Magnetic nanoparticle-supported crown ethers

Article abstract of DOI:10.1039/b705640k

Magnetic nanoparticle (MNP)-supported crown ethers were successfully prepared and evaluated as catalysts for solid-liquid phase-transfer reactions; the catalytic activities of the MNP-supported crown ethers were not inferior to those of non-supported crown ethers; after the reactions, the catalysts could be readily separated using an external magnet and reused without significant loss of catalytic efficiency. The Royal Society of Chemistry.

Full text of DOI:10.1039/b705640k

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Magnetic nanoparticle-supported crown ethers{
Masato Kawamura and Kazuhiko Sato*
Received (in Cambridge, UK) 13th April 2007, Accepted 30th May 2007
First published as an Advance Article on the web 13th June 2007
DOI: 10.1039/b705640k
Magnetic nanoparticle (MNP)-supported crown ethers were
successfully prepared and evaluated as catalysts for solid–liquid
phase-transfer reactions; the catalytic activities of the MNP-
supported crown ethers were not inferior to those of non-
supported crown ethers; after the reactions, the catalysts could
be readily separated using an external magnet and reused
without significant loss of catalytic efficiency.
Since Pedersen’s discovery of crown ethers in 1967,1 a large
number of crown ethers and related compounds have been
extensively developed because of their remarkable cation-binding
properties that is termed ‘‘host–guest complexation’’.2 In the field
of organic synthesis, crown ethers have been utilized as versatile
phase-transfer catalysts.3–5 It is well known that ‘‘host–guest
Scheme 1 Preparation of MNP-supported crown ethers 3–4.
complexation’’ is the origin of the concept of solid–liquid (S–L)
phase transfer catalysis (PTC).5a By binding a metal cation, crown
ethers allows an ionic solid reagent to be dissolved in a non-polar
organic solvent, in which a weakly solvated counter anion (‘‘naked
anion’’) shows high reactivity.
As MNPs for catalyst supports, magnetite (Fe3O4) nanoparticles
(average diameter = 12 nm) were used and prepared by a common
co-precipitation method.11 Immobilization of crown ethers on the
surface of magnetite nanoparticles was carried out according to a
silanization protocol.12 As described in Scheme 1, magnetite
nanoparticles were treated by organosilane 113 and 2 to give the
MNP-supported crown ethers 3 and 4, where the crown ethers
were covalently bonded to the surface of magnetite nanoparti-
cles.14 The loading of the crown ethers on magnetite nanoparticles
was determined by elemental microbial analysis of nitrogen, and
However, there have been two major problems with using
crown ethers as phase-transfer catalysts. One is that they are
generally highly toxic; hence, careful handling is required.3
Another is the fact that they are both difficult to reuse and very
expensive. Immobilization of crown ethers on a polymer support,
such as cross-linked polystyrene (PS) resin, improves their
recyclability and ease of handling.6 The advantage of polymer-
supported catalysts is that they can be recovered by filtration.
However, phase-transfer reactions often require vigorous stirring,
which leads to mechanical destruction of the supports; con-
sequently, catalyst recovery becomes troublesome and difficult.7 In
addition, the catalytic performance of polymer-supported catalysts
is generally inferior to that of non-supported catalysts.8
the values ranged from 0.23 to 0.27 mmol g21
.
To evaluate the catalytic activities of the MNP-supported crown
ethers, a halogen exchange reaction of 1-bromooctane with KI was
examined under S–L PTC conditions. The results are summarized
in Table 1. It should be noted that the MNP-supported crown
ethers showed almost identical catalytic activity to the non-
supported controls (entries 2 vs. 4 and 3 vs. 5). Their catalytic
performance depended strongly on the ring size of the crown
In this communication, we propose a new solution for recycling
crown ethers: magnetic nanoparticle (MNP)-supported crown
ethers.9,10 The MNP-supported crown ethers are easily separable
using magnetism; therefore, filtration is not required for catalyst
recovery. Moreover, MNPs are expected to be more tolerant of
vigorous stirring than PS-resin because MNPs are extremely small
spheres of metal compounds. We have developed MNP-supported
crown ethers through immobilization of crown ethers on the
surfaces of MNPs, and evaluated their catalytic performance and
recyclability.
Table 1 Halogen exchange reaction of n-C8H17Br with KI under S–L
PTC conditions
Entrya
Catalyst
Yieldb (%)
1
2
3
4
5
a
None
Benzo-15-crown-5
Benzo-18-crown-6
3
4
2
34
88
48
81
National Institute of Advanced Industrial Science and Technology
(AIST), Central 5 Higashi 1-1-1, Tsukuba, Ibaraki, Japan.
