Transesterification of Isosorbide with Dimethyl Carbonate Catalyzed by Task-Specific Ionic Liquids

Article abstract of DOI:10.1002/cssc.201802572

Green synthesis of high-molecular-weight isosorbide-based polycarbonate (PIC) with excellent properties is a tremendous challenge and is profoundly influenced by the precursor. Herein, an ecofriendly catalyst was employed to obtain the more reactive PIC p

Full text of DOI:10.1002/cssc.201802572

Accepted Article
Title: Transesterfication of isosorbide with dimethyl carbonate
catalyzed by task-specific ionic liquids
Authors: Wei Qian, Xin Tan, Qian Su, Weiguo Cheng, Fei Xu, Li Dong,
and Suojiang Zhang
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10.1002/cssc.201802572
ChemSusChem
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Transesterfication of isosorbide with dimethyl carbonate
catalyzed by task-specific ionic liquids
Wei Qian,[a,b] Xin Tan,[a,b] Qian Su,[a] Weiguo Cheng,*[a] Fei Xu,[a] Li Dong,[a] and Suojiang Zhang*[a]
Abstract: Green synthesis of high-molecular-weight isosorbide-
based polycarbonate (PIC) with excellent properties is a tremendous
challenge, which is profoundly influenced by the precursor. Herein,
we employed an eco-friendly catalyst to achieve the more reactive
PIC precursor dicarboxymethyl isosorbide (DC) selectivity of 99.0%
through the transesterfication reaction of isosorbide with dimethyl
carbonate (DMC), which is the indispensable stage of one-pot green
has been used for synthesizing polymer, e.g., polyurethanes,[7]
polyesters[8] and polycarbonates,[5a, 9] etc. As for polycarbonate,
isosorbide is a prominent alternative to bisphenol A (BPA),
which is originated from petroleum and a well-known endocrine
disruptor.[10] Nevertheless, isosorbide is
a weakly reactive
secondary alcohol, resulting in difficulty in synthesizing high-
11]
molecular-weight isosorbide-based polycarbonate (PIC).[5a,
synthesis of PIC, playing
polymerization mechanism of polymers synthesis by melt
transesterfication reaction. For this, series of 4-substituted
a
critical role in insight into the
Nonetheless, the molecular weight of polymer should be large
enough to sustain sufficient mechanical properties for
engineering application.[12] At present, the most promising green
synthetic pathway is the melt polycondensation of isosorbide
with dimethyl carbonate (DMC) in bulk.
a
phenolate ionic liquids (ILs) were developed as a new type of high-
efficiency catalyst for this reaction. These homogeneous ILs exhibit
outstanding catalytic performance. The DC selectivity is gradually
increasing with the decrease of basicity of ILs, among of which
trihexyl(tetradecyl)phosphonium 4-iodophenolate ([P66614][4-I-Phen])
shows the highest catalytic activity. Additionally, according to the
experiment and DFT calculations results, a plausible nucleophilic
activation mechanism was put forward, which confirmed that the
reaction was activated through formation of H-bond and electrostatic
interactions with IL catalyst. This strategy of tunable basicity and
structure of anions in ILs affords an opportunity to develop other ILs
for transesterfication reaction, thereby conveniently providing a
variety of polymers via a greener synthetic pathway.
Thereinto, DMC, a genuinely green reagent, is a fascinating
substitute of highly toxic phosgene for carboxylation reaction
and methyl halides (or dimethyl sulfate) for methylation
reaction[13]
.
Generally, DMC can play the role of
a
methoxycarboxylating agent or a methylating agent with the
presence of a nucleophile (Y-), intensively relying on the reaction
condition simultaneously, e.g., the nature of catalyst and
reaction temperature.[13-14] Thereby, it is an enormous
challenging study to synthesize high-molecular-weight PIC
through melt polycondensation of isosorbide and DMC.
Currently, this synthetic pathway was relatively infrequently
investigated, and the study on the reaction process was still not
full-scale and in-depth. This process bears two stages including
the transesterfication reaction stage and the polycondensation
reaction stage. Thus, it is necessary to investigate the
transesterfication reaction stage, due to high levels of the
transesterfication product of dicarboxymethyl isosorbide (DC,
Figure 1) is the prerequisite of synthesizing high-molecular-
weight PIC.[15]
Introduction
Getting polymers and materials from biorefineries instead of
refineries is presently considered a requirement for sustainable
development.[1] Recently, bio-derived polymers have aroused
extensive interest and attention of scientific research personnel.
A number of bio-based building blocks for the synthesis of
polymers have exhibited potential future applications, such as,
limonene oxide[2], pinene oxide,[3] lactic acid,[4] isosorbide,[5] and
so on. Among them, isosorbide (1,4:3,6-dianhydro-D-sorbitol, IS),
obtained from the double dehydration of D-sorbitol, which is
identified as
a promising monomer for polycondensation
attributing to its intriguing rigidity, chirality and nontoxicity.[6] It
[a]
[b]
W. Qian, X. Tan, Q. Su, W.G. Cheng, F. Xu, L. Dong, S.J. Zhang
Key Laboratory of Green Process and Engineering, State Key
Laboratory of Multiphase Complex Systems, Beijing Key Laboratory
of Ionic Liquids Clean Process, Institute of Process Engineering,
Chinese Academy of Sciences, Beijing 100190, China. E-mail:
wgcheng@ipe.ac.cn, sjzhang@ipe.ac.cn
School of Chemistry and Chemical Engineering, University of
Chinese Academy of Sciences, 19A Yuquan Road, Shijingshan
District, Beijing 100049, China.
