Non-corrosive heteropolyacid-based recyclable ionic liquid catalyzed direct dehydrative coupling of alcohols with alcohols or alkenes

Article abstract of DOI:10.1016/j.mcat.2019.02.019

A non-corrosive, recyclable and efficient heterogeneous catalyst based on Keggin-type polyoxometalate, i.e., [NMPH]H₃[SiW₁₂O₄₀] was developed for the direct dehydrative coupling of alcohols with alcohols (or alkenes) to synthesize various polysubstituted olefins in good to excellent yields. Furthermore, this reaction could be scaled up and the catalyst could be used for seven runs without significant loss of activity. The kinetic competition experiment shows that the C–H bond cleavage might be involved in the rate-determining step.

Full text of DOI:10.1016/j.mcat.2019.02.019

MolecularCatalysis468(2019)80–85
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Editor's choice paper
Non-corrosive heteropolyacid-based recyclable ionic liquid catalyzed direct
dehydrative coupling of alcohols with alcohols or alkenes
Guo-Ping Yanga,d, Nan Jianga, Xian-Qiang Huangb, Bing Yuc,⁎, Chang-Wen Hua,⁎
a Key Laboratory of Cluster Science of Ministry of Education, Beijing Key Laboratory of Photoelectronic/Electrophotonic, School of Chemistry and Chemical Engineering,
Beijing Institute of Technology, Beijing 100081, PR China
b School of Chemistry & Chemical Engineering, Liaocheng University, Liaocheng, PR China
c College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, Henan Province, PR China
d China Academy of Engineering Physics, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
A non-corrosive, recyclable and efficient heterogeneous catalyst based on Keggin-type polyoxometalate, i.e.,
[NMPH]H3[SiW12O40] was developed for the direct dehydrative coupling of alcohols with alcohols (or alkenes)
to synthesize various polysubstituted olefins in good to excellent yields. Furthermore, this reaction could be
scaled up and the catalyst could be used for seven runs without significant loss of activity. The kinetic com-
petition experiment shows that the C–H bond cleavage might be involved in the rate-determining step.
Heteropolyacid
Ionic liquid
Dehydrative coupling
Alcohol
Alkene
1. Introduction
acidic ionic liquids systems required stoichiometric amount of corrosive
acid such as TfOH and H2SO4. From the perspective of green and sus-
Conventional transition-metal-catalyzed coupling reactions gen-
erally employed organic halides as starting materials [1–10]. Accord-
ingly, waste was inevitably produced by using the stoichiometric
amount of base in these methods. On the other hand, the ubiquitous
hydroxy-containing substrates like aryl alcohols and phenols are useful
building blocks for organic synthesis due to their ready availability and
low toxicity. From the standpoint of green chemistry, the application of
alcohols as an economical alternative to organic halides in coupling
reactions is an attractive strategy, which only produce water as the sole
by-product [11–20].
The Bronsted acid-catalyzed reaction of 9-fluorenol with 9-alkyli-
denefluorenes was firstly reported by Wawzonek albeit with poor yields
[21]. Recently, more efficient procedures have been developed for the
dehydrative coupling of alcohols and terminal alkenes (or alcohols). For
example, the transition-metal (Cu, Fe, Pd, Ru, etc.)-based Lewis acids
[22–27], TfOH [28], iodine [29], supported acid catalyst i.e., NaHSO4/
SiO2 [30], and silica-supported policresulen [31] were found to be ef-
fective catalysts for this dehydrative reactions. On the other hand, the
application of ionic liquids in organic synthesis has drawn much at-
tention in the recent years [32–42]. Subsequently, the heterogeneous
acidic ionic liquid systems including SO3H-functionlization ionic liquid
[BsOdP][OTf] [43], Bronsted acidic ionic liquid [NMPH][HSO4] [44]
were established. Nevertheless, the preparation of these traditional
tainable chemistry [45–49], the previous reported systems remain to be
substantially improved to reduce the dosage for catalysis, shorten the
reaction time, and avoid the conventional corrosive acids for catalyst
preparation.
