Dihydroxylation of olefins catalyzed by zeolite-confined osmium(0) nanoclusters: An efficient and reusable method for the preparation of 1,2-cis-diols

Article abstract of DOI:10.1039/c2gc16616j

Addressed herein is a novel, eco-friendly, recoverable, reusable and bottleable catalytic system developed for the dihydroxylation of various olefins yielding 1,2-cis-diols. In our protocol, zeolite-confined osmium(0) nanoclusters (zeolite-Os⁰) are used as reusable catalyst and H ₂O₂ served as a co-oxidant. Zeolite-Os⁰ are found to be highly efficient and selective catalysts for the dihydroxylation of a wide range olefins in an aqueous acetone mixture at room temperature. In all of the olefins surveyed, the catalytic dihydroxylation reaction proceeds smoothly and the corresponding 1,2-cis-diols are obtained in excellent chemical yield under the optimized conditions. The present heterogeneous catalyst system provides many advantages, such as being eco-friendly and industrially applicable over the traditional homogenous OsO₄-NMO system for the dihydroxylation of olefins.

Full text of DOI:10.1039/c2gc16616j

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PAPER
Dihydroxylation of olefins catalyzed by zeolite-confined osmium(0)
nanoclusters: an efficient and reusable method for the preparation of
1,2-cis-diols†
Önder Metin,*a Nurdan Alcan Alp,a Serdar Akbayrak,b Abdullah Biçer,a Mehmet Serdar Gültekin,*a
Saim Özkarb and Uğur Bozkayaa
Received 16th December 2011, Accepted 13th March 2012
DOI: 10.1039/c2gc16616j
Addressed herein is a novel, eco-friendly, recoverable, reusable and bottleable catalytic system developed
for the dihydroxylation of various olefins yielding 1,2-cis-diols. In our protocol, zeolite-confined osmium
(0) nanoclusters (zeolite-Os0) are used as reusable catalyst and H2O2 served as a co-oxidant. Zeolite-Os0
are found to be highly efficient and selective catalysts for the dihydroxylation of a wide range olefins in
an aqueous acetone mixture at room temperature. In all of the olefins surveyed, the catalytic
dihydroxylation reaction proceeds smoothly and the corresponding 1,2-cis-diols are obtained in excellent
chemical yield under the optimized conditions. The present heterogeneous catalyst system provides many
advantages, such as being eco-friendly and industrially applicable over the traditional homogenous
OsO4–NMO system for the dihydroxylation of olefins.
heterogenization of osmium catalysts. In all immobilization pro-
tocols used for the heterogenization of osmium catalyst so far,
Introduction
The oxidation of olefins in the presence of transition metal cata-
lysts is widely used in cyclitols chemistry for the production of
important pharmaceuticals, drugs and fine chemicals.1 Several
methods have been developed for the synthesis of compounds
bearing cis-diol group from the olefin oxidation.2 Among them,
the osmium catalyzed dihydroxylation of olefins is one of the
most powerful synthetic protocols for the preparation of vicinal
diols.3 Despite of the widespread application of this type of reac-
tion in organic synthesis, the high cost, hazardous toxicity, vola-
tility and contamination of products with osmium are the main
drawbacks for their use in industry on the subject of green chem-
istry.4 One promising way of handling these problems is to use a
heterogeneous osmium catalyst obtained by immobilization of
the catalytic species on suitable inorganic or organic supports so
that the recovery of the catalyst can be achieved by simple fil-
tration of reaction mixture. As supports chiral ligands,5 poly-
mers,6 dendrimers,7 iox-exchange resins,8 silica gels,9
hydroxyapatites10 and fullerenes11 have been used for the immo-
bilization of osmium catalysts. The microencapsulation12 and
miscellaneous13 approaches have also been reported for the
the osmium tetraoxide (OsO4) was immobilized via molecular
interaction with the support materials. However, in all cases, the
immobilized OsO4 catalyst leaches into the reaction solution as
the weak molecular interaction between the support and catalyst
can easily be broken, which creates problems in recovery and
reusability of the catalyst.14 It is obvious that the development of
a new type of heterogeneous osmium catalyst system is still
needed for the dihydroxylation of olefins due to continuing pro-
blems such as the lower activity and complete recovery of the
immobilized catalysts from the reaction medium.
Transition metal nanoclusters are highly active catalysts as
they have large surface areas, providing a high density of active
sites. They have attracted much attention in organic synthesis
due to their distinct catalytic activities for various transform-
ations.15 However, one of the most important problems in their
catalytic applications is the aggregation of nanoclusters into
clumps and ultimately to the bulk metal, despite using the best
stabilizers,16 which leads to a vital decrease in their catalytic
activity and lifetime. One of the most promising ways to prevent
the aggregation is the generation of metal nanoclusters in the
