Efficient direct ester condensation between equimolar amounts of carboxylic acids and alcohols catalyzed by trifluoromethanesulfonic acid (TfOH) in Solkane365mfc

Article abstract of DOI:10.1039/c3ra00132f

A simple, practical, and environmentally benign esterification protocol has been devised on the basis of TfOH as the catalyst and Solkane365mfc as the reaction medium. The direct condensation of equimolar amounts of various carboxylic acids and alcohols was conveniently carried out without recourse to any additional water removal technique, giving the desired carboxylic esters in excellent yields.

Full text of DOI:10.1039/c3ra00132f

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Efficient direct ester condensation between equimolar
amounts of carboxylic acids and alcohols catalyzed by
trifluoromethanesulfonic acid (TfOH) in
Solkane365mfc3
Cite this: RSC Advances, 2013, 3, 3848
Received 9th December 2012,
Accepted 22nd January 2013
DOI: 10.1039/c3ra00132f
Xiu-Hua Xu, Ayaka Azuma, Misaki Taniguchi, Etsuko Tokunaga and Norio Shibata*
www.rsc.org/advances
A simple, practical, and environmentally benign esterification
protocol has been devised on the basis of TfOH as the catalyst and
Solkane365mfc as the reaction medium. The direct condensation
of equimolar amounts of various carboxylic acids and alcohols
was conveniently carried out without recourse to any additional
water removal technique, giving the desired carboxylic esters in
excellent yields.
p-toluenesulfonic acid (TsOH) and 10-camphorsulfonic acid (CSA),
can efficiently catalyze ester condensation under solvent-free and
drying agent-free conditions.4 Sulfonic acid is a promising
candidate for the esterification reaction in process chemistry
due to its simplicity, ready availability and low cost. We present
herein another sulfonic acid, trifluoromethanesulfonic acid
(TfOH), as an efficient catalyst in the esterification reaction. The
unique property of TfOH5 makes it more active than other sulfonic
acids, and in most cases only 0.2 mol% of TfOH is sufficient for
excellent conversion.6
Introduction
The reaction medium is also very important for practical
catalytic esterification. Solvents, which are usually used in great
quantities, define a major part of the environmental performance
of processes in the chemical industry and also have an impact on
cost, safety and health issues.7 Conventionally, ester condensa-
tions were performed in flammable and toxic non-polar solvents,
such as heptane, toluene and xylene. Replacing these solvents with
more environmentally acceptable alternatives appeared to be a
subject of significant academic and commercial interest. Up to
now, the most commonly explored green solvents for the
esterification reaction are ionic liquids.3e,i,8 However, whether
ionic liquids can be classed as green solvents is still highly
debateable.9 The esterification reaction in other environmentally
benign solvents water3c,d,k,10 and fluorinated phases2d,f,3j,11 have
been primarily investigated. Although solvent-free conditions12
seem to be a good alternative, in most cases, a solvent is often
required to assist in working-up processes and transporting of
materials. Thus, the development of other green media for an
extremely practical catalytic esterification is still worthy of
investigation. Recently, fluorous organic hybrid solvents have
received more and more attention to be green substitutes for
traditional organic solvents.13 Solkane365mfc, 1,1,1,3,3-pentafluor-
obutane, is a typical fluorous organic hybrid solvent developed by
Solvay Fluoro GmbH.14 It is non-toxic and has no impact on the
ozone layer. Although Solkane365mfc has a flash point below 227
uC, it is difficult to ignite. Its thermal and chemical stability make
The ester condensation reaction is one of the most fundamental
transformations in various fields of organic synthesis.1
Conventional esterifications are conducted with excess carboxylic
acids (or alcohols) against a counterpart in the presence of an acid
catalyst, or with stoichiometric dehydrating reagents or activated
carboxylic acid derivatives in the presence of a stoichiometric base.
In view of the rapidly increasing demands of green and
sustainable chemistry, more environmentally benign alternative
approaches to those currently used by the chemical industry are in
strong demand. Recently, much attention has been focused on
developing different efficient catalysts, which can activate the ester
condensation of equimolar mixtures of carboxylic acids and
alcohols. Impressive examples, mainly including metal salt
catalysts2 and Brønsted acid catalysts,3 have been exploited by
different groups. The principal drawbacks of the majority of
metallic catalysts are moisture-sensitivity, high cost, and toxicity.
