Synthesis of cyclic carbonates from diols and CO2 catalyzed by carbenes

Article abstract of DOI:10.1039/c6cc05730f

The synthesis of cyclic carbonates from epoxides and CO₂ is a well-established reaction, whereas the synthesis of cyclic carbonates from diols and CO₂ is considerably more challenging, and few efficient catalysts are available. Here, we describe heterocyclic carbene catalysts, including one derived from a cheap and efficient thiazolium salt, for this latter reaction. The reaction proceeds at atmospheric pressure in the presence of an alkyl halide and Cs₂CO₃. Reaction mechanisms for the transformations involved are also proposed.

Full text of DOI:10.1039/c6cc05730f

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Synthesis of cyclic carbonates from diols and CO2 catalyzed by
carbenes
Felix D. Bobbink,a Weronika Gruszka,a Martin Hulla, Shoubhik Das and Paul J. Dyson*
a the authors contributed equally to this work
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The synthesis of cyclic carbonates from epoxides and CO2 is a well- problem by the implementation of a suitable catalyst system and a
established reaction, whereas the synthesis of cyclic carbonates dehydrating agent. Both heterogeneous and homogeneous
from diols and CO2 is considerably more challenging, and few catalysts have been proposed for this reaction. For example, a
efficient catalysts are available. Here, we describe heterocyclic heterogeneous cascade catalysis comprising CeO2 and 2-
carbene catalysts, including one derived from
a
cheap and cyanopyridine is arguably the most efficient system.15 However, this
efficient thiazolium salt, for this latter reaction. The reaction process requires harsh reaction conditions (150 °C and 50 bars of
proceeds at atmospheric pressure in the presence of an alkyl CO2), an expensive reagent (2-cyanopyridine) and the activity is
halide and Cs2CO3. Reaction mechanisms for the transformations highly sensitive to the size of ceria particles.
A
number of
involved are also proposed.
homogeneous metal-free catalysts run under milder conditions and,
interestingly, all are based on the 1,8-diazabicyclo[5.4.0]undec-7-
ene (DBU) aided insertion of CO2. Different reagents are used to
facilitate the subsequent alkylation step to afford cyclic carbonates
in good yield under only 10 bars of CO2.16 The reaction may even
proceed at an atmospheric pressure of CO2 if DBU and the alkyl
halide are used in large excess.17 The same mild conditions are
employed in a system in which tosyl chloride and triethylamine are
used to afford cyclic carbonates with 6-membered rings in good
yields.18 Ultimately, only a few efficient processes exist and finding
an increasingly sustainable process for this reaction remains
important.
Utilization of carbon dioxide (CO2) in the production of fine
chemicals and synthetic fuels would contribute towards a more
sustainable chemical industry. However, CO2 is
a challenging
molecule to activate as it is thermodynamically stable and
kinetically inert in many transformations. Accordingly, only a few
energy-efficient processes which employ CO2 have been
commercialized.1
From a thermodynamic perspective, oxygenated cyclic carbonates
are particularly suitable synthetic targets from CO2. These
compounds have been exploited as electrolytes for lithium ion
batteries,2 building blocks for polymeric materials,3,4 solvents5,6 and
intermediates in the synthesis of compounds such as dimethyl
carbonate (DMC)7 and ethylene glycol.8 Industrial production of
cyclic carbonates involves either the transesterification of diols with
phosgene in an energy-intensive process9 or the cycloaddition of
CO2 to epoxides.10–12 Despite the latter route exhibiting 100 % atom
economy and industrial scalability, the synthesis of epoxides
combined with their high reactivity and volatility are problematic.