Following our recent development of polyoxometalate-based cata-
lytic systems for various reactions [50–53], we envisioned that the
application of inexpensive, noncorrosive, acidic polyoxometalate-based
catalysts could be a practical and green strategy for the dehydrative
coupling of alcohols and alcohol/alkenes. Although the Wells-Dawson
tungsten heteropolyacid H6P2W18O62 has been applied to catalyse the
dehydrative coupling of benzylic alcohols to generate the corre-
sponding alkenes [54], only moderate yields were observed and the
reusability of catalyst was not investigated. Herein, we disclose a
practical protocol for the dehydrative coupling of alcohols and alcohols
(or alkenes) under the mild conditions with the catalysis of hetero-
polyacid-based ionic liquid, i.e. [NMPH]H3[SiW12O40] (Scheme 1). This
catalyst could be easily recycled and reused at least for 7 times without
significantly loss in activity or selectivity.
⁎ Corresponding authors.
E-mail addresses: bingyu@zzu.edu.cn (B. Yu), cwhu@bit.edu.cn (C.-W. Hu).
https://doi.org/10.1016/j.mcat.2019.02.019
Received 3 December 2018; Received in revised form 14 February 2019; Accepted 18 February 2019
2468-8231/©2019ElsevierB.V.Allrightsreserved.

G.-P. Yang, et al.
MolecularCatalysis468(2019)80–85
Scheme 1. The heteropolyacid-based ionic liquid catalyzed dehydrative coupling.
250 mL flask, followed by the dropwise addition of aqueous solution of
12-silicotungstic acid (1 mmol), then it was further stirred at ambient
temperature for 12 h. Afterwards, the solvent was removed by rotary
evaporator and the residue solid was dried under vacuum at 80 °C for
12 h to obtain the final product [NMPH]4[SiW12O40].
Table 1
Optimization of the reaction conditions.a
[NMPH]3H[SiW12O40], [NMPH]2H2[SiW12O40], and [NMPH]
H3[SiW12O40] were prepared by similar procedures with their re-
spective stoichiometric compositions. i.e., 3 mmol, 2 mmol and 1 mmol
Entry
Catalyst
Solvent
Conv. (%)
Yieldb [%]
1
2
3
4
5
6
7
8
–
DCE
DCE
DCE
DCE
DCM
PhCl
Cyclohexane
CH3NO2
PhCH3
THF
CH3CN
DMF
15
> 99
97
> 99
> 99
> 99
> 99
> 99
> 99
98
0
of
N-methyl-2-pyrrolidinone,
respectively.
Accordingly,
H3PW12O40
H3PMo12O40
H4SiW12O40
H4SiW12O40
H4SiW12O40
H4SiW12O40
H4SiW12O40
H4SiW12O40
H4SiW12O40
H4SiW12O40
H4SiW12O40
H4SiW12O40
H4SiW12O40
40
36
76
71
52
33
29
18
5
1
0
0
98
[MImH]4[SiW12O40], [MPyH]4[SiW12O40], and [DBUH]4[SiW12O40
]
were prepared using 1-methylimidazole (MIm), 4-methylpyridine
(MPy), DBU and 12-silicotungstic acid as the raw materials with the
similar procedure. All of the ionic liquids were characterized and con-
sistent with those reported in the literature [55].
9
2.3. Typical procedure for direct dehydrative coupling of alcohols with
alcohols
10
11
12
13
14c
72
9
26
> 99
DMSO
DCE
To a 4 mL reaction vial, diphenylmethanol (0.6 mmol), α-phenethyl
alcohol (1.2 mmol), [NMPH]H3[SiW12O40
] (4 mol%) and 1,2-di-
a
chloroethane (3 mL) were added. Then the reaction was carried out in
screw cap vials with a Teflon seal at 80 °C for desired time. After cooling
to room temperature, the mixture was further purified by column
chromatography (petroleum ether/EtOAc) to afford the desired pro-
ducts.