Many esterification methods require the azeotropic removal of
water, or at least the presence of a drying agent. For Brønsted acid
catalysts, it is not necessary to remove water, but most of them
suffer from complex structures and are not commercially
available. It is still highly desirable to search for a simpler, facile,
inexpensive and efficient catalyst. In 2008, Ishihara and co-workers
reported that catalytic amounts of sulfonic acids (5 mol%), such as
Department of Frontier Materials, Graduate School of Engineering, Nagoya Institute
of Technology, Gokiso, Showa-ku, Nagoya 466-8555, Japan.
E-mail: nozshiba@nitech.ac.jp; Fax: +81-52-735-5442
it
a good alternative solvent. Previously, we proved that
Solkane365mfc was an environmentally benign solvent for several
transformations.15 To further explore the reaction range, we
Electronic supplementary information (ESI) available: experimental details. See
3
DOI: 10.1039/c3ra00132f
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taking advantage of its mildness, nonoxidative character, and ease
of handling. The result of TfOH was exciting since the conversion
reached as high as 99% at 100 uC, and even at 80 uC (entries 7 and
8). Decreasing the temperature to 60 or 50 uC, both the conversion
and yield dropped slightly (entries 9 and 10). To our delight,
excellent conversion and yield were obtained when only 0.2 mol%
of TfOH was used at 80 uC (entry 11). It is noteworthy that when
the reaction was performed in toluene or heptane, slightly lower
conversion and yield were obtained (entries 12 and 13). The most
conventional acid, HCl, was not effected and furnished a complex
mixture (entry 14).
Under the optimized reaction condition, the ester condensa-
tion of various carboxylic acids 1 and alcohols 2 was carried out in
the presence of 0.2 mol% TfOH in Solkane365mfc at 80 uC to
explore the generality and scope of the substrates. First, the
esterification of phenylpropanoic acid 1a and a variety of alcohols
2a–g were investigated. As shown in Table 2, alcohols 2a–c with
different chain lengths (C8, C6, and C2) were converted to ester
3aa–ac in excellent yields. Several functionalities in alcohols such
as an allylic group (2d), a phenyl group (2e), and a halogen (2f)
were tolerated. The short chain alcohol (2c) and sterically crowded
secondary alcohol (2g) required more catalyst and/or longer time.
Then, the esterification of different carboxylic acids 1b–o and
octan-1-ol 2a was examined. The results are shown in Table 3.
2-Unsubstituted carboxylic acids, 2-monosubstituted carboxylic
acid, and sterically demanding 2,2-disubstituted carboxylic acid
were smoothly condensed to produce the corresponding esters
3ba–ea in excellent yields. 4-Oxopentanoic acid (1f) was selectively
esterified without a protecting ketone moiety. The reactions using
a,b-unsaturated carboxylic acid (1g) and benzoic acid (1h) were
also successful, although more catalyst and/or a longer time was
needed. A wide range of functional groups, including electron-
donating, electron-withdrawing, and neutral groups in carboxylic
Scheme 1 Ester condensation between equimolar amounts of carboxylic acids 1
and alcohols 2 catalyzed by TfOH in Solkane365mfc.
present here the first thermal reaction with Solkane365mfc as the
medium. The ester condensation between different carboxylic
acids 1 and alcohols 2 can proceed efficiently when catalyzed by
TfOH in Solkane365mfc, giving the product carboxylic esters 3 in
excellent yields (Scheme 1).
Results and discussion
Initially, several reported catalysts (1.0 mol%) were evaluated in
the condensation reaction of equimolar amounts of 3-phenylpro-
panoic acid (1a) and octan-1-ol (2a) in Solkane365mfc at 100 uC
(Table 1, entries 1–6). Neither the metal salt catalyst
[{Cl(C6F13C2H4)2SnOSn(C2H4C6F13)2Cl}2] (Otera’s cat.)2d or
HfCl2?2THF2c gave good results (entries 1 and 2). Ammonium
triflate (DPAT)3b worked well giving the desired product 3aa in
87% yield (entry 3). All the sulfonic acids TsOH?H2O, CSA, and
methanesulfonic acid (MsOH)4 promoted the condensation in
high conversion (entries 4–6). This result encouraged us to search
for a more efficient sulfonic acid catalyst. TfOH was then chosen
Table 1 Optimization of reaction conditions in Solkane365mfc
Table 2 Ester condensation between phenylpropanoic acid 1a and various
alcohols 2 catalyzed by TfOH in Solkane365mfc
Entry
Catalyst
Temp. (uC)
Conv. (%)a
Yield (%)b
1
2
3
4
5
6
7
8
Otera’s cat.