Recently, more stable, biodegradable 1,2-diols have been proposed
as promising alternatives for the synthesis of cyclic carbonates with
CO2.13 Their reaction with CO2 is, however, neither kinetically nor
Recently, N-heterocyclic carbenes (NHCs) have gained interest as
catalysts for reactions which employ CO2 as a substrate.19–23 This
stems from their ability to act as nucleophiles which activate CO2
via the formation of imidazolium carboxylates.24,25 Interestingly,
these intermediates have been previously reported to catalyze the
synthesis of cyclic carbonates from diols employing DMC as the
carbonyl source rather than CO2.26 Herein, we show the utility of
carbene catalysts for the synthesis of cyclic carbonates from diols
and CO2 and, based on key experiments, propose plausible
mechanisms for this transformation.
thermodynamically-favored due to the formation of water as the Initially, reaction conditions were optimized using 1-phenyl-1,2-
sole by-product.14 Attempts have been made to by-pass this ethanediol (1a) as the substrate, see Table 1. Several imidazolium
and thiazolium carbene catalysts (1c-4c) were evaluated. NHCs 1c
and 3c19 and the thiazolium carbene catalysts 1b and 1d27 have
been previously shown to catalyze the N-methylation of amines
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using CO2 as the carbon source. The efficiency of a variety of bases also be used (Table 1, entries 21 and 22), whereas no reaction was
and alkyl halides was also studied as they are essential for the observed in toluene (Table 1, entry 23).
reaction to proceed (see below).16,18
The optimum reaction temperature is 90 °C, with lower
The ability of cesium carbonate (Cs2CO3) to activate CO2 and other temperatures leading to a decrease in product yield (Table 1,
small molecules28–32 encouraged us to employ it as a base in the entries 13 and 14) and with more elevated temperatures, e.g. 110
reaction. Dibromomethane (CH2Br2) was also used due its efficiency °C, leading to deactivation of the catalytic system (Table 1, entry
in forming an effective leaving group.16 The activity of 1c–4c was 15). The alkyl halide also affects the reaction, in particular, 2 eq. of
investigated in the presence of 2 eq. of CH2Br2 and 2 eq. of Cs2CO3. bromobutane (C4H9Br) results in a higher yield than 5 eq. of CH2Br2
The highest yields of styrene carbonate (1b) were obtained with (Table 1, entries 8 and 20). The other alkyl halides evaluated were
catalysts 1c and 2c (Table 1, entries 1 and 2). In contrast, 3c and 4c less effective (Table 1, entries 16, 17 and 19).
resulted in lower product yields (Table 1, entries 3 and 4). The
Table 1 Optimization of the reaction conditions for the transformation of 1-phenyl-1,2-
effect of quantities of CH2Br2 and Cs2CO3 on the reaction was
ethanediol (1a) used as a model substrate.
studied. Increasing CH2Br2 to 5 eq. resulted in 61 % yield of styrene
carbonate 1b (Table 1, entry 6). Interestingly, a larger excess of the
base (3 eq. instead of 2 eq.) led to a slight decrease in the yield of
1b (Table 1, entry 7). It should be noted that the reaction proceeds
in low yield using Cs2CO3 as the base in the absence of CO2 (Table
S2, entry 2). However, 13C labeled CO2 was used to confirm that the
main source of the carbonyl group incorporated in the cyclic
carbonate product originates from CO2 (see NMR spectra
comparing the 13C NMR spectra of non-labeled and 13C labeled
products in the SI).
Entry
Catalyst
Alkyl halide
(Eq.)
Base (Eq.)
Yield (%)
The enhanced activity of catalyst 2c might be due to a greater
stability to moisture; note that 1b was not observed in a control
experiment in which water was introduced into the system (Table
S2, entry 6). In the initial catalytic runs the active carbene catalyst
was generated prior to reaction by the deprotonation of the
corresponding salt with NaH. Subsequently, we found that the in
situ generation of the carbene catalyst yielded 1b in 71 % in
presence of 3 eq. of Cs2CO3 (Table 1, entry 8). Interestingly, in a
previous study using the ionic liquid 1-butyl-3-methylimidazolium
hexafluorophosphate ([bmim][PF6]) as solvent, the increased
carbonate yield was attributed to the increased solubility of CO2.16
Presumably, an active carbene was also generated by the
deprotonation of the imidazolium salt by DBU – the ability of DBU
and Cs2CO3 to deprotonate [bmim][BF4] to form a NHC has been
reported.33 DBU was evaluated under our conditions, but yielded 1b
in a significantly lower yield (Table 1, entry 11).16 Na2CO3 and K2CO3
were evaluated in place of Cs2CO3, but afford the product in 0 and 5
% yield, respectively, presumably due to the lower solubility of
these carbonates in DMF (Table 1, entries 9 and 10). No product
was observed with Et3N (Table 1, entry 12). We speculate that Et3N,
which is often employed in the Stetter reaction, may undergo a
Menshutkin reaction with CH2Br2 thereby inhibiting the reaction.
