Catalytic epoxidation activity of keplerate polyoxomolybdate nanoball toward aqueous suspension of olefins under mild aerobic conditions

Article abstract of DOI:10.1021/ja405852s

Catalytic efficiency of a sphere-shaped nanosized polyoxomolybdate {Mo ₁₃₂} in the aerobic epoxidation of olefins in water at ambient temperature and pressure in the absence of reducing agent is exploited which resulted good-to-high yields and desired selectivity.

Full text of DOI:10.1021/ja405852s

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Catalytic Epoxidation Activity of keplerate Polyoxomolybdate Nanoball
towards Suspension of Olefins under Mild Aerobic Conditions
Abdolreza Rezaeifard, Reza Haddad, Maasoumeh Jafarpour, and Mohammad Hakimi
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja405852s • Publication Date (Web): 25 Jun 2013
Downloaded from http://pubs.acs.org on June 26, 2013
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Catalytic Epoxidation Activity of Keplerate Polyoxomolybdate
Nanoball towards Aqueous Suspension of Olefins under Mild
Aerobic Conditions
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Abdolreza Rezaeifard*†, Reza Haddad†, Maasoumeh Jafarpour*†, and Mohammad Hakimi‡
†Catalysis Research Laboratory, Department of Chemistry, Faculty of Science, University of Birjand, Birjand, 97179ꢀ414
Iran.;
‡ Payame Noor University of Tehran, Iran
Supporting Information
treme interest as oxidation catalysts due to both their resistance
toward oxidation and compatibility with various oxygen sources.7
Giant polyoxomolybdates, especially {Mo132} spherical ballꢀlike
cluster, are nanoscale architectures with more than 500 atoms and
the highest Euclidean symmetry,8 which gained significant interꢀ
est as cation carriers, nanosponges and for various other material
science applications in the solid state.8d,9 However, their catalytic
properties in the oxidation of organic reactions really lag behind.10
In continuation of our ongoing research on the development of
novel oxidation methods for oxidation of organic compounds11
and especially in sustainable media,12 herein we report a simple,
mild and efficient aerobic epoxidation procedure catalyzed by
ABSTRACT: Catalytic efficiency of a sphereꢀshaped nanosized
polyoxomolybdate {Mo132} in the aerobic epoxidation of olefins
in water at ambient temperature and pressure exploited which
resulted good/high yields and desired selectivity.
Metalꢀcatalyzed aerobic oxidation represents the most elegant
and environmentally friendly route for a largeꢀscale production of
invaluable oxidation products.1 The utilization of molecular oxyꢀ
gen for a catalytic oxidation without reducing reagents or radical
initiators is a rewarding goal. This is due to the highest content of
active oxygen in molecular oxygen, and lack of byproducts, conꢀ
trary to various other oxidants. However, only a few ideal homoꢀ
geneous epoxidations of olefins with molecular oxygen at 1 atm
without reducing agents or radical initiators have been developed
to resolve difficulty with either persuasion of the catalysts to act
as a stoichiometric oxidant or degradation of their organic ligꢀ
ands.2 Moreover, the poor product selectivity obtained in aerobic
processes is one of the major drawback for using molecular oxyꢀ
gen as the terminal oxidant for the formation of epoxides.3 In adꢀ
dition, the limited number of catalysts available for direct activaꢀ
tion of molecular oxygen, effectively restricts its use. Thus, the
innovation and improvement of selective catalytic epoxidation
methods where molecular oxygen is employed as terminal oxiꢀ
dants without any reducing reagents or radical initiators is highly
desirable.1 Nevertheless, a major challenge is to accomplish such
process in nonꢀtoxic solvents, particularly in aqueous media. It is
known that water as a solvent induces attractive patterns of reacꢀ
tivity and selectivity compared with those observed in common
organic solvents, which makes green chemistry an active field.4
Recently, reactions of waterꢀinsoluble organic compounds that
take place in aqueous suspensions (“on water”) have received a
great deal of attention because of their high efficiency and
straightforward synthetic protocols.5 Nevertheless, such reactions
are still rare and lack generality.
nanoball
Keplerate
{Mo132}
{(NH4)42[MoVI72MoV60O372(CH3COO)30(H2O)72}8a that takes
place “on water” in the absence of reducing agent or radical initiaꢀ
tors (Scheme 1). The catalyst was reused in the procedure for
many times without any loss of activity.