Reaction conditions: 1a (0.2 mmol), 2a (0.21 mmol), catalyst (3 mol%),
solvent (1.0 mL), 80 °C, 1.5 h. DCE = 1,2-dichloroethane.
b
The conversions and yields were determined by GC with biphenyl as the
internal standard. The main by-product is benzhydrol ether.
c
Molar ratio: 1a/2a = 1/2.
2. Experimental
3. Results and discussion
2.1. Materials and methods
We initiated our study by examining the reaction of diphe-
nylmethanol 1a and 1-phenylethanol 2a in DCE as solvent at 80 °C for
1.5 h (Table 1). Obviously, the model reaction without any catalyst did
not afford the desired product 3a (Table 1, entry 1). By using different
Keggin-type heteropolyacid including H3PW12O40, H3PMo12O40, and
H4SiW12O40 as catalyst, the product 3a was obtained in yield of 40%,
The starting materials were commercially available and were used
without further purification. The products were isolated by column
chromatography on silica gel (200–300 mesh) using petroleum ether
(60–90 °C) and ethyl acetate. All compounds were characterized by 1H
NMR, 13C NMR and mass spectroscopy, which are consistent with those
reported in related literatures. NMR spectra were determined on Bruker
Ascend 400 in CDCl3. 1H NMR chemical shifts were referenced to re-
sidual solvent as determined relative to CDCl3 (7.26 ppm). The 13C
NMR chemical shifts were reported in ppm relative to the carbon re-
sonance of CDCl3 (central peak is 77.0 ppm). 1H NMR peaks are labelled
as singlet (s), doublet (d), triplet (t), and multiplet (m). The coupling
constants, J, are reported in Hertz (Hz). EIeMS data were performed on
Agilent 7890 A. GC analyses were performed on a Shimadzu GC-2014C
equipped with a capillary column (HP-5, 30 m × 0.25 μm) using a
flame ionization detector. Powder X-ray diffraction (PXRD) data on
samples were recorded on a Bruker instrument equipped with graphite-
monochromatized Cu Kα radiation (λ = 0.154060 nm; scan speed = 8°
min−1; 2θ = 10-50°) at room temperature. Scanning electron micro-
scopy (SEM) of the samples was obtained on a JSM-6490LV unit.
36%, and 76%, respectively (Table 1, entries 2–4). Given that the
4−
[SiW12O40
]
possess greatest softness among those heteropolyacid
anions [56,57], it is assumed that the softness of [SiW12O40
]
4- played an
important role in stabilizing the reaction intermediates. Then
H4SiW12O40 was applied as catalyst for the screening of different sol-
vents. The reactions in solvents like DCM, PhCl gave 3a in 52–71%
yields (Table 1, entries 5 and 6). While in cyclohexane, CH3NO2,
PhCH3, THF, CH3CN, DMF, and DMSO, even poor yields were achieved
for the dehydrative reaction (Table 1, entries 7–13). Therefore, DCE
was found to be the optimal solvent for the reaction of 1a and 2a using
H4SiW12O40 as catalyst. After extensive examination of the ratio of
reactants (see Table S1), it was pleased to find that when the molar
ration of 1a and 2a was 1:2, almost quantitative yield of 3a was ob-
tained with 3 mol% of H4SiW12O40 as catalyst in DCE at 80 °C for 1.5 h
(Table 1, entry 14).
The above-mentioned results suggested that the H4SiW12O40
showed high catalytic activity for the dehydrative coupling, however,
the homogeneous nature might hinder its practical applications because
it would be difficult to separate the catalyst from the reaction mixture
2.2. Synthesis of POM-based ionic liquids
N-Methyl-2-pyrrolidinone (NMP) (4 mmol) was charged into a
81

G.-P. Yang, et al.
