Selective Catalytic Isomerization of β-Pinene Oxide to Perillyl Alcohol Enhanced by Protic Tetraimidazolium Nitrate

Article abstract of DOI:10.1002/open.202000318

A series of tetraimidazolium salts with different anions was prepared and applied in the isomerization of β-pinene oxide. After examining the activity of different catalysts, a remarkable enhancement of the selectivity of perillyl alcohol (47 %) was obtained over [PEimi][HNO₃]₄ under mild reaction conditions and using DMSO as the solvent. Furthermore, noncovalent interactions between solvent molecules and the catalyst were found by FT-IR spectroscopy and confirmed by computational chemistry. The homogeneous catalyst showed excellent stability and was reused up to six times without significant loss.

Full text of DOI:10.1002/open.202000318

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Selective Catalytic Isomerization of β-Pinene Oxide to
Perillyl Alcohol Enhanced by Protic Tetraimidazolium
Nitrate
Hui Li, Jian Liu, Juan Zhao, Huiting He, Dabo Jiang, Steven Robert Kirk, Qiong Xu,
[a]
Xianxiang Liu, and Dulin Yin*
A series of tetraimidazolium salts with different anions was
prepared and applied in the isomerization of β-pinene oxide.
After examining the activity of different catalysts, a remarkable
enhancement of the selectivity of perillyl alcohol (47%) was
obtained over [PEimi][HNO3]4 under mild reaction conditions
and using DMSO as the solvent. Furthermore, noncovalent
interactions between solvent molecules and the catalyst were
found by FT-IR spectroscopy and confirmed by computational
chemistry. The homogeneous catalyst showed excellent stability
and was reused up to six times without significant loss.
1. Introduction
Conversion of biomass into high value-added chemicals has
become one of the most significant topics in ‘green chemistry’
in recent years. Extraction is the main way to obtain fine
chemicals from biomass. However, these extraction processes
suffer from several significant drawbacks, such as a demanding
equipment, difficult purification, and high costs. To overcome
these drawbacks, the development of chemical synthesis has
become an important alternative method.
Figure 1. Reaction scheme of β-pinene oxide isomerization.
the isomerization of β-pinene oxide over solid acid catalysts is
[
9]
β-pinene oxide is a key intermediate in fine chemistry
because it can undergo selective ring-opening reaction and
rearrangement, obtaining a variety of scented, fragrant and
antimicrobial chemicals. β-pinene oxide isomerization catalyzed
by solid acid catalysts is one of the most efficient methods for
obtaining the high value-added products (Figure 1) such as
66%, but achieving this yield takes a long time.
[10,11]
A pseudo-homogeneous method
has the characteristics
of “homogenized reaction, heterogenized recovery”, which is of
great significance to the industrial application of the catalyst.
Recently, task-specific ionic compounds have attracted sus-
[12–18]
tained attention
due to their tailorable properties. Typically,
[1]
myrtanal, myrtenol and perillyl alcohol.
Most of the studies on the catalytic isomerization of β-
this kind of compound is considered as a pseudo-homogeneous
[19,20]
catalyst. For example, Di-cation imidazolium
ionic liquids
[2–6]
pinene oxide
have been directed toward the synthesis of
can achieve homogeneous catalytic reactions and heteroge-
neous separation by initially regulating its own structure. Based
on the concept of such protic multi-cation arrays, we inves-
tigated protic tetraimidazolium salts with distinct and control-
lable backbone and different, in part protic anions as potential
catalysts for β-pinene oxide isomerization. Thus far, there has
been no published research literature on the highly selective
synthesis of perillyl alcohol over imidazole ionic liquid catalysts
by β-pinene oxide isomerization.
myrtanal and myrtenol, which are used in the flavor and
fragrance field. In general, perillyl alcohol occurs as a by-
product of the isomerization of β-pinene oxide to myrtenol.
