Synthesis of Carbonate Esters by Carboxymethylation Using NaAlO2 as a Highly Active Heterogeneous Catalyst

Article abstract of DOI:10.1021/acs.oprd.8b00333

Sodium aluminate is presented as a highly active heterogeneous catalyst that is able to convert a range of alcohols into the corresponding unsymmetrical carbonate esters by reaction with dimethyl carbonate. Preparing NaAlO₂ via spray drying boosts the basic properties and the activity of the catalyst.

Full text of DOI:10.1021/acs.oprd.8b00333

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Communication
Synthesis of carbonate esters by carboxymethylation
using NaAlO2 as a highly active heterogeneous catalyst
Sreerangappa Ramesh, Kiran Indukuri, Olivier Riant, and Damien P. Debecker
Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.8b00333 • Publication Date (Web): 20 Nov 2018
Downloaded from http://pubs.acs.org on November 20, 2018
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Synthesis of carbonate esters by carboxymethylation
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using NaAlO as a highly active heterogeneous
2
catalyst
Sreerangappa Ramesh, Kiran Indukuri, Olivier Riant, and Damien P. Debecker*
UCLouvain, Institute of Condensed Matter and Nanosciences (IMCN), Place Louis Pasteur 1,
box L4.01.09, 1348 Louvain-La-Neuve, Belgium, damien.debecker@uclouvain.be
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TOC Figure
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O
R
O
OMe
Large scope of
alcohols
+
O
O
O
OMe
O
DMC
R2
O
O
O
R
OH
R1
O
OMe
Alkyl
R2
O
OMe
OH
R1
R1
O
O
OH
Unsymmetrical
carbonate esters
OH
R1
Alkyl
Spray-dried
NaAlO catalyst
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Abstract. Sodium aluminate is presented as a highly active heterogeneous catalyst able to
convert a range of alcohols into the corresponding unsymmetrical carbonate esters by reaction
with dimethyl carbonate. Preparing NaAlO via spray drying allows to boost the basic properties
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and the activity of the catalyst.
Keywords: Spray drying; sodium aluminate; dimethyl carbonate; mixed carbonate esters;
carboxymethylation
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Carbonate esters are important compounds in the bulk chemicals industry, as they find
applications as green solvents, lubricating oils and fuel additives.1-2 Carbonates esters are also
key compounds in organic synthesis.3-5 They serve as protecting groups of alcohols owing to
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their higher stability compared to the corresponding esters under basic conditions.6-7 In
particular, allylcarbonates are valuable intermediates in asymmetric allylic substitution
reactions.8
Unfortunately, the currently applied routes toward these chemicals rely on toxic reagents –
e.g. chloroformates, trimethylamine, pyridine, phosgene – and on the use of homogeneous
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catalysts, which are non-reusable and require onerous separation procedures (Scheme 1A). A
greener alternative is the carboxymethylation reaction using (i) environmental friendly reagents
and (ii) heterogeneous catalysts.
Dialkyl carbonates are interesting reagents for the synthesis of mixed carbonate esters from
alcohols. For example, dimethyl carbonate (DMC) – which can be obtained by the reaction of
methanol with CO210-12 – is emerging as a green and versatile reagent for a wide range of
chemical transformations, including carboxymethylation.13-14 Various homogeneous acid or base
catalysts have been reported for the carboxymethylation reactions using DMC (Scheme 1B). For
example, in situ generated lanthanum(III) nitrate alkoxide was proposed by Hatano et al.15
Recently, McElroy et al. reported various homogeneous acids as efficient catalysts.16 While
primary alcohols were efficiently converted, secondary alcohols required stoichiometric amounts
of acids to undergo the carboxymethylation reaction. An attractive route was proposed by Mutlu
et al., using an organo catalyst (1,5,7,-triazabicyclo[4.4.0]dec-5-ene) able to convert a large
range of alcohols – including tertiary alcohols – to the corresponding unsymmetrical carbonates
in a solvent-less reaction with DMC.17
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Heterogeneous catalysts would offer an even greener pathway, provided they can demonstrate
high efficiency, selectivity, and recyclability. However, up until now, only a handful of
heterogeneous catalysts showed significant activity for the synthesis of mixed carbonate esters
using di alkylcarbonate (Scheme 1B). Sartorio et al. developed an efficient hybrid organic-
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inorganic catalyst by anchoring Triazabicyclodecene (TBD) onto a MCM-41 silica. Figueras et
al. used fluorinated hydrotalcites, MgLa mixed oxides and CsF/α-Al O as basic solid catalysts,
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demonstrating that the rate of the reaction was roughly proportional to the number of strongly
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basic sites present on the catalyst. Kantam et al. proposed a nano crystalline magnesium oxide
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showing good efficiency, selectivity and stability. Stanley et al. used various basic catalysts for
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the carboxymethylation of cinnamyl alcohol. Tabanelli et al. used MgO for the synthesis of
aliphatic mixed alkyl-methyl carbonates and catechol carbonate.22
O
Organic base
O
OMe
OMe
OH
OH
R
+
HCl
+
+
A)
R
Cl
OMe
OMe
O
O
acid/base (cat.)