Figure 1. Carboxymethyl and methyl products of the reaction of isosorbide
with DMC
It is important to mention that the reaction of isosorbide with
DMC has eight possible products (carboxymethyl compounds:
DC, MC-1, MC-2; methyl compounds: DMI, MMI-1, MMI-2;
carboxymethyl methyl compounds: MCE-1, MCE-2, Figure 1),
which are attributed to the endo and exo –OH groups of
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/cssc.201802572
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isosorbide showing non-identical reactivity.[15c] The resultants
depend intensively on the properties of catalyst. As reported by
Tundo,[16] isosorbide could be quantitatively methylated by
reacting with DMC in the presence of a strong base, which led
mostly to the formation of carboxymethylated products in the
presence of a weak base. To date, there are only several
catalysts (K2CO3, Cs2CO3, lithium acetylacetonate (LiAcac))
experiment results and DFT calculations,
a
plausible
nucleophilic activation mechanism, the transesterfication
reaction was activated through formation of H-bond and
electrostatic interactions with IL catalyst, was propounded. This
strategy of tunable basicity and structure of anions in ILs
provides an opportunity to develop other ILs for
transesterfication reaction, thereby conveniently providing a
variety of polymers via a greener synthetic pathway.
15c, 17]
showing good selectivity for the synthesis of DC.[5a,
Employing these traditional metal-based catalysts generally
suffered from disadvantages such as chemical waste, corrosion
and environmental issues.[18] As far as we know, no report
revealed that metal-free catalysts could promote this reaction.
Nowadays, ionic liquids (ILs) have received tremendous
attention in many scientific and engineering fields.[19] More
significantly, ILs display highlighted performance in catalysis and
are identified as promising substitutes for conventional metal-
based catalysts ascribing to their readily tunable properties and
hypotoxicity.[20] IL catalysts possess good dispersibility and have
no volatileness in the reaction substrates, which can be
decomposed completely and no residue at the end of high
temperature polycondensation reaction stage. But so far, ILs
have never been employed for catalyzing the reaction of
isosorbide with DMC. It is reported that, as catalysts, anions of
ILs commonly have more significant impacts on the reaction
than cations, thereby prominently affecting the selectivity and
conversion of the reaction.[21] Consequently, it is probably
significant to modify the structure of the anion for improving the
expected reaction. Satisfactorily, phenolic-based ILs, namely,
task-specific ILs (TSIL),[22] are a type of promising ILs since their
physicochemical properties could be readily tuned by grafting
various electron-withdrawing or electon-donating groups to the
benzene ring.[23] Moreover, phosphonium phenolic-based ILs
have better solubility, good thermal stability and appropriate
basicity and no effect on the color of polycarbonate.[24]
Results and Discussion
Characterization of ILs
These task-specific phenolic ILs (Scheme 1) were prepared by
neutralizing
trihexyl(tetradecyl)phosphonium
hydroxide
([P66614]OH) with corresponding 4-substituted phenols. The
decomposition temperature and structure of ILs were analyzed
and detected with different characterization methods.
Figure 2. TGA curves of task-specific phenolic ILs
Firstly, TGA curves of these phenolic ILs were obtained. As
illustrated in Figure 2, all of these phenolic ILs displayed
favorable thermal stability above 240 oC and possessed one
weight loss peak which implied these ILs were likely to be pure
state without other unstable impurities. Thereafter, FT-IR spectra
for all these phenolic ILs were presented in Figure 3. The
Scheme 1. Structure of cation and anions of ILs for the transesterification
reaction of IS and DMC
absorption band at 2810-3020 cm-1 could be assigned to the
+
stretching vibration of saturated C-H bond of [P66614
]
and the
unsaturated C-H bond of the benzene ring. In the region of
1420-1615 cm-1, the peaks could be subject to the benzene ring
skeleton vibration. The absorption band of C-O bond stretching
vibration was at 1160-1140 cm-1. The out-plane flexural vibration
of C-H bond of benzene ring was in the range from 718 cm-1 to
835 cm-1. This result was consistent with the previous
In this prospect, we studied the transesterfication reaction
stage of isosorbide and DMC in the presence of IL catalysts.
Various task-specific phenolic ILs were prepared through the
neutralization reaction of phosphonium hydroxide with
corresponding 4-substituted phenols (Scheme 1). As we
expected, the reaction was able to be catalyzed effectively by
the 4-substituted phenolate based ILs. The basicity of anions in
ILs was readily tunable by different substituents and could
influence the selectivities of the products of the transesterfication
reaction. By tuning the structure of anions in ILs and optimizing
the experimental conditions, the more reactive PIC precursor DC
selectivity of 99.0% was achieved. Additionally, based on the
1
literatures.[23-24] The H NMR and ESI-MS data were presented
in the experimental section. These results proved the structure
of ILs were correct. The basicity of the anion in IL could affect
the transesterification reaction of isosorbide with DMC, for which
the pKa values of the conjugate of acids of the anions in ILs are
shown in Table
1 and given according to the previous
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10.1002/cssc.201802572
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literature.[25] As indicated in Table 1, the pKa of the conjugate of
acid of anion in IL is decreasing with the decrease of the
electronegativity of substituent group. That is to say, among
these phenolic ILs, the basicity of [4-I-Phen]- in [P66614][4-I-Phen]
is the weakest.
optimization section, we would investigate the optimal DMC/IS
molar ratio.
