube with a magnetic stirring bar. The progress
of the reaction was monitored using TLC and GC‐MS. After the
reaction, the mixture was cooled to room temperature, and
CH2Cl2 (5 mL) and water (5 mL) were added to the mixture.
The aqueous and organic layers were separated, and the aque‐
ous phase was extracted with CH2Cl2 (5 mL  3). The combined
organic extracts were washed with a saturated NaCl solution,
dried over Na2SO4, and concentrated under reduced pressure.
Then the product acetophenone was abtained.
d Second run, isolated yield.
e Third run, isolated yield.
f Phenylacetylene 10.0 mmol, isolated yield.
salen‐Co(III) complex C3 was developed as an ideal catalyst for
the hydration of terminal alkynes. As expected, the hydration of
phenyl acetylene to the corresponding acetophenone proceed‐
ed uneventfully in the presence of the water‐soluble
salen‐Co(III) complex C3 (2.0%) and 2.0% H2SO4 (Table 1, en‐
triey 3). Preliminary results indicated that including an acid as
a co‐catalyst greatly influenced the hydration, but among the
acids tested, only HCl provided a comparable yield to H2SO4
(Table 1, entries 3–6). As Naka group reported [25], alkyne
hydration was even faster in the presence of HNTf2. However,
for the water‐soluble salen‐Co(III) complex, HNTf2 could not
promote the hydration efficiently. In addition, when the reac‐
tion was carried out under an Ar atmosphere, the ketone yield
decreased (Table 1, entry 7). As reported by Naka group [25],
methanol was an excellent solvent for the hydration of terminal
alkynes catalyzed by water‐soluble porphyrin‐Co(III) com‐
plexes (Table 1, entry 3 vs 8).
3. Results and discussion
3.1. Optimization of reaction conditions
As we previously reported, the salen‐Co(III) complex C1 and
C2 could catalyze the hydration of terminal alkynes in the
presence of H2SO4 (Table 1, entries 1 and 2) [26]. Taking into
account the problem of product isolation, a water‐soluble
N
O
N
O
N
O
N
O
N
O
N
O
Co
Co
Co
NaO3S
SO3Na
OAc
OAc
OAc
C1
C2
C3
(C1, C2, C3)/H+
O
R
R
CH3OH, H2O
Scheme 1. Hydration of alkynes catalyzed by salen‐Co(III) complexes.

Shoufeng Wang et al. / Chinese Journal of Catalysis 35 (2014) 1695–1700
1697
Table 2
3.2. Catalyst recyclability and a gram‐scale hydration
Substrate scope for alkyne hydration catalyzed by C3.
Entry
Substrate
Product
Yielda (%)
An important feature of this substituted sulfonato‐sal‐
en‐Co(III) complex‐catalyzed hydration of terminal alkynes is the
facile separation of the catalyst and the product. After every hy‐
dration reaction, the solution pH was adjusted to 7 with a NaOH
solution, the cooled mixture was partitioned between water and
n‐hexane or dichloromethane, and the organic layer was sepa‐
rated. The water‐soluble Co complex in the aqueous layer was
recovered by evaporating water, and the complex was then re‐
used for further reactions. The catalyst C3 could be reused three
times with only a slight loss of activity (Table 1, entry 9). To
evaluate the scalability of the reaction, a gram‐scale hydration of
phenylacetylene (10 mmol) was carried out under the optimal
conditions (Table 1, entry 10). The hydration proceeded
smoothly with acetophenone obtained in 96% yield.
O
R
R
1
2
3
4
5
6
7
8
1a, R = H
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
—
O
91
95
96
94
98
97
97
96
1b, R = p‐Me
1c, R = m‐Me
1d, R = o‐Me
1e, R = p‐tBu
1f, R = p‐Ph
1g, R = p‐OMe
1h, R = 3,4‐OMe
1i, R = p‐F
9
65, 96b
51, 91b
54, 90b
10
11
12
1j, R = p‐Cl
1k, R = p‐Br
1m, R = p‐CN
b
—
3.3. Substrate scope
13
96b
98b
To explore the synthetic utility of the catalyst, we investi‐
gated the substrate scope under the optimized conditions. A
series of aromatic alkynes with different substituent groups on
the phenyl ring, heteroaromatic alkynes, and aliphatic alkynes
were subjected to this hydration process with catalyst C3 (Ta‐
ble 2). The results clearly demonstrated that the reaction was
influenced by the electronic effect. Aromatic alkyne derivates
with electron‐donating substitutents, such as methyl, t‐butyl,
phenyl, and methoxy, reacted smoothly to afford the desired
methyl ketones in high yields (Table 2, entries 2–8). However,
those with electron‐withdrawing substituents, such as –F, –Cl,
or –Br groups, showed decreased conversion. Interestingly,
when the fraction of H2SO4 was increased from 2% to 10%, the
effectiveness of the cobalt catalyst for these substrates also
increased (Table 2, entries 9–11). By contrast, for phenylacety‐
lene with a strong electron‐withdrawing group (–CN), no hy‐
dration took place (Table 2, entry 12). Heterocyclic aromatic
alkynes were also efficiently hydrated in this reaction (Table 2,
entries 14 and 15). For example, the reaction for 4‐ethyny‐
lpyridine proceeded smoothly under typical conditions, af‐
fording 4‐acetylpyridine in 98% yield (Table 2, entry 14). Ali‐
phatic alkyne such as 1‐decyne was less active than aromatic
alkynes, with only 37% yield (Table 2, entry 16). The hydration
product was not obtained with the simple internal alkyne
3‐phenyl‐1‐ propyne (Table 2, entry 17).
