Iron(III)-zirconium(IV) combined salt immobilized on N-(polystyrylbutyl) pyridinium triflylimide as a reusable catalyst for a dehydrative esterification reaction

Article abstract of DOI:10.1002/adsc.200606126

N-(Polystyrylbutyl)pyridinium triflylimide is prepared by a coupling reaction of commercially available 4-bromobutyl-polystyrene with pyridine and a subsequent anion exchange reaction with lithium triflylimide. This new polystyrene-bound pyridinium salt is useful as a solid support to immobilize a zirconium(IV)-iron(III) binary metal complex, which is a homogeneous catalyst for the dehydrative ester condensation of an equimolar mixture of carboxylic acids and alcohols. The immobilized zirconium(IV)-iron(III) complex is easily recovered by filtration after complete esterification, and is reusable without any loss of activity.

Full text of DOI:10.1002/adsc.200606126

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DOI: 10.1002/adsc.200606126
Iron(III)–Zirconium(IV) Combined Salt Immobilized on
A
N-(Polystyrylbutyl)pyridinium Triflylimide as a Reusable Catalyst
for a Dehydrative Esterification Reaction
Yuka Nakamura,a Toshikatsu Maki,a Xiaowei Wang,a Kazuaki Ishihara,a,*
and Hisashi Yamamotob,*
a
Graduate Schoolof Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan
Fax : (+81)-52-789-3222; e-mail: ishihara@cc.nagoya-u.ac.jp
Department of Chemistry, The University of Chicago, 5735 South Ellis Avenue, Chicago, IL 60637, USA
b
Fax : (+1)-773-703-0805; e-mail: yamamoto@u.chicago.edu
Received: March 22, 2006; Accepted: May 22, 2006
Abstract: N-(Polystyrylbutyl)pyridinium triflylimide
is prepared by a coupling reaction of commercially
available 4-bromobutyl-polystyrene with pyridine
and a subsequent anion exchange reaction with lith-
ium triflylimide. This new polystyrene-bound pyridi-
nium salt is useful as a solid support to immobilize
for immobilizing this binary metal complex. Since sev-
eral anions such as tetrafluoroborate and hexafluoro-
phosphate in ionic liquids strongly suppress the pres-
ent esterification catalysis, the anion species selected
is significant.[1e]
The distribution of Zr(IV)–Fe(III) combined cata-
A
a zirconium(IV)–iron
A
lysts, carboxylic acids, alcohols, and esters in a bilayer
which is a homogeneous catalyst for the dehydra-
tive ester condensation of an equimolar mixture of
carboxylic acids and alcohols. The immobilized zir-
solvent of heptane and 1 in each step of this reaction
process [Eq. (1)] is shown in Figure 1. Zr
(O-i-Pr)4
and Fe(O-i-Pr)3 are present not in the ionic liquid
AHCTREUNG
conium(IV)–iron
A
layer but rather in the heptane layer before the reac-
tion mixture is heated. After complete conversion to
by filtration after complete esterification, and is re-
usable without any loss of activity.
the desired esters, however, the Zr(IV)–Fe(III) com-
U
plex is completely transferred from the heptane layer
to the ionic liquid layer. Therefore, the Zr(IV)–Fe-
Keywords: dehydration; esterification; esters; green
chemistry; ionic liquids; recycling catalyst
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AHCTREUNG
Recently, we found that Zr(IV)–Fe
G
[1]
salt, which is prepared from Zr(O-i-Pr)4 and Fe(O-
A
i-Pr)3 in situ, is a highly effective homogeneous cata-
lyst for the dehydrative esterification of an equimolar
mixture of carboxylic acids and alcohols not only in
non-polar solvents such as heptane and octane but
also in hydrocarbon-ionic liquid bilayer solvents
under azeotropic reflux conditions with the removal
of water [Eq. (1)].[1e] To the best of our knowledge,
N-butylpyridinium trifluoromethanesulfonimide (tri-
flylimide) (1) is one of the most suitable ionic liquids
Figure 1. Recovery and reuse of Zr(IV)–Fe(III) combined
H
salt catalysts using ionic liquid 1 for the dehydrative conden-
sation of an equimolar mixture of carboxylic acids and alco-
hols.
