Task-specific ionic liquid and CO2-cocatalysed efficient hydration of propargylic alcohols to α-hydroxy ketones

Article abstract of DOI:10.1039/c5sc00040h

The hydration of propargylic alcohols is a green route to synthesize α-hydroxy ketones. Herein a CO₂-reactive ionic liquid (IL), [Bu₄P][Im], was found to display high performance for catalyzing the hydration of propargylic alcohols in the presence of atmospheric CO₂, and a series of propargylic alcohols could be converted into the corresponding α-hydroxy ketones in good to excellent yields. In the IL/CO₂ reaction system, CO₂ served as a cocatalyst by forming α-alkylidene cyclic carbonates with propargylic alcohols, and was released via the rapid hydrolysis of the carbonates catalysed by the IL. This is the first example of the efficient hydration of propargylic alcohols under metal-free conditions. This journal is

Full text of DOI:10.1039/c5sc00040h

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Task-specific ionic liquid and CO2-cocatalysed
efficient hydration of propargylic alcohols to a-
hydroxy ketones†
Cite this: DOI: 10.1039/c5sc00040h
Yanfei Zhao, Zhenzhen Yang, Bo Yu, Hongye Zhang, Huanjun Xu, Leiduan Hao,
*
Buxing Han and Zhimin Liu
The hydration of propargylic alcohols is a green route to synthesize a-hydroxy ketones. Herein a CO2-
reactive ionic liquid (IL), [Bu4P][Im], was found to display high performance for catalyzing the hydration
of propargylic alcohols in the presence of atmospheric CO2, and a series of propargylic alcohols could
be converted into the corresponding a-hydroxy ketones in good to excellent yields. In the IL/CO2
reaction system, CO2 served as a cocatalyst by forming a-alkylidene cyclic carbonates with propargylic
alcohols, and was released via the rapid hydrolysis of the carbonates catalysed by the IL. This is the first
example of the efficient hydration of propargylic alcohols under metal-free conditions.
Received 6th January 2015
Accepted 15th January 2015
DOI: 10.1039/c5sc00040h
www.rsc.org/chemicalscience
indicates that there is no report on the hydration of propargylic
alcohols to a-hydroxy ketones under metal-free conditions.
Introduction
Ionic liquids (ILs), possessing unique features such as high
thermal and chemical stability, negligible vapor pressure, and
tunable properties, have been applied in many areas.6 Particu-
larly, task-specic ILs have displayed superior performances in
catalysis (e.g., hydrolysis reactions, CO2 conversion) and gas
capture via the careful design and selection of novel component
ions to endow unique properties upon them.6f,g,7 For example,
CO2-reactive ILs have shown excellent performances for the
capture and conversion of CO2 under mild conditions.6b,7a The
supported basic IL [Emim][HCO3À] exhibits a high activity
for catalyzing the hydrolysis of propylene carbonate to 1,2-
propylene glycol.6f
The selective addition of water to unsaturated bonds is of
paramount importance in the production of building blocks for
the synthesis of specialty chemicals.1 Especially the hydration of
acetylenic compounds, involving simple addition of a water
molecule with 100% atom efficiency for generating carbonyl
compounds, has received much attention in the past decades,2
notably with the requirements of green chemistry and sustain-
able development. The key issue of the hydration of alkynes
relies on the effective activation of the carbon–carbon triple
bond, followed by the rapid addition of a water molecule. So far,
a variety of metal (e.g., Fe, Au, Ag, Ru)-based catalysts have been
developed to replace traditional toxic Hg(II) catalysts for the
hydration of alkynes, where cocatalysts such as strong acids and
organic ligands are generally required and/or side reactions
including the Meyer–Schuster and Rupe rearrangements
usually occur in these catalytic systems.3 The hydration of
propargylic alcohols is an efficient and green route to produce
a-hydroxy ketones, which are important building blocks for
more elaborate molecules;3a–c,4 however, it is very difficult to do
this under mild and metal-free conditions. Recently, Qi et al.
reported a CO2-promoted route for the hydration of propargylic
alcohols to a-hydroxy ketones using silver acetate (Ag2COO) and
1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as the catalysts at
120 ꢀC and under high CO2 pressure.5 Our literature survey
Herein, we report the hydration of propargylic alcohols to a-
hydroxy ketones cocatalysed by task-specic ILs and CO2. The
structures of all the ILs used are given in Scheme 1 and in the
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid,
Interface and Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, China. E-mail: liuzm@iccas.ac.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5sc00040h
Scheme 1 The CO2-reactive ILs used.
