Triflic acid-functionalized silica-coated magnetic nanoparticles as a magnetically separable catalyst for synthesis of gem-dihydroperoxides

Article abstract of DOI:10.1002/adsc.201100561

Triflic acid-functionalized silica-coated magnetic nanoparticles [γ-Fe₂O₃@SiO₂-TfOH] were readily prepared and identified as an effictive catalyst for the transformation of aldehydes or ketones into their corresponding gem-dihydroperoxides with 30% aqueous hydrogen peroxide. The catalyst was easily separated by magnetic decantation and the recovered catalyst was reused for seven cycles without significant loss of catalytic activity. Copyright

Full text of DOI:10.1002/adsc.201100561

FULL PAPERS
DOI: 10.1002/adsc.201100561
Triflic Acid-Functionalized Silica-Coated Magnetic
Nanoparticles as a Magnetically Separable Catalyst for Synthesis
of gem-Dihydroperoxides
Yong-Hui Liu,a Jia Deng,a Jian-Wu Gao,a and Zhan-Hui Zhanga,*
a
Key Laboratory of Inorganic Nanomaterial of Hebei Province, College of Chemistry & Material Science, Hebei Normal
University, Shijiazhuang 050024, Peopleꢀs Republic of China
Fax : (+86)-311-8963-2795; e-mail: zhanhui@mail.nankai.edu.cn
Received: July 14, 2011; Revised: September 18, 2011; Published online: January 19, 2012
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201100561.
Abstract: Triflic acid-functionalized silica-coated covered catalyst was reused for seven cycles without
magnetic nanoparticles [g-Fe2O3@SiO2-TfOH] were significant loss of catalytic activity.
readily prepared and identified as an effictive cata-
lyst for the transformation of aldehydes or ketones
into their corresponding gem-dihydroperoxides with Keywords: aldehydes; gem-dihydroperoxides; func-
30% aqueous hydrogen peroxide. The catalyst was tionalized silica-coated magnetic nanoparticles; ke-
easily separated by magnetic decantation and the re- tones; magnetic catalysts
Introduction
endoperoxides,[12] and acylic analogues with a variety
of functional groups.[13] These cyclic peroxides have
With increasing environmental concerns, the develop- been found to exhibit significant antimalarial activi-
ment of efficient and recoverable heterogeneous cata- ties.[14] gem-Dihydroperoxides have also been utilized
lysts has become an important research field. In this as oxidants for epoxidation,[15,16] sulfoxidation,[17] and
context, nanoparticles as heterogeneous catalysts synthesis of oligodeoxyribonucleotides,[18] and as ini-
have attracted a great deal attention due to their in- tiators for radical polymerization.[19] Generally, ozo-
teresting structures and high catalytic activities.[1] Fur- nolysis of ketone enol ethers or a-olefins in the pres-
thermore, nanometer-sized particles are easily disper- ence of H2O2 had been widely used for the synthesis
sible in solution by forming stable suspensions.[2] In of gem-DHPs.[9a] The preparation of gem-DHPs has
spite of these advantages, the tedious recycle of such also been accomplished through the reaction of ace-
small particles via expensive ultra-centrifugation has tals and enol ethers with H2O2 in the presence of
limited their application. To further address the issues tungstic acid,[20] or BF3·OEt2,[21] or hydroperoxide re-
of recyclability and reusability, magnetic nanoparticles arrangement of bicyclic hydroperoxides.[22] In spite of
(MNPs) have emerged as excellent supports amena- their potential utility, these methods involved the
ble to simple magnetic separation.[3] Moreover, mag- need for the prior synthesis of the suitable starting
netic nanoparticles can be functionalized easily substrates.[20–22] In addition, the selectivity of ozonoly-
through appropriate surface modifications, which are sis is poor and the process cannot be used for sub-
able to load various functionalities.[4–6]
strates containing ozone-sensitive functional groups.[9a]
Organic gem-dihydroperoxides (DHPs) have been In this respect, directly employing ketones and alde-
shown to possess intriguing biological and pharma- hydes under oxidative conditions turned out to be the
ceutical properties such as antiparasitic and antimi- most facile and effective method to synthesize gem-
crobial activity.[7] They are also used as allergens, anti- DHPs. Some catalysts such as stannous chloride,[23]
fungal and antitumour agents, as well as inhibitors of strontium chloride,[24] ceric ammonium nitrate,[25]
enzymes.[8] In addition, these compounds are synthetic phosphomolybdic acid,[26] camphorsulfonic acid,[27]
[31]
intermediates for the preparation of various cyclic H2SO4,[28] NaHSO4·SiO2,[29] iodine,[30] or Re2O7 have
peroxides such as 1,2,4,5-tetraoxacycloalkanes,[9] 1,2,4- been employed to accomplish this reaction. However,
trioxane,[10] 1,2,4,5,7,8-hexaoxonane,[11] macrocyclic some of the reported methods involved the use of
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Yong-Hui Liu et al.
