Download (1125) Catalytic Dehydration Reactions for Green Synthesis of

Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)
Catalytic Dehydration Reactions for Green Synthesis of Bioactive
Natural Products
Akira Sakakura1
1. EcoTopia Science Institute, Nagoya University, Nagoya, Japan
Abstract: More environmentally benign alternatives to current chemical processes, especially large-scale
fundamental reactions, are in strong demand for many reactions. We have developed catalytic dehydration processes based on acid–base combination chemistry. Bulky diarylammonium pentafluorobenzenesulfonates are mild and extremely active ester condensation catalysts. In the presence of the catalysts, ester condensation of carboxylic acids with an equimolar amount of alcohols is performed in heptane by heating at 80 °C. Even though the ester condensation is performed under heating conditions
without the removal of water, the reaction proceeds well without any deceleration due to the generated
water. Furthermore, the steric bulkiness of the N-aryl groups and the S-pentafluorophenyl group in the
catalyst suppresses the dehydrative elimination of secondary alcohols to produce alkenes. In addition,
the immobilization of bulky diarylammonium pentafluorobenzenesulfonate on a polymer support
provides an efficient atom-economical esterification catalyst that can be easily recovered and reused.
Phosphoric acid monoesters are selectively synthesized by the catalytic dehydrative condensation of an
equimolar mixture of phosphoric acid and alcohols. The reaction is catalyzed by a mixture of perrhenic
acid (1 mol%) and dibutylamine (20 mol%) under azeotropic reflux conditions with the removal of water
in NMP–o-xylene (1:1 v/v). To the best of our knowledge, this is the first successful example for the
direct catalytic condensation of an equimolar mixture of phosphoric acid and alcohols for the selective
synthesis of phosphoric acid monoesters.
Keywords: Catalysis, Dehydration Condensation, Ester, Phosphoric Acid Monoester
1. BULKY DIARYLAMMONIUM PENTAFLUOROBENZENESULFONATES
AS
EFFECTIVE
CATALYSTS FOR ESTER CONDENSATION
REACTION
The ester condensation reaction is among the most
fundamental organic transformations, and more environmentally benign alternative synthetic approaches to the
ones currently used are in strong demand by the chemical
industry [1]. With regard to green chemistry, in particular with respect to atom economy and E-factor, several catalytic methods for the ester condensation reaction
between equimolar amounts of carboxylic acids and alcohols have been developed [2–6]. Conventionally in
fact esterifications are conducted with an excess of carboxylic acids or alcohols against its reaction counterpart
in the presence of an acid catalyst, or with a stoichiometric dehydrating reagent or an activated carboxylic acid
derivative in the presence of a stoichiometric amount of
base. The use of excess amounts of substrates is a
wasteful practice in itself. Furthermore, the use of
stoichiometric dehydrating reagents or activated carboxylic acid derivatives leads to the formation of significant
amounts of undesired by-products. Purifying the crude
products from (excess) substrates or from reaction
by-products is a rather demanding task. It is therefore
easy to see why the direct catalytic condensation between
equimolar amounts of carboxylic acids and alcohols that
does not require the presence of dehydrating agents is, at
least in principle, such an attractive synthetic goal.
Among these ‘green’ catalytic condensations, metal-free
organocatalytic methods are particularly desirable, especially for industrial processes. In 2000, Tanabe and
coworkers reported that diphenylammonium triflate
([Ph2NH2]+[OTf]–, 1.0–10 mol%) efficiently catalyzed
the ester condensation reaction at 80 °C without need for
the removal of water [2]. Unfortunately however, since
[Ph2NH2]+[OTf]– is the salt of a superacid (TfOH) and a
weak base (Ph2NH), it is a strong Brønsted acid, and as
such difficult to use in the reaction of sterically demanding and acid-sensitive alcohols.