Scheme 2. The transesterification reaction of isosorbide and DMC.
The results are summarized in Table 2, indicating that the
reaction is able to be catalyzed effectively by the 4-substituted
phenolate based ILs. Isosorbide can completely convert into its
carboxymethylated products. Under anyone of abovementioned
IL catalysts, the selectivity of DC is more than 90%. Thereinto,
[P66614][4-I-Phen] demonstrates the highest DC selectivity
(>99.0%) and the monocarboxymethyl compounds (MC-1 and
MC-2 ) are presented in trace amount (Entry 5, Table 2). For
comparison, LiAcac and 4-I-Phen were also introduced to
catalyze the reaction (Entry 6 and 7, Table 2). The DC
selectivity is only 77.7%, the monocarboxymethyl products
selectivities (MC-1 and MC-2) are 15.1% and 7.1%, respectively
for the metal complex catalyst of LiAcac. The weak acid of 4-I-
Phen reveals no catalytic activity for the transesterification
reaction. Moreover, we have also investigated the influence of
reaction time on the IS conversion and the selectivity towards
DC (Figure S1). By analyzing these data, in general, we can
confirm that the DC selectivity is gradually increasing with the
diminution of the pKa of the conjugate of acid of anion in IL
(Table 1, 2 and Figure S1). That is to say that DC selectivity is
gradually increasing with the diminution of the anion alkalinity of
IL apart from [P66614][4-F-Phen] (Entry 2, Table 2). Presumably
this exception is related with fluorine atom of [P66614][4-F-Phen].
The most electronegative element of fluorine may form hydrogen
bond with the hydroxyl group of isosorbide.[26] The
undermentioned result of DFT calculations can demonstrate it.
Due to the pKa values of the conjugate of acids of anions in ILs
with small spacing, the variation of DC selectivity is not
particularly remarkable. In the five ILs, [P66614][4-I-Phen] is the
most high-efficient catalyst for the transesterification reaction of
isosorbide and DMC. As reported by Tundo[16], the reaction of
isosorbide and DMC in the presence of weak bases, which
mainly led to the formation of carboxymethylated products (DC,
MC-1, MC-2). Interestingly, the selectivity of MC-1 is always
greater than MC-2. This result is consistent with the previous
literature.[15c] This can be explained by the following description.
In isosorbide molecule, endo–OH group is refered to a strong
intramolecular hydrogen bond and exo–OH group is not involved,
which results in –OH groups presenting unequal reactivity.
Sterically hidered carboxymethyl group tends to stand on the
exo-position with small steric hindrance than the endo one.
Namely, the exo position of isosorbide is easier to
carboxymethylation than endo one.[6d] Additionally, the
undermentioned DFT simulation results also can confirm this
Figure 3. FT-IR spectra of task-specific phenolic ILs
Table 1. The pKa of the conjugate of acids of anions in
ILs.[a]
Entry
Catalyst
pKa
9.99
9.89
9.41
9.37
9.33
1
2
3
4
5
[P66614][4-H-Phen]
[P66614][4-F-Phen]
[P66614][4-Cl-Phen]
[P66614][4-Br-Phen]
[P66614][4-I-Phen]
[a] The pKa values are in H2O.
Influence of the basicity of anions of ILs
Taking into consideration the influence of the basicity of the
anion in IL on the transesterification reaction of isosorbide with
DMC (Scheme 2), five types of 4-substituted phenol (weak
proton donors) with different pKa were chosen (Table 1). Herein,
these synthesized trihexyl(tetradecyl)phosphonium 4-substituted
phenolate ILs (0.88 mol % based on isosorbide) were employed
as catalysts to investigate the effect of the basicity of the anion
in IL on the transesterification reaction. In this reaction, DMC
acts as not only a reagent but also a solvent, which can form an
azeotrope with the by-product methanol. However, for the
reversible transesterification reaction, removing the by-product
CH3OH continuously is essential to shift the chemical equilibrium
to the products and achieving the complete conversion of
isosorbide. As a result, DMC would volatize with the methanol
elimination. Thus, DMC/IS molar ratio must be greater than 1,
and firstly the molar ratio was set as 7.5. In the condition
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Table 2. Activities of various catalysts for the transesterification reaction[a]
.
Entry
Catalyst
IS Conversion (%)
DC Selectivity (%)
MC-1 Selectivity (%)
MC-2 Selectivity (%)
1
2
3
4
5
6
7
[P66614][4-H-Phen]
[P66614][4-F-Phen]
[P66614][4-Cl-Phen]
[P66614][4-Br-Phen]
[P66614][4-I-Phen]
LiAcac
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
0
91.6
97.6
91.8
94.3
>99.0
77.7
4.9
1.9
4.7
3.2
trace
15.1
-
3.5
0.5
3.5
2.5
trace
7.1
-
[b]
4-I-Phen
-
[a] Reaction conditions: IS:DMC 1:7.5 eq. mol.; Catalyst amount: 8.8×10-3 eq. mol. based on isosorbide; Reaction time 13h;
Reaction temperature: 98 oC. IS conversion is determined by GC. [b] “-” means not detectable.
deduction. Afterwards, the most high-efficient catalyst of
[P66614][4-I-Phen] was selected for the following research.
by the data of Table 4. Augmenting the catalyst amount
continuously, the DC selectivity is unchanged (Entry 5, Table 3).