2.9 nm
{Mo132
}
O
"
O2/rt, On Water"
Scheme 1. Aerobic epoxidation of olefins in water catalyzed
by {Mo132 naoball
}
Catalytic experiments were initiated with the oxidation of cyꢀ
clooctene (0.5 mmol) with O2 (1 atm) in the presence of 1 mol%
of {Mo132} in bidistilled water at 25°C. It led to 14, 24, 37 and
100% cyclooctene oxide (GC yield based on toluene as internal
standard) after 30, 40, 60 and 120 min, respectively, without any
Metalꢀoxygen cluster molecules, polyoxometalates (POMs) are
attractive compounds for a wide range of chemistry because of
their high ability to control the molecular properties (composition,
size, shape, acidity, and redox potential).6 They have been of exꢀ
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reducing reagent. To confirm the ineffectiveness of reducing
Table 1. Aerobic olefin epoxidation in water catalyzed by
a
1
2
3
4
5
6
7
8
agent, isoꢀbutyraldehyde as a commonly used reducing agent was
added and no improvement in the reaction rate was observed
(35% after 60 min). Our investigation on the catalyst loading
demonstrated a high atom efficiency of title nanopolyoxomolybꢀ
date. Only after 2 h cyclooctene converted completely using a few
minute of catalyst (0.2 mol%) and cyclooctene oxide was secured
exclusively in 96% yield by an easy isolation with ethyl acetate as
a safe solvent. More interestingly, the use of excess amount of
cyclooctene (1 mmol) rather than catalyst (1×10ꢀ5 mmol, 0.001
mol%) gave 57% epoxide yield after 24 h, illustrating a high turnꢀ
over number of 57000 for this nanostructured catalyst.[2b] This
desired conversion rate can be rationalized in terms of the reaction
conditions as a heterogeneous suspension of organic droplets in
water (‘‘onꢀwater’’).5b However, the excellent dispersity of
{Mo132}ꢀtype cluster in aqueous solution generating discrete sinꢀ
gle molecular clusters, should be taken into account for such an
activity.13 It should be noted that when the catalyst was replaced
by MoO3, Na2MoO4.4H2O and (NH4)6MO7O24.4H2O as simple
salts of Mo(VI), no oxidation product was observed with longer
reaction times up to 24h. The time course of oxygen uptake by
cyclooctene verified these results (Figure 1).
{Mo132
}
Entry
Olefin
Product
Yield%b
Time (h)
1
2
3
4
5
6
7
8
9
96
92
60
33
95
93
94
45
89
90
96
2
3
5
5
2
3
4
5
4
4
5
O
9
O
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
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30
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O
Ph
Ph
O
O
O
O
14
12
10
8
O
O
6
O
4
10
11
2
0
OH
OH
0
15
30
45
60
75
90
105
120
O
OH
55c (erythro)
43c (threo)
Time(min)
12
13
5
5
O
OH
OH
OH
Figure 1. Time course of oxygen uptake by cyclooctene in the
presence of {Mo132} (black line), (NH4)6Mo7O24.6H2O (blue line)
and NaMoO4.2H2O (red line) in water.
O
OH
80
O
O
Since, the structure of POMs is pHꢀdependent;14 the epoxidation
of cyclooctene was run at different pH values. When, pH value
was adjusted to less than or equal 8, exactly the same conversions
and selectivity were obtained. Nevertheless, at higher pH values
no oxidation activity was observed because {Mo132} cluster beꢀ
come less stable consequently they are broken as established by
UVꢀVis study (Figure 2). Note that all polyoxometalates decomꢀ
pose at high pH values while smaller species are formed.14
O
14
15
40
97
5
5
O
O
16
25
5
O
aThe reactions were run at 25°C in 2 ml aqueous suspension of
olefin (0.5 M), containing 2 ꢁmol {Mo132} as catalyst under 1
atm O2 (7ꢀ10 ml/min). Yield of isolated products. All the reacꢀ
tions were run at least in triplicate, and the yields represent an
average of these reactions. The selectivity of epoxides were >99%
except for entry 6 which was 98% (2% benzaldehyde). Deterꢀ
2.5
b
pH=2
2
pH=4
pH=6
1.5
pH=7
c
pH=9
mined by NMR.
1
pH=10
To establish the general applicability of the method, various
olefins were subjected to the oxidation protocol using O2 (1 atm)
and 0.2 mol% of {Mo132} at 25ºC (Table 1). As summarized in
Table 1, different olefins are generally good substrates for this
nanocatalyst forming solely the related epoxides within desired
reaction times. Our method led to complete conversion of cyꢀ
clooctene, cyclohexene, norbornene, styrene, αꢀmethylstyrene and
indene (entries 1, 2, 5, 6, 7 and 9) within 2ꢀ4 h. It is worth to menꢀ
tion that no allylic oxidation was occurred and only a trace
0.5
0
250
300
350
400
450
500
550
600
Wavelength/nm
Figure 2. The screening of pH in the aerobic epoxidation of cyꢀ
clooctene in water catalyzed by {Mo132} nanoball.
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amount of benzaldehyde was detected (2%) resulting from ring
of cyclooctene in the presence of {Mo132} led to isolation of the
1
2
3
4
5
6
7
8
opening reaction of styrene oxide (entry 6). The reaction also
proceeded smoothly with less reactive 1ꢀoctene (entry 10) and
90% of the corresponding epoxide was secured as a sole product
within 4 h. However, the poor reactivity of 1, 1, 2ꢀtribustitutedꢀ as
well as Eꢀolefins (entries 3, 4, 8, 16) were observed demonstrating
the steric hindrance around the active site of catalyst. It is worth to
mention that the catalyst was able to oxidize electronꢀdeficient 2ꢀ
cyclohxeneꢀ1ꢀone to the related epoxide in moderate yield (40%,
entry 14).
related epoxide in 93% yield within 2 h.