E-mail: k.sato@aist.go.jp; Fax: +81-29-861-4852; Tel: +81-29-861-4852
{ Electronic supplementary information (ESI) available: SEM and TEM
images, particle size distribution histograms, experimental procedures for
the synthesis of compounds 2–4 and phase-transfer reactions. See DOI:
10.1039/b705640k
The reactions were performed in toluene (1 ml) at 100 uC for 8 h.
Catalyst : n-C8H17Br : KI = 3 : 100 : 500. Determined by GC
analysis using n-tetradecane as an internal standard.
b
c
3404 | Chem. Commun., 2007, 3404–3405
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efficient for S–L phase-transfer reactions, easily recoverable using
magnetism, and reusable without loss of catalytic activity. To the
best of our knowledge, this is the first successful example of
magnetically separable and reusable crown ethers.
Table 2 Substitution reaction of BnBr with AcOK under S–L PTC
conditions
This work was supported by a Grant-in-Aid for Scientific
Research on Priority Areas ‘‘Advanced Molecular Transforma-
tions of Carbon Resources’’ from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
Entrya,b
Catalyst
Time/h
Yieldc (%)
1
2
3
4
5
a
None
Benzo-15-crown-5
Benzo-18-crown-6
3
4
8
6
6
6
6 (8)
Trace
3
71
78
95 (96)
Notes and references
1 (a) C. J. Pedersen, J. Am. Chem. Soc., 1967, 89, 2495; (b) C. J. Pedersen,
J. Am. Chem. Soc., 1967, 89, 7017.
2 For reviews, see: (a) H. K. Frensdorff, J. Am. Chem. Soc., 1971, 93, 600;
(b) J. J. Christensen, D. J. Eatough and R. M. Izatt, Chem. Rev., 1974,
74, 351; (c) R. M. Izatt, K. Pawlak, J. S. Bradshaw and R. L. Bruening,
Chem. Rev., 1995, 95, 2529.
3 G. W. Gokel and H. Durst, Synthesis, 1976, 168.
4 For recent reviews on phase-transfer catalysis, see: (a) D. Albanese,
D. Landini, A. Maia and M. Penso, J. Mol. Catal. A: Chem., 1999, 150,
The reactions were performed in toluene (1 ml) at 80 uC.
Catalyst : BnBr : AcOK = 3 : 100 : 300. Determined by GC
analysis using n-tetradecane as an internal standard.
b
c
Table 3 Recycling of 4 for the substitution reaction of BnBr with
AcOKa
Recycle
1st
2nd
99
3rd
99
4th
99
5th
6th
99
7th
8th
´
113; (b) M. Ma˛kosza and M. Fedorynski, Catal. Rev.-Sci. Eng., 2003,
Yieldb (%)
a
.99
.99
.99
.99
45, 321; (c) D. Albanese, Catal. Rev.-Sci. Eng., 2003, 45, 369.
5 For representative examples for S–L PTC, see: (a) D. J. Sam and
H. E. Simmons, J. Am. Chem. Soc., 1974, 96, 2252; (b) C. L. Liotta,
H. P. Harris, M. McDermott, T. Gonzalez and K. Smith, Tetrahedron
Lett., 1974, 15, 2417; (c) K. Nakamura, S. Nishiyama, S. Tsuruya and
M. Masai, J. Mol. Catal., 1994, 93, 195; (d) B. Łe˛ska, R. Pankiewicz,
G. Schroeder and A. Maia, Tetrahedron Lett., 2006, 47, 5673.
6 For reviews, see: (a) S. L. Regen, Angew. Chem., Int. Ed. Engl., 1979, 18,
421; (b) M. Benaglia, A. Puglisi and F. Cozzi, Chem. Rev., 2003, 103,
3401; (c) S. D. Alexandratos and C. L. Stine, React., Funct. Polym.,
2004, 60, 3.
All reactions were performed in toluene (1 ml) at 80 uC for 8 h
using 0.75 mmol of BnBr, and 2.25 mmol of AcOK. Determined
by GC analysis using n-tetradecane as an internal standard.
b
ethers, and the reaction with 4 gave better results than that with 3
(entry 4 vs. 5). Clearly, this result can be attributed to the fact that
18-membered crown ethers are generally more suitable for
extracting K+ than 15-membered crowns.1,2a,15 Furthermore, the
MNP-supported crown ethers did not require any pretreatment,
while PS-resin supported catalysts generally require a long pre-
conditioning time.16
ˇ
7 (a) B. Corain, M. Zecca and K. Jera´bek, J. Mol. Catal. A: Chem., 2001,
177, 3; (b) U. Sa´nchez, A. L-Claver´ıe, J. Gonza´lez, L. Cota and
F. Castillon, Polym. Bull., 2002, 49, 39.
8 (a) A. Guyot, Pure Appl. Chem., 1988, 60, 365; (b) F. Cozzi, Adv. Synth.