Consequently, the optimal catalyst amount of [P66614][4-I-Phen] is
4.4×10-3 mol/mol (based on isosorbide). Under the optimal
catalyst amount (4.4×10-3 mol/mol, based on isosorbide), the
influence of DMC/IS molar ratio was surveyed.
Influence of reation conditions
For seeking optimum reaction conditions, we have investigated
the influences of the catalyst amount of [P66614][4-I-Phen] and
DMC/IS molar ratio on the selectivities of products. As shown in
Table 3, the DC selectivity increases from 83.2% to more than
99.0% with the catalyst amount increased from 0.5×10-3 mol/mol
(based on isosorbide) to 4.4×10-3 mol/mol. When the catalyst
content is low, the active site of the reaction is relatively less.
Hence, the chemical reaction rate is slower, which results in that
the DC selectivity is lower and isosorbide is not able to be
converted completely at the same reaction time (Entry 1, Table
3). Simultaneously, the monocarboxymethyl products (MC-1 and
MC-2) selectivities are 12.3% and 4.5%, respectively (Entry 1,
Table 3). The influence of catalyst amount on this reaction
implies that the transesterfication reaction is starting by
producing the monocarboxymethyl compounds (MC-1 and MC-
2). After that, under the molar ratio of DMC/IS is large enough,
the monocarboxymethyl products can be further converted into
dicarboxymethyl compound (DC), instead of DC, MC-1 and MC-
2 producing at the same time. This conclusion can be testified
As demonstrated in Table 4, the DMC/IS molar ratio has a
great effect on the selectivities of carboxymethyl products. The
DC selectivity is increasing from zero to more than 99.0% with
the DMC/IS molar ratio increased from 1.5/1 mol/mol to 7.5/1
mol/mol. Simultaneously, the selectivity of MC-1 and MC-2 is
decreasing from 60.4% and 39.6% to trace amount. The
isosorbide conversion is increasing from 56.1% to 99.9%. The
result can be explained in the following expression. As
mentioned previously, DMC is bound to evaporate continuously
with the proceeding of the transesterfication reaction. As a result,
when the DMC/IS molar ratio is less than 7.5/1 mol/mol, the
amount of DMC is insufficient to make isosorbide to be
transformed completely. DC cannot be detected and a high
amount of monocarboxymethyl compounds (MC-1 and MC-2)
are formed when the DMC/IS molar ratio is less than or equal to
2.5/1 mol/mol. This result is consistent with the above
statement.[5a] That is to say that the smaller DMC/IS molar ratio,
Table 3. The influence of catalyst amount ([P66614][4-I-Phen]) on the transesterification reaction[a]
.
Entry
Cat. amount (mol/mol)[b]
0.5×10-3
IS Conversion (%)
95.6
DC Selectivity (%)
MC-1 Selectivity (%)
MC-2 Selectivity (%)
1
2
3
4
5
83.2
91.3
12.3
5.2
4.5
3.6
1.1×10-3
>99.9
2.2×10-3
>99.9
98.1
1.2
0.7
4.4×10-3
>99.9
>99.0
>99.0
trace
trace
trace
trace
8.8×10-3
>99.9
[a] Reaction conditions: IS:DMC 1:7.5 eq. mol.; Reaction time 13h; Reaction temperature: 98 oC. IS conversion is determined by
GC. [b] Cat. amount based on isosorbide (mol/mol).
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Table 4. The influence of DMC/IS molar ratio on the transesterification reaction[a]
.
Entry
DMC/IS (mol/mol)
IS Conversion (%)
DC Selectivity (%)
MC-1 Selectivity (%)
MC-2 Selectivity (%)
[b]
1
2
3
4
5
6
1.5/1
2.5/1
5/1
56.1
82.6
-
60.4
65.9
5.1
39.6
34.1
4.1
-
96.4
90.2
7.5/1
10/1
20/1
>99.9
>99.9
>99.9
>99.0
>99.0
>99.0
trace
trace
trace
trace
trace
trace
[a] Reaction conditions: Reaction temperature: 98 oC. IS conversion is determined by GC. Catalyst amount: 4.4×10-3 eq.
molbased on isosorbide. [b] “-” means not detectable.
the product content of DC is lower. DC is obtained from the
further reaction of monocarboxymethyl products with DMC. In
order to acquire a high amount of DC, the DMC/IS molar ratio
should increase to at least 7.5/1 mol/mol. Further increasing the
content of DMC, the DC selectivity is almost invariable. For
economizing the raw material, the optimum DMC/IS molar ratio
is 7.5/1 mol/mol.
respectively, when the molar ratio of DMC to [P66614][4-H-Phen]
increased from 0:1 to 20:1 (Figure 4b). This indication
suggested that DMC were activated by the nucleophilic reagent
of [4-H-Phen]-. For verifying the deduction, the FT-IR analysis
was also implemented. It showed that the O atom of [4-H-Phen]-
had interacted with the –OH in isosorbide (Figure 4c). Since the
absorption peak of the C-O bond in [4-H-Phen]- and –OH in
isosorbide were moving from 1147 cm-1, 3367 cm-1 to 1122 cm-1,
3359 cm-1, respectively, once they were mixed. Similarly, the O
atom of [4-H-Phen]- had interacted with the C-O bond in DMC
(Figure 4d). The absorption peak of the C-O bond in [4-H-Phen]-
and C-O bond in DMC were shifting from 1147 cm-1 and 1261
cm-1 to 1158 cm-1 and 1266 cm-1, respectively, once they were
mixed. FT-IR patterns are in good agreement with the HNMR
results. Additionally, the HNMR and FT-IR analysis of [P66614][4-
F-Phen] and [P66614][4-I-Phen] were also simlar to that of
[P66614][4-H-Phen] (Figure S2, S3 and S4).