In the course of our continuing study on environmentally benign dehydration catalysts, we have developed
N,N-dimesitylammonium pentafluorobenzenesulfonate 1
and N-(2,6-diphenylphenyl)-N-mesitylammonium pentafluorobenzenensulfonate 2 as mild and selective ester
condensation catalysts (Figure 1) [7–10]. C6F5SO3H
[pKa(CD3 CO2D) = 11.1, H0 = –3.98] is a weaker acid
than trifluoromethansulfonic acid [TfOH, pKa(CD3 CO2D)
= –0.74, H0 = –14.00], concentrated H2SO4
Ph
NH2
1
NH2
[O3SC6F5]
2
Fig 1. Bulky diarylammonium pentafluorobenzenesulfonates.
Corresponding author: A. Sakakura, [email protected]
772
[O3SC6F5]
Ph
Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)
OH
Ph(CH2)3CO2
catalyst
(5 mol%)
Ph(CH2)3CO2H +
+
heptane
(bp. 98 °C)
reflux
3
conv. (%)
100
80
conv. (%)
100
A
4
catalyst:
+
[Ph2NH2] [OTf]–
40
0
conv. (%)
100
2
4
6
time (h)
60
8
10
0
0
catalyst: 1
40
5
20
2
4
6
time (h)
8
4
60
catalyst:
[Mes2NH2]+[OTf]–
40
C
80
4
3
5
20
5
B
80
3
60
0
4
3
20
0
10
5
0
2
4
6
time (h)
8
10
Fig 2. Ester condensation of 4-phenylbutyric acid with cyclododecanol (3). The proportions of 3 (red circle), 4
(green square) and 5 (blue triangle) in the reaction mixture over time are shown.
Table 1. Esterification reaction of carboxylic acids with equimolar amounts of alcohols catalyzed by 1.a
R1CO2H + HOR2
1:1 molar ratio
1
2
1 (1 mol%)
entry
1
product (R CO2R )
Ph(CH2)3CO2-n-C8H17
2
c-C6H 11CO2-n-C8H17
5
98
10
3
t-C4H9CO2-n-C8H17
6
93
11c
4
5
Ph
Ph
time (h) yield (%)
1
99
CO2-n-C8H17
5
CO2-n-C8H17
24
R1CO2R2
heptane, 80 °C
1
96
12
96
2
product (R CO2R )
Ph(CH2)3CO2Bn
entry
9
time (h) yield (%)
2
95
Ph(CH2)3CO2
Ph(CH2)3CO2
MeO
CO2
99
24
85 (0)b
4
95 (0)b
24
97 (0)b
24
73
24
99
72
92
C5H11-n
C5H11-n
C5H11-n
C5H11-n
Ph(CH2)3CO2
13
3
Ph(CH2)3CO2
6
7
PhCO2-n-C8H17
MeO
CO2-n-C8H17
24
91
14d
1
99
15d
3
99
16
MeO
MeO
CO2
CO2
OMe
OMe
8
Ph
CO2-n-C8H17
MeO
CO2
a
Unless otherwise noted, a solution of carboxylic acids (2 mmol) and alcohols (2 mmol) in heptane (4 mL) was heated at 80 °C
in the presence of 1 (1 mol%). b Yield of alkenes is shown in parentheses. c 2 (1 mol%) was used. d 1 (10 mol%) was used.
[pKa(CD3 CO2D) = 7.5, H0 = –11.93] and
p-toluenesulfonic acid [pKa(CD3CO2D) = 8.5, H0 = –4.5].
This means that 1 and 2 are milder acids than the corre-
sponding ammonium triflates, sulfates, and tosylates.