MolecularCatalysis468(2019)80–85
Table 2
Table 3
Heterogenization of the heteropolyacid.a
Substrates scope of the coupling of alcohols and alcohols.a
Entry
1
1
2
Yield of 3b
Entry
Catalyst
Conv. (%)
Yieldb [%]
1
2
H4SiW12O40
[MImH]4[SiW12O40]
> 99
2
98
0
3
4
5
6
7
8
9
[MPyH]4[SiW12O40
[DBUH]4[SiW12O40
[NMPH]4[SiW12O40
[NMPH]3H[SiW12O40
[NMPH]2H2[SiW12O40
]
]
]
6
3
95
95
> 99
> 99
90
0
0
2
3
4
26
30
76
84
8
]
]
[NMPH]H3[SiW12O40
[NMPH][HSO4]
]
10
[NMPH][TsO]
[NMPH]H3[SiW12O40
83
> 99
0
95
11c
]
5
6
7
a
Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), catalyst (3 mol%), DCE
(1.0 mL), 80 °C, 1.5 h. MIm = 1-methylimidazole, MPy = 4-methylpyridine,
DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene, NMP = N-methyl-2-pyrrolidinone.
b
The conversions and yields were determined by GC with biphenyl as the
internal standard. The main by-product is benzhydrol ether.
c
4 mol% of catalyst was used.
for recycling. Then the heterogenization of the Keggin heteropolyacid
(H4SiW12O40) was further explored. Therefore, a series of ionic liquids
was synthesized as catalyst according to the previous reported proce-
dures [55,58,59]. As showed in Table 2, the ionic liquids synthesized
from H4SiW12O40 and 1-methylimidazole (MIm), 4-methylpyridine
8
9c
10c
(MPy),
1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU),
i.e.
[MImH]4[SiW12O40], [MPyH]4[SiW12O40], and [DBUH]4[SiW12O40
]
did not display any catalytic activity in the reaction (Table 2, entries
2–4). Fortunately, the ionic liquids, prepared from N-methyl-2-pyrro-
11d
a
lidone (NMP) and H4SiW12O40
[NMPH]3H[SiW12O40], [NMPH]2H2[SiW12O40],
,
including [NMPH]4[SiW12O40],
and [NMPH]
H3[SiW12O40] were found to be efficient for the reaction of 1a and 2a,
giving the desired product 3a in yields of 26%, 30%, 76% and 84%,
Reaction conditions: 1 (0.6 mmol), 2 (1.2 mmol), catalyst (4 mol%), DCE
(3 mL), 80 °C, 1.5 h.
b
Isolated yields.
2c (1.8 mmol) was applied.
2a (1.2 mmol) as the sole reactant.
respectively (Table 2, entries 5–8). These results suggested that the
c
4−
protons on the [SiW12O40
]
anion are also critical for the catalytic
d
activity for this dehydrative reaction. Additionally, the salts [NMPH]
[HSO4] and [NMPH][TsO] synthesized from NMP and H2SO4, p-TsOH,
respectively, were also applied as catalyst (3 mol%) for this model re-
action at 80 °C for 1.5 h. Although high conversions of 1a were ob-
served, the yields of desired product 3a were poor, delivering benz-
hydrol ether as by-product (Table 2, entries 9 and 10). Moreover, the
yield of 3a could be further improved to 95% by increasing the catalyst
dosage of [NMPH]H3[SiW12O40] to 4 mol% (Table 2, entry 11). After
investigation of the reaction temperature (see Table S3), the optimal
condition for this dehydrative reaction was established by using 4 mol%
of [NMPH]H3[SiW12O40] as catalyst in DCE as solvent at 80 °C for 1.5 h.