Possible reaction mechanism
For insight into the catalytic reaction mechanism of
trihexyl(tetradecyl)phosphonium 4-substituted phenolate ILs for
the transesterfication reaction of isosorbide and DMC, the
interactions between phenolic ILs and isosorbide or DMC were
investigated by HNMR analysis, FT-IR analysis, and Density
Functional Theory (DFT) calculations. According to the
experiment results, simulation results, and the previous
reports,[13, 27] a plausible nucleophilic activation mechanism was
proposed (Scheme 3). The oxygen atom of [4-R-Phen]- (R = H,
F, Cl, Br or I) was anticipated to facilitate the nucleophilic
To further investigate the detailed interactions between
[P66614][4-R-Phen] (R = H, F or I) and isosorbide or DMC, DFT
calculations were also carried out using the Gaussian 09
activation of isosorbide through a hydrogen bond, similar to what
28]
was mentioned in other literatures.[27a,
This deduction was
program. According the crystal structure data of isosorbide[31]
,
verified by the 1H NMR chemical shift of the mixture of
isosorbide and [P66614][4-H-Phen]. As exhibited in Figure 4a, the
proton signals of [4-H-Phen]- shifted from 6.89 ppm, 6.51 ppm,
6.31 ppm to 6.96 ppm, 6.55 ppm, 6.43 ppm, respectively, when
the molar ratio of isosorbide to [P66614][4-H-Phen] increased from
0:1 to 10:1. As expected, the chemical shifts of the H atoms of
[4-H-Phen]- were all toward the downfield shift.[29] It is explained
by the deshielding effect that the O atom of [4-H-Phen]- forms H-
bond with the –OH group of isosorbide causing a substantial
change of aromaticity of [4-H-Phen]-,[30] namely the electron
density of [4-H-Phen]- is changed. Once the IL was introduced,
the H signals of –OH groups in isosorbide at 5.11 ppm and 4.72
ppm could not be displayed any more (Figure S2), implying the
H atoms of –OH groups were activated by forming a H-bond with
O atom of [4-H-Phen]-. This is similar to the previous literature.
[27a] Moreover, DMC was also activated by the IL, which could be
deduced from the 1H NMR information of the mixture of DMC
and [P66614][4-H-Phen]. The protons of benzene ring shifted from
6.89 ppm, 6.51, 6.31 ppm to 6.92 ppm, 6.54 ppm, 6.37 ppm,
we calculated the geometry and energy optimizations at the
B3LYP/6-31+G(d,p) level. The isosorbide conformer with the
global energetic minimum are shown in Figure 5a.
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As shown in Figure 5 and Figure S5, the H-bond is formed
between the oxygen atom of [4-R-Phen]- (R = H, F or I) and the
hydrogen atom of exo or endo –OH group in isosorbide. The H-
bond interaction will bring changes to the bond lengths. The O–
H
bond length of the exo–OH group in isosorbide after
interaction with the oxygen atom in [4-H-Phen]-, [4-F-Phen]-, and
[4-I-Phen]- become longer than those before interaction (original
0.966 Å to 1.028 Å, 1.027 Å and 1.022 Å), which causes the
hydrogen atom to be lost more easily.[32] The H–O bond lengths
of H-bonds are 1.538 Å, 1.539 Å and 1.557 Å, for [4-H-Phen]-,
[4-F-Phen]-, and [4-I-Phen]-, respectively, implying the ease of
the dissociation of [4-R-Phen]- (R = H, F or I) is distinct. Thus,
the catalytic activity of these catalysts is not identical, namely,
the selectivity of DC is not equal under different 4-substituted
phenolate ILs. The endo–OH group in isosorbide after
interaction with the oxygen atom in [4-R-Phen]- (R = H, F or I)
has the similar consequence (Figure S5). Moreover, F atom of
[4-F-Phen]- can also form H-bond with the hydrogen atom of exo
or endo –OH group in isosorbide and the H–F bond lengths of
H-bond is 1.826 Å or 1.885 Å, respectively (Figure S6). This
result is consistent with abovementioned experiment result.
Table 5 and Figure S7 indicate that the H-bond interaction
between isosorbide and [4-R-Phen]- (R = H, F or I) will change
the charge density of exo–OH group in isosorbide and will
induce the electronegativity of the oxygen of exo–OH group in
isosorbide to be stronger than the original -0.776 to -0.840, -
0.840, or -0.838, which makes the nucleophilicity of the oxygen
atom of exo–OH in isosorbide stronger, causing the oxygen
atom to preferentially attack the carbon atom of the C=O in DMC.
For endo–OH group, the H-bond interaction has a similar result.