[7,8]
Despite its antibacterial importance, only a few reports have
investigated the selective synthesis of perillyl alcohol from β-
pinene oxide. The highest reported yield of perillyl alcohol in
[a] H. Li, J. Liu, J. Zhao, H. He, Dr. D. Jiang, Prof. S. R. Kirk, Prof. Q. Xu, Dr. X. Liu,
Prof. D. Yin
In this work, a series of tetracationic imidazolium salts with
different anions was synthesized and successfully applied in the
selective synthesis of perillyl alcohol by β-pinene oxide isomer-
ization. The effect of different anions on the formation of the
perillyl alcohol was proven, and the effect of different solvents
on the product distribution was explored. Furthermore, the
weak interaction between catalyst and solvent (DMSO) was
studied by using computational chemistry and chemical
characterization. Finally, a possible mechanism for the synthesis
of perillyl alcohol by solvo-assisted catalysts was proposed.
National & Local Joint Engineering Laboratory for New Petro-chemical
Materials and Fine Utilization of Resources
College of Chemistry and Chemical Engineering
Hunan Normal University
Changsha, 410081 (China)
E-mail: dulinyin@126.com
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/open.202000318
©
2021 The Authors. Published by Wiley-VCH GmbH. This is an open access
article under the terms of the Creative Commons Attribution Non-Com-
mercial License, which permits use, distribution and reproduction in any
medium, provided the original work is properly cited and is not used for
commercial purposes.
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2
. Results and Discussion
with the stretching vibration mode of CÀ Cl in curve(1), The
À 1 À 1
characteristic bands at around 1245 cm and 1030 cm are
associated, respectively, with the stretching vibration mode of
2
.1. Catalyst Characterization
À
CÀ F and S=O in CF SO group in curve(2). The characteristic
3
4
À 1
À 1
The imidazolium nitrate with different cations was analyzed by
FT-IR. As shown in Figure 2, there are three characteristic
bands at around 1300 cm and 1085 cm are associated with
À
the stretching vibration mode of P=O and PÀ O in H PO group
2
4
À 1
À 1
À 1
vibration bands around 3103 cm , 1632 cm and 1385 cm in
all curves, which are respectively attributed to the characteristic
vibration band of CÀ H on the imidazole ring, the characteristic
vibration band of the imidazole ring skeleton and the stretching
vibration band of N=O on the nitrate, indicating that three
substances are ascribed to imidazole nitrate. In addition, as the
number of imidazole ring increases, the intensity of the
corresponding stretching vibration band also increases, which
further confirms the synthesis of the imidazolium nitrate with
different cations.
in curve(3). When the hydrogen sulfate is modified as the anion,
the intensity of the stretching vibration band (at around
À 1
1170 cm ) is stronger than before, indicating that the charac-
teristic band of stretching vibration of S=O in hydrogen sulfate
coincides with the bending vibration band of CÀ H on imidazole
À 1
ring. Additionally, The characteristic band at around 854 cm is
À
associated with the stretching vibration mode of SÀ O in HSO4
À 1
group in curve(4). The characteristic band at around 1380 cm
À
is attributed to the stretching vibration mode of NÀ O in NO3
group in curve(5).
1
Tetraimidazolium ionic liquids with different anions were
investigated by FT-IR. As shown in Figure 3, the characteristic
bands (at around 3070 cm ,1170 cm and 1158 cm ) are
associated with the stretching vibration mode of the tetraimida-
H NMR spectroscopy was employed to gain further insight
into the structural aspects of protic tetraimidazolium salts with
different anions. As shown in Figure 4, it can be seen that the
solubility of products of different anions in deuterium oxide is
relatively different from the relative ratio of the characteristic
peak to the solvent peak. The chemical shift of hydrogen atoms
À 1
À 1
À 1
À 1
zolium moiety. The characteristic band at 620 cm is associated
(at above 6 ppm) is associated with the type of hydrogen atom
on the imidazole ring. After pentaerythrityl tetraimdazole was
neutralized with the corresponding acids to obtain tetraimida-
zolium salts with different anions, the chemical shifts of the
corresponding hydrogen atoms on the imidazole ring are blue-
shifted, indicating that the introduction of different anions
produce corresponding weak interaction forces in catalysts,
changing the chemical shift of hydrogen atoms in the raw
materials.
The acid-base titration results were shown in Table 1.