With the optimized conditions in hand, we further explored the
reaction using various alcohols as substrates by using the ionic liquid
[NMPH]H3[SiW12O40] as catalyst. As shown in Table 3, the reaction of
1a and 2a delivered the product 3a in 91% of isolated yield (Table 3,
entry 1). Subsequently, the electron-withdrawing (-Cl) and electron-
donating group (-Me) substituted diarylmethanols 1b and 1c reacted
smoothly with the alcohol 2a to form the corresponding alkene pro-
ducts 3b and 3c in 83% and 84% yields, respectively (Table 3, entries 2
and 3). Notably, when 1,2,3,4-tetrahydro-1-naphthol 2b was employed
as substrate, various diarylmethanols 1a-e were found to be effective
reaction partners for the construction of the corresponding unsaturated
products 3d-h (Table 3, entries 4–8). Following the success of the
above-mentioned aromatic alcohols (2a and 2b), we explored the
possibility of the reaction of diarylmethanols (1a and 1d) and aliphatic
tert-butanol 2c. To our delight, the desired products 3i and 3 j were
observed in good yields (73% and 71%) when 3 equivalents of 2c were
employed under the standard conditions (Table 3, entries 9 and 10).
Additionally, when 1-phenylethanol 2a was applied as the sole reactant
under this catalytic system, the product 3k was isolated in 62% yield
(Table 3, entry 11). Unfortunately, the reaction of 1a and phenyl me-
thanol (2d) did not give the desired product, indicating that primary
alcohol was not suitable in this procedure. In contrast to reaction 1a
and 2a, the reaction of 1a and 2c gave a lower yield of corresponding
product. It could be reasoned that the generation of conjugated alkene
4a from 2a as intermediate is more favorable than the production of
aliphatic alkene 10 from 2c, due to the conjugation effect of aryl group
in 4a. Similarly, when compared the results of entry 1 and entry 11 in
Table 3, the reaction of 1a and 2a gave a higher yield than the reaction
of 2a itself, revealing that the diaryl groups are beneficial to stabilize
the in situ generated carbocation intermediate (see the Supporting In-
formation).
After demonstrating that this catalytic system was compatible with
various alcohols, investigation of the scope with respect to the styrene
derivatives was undertaken. As can be seen in Table 4, the reactions of
styrene 4a and various diarylmethanols 1a-c were firstly investigated.
The coupling products 5a-c were obtained in 84–87% yields (Table 4,
entries 1–3). Moreover, the styrene derivatives 4b-e, bearing electron-
withdrawing and electron-donating groups, were also applied as sub-
strates for the coupling reaction with various diarylmethanols 1a-c,
affording the corresponding products 5d-l in good to excellent yields
82

G.-P. Yang, et al.
MolecularCatalysis468(2019)80–85
Table 4
Substrates scope of the coupling of alcohols and alkenes.a
Entry
1
1
4
Yield of 5b
2
3
4
5
6
Fig. 1. Recyclability of [NMPH]H3[SiW12O40] in direct dehydrative coupling of
1a and 2a under the optimized reaction conditions.
7
8
9
under the optimal reaction conditions. After each cycle, the product
was extracted by ethyl ether for several times. Subsequently, the cata-
lyst layer was dried under vacuum for 3 h at 50 °C for next run. The
results shown in Fig. 1 indicate that the yields of the product 3a were
almost consistent after 7 runs, suggesting that the ionic liquid [NMPH]
H3[SiW12O40] can be reused at least 7 times without significant loss of
activity.
10
To further study the structure of catalyst [NMPH]H3[SiW12O40],
characterization including FT-IR, PXRD, and SEM were conducted. The
11
12
SEM image suggests that the primary particles of [NMPH]H3[SiW12O40
]
are in the form of block-shaped morphology. The particle sizes range
from several hundred nanometers to a few microns (see the Supporting
Information). The powder X-ray diffraction (PXRD) patterns of [NMPH]
H3[SiW12O40] and pure H4[SiW12O40] revealed that the featured peaks
of H4[SiW12O40] were maintained in the [NMPH]H3[SiW12O40] but
with lower peak intensities (see the Supporting Information). It was
reasoned that the long-range order of the crystal lattice was sig-
nificantly decreased resulting from the replacement of protons by the
larger NMP cation, which is consistent with the previous reports [55].