Nevertheless, 1 and 2 have much higher catalytic activities than Tanabe’s catalyst ([Ph2NH2]+[OTf]–) (Figure 2),
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Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)
Table 2. Esterification reaction under solvent-free
conditions at room temperature.a
R1CO2H + HOR2
1 (1 mol%)
(1.1 equiv) no solvent, rt
entry
1
2
3b
1
2
product (R CO2R )
Ph(CH2)3CO2Me
time (h)
24
yield (%)
95
CO2Me
8
24
72
68
CO2Me
11
24
91
82
48
90
CO2-n-C8H17
48
69
CO2-n-C8H17
24
48
74
71
42
69
MeO
O
4
5b
R1CO2R2
O
6
7b
8
9b
CO2Me
MeO
O
OMe
10
Ph
CO2-n-C8H17
ing esters (entries 4–6). 2-Alkoxycarboxylic acids were
very reactive substrates probably due to favorable chelation between the substrates and 1 (entries 7, 8, 12 and
14–16). 1 could be used for acid-sensitive alcohols such
as benzyl alcohol, allylic alcohols, and secondary
alcohols (entries 9–13). In particular, esterification with
sterically demanding alcohol 6-undecanol gave the desired esters in good yield without the production of alkenes (entries 11 and 12). Although Lewis acidic metal
salts such as Hf(IV) and Zr(IV) were not adapted to
1,2-diols due to tight chelation with metal ions [12], these
diols were also esterified in high yield by 1 (entry 13).
Less-reactive aryl alcohols and 1-adamantanol were also
esterified in high yields (entries 14–16).
Surprisingly, ester condensation reactions with
more-reactive primary alcohols proceeded even at room
temperature (22°C) without solvents (Table 2). Most
carboxylic acids were esterified with 1.1 equivalents of
methanol in good yield in the presence of 1 mol% of 1
(entries 1–6). 1-Octanol was also reactive, although the
reactivity was lower than methanol (entries 7–10). As
far as we know, this is the first example of an ultimate
green esterification process.
One major problem associated with the use of soluble
catalysts lies in recovery of the catalyst from the reaction
a
Unless otherwise noted, a solution of carboxylic acids (2
mmol) and alcohols (2.2 mmol) was reacted at room temperature in the presence of 1 (1 mol%). b 2 (1 mol%) was
used.
Br
(2.71 mmol Br/g)
+
NH2
due to the hydrophobic environment created around the
ammonium protons in the catalyst [11]. Even though
the ester condensation was performed under heating
without the removal of water, the reaction proceeded well
without any deceleration due to the generated water.
When the ester condensation of 4-phenylbutyric acid with
cyclododecanol (3) was conducted in the presence of Tanabe’s catalyst (5 mol%) in heptane under reflux conditions (bath temperature 115 °C), a significant amount of
the undesired cyclododecene (5) was produced along with
cyclododecyl 4-phenylbutyrate (4) (Figure 2, graph A).
The
use
of
dimesitylammonium
triflate
([Mes2NH2]+[OTf]–) showed higher catalytic activity than
[Ph2NH2]+[OTf]– and reduced the production of 5 (Figure
2, graph B). Furthermore, the ester condensation catalyzed by 1 (5 mol%) proceeded more rapidly and the
production of 5 decreased (Figure 2, graph C). The use
of less-polar solvents such as heptane is important. The
catalytic activities of 1 and 2 increased in such less-polar
solvents, to give esters in good yields.
To explore the generality and scope of the selective
esterification catalyzed by 1 (1 mol%) at 80 °C, the condensation was examined with an equimolar mixture of
various structurally diverse carboxylic acids and alcohols
(Table 1).
2-Unsubstituted carboxylic acids,
2-monosubstituted carboxylic acids, and sterically demanding 2,2-disubstituted carboxylic acids were
smoothly condensed to produce the corresponding esters
(entries 1–3). α,β-Unsaturated carboxylic acids and
benzoic acids were also transformed into the correspond-
Pd(dba)2 (20 mol%)
BINAP (45 mol%)
t-BuONa (36 equiv)
NH
toluene
120 °C, 3 days
(75%)
6
(1.76 mmol N/g)
C6F5SO3H
(3 equiv)
NH2
toluene, rt
(97%)
[O3SC6F5]
7
Ph(CH2)3CO2H
7 (5 mol%)
+
Ph(CH2)3CO2-n-C8H17
heptane
80 °C,1 h
HOC8H17
run
1
2
3
4
5
6
7
8
9
10
conv. (%) 99 99 99 99 99 99 99 99 98 99
Fig 3. Preparation of polymer-supported catalyst 7 and
its recovery and reuse for the ester condensation.