Additionally, as illustrated in Table 6 and Figure S8, [4-R-Phen]-
(R = H, F or I) also has weak interactions with DMC and induce
the electronegativity of the carbon of C=O in DMC to be weaker
than the original 1.037 to 1.068, 1.068, or 1.083, which makes
the eletrophilicity of the carbon of C=O in DMC stronger, causing
the carbon atom of C=O to be more easily attacked.
Figure 4. (a) 1H NMR chemical shift of proton of [4-H-Phen]- after addition of
different amounts of isosorbide in DMSO-d6; (b) 1H NMR chemical shift of
proton of [4-H-Phen]- after addition of different amounts of DMC in DMSO-d6;
(c) FT-IR spectra of isosorbide and [P66614][4-H-Phen] with isosorbide or
without isosorbide; (d) FT-IR spectra of DMC and [P66614][4-H-Phen] with DMC
or without DMC.
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10.1002/cssc.201802572
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FULL PAPER
more opportunities to participate in the chemical reaction.
Combined with the abovementioned experinment results, the
amount of MC-1 is always more than that of MC-2, we can
conclude that the exo–OH group is activated by IL more faster
than the endo–OH group.
Table 7. The complex engergy of isosorbide exo or endo –OH group
interaction with [4-R-Phen]-
Entry
complex engergy for
complex engergy for
exo–OH (kcal/mol)
endo–OH (kcal/mol)
[P66614][4-H-Phen]
[P66614][4-F-Phen]
[P66614][4-I-Phen]
-528690.2537
-590966.3129
-715181.7844
-528683.7320
-590959.7986
-715175.3198
Briefly, this mechanism of the transesterfication reaction
(Scheme 3) is assumed to proceed as follows: initially, the O
atom of [4-R-Phen]- of one IL molecule nucleophilic attacked the
C atom of the C=O group in DMC and electron transfer occured,
making the possible intermediates of 4-R-phenyl methyl
carbonate (R = H, F, Cl, Br or I) and [P66614][OCH3] generated
(Step 1, 2 and 3). Thereinto, the compound of 4-H-phenyl
methyl carbonate can been obtained from the transesterfication
reaction of phenol and DMC.[33] The DFT simulation result shows
the C-O bond length of the C atom of C=O group with the O
atom of phenoxyl in phenyl methyl carbonate (1.352 Å) is longer
than that of the C atom of C=O group with the O atom of
methoxyl in DMC (1.340 Å) (Figure S9), implying that the C-O
bond of phenyl methyl carbonate is more unstable and shows
more reactivity than DMC. Activation of –OH group of isosorbide
by the H-bond interactions between the O atom of [4-R-Phen]- of
another IL molecule and the –OH group of isosorbide, which
could make isosorbide transformed into an alkoxide anion and
the 4-R-Phen molecule was generated (Step 1’ and 2’).
Subsequently, the 4-R-Phen molecule reacted with pre-
generated [P66614][OCH3] molecule generating highly volatile
CH3OH and one [P66614][4-R-Phen] molecule regenerated (Step
3’). This case is similar to the previous literature.[33a] Moreover,
the pre-generated alkoxide anion of isosorbide (nucleophilic
reagent) attacked the intermediate of 4-R-phenyl methyl
carbonate (Step 4’). With the electron transfer and the
electrostatic interactions, [4-R-Phen]- dissociated from the
carboxymethyl compound, and the deprotonated isosorbide was
bonding with the C=O group of 4-R-phenyl methyl carbonate
resulting in the formation of monocarboxymethyl isosorbide (MC-
1 or MC-2). Simultaneously, the another [P66614][4-R-Phen]
molecule completed the catalytic cycle (Step 4 and 5). In the
presence of [P66614][4-R-Phen], excess DMC participated in the
transesterfication reaction of DMC and monocarboxymethyl
isosorbide. Repeating the reaction process of Step 1, 2, 3, 1’, 2’,
3’, 4’, 4 and 5, DC was produced smoothly (Step 6, 7, 8, 6’, 7’,
8’, 9’, 9 and 10). Isosorbide and DMC were both activated by the
IL catalyst, thereby promoting the high-efficiency synthesis of
DC. Isosorbide and DMC were all activated by the IL catalyst,
thereby promoting the high-efficiency synthesis of DC.
Figure 5. Optimized structure of (a) isosorbide; (b) isosorbide exo–OH group
interaction with [4-H-Phen]-; (c) isosorbide exo–OH group interaction with [4-F-
Phen]-; (d) isosorbide exo–OH group interaction with [4-I-Phen]-. C atoms
(dark gray), H atoms (light gray), O atoms (red), F atom (cyan), I atom (purple).
Table 5. Change of the calculated NPA atomic charge of the oxygen of
-OH in isosorbide influenced by ILs
Entry
Charge of O of exo–
Charge of O of endo–
OH in isosorbide
OH in isosorbide
isosorbide
-0.776
-0.840
-0.840
-0.838
-0.784
-0.848
-0.848
-0.845
[P66614][4-H-Phen]
[P66614][4-F-Phen]
[P66614][4-I-Phen]
Table 6. Change of the calculated NPA atomic charge of the carbon of
the C=O in DMC influenced by ILs
Entry
Charge of C of C=O in DMC
DMC
1.037
1.068
1.068
1.083
[P66614][4-H-Phen]
[P66614][4-F-Phen]
[P66614][4-I-Phen]
To compare the influence of IL on the exo and endo –OH
groups, we also calculated the complex engergy of isosorbide
exo or endo –OH group interaction with [4-R-Phen]- (R = H, F or
I). As shown in Table 7, the complex engergy of isosorbide exo–
OH group interaction with [4-R-Phen]- (R = H, F or I) is always
approximately 6.5 kcal/mol lower than that of endo–OH group,
which suggests the complex engergy of isosorbide exo–OH
group interaction with [4-R-Phen]- (R = H, F or I) is more stable
and the probability of its existence is bigger than that of endo–
OH group.[32] That is to say, exo–OH group in isosorbide has
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10.1002/cssc.201802572
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FULL PAPER
Scheme 3. Plausible mechanism of transesterification of DMC and isosorbide catalyzed by [P66614][4-R-Phen], R represents H, F, Cl, Br or I.
