Synthesis of dialkyl carbonates from CO2 and alcohols via electrogenerated N-heterocyclic carbenes

Article abstract of DOI:10.1016/j.elecom.2012.09.028

Here we describe the utilization of NHC-CO₂ which transfers CO₂ to alcohols for the synthesis of dialkyl carbonates under mild conditions with high conversion and excellent selectivity. In addition NHC-CO₂ formation was supported by electrochemical analysis. This study provides a new procedure for the synthesis of dialkyl carbonates of chemical and pharmaceutical interest, as well as expands the applications of NHCs in CO₂ fixation.

Full text of DOI:10.1016/j.elecom.2012.09.028

Electrochemistry Communications 25 (2012) 116–118
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications
journal homepage: www.elsevier.com/locate/elecom
Synthesis of dialkyl carbonates from CO2 and alcohols via electrogenerated
N-heterocyclic carbenes
⁎
La-Xia Wu, Huan Wang, Yan Xiao, Zhuo-Ying Tu, Bin-Bin Ding, Jia-Xing Lu
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, Shanghai 200062, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 July 2012
Received in revised form 18 September 2012
Accepted 18 September 2012
Available online 24 September 2012
Here we describe the utilization of NHC–CO2 which transfers CO2 to alcohols for the synthesis of dialkyl carbon-
ates under mild conditions with high conversion and excellent selectivity. In addition NHC–CO2 formation was
supported by electrochemical analysis. This study provides a new procedure for the synthesis of dialkyl carbon-
ates of chemical and pharmaceutical interest, as well as expands the applications of NHCs in CO2 fixation.
© 2012 Elsevier B.V. All rights reserved.
Keywords:
N-heterocyclic carbenes
Electrosynthesis
CO2
Dialkyl carbonates
Ionic liquid
1. Introduction
2. Experimental
The first stable nucleophilic carbenes were reported by Arduengo et
al. [1] in the early 1990s. Since then N-heterocyclic carbenes (NHCs)
have been extensively applied in organic synthesis, particularly as ver-
satile ligands for various organometallic complexes [2]. NHCs have
also been used as effective organocatalysts, reviewed by Ender et al.
[3]. NHCs are usually generated by the deprotonation of corresponding
imidazolium salts with strong bases [4,5]. However, for certain synthet-
ic applications of NHCs, forming the carbene only as an intermediate is
easier. In these cases, the chemical reduction route may not be desir-
able. Electrogeneration by the cathodic reduction of imidazolium or
thiazolium-based room temperature ionic liquids (RTILs) [6–10] offers
an alternative route to NHC synthesis that is both rapid and efficient.
NHCs have been recently used as CO2 transformation catalysts
[11–13] with epoxides or propargyl alcohols, thereby providing a new
approach to the synthesis of cyclic carbonates. During our studies on
CO2 fixation for the production of value-added chemicals [14–16], CO2
was first electro-activated with alcohols to synthesize dialkyl carbonates
[15] in CO2-saturated RTILs. Prompted by these applications of NHC–CO2
in organic synthesis and our continuous research on CO2 fixation in
RTILs, NHC–CO2 was used in this study to synthesize dialkyl carbonates
from CO2 and alcohols under mild conditions.
2.1. Apparatus and reagents
Voltammetric measurements were conducted using the CHI650C
electrochemical station in a conventional three-electrode cell. The work-
ing electrode was a glassycarbon (GC) disk (r=1 mm). The auxiliary
electrode was a platinum spiral wire. The reference electrode was Ag/
AgI/0.1 M n-Bu4NI in N,N′-dimethylformamide. Galvanostatic electroly-
sis was performed using a direct current-regulated power supply
(HY3002D, HYelec®,China). 1HNMR spectra were recorded on an
AVANCE 500 (500 MHz, Bruker, Germany) spectrometer in CDCl3 with
Me4Si as an internal standard. Mass spectra were obtained using a
5973-N spectrometer connected with an HP 6890 gas chromatograph
(Agilent, USA). Alcohols were used as received (J&K). The ionic liquids
used were prepared as described in literature [17].
2.2. General electrochemical procedure
Preparative electrolyses were conducted at 40 °C in a two-
compartment cell. An ionic liquid (5 mL) solution containing benzyl
alcohol 1a (1.0 mmol) was electrolyzed under galvanostatic con-
trol (j=18.9 mA cm−2) under N2 atmosphere. After consuming a
charge of 2.0 Fmol−1, the current was switched off; CO2 bubbling
was started and stirring was performed for 2 h. The electrolyte
was then esterified by adding anhydrous K2CO3 (1.0 mmol) and
MeI (3.0 mmol), and the mixture was stirred at 60 °C for 5 h.
After all these reactions, the solution was extracted with ethyl
ether. The products were analyzed by gas chromatography to ob-
tain the yields based on the starting materials.
⁎
Corresponding author. Tel.: +86 21 62233491.
E-mail address: jxlu@chem.ecnu.edu.cn (J.-X. Lu).
1388-2481/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.elecom.2012.09.028