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loss of solid catalyst in the separation are some of the
drawbacks. Therefore, we decided to design a novel
version of silica-supported TfOH to facilitate a more
efficient recycling of this catalyst. For this purpose,
we prepared the triflic acid-functionalized silica-
coated magnetic nanoparticles [g-Fe2O3@SiO2-TfOH]
and, after characterization, the peroxidation of car-
bonyl compounds was selected to investigate its cata-
Scheme 1. Synthesis of gem-DHPs with 30% H2O2 in the
presence of g-Fe2O3@SiO2-TfOH.
Lewis acids or Brønsted acids. These traditional acids lytic activity and recyclability.
are corrosive and produce significant amounts of
To prepare g-Fe2O3@SiO2-TfOH, we have first
waste, which limits their usefulness and causes serious chosen a known nanomagnet g-Fe2O3, which can be
environmental concerns. Furthermore, long reaction easily prepared by coprecipitation of ferrous (Fe2+)
times,[27,30] unsatisfactory yields,[28] the use of highly and ferric (Fe3+) ions in a basic aqueous solution fol-
[26,27]
concentrated H2O2
or relatively expensive re- lowed by thermal treatment according to the reported
agents,[31] are other disadvantages of some of these procedure.[37] Coating of a layer of silica on the sur-
methods. Besides, in these methods, catalysts are de- face of g-Fe2O3 nanoparticles was achieved by sonica-
stroyed during the work-up and cannot be recov- tion of a g-Fe2O3 suspension in an alkaline ethanol-
ered.[23–28] The catalyst-free synthesis of gem-DHPs water solution of tetraethyl orthosilicate (TEOS).
from ketones and aldehydes with hydrogen peroxide Then, triflic acid was added to a suspension of g-
in dimethoxyethane[32] or with molecular oxygen Fe2O3@SiO2 in Et2O, while dispersed by sonication.
under light irradiation[33] proceeded under mild condi- The mixture was concentrated and the residue was
tions, but required long reaction times and gave low heated at 708C for 24 h under vacuum to obtained
yields of products. Due to their importance and useful triflic acid-functionalized silica-coated magnetic nano-
properties, the development of an efficient, general particles [g-Fe2O3@SiO2-TfOH] (Scheme 2). The load-
and environmentally benign method for the prepara- ing of triflic acid moiety on surface was determined
tion of these widely used gem-DHPs is a major chal- by means of back tritration and pH analysis
lenge in synthetic organic chemistry.
(0.30 mmolgÀ1, 10 mg=0.003 mmol of CF3SO3H).
The FT-IR spectra of g-Fe2O3@SiO2-TfOH dis-
Recently, we have discovered that magnetic Fe3O4
nanoparticles are excellent catalysts for the synthesis played strong bands around 3440, 1078, and 639 cmÀ1,
of quinoxalines[34] and 2,3-dihydroquinazolin-4(1H)- which are ascribed to the characteristic bands of
ones.[35] We have developed an efficient method for n (O H), n (Si O), and n (Fe O) in magnetic nano-
one-pot reductive amination of carbonyl compounds particles. This demonstrated the successful in loading
using NaBH4 in the presence of the magnetically re- of TfOH onto the magnetic nanoparticles, whereby a
coverable g-Fe2O3@HAP-SO3H.[36] Immobilization of relatively weak absorption band around 1170 cmÀ1 for
À
À
À
[37]
[38]
HClO4 or H3PW12O40 onto the silica-coated mag- n (S=O) was derived from TfOH (Figure 1). The
netic nanoparticles has also been successfully energy dispersive spectrum (EDS) showed that the
achieved. As a continuation of our interest in devel- mass percentages of Fe, Si, O, C and S are 1.89%,
oping efficient and environmental benign synthetic 38.62%, 55.31%, 3.85% and 0.33%, respectively
methodologies,[39] we report herein on the preparation (Figure 2). XRD measurements of g-Fe2O3@SiO2 and
of
a
new type of magnetically separable g- g-Fe2O3@SiO2-TfOH exhibit diffraction peaks at
Fe2O3@SiO2-TfOH nanoparticles and their applica- around 35.58, 43.18, 62.88, 548 corresponding to the
tion for the synthesis of gem-DHPs at room tempera- (311), (400), (511) and (440) faces (Figure 3). The
ture (Scheme 1).