Considering the degree of ionization of anions in the catalysts,
its theoretical protons value can be calculated. Comparing the
above results, it is found that these protic imidazolium salts in
water can be ionized to the protons to be equal to theoretical
ones, suggesting that these imidazolium salts have great
quality.
3 3 2 3 4
Figure 2. FT-IR spectra of [Imi][HNO ], [Bis-imi][HNO ] and [PEimi][HNO ] .
1
Figure 3. FT-IR spectra of (1) [PEimi][HCl]
4
, (2) [PEimi][CF
3
SO
4
H]
4
, (3)
Figure 4. H NMR spectra of (1) PEimi, (2) [PEimi][HNO
3
]
4
, (3) [PEimi][H
2
SO
4
]
4
,
[
PEimi][H PO , (4) [PEimi][H SO , (5) [PEimi][HNO
3
4
]
4
2
4
]
4
3
]
4
.
3 4 4 3 4 4 4
(4) [PEimi][H PO ] , (5) [PEimi][CF SO H] , (6) [PEimi][HCl] .
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+
results show the Brønsted acidity of all catalysts can be given,
which is different from the acidity of catalysts in DMSO.
Table 1. The amount of H of different catalyst.
Entry Catalyst Amount Amount Determined Acid
+
+
[
g]
H
H
[mmol]
amount
[mmol/
g]
[
mmol]
2
.2. Catalytic Performance
1
2
3
4
5
6
7
[Imi]HNO
[Bis-imi][HNO
[PEimi][HNO
[PEimi][HCl]
[PEimi][CF SO
3
0.101
0.100
0.100
0.100
0.100
0.102
0.102
0.696
0.365
0.680
0.833
0.427
1.120
1.120
0.697
0.366
0.680
0.834
0.428
1.100
1.130
6.901
3.660
6.800
8.340
4.280
10.784
11.078
3 2
]
]
3 4
The catalytic performance of different catalysts in the selective
synthesis of perillyl alcohol by β-pinene oxide isomerization, is
summarized in Table 3. Notably, with the increase of imidazole
moiety, the yield of perillyl alcohol has significantly increased
4
3
4
H]
4
[
a]
[PEimi][H
[PEimi][H
2
SO
PO
4
]
4
[
b]
3
4 4
]
(Table1, entry 1 vs.2 and 3), indicating that multi-cation
θ
θ
À
θ
[a] H
2
SO
4
: pK
a
= 1.99, [b] H
3
PO
4
: pKa =2.148, H
2
PO
4
: pKa =7.198,
imidazolium plays an important role in facilitating the multi-
active sites of Brønsted acid, which can accelerate to form more
C6 product in the rearrangement of β-epoxide pinene.
Furthermore, the effect of tetraimidazolium with different
anions was investigated (Table1, entry 3 vs.4 5, 6, and 7): the
results showed that the catalytic activity of different catalysts
follows the following rules: [PEimi][HNO ] >[PEimi][CF SO H] >
2
À
θ
HPO
4
: pKa =12.32.
The Brønsted acidity of the catalyst was measured by UV-
visible spectroscopy of the 4-Nitroaniline indicator (pK(I)aq=
0
.99), evaluating from the determination of the Hammett acidity
3
4
3
4
4
functions. The detailed results are shown in Table 2. Notably,
[PEimi][H SO ] >[PEimi][H PO ] >[PEimi][HCl] ,which was con-
2 4 4 3 4 4 4
with the increase of imidazole moiety, the H of the catalyst
sistent with the acidic strength of the catalysts. In summary, we
selected [PEimi][HNO ] as the optimum catalyst for catalytic
performance in the selective synthesis of Perillyl alcohol by β-
0
shouws no noticeable change (Table S2, entry 2 vs.3 and 4),
Furthermore, the acidity of protic tetraimidazolium salts with
different anions was investigated (Table S2, entry 4 vs.5, 6, 7,
and 8): the results indicated that the acidity of different
catalysts follows the rules: [PEimi][HNO ] >[PEimi][CF SO H] >
3
4
pinene oxide isomerization. In addition, the [PEimi][HNO ] has
3
4
a very good thermal stability by TG-DTG analysis: for details see
Fig. S1(see ESI+).