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Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)
medium. A simple solution is to immobilize the catalyst
on a polymeric matrix. Figure 3 describes the preparation
of
immobilized
catalyst
7.
4-(N-Mesitylamino)polystyrene resin (6) was prepared by
palladium-catalyzed
cross-coupling
of
4-bromopolystyrene resin cross-linked with 2% divinyl
benzene (2.71 mmol Br/g, 200–400 mesh) with
2,4,6-trimethylaniline in 75% yield. 7 was then readily
prepared in 97% yield by treating 6 with C6F5SO3H. In
contrast, an immobilized catalyst could not be prepared
from 6 and TfOH since the resin decomposed with superacidic TfOH. 7 was recovered by filtration and reused more than ten times as the catalyst for the direct ester condensation reaction of 4-phenylbutyric acid with
octanol, and no loss of activity was observed for the recovered catalyst.
Table 3. Catalytic activities of metal oxides for the
direct condensation of phosphoric acid with an equimolar amount of stearyl alcohol.a
n-C18H37OH
+
catalyst (10 mol%)
NMP–PhCl (1:1 v/v)
azeotropic reflux
1:1 molar ratio
10 h
n-C18H37OPO3H2
H3PO4
entry
1
catalyst
HOReO3
yield (%) entry
97
6
catalyst
ReO2
yield (%)
8
2
TMSOReO3
94
7
MoO3
8
3b
Re2O7
81
8
WO3
12
4
MeReO3
12
9b
V2 O5
17
5
35
10 no catalyst
7
ReO3
A solution of phosphoric acid (2 mmol) and alcohols (2
mmol) in NMP–PhCl (1:1 v/v) was heated in the presence
of a metal oxide (10 mol%) under azeotropic reflux conditions for 10 h. b The amount (10 mol%) of the metal oxide
was based on the metal atom.
2. SELECTIVE SYNTHESIS OF PHOSPHORIC
ACID MONOESTERS BY THE DEHYDRATIVE
CONDENSATION OF PHOSPHORIC ACID
Phosphoric acid monoesters are some of the most important substances in materials chemistry, medicinal
chemistry, and so on. Many phosphoric acid monoesters have been synthesized on an industrial scale and
are used as necessities in our daily life [13, 14]. From
the perspective of green chemistry, the direct catalytic
condensation of phosphoric acid with equimolar amounts
of alcohols is attractive for the synthesis of phosphoric
acid monoesters, especially for the industrial scale synthesis, since the reaction produces only water as a byproduct. Although various methods for the direct catalytic condensation of carboxylic acids with equimolar
amounts of alcohols have been reported [1–9], there is no
successful method for synthesizing phosphoric acid
monoesters by the direct catalytic condensation of phosphoric acid with equimolar amounts of alcohols [15, 16].
The esterification of phosphoric acid is much more difficult than that of carboxylic acids due to the stronger acidity of phosphoric acid. Previously, we reported that a
catalytic amount of nucleophilic bases such as
N-butylimidazole promoted the dehydrative condensation
of phosphoric acid and alcohols [17]. However, this
condensation is not a true catalytic process, since the reaction requires one equivalent of tributylamine (n-Bu3N).
Here we report our recent development of the catalytic
condensations between phosphoric acid and alcohols for
the selective synthesis of phosphoric acid monoesters.
