Significant heterogeneous carbonate salt catalyzed acetylation of alcohols via a transesterification process with carbonate salt-activated alcohol 1H NMR evidence

Article abstract of DOI:10.1002/cjoc.201190214

Heterogeneous carbonate salt catalyzed acetylation of alcohols via a transesterification process has been developed. Various esters are furnished up to 97% yield. Established procedure is simple and air-tolerant with readily available reagents. Ethyl acetate and isobutyl acetate are used as not only acetylating agents, but also reaction solvents in transesterification. Aliphatic linear alcohols, allylic alcohols and benzyl alcohols show high reactivities in the presence of 1 or 5 mol% Cs₂CO₃ at 125°C. Cesium carbonate can be recycled by pumping liquid phase out of reactor after reaction. During four cycle runs for reaction of 2-phenylethanol and ethyl acetate, high yields of phenethyl acetate are provided (>60% yield). Based on experiments and ¹H NMR investigation, bifunctional catalysis is proposed, alcohol activated by carbonate ion is confirmed, and higher activity of catalytic amount than stoichiometric cesium carbonate is interpreted. Copyright

Full text of DOI:10.1002/cjoc.201190214

FULL PAPER
Significant Heterogeneous Carbonate Salt Catalyzed
Acetylation of Alcohols via a Transesterification Process with
Carbonate Salt-activated Alcohol 1H NMR Evidence
Xiong, Yan*(熊燕)
Zhang, Xueqiang(张学强)
College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400030, China
Heterogeneous carbonate salt catalyzed acetylation of alcohols via a transesterification process has been devel-
oped. Various esters are furnished up to 97% yield. Established procedure is simple and air-tolerant with readily
available reagents. Ethyl acetate and isobutyl acetate are used as not only acetylating agents, but also reaction sol-
vents in transesterification. Aliphatic linear alcohols, allylic alcohols and benzyl alcohols show high reactivities in
the presence of 1 or 5 mol% Cs2CO3 at 125 ℃. Cesium carbonate can be recycled by pumping liquid phase out of
reactor after reaction. During four cycle runs for reaction of 2-phenylethanol and ethyl acetate, high yields of
1
phenethyl acetate are provided (＞60% yield). Based on experiments and H NMR investigation, bifunctional ca-
talysis is proposed, alcohol activated by carbonate ion is confirmed, and higher activity of catalytic amount than
stoichiometric cesium carbonate is interpreted.
Keywords acetylation, alcohols, cesium carbonate, ester, NMR analysis, transesterification
Introduction
to effectively catalyze the transesterification, but Brøn-
sted acid or base catalyst is strongly corrosive to facility,
especially in industry. Utilizing Brønsted base catalysts
such as sodium hydride and potassium tert-butoxide
leads to high conversions, but the use of such species is
problematic for base-sensitive substrates. Lewis acid
catalysts can cleave sensitive functional groups such as
acetals, dienes and epoxides.7 They may also lead to
formation of side products and deterioration of primary
products during the prolonged reaction times.8 Nolan’s
group have paid plenty of attentions on N-heterocyclic
carbene (NHC) catalyzed transesterification and shown
that NHCs act as efficient organocatalysts to mediate
this transformation.9 Due to its highly catalytic activity,
most of transesterifications provide the product esters
quantatively in short time. However, argon atmosphere
in NHC participation and multi-step synthesis of or-
ganocatalyst NHC is required.9
Inspired by NHC catalysis in transesterification, an
attempt has been made to simple carbonate salt catalysis
for high efficiency. Carbonate salt is a weak base cata-
lyst. The advantages of them are: (1) base is a potential
catalyst to transesterification, and its catalytic ability
could be improved by simply elevating temperature; (2)
heterogeneous catalysis makes the carbonate salt con-
veniently recoverable; and (3) carbonate salt is generally
cheap and readily available. However, to the best of our
knowledge the cesium carbonate catalyzed transesterifi-