3
4
3
4
4
[
PEimi][H SO ] . Considering that all the catalysts have better
Given that the reaction solvent plays a critical role in the
catalytic performance of the [PEimi][HNO ] in the synthesis of
2
4 4
solubility in water, the Brønsted acidity of the catalyst was
evaluated by Hammett functions of different acids in water. The
3
4
perillyl alcohol by β-pinene oxide isomerization, Table 4 shows
in detail the comparative results of the isomerization of β-
pinene oxide over [PEimi][HNO3]4 in various solvents. It was
found that the non-polar toluene has high conversion with low
selectivity of perillyl alcohol, An the increase of the solvent
polarity, such as chloroform, ethyl acetate and acetone, used for
the isomerization of β-pinene oxide, results in low conversion
and more by-products. The difference should be associated
with the expectation that the high polar catalyst is easily
dissolved in a solvent of similar polarity, avoiding the problem
of mass transfer obstruction. In addition, although the catalyst
can be well dissolved in methanol, the results indicate that the
reaction was completely transformed and yield more of the
isomers of perillyl alcohol. Furthermore, 28.6% selectivity of
perillyl alcohol was obtained in nitromethane: this can be
attributed to the weakly basic nature of the solvent, which
Table 2. Hammett functions of different Acids in DMSO.
+
[a]
Entry
Catalyst
Amax
[I]
[IH ]
H
0
H
0
[
%]
[%]
1
2
3
4
5
6
7
8
None
[Imi]HNO
[Bis-imi][HNO
[PEimi][HNO
[PEimi][H
[PEimi][H
1.059
0.546
0.511
0.571
0.617
–
100.00
51.55
48.23
53.92
58.26
–
0.00
48.45
51.77
46.08
41.74
–
0.00
1.02
0.96
1.06
1.13
–
–
3
1.00
0.99
1.18
0.95
1.06
1.09
1.66
3
]
4
2
3
]
2
SO
PO
4
]
4
3
4 4
]
[PEimi][HCl]
[PEimi][CF SO
4
–
–
–
–
0.96
3
4
H]
4
0.511
48.23
51.77
+
Indicator: 4-Nitroaniline (pK(I)aq =0.99), H
[
[
0
=pK(I)aq +log([I]/[IH ]), [PEimi]
4
are insoluble in DMSO.
H
3
PO
4
]
4
and [PEimi][HCl]
a] H
0
is expressed as the Hammett functions of different acids in water.
Table 3. Effect of different catalysts on the isomerization of β-pinene oxide.
[a]
0
À 3[b]
Entry
Catalyst
H
Conv.
%]
TOF×10
Selectivity [%]
M-al
À 1
[
[s
]
M-ol
PA
P-ol
Others
1
2
3
4
5
6
7
[Imi]HNO
[Bis-imi][HNO
[PEimi][HNO
3
1.02
0.96
1.06
1.13
–
–
1.09
78.6
94.8
100.0
85.1
71.5
53.9
99.6
3.41
4.11
4.34
3.69
3.10
2.34
4.32
9.5
7.1
7.1
8.8
10.8
7.8
8.1
27.0
24.5
18.6
20.6
35.1
41.3
22.4
37.0
44.2
47.3
37.7
32.3
29.2
43.6
14.5
15.8
15.8
12.4
9.6
8.3
16.7
12.0
8.4
11.2
20.5
12.2
13.5
9.3
3
]
4
2
3
]
[PEimi][H
2
SO
PO
4
]
4
[PEimi][H
3
4 4
]
[PEimi][HCl]
[PEimi][CF SO
4
3
4
H]
4
Reaction conditions:Catalyst (4.8 mol% β-pinene oxide), β-pinene oxide (5 mmol), DMSO (2.5 ml), 80 min, 40°C.
[
[
a] H
0
is represented as Hammett functions of different acids in DMSO: [PEimi][H
3
PO
4
]
4
and [PEimi][HCl]
4
are insoluble in DMSO.
moles of BPO
À 1
b] TOF is calculated by the TOF ¼
ðs Þ
moles of catalyst�time
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Table 4. Effect of solvent polarity on the isomerization of β-pinene oxide.
À 3[a]
Solvent
Dielectric constant
Conv.