Characterization Method
Conclusions
1H NMR and 13C NMR spectra were collected in deuterated chloroform
In summary, we have successfully achieved PIC precursor DC
selectivity of 99.0% through the transesterfication reaction of
isosorbide with DMC by developing a new type of IL catalysts.
The basicity of abovementioned ILs can be readily tuned
through altering the basicity of the anions in ILs. Additionally,
according to the experiment results, and DFT calculations, a
possible nucleophilic activation mechanism has been proposed,
which demonstrated that this reaction was activated through
formation of H-bond and electrostatic interactions with IL catalyst.
Isosorbide and DMC were both activated by the IL catalyst, thus
promoting the high-efficiency synthesis of DC. This strategy of
tunable basicity and structure of anions in ILs affords an
opportunity to develop other ILs for transesterfication reaction,
thereby conveniently providing a variety of polymers via a
greener synthetic pathway. Moreover, in the presence of IL
catalysts, further investigating the polycondensation reaction of
PIC precursor (DC) is underway.
(CDCl3) or deuterated dimethyl sulfoxide (DMSO-d6) on
a Bruker
AVANCE III HD 600 NMR spectrometer with tetramethylsilane (TMS) as
the internal standard. Mass spectra data was determined on a Bruker
micrQTOF-Q II mass spectrometer. Elemental analysis (C, H and O) was
performed on an Elementar vario EL cube Elemental analyzer. The
chemical structures were identified using a Thermo Nicolet 380 FT-IR.
The selectivities of carboxymethyl resultants were calculated using a
SHIMADZU LC-20AT HPLC with an InertSustain C18 (size: 5μm, 4.6 ×
250mm) chromatographic column and a differential refraction detector
(RID). The carboxymethyl isosorbide resultants were separated with the
mobile phase of methanol and water. The calibration curves were
implemented using MC-1, MC-2 and DC concentrations between 20 ppm
and 2500 ppm with isosorbide 5-mononitrate as internal standard (Figure
S10). The reaction extent was detected using an Agilent Technologies
7890B GC. Thermogravimetric analysis (TGA) was implemented on a
Diamond TG/DTA6300 thermal analyzer under a N2 atmosphere with a
heating rate of 10 oC min-1.
Synthesis of ILs
These task-specific phenolic ILs (Scheme 1) were prepared by
neutralizing trihexyl(tetradecyl)phosphonium hydroxide ([P66614]OH) with
corresponding 4-substituted phenols according to a reported method.[34]
For instance, a solution of [P66614]OH in ethanol was obtained by a
solution of [P66614]Br in ethanol flowing past the hydroxyl ion exchange
resin column and the column was eluted with ethanol. Then, equimolar
phenol was incorporated into the abovementioned ethanol solution and
the mixed solution was stirred at 25 oC for 24h. Subsequently, ethanol
and water were removed by applying vacuum-rotary evaporation at 55 oC.
The obtained viscous liquid, [P66614][4-H-Phen], was vacuum-dried for
24h at 55 oC.
Experimental Section
Materials
Isosorbide (IS, 98%), lithium acetylacetonate (LiAcac, 99.5%), and anion
exchange resin Ambersep 900(OH) were purchased from Alfa Aesar.
DMC (≥ 99%, anhydrous) was received from Aladdin Chemistry Co., Ltd.
(shanghai, China). Trihexyl(tetradecyl)phosphonium bromide ([P66614]Br,
≥ 97%) was obtained from J&K Scientific Ltd. (Beijing, China). P-
substituted phenols, including phenol (4-H-Phen, AR), 4-fluorophenol (4-
F-Phen, 99%), 4-chlorophenol (4-Cl-Phen, 99%), 4-bromophenol (4-Br-
Phen, 98%), and 4-iodophenol (4-I-Phen, 98%), as well as, isosorbide 5-
mononitrate (98%) were purchased from Shanghai Macklin Biochemical
Co., Ltd. Among them, isosorbide was refined by recrystallization three
times in moderate quantities of acetone and anion exchange resin was
washed to neutral using deionized water. The other reagents were
employed as received.
Trihexyl(tetradecyl)phosphonium phenolate ([P66614][4-H-Phen]): 1H
NMR (600MHz, CDCl3): δ = 7.07 (m, 2H), 6.85 (d, 2H), 6.59 (m, 1H),
2.34 (m, 8H; 4 × PCH2), 1.68 – 1.22 (m, 48H; 24 × CH2), 0.86 ppm (m,
12H; 4 × CH3). MS (ESI): m/z: 483.51 [P66614], 93.02 [4-H-Phen].