O
R
B)
R
+ MeOH
MeO
O
Scheme 1. Methods for the carboxymethylation of alcohols.
These reports prompted us to investigate strongly basic catalysts. Over the past few decades,
research on solid basic catalysts, such as magnesium oxides, apatites, layered double hydroxides,
alkali doped zeolites and basic metal-organic frameworks has gained interest.23-29 Sodium
aluminate (NaAlO ) is a highly basic material, inexpensive and readily available in the solid
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form or as a concentrated solution. It finds applications in water purification, paper industry and
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0
as a precursor in zeolite synthesis. Being insoluble in various organic solvents, NaAlO can be
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used as a true heterogeneous catalyst. Nevertheless, only a small number of publications have
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reported NaAlO as an active heterogeneous catalyst for a limited number of base-catalyzed
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reactions such as isomerisation,31-32 transesterification,33-34 and condensation. Recently, we
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showed that NaAlO is an excellent catalyst for the production of glycerol carbonate in mild
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conditions.36-37 Earlier, the in-situ formation of NaAlO from NaOH and γ-alumina was indeed
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also reported during this reaction.38 Thus, we anticipated that NaAlO would also catalyze
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transcarbonation reactions between DMC and other types of alcohols.
Here, NaAlO is probed as a catalyst for the carboxymethylation reaction. Various mixed
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carbonate esters are synthesized by reacting the corresponding alcohols with DMC, acting both
as the reagent and the solvent. The NaAlO catalysts are either readily acquired from commercial
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sources or prepared using a spray drying process aiming at increasing the concentration of
defects, responsible for the formation of strong basic sites.
Table 1. Screening of common heterogeneous and homogeneous basic catalysts for the
carboxymethylation of cinnamyl alcohol (1) with DMC and comparison with sodium aluminate.a
Ph
O
OMe Ph
O
O
Ph
DMC 2 (10 eq.)
Ph
OH
+
O
4
3
O
Catalyst, 90 °C
1
Entry
Catalyst
Time Conversion Selectivity
b
b
(h)
(%)
for 3:4 (%)
1
2
3
4
5
6
7
Blank
24
24
24
96
50
24
24
0
---
Triethylaminec
NaOHc
100
15
91
94
0
97:3
100:0
90:10
94:6
d
K CO
2
3
d
CsF/Al O
2
3
NaX, NaYd
MgO
---
30
100:0
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Hydrotalcite
NaAlO2
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24
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40
52
76
99:1
99:1
99:1
0
SD-NaAlO2
a
b
Reaction conditions: 5 mmol of 1, 50 mmol of DMC, 0.10g of catalyst at 90 °C. Conversion
and selectivity were determined by gas chromatography (relative experimental error is about
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c
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%). 1.2 equivalent basic homogeneous catalyst. Results from Stanley et al.
The carboxymethylation of cinnamyl alcohol (CA, 1) with DMC in the liquid phase was used
as a probe reaction (Table 1). After 24h reaction at 90°C the reaction did not proceed at all in the
absence of a catalysts (Tableꢀ1, entryꢀ1). Using an excess of triethylamine as a conventional
homogeneous basic catalyst, a total conversion of could be achieved with 97% selectivity. Using
NaOH, the conversion reached only 15%. K CO was also reported to catalyze the reaction but
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longer reaction times had to be used to reach decent conversion. In the same conditions, basic
NaX and NaY zeolites were reported to be inactive but CsF/Al O showed significant activity
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after a long reaction time (50h).21 Here, common solid base catalysts such as MgO and
hydrotalcites were tested as benchmarks and showed only moderate CA conversion (entries 7
and 8). On the contrary, sodium aluminate showed significantly higher CA conversion (entries 9
and 10). Interestingly, the NaAlO catalyst prepared by spray drying (vide infra) was even more
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active than the commercial NaAlO . After standard workup, H-NMR analysis of the crude
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mixture showed purity well above 95% (see ESI), excluding the need for purification. Selectivity
towards cinnamyl methyl carbonate (3) was close to 100%, meaning that no further purification
would be needed to obtain the product in this case. Thus, among the tested catalysts, NaAlO2
showed the best performance and appeared as a highly promising catalyst for the
carboxymethylation reaction.
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Figure 1. CO -DRIFT spectra of NaAlO and SD-NaAlO catalysts
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2
The superior activity of NaAlO catalysts has to be related to the strongly basic properties of
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this crystalline material.30 XRD confirmed that the main crystalline phases were
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NaAlO ·1.25H O, NaAlO and Na CO (see Figure S1).