%]
TOF×10
Selectivity [%]
À 1
[
[s
]
M-al
M-ol
PA
P-ol
Others
Toluene
2.38
4.70
6.03
20.5
32.6
38.6
48.9
62.0
30.9
37.9
34.3
99.9
51.8
100.0
2.69
1.34
1.64
1.49
4.33
2.25
4.34
14.0
10.3
13.8
11.1
2.8
6.8
7.1
14.6
17.5
12.1
16.0
8.4
15.1
18.6
9.6
7.8
8.6
8.2
12.2
28.6
47.3
0.4
2.3
1.8
1.9
14.0
2.7
15.8
61.4
62.1
63.7
62.8
62.6
46.8
11.2
Trichloromethane
Ethyl acetate
Acetone
Methanol
Nitromethane
DDimethyl sulfoxide(DMSO)
Reaction conditions:Catalyst (4.8 mol% β-pinene oxide), β-pinene oxide (5 mmol), solvent (2.5 ml), 80 min, 40°C.
moles of BPO
moles of catalyst�time
À 1
[a] TOF is calculated by the TOF ¼
ðs Þ.
promotes ring-opening for the formation of C6 products. The
dimethyl sulfoxide (DMSO) was the optimal solvent for the
isomerization of β-pinene oxide, obtaining an excellent yield of
perillyl alcohol due to the high polarity and weakly basic
oxide isomerization was studied. As shown in Figure 5, the
results showed that the conversion rate of β-pinene oxide
gradually increases with increasing temperature. When the
temperature reaches 40°C, the reaction is completed. As the
reaction temperature continues to increase, more perillyl
alcohol is generated at the expense of myrtenal and myrtenol
formation. The remarkable change in the trend of the product
distribution might be due to the product of a reaction under
[4]
solvent.
In the next experiment, the effect of reaction temperature
and reaction time in the synthesis of perillyl alcohol by β-pinene
[8]
thermodynamic control. As shown in Figure 6, with the
increase of reaction time from 20 to 80 min, the conversion of
β-pinene oxide shows an obvious increase from 67% to 95%
with over 44% selectivity for perillyl alcohol. Upon further
extending the reaction time, the selectivity for perillyl alcohol
increases slightly. Furthermore, the product distribution follows
a rule that a significant increase in the selectivity to perillyl
alcohol occurs at the expense of myrtenol and myrtenal
formation during the whole reaction time. This might be due to
the reaction system being more favorable toward generation of
more perillyl alcohol over the catalysis for a long time.
2
.3. Catalyst Reusability
Although the [PEimi][HNO3]4 is used in a reaction as a
homogeneous catalyst, it can be easily recovered from the
reaction mixture by adjusting the polarity of the solvent system:
it was added in a large amount of ethyl acetate, centrifuged,
washed with ethyl acetate and reused. Figure 7 shows the
results of the recyclability of [PEimi][HNO ] in the synthesis of
Figure 5. Effect of temperature on the isomerization of β-pinene oxide.
3
4
perillyl alcohol by β-pinene oxide isomerization. It was gratify-
ing to observe that; the catalyst could be reused six times
without a significant decrease in the conversion and selectivity
of perillyl alcohol. In addition, the reused catalyst was
1
characterized by H NMR and FT-IR, as shown in Figure 8–9. The
results suggested that the reused catalyst still maintained its
original structure, indicating that [PEimi][HNO ] exhibits great
3
4
stability and recyclability.
2
.4. Microscopic Interaction Between Solvent and Catalyst
In order to visually investigate the microscopic interaction
between solvent and catalyst, a few catalysts were dissolved in
Figure 6. Effect of reaction time on the isomerization of β-pinene oxide.
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6 6 3 4
Figure 10. FT-IR spectra of DMSO-d and DMSO-D + [PEimi][HNO ] .
3 4
Figure 7. Recyclability of [PEimi][HNO ] on the isomerization of β-pinene
oxide.