In our previous work, a 1:1 (v/v) mixture of
N,N-dimethylformamide (DMF) and nitroethane (EtNO2)
was used as a solvent [17]. Since phosphoric acid can
not be dissolved in that solvent, 100 mol% of n-Bu3N is
required to dissolve phosphoric acid and promote the reaction. We considered that Lewis acidic metal oxides
might
catalyze
the
condensation
in
N-methyl-2-pyrrolidone (NMP)–o-xylene (1:1 v/v) or
NMP–chrolobenznene (PhCl) (1:1 v/v) in the absence of
any auxiliary base, since phosphoric acid is dissolved
well in NMP–o-xylene (1:1 v/v) and NMP–PhCl (1:1 v/v)
even without an auxiliary base. Some metal oxides have
been shown to be versatile green catalysts for dehydration
reactions [18–21]. Therefore, we first investigated the
catalytic activities of various metal oxides (10 mol%) that
promoted the dehydrative condensation of phosphoric
a
acid with an equimolar amount of stearyl alcohol. By
screening of reactivities of several metal oxides, we
found that some rhenium(VII) oxo compounds such as
perrhenic acid (HOReO3), trimethylsilyl perrhenate
(TMSOReO3) and rhenium(VII) oxide (Re2O7) efficiently
promoted the dehydrative condensation of phosphoric
acid (1.0 equiv) with stearyl alcohol in NMP–PhCl (1:1
v/v), to selectively give the corresponding phosphoric
acid monoester (Table 3, entries 1–3) [22]. In contrast,
the reactivities of methyltrioxorhenium (MTO, CH3ReO3),
rhenium(VI) oxide (ReO3) and rhenium(IV) oxide (ReO2)
were very low (entries 4–6). The other metal oxides
such as molybdenum(VI) oxide (MoO3), tungsten(VI)
oxide (WO3) and vanadium(V) oxide (V2O5) were almost
inert (entries 7–9).
Under the reaction conditions described above, rhenium(VII) oxo comounds gradually decomposed to dark
insoluble species (probably oligomeric low-valent rhenium oxides) to be inactivated. Actually, when the reaction was conducted in the presence of smaller amount
(1 mol%) of perrhenic acid at azeotropic reflux in
NMP–o-xylene (1:1 v/v) for 12 h, stearyl phosphate was
obtained in very low yield (16%, Figure 4, red circles).
The same reactivity was observed even in the absence of
perrhenic acid showed (black squares). Therefore, we
examined additives that could stabilize perrhenic acid
under the present reaction conditions, and found that
10–20 mol% of several organic amines could stabilize
perrhenic acid to promote the condensation efficiently.
Especially, sterically less hindered secondary and tertiary
amines, such as dibutylamine (Bu2NH) and dimethyloctylamine, gave excellent results. In the presence of 20
mol% of Bu2NH, the reaction catalyzed by 1 mol% of
perrhenic acid gave stearyl phosphate in excellent yield
(98%, green squares). The reaction mixture was a clear
dark blown solution when the reaction was conducted in
the presence of Bu2NH. When the reaction was conducted with Bu2NH (20 mol%) in the absence of perrhenic acid, stearyl phosphate was obtained in only 39%
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Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)
n-C18H37OH
+
HOReO3, Bu2NH
NMP–o-xylene
H3PO4
(1:1 v/v)
1:1 molar ratio azeotropic reflux
Table 5. Dehydrative condensation of phosphoric acid
with alcohols catalyzed by HOReO3 and n-Bu2NH.a
n-C18H37OPO3H2
ROH
+
H3PO4
(1.1 equiv)
yield (%)
100
HOReO3 (1 mol%)
n-Bu2NH (20 mol%)
ROPO3H2
NMP–o-xylene (1:1 v/v)
azeotropic reflux, 12 h
80
entry
1
product (ROPO3H2)
n-C18H37OPO3H2
yield (%)
98
2
(Z)-n-C8H17CH=CH(CH2)8OPO3H2
100
60
3b
(Z)-n-C8H17CH=CH(CH2)8OPO3H2
96
4
(E)-n-C8H17CH=CH(CH2)8OPO3H2
100
5
n-C12H25(OCH2CH2)2OPO3H2
100
40
6
20
n-C9H19
(OCH2CH2)2OPO3H2
0
0
2
4
6
8
time (h)
10
100
H
12
H
7
Fig 4. Yield versus time for the dehydrative condensation of phosphoric acid with an equimolar amount
of stearyl alcohol. Green squares, HOReO3 (1
mol%) and Bu2NH (20 mol%); blue triangles, Bu2NH
(20 mol%); red circles, HOReO3 (1 mol%); black
squares, without HOReO3 and Bu2NH.