HfCl2?2THF
DPAT
TsOH?H2O
CSA
MsOH
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
100
100
100
100
100
100
100
80
60
50
80
80
6
16
88
93
70
96
99
99
97
96
98
96
93
complex
—
—
87
93
70
91
96
98
94
93
98
95
93
—
9
10
11c
12c,d
13c,e
14
TfOH
HCl
80
80
a
b
Determined by crude 1H NMR. Isolated yield by silica-gel column
c
d
chromatography. TfOH (0.2 mol%) was used as the catalyst. The
reaction was carried out in toluene. The reaction was carried out in
a
b
e
Isolated yield by silica-gel column chromatography. TfOH
c
(1.0 mol%) was used. The reaction was carried out for 48 h.
heptane.
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Table 3 Ester condensation between various carboxylic acids 1 and octan-1-ol
2a catalyzed by TfOH in Solkane365mfc
Scheme 3 Ester condensation reaction catalyzed by TfOH in Solkane365/227.
Scheme 3, several different carboxylic acids and octan-1-ol 2a
reacted under the catalysis of TfOH in Solkane365/227 giving the
corresponding carboxylic esters in excellent yields.
Conclusions
We have developed a new practical and environmentally benign
method for the esterification reaction. Only 0.2–1.0 mol% TfOH
could efficiently catalyze the ester condensation of equimolar
mixtures of various carboxylic acids and alcohols at 80 uC with
Solkane365mfc as the medium (23 examples), although tertiary
alcohols are not acceptable as partners of carboxylic acids. All of
the desired carboxylic esters were obtained in excellent yields (92–
99%). Even in Solkane365/227, a non-flammable solvent, the ester
condensation reaction also proceeded efficiently. Note that the
present method did not necessitate the use of a dehydration
reagent and/or technique for the removal of azeotropic water. We
hope that Solkane365mfc can be the green organic solvent of
choice after water and ionic liquids in industrial process
chemistry. The extension to other ubiquitous organic reactions
with Solkane365mfc as the medium is currently under investiga-
tion.
a
Isolated yield by silica-gel column chromatography.
TfOH (1.0 mol%) was used. The reaction was carried out
b
c
for 48 h.
acids 1i–o were well tolerated, and all the ester condensation
products 3ia–oa were obtained in excellent yields.
To further investigate the substrate generality, ester condensa-
tion of either diacid (1p) or diol (2h) was investigated (Scheme 2).
Both of them were smoothly converted to the corresponding
diesters 3pa or 3ah in excellent yields without a prolonged reaction
time.
Experimental
Typical procedure for ester condensation in Solkane365mfc
A mixture of phenylpropanoic acid 1a (750.9 mg, 5.00 mmol),
octan-1-ol 2a (784.5 mL, 5.00 mmol), TfOH (2.2 mL, 0.01 mmol),
and Solkane365mfc (5.0 mL) in a sealed tube was stirred at 80 uC
for 18 h. After cooling, the solvent was evaporated, and the residue
was purified by column chromatography on silica-gel (n-hexane/
ethyl acetate = 20/1) to give ester 3aa as a clear oil (1.2834 g, 98%).
Finally, we examined the ester condensation reactions in
commercially available Solkane365/227 blend solvent (Solkane227:
1,1,1,2,3,3,3-heptafluoropropane).
It
is
disclosed
that
Solkane365mfc has a flash point ¡227 uC; however, its blend
form (93/7 mixture of Solkane365mfc and Solkane227) as
Solkane365/227 is known to have no flash point (non-flammable)
and could provide the benefits of Solkane365mfc.16 As shown in
Acknowledgements
This study was financially supported in part by Grants-in-Aid
for Scientific Research from the Ministry of Education,
Culture, Sports, Science and Technology (Project name:
Advanced Molecular Transformation by Organocatalysts). We
thank Dr Max Braun, Solvay Fluor GmbH for the generous gift
of Solkane365mfc and thank Professor Junzo Otera for
providing us with Otera’s catalyst.
Notes and references
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3850 | RSC Adv., 2013, 3, 3848–3852
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mfc.aspx.
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227-Blends.aspx.
3852 | RSC Adv., 2013, 3, 3848–3852
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