observed diffraction peaks agree well with the tetrag-
onal structure of maghemite.[33] It is clear that no
other phases except the maghemite are detectable.
The average crystallite size of g-Fe2O3@SiO2-TfOH
particles was calculated to be ~50 nm by Scherrerꢀs
Results and Discussion
Although silica-supported triflic acid (TfOH-SiO2) equation for the (311) reflection, which is in good ac-
was introduced by Liu and co-workers,[40] the tedious cordance with transmission electron microscopy
recycling of the catalyst by filtration and inevitable (TEM) results (Figure 4). The specific surface area of
Scheme 2. Synthesis of g-Fe2O3@SiO2-TfOH.
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Triflic Acid-Functionalized Silica-Coated Magnetic Nanoparticles as a Magnetically Separable Catalyst
Figure 4. TEM image of g-Fe2O3@SiO2-TfOH.
Figure 1. IR spectra of g-Fe2O3@SiO2-TfOH.
Table 1. Influence of different catalysts on the reaction of
cyclohexanone with 30% aqueous H2O2.[a]
Entry
Catalyst (10 mol%)
Time [min]
Yield [%][b]
1
2
3
4
5
6
7
8
nano ZnO
nano TiO2
nano CuO
nano Al(OH)3
nano ZnFe2O4
nano Fe3O4
nano g-Fe2O3
g-Fe2O3@HAP-SO3H
SiO2 (50mg)
TfOH
240
240
240
240
240
240
240
240
240
120
120
80
0
0
0
0
0
0
0
18
0
9
10
11
12
80
86
96
SiO2-TfOH
g-Fe2O3@SiO2-TfOH
Figure 2. EDS spectrum of g-Fe2O3@SiO2-TfOH.
[a]
Experimental conditions: cyclohexanone (1 mmol), 30%
H2O2 (4 mmol), and catalyst (0.10 mmol) in 5 mL of
CH3CN at room temperature.
[b]
Isolated yields.
the powders was determined by use of the Brunauer–
Emmett–Teller (BET) method. BET results showed
that the average surface area was 50 m2 gÀ1.
In initial attempts, the activity of the prepared cata-
lyst as described above was measured in the model re-
action between cyclohexanone and 30% aqueous
H2O2 in acetonitrile. From Table 1, it was clear that g-
Fe2O3@SiO2-TfOH worked remarkably well to give
the desired product within 80 min in 96% yield. For
comparison, the model reaction was carried out using
liquid TfOH as the homogeneous catalyst, and only
80% yield was obtained (Table 1, entry 10). This
result suggested that g-Fe2O3@SiO2-TfOH was a more
effective catalyst than the original homogeneous one.
Besides, this novel catalyst was superior to triflic acid
Figure 3. XRD pattern of g-Fe2O3@SiO2 and g-Fe2O3@SiO2-
TfOH.