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ESI.† It was discovered that the IL [Bu4P][Im] was able to of these ILs was signicantly affected by their anions, and may
catalyse the hydration of propargylic alcohols in the presence of be ascribed to the nucleophilicity of the anions of these ILs as a
CO2 at atmospheric pressure, and various propargylic alcohols result of their different basicity.7a
could be transformed into the corresponding a-hydroxy ketones
[Bu-DBU][Im] and [Bmim][Im], with the same anion as
in good to excellent yields. Moreover, it was found that both CO2 [Bu4P][Im], gave no product (Table 1, entries 8 and 9), suggesting
and [Bu4P][Im] were indispensable for the hydration reactions, that the cations of the ILs affected the activities of the catalysts
and they cocatalyzed these reactions efficiently. In addition, the more signicantly that the anion. In addition, [Bu4P][Br], and
IL catalyst could be easily recovered and reused without [Bu4P][NO3] with the same cation as [Bu4P][Im] also showed no
considerable activity loss. To the best of our knowledge, this is activity (Table 1, entries 10 and 11), which may be ascribed to
the rst example of the efficient hydration of propargylic alco- the very weak interactions between their anions and CO2. From
hols under mild and metal-free conditions. Meanwhile it is also the above results, it can be concluded that both CO2 and the
the rst time it has been found that CO2 can serve as a task-specic ILs were indispensable, and their synergistic
cocatalyst.
effects resulted in the formation of the nal products. In addi-
tion, the chemical structures (both cations and anions) of the ILs
played important roles in the hydration of 2-methylbut-3-yn-2-ol.
Encouraged by the above results, the generality of [Bu4P]-
[Im]/CO2-catalyzed hydration reactions of diverse propargylic
alcohols was evaluated. The results indicated that most reac-
tions proceeded smoothly, producing corresponding a-hydroxy
ketones in good to excellent yields under experimental condi-
tions (Table 2, entries 1–15). For example, 3-methyl-1-pentyn-3-
ol afforded the corresponding product in a yield of 77% within
24 h (Table 2, entry 2), comparable to that reported previously
using Ag2COO and DBU as catalysts at 120 ꢀC and 2 MPa of CO2
pressure (Table 2, entry 16).5 Moreover, prolonging the reaction
time to 48 h, the product yield reached 93% (Table 2, entry 3).
Notably, the steric effects of the substituents in the substrates
had signicant effects on their activities in the formation of the
corresponding a-hydroxy ketones. This was conrmed by the
fact that the product yields decreased when the lengths of the
substituent chains in the propargylic alcohols increased (Table
2, entries 1, 2, 4, and 6). Moreover, large scale reactions of
substrates including 3-methyl-1-nonyn-3-ol, 2-phenyl-3-butyn-2-
ol and 2-methyl-4-phenyl-3-butyn-2-ol were carried out, and the
corresponding a-hydroxy ketones were isolated in yields of 84%,
87%, and 85%, respectively (Table 2, entries 8, 12 and 15,
Results and discussion
The hydration of 2-methylbut-3-yn-2-ol was carried out both in
the absence and presence of ILs, and the results are listed in
Table 1. It was demonstrated that this reaction did not occur in
the absence of CO2 and/or [Bu4P][Im] (Table 1, entries 1–3).
