Magnetically separable phase-transfer catalysts

Article abstract of DOI:10.1039/b611906a

Magnetic nanoparticles-supported quaternary ammonium and phosphonium salts were prepared and evaluated as phase-transfer catalysts. Some of them exhibited good activities that were comparable to that of tetra-n-butylammonium iodide. The catalysts were readily separated using an external magnet and reusable without significant loss of their catalytic efficiency. The Royal Society of Chemistry.

Full text of DOI:10.1039/b611906a

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Magnetically separable phase-transfer catalysts{
Masato Kawamura and Kazuhiko Sato*
Received (in Cambridge, UK) 17th August 2006, Accepted 11th September 2006
First published as an Advance Article on the web 27th September 2006
DOI: 10.1039/b611906a
Magnetic nanoparticles-supported quaternary ammonium and
phosphonium salts were prepared and evaluated as phase-
transfer catalysts. Some of them exhibited good activities that
were comparable to that of tetra-n-butylammonium iodide. The
catalysts were readily separated using an external magnet and
reusable without significant loss of their catalytic efficiency.
Recently, much attention has been focused on phase-transfer
reactions since they have been recognized as environmentally
1
benign alternatives to many homogeneous reactions. A practical
Scheme 1 Preparation of magnetite nanoparticles-supported ammonium
and phosphonium salts 3–4.
problem associated with phase-transfer catalysis technology is the
difficulty of separating conventional phase-transfer catalysts
9
according to a previously described method, and magnetite
nanoparticles were treated by excess 1–2 to give magnetite-
supported ammonium salts 3 and phosphonium salts 4. The
catalyst loadings of 3–4 were determined by iodide-ion analysis to
(
PTCs) such as quaternary ammonium salts. The immobilization
of a PTC on a solid support has afforded a solution to overcome
2,3
this problem. So far, cross-linked polystyrene resin (PS resin) is
the most common catalyst support and has been utilized for many
4
phase-transfer reactions. However, PS resin-supported PTCs have
21
range from 0.10 to 0.25 mmol g . A SEM image of 3c is shown in
Fig. 1. It was confirmed that the catalyst was made up of
nanometer-sized particles. The PS resin-supported counterpart 5c
was also synthesized from commercially available chloromethy-
several drawbacks including swelling in organic solvents, which
usually requires a long conditioning time prior to use, and their
5
activities being highly dependent on the solvent used. Moreover,
10
lated polystyrene (Argonaut PS-Cl, 1% cross-linked).
the activities of PS resin-supported PTCs are often lower than
those of their non-supported counterparts, and the deactivation
To evaluate the catalytic activity of 3–4 as PTCs, the reaction of
PhONa with n-BuBr was examined as a model reaction. As
control experiments, tetra-n-butylammonium iodide (TBAI) and
6
often occurs in the reuse process.
Magnetic nanoparticles (MNPs) have emerged as new catalyst
7
supports. A striking feature of the MNPs-supported catalysts is
1
1
5c were employed for the same reaction. The results are
summarized in Table 1. As can be seen from entries 3–8, the
catalytic activity was strongly affected by the alkyl group of
that they can be readily separated using an external magnet, which
achieves simple separation of the catalysts without filtration.
Additionally, the MNPs-supported catalysts showed not only high
catalytic activity but also a high degree of chemical stability, and
they do not swell in organic solvents. Although the use of MNPs
has been promising, the application of MNPs to catalyst support is
still limited as compared with that of PS resin.
1
2
ammonium moiety. Although 3a did not catalyze the reaction
entry 3), the catalytic activities of 3b–d were comparable to that of
(
TBAI (entries 4–6 vs 2). It seemed that lipophilicity of the
ammonium moiety initially increased the catalytic activities.
However, the reactions with 3e and 3f produced the ether in
moderate yields, which were inferior to that using 3c. We thus
Herein, we report the first example of magnetic nanoparticles-
supported PTCs through the immobilization of a quaternary
ammonium and phosphonium salt on magnetite and the
preliminary evaluation of their catalytic activity in phase-transfer
reactions.
7 8
concluded that the long alkyl groups (n-C H15- and n-C H17-) were
hydrophobic to the extent that the diffusion of the catalyst was
restrained. Phosphonium salt 4b was less effective than the
corresponding ammonium salt 3b (entry 4 vs 9), and the activity
Magnetite nanoparticles were chosen for use as magnetic
supports since they could be readily prepared by the conventional
8
coprecipitation method. As described in Scheme 1, the silane
coupling agents 1–2 were initially prepared by reaction of the
corresponding amines or phosphines with (3-iodopropyl)
trimethoxysilane. The immobilization of 1–2 was performed
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: Experimental
{
procedures for the synthesis of compounds 1–4 and phase-transfer
reactions. See DOI: 10.1039/b611906a
Fig. 1 SEM image of 3c.
4
718 | Chem. Commun., 2006, 4718–4719
This journal is ß The Royal Society of Chemistry 2006