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Yuka Nakamura et al.
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liquid layer have been estimated by ICP emission
spectroscopic analysis.[1e] On the other hand, the de-
sired esters are present in both layers. Therefore,
esters are collected from not only the heptane layer
but also the ionic liquid layer by repeated extraction
with a 1:1 (v/v) mixed solution of hexane and diethyl
ether. Extraction from the liquid layer with more
polar solvents such as 100% diethyl ether and 100%
ethyl acetate is not allowed because of their high solu-
bility for 1. This limitation gives rise to a serious
problem when polar ester products are produced.
We report here a Zr(IV)–Fe(III) catalyst immobi-
A
lized on N-(polystyrylbutyl)pyridinium triflylimide for
the dehydrative condensation of an equimolar mix-
ture of carboxylic acids and alcohols.[2,3] This new
solid catalyst can be easily recovered by filtration,
washed with polar solvents such as diethyl ether and
ethyl acetate, and reused repeatedly without any loss
of activity. Very interestingly, the Zr(IV)–Fe(III) cata-
A
lyst was released from its supporting resin in the pres-
ence of carboxylic acids, and acted as a homogeneous
catalyst.[4] The Zr(IV)–Fe
(III) catalyst was immobi-
A
Scheme 1. Preparation of polymer-bound pyridinium triflyli-
mides 4 and 6.
lized on its supporting resin again upon complete con-
version to the desired esters. To the best of our knowl-
edge, this is the first example of a solid-supported ho-
mogeneous catalyst.[4]
two steps [nucleophilic substitution reaction of 4-bro-
Initially, we attempted to recover the Zr(IV)– mobutylpolystyrene (5) with pyridine and a subse-
Fe(III) complex by filtration after complete conver- quent anion exchange reaction of N-(polystyrylbutyl)-
ACHTREUNG
sion of the esterification reaction using an ionic solid pyridinium bromide with lithium triflylimide] to
such as N-methylpyridinium triflylimide (2) (mp afford 6 in 87% yield.
438C) instead of ionic liquid 1. The esterification of
an equimolar mixture of 4-phenylbutyric acid and ture of 4-phenylbutyric acid and benzyl alcohol was
benzyl alcohol proceeded well in heptane in the pres- examined in the presence of Zr(O-i-Pr)4 (1 mol%),
ence of 2 under azeotropic reflux conditions. Al- Fe(O-i-Pr)3 (1 mol%) and 4 (3.5 mmol) in heptane
The dehydrative esterification of an equimolar mix-
AHCTREUNG
AHCTREUNG
though esterification proceeded in a liquid/liquid bi- under reflux conditions with the removal of water
layer solution, 2 was solidified again by cooling to am- (Scheme 2). Although the reaction proceeded well, 4
bient temperature after complete conversion to was gradually degraded and turned from a solid to a
benzyl4-phenybl utyrate. Not onyl the Zr(IV)–Fe
A
complex but also the ester product was included in through deprotonation at its benzylic positions, which
solid 2. It was much more difficult to extract the ester were conjugationally activated by pyridinium cations.
product from the solid layer than from the corre- Another possibility is degradation with a super
sponding liquid layer of 1.
Brønsted acid HNTf2 which would be generated by
Next, we designed poly(N-butyl-4-vinylpyridinium the above deprotonation.
triflylimide) (4) and N-butyl-4-(polystyryl)pyridinium
Next, we examined pyridinium triflylimide 6 which
triflylimide (6) as supporting resins to realize the was non-conjugationally linked with polystyrene in-
practicalrecovery and reuse of Zr(IV)–Fe (III) com- stead of 4 for the same dehydrative esterification re-
G
bined catalyst (Scheme 1). These polymer-bound ionic action. As expected, the esterification proceeded
liquids were designed based on the introduction of quantitatively under the same conditions without any
polyethylene or polystyrene resin to the 4-position degradation of 6. Furthermore, the Zr(IV)–Fe(III)
E
and the N-butylterminus of N-butylpyridinium triflyl- complex was easily recovered as 6-immobilized cata-
imide 1, respectively. The former was prepared in two lyst from the resultant mixture by filtration and wash-
steps [N-butylation of poly(4-vinylpyridine) (3), which ing with diethylether. Thus, 6-immobilized Zr(IV)–
was neutralized from poly(4-vinylpyridine hydrochlo- Fe(III) catalyst could be reused at least 10 times with-
A
ride), with 4-bromobutane, and a subsequent anion out any loss of activity (Table 1). It was ascertained
exchange reaction of poly(N-butyl-4-vinylpyridinium that 6 did not activate the esterification reaction in
bromide) with lithium triflylimide] to afford 4 in 93% the absence of Zr(IV) and Fe
A
yield. On the other hand, the latter was prepared in same reaction conditions (run 0).