H
H2O3PO
95 (0)c
H
H
a
Reaction of phosphoric acid (2.2 mmol) with alcohols (2
mmol) was conducted in the presence of HOReO3 aq. (1
mol%) and n-Bu2NH (20 mol%) in NMP–o-xylene (1:1 v/v,
10 mL) at azeotropic reflux for 12 h. b The reaction of
H3PO4 (110 mmol) with oleyl alcohol (100 mmol) was
conducted in the presence of HOReO3 aq. (0.5 mol%) and
n-Bu2NH (20 mol%). c The reaction was conducted in the
presence of HOReO3 aq. (10 mol%) without n-Bu2NH.
yield (blue triangles). Therefore, Bu2NH primarily contributed to the stabilization of perrhenic acid. It is very
interesting that a mixture of perrhenic acid (1 mol%) and
Bu2NH (20 mol%) showed good catalytic activity,
although Lewis acidic metal oxides were generally inactivated by bases.
To explore the generality and scope of the dehydrative
condensation catalyzed by the mixture of perrhenic acid
and n-Bu2NH, the reactions of phosphoric acid (1.1
equiv) with various alcohols were examined (Table 5).
The activity of this catalyst was much higher than that of
8, and various phosphoric acid monoesters were synthesized in almost quantitative yields. Unsaturated primary
alcohols such as (Z)-oleyl alcohol and its (E)-isomer
could be easily converted to the corresponding phosphoric acid monoesters in excellent yields (100%, entries
2 and 4). The geometries of the C–C double bonds in
oleyl alcohol and its (E)-isomer were maintained during
the reaction. The present protocol could be easily applied to a large-scale process, and the condensation of
phosphoric acid (1.1 equiv) with oleyl alcohol (100
mmol) catalyzed by HOReO3 (0.5 mol%) and n-Bu2NH
(20 mol%) gave oleyl phosphate in 96% yield (entry 3).
Ethylene glycol dodecyl ether and ethylene glycol
p-nonylphenyl ether (Igepal® CO-210) also showed high
reactivities (100%, entries 5 and 6). Phosphoric acid
monoesters of ethylene glycol dodecyl ether and ethylene
glycol p-nonylphenyl ether are useful surfactants as ingredients in detergents. A secondary alcohol such as
β-cholestanol was also converted to the corresponding
phosphoric acid monoester in 95% yield (entry 7), while
the product was completely decomposed when the reac-
tion was conducted in the absence of n-Bu2NH because of
the high acidity of the reaction media. n-Bu2NH
contributed to the stabilization of the acid-sensitive
substrate as well as perrhenic acid.
3. CONCLUSION
We developed the direct catalytic dehydrative condensation of carboxylic acid and phosphoric acid with an
equimolar amount of alcohols, to selectively give carboxylic acid esters and phosphoric acid monoesters in excellent yields. The present methods would provide the environmentally and industrially ideal synthesis of the condensation products.
ACKNOWLEDGEMENT
The author thanks Professor Kazuaki Ishihara, Graduate School of Engineering, Nagoya University, for his
helpful discussion. All experiments in this manuscript
were operated by Miss Shoko Nakagawa (ester condensation reaction) and Mr. Mikimoto Katsukawa (synthesis of
phosphoric acid monoesters). The author is also grateful
to these two students at Graduate School of Engineering,
Nagoya University.
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