À 1
2
251.97 cm . After adding the catalyst, the band correspond-
ing to the CÀ D in DMSO-D was slightly displaced, and the
6
2
characteristic bands of C -H in imidazole ring groups (at around
À 1
1
630 cm ) have a significant red shift, indicating the gener-
2
ation of hydrogen bonds between C -H (in catalyst) and S=O in
DMSO-D . Furthermore, we used computational chemistry to
6
further illustrate the weak interaction between the catalyst and
solvent. Considering the catalyst itself as a symmetric structure,
using explicit basic units and restricted computing devices, the
structure of catalyst was calculated by using basic units instead
of catalyst, as shown in Figure 11.
In order to visualize the noncovalent interaction between
solvent and catalyst, we thereby using the reduced density
[21]
gradient (RDG)
to examine such effect, The geometrical
3 4
Figure 8. FT-IR spectra of fresh (a) and reused (b) of [PEimi][HNO ] .
calculation was optimized under density functional theory M06-
[22]
[23]
2
X
with Ahlrichs’ triple-zeta basis sets def-TZVP, following
tight SCF convergence and ultrafine integration grids, by using
[
24]
the Gaussian 09
package, version B01. The Multiwfn 3.7
[25]
program was employed to do RDG calculations using the
formatted checkpoint file generated from Gaussian calculation
as the inputs. The gradient isosurface can be colored according
to the sign(λ ) values in plot of RDG, which is a straight
2
illustration of noncovalent interaction between solvent mole-
cule and catalyst. Figure 12 shows in detail the results of
gradient isosurface (a) and scatter plot of the RDG (b) for one
I
Figure 9. H NMR of reused [PEimi][HNO
3
]
4
.
dimethyl sulfoxide-D to simulate the actual catalytic system:
6
the sample and blank were measured using FT-IR spectra. The
detailed results are shown in Figure 10. The characteristic
Figure 11. The weak interaction between the basic unit of the catalyst and
DMSO.
vibration bands of CÀ D in DMSO-D at around 2125.37 and
6
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Figure 12. Reduced density gradient versus the electron density multiplied by the sign of the second Hessian eigenvalue, data evaluated at M06-2X/def-TZVP
level of theory.
basic unit of the catalyst and DMSO. According to the evalues
2.5. Proposed Mechanism
[26]
of sign(λ )1,
ranging from À 0.05 to 0.05 au., the gradient
2
isosurfaces are colored on a blue-green-red scale, which
respectively indicates strong attractive interactions(hydrogen
bonds), Van der Waals force and strong non-bonded overlap
A plausible mechanism of acid-catalyzed transformations of β-
pinene oxide is presented in Scheme 1. A tertiary carbocation
(A) is formed by protonation of the exo-epoxide with H
+
(strong site-resistance effects). Thus, the results clearly show the
released by the imidazole ring in [PEimi][HNO ] . Transformation
3
4
evidence for the hydrogen bond between the S=O (in the
of the tertiary carbocation (A) results in two reaction paths (I)
and (II) due to the polarity and basicity of solvent. In path (I),
the electron pair quickly transfer from C by proton release
2
[27–29]
DMSO group) and C -H(on the imidazole ring).
Same results
3
could also be evaluated from the 2D plot (b), where the most
spikes are lying at negative values suggesting that some
interactions are playing a stabilizing role in the system.
giving myrtenol (M-ol) and myrtenal (M-al). On the other hand,
2
the hydrogen bond between the S=O (in DMSO group) and C -
H (on the imidazole ring) can further stabilize the catalyst. In
path (II), the tertiary carbocation (A) can involve the same
electron pair. However, the DMSO as a solvent with high
Scheme 1. Plausible mechanism of acid-catalyzed transformations of β-pinene oxide.
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polarity and basicity, can facilitate the deprotonation of the
Preparation of the Imidazole Skeleton
CÀ C bond. Thus, the allylic carbocation (C) is formed via the
Synthesis of gemini imidazole. imidazole (6.816 g, 0.1 mol), KOH
1
6
3
breaking of the C À C bond in (B) and the deprotonation of C .
Subsequently, the proton is released to obtain perillyl alcohol:
in addition, the presence of a small amount of residual water is
sufficient to favor the formation of P-ol.