L.-X. Wu et al. / Electrochemistry Communications 25 (2012) 116–118
117
3. Results and discussion
3.1. Cyclic voltammogram of BMIMBF4
Fig. 1 shows the cyclic voltammogram (CV) of BMIMBF4 in the pres-
ence of CO2 and 1a. For comparison, the electrochemical behavior of
BMIMBF4 saturated with N2 was recorded on a GC electrode which
showed the appearance of an irreversible oxidation peak at −0.261 V
(curve a) corresponding to NHC oxidation. After saturation with CO2,
BMIMBF4 reduction became much easier than that under N2 atmosphere.
The oxidation peak moved positively to −0.083 V with sharply de-
creased peak current (curve b), indicating that CO2 reaction with NHCs
to form NHC–CO2 resulted in free NHCs with decreased electroactivity.
Consequently, the peak current of oxidation also decreased.
To determine whether alcohol adducts of NHC were formed from
NHCs and alcohols, 1a (100 mM) was added to BMIMBF4 under N2
atmosphere (curve c). To avoid concentration effects on adduct forma-
tion, the 1a concentration was made equal to the CO2 solubility in
BMIMBF4 under the experimental conditions [18]. After adding 1a, the
behavior of NHCs slightly changed (curve c vs. a), which indicated that
NHCs cannot react with 1a. The CVs also showed that 1a cannot be
reduced or oxidized under the experimental conditions.
Scheme 1. Synthesis of dialkyl carbonates from CO2 and alcohols via electrogenerated NHCs.
The nature of cathode materials strongly influenced the conversion
of 1a, but insignificantly affected the selectivity of 2a. Ti was the most
efficient cathode material, which resulted in 90% conversion and excel-
lent selectivity (96%, Table 1, entry 5). The other cathode materials,
including stainless steel (Ss), Ni, Cu and Pt, gave moderate conversions
(Table 1, entries 1–4).
The current density and charge passed during electrolyses also
influenced the conversion of 1a, but minimally affected the selectivity of
product 2a. In fact, both low and high current densities led to lower con-
versions (Table 1, entries 6–9). The results obtained at low current densi-
ties can probably be ascribed to a longer electrolysis time, which can lead
to side reactions such as dimerization and dealkylation of imidazolium
cations [6]. The results at high current densities can be ascribed to a
lower current efficiency with increased ohmic component. The best result
was obtained using 18.9 mA cm−2 current density and 2.0 Fmol−1
charge of 1a (Table 1, entry 5). Further increased consumed charge did
not improve the conversion (Table 1, entry 13).
The temperature can be a crucial factor for this reaction. High tem-
peratures can decrease the RTIL viscosity and increase the conductivity
[19], as well as affect the CO2 solubility. Therefore, the effects of the
reaction temperature were examined (Table 1, entries 5, 14–17).
Increased temperature from 20 °C to 40 °C favored the reaction, but fur-
ther increased temperature resulted in lower conversion and selectivity
(Table 1, entries 16 and 17). The highest conversion and selectivity
were achieved at 40 °C.
Extremely high RTIL viscosities can strongly affect the rate of mass
transport within a solution. Low-rate mass transport may result in a
long reaction time to obtain a satisfactory value. Thus, the effects of the
stirring time (Table 1, entries 5 and 18–22) were examined. The highest
3.2. Electrosynthesis of benzyl methyl carbonate from CO2 with benzyl
alcohol
This investigation aimed to establish a new procedure for the
electrosynthesis of dialkyl carbonates in RTILs (Scheme 1). Before
optimizing the procedure, the addition time of 1a was examined.
Based on the above-discussed CVs, the addition time of 1a was speculat-
ed to have minimal influence on the reaction. Parallel tests were carried
out under the same conditions, expect for the addition time of 1a. About
96% selectivity and 90% conversion were achieved when 1a was added
before electrolysis, whereas 94% selectivity and 89% conversion were
achieved when 1a was added after electrolysis. These results agreed
with our speculation. However, free NHCs are sensitive to air- and mois-
ture. To avoid the introduction of air and water, 1a was added before
electrolysis.
To optimize the procedure, the effects of different parameters on the
selectivity of the target product and substrate conversion were investi-
gated using 1a as a model compound. Accordingly, several electrolyses
were carried out using different cathodes, current densities (j), num-
bers of Faradays per mole of 1a(Q), temperatures of electrolysis and
stirring (T), stirring times (t), and RTILs. The electrolysis results are
summarized in Table 1.
Table 1
a
Electrolyses of solutions of benzyl alcohol 1a in BMIMBF4 followed by bubbling CO2
under galvanostatic conditions.
Entry Cathode
Q
j
T
t
Conversion Selectivity
(F mol−1
)
(mA cm−2
)
(°C) (h)
(%)
(%)
1
2
3
4
5
6
7
8
Ss
Ni
Cu
Pt
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
Ti
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
1.0
1.2
1.5
2.5
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
18.9
18.9
18.9
18.9
18.9
13.9
16.7
21.1
23.9
18.9
18.9
18.9
18.9
18.9
18.9
18.9
18.9
18.9
18.9
18.9
18.9
18.9
18.9
18.9
40
40
40
40
40
40
40
40
40
40
40
40
40
20
30
50
60
40
40
40
40
40
40
40
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
0.5
57
62
59
73
90
73
82
70
52
63
69
82
79
56
72
70
45
44
91
94
92
85
96
93
94
91
88
94
94
94
92
25
64
76
31
75
91
92
93
96
33
30
0.16
0.12
0.08
0.04
0.00
b
c
a
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23b
24c
0.75 54
1.0
1.5
2.5
2.0
2.0
78
89
92
63
69
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
E (V vs. Ag/AgI/I-)
a
Fig. 1. Cyclic voltammograms recorded at 100 mV s−1 in BMIMBF4 with GC electrode
at 25 °C (a) neat BMIMBF4 saturated with N2, (b) as (a) saturated with CO2, (c) as
(a) +100 mM benzyl alcohol.
Electrolyses carried out in divided cell, alcohols=0.2 mol L−1, PCO2=1 atm.
Electrolyses carried out in BMIMPF6.
Electrolyses carried out in BDMIMBF4.
b
c