Consistently, DRIFT spectroscopy
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coupled with CO adsorption showed the abundance of surface basic sites (Figure 1). Bridged
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bidentate carbonate (1690 and 1642 cm ) and chelating bidentate carbonate (1536 cm )
adsorption can be assigned to moderate or weak basic sites.42-43 CO adsorbed on oxygen ions
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with the lowest coordination number (usually monodentate) leads to strong basic sites (1319 cm-
1)
-1
. FTIR spectra obtained on NaAlO catalysts feature an intense band at 1420 cm , attributed to
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−
1
carbonates (Figure S2A). The other main IR peaks correspond to O-O bonds at 794 cm and
vibrations of the Al-O bond at 558 cm−1 and 627 cm , and O-Na-O bonds at 454 cm .
−1
−1
Compared to the commercial sodium aluminate (NaAlO ), the catalyst prepared by spray drying
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(
denoted SD-NaAlO ; see supporting information for the preparation procedures) was
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significantly more active. This catalyst consists of spherical particles in the 0.1–5.0 μm size
range (Figure 2). Their spherical shape is linked to the preparation process in which particles are
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produced after the evaporation of the solvent contained in aerosol droplets. Such spray drying
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method does not allow to increase the specific surface area (~2 m².g in both cases), but favors
the formation of smaller crystallites (Figure S1), which leads to a higher concentration and/or
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accessibility of strongly basic edge ions (defects).36 Indeed, CO -DRIFTS shows that SD-
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NaAlO globally exhibits higher basicity compared to the commercial NaAlO , and especially a
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2
larger contribution of strong basic sites (Figure 1). CO -temperature programmed desorption
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(
TPD) experiments (Figure S2B and Table S1) provide a more quantitative analysis of the total
basicity of the catalysts, and confirm that the spray dried catalysts showed higher surface basicity
-1
-1
(
0.80 mmolCO2.g for SD-NaAlO vs. 0.47 mmol .g for commercial NaAlO ). With this
2 CO2 2
highly efficient catalyst in hand, the reaction was further studied and optimized to boost
conversion levels, check for the stability of the catalyst, and expand its scope.
Figure 2. Scanning electron micrograph of SD-NaAlO catalyst (for comparison, a SEM
2
micrograph of the commercial NaAlO is available in the supplementary information, Figure S3)
2
Under atmospheric pressure, the maximum reaction temperature is 90°C (the boiling
temperature of DMC). Lower conversion was obtained at lower temperature and the selectivity
remains very high (100%) (Figure S4). Almost quantitative conversion of CA into cinnamyl
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methyl carbonate can be reached by increasing the catalyst amount to 0.2 g (Table S2). The
recyclability of SD-NaAlO was tested by performing 5 consecutive runs of reaction (Figure 3
2
and Figure S5). After each reaction, the catalyst was centrifuged, washed three times with 5 mL
ethanol, and dried at 120°C for 6h before being used with fresh reactants. In these recycling
experiments, no significant activity drop was observed. Additionally, a “hot filtration”
experiment was run, demonstrating that the activity is strictly arising from the solid catalyst
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(
Figure S6). The integrity of the spent SD-NaAlO catalyst was also verified by XRD, FTIR and
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CO -TPD. Both the crystalline structure (Figure S1) and the surface basicity (Figure S2B) of the
2
used solid were unaffected after the reaction. Thus, the catalyst can be described as truly
heterogeneous, stable, resistant to leaching during the reaction, and therefore recyclable.
The scope of the catalyst was evaluated with a large series of alcohols, at 8h and 24h of
reaction. Various primary allylic alcohols 2a-d afforded the corresponding carbonate esters with
high conversion and selectivity after only 8 hours of reaction. Secondary allylic alcohol 2f gave
only 31% of conversion after 8h and 45% by increase in reaction time to 24h. In this case, we did
not observe any diallylcarbonate product 4f. No conversion was observed from the tertiary allylic
alcohol, linalool (2e). Steric hindrance is probably the reason for the lower activity when going
from the primary to the secondary and then tertiary allyl alcohols. Primary benzylic alcohols like
3
,5-dimethoxy benzyl alcohol and 3,5-di(trifluoromethyl) benzyl alcohol 2h-j afforded the
corresponding carbonate esters in good yields. Secondary benzylic alcohols such as, 1-
phenylpropan-1-ol and 1-(naphthalen-2-yl)ethan-1-ol were also converted to the corresponding
carbonate esters 3k and 3l with 100% selectivity but with moderate conversion. Interestingly,
alcohols bearing a hetero aryl group (e.g. furfural 1m) can also be converted. 2-furfurmethyl
carbonate 3m was obtained after 8h with 78% conversion and full selectivity. The same reaction
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after 24h gave 100% conversion with 93% selectivity. The carboxymethylation reaction of
primary aliphatic alcohols (1n) was also established, with quantitative conversion and selectivity
at 24h reaction time. Menthol 1o reacted with 100% of selectivity, with 45% of conversion. We
also tested this reaction for obtaining propargyl carbonates, which are important synthetic
precursors in the synthesis of allenes and various heterocyclic compounds.45 After 24h, the
reaction showed 93% conversion of 2-butyne-1-ol 2g with 100% selectivity to 3g.