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Iron(III)–Zirconium(IV) Salt Immobilized on N-(Polystyrylbutyl)pyridinium Triflylimide
A
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Scheme 2. Proposed mechanism for the degradation of 4 in
the esterification catalyzed by Zr
N
N
in the presence of 4.
Figure 2. Recovery and reuse of the 6-immobilized Zr(IV)–
Fe(III) complex for the dehydrative condensation of an
equimolar mixture of carboxylic acids and alcohols.
A
Table 1. Recovery and reuse of 6-immobilized Zr(IV)–
Fe(III) catalyst for dehydrative esterification.
ACHTREUNG
hols. Thus, the color of the reaction solution changed
from colorless to light brown. However, after com-
plete conversion to the desired esters, the color of the
reaction solution changed to almost colorless again.
This observation demonstrates that the Zr(IV)–Fe-
Run
0[a] 1[b] 2[c] 3[c] 4[c] 5[c] 6[c] 7[c] 8[c] 9[c] 10[c]
(III) complex served as a homogeneous catalyst even
in the presence of 6. Thus, this is a unique example of
a reusable homogeneous catalyst which can be recov-
ered by filtration.[4]
Yiel d [%] 2
91 90 93 91 92 93 95 91 95 93
[a]
As a blank experiment, 6 was used in the absence of
Zr(O-i-Pr)4 and Fe(O-i-Pr)3.
[b] Zr
(O-i-Pr)4 and Fe(O-i-Pr)3 were used as catalysts in the
first run.
Zr(IV) and Fe
washing with acidic solution which included the car-
boxylic acids. In contrast, no Zr(IV) or Fe(III) species
(III) species were released from 6 by
E
ACHTREUNG
A
ACHTREUNG
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[c]
Recovered 6-immobilized Zr(IV)–Fe(III) complex was
A
were released from 6 even upon washing with not
only less polar solvents such as hexane and toluene
but also polar solvents such as diethyl ether, ethyl
acetate and alcohols under neutral conditions. There-
used in and after the second run.
The distribution of Zr
N
N
Zr(IV)–Fe(III) complex, carboxylic acids, alcohols, with regard to carboxylic acids should be used to pre-
ACHTREUNG
and esters in heptane in each step of the dehydrative vent the leaching of Zr(IV) and Fe(III) species from
G
esterification reaction in the presence of 6 is shown in 6.
Figure 2. Zr
bilized in 6 as with 1, before heating. After the com- ity of Zr(IV)–Fe
plete esterification reaction, however, Zr(IV) and Fe- eralsubstrates were examined in the presence of 1.5
(III) species were recovered as a 6-immobilized or 2.0 mol% of Zr(O-i-Pr)4 and Fe(O-i-Pr)3 with 6
binary metal complex by filtration and washed with under azeotropic reflux conditions with the removal
diethylether to extract ester products from the soilds. of water in heptane or octane (Table 2). Not only
The solid complex, which was brown in color, was simple primary alcohols such as benzyl alcohol and 2-
reused as a catalyst. Unexpectedly, the Zr(IV)–Fe(III) phenylethanol but also acid-sensitive primary and sec-
(O-i-Pr)4 and Fe
(O-i-Pr)3 were not immo-
To explore the generality and scope of the reusabil-
AHCTREUNG
G
A
ACHTREUNG
AHCTREUNG
complex in 6 was again transferred into the heptane ondary alcohols such as 3-phenyl-2-propyn-1-ol, cyclo-
layer which included the carboxylic acids and alco- dodecanoland 6-undecanolcoudl be used for the cat-
Adv. Synth. Catal. 2006, 348, 1505 – 1510
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Yuka Nakamura et al.