Excitingly, [Bu4P][Im] allowed the desired hydration reaction to
proceed efficiently in the presence of CO2 at atmospheric
pressure (Table 1, entry 4), solely producing 3-hydroxy-3-methyl-
2-butanone in a yield of 92% (entry 4). For comparison, the
other seven ILs, including [Bu4P][Triz], [Bu4P][BenzIm], [Bu4P]-
[BenzTriz], [Bu4P][Br], [Bu4P][NO3], [Bu-DBU][Im], and [Bmim]-
[Im], were examined for catalysis of this reaction. The results
indicated that [Bu4P][Triz], [Bu4P][BenzIm] and [Bu4P][Benz-
Triz] were also effective (Table 1, entries 5–7), but showed lower
activities compared to [Bu4P][Im], while the other ILs exhibited
no activity. The signicant differences in the chemical struc-
tures of these ILs may be responsible for their catalytic perfor-
mances. [Bu4P][Triz], [Bu4P][BenzIm] and [Bu4P][BenzTriz], with
the same cation as [Bu4P][Im], afforded lower product yields,
following the order: [Bu4P][BenzTriz] < [Bu4P][Triz] < [Bu4P]-
[BenzIm] < [Bu4P][Im]. This indicated that the catalytic activity
a
Table 1 Hydration of propargylic alcohol promoted by different IL catalysts in the presence of CO2
Entry
Catalyst
CO2 pressure (MPa)
Anion pKa in DMSO7a
Yieldb [%]
1
—
—
—
—
—
—
18.6
16.4
13.9
11.4
—
0
0
0
92
83
82
39
0
2c
3
[Bu4P][Im]
—
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
4
5
6
7
8
9
[Bu4P][Im]
[Bu4P][BenzIm]
[Bu4P][Triz]
[Bu4P][BenzTriz]
[Bu-DBU][Im]
[Bmim][Im]
[Bu4P][Br]
[Bu4P][NO3]
—
—
—
0
0
0
10d
11
a
Reaction conditions: substrate (1 mmol), H2O (2 mmol), IL (3 mmol), 353 K, 24 h. b Determined by 1H NMR ([D6]DMSO) using tert-butyl alcohol as
an internal standard. c Under N2 atmosphere. d H2O (0.5 mL) was added to the reaction mixture.
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Table 2 Hydration of various propargylic alcohols over the [Bu4P][Im]/
(Table 2, entry 17; Fig. S8†), suggesting that [Bu4P][Im] was
stable and the designed catalytic system was recyclable.
CO2 catalysta
The above results indicate that CO2 plays an important role
in the hydration of various propargylic alcohols. To explore the
reaction mechanism, the IL, the IL exposed to CO2, and the
reaction solution for 2-methylbut-3-yn-2-ol hydration in the
presence of atmospheric CO2 at 80 ꢀC for 7 h were examined by
NMR analysis. As illustrated in Fig. 1, a new signal appeared at
161.6 ppm in the 13C NMR spectrum of the IL exposed to
atmospheric CO2, which was attributed to the carbonyl carbon
of carbamate, indicating that CO2 could react with the anion
[ImÀ] of the IL to form an intermediate ([Bu4P][ImCOO]), in
agreement with previous reports.7a This signal also appeared at
161.5 ppm in the spectrum of the reaction solution, suggesting
that in the reaction process the IL could activate CO2 to form a
[Bu4P][ImCOO] intermediate. The carbamate anion moiety in
the [Bu4P][ImCOO] intermediate may attack the triple bond in
2-methylbut-3-yn-2-ol,8a thus facilitating the conversion of this
propargylic alcohol. The 13C NMR analysis indicated that
besides 2-methylbut-3-yn-2-ol and 3-hydroxy-3-methyl-2-buta-
none, the species from CO2 and the anion of the IL were
detectable in the reaction solution. This implies that the IL
served as a catalyst for activating CO2 in the reaction process,
Entry
1
Substrate
Time/h
24
Product
Yield/%
92
2
3
24
48
77
93
4
5
40
24
62
86b
6
7
8
40
40
40
51
90b
84c
9
48
90
10
11
12
40
24
24
68
90b
87c
and the resultant [Bu4P][ImCOO] intermediate played
a
13
48
88b
14
15
48
48
90b
85c
16
24
24
78d
88e
17
a
Reaction conditions: substrate (1 mmol), [Bu4P][Im] (3 mmol), CO2
(0.1 MPa), H2O (2 mmol), 353 K. Yield was determined by 1H NMR
([D6]DMSO) using tert-butyl alcohol as an internal standard. CO2 (1
MPa). Isolated yield, substrate (7 mmol), [Bu4P][Im] (21 mmol), CO2
(1 MPa), H2O (14 mmol). Substrate (0.5 mmol), AgOAc (10% mol),
b
c
d
DBU (0.25 mmol), CO2 (2 MPa), H2O (0.3 mL), MeCN (1 mL), 393 K.
e
[Bu4P][Im] was used for the h time.
respectively), showing that the present system has great
potential for applications. In addition, 3-butyn-2-ol, 3,3-
dimethyl-1-butyne and 3-chloro-3-methylbut-1-yne were also
employed as substrates; however, the hydration reactions did
not occur, and no products were obtained in these cases. This
suggests that the presence of an –OH linked to the C next to the
alkynyl C is necessary for the hydration of propargylic alcohols.