(
6.732 g, 0.12 mol) were stirred in methanol (100 ml) at 50°C for
1 h. After removing methanol, dibromomethane (8.592 g, 0.05 mol)
and tetrabutyl ammonium bromide (0.1 g) were slowly added and
stirred in acetonitrile (100 ml) at 80°C for 6 h. The reaction mixture
was filtered to remove potassium bromide, the filtrate was cyclically
steamed to remove the solvent, The crude product was recrystal-
lized with acetone at a yield of 31.3%.
3. Conclusion
1
H NMR (500 MHz, DMSO) δ 7.93 (s, 2H), 7.39 (s, 2H), 6.91 (s, 2H),
1
3
6
.21 (s, 2H); C NMR (126 MHz, CDCl ) δ 136.62 (s), 131.19 (s), 118.12
3
In this work, a series of tetraimidazolium salts with different
anions was successfully synthesized by simple nucleophilic
substitution and neutralization and used in the synthesis of
perillyl alcohol by β-pinene oxide isomerization. Among the
different catalysts examined, [PEimi][HNO ] , as a pseudo
(
(
s), 77.30 (s), 77.05 (s), 76.79 (s), 56.35 (s); Elem. Anal.(calc.): C, 56.73
56.74); H, 5.42 (5.44); N, 37.85 ( 37.81).
[31,32]
Synthesis of pentaerythrityl tetraimdazole:
Imidazole (6.816 g,
0
.1 mol) was dissolved in toluene(12 ml) and dimethyl sulfoxide
3
4
(12 ml), 50% sodium hydroxide(16 g) was slowly added dropwise
homogeneous catalyst, exhibits remarkable yield of perillyl
alcohol (47%) under the mild reaction conditions of 5 mmol of
β-pinene oxide, 2.5 ml of DMSO as solvent, 4.8 mol% catalyst,
°
and stirred at room temperature for 30 min, then heated to 110 C
until the generated water was removed. Pentaerythrityl tetrabro-
mide (7.672 g, 0.02 mol) was added slowly, then the reaction
mixture was heated to 110°C for 4 h, then filtered to remove
4
0°C and 80 min. Importantly, the catalyst was reused in six
sodium bromide. The filtrate was poured into a large amount of ice
water, The crude product was obtained after sitting at room
temperature overnight. After filtration, the product was washed
with pure water and dried at 65°C vacuum for 12 h. The white solid
was obtained with a yield of 26%.
runs without a significant loss, being recovered by a simple
centrifuge. Moreover, the weak interaction between catalyst
and solvent (DMSO) was studied theoretically and experimen-
tally, providing evidence for an imidazole-buffered acid-base
mechanism.
1
H NMR (500 MHz, DMSO) δ 7.49 (s, 4H), 6.97 (s, 4H), 6.93 (s, 4H),
.16 (s, 8H); C NMR (126 MHz, DMSO) 139.44 (s), 129.36 (s), 121.60
1
3
4
(s), 49.84 (s), 42.51(s). Elem. Anal.: C,60.70; H,5.99; N,33.31.
Experimental Section
Preparation of Catalyst
Materials and Reagents
The synthesis of the imidazolium, Di-cation imidazolium and
tetracation imidazolium salts is presented in Scheme 2. The specific
synthesis steps were similar, the structure and denominate of
catalyst was presented in Table S1.
Imidazole, 1-methylimidazole, sodium hydroxide, toluene, dimethyl
sulfoxide, dibromomethane, nitric acid, sulfuric acid, trifluorometha-
nesulfonic acid, phosphoric acid, 4-Nitroaniline, acetone and
methanol were AC grade purchased from Sinopharm Chemical
Reagent Co., Ltd. Pentaerythrityl tetrabromide were AC grade
purchased from Alfa Aesar.
Preparation of tetracation imidazolium nitrate as an example:
pentaerythrityl tetraimdazole (1 mmol) was dissolved in methanol
(10 ml), then nitric acid (4 mmol) was added slowly under the ice
bath. The mixture was stirred at room temperature for 1 h, then
heated to 60°C for 2 h. The product was obtained by removing the
methanol, then drying overnight at 70°C under vacuum.