The ester moiety is a common functional group in
drugs and natural products, and sometimes, the ester
functionality serves as a protecting group for alcohols.1,2
Preparation of esters may be achieved through reactions
of alcohols with acylation reagents, such as carboxylic
acids, acyl chlorides or anhydrides. On some occasions,
these methods are unusable. For example, some carbox-
ylic acids are sparingly soluble in organic solvents and
accordingly difficult to proceed in homogeneous esteri-
fication; acyl chloride and anhydride are mois-
ture-sensitive and decompose to acids, so they are unal-
ternative to acid-sensitive alcohol participated acylation;
moreover, acyl chloride and anhydride are highly active,
which leads to poor chemoselective acylation among
primary, secondary and tertiary alcohols. Due to its
solubility in most of organic solvents and relative stabil-
ity, ester is commonly considered as a popular acylation
reagent for acylation of alcohols via transesterification
under neutral condition.2
Transesterification is an equilibrium process, and the
transformation occurs essentially by simply mixing the
two components, alcohol and starting ester. However, it
has long been known that the reaction is accelerated by
catalysts, such as Brønsted acid catalysts, Brønsted base
catalysts and Lewis acid catalysts.3-6 Brønsted acid,
Brønsted base and Lewis acid catalysts have been found
*
E-mail: xiong@cqu.edu.cn
Received October 31, 2010; revised January 22, 2011; accepted February 26, 2011.
Project supported by the Natural Science Foundation Project of CQ CSTC (No. 2010BB5064), Sharing Fund of Chongqing University’s Large-Scale
Equipment (No. 2010063040), Scientific Research Fund of Chongqing University for the Introduction of Talent (No. CDJRC10220004), Innovative
Talent Training Project of Chongqing University, the Third Stage of “211 Project” (No. S-09103) and Chongqing Municipal Education Commission
Project (No. KJ-091201).
Chin. J. Chem. 2011, 29, 1143— 1148
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FULL PAPER
a
cation has not been described systematically yet. In this
work, we report the carbonate salt-promoted transesteri-
fication with commercially available methyl, ethyl and
i-butyl acetate.
Table 2 Investigation on amount of K2CO3
Results and discussion
Entry
Amount of K2CO3 (x equiv.)
t/h
1.5
1.5
1.5
1.5
1.5
Yieldb/%
1
2
3
4
5
3.00
0.20
0.15
0.10
0.05
48
57
65
62
43
We chose the reaction of benzyl alcohol with ethyl
acetate as a model reaction, and the results were listed in
Table 1. Ethyl acetate was used not only as acetylation
reagent but also as solvent. In the presence of 3.0
equivalent K2CO3, at 125 ℃, the reaction gave a mod-
erate yield (48%) of the desired product benzyl acetate
(Table 1, Entry 3). With the temperature dropped, the
yields reduced dramatically. At 85 and 45 ℃, the yields
of 10% and trace were obtained, respectively (Table 1,
Entries 1 and 2). Higher temperature (＞125 ℃) was
disfavorable to transesterification. At 165 ℃, the yield
of 38% was made (Table 1, Entry 4). Continuously in-
creasing the temperature to 205 ℃, the reaction rate
was identical (Table 1, Entry 5).
a Reaction conditions: 2 mmol benzyl alcohol, 2 mL ethyl acetate,
b
amount-indicated potassium carbonate, 125 ℃ and 1. 5 h. Iso-
lated yield after silica gel column chromatography.
Table 3 Catalyst screensa
Table 1 Temperature effectsa
Entry
Catalyst
None
t/h
1.5
1.5
1.5
1.5
1.5
1.5
1.5
Yieldb/%
No product
Trace
Trace
65
1
2
Li2CO3
Na2CO3
K2CO3
Cs2CO3
Cs2CO3
Cs2CO3
3
4
Entry
T/℃
45
t/h
1.5
1.5
1.5
1.5
1.5
Yieldb/%
Trace
10
5
81
1
2
3
4
5
6c
7d
77
85
65
125
165
205
48
a Reaction conditions: 2 mmol benzyl alcohol, 2 mL ethyl acetate,
b
38
15 mol% catalyst loading, 125 ℃ and 1. 5 h. Isolated yield
c
d
37
after silica gel column chromatography. 5 mol% Cs2CO3. 10
mmol benzyl alcohol, 10 mL ethyl acetate and 1 mol% Cs2CO3.
a Reaction conditions: 2 mmol benzyl alcohol, 2 mL ethyl acetate,
6 mmol potassium carbonate and 1.5 h. b Isolated yield after silica
gel column chromatography.