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10.1002/cssc.201802572
ChemSusChem
FULL PAPER
Trihexyl(tetradecyl)phosphonium 4-fluorophenolate ([P66614][4-F-
Phen]): 1H NMR (600MHz, CDCl3): δ = 6.76 - 6.75 (m, 4H), 2.34 (m, 8H;
4 × PCH2), 1.68 – 1.22 (m, 48H; 24 × CH2), 0.86 ppm (m, 12H; 4 × CH3).
MS (ESI): m/z: 483.51 [P66614], 111.01 [4-F-Phen].
C: 47.06, H: 5.92, O: 47.02; Found: C: 47.08, H: 5.88, O: 46.98. Isolated
yield (g): 0.4.
MC-2: C8H12O6; MS (ESI): ([MC-1+Na]+) m/z: 227.05; 1H NMR (600MHz,
CDCl3): δ = 5.05-5.02(m, 1H), 4.87(t, 1H), 4.38(d, 1H), 4.31(d, 1H), 3.91-
3.82(m, 4H), 3.80(s, 3H), 2.41(s, 1H) ppm. 13C NMR (600MHz, CDCl3): δ
Trihexyl(tetradecyl)phosphonium 4-chlorophenolate ([P66614][4-Cl-
Phen]): 1H NMR (600MHz, CDCl3): δ = 6.97 (d, 2H), 6.69 (d, 2H), 2.31
(m, 8H; 4 × PCH2), 1.68 – 1.22 (m, 48H; 24 × CH2), 0.86 ppm (m, 12H; 4
× CH3). MS (ESI): m/z: 483.51 [P66614], 126.98 [4-Cl-Phen].
=
155.41, 88.57, 80.63, 77.46, 76.28, 75.86, 70.54, 55.32 ppm.
Elemental analysis (%) calculated for (C8H12O6): C: 47.06, H: 5.92, O:
47.02; Found: C: 47.09, H: 5.87, O: 47.05. Isolated yield (g): 0.2.
Trihexyl(tetradecyl)phosphonium 4-bromophenolate ([P66614][4-Br-
Phen]): 1H NMR (600MHz, CDCl3): δ = 7.09 (d, 2H), 6.63 (d, 2H), 2.30
(m, 8H; 4 × PCH2), 1.68 – 1.25 (m, 48H; 24 × CH2), 0.86 ppm (m, 12H; 4
× CH3). MS (ESI): m/z: 483.50 [P66614], 172.93 [4-Br-Phen].
Then, these purified compounds were used for making HPLC calibration
curves. It should be noted that these transesterfication products have no
standard substances. Therefore, the internal standard method was
adopted for the HPLC analysis. Subsequently, in the presence of IL
catalysts, we investigated the transesterfication reaction of isosorbide (IS)
and DMC.
Trihexyl(tetradecyl)phosphonium
4-iodophenolate
([P66614][4-I-
Phen]): 1H NMR (600MHz, CDCl3): δ = 7.25 (d, 2H), 6.54 (d, 2H), 2.30
(m, 8H; 4 × PCH2), 1.68 – 1.26 (m, 48H; 24 × CH2), 0.86 ppm (m, 12H; 4
× CH3). MS (ESI): m/z: 483.50 [P66614], 218.92 [4-I-Phen].
Acknowledgements
Synthesis of PIC precursor
This work is financially supported by the National Natural
Science Foundation of China (21676274), the Transformational
Technologies for Clean Energy and Demonstration, Strategic
Priority Research Program of the Chinese Academy of Sciences,
Grant No. XDA 21030500 and the National Natural Science
Foundation of China (21878316).
The transesterification reaction was operated in a 250 mL four-necked
round-bottom flask equipped with a feeding funnel, a nitrogen inlet, a
mechanical stirrer and a dephlegmator connected to a liquid dividing
head. In an instantial experiment, under a N2 atmosphere, isosorbide (10
g, 0.0684mol), DMC (46.23 g, 0.5132mol) and [P66614][4-H-Phen] (0.346
g, 0.88 mol% based on isosorbide) were added into the flask with stirring
and the oil bath temperature was gradually increased to 98 oC. For
removing methanol and preventing excess volatilization of DMC, the
temperature of column top was maintained between 40 oC and 65 oC. To
confirm the reaction extent, the column top distillate was analyzed by GC
once 1h to calculate the CH3OH content. The reaction was stopped when
the methanol was trace quantity. The reaction liquid was concentrated,
and then taking appropriate amount of the viscous product dissolved in
methanol and isosorbide 5-mononitrate was added as internal standard.
Subsequently, the sample was injected into the HPLC and the
selectivities of products were determined using calibration curves.
Keywords: Dicarboxymethyl isosorbide • Dimethyl carbonate •
Ionic liquids • Isosorbide • Polymers
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Wei Qian, Xin Tan, Qian Su, Weiguo
Cheng,* Fei Xu, Li Dong, and Suojiang
Zhang*
[P66614][4-R-Phen] (R=H, F, Cl, Br, I)
with weak alkalinity not only can
activate isosorbide but also can
activate DMC, thereby efficiently
promoting this reaction and achieving
the more reactive PIC precursor DC
with a high level. This strategy affords
an opportunity to develop other ILs for
transesterfication reaction, thereby
Page No. – Page No.
Transesterfication of isosorbide with
dimethyl carbonate catalyzed by task-
specific ionic liquids
conveniently
polymers via
pathway.
providing
various
a
greener synthetic
This article is protected by copyright. All rights reserved.