O
14
15
N
N
O
O
O
95c
37c
16
17
O
b
—
—
Reaction conditions: substrate 0.5 mmol, catalyst 10 μmol (2.0%),
H2SO4 10 μmol (2.0%), H2O 4.4 eq. (40 μL), CH3OH 0.625 mL ([Alkyne]0
= 0.8 mol/L), 80 °C, 20 h.
a Isolated yield.
b H2SO4 50 μmol (10%).
c The yield was determined by GC.
d 50 °C for 20 h, the yield was determined by GC.
complex and phenylacetylene. Taking into account our previ‐
ous work and other reports [30,34], the Co(III) alkyne complex
intermediate is most likely then attacked by methanol to gen‐
erate enol ether in the presence of the acid co‐catalyst (Scheme
2(1)). The enol ether will then be transformed into the desired
ketone in the presence of the acid co‐catalyst and water under
acidic conditions (Scheme 2(2)).
N
O
N
O
3.4. Mechanism
N
O
N
Co
+
O
Co
To gain insight into the mechanism of Co‐catalyzed hydra‐
tion of alkynes, quadrupole‐time‐of‐flight‐mass spectrometry
was used to investigate the hydration of terminal alkynes.
Salen‐Co(III) complex C1 was used as the catalyst. Phenyla‐
cetylene (0.5 mmol) and catalyst C1 (2%) in methanol(0.6 mL)
were stirred at 50 °C for 1 h. After this reaction, peaks appeared
at Mr/z = 325.0379 and 449.0652, and could be assigned to
[salen‐Co(III)]+ and [salen‐Co(III)‐phenylacetylene+Na]+ (Fig.
1), respectively. These results indicate that a cobalt(III) alkyne
complex intermediate was formed between the salen‐Co(III)
+
Na
Mr/z
Fig. 1. HRMS spectra of the reactants. Phenylacetylene 0.5 mmol, C1
2%, methanol 0.6 mL, 50 °C, 1 h.

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Shoufeng Wang et al. / Chinese Journal of Catalysis 35 (2014) 1695–1700
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5816
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CH3
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Scheme 2. Proposed mechanism for salen‐Co(III) catalyzed hydration.
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In conclusion, a water‐soluble salen‐Co(III) complex was
developed for the hydration of terminal alkynes using H2SO4 as
a co‐catalyst. This catalyst transformed various alkynes to me‐
thyl ketones in good to excellent yields. The products could be
isolated by simple extraction from the reaction mixture, which
avoided column chromatography. The catalyst could be reused
three times with only a slight loss of activity for the reaction.
Overall, this water‐soluble salen‐Co(III) complex catalyst
makes the hydration of terminal alkynes very practical. Appli‐
cations of this water‐soluble salen‐Co(III) complex to other
reactions and further experimental are ongoing in our labora‐
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Graphical Abstract
Chin. J. Catal., 2014, 35: 1695–1700 doi: 10.1016/S1872‐2067(14)60105‐4
Hydration of terminal alkynes catalyzed by water‐soluble salen‐Co(III) complex
Shoufeng Wang, Chengxia Miao, Wenfang Wang, Ziqiang Lei*, Wei Sun*
Northwest Normal University; Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences
N
O
N
O
Co
OAc
NaO3S
SO3Na
H2O
R
O
+
R
water-soluable cobalt(III)-salen complex
H+, CH3OH
R = aryl, alkyl
15 examples
up to 98% yield
A water‐soluble sulfonato‐Co(salen) complex was studied as a catalyst for hydration of terminal alkynes to give methyl ketones in high
yields. The product can be easily separated from the catalyst by extraction using water and an organic solvent, and the catalyst can be
recovered and reused.