Notably, Cs2CO3 was found to be the optimal base in this reaction
owing to its ability to generate the active carbene catalyst as well as
to act as a minor carbonyl donor and a dehydrating agent.
1
1ca
2ca
3ca
4ca
1ca
2ca
2ca
2c
2c
2c
2c
CH2Br2 (2)
CH2Br2 (2)
CH2Br2 (2)
CH2Br2 (2)
CH2Br2 (5)
CH2Br2 (5)
CH2Br2 (5)
CH2Br2 (5)
CH2Br2 (5)
CH2Br2 (5)
CH2Br2 (5)
CH2Br2 (5)
CH2Br2 (5)
CH2Br2 (5)
CH2Br2 (5)
(CH2Br)2 (5)
(C2H4Br)2 (5)
C4H9Br (5)
C4H9Cl (5)
C4H9Br (2)
C4H9Br (2)
C4H9Br (2)
C4H9Br (2)
Cs2CO3 (2)
Cs2CO3 (2)
Cs2CO3 (2)
Cs2CO3 (2)
Cs2CO3 (2)
Cs2CO3 (2)
Cs2CO3 (3)
Cs2CO3 (3)
Na2CO3 (3)
K2CO3 (3)
DBU (3)
44
45
29
32
42
61
53
71
0
2
3
4
5
6
7
8
9
10
11
5
21
0
12
2c
Et3N (3)
13 (50 °C)
14 (70 °C)
15 (110 °C)
16
2c
Cs2CO3 (3)
Cs2CO3 (3)
Cs2CO3 (3)
Cs2CO3 (3)
Cs2CO3 (3)
Cs2CO3 (3)
Cs2CO3 (3)
Cs2CO3 (3)
Cs2CO3 (3)
Cs2CO3 (3)
Cs2CO3 (3)
5
2c
12
25
37
32
59
20
81
33
50
0
2c
2c
17
2c
18
2c
19
2c
20
2c
21 (DMSO)
22 (DMA)
23 (Toluene)
2c
2c
2c
Reaction conditions: 1a (0.5 mmol), catalyst (20 mol%), alkyl halide (1–2.5 mmol),
base (1– 1.5 mmol), DMF (4 mL), CO2 (1 atm.). Yields were determined by GC-FID
using n-decane as internal standard. [a] The carbene catalyst was generated with
NaH. Otherwise, the carbene is generated in situ using an extra 20 mol% base.
Based on the optimized conditions, which afford 1b in up to 81 %
yield, the scope of the reaction was explored using catalyst 2c
(Table 2). The substrates varied from 1,2-diols to 1,3-diols (2a – 4a)
bearing functional groups with varying steric influence. The diols
Dimethylformamide (DMF) was selected as a reaction solvent as it
can activate CO2.34 As expected, other polar aprotic solvents
(dimethyl sulfoxide (DMSO) or dimethylacetamide (DMA)) could
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were subjected to the optimized conditions of 2 eq. bromobutane, that while 25 % of 1b was obtained in the absence of CO2 (Table S1,
3.2 eq. Cs2CO3 at 90 °C and 1 atm. CO2 pressure.
entry 2), addition of CO2 increased the yield of 1b to 81 % (Table 1,
entry 17).