118
L.-X. Wu et al. / Electrochemistry Communications 25 (2012) 116–118
Table 2
also afforded dialkyl carbonates 2d–2f with excellent selectivity but low
conversion (Table 2, entries 4–6). In the case of tertiary alcohol 1g and
phenol 1h no carbonate was detected (Table 2, entries 7 and 8). For tertia-
ry alcohol 1g, the considerably large steric hindrance may have rendered
it unable to undergo reaction to form the carbonate. For phenol 1h,
PhOMe (2h) was obtained. The poor nucleophilicity of the 1h anion is
probably responsible for the unsuccessful formation of the corresponding
carbonate.
Synthesis of dialkyl carbonates via electrogenerated NHCs according to the conditions
(Table 1, entry 5).
Entry Substrate
1
Products
Conv. (%) S (%)
1a
1b
1c
1d
1e
1f
2a 90
96
95
97
94
95
93
2
3
4
5
6
2b 89
2c 90
2d 47
2e 44
2f 24
4. Conclusions
A new procedure for dialkyl carbonate synthesis from CO2 and alco-
hols in RTILs was established. The dual role of RTILs as green solvent and
NHC precursor avoided the use of organic solvents and the addition of
supporting electrolytes.
Acknowledgments
7
8
1g
1h
2g
–
–
This work was financially supported by the Project for the Nation-
al Natural Science Foundation of China (20973065, 21173085), and
Key Project for Subject Construction of Shanghai (B409).
2h 100
>96
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To examine the effectiveness and generality of the proposed meth-
odology, the investigation was extended to other alcohols (1b–1h)
under optimized conditions (Table 1, entry 5). The analysis results are
reported in Table 2, from which some conclusions were drawn. The
primary alcohols 1a–1c were converted into the corresponding dialkyl
carbonates 2a–2c with excellent conversion and selectivity (Table 2,
entries 1–3). Under the same conditions, the secondary alcohols 1d–1f