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Table 2. Generality and scope of the reusability of Zr(IV)–Fe(III) complex immobilized on 6 for the dehydrative esterifica-
G
tion reaction.[a]
Entry
Product[b]
Zr(IV)–Fe
2.0
A
Solvent
C7H16
Time [h]
24
Yield [%]
Run 1[c]
Run 2[d]
Run 3[d]
Run 4[d]
1
>99
>99
>99
>99
-
2
3
1.5
1.5
C7H16
24
17
>99
>99
>99
>99
-
-
C8H18
>99
4
5
2.0
C8H18
24
82
98
>99
99
2.0
1.5
C8H18
C8H18
24
8
99
>99
>99
99
>99
>99
6[e]
>99
>99
[a]
See ExperimentalSection for the experimentalprocedure.
[b] All products were identified by comparison of their IR, H NMR, 13C NMR, and TLC results with those of authentic sam-
1
ples.
[c]
Zr
G
E
[d] Recovered 6-immobilized Zr(IV)–Fe
(III) complex was used in and after the second run.
G
[e]
(R)-2-Phenylpropionic acid (98% ee) was used. No epimerization occurred.
alytic esterification reaction. Furthermore, no epime- contrast, the Fe
ACHTREUNG
rization occurred in the esterification of (R)-2-phenyl- ter anion, [Fe
A
ACHTREUNG
A
ACHTREUNG
were observed at least three times without any trou-
ble for these esterification reactions.
According to our previous report,[1e] the Zr(IV) ion
is not immobilized at all in N-butylpyridinium triflyli-
mide 1 in the absence of Fe(III) ions [Eq. (2)]. In
A
ions in 1 quantitatively. Based on a consideration of
these experimentalresutls, we supposed that Zr(IV)–
Fe
A
anion, [Zr(IV)–Fe
N
N
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Adv. Synth. Catal. 2006, 348, 1505 – 1510

Iron(III)–Zirconium(IV) Salt Immobilized on N-(Polystyrylbutyl)pyridinium Triflylimide
A
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butylpyridinium bromide; yield: 4.8 g (2.04 mmol of pyridi-
nium).
dinium cation in a similar manner [Eq. (4)]. A struc-
turalanaylsis of the Zr(IV)–Fe (III) complex and its
AHCTREUNG
Next, a mixture of 3 and 1-bromobutane (78 mmol, 3.0
equivs., 8.4 mL) in 12 mL of acetonitrile was heated under
reflux conditions for 12 h. After being cooled to ambient
temperature, the resultant mixture was filtered and the solid
product was washed with THF (50 mL ” 2) and diethyl
ether (50 mL ” 2) to give poly(4-vinyl-N-butylpyridinium
bromide); yield: 7.4 g [~4.3 mmolof BuBr/g as determined
based on the yield (weight); quantitative].
ionic liquid-immobilized complex is currently under-
way in our laboratory.
In conclusion, we have developed a practical
system which uses polymer-bound ionic liquid 6 for
the recovery and reuse of a homogeneous Zr(IV)–
The reaction mixture of poly(4-vinyl-N-butylpyridinium
bromide) and lithium triflylimide (7.5 g, 26 mmol, 1.0 equiv.)
was heated in 80 mL of water at 808C for 12 h. After being
cooled to ambient temperature, the resultant mixture was
filtered and the solid compound was washed with THF
(50 mL ” 2), water-THF (50 mL ” 2), THF (50 mL ” 2)
and diethylether (50 mL ” 2), and dried at 80 8C under
vacuum for 12 h to give 4; yield: 11 g [~2.2 mmolof
BuNTf2/g as determined based on the yield (weight);
~93%]; IR (KBr): n=2971, 1643, 1518, 1473, 1351, 1131,
Fe(III) combined catalyst for the dehydrative conden-
A
sation of an equimolar mixture of carboxylic acids
and alcohols. In general, homogeneous catalysts are
more active than heterogeneous catalysts due to the
difference in the surface area of the active site. How-
ever, the latter can be more easily recovered and
reused by filtration than the former. This new method
offers great advantages as both a homogeneous cata-
lyst and a heterogeneous catalyst.
1056 cmÀ1
.