The model product 4-phenyl-1,3-dioxolan-2-one was isolated in 61
% yield (Table 2, entry 1). Five-membered cyclic carbonates, 4,5- In the mechanism in Scheme 1, step 2 involves the generation of an
diphenyl-1,3-dioxolan-2-one (2b) and propylene carbonate (3b) alkoxide I and the parallel attack of the carbene-CO2 adduct on
were obtained in yields of 63 and 54 %, respectively (Table 2, CH2Br2 after activation of CO2 by the carbene in step 1. Nucleophilic
entries 2 and 3). The six-membered cyclic carbonate, 5-phenyl-1,3- attack of the alkoxide I on intermediate II in step 3 results in the
dioxan-2-one (4b) was produced in 53 % (Table 2, entry 4). These elimination of the leaving group and formation of intermediate III.
yields are comparable to those obtained using alternative methods In step 4, the secondary hydroxyl group of the diol is deprotonated,
(see Table S1 for a comparison).16,17
leading to the generation of intermediate IV and, in the final step
(step 5), the intramolecular addition of the alcohol occurs in
intermediate IV, which affords the cyclic carbonate and regenerates
the catalyst. Notably, bromomethanol is eliminated as a leaving
group, however bromomethanol is unstable, and hence it is
believed to decompose to a mixture of hydrogen bromide (HBr) and
formaldehyde (CH2O).37 Note, the formation of these side-products
was not detected by spectroscopic or chromatographic studies,
possibly due to neutralization of HBr by Cs2CO3 and the volatility of
CH2O.
Table 2 Reaction of various diols with CO2 under optimized conditions.
Entry
1
Reactant
Product
Yield (%)
61a
2
63a
3
4
54b
53b
HO
OH
Ph
4a
Reaction conditions: substrate (0.5 mmol), cat. 2c (20 mol%), C4H9Br (1.0 mmol),
Cs2CO3 (1.6 mmol), DMF (4 mL), CO2 (1 atm), 24 h, 90 °C. [a] Isolated yield. [b] GC
yield.
Scheme 1 Tentative mechanism for the carbene-catalyzed reaction of diols and CO2 to
form cyclic carbonates. The substituents of the catalyst are omitted for clarity.
On the basis of our results and previous literature, two plausible
reaction mechanisms in Schemes 1 and 2 are suggested.16,26,35,36
Scheme 1 presents the principal mechanism for the carbene-
catalyzed reaction. As mentioned above, both the base and alkyl
halide are essential in the reaction, as confirmed in control
experiments in which no carbonate was formed in their absence
(Table S1, entries 4 and 5). Scheme 2 represents the mechanism for
the minor non-catalytic formation of cyclic carbonate in the
absence of CO2 (Table S1, entry 2). C4H9Br is included in the second
mechanism due to detection of dibutyl carbonate and n-butanol in
the reaction mixture using GC-MS, see SI. However, a similar
mechanism is likely to take place in presence of other alkyl halides.
Moreover, both of these mechanisms appear to occur concurrently
to form the cyclic carbonate. This hypothesis is based on our finding
Scheme 2 Proposed mechanism for the non-catalytic reaction of diols and CO2 to form
cyclic carbonates.
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The secondary (non-catalytic) reaction in Scheme 2 proceeds by the 13
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attack of Cs2CO3 on C4H9Br in step 1, leading to the formation of
14
intermediate II (dibutyl carbonate was observed by GC-MS). Similar
to the mechanism in Scheme 1, the reaction of the alkoxide I with
15
intermediate II in step 2 leads to the elimination of butanol
(observed by GC-MS) and the formation of intermediate III. Again, 16
the deprotonation of the secondary hydroxyl group in intermediate
17
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III in step 3 results in the formation of intermediate IV. The final
cyclization in step 4 leads to the elimination of the second leaving
18
group and the formation of the cyclic carbonate.
19
S. Das, F. D. Bobbink, G. Laurenczy and P. J. Dyson, Angew.
Chem., Int. Ed., 2014, 53, 12876–12879.
In summary, the work presented here offers an approach for the
synthesis of cyclic carbonates from diols and CO2. The proposed
system benefits from the use of environmentally-friendly metal-free
carbene catalysts. Using this methodology cyclic carbonates were
obtained under mild conditions (90 °C and atmospheric pressure of
CO2) in good yield and comparable or better to those obtained with
other catalysts that operate under more forcing conditions. Based
on labelling studies and other experiments two-mechanisms are
proposed, one non-catalytic and one catalytic that account for the
overall reaction.
20
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Acknowledgements
The EPFL, the CTI Swiss Competence Center of Energy
Research (SCCER) on Heat and Electricity Storage and the
Fondation Claude et Juliana are thanked for funding.
27
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