Having established that Cs2CO3 was an excellent
catalyst for the transesterification reaction of ethyl ace-
tate with benzyl alcohol even with 1 or 5 mol% catalyst
loading, we investigated a variety of alcohol substrates,
with the results summarized in Table 4. In order to ex-
plore the application of this methodology in the indus-
trial scale, the reactions between alcohols and ethyl (or
isobutyl) acetate were scaled up using 10 mmol starting
material (alcohol). Primary alcohol showed higher reac-
tivity than secondary alcohol and aliphatic alcohols gave
better results than benzyl alcohols. Benzyl alcohol gave
the product in 71% yield (Table 4, Entry 1).
Methyl-substituted benzyl alcohol afforded the product
ester in 79% yield after 31 h (Table 4, Entry 2). Secon-
dary benzyl alcohols gave lower yields. For example,
1-phenylethanol and diphenylmethanol produced the
esters in 32% and 30% yields, respectively (Table 4,
Entries 3 and 4). For these alcohols, due to fewer side
reaction, based on recovery of starting materials, the
total yield would be greatly improved. For instance,
yield of benzhydryl acetate was increased to 70% after
11 runs, each for 10 h (Table 4, Entry 4). Heteroaro-
As seen in Table 2, at 125 ℃, more K2CO3 was used,
and less product was detected, which indicated that sub-
strate adsorption influenced the reaction rate directly.
Excessive substrate adsorption would keep back the
molecular collision of reactants. So decrease of K2CO3
from stoichiometric to catalytic amount showed growing
catalytic activity (Table 2, Entries 1—3). In the presence
of 15% K2CO3, the desired product was obtained in 65%
yield. Further decreasing the catalyst loading, the reac-
tions became slower (Table 2, Entries 4 and 5).
Other carbonate salts were subsequently screened
with 15% catalyst loading at 125 ℃ (Table 3). When
Cs2CO3 was employed as catalyst, 15% catalyst loading
gave the product in higher yield of 81% (Table 3, Entry
5). No catalyst was used and no product was obtained
(Table 3, Entry 1). The uses of Li2CO3 and Na2CO3 only
gave trace product (Table 3, Entries 2 and 3). With prac-
tical consideration, it is attractive that the catalyst load-
ing of cesium carbonate was reduced to 1% and 65%
yield was produced (Table 3, Entry 7).
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Chin. J. Chem. 2011, 29, 1143— 1148

Significant Heterogeneous Carbonate Salt Catalyzed Acetylation of Alcohols
matic alcohol such as furan-2-ylmethanol afforded the
corresponding ester in 51% yield (Table 4, Entry 5).
Aliphatic esters were obtained in good yields, i.e.
2-phenylethanol acetate in 94% yield (Table 4, Entry 6),
2-phenoxyethyl acetate in 76% yield (Table 4, Entry 7),
octyl acetate in 78% yield (Table 4, Entry 8) and dodecyl
acetate in 79% yield (Table 4, Entry 9). Unsaturated al-
cohols such as substituted 2-propen-1-ol are of interest
since their corresponding esters were used as herbicide
or antimycotic agents.10 Unsaturated alcohols geraniol
and cinnamyl alcohol reacted smoothly with ethyl ace-
tate in the presence of 5 mol% of Cs2CO3, yielding the
desired products in 97% and 74% yields, respectively
(Table 4, Entries 10 and 11). It was concluded that less
steric hindrances of aliphatic alcohols and unsaturated
alcohols contributed to their higher reactivities. The
sterically hindered isobutyl acetate presented a similar
trend, reacting with above alcohols and giving corre-
sponding esters in lower or identical yields in most
cases. Noteworthily, when methyl acetate served as
acylation reagent, the solid cesium carbonate was dis-
solved and no product was observed. Moreover, for ter-
tiary alcohol with highest steric hindrance such as
t-BuOH, transesterification did not take place.