Catal., 2006, 348, 1367.
We also examined a substitution reaction of benzyl bromide
with potassium acetate under S–L PTC conditions (Table 2). It is
well known that the acetate anion in organic solvents exhibits high
nucleophilicity in the presence of crown ethers.5b Remarkably, the
MNP-supported catalyst 4 was much more efficient than the
benzo crown ethers (entries 2 and 3 vs. 5), and gave the best result.
Furthermore, 3 showed good catalytic activity, slightly superior to
that of benzo-18-crown-6 (entry 3 vs. 4), while benzo-15-crown-5
did not promote the reaction at all (entry 2).17 When the reaction
was conducted under liquid–liquid (L–L) PTC conditions (in
toluene/H2O) with 3 mol% of benzo-18-crown-6, the yield was
drastically decreased to 40% (55% conversion), and ester hydrolysis
occurred partially.18
9 Recently magnetic nanoparticles (MNPs) have been utilized as catalyst
supports, see: (a) H. M. R. Gardimalla, D. Mandal, P. D. Stevens,
M. Yen and Y. Gao, Chem. Commun., 2005, 4432; (b) A. Hu, G. T. Yee
and W. Lin, J. Am. Chem. Soc., 2005, 127, 12486; (c) Y. Zheng,
P. D. Stevens and Y. Gao, J. Org. Chem., 2006, 71, 537; (d) D. Lee,
J. Lee, H. Lee, S. Jin, T. Hyeon and B. M. Kim, Adv. Synth. Catal.,
2006, 348, 41; (e) R. Abu-Rezig, H. Alper, D. Wang and M. L. Post,
J. Am. Chem. Soc., 2006, 128, 5279; (f) N. T. S. Plan, C. S. Gill, J. V.
Nguyen, Z. J. Zhang and C. W. Jones, Angew. Chem., Int. Ed., 2006, 45,
2209; (g) M. Kawamura and K. Sato, Chem. Commun., 2006, 4718.
10 For the recent review for applications of MNPs, see: A.-H. Lu,
E. L. Salabas and F. Schu¨th, Angew. Chem., Int. Ed., 2007, 46, 1222.
11 X. Liu, Z. Ma, J. Xing and H. Liu, J. Magn. Magn. Mater., 2004,
270, 1.
12 L. Josephson, US Pat., 4 672 040, 1987.
13 M. Barboiu, S. Cerneaux, A. van der Lee and G. Vaughan, J. Am.
Chem. Soc., 2004, 126, 3545.
14 The effect of silane surface coating on average diameter of magnetite
nanoparticle would be almost negligible. According to TEM observa-
tions, the average diameter of 4 was estimated to be 12 nm (see ESI{).
15 When the reaction was conducted using NaI instead of KI, the reaction
with 3 gave a better result (8 h, 68% yield).
Recycling of the MNP-supported crown ethers was indeed
simple and easy. After the reaction, the toluene phase could be
separated without filtration since the MNP-supported crown ether
was rapidly concentrated as soon as an external magnet was set
close to the sidewall of the reaction vessel. The residual catalyst
was washed and dried, then immediately reused for the next run.
We examined recycling of 4 for the reaction of BnBr with AcOK
under identical reaction conditions (80 uC, 8 h). As shown in
Table 3, the catalyst could be recycled for at least 8 times without
loss of catalytic activity.
16 (a) H. Molinari, F. Montanari and P. Tundo, J. Chem. Soc., Chem.
Commun., 1977, 639; (b) P. L. Anelli, B. Czech, F. Montanari and
S. Quici, J. Am. Chem. Soc., 1984, 106, 861.
17 During the reactions with the non-supported crown ethers, the reaction
mixture became so viscous that efficient stirring was retarded. On the
other hand, the viscosities of the mixture were sufficiently low to permit
vigorous stirring when the MNP-supported crown ethers were
employed.
In summary, magnetically separable crown ethers were success-
fully prepared through the immobilization of crown ethers on
magnetite nanoparticles. The MNP-supported crown ethers were
18 Benzyl alcohol, a hydrolysis product, was detected by GC analysis
(9% yield).
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