View Article Online
Table 1 O-Alkylation reaction of PhONa with n-BuBr under phase-
transfer conditions
Scheme 2 Halogen exchange reaction of 1-bromooctane under phase-
a
b
transfer conditions.
Entry
Catalyst
%Yield
1
2
3
4
5
6
7
8
9
None
TBAI
3a
3b
3c
3d
3e
3f
4b
28
86
36
86
93
84
69
70
80
79
50
without loss of catalytic activity. To the best of our knowledge, this
is the first example of magnetically separable PTCs. We believe
that these results expand the scope of phase-transfer catalysis
technology. Further investigation of magnetically collectable PTCs
is currently in progress.
This work was supported by a Grant-in-Aid for Scientific
Research on Priority Areas ‘‘Advanced Molecular Transfor-
mations of Carbon Resources’’ from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
1
1
a
0
1
4g
5c
All reactions were performed at 100 uC for 12 h in toluene/H
1/1) with 0.75 mmol of PhONa?3H O and n-BuBr (1.5 equiv.).
Determined by GC analysis using n-tetradecane as an internal
2
O
(
2
b
Notes and references
standard.
1
For recent reviews, see: (a) M. Makosza, Pure Appl. Chem., 2000, 72,
399; (b) M. J. O9Donnell, Aldrichimica Acta, 2001, 24, 3; (c) B. Lygo
and B. I. Andrews, Acc. Chem. Res., 2004, 37, 518.
1
Table 2 Recycling of 3c for the alkylation reaction of PhONa with
n-BuBr
2 For reviews, see: (a) S. L. Regen, Angew. Chem., Int. Ed. Engl., 1979, 18,
421; (b) T. J. Dickerson, N. N. Reed and K. D. Janda, Chem. Rev.,
2
002, 102, 3325; (c) D. E. Bergbreiter, Chem. Rev., 2002, 102, 3345; (d)
a
b
Entry
Recycle
%Yield
M. Benaglia, A. Puglisi and F. Cozzi, Chem. Rev., 2003, 103, 2401.
For recent examples of phase-transfer reactions with solid supported
PTCs, see: (a) R. Annunziata, M. Benaglia, M. Cinquini, F. Cozzi and
G. Tocco, Org. Lett., 2000, 2, 1737; (b) B. Thierry, J.-C. Plaquevent and
D. Cahard, Tetrahedron: Asymmetry, 2000, 11, 3277; (c) M. Benaglia,
M. Cinquini, F. Cozzi and G. Tocco, Tetrahedron Lett., 2002, 43, 3391;
(d) B. Tamami and H. Mahdavi, Tetrahedron Lett., 2002, 43, 6225; (e)
L. Li, J. Shi, J. Yan, H. Chen and X. Zhao, J. Mol. Catal. A: Chem.,
2004, 209, 227.
3
1
2
3
4
a
1st
94
90
90
89
2nd
3rd
4th
All reactions were performed at 100 uC for 12 h in toluene/H
1/1) with 0.75 mmol of PhONa?3H
2
O
(
2
O and n-BuBr (1.5 equiv.).
Determined by GC analysis using n-tetradecane as an internal
b
standard.
4 For recent examples of PS-resin supported PTCs, see: (a) M. Xu, Z. Ou,
Z. Shi, M. Xu, H. Li, S. Yu and B. He, React. Funct. Polym., 2001, 48,
8
5; (b) H.-S. Wu and C.-S. Lee, J. Catal., 2001, 199, 217; (c) Z.-T. Wang,
L.-W. Xu, C.-G. Xia and H.-Q. Wang, Helv. Chim. Acta, 2004, 87,
958.
of 4g was identical to that of 4b. It should be noted that the
immobilization on magnetite-nanoparticles did not decrease the
catalytic activity whereas the reaction with PS resin-supported
catalyst 5c was much less effective under the same conditions
1
ˇ
5
6
(a) A. Guyot, Pure Appl. Chem., 1988, 60, 365; (b) F. Svec, Pure Appl.