Preparation of N-Polystyrenylbutylpyridinium
Triflylimide (6)
Experimental Section
4-Bromobutyl-polystyrene (5, 4.0 g, Novabiochem, 8.8 mmol
of Br, 2.2 mmolof Br/g resin, cross-ilnked with 2% DVB,
200–400 mesh) was washed with THF (200 mL ” 3) and
dried under vacuum for 12 h. The mixture of 5 and pyridine
(7.1 mL, 88 mmol) in 64 mL of a 1:1 (v/v) mixed solvent of
THF and acetonitrile was heated at reflux conditions (bath
temp. 908C) for 24 h. After being cooled to ambient temper-
ature, the resultant mixture was filtered and the solid prod-
uct was washed with THF (100 mL ” 4) and diethylether
(50 mL ” 2), and dried at 808C under vacuum for 13 h to
give N-polystyrenylbutylpyridinium bromide; yield: 4.8 g
[2.04 mmolof pyridinium bromide/g as determined based on
the yield (weight); quantitative].
Next, N-polystyrenylbutylpyridinium bromide was treated
with lithium triflylimide (12.6 g, 44 mmol, 5.0 equivs.) in
88 mL of a 1:1 (v/v) mixed solvent of THF and water under
heating conditions at 808C for 1 day. After being cooled to
ambient temperature, the solid product was filtered, and
washed with THF (50 mL), THF-H2O (100 mL ” 4; lithium
bromide-free resin was confirmed by titration using aqueous
AgNO3 solution), THF (50 mL ” 2), ethyl acetate (50 mL ”
2), hexane (50 mL ” 2), and diethylether (50 mL ” 2), and
dried at 808C under vacuum for 12 h to give 6; yield: 6.1 g
(1.2 mmolof pyridinium trifylilmide/g, 0.048 mmolof Br/g,
and 0.050 mmolof pyridinium bromide/g as determined by
elemental analysis); 87%; IR (KBr): n=2927, 1637, 1511,
1491, 1356, 1194, 1132, 1053 cmÀ1; anal. found: N 3.35, Br
0.79, F 13.38.
Preparation of N-Methylpyridinium Triflylimide (2)
An equimolar mixture of N-methylpyridinium bromide
(1.74 g, 10 mmol)[7] and lithium triflylimide (2.87 g,
10 mmol) in water (50 mL) was heated at 808C for 12 h. The
resulting mixture was extracted with 100 mL of CH2Cl2 and
concentrated. The residue was washed with a linear gradient
of EtOAc in hexane, purified by column chromatography on
alumina oxide (neutral) using EtOAc, and dried under
vacuum to give 2 as a white solid; yield: 2.88 g (7.7 mmol,
77%); mp 438C; IR (KBr): n=3087, 1639, 1355, 1290, 1194,
1135, 1057, 795, 774, 741, 680, 618, 571, 516 cmÀ1; H NMR
1
(DMSO-d6, 300 MHz): d=4.33 (s, 3H), 8.11 (t, J=7.2 Hz,
2H), 8.56 (t, J=7.5 Hz, 1H), 8.97 (d, J=6.0 Hz, 2H);
13C NMR (DMSO-d6, 75 MHz): d=48.2 (1C), 117.7 (1C),
121.9 (1C), 128.0 (2C), 145.3 (1C), 145.9 (2C); 19F NMR
(DMSO-d6, 282 MHz): d=À81.5; anal. calcd. for
C8H8F6O4S2: C 25.67, H 2.15, N 7.48; found: C 25.67, H
2.05, N 7.47.
Preparation of Poly(4-vinyl-N-butylpyridinium
triflylimide) (4)
After poly(4-vinylpyridine hydrochloride) (4.0 g, Fluka,
~26 mmolof pyridine, ~6.5 mmol of Cl/g resin, cross-linked
with 2% DVB, 100–200 mesh) had been neutralized with a
3.0m aqueous solution of NaOH at ambient temperature,
poly(4-vinylpyridine) (3) was filtered and the solid product
was washed with water (50 mL ” 2), water-THF (50 mL ”
2), THF (50 mL ” 2) and diethylether (50 mL ” 2), and
dried at 808C under vacuum for 13 h to give N-polystyrenyl-
Adv. Synth. Catal. 2006, 348, 1505 – 1510
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Yuka Nakamura et al.