Moreover, for the recycled [NMPH]H3[SiW12O40], the featured peaks
were also remained albeit with higher peak intensities.
To gain insights into the mechanism of this dehydrative reaction,
some control experiments were conducted as shown in Scheme 3. Under
the typical conditions, the reaction of 1a and 4a for 1 min gave the
product 5a and by-product 6a in yields of 11% and 78%, respectively.
By contrast, after a prolonged reaction time (120 min), the compound
6a was disappeared, and only 5a was obtained in 83% yield. These
results revealed that the ether 6a might be the intermediate of this
dehydrative coupling. This hypothesis was further confirmed by the
reaction of 6a and 4a under the standard conditions, which led the
desired product 5a in 88% yield (Scheme 3b). Then, a competing
a
Reaction conditions: 1 (0.6 mmol), 4 (1.2 mmol), catalyst (4 mol%), DCE
(3 mL), 80 °C, 2 h.
b
Isolated yields.
(71–89%, Table 4, entries 4–12).
Additionally, the present protocol was found to be scalable at a 5-
mmol scale to synthesize substituted olefins via the direct dehydrative
coupling of alcohols and alcohols (or olefins), which is significant from
the standpoint of practical applications. As shown in Scheme 2, the
reaction of 1b (5 mmol) and 1,2,3,4-tetrahydro-1-naphthol 2b
(10 mmol) as substrates produced the corresponding product 3e in 89%
of isolated yield using [NMPH]H3[SiW12O40] as catalyst at 80 °C for 4 h
(Scheme 2a). Similarly, the gram-scale reaction of 1a and 4c led to the
product 5e in yield of 78% under the identical conditions (Scheme 2b).
These results indicated that this method is operationally simple, scal-
able, and fast under mild conditions.
To test the reusability of the catalyst, the model reaction of 1a and
2a was carried out in the presence of 10 mol% of [NMPH]H3[SiW12O40
]
Scheme 2. Gram-scale synthesis of polysubstituted olefins.
83

G.-P. Yang, et al.
MolecularCatalysis468(2019)80–85
4. Conclusions
In summary, we have successfully developed a heterogeneous acidic
ionic liquid catalytic system on the basis of homogeneous catalysis. A
recyclable and efficient heterogeneous catalyst based on Keggin-type
polyoxometalate, i.e., [NMPH]H3[SiW12O40], was proven to be efficient
for the direct dehydrative coupling of alcohols with alcohols (or al-
kenes) to synthesize various polysubstituted olefins in good to excellent
yields [60]. Furthermore, this reaction could be scaled up. The kinetic
competition experiment showed that the C–H bond cleavage might be
involved in the rate-determining step. Our route employs readily ac-
cessible and noncorrosive heteropolyacid as a green precursor for the
preparation of acidic ionic liquid catalyst. Such findings may pave the
way for the development of new green chemistry transformations.
Acknowledgements
We thank the Natural Science Foundation of China (21501010,
21231002, 21671019), 973 Program (2014CB932103) and the 111
Project (B07012) for financial support.
Appendix A. Supplementary data
Scheme 3. Control experiments and KIE experiment.
Supplementary data associated with this article can be found, in the
online version, at https://doi.org/10.1016/j.mcat.2019.02.019.
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kinetic isotope effect (KIE) experiment was carried out (Scheme 3c). As
a result, a typical KIE was observed (KH/KD = 3.13), suggesting that the
C–H bond cleavage might be the rate-determining step [22].
On the basis of these results, the mechanism for the dehydrative
coupling was proposed in Scheme 4. By using the heteropolyacid based
ionic liquid [NMPH]H3[SiW12O40] as catalyst, carbocation species 8
could be generated from the ether 6 or directly from the alcohol 1 via
protonation-dehydration process. On the other hand, the [NMPH]
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