On the other hand we have also investigated the pos-
sibility of reusing the catalyst Cs2CO3. Figure 1 shows
the efficiency of the catalyst employed during four cycles
to promote the reaction of 2-phenylethanol and ethyl
acetate under the same conditions: 10 mmol
2-phenylethanol, 10 mL ethyl acetate, 5 mol% cesium
carbonate, 125 ℃ and 9 h. After one run was complete,
the reaction mixture was pumped out by syringe, het-
erogeneous catalyst Cs2CO3 remained in reactor and
subsequently reactants of another run were injected into
the reactor for next catalytic process. Good yields were
observed, 69% for first cycle, 77% for second cycle,
87% for third cycle and 78% for forth cycle, which
suggested no significant catalyst poisoning and potential
protocol for industrial applications.
1
From H NMR investigations below (Figure 2), for
the mixture of benzyl alcohol and ethyl acetate in CDCl3,
hydrogens of hydroxyl OH and methylene CH2 respec-
tively correspond to two singlets (δ 2.52 and 4.65) and
no split was detected. In the presence of 15 mol% or 300
mol% K2CO3, slower H-D exchange of hydroxyl group
resulting from interaction of carbonate ion with
Table 4 Cesium carbonate catalyzed acetylation of various alcoholsa
AcOEt
AcOBu-i
Yield/%
Entry
Substrate
Product
Catalyst loading/mol%
t/h
11
Yield/%
71
79 (53)b
t/h
16
31
30
27
28
19
19
29
29
30
25
1
2
1a
1b
1c
1d
1e
1f
2a
2b
2c
2d
2e
2f
1
76
58c
Trace c
57c
7c
1 (15)
31 (11)
22
3
1
32
4
1
24
30 (70)d
51
94 (86)b
5
1
22
6
1 (15)
16 (11)
15
93
7
1g
1h
1i
2g
2h
2i
1
1
1
5
5
76
59
8
25
78
63
9
26
79
19
10
1j
2j
25
97
96
11
1k
2k
25
74
62
a Reaction conditions: 10.0 mmol alcohol and 10 mL AcOR' (R'＝Et or i-Bu). b Catalyzed by K2CO3. c Reaction conditions: 2.0 mmol
alcohol, 2.0 mL AcOR' (R'＝Et or i-Bu) and Cs2CO3 5 mmol%. d Total yield of eleven runs. 10 mmol diphenylmethanol for the first run
and recovered starting material for next run.
Chin. J. Chem. 2011, 29, 1143— 1148
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Figure 3 Proposed cesium carbonate-catalyzed transesterifica-
tion mechanism.
Figure 1 Catalyst recycling experiments for transesterification
of benzyl alcohol and ethyl acetate.
1
All products are known and their H NMR data were
3
hydroxyl group led to strong J coupling (3J＝5 Hz for
obtained using a Brüker AV-400 and AV-500 spectram-
eters (1H 400 MHz and 500 MHz). Chemical shifts are
reported as follows: chemical shift, multiplicity (s＝
singlet, d＝doublet, t＝triplet, m＝multiplet), coupling
constants (Hz), integration, and assignment.
3
15 mol% K2CO3 and J＝6 Hz for 300 mol% K2CO3).
Meanwhile, in the cases of 15 mol% or 300 mol%
K2CO3, hydrogen peaks of hydroxyl groups both shifted
to low magnetic field (δ 2.55 and 2.53) which indicated
that hydroxyl group of alcohol was activated by carbon-
ate ion and revealed 15 mol% K2CO3 led to higher cata-
lytic activity than 300 mol% K2CO3 (δ 2.55＞2.53).