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Green Chem. 2002, 4, 88–93; b) A. Wolfson, I. F. J. Van-
kelecom, P. A. Jacobs, Tetrahedron Lett. 2003, 44, 1195–
1198; c) B. S. Lee; S. Mahajan, K. D. Janda, Tetrahedron
Lett. 2005, 46, 807–810; d) B. S. Lee; S. Mahajan, K. D.
Janda, Tetrahedron Lett. 2005, 46, 4491–4493; e) S.
Miao, Z. Liu, B. Han, J. Huang, Z. Sun, J. Zhang, T.
Jiang, Angew. Chem. Int. Ed. 2006, 45, 266–269; f) B.
Karimi, D. Enders, Org. Lett. 2006, 8, 1237–1240.
[3] For immobilized ammonium sulfonate catalysts for de-
hydrative esterification reactions, see: a) K. Ishihara, S.
Nakagawa, A. Sakakura, J. Am. Chem. Soc. 2005, 127,
4168–4169; b) A. Sakakura, S. Nakagawa, K. Ishihara,
Tetrahedron 2006, 62, 422–433.
[4] For a review for microcapsulated catalysts, see: S. Ko-
bayashi, R. Akiyama, Chem. Commun. 2003, 449–460.
According to Kobayashi et al. metal species of microcap-
sulated catalysts are strongly immobilized by physical
envelopment by polymer backbones and interaction be-
tween p electrons of benzene rings of the polystyrenes
used as polymer backbones and vacant orbitals of the
metalspecies. Therefore, the metalspecies are not re-
leased from polymer backbones even during the cataly-
sis.
Typical Procedure for Direct Esterification Catalyzed
by 6-Immobilized Zr(IV)–Fe(III) Complex
G
A 20 mL, single-necked, round-bottomed flask equipped
with a Teflon-coated magnetic stirring bar and a Dean–
Stark apparatus surmounted by a reflux condenser was
charged with carboxylic acid (5 mmol) and alcohol (6 mmol)
as substrates and Zr
A
Fe(O-i-Pr)3 (11.7 mg, 0.05 mmol) as catalysts in 6 (2.9 g,
ACHTREUNG
3.5 mmol, 1.2 mmol of pyridinium triflylimide/g) and hep-
tane or octane (20 mL). The reaction mixture was heated
under azeotropic reflux conditions with the removal of
water. After the reaction was complete, the resulting mix-
ture was cooled to ambient temperature and a colorless
liquid layer was separated from 6, which was colored brown,
by filtration, and 6 was washed with diethylether (50 mL ”
2). After the combined liquid layers were concentrated
under reduced pressure, the desired ester was isolated from
the crude oil by column chromatography on silica gel. The
Zr(IV)–Fe(III) catalyst immobilized by 6 was used directly
G
in the next reaction without further purification.
[5] For [bmim][FeCl4], see: a) S. Hayashi, H. Hamaguchi,
G
Acknowledgements
Chem. Lett. 2004, 33, 1590–1591 and 2005, 34, 740 (addi-
tions and corrections); b) T. Sasaki, C. Zhong, M. Tada,
Y. Iwasawa, Chem. Commun. 2005, 2506–2508, and ref-
erences therein.
This work was supported by SORST, Japan Science and
Technology Corporation (JST). T.M. also acknowledges a
JSPS Fellowship for Japanese Junior Scientists. We thank At-
sushi Sato for the synthesis of 2.
[6] For examples of Zr(IV)–Fe(III) combined complexes,
A
see: a) A. Shah, A. Singh, R. C. Mehrotra, Ind. J. Chem.
1987, 26A, 485–488; b) S. S. Berdonosov, B. I. Kharisov,
I. V. Melikhov, Zh. Neorganicheskoi Khimii 1988, 33,
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miya 1994, 35, 520–524; e) P. S. Ammala, J. D. Cashion,
C. M. Kepert, K. S. Murray, B. Moubaraki, L. Spiccia,
B. O. West, J. Chem. Soc., Dalton Trans. 2001, 2032–
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ty, T. J. Meyer), Elsevier, Oxford, 2004, pp. 105–173.
[7] W. F. Reynolds, U. R. Priller, Can. J. Chem. 1968, 46,
2787–2791.
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