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5
5
5
5
5
5
6
0
1
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6
7
8
9
0
1
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8
9
0
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9
0
1
2
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4
5
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7
8
9
0
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2
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9
0
It is noteworthy that for many of the screened substrates, selectivity is close to 100% meaning
that the work-up to get the pure product would be facile. For the other compounds like some allyl
and benzyl alcohols, specific purification procedure would be required.
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Table 2: Scope of the reaction for the NaAlO -catalysed carboxymethylation of alcohols.
2
O
O
OMe
O
O
NaAlO2
R
+
R
R
OH
+
R
90 °C
MeO
OMe
O
3
O
1
2
8 or 24h
4
Entrya
Time (h) Conversionb
Selectivity
3/4 (%)b
Alcohol (1)
Product (3)
0
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
8
97%
97%
99:1
97:3
96:4
90:10
a
O
OMe
OMe
OH
24
8
O
O
85%
92%
OH
b
c
O
24
O
O
OMe
OMe
OMe
8
93%
95%
97:3
97:3
OH
OH
24
O
O
8
53%
76%
0%
100:0
100:0
---
d
e
24
OH
OH
O
O
8
24
0%
---
8
31%
45%
100:0
100:0
f
O
OMe
24
Ph
Ph
O
O
8
70%
93%
100:0
100:0
OH
OH
OH
g
h
O
O
O
OMe
OMe
OMe
Me
24
Me
O
O
8
100%
100%
97:3
96:4
24
MeO
MeO
F3C
8
2
79%
81%
98:2
98:2
i
4
OMe
CF3
OMe
CF3
O
F3C
OH
8
94%
92%
94:6
91:9
O
OMe
OMe
j
24
Et
Et
O
O
8
17%
69%
100:0
100:0
k
l
OH
OH
O
O
24
Me
Me
8
39%
65%
100:0
100:0
OMe
OMe
24
8
78%
100:0
93:7
O
OH
m
n
O
O
24
8
100%
93%
O
O
OMe
OMe
100:0
100:0
OH
OH
Ph
Me
Ph
Me
O
O
24
100%
Me
Me
8
45%
45%
100:0
100:0
O
o
24
Me
Me
a
b
Conditions: Alcohol (5.0 mmol), DMC (50 mmol) and NaAlO (0.20g) at 90 °C. Conversion
2
1
and selectivity were measured by H NMR (300MHz) using the NMR integration ratio between
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the desired product and the by-product (see ESI). For entry a, conversion and selectivity were
measured by GC and confirmed by NMR. For entries b, c, and d, selectivity was evaluated by
GC.
1
1
1
1
1
1
1
1
1
1
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0
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9
0
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9
0
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9
0
1
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9
0
1
2
3
4
5
6
7
8
9
0
40
30
20
10
0
Fresh
Cycle 1
Cycle 2
Cycle 3
Cycle 4
Figure 3. Regeneration studies (Reaction conditions: 5mmol of CA, 50 mmol of DMC, 0.10 g of
catalyst at 90 °C for 2 h). Under these conditions, the conversion is far from equilibrium. A
recycling experiment was also done at high conversion (8h reaction, 0.20 g of catalyst) to show
that high yields can be achieved over several runs as well (see Figure. S5).
In conclusion, the heterogeneously catalyzed synthesis of mixed carbonate esters using sodium
aluminate and DMC is demonstrated for a large range of substrates. The reaction can be
considered greener than the classical routes because it is run with environmentally friendly
reactants and with a recyclable solid catalyst. The preparation of NaAlO by a simple spraying
2
method yield a highly basic catalyst which shows excellent performance levels and acts as a truly
heterogeneous and recyclable catalyst.
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Supporting Information. General experimental procedures, additional catalyst
characterization, characterization of the spent catalyst, optimization of reaction conditions,
heterogeneity and leaching test, NMR spectra of the products
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AUTHOR INFORMATION
Corresponding Author
*
Damien P. Debecker Damien.debecker@uclouvain.be
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.
ACKNOWLEDGMENT
Authors gratefully acknowledge the Walloon Region for the financial support (BEWARE
programme, convention no.1410279). KI would like to acknowledge the ‘MOVE-IN Louvain’
Incoming post-doc Fellowship programme for the financial support. F. Devred is acknowledged
for the logistical and technical support.
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