General procedure for transesterification
To a round flask equipped with a stirring bar carbon-
ate salt (1 mol%—300 mol%) was added. Ethyl acetate
or isobutyl acetate (2 mL or 10 mL) was injected to re-
actor by syringe, turned on the magnetic stirrers and
then corresponding alcohol (2 mol or 10 mol) was
dropped into the mixture. The reaction was moved to oil
bath at 125 ℃ and kept stirring for the indicated time in
Tables 1—4. After that time, the solvent was removed
under reduced pressure. The reaction crude was purified
by flash chromatography (EtOAc/PE＝1∶100—70, V/V),
yielding corresponding transesterification adduct.
1
Using Cs2CO3 instead of K2CO3 in H NMR investiga-
tions, similar results appeared.
A typical procedure for preparation of benzyl ace-
tate
To a round flask equipped with a stirring bar cesium
carbonate (32.6 mg, 0.1 mmol) was added. Ethyl acetate
(10 mL) was injected to reactor by syringe, turned on
the magnetic stirrers and then benzyl alcohol (1.0814 g,
10 mmol) was dropped into the mixture. The reaction
was moved to oil bath at 125 ℃ and kept stirring for
11 h. After that time, the solvent was removed under
reduced pressure. The reaction crude was purified by
flash chromatography (EtOAc/PE＝1∶70, V/V), yield-
ing corresponding transesterification adduct benzyl ace-
tate.
Figure 2 1H NMR analysis on potassium carbonate activated
alcohol.
Bifunctional catalysis was proposed: while metal ion
coordinates with carbonyl group of ethyl acetate, car-
bonate ion activates alcohol (see intermediate A, in Fig-
ure 3). The negative charge is relatively gathered on
oxygen of hydroxyl group. The oxygen of activated al-
cohol attacks positive carbon of carbonyl group to form
intermediate B. With ethanol leaving, ester adduct is
produced. Then cesium carbonate performs to the next
catalytic cycle.
Benzyl acetate Colorless liquid, 1.0662 g, 71%
1
yield. H NMR (CDCl3, 400 MHz) δ: 7.40—7.36 (m,
5H), 5.14 (s, 2H), 2.12 (s, 3H).
4-Methylbenzyl acetate Colorless liquid, 1.2972 g,
79% yield. 1H NMR (CDCl3, 500 MHz) δ: 7.20 (d, J＝8
Hz, 2H), 7.11 (d, J＝8 Hz, 2H), 5.02 (s, 2H), 2.29 (s,
3H), 2.01 (s, 3H).
Experimental
Chemical reagents were purchased from different
commercial sources and used without further purification.
Flash chromatographies were performed using silica gel.
1-Phenylethyl acetate Colorless liquid, 0.5254 g,
1
32% yield. H NMR (CDCl3, 400 MHz) δ: 7.39—7.31
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Significant Heterogeneous Carbonate Salt Catalyzed Acetylation of Alcohols
(m, 5H), 5.92 (q, J＝6.4 Hz, 1H), 2.10 (s, 3H), 1.57 (d,
J＝6.8 Hz, 3H).
isobutyl) acetate has been extensively studied, finding
that cesium carbonate can be recycled and reused for
several times without a significant loss of activity. Ad-
ditionally we are able to scale up the reaction on a gram
Benzhydryl acetate Colorless liquid, 0.6788 g,
1
30% yield. H NMR (CDCl3, 400 MHz) δ: 7.40—7.30
1
scale, yielding the desired esters in high yields. In H
(m, 10H), 6.93 (s, 1H), 2.20 (s, 3H).
NMR studies, carbonate ion activating alcohol was
proposed.
Furan-2-ylmethyl acetate Yellow liquid, 0.7147 g,
51% yield. 1H NMR (CDCl3, 500 MHz) δ: 6.40 (d, J＝3
Hz, 1H), 6.36—6.35 (m, 1H), 7.42 (d, J＝1 Hz, 1H),
5.05 (s, 2H), 2.07 (s, 3H).
References
Phenethyl acetate Colorless liquid, 1.5435 g, 94%
1
1
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2.06 (s, 3H).
2
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heterogeneous catalysis rather than homogeneous ca-
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Xiong & Zhang
FULL PAPER
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Chin. J. Chem. 2011, 29, 1143— 1148