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Table 2. Optimization of the reaction conditions.[a]
Having these results in hand, we then investigated
the scope and limitations of the method. As shown in
Table 3, many acyclic aliphatic ketones (Table 3, en-
tries 1–9) were effectively transformed into their cor-
responding gem-dihydroperoxides in 86–96% yields
within 80–120 min at room temperature. Steric effects
in the a-position of the carbonyl group play a key
role in facilitating the reaction because of the fact
that replacing a hydrogen atom with a methyl group,
for example, pentan-3-one (1g), results in a decrease
of the reaction yield (88%, Table 3, entry 7). Similarly,
various cyclic ketones such as cyclopentanone, cyclo-
hexanone and its derivatives, cycloheptanone, cyclo-
dodecanone, and 1H-inden-2(3H)-one (Table 3, en-
tries 10–21) furnished the products in high to excel-
lent yields. 2-Adamantanone (entry 22) and 2-norbor-
nanone (entry 23), having high steric hindrance, were
still reactive and the corresponding gem-dihydroper-
oxides were obtained in 95% and 94% yields, respec-
tively. Furthermore, no products of ring expansion
were observed.[43] When aromatic ketones were used,
the yields were somewhat lower (entries 24 and 25).
In addition, seven aliphatic aldehydes were suitable
Entry Catalyst load- Solvent 30% H2O2 Time Yield
ing [mol%]
(equiv.)
[min] [%][b]
1
2
3
4
5
6
7
8
no
10
10
10
10
5
15
20
10
10
10
10
MeCN
CH2Cl2
EtOAc
Et2O
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
4
4
4
4
4
4
4
4
2
3
5
6
180
120
120
120
80
120
120
120
180
120
120
120
0
81
77
50
96
84
96
95
66
90
96
92
9
10
11
12
[a]
Experimental conditions: cyclohexanone (1 mmol) in
5 mL of solvent at room temperature.
Isolated yields.
[b]
absorbed on silica gel (TfOH-SiO2)[40] with respect to substrates to produce gem-dihydroperoxides in high
yield of the product under identical conditions yield, as shown in entries 26–28. The reaction of aro-
(Table 1, entry 11). The reason for the improved cata- matic aldehydes having electron-donating groups also
lytic activity may due to higher surface areas of nano- proceeded efficiently (Table 3, entries 29–32), while
scale heterogeneous catalysts.[41] Although g- with a compound having an electron-withdrawing
Fe2O3@HAP-SO3H is well known to be a good cata- group, such as 4-nitrobenzaldehyde (Table 3,
lyst for some organic reactions,[36,42] its catalytic activi- entry 33), no corresponding gem-dihydroperoxide was
ty for this reaction was lower than g-Fe2O3@SiO2- isolated.
TfOH (Table 1, entry 8). Various other nano materials
The reusability is one of the important properties
such as ZnO, TiO2, CuO, Al(OH)3, ZnFe2O4, Fe3O4 of this catalyst. In this work, we examined the possi-
and g-Fe2O3 were also tested for this transformation, bility of recovery and reuse of g-Fe2O3@SiO2-TfOH.
however, no expected product formation was ob- After completion of the model reaction, the catalyst
served.
was recovered from the reaction mixture simply by
For further studies regarding the effect of various applying an external magnet. Then the recovered cat-
solvents, the above reaction was conducted in the alyst was washed with diethyl ether, and dried at
presence of g-Fe2O3@SiO2-TfOH with various sol- room temperature. The recovered catalyst could be
vents such as acetonitrile, dichloromethane, ethyl ace- added to fresh substrates under identical conditions
tate, and diethyl ether. The results indicated that dif- for seven runs without a noticeable drop in the prod-
ferent solvents affected the efficiency of the reaction uct yield and its catalytic activity. (Figure 5). The
(Table 2). The best conversion was observed when the slight decrease of catalytic activity should be due to
reaction was performed in acetonitrile in which prob- the normal loss of the catalyst during the work-up
ably solvent polarity plays an important role.[24,31] stage. No quantifiable amount of leached Fe was de-
Moreover, we found that the yields were obviously af- tected in the filtrates as determined by inductively
fected by the amount of catalyst. It was found that coupled plasma atomic emission spectroscopy (ICP-
10 mol% of g-Fe2O3@SiO2-TfOH was sufficient to AES). To determine of the leaching of the acid, the
catalyze the reaction and the use of higher amounts model reaction was carried out in the presence of g-
of catalyst did not increase the yields significantly. On Fe2O3@SiO2-TfOH for 30 min and at that point the
the other hand, when the reaction was attempted catalyst was removed by external magnet. The residu-
without the addition of catalyst, no desired product al solution was then allowed to react, but no signifi-
was obtained (Table 2, entry 1). 4 equivalents of H2O2 cant progress was observed after 1 h. Therefore, these
were necessary to produce the product in satisfying experiments are a further testimony to the heteroge-
yield, and lower yields of the product were observed neous nature of the catalytic system. Furthermore, the
when using only 2 or 3 equivalents of H2O2 (Table 2, TEM images of the used catalyst did not show any
entries 9 and 10).