In conclusion, the catalytic performance of keplerate type giant
ball nanopolyoxomolybdate {Mo132} in the aerobic epoxidation of
different olefins that takes place “on water” in the absence of
reducing agent or radical initiators was demonstrated. Desired
yields and chemoꢀ and stereoꢀselectivity were obtained. These
favorable characteristics along with easy and safe workꢀup proceꢀ
dure as well as excellent reusability of catalyst are environmentalꢀ
ly benign and cost effective. Thus, our method possesses high
generality which makes it suitable for industrial goal.
9
The method possesses novelty regarding chemoselectivity. The
hydroxyl group which is sensitive to oxidation remained comꢀ
pletely intact under the influence of the catalyst producing the
pertinent epoxides in high yields (entries 11ꢀ13). To confirm our
claim, a competitive reaction between cyclooctene and benzyl
alcohol was run. After 2h, cyclooctene oxide was obtained quantiꢀ
tatively as the only product.
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Then, we applied this protocol to sulfur containing olefins.
Electronꢀrich allyl sulfides namely diallyl and allyl phenyl sulfide
were converted completely to the corresponding sulfones after 5ꢀ6
h, while, olefin moiety was tolerated in the reaction. Nevertheless,
other sulfides displayed actually no oxidation activity under the
same conditions even after a long period of time.
The stereoselectivity of the method was notable. Norbornene
gave exclusively the related exoꢀepoxide (entry 5). Moreover,
epoxidation of cisꢀ and transꢀolefins proceeded with absolute
stereospecificity (entries 8, 15 and 16). However, 1ꢀoctenꢀ3ꢀol
(entry 12) gave approximately equal yield of diastereoisomers of
erythro:threoꢀ1,2ꢀepoxyꢀ3ꢀoctanol (55:43 ratio).
Figure 3. Recycling of the catalytic system for aerobic epoxidaꢀ
tion of cyclooctene in water according to procedure mentioned in
Table 1.
In addition, epoxidations of cisꢀstilbene was faster than that of
the corresponding transꢀolefin. This result was similar to that
observed in the presence of porphyrin systems which proceeded
differently from a radical pathway.15 Moreover, oxygenation of
cyclooctene was not inhibited by the presence of radical scavenꢀ
gers such as 2,6ꢀdiꢀtertꢀbutylꢀ4ꢀmethylphenol (BHT). For examꢀ
ple, the conversion and selectivity for cyclooctene oxide after 2 h
in the presence of 5 and more equivalents of BHT (rather than
catalyst) were ~97 and 100%, respectively according to GC analꢀ
ysis. These results demonstrate that nonradical processes prevail
to high extent in the present system, although some contribution
of free radicals could exist.
Recovery of catalyst was easy and efficient. When the reaction
completed, hydrophobic organic products were isolated by adding
ethyl acetate as a safe solvent. Then an aqueous solution of cataꢀ
lyst was reused directly for next round of reaction without further
purification. The solid catalyst {Mo132} could also be obtained
easily by removing the water followed by washing with ethyl
acetate or ethanol, dried under vacuum. The ease of recovery,
combined with the intrinsic stability of the {Mo132}, allows the
catalyst to be recovered efficiently over at least ten times in the
epoxidation of cyclooctene (Figure 3).Only after the ninth run was
a negligible decrease in catalyst performance (<2%). Raman
bands of the original skeletal vibration of {Mo132} changed only
slightly after many times recovering (Figure 4). Also the compariꢀ
son of UVꢀVis and IR spectra of used catalyst with fresh one
(Figures S1 and S2) illustrated that the structure and morphology
of the catalyst remained completely intact. Therefore, title methꢀ
odology is environmentally benign because of using molecular
oxygen as an oxygen source, water as a reaction media, reusing of
an active catalyst with very low catalyst loading, easy isolation of
hydrophobic organic products and finally, no need for reducing
agent, radical initiator, toxic reagents or solvents. These adꢀ
vantages make the method amenable to scalability readily. As an
example, a semi scaleꢀup procedure (20.0 mmol) for epoxidation
Figure 4. The Raman spectra of fresh (A) and used {Mo132} after
10 times reuses (B) in aerobic epoxidation of cyclooctene in water
according to procedure mentioned in Table 1.
ASSOCIATED CONTENT
Supporting information
Experimental procedures, synthesis and characterization of the
{Mo132} and GC trace. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
Support for this work by the Research Council of the University
of Birjand is highly appreciated.
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Graphical Abstract
OH
2.9 nm
OH
O
O2
O2
O2
O2
O
O2
O2
O2
{Mo132
}
O2
4
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