To explore the reusability of the IL catalyst, ve catalytic
cycles of 2-methylbut-3-yn-2-ol hydration were performed over
the [Bu4P][Im] catalyst in the presence of CO2. It was demon-
strated that the product yield almost remained unchanged
Fig. 1 13C NMR spectrum of pure [Bu4P][Im], the intermediate of
[Bu4P][Im] exposed to CO2 (0.1 MPa), and the reaction solution of 2-
methylbut-3-yn-2-ol hydration in the presence of atmospheric CO2 at
80 ꢀC for 7 h (D2O, 0.6 mL, 298 K, using DMSO as the internal
standard).
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signicant role in the formation of nal product. Considering IL; rapid hydrolysis of intermediate d catalysed by [Bu4P][Im]
that [Bu-DBU][Im] and [Bmim][Im] showed no activity for the produces e, which subsequently converts to the product
hydration of 2-methylbut-3-yn-2-ol (Table 1, entries 8 and 9), 13
C
a-hydroxy ketone via keto–enol tautomerization and the release
NMR analyses of [Bu-DBU][Im] and [Bmim][Im] exposed to of CO2. In this reaction process, CO2 is involved in the forma-
atmospheric CO2 were performed, respectively, and no obvious tion of the key intermediates (i.e., a-alkylidene cyclic carbon-
peaks attributable to carbamates formed between the ILs and ates), and is rapidly released via the hydrolysis of these
CO2 were observed in the corresponding 13C NMR spectra, intermediates. CO2 is required, but not consumed in the whole
suggesting that these two ILs could not activate CO2 efficiently. reaction process, and plays a catalyst-like role in the formation
The steric hindrance around the P or N atoms in these ILs may of the nal product.
be responsible for their signicantly different performances for
In addition, it should be pointed out that CO2 was able to
activating CO2. From the above ndings, it can be further react with propargylic alcohols catalyzed by [Bu4P][Im] in the
deduced that in the [Bu4P][Im]/CO2 reaction system the [Bu4P]- absence of water, producing a-alkylidene cyclic carbonates at
[ImCOO] intermediate was crucial for the synthesis of the nal atmospheric pressure. This is a new metal-free catalytic route
product.
for the synthesis of a-alkylidene cyclic carbonates through
It was reported that CO2 could react with propargylic alco- coupling reactions of CO2 with propargylic alcohols under mild
hols to produce a-alkylidene cyclic carbonates catalyzed by conditions.
metal and base catalysts.8 The control experiment in this study
demonstrated that a-alkylidene cyclic carbonate could be
formed from CO2 and 2-methylbut-3-yn-2-ol catalyzed by [Bu4P]-
Conclusions
[Im] with a trace amount of H2O at atmospheric pressure, In summary, we have developed a green, metal-free and efficient
1
conrmed by H NMR analysis (see Fig. S9†). However, a-alky- method for the hydration of propargylic alcohols to generate
lidene cyclic carbonates were not detectable in the reaction a-hydroxy ketones using [Bu4P][Im]/CO2 as the catalytic system,
solutions for the hydration of various propargylic alcohols. which enables the reactions to proceed smoothly and to afford
Another control experiment, hydrolysis of a-alkylidene cyclic good to excellent yields of products. The IL and CO2 have an
carbonate in [Bu4P][Im], was performed. To our delight, this excellent synergistic effect on catalyzing the reactions, and CO2
cyclic carbonate was rapidly hydrolysed at 80 ꢀC within 1 h, serves as a cocatalyst. In addition, the multifunctional IL can be
affording 3-hydroxy-3-methyl-2-butanone in a yield approaching easily recovered and reused without obvious loss in its activity.
100%. The above ndings suggest that the a-alkylidene cyclic We believe that this kind of highly-efficient and greener CO2–IL
carbonate formed from CO2 reacting with 2-methylbut-3-yn-2-ol catalytic system has great potential for applications.
may be the key intermediate for the formation of 3-hydroxy-3-
methyl-2-butanone.
Acknowledgements
On the basis of the experimental results and previous
reports,5,8,9 a possible mechanism for IL and CO2-cocatalyzed The authors thank the National Natural Science Foundation of
hydration of propargylic alcohols was proposed, as shown in China (no. 21125314, 21321063, 21403252) for nancial support.
Scheme 2. CO2 is rst activated by the anion [ImÀ] to form
intermediate a, [Bu4P][ImCOO]. The nucleophilic O atom of
intermediate a attacks the triple bond of the propargylic alcohol
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