significant change in the shape and size of catalyst
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Triflic Acid-Functionalized Silica-Coated Magnetic Nanoparticles as a Magnetically Separable Catalyst
Table 3. Synthesis of gem-dihydroperoxides in the presence
of g-Fe2O3@SiO2-TfOH.
Table 3. (Continued)
Entry Substrate
Product Time
[min]
Yield
[%][a]
Ref.
Entry Substrate
Product Time
[min]
Yield
[%][a]
Ref.
[24]
24
25
2x
2y
480
480
41
42
[29]
[29]
[30b]
[29]
[24]
1
2
3
4
5
2a
2b
2c
2d
2e
80
80
80
80
80
92
96
94
93
92
[24]
[27]
26
27
28
2z
480
480
480
76
78
75
2aa
2ab
[32]
[29]
[30a]
[33]
6
7
8
2f
2g
2h
100
100
100
89
88
87
29
30
31
32
2ac
2ad
2ae
2af
480
480
480
480
66
69
62
78
[30a]
[32]
[24]
[24]
[23]
9
2j
90
86
95
[33a]
10
2k
120
[33a]
[33a]
11
12
13
2k
2l
80
96
93
95
100
90
[33a]
[33a]
2m
33
2ag
480
0
14
15
16
17
2n
2o
2p
2q
80
80
80
80
96
95
96
90
[a]
Isolated yield.
[32]
18
2r
80
81
[32]
[32]
19
20
2s
2t
80
80
91
93
21
22
23
2u
2v
2w
80
80
80
84
95
94
Figure 5. Reusability of the catalyst using cyclohexanone as
substrate.
[32]
particles even after seven times reaction cycles, which
proved its robustness (see Supporting Information,
Figure S1).
Adv. Synth. Catal. 2012, 354, 441 – 447
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Yong-Hui Liu et al.
FULL PAPERS
Conclusions
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In conclusion, we have immobilized, for the first time,
triflic acid onto magnetic g-Fe2O3@SiO2 and applied
the immobilized catalyst (g-Fe2O3@SiO2-TfOH) in the
dihydroperoxidation of a wide variety of carbonyl
compounds. The corresponding gem-dihydroperoxides
were obtained in moderate to high yields with aque-
ous H2O2 (30%) as oxidant. Product separation and
catalyst recycling are easy and simple with the assis-
tance of an external magnet. The catalyst can be re-
covered and reused for seven times with little loss of
activity.
Experimental Section
Synthesis of Triflic Acid-Functionalized Silica-Coated
Magnetic Nanoparticles
To a suspension of g-Fe2O3@SiO2 (1 g) in Et2O (50 mL),
TfOH (0.30 mmol) was added. The mixture was irradiated
in the ultrasonic bath for 60 min at room temperature. The
Et2O was removed and the residue was heated at 708C
under reduced pressure for 24 h to afford g-Fe2O3@SiO2-
TfOH (0.30 mmolgÀ1, 10 mg=0.0030 mmol of TfOH).
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General Procedure for Synthesis of gem-
Dihydroperoxides
A mixture of carbonyl compound (1 mmol), 30% aqueous
H2O2 (4.0 mmol) and g-Fe2O3@SiO2-TfOH (0.1 mmol) in
MeCN (5 mL) was stirred at room temperature. The prog-
ress of reaction was monitored by TLC using N,N,N’,N’-tet-
ramethyl-p-phenylenediamine dihydrochloride as a chroma-
togenic reagent. After completion of the reaction, the cata-
lyst was separated with the aid of an external magnet. The
mixture was diluted with water (5 mL), extracted with ethyl
acetate. The combined organic layer was washed with brine,
dried (MgSO4) and the solvent was evaporated under
vacuum. The residue was purified by short column chroma-
tography on silica gel using ethyl acetate/petroleum ether as
solvent to provide the pure products.
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