Chem., 1988, 60, 377; (c) A. R. Vaino and K. D. Janda, J. Comb. Chem.,
2000, 2, 579.
It was reported that significant decomposition of a PS resin-supported
ammonium salt occurred through a dequaternarization process:
H. J.-M. Fou, R. Gallo, P. Hassanaly and J. Metzger, J. Org. Chem.,
1977, 42, 4275 and ref. 2a.
(a) T.-J. Yoon, W. Lee, Y.-S. Oh and J.-K. Lee, New J. Chem., 2003, 27,
227; (b) A.-H. Lu, W. Schmidt, N. Matoussevitch, H. B o¨ nnemann,
B. Spliethoff, B. Tesche, E. Bill, W. Kiefer and F. Sch u¨ th, Angew.
Chem., Int. Ed., 2004, 43, 4303; (c) H. M. R. Gardimalla, D. Mandal,
P. D. Stevens, M. Yen and Y. Gao, Chem. Commun., 2005, 4432; (d)
A. Hu, G. T. Yee and W. Lin, J. Am. Chem. Soc., 2005, 127, 12486; (e)
Y. Zheng, P. D. Stevens and Y. Gao, J. Org. Chem., 2006, 71, 537; (f)
D. Lee, J. Lee, H. Lee, S. Jin, T. Hyeon and B. M. Kim, Adv. Synth.
Catal., 2006, 348, 41; (g) R. Abu-Rezig, H. Alper, D. Wang and
M. L. Post, J. Am. Chem. Soc., 2006, 128, 5279.
(entry 11).
Easy and rapid separation of the catalyst by magnetism is the
most advantageous feature of these catalysts. After the reaction,
the catalysts were concentrated on the sidewall of the reaction
vessel using an external magnet, the aqueous and the organic
phases were separated by decantation, and the residual catalyst in
the reaction vessel was washed and dried and then subjected to the
next run directly. We examined the recycling of 3c for the reaction
of PhONa with n-BuBr. As shown in Table 2, 3c was reusable
without any significant loss of activity for the 4th recycling.
The halogen exchange reaction of 1-bromooctane was also
examined as another model reaction (Scheme 2). This reaction
proceeded negligibly without PTC (12 h, 6%), whereas the reaction
with 3 mol% TBAI gave 1-iodooctane in good yield (6 h, 92%).
The activity of 3c was comparable to that of TBAI, although the
yield was slightly reduced under the same reaction conditions (6 h,
7
8
9
S. Giri, B. G. Trewyn, M. P. Stellmaker and V. S.-Y. Lin, Angew.
Chem., Int. Ed., 2005, 44, 5038.
M. Ma, Y. Zhang, W. Yu, H.-Y. Shen, H.-Q. Zhang and N. Gu,
Colloids Surf., A, 2003, 212, 219.
1
0 H. Molinari, F. Montanari, S. Quici and P. Tundo, J. Am. Chem. Soc.,
979, 101, 3920.
1 Control experiments are essential for the evaluation of solid-supported
1
1
8
5%; 9 h, 92%). On the other hand, 5c showed low activity, and
PTCs since the reaction rates are highly affected by various factors:
M. Tomoi and W. T. Ford, J. Am. Chem. Soc., 1981, 103, 3821 and
the yield of 1-iodooctane was drastically decreased (40%, 6 h).
In summary, we have developed magnetically separable PTCs
through the immobilization of ammonium and phosphonium salts
on magnetic nanoparticles. The catalysts are readily reusable
3828 and ref. 5.
1
2 A similar structural dependency of catalytic activity was reported:
M. Cinouni, S. Colonna, H. Molinari, F. Montanari and P. Tundo,
J. Chem. Soc., Chem. Commun., 1976, 394.
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 4718–4719 | 4719