Methods
The structure of the materials was characterized by using Fourier
transform infrared (FT-IR) spectra. These were measured using KBr
pellets with a resolution of 4 cm and 32 scans in the range of
1
[Imi][HNO ]: H NMR (500 MHz, D O) δ 8.64 (s, 1H), 7.42 (s, 2H), 3.91
3
2
À 1
13
(s, 3H), C NMR (126 MHz, D O) δ 134.97 (s), 122.93 (s), 119.44 (s),
2
À 1
4
00–4000 cm using a Nicolet Nexus 670 spectrometer. NMR
35.38 (s), 20.93 (s). Elem. Anal.: C, 33.12 (33.11); H, 4.87 (4.86); N,
28.98(28.96); O,33.03 (33.07).
spectra data were obtained on
a Bruker Avance-500 MHZ
spectrometer with tetrakis (trimethylsilyl) silane (TMS) as the
internal standard. Thermogravimetric and differential thermogravi-
metric (TG-DTG) curves were recorded on a Netzsch-STA409PC
thermalgravimetric analyzer, and the samples were heated from
À 1
3
0°C to 800°C with 10 K min under nitrogen flow.
The acidity of the materials was evaluated by a traditional acid-base
titration. Samples 0.1 g were dissolved in 25 ml distilled water and
titrated by the calibrated 0.01 mol/L NaOH solution with phenolph-
thalein as an indicator. The average titration result is considered as
the acid amount of the catalyst. UV-visible spectroscopy measure-
ments were recorded on a UV-vis Agilent 8453 spectrophotometer
with 4-Nitroaniline (pK(I)aq=0.99) as the indicator and using the
[
30]
method of Hammett acidity functions.
Scheme 2. Synthesis of protic imidazolium salts.
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1
[
7
Bis-imi][HNO ] : H NMR (500 MHz, D O) δ 9.18 (s, 2H), 7.75 (s, 2H), Acknowledgements
3
2
2
13
.56 (s, 2H), 6.74 (s, 2H); C NMR (126 MHz, D O) δ 136.23 (s), 121.39
2
(
s), 121.12 (s), 58.66 (s). Elem. Anal.: C, 30.65 (30.66); H, 3.67 (3.68);
We acknowledge the financial support for this study by the
National Natural Science Foundation of China (Grant
Nos. 21975070, 21776068).
N, 30.66 (30.65); O, 35.02 (35.01).
1
[PEimi][HNO ] : H NMR (500 MHz, D O) δ 8.91 (s, 4H), 7.58 (s, 8H),
3
1
4
2
3
4
.87 (s, 8H); C NMR (126 MHz, D O) δ 136.86 (s), 123.04 (s), 121.36
2
(
(
s), 50.69 (s), 42.16 (s); Elem. Anal.(calc.): C, 34.73 (34.70); H, 4.15
4.11); N, 28.47 (28.56); O, 32.65 (32.63).
Conflict of Interest
1
[
4
(
PEimi][H SO ] : H NMR (500 MHz, D O) δ 8.94 (s, 4H), 7.57 (s, 8H),
2
4 4
3
2
.87 (s, 8H); 1 C NMR (126 MHz, D O) δ 136.83 (s), 123.06 (s), 121.27
2
The authors declare no conflict of interest.
s), 50.76 (s), 42.01 (s); Elem. Anal.: C, 28.04 (28.02); H, 3.88 (3.87); N,
15.40 (15.38); O, 35.11 (35.13); S, 17,57 (17.63).
1
Keywords: tetraimidazolium salts
·
green chemistry
·
[
4
(
PEimi][H PO ] : H NMR (500 MHz, D O) δ 8.84 (s, 4H), 7.57 (s, 8H),
3
4 4
2
1
3
isomerization reactions
catalysis
·
Perillyl alcohol homogeneous
·
.85 (s, 8H); C NMR (126 MHz, D O) δ 136.78 (s), 122.97 (s), 121.54
s), 50.70 (s), 42.02 (s); Elem. Anal.: C, 28.02 (28.03); H,4.40 (4.43); N,
2
15.40 (15.38); O, 35.16 (35.14); P, 17.02 (17.07).
1
[
9
PEimi][HCl] : H NMR (500 MHz, D O) δ 8.85 (s, 4H), 7.48 (d, J=
4
2
13
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Revised manuscript received: February 17, 2021
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