void spaces of mesoporous and microporous solids.17 Zeolite-Y,
a FAU-type zeolite, is considered to be a suitable host providing
highly ordered large cavities with an inner diameter of 1.3 nm,18
and can be used for the assembly of metal and metal oxide
nanoclusters as the pore size could limit the growth of par-
ticles.19 Recently, we reported that zeolite-confined osmium(0)
nanoclusters, hereafter referred to as zeolite-Os0, are highly
active and selective catalysts in the aerobic oxidation of alcohols
under mild conditions.20 In addition to their activity and
aDepartment of Chemistry, Faculty of Science, Atatürk University, 25240
Erzurum, Turkey. E-mail: ometin@atauni.edu.tr, gultekin@atauni.edu.tr
bDepartment of Chemistry, Middle East technical University, 06800
Ankara, Turkey
†Electronic supplementary information (ESI) available: Copy of
1H-NMR and 13C-NMR spectra and spectral data for all cis-diols. The
details of theoretical molecular volume computations using density func-
tional theory. Additional characterization data for the catalyst. See DOI:
10.1039/c2gc16616j
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selectivity, zeolite-Os0 are isolable, reusable and bottleable,
which are very important criteria for their application in many
heterogeneous catalytic system on the subject of green chemistry.
Herein we report the use of zeolite-Os0 as catalysts in the dihy-
droxylation of various olefins yielding 1,2-cis-diols in aqueous
acetone at room temperature. The zeolite-Os0 are highly efficient
and selective catalyst providing excellent yield to 1,2-cis-diols as
well as eco-friendly, recoverable, reusable and bottleable cataly-
tic system for the dihydroxylation of olefins. The catalytic dihy-
droxylation of olefins was performed in the presence of
hydrogen peroxide as a co-oxidant in aqueous acetone at room
temperature. After reaction, the solid catalyst materials were sep-
arated from the reaction solution by centrifugation easily, which
is one of the major problems of the traditional OsO4/NMO cata-
lyst system used for the dihydroxylation of olefins. The conver-
sion of the starting olefins to the corresponding 1,2-cis-diols was
monitored by 1H-NMR and 13C-NMR spectra of supernatant sol-
ution. The solid zeolite-Os0 catalyst recovered by centrifugation
can be bottled and stored under ambient conditions. The reusa-
bility experiments performed on different types of olefin dihy-
droxylation revealed that zeolite-Os0 catalyst can be reused
efficiently and preserve its initial activity with minimal loss up
to 5 catalytic cycles.
150 mM NaBH4 solution at room temperature. The solid
powders were isolated again by suction filtration when the
hydrogen generation from the reaction solution ended (∼3 h),
washed three times with 20.0 mL of deionized water to remove
metaborate and chloride anions and were dried under vacuum at
ambient temperature. The samples of zeolite-Os0 were bottled as
black powders.
General procedure for the cis-dihydroxylation of olefins
catalyzed by zeolite-confined osmium(0) nanoclusters
In a typical procedure, 2.0 mmol of a selected olefin was trans-
ferred into a single-necked flask and then 10.0 mL of aqueous
acetone (acetone/H2O = 9 : 1 v/v) was added into the alkene sol-
ution at room temperature. The resulting mixture was vigorously
stirred until a homogenous mixture obtained, while 0.18 mL of
H2O2 (1 equiv to alkene) was gradually added by syringe. Next,
100.0 mg of dried zeolite-Os0 catalyst was added into the reac-
tion mixture and was stirred vigorously to obtain a well dis-
persed mixture, which was stirred at room temperature for
24 hours. Next, another 0.18 mL of H2O2 was gradually added
by syringe into the reaction mixture after 24 hours. The resulting
mixture was stirred for a further 24 hours at room temperature.
At the end of the 48 hours, the reaction mixture was centrifuged
at 6000 rpm for 10 min. The supernatant of the centrifuged
sample was transferred into a glass vial containing NaHSO3 and
was agitated for 5 min to remove excess H2O2. Hereafter, the
resulting heterogeneous mixture was isolated via suction fil-
tration, the aqueous acetone was removed on a rotary evaporator.
The desired cis-diols were obtained in good yields. The precipi-
tate (zeolite-Os0 catalyst) was washed with 20.0 mL of acetone 2
times to remove residues and was bottled for re-use. A copy of
Experimental
General remarks
Osmium(III) chloride trihydrate (OsCl3·3H2O), sodium borohy-
dride (NaBH4, 98%), zeolite-Y (Na56[(AlO2)56(SiO2)140]·
250H2O) and all organic compounds were purchased from
Sigma-Aldrich® and were used without further purification.
Deionized water was distilled by water purification system
(Milli-Q system). The osmium content of zeolite-Os0 samples
was determined by inductively coupled plasma optical emission
spectroscopy, ICP-OES (Leeman Labs.). All glassware and
Teflon coated magnetic stir bars were cleaned with acetone, fol-
lowed by copious rinsing with distilled water before drying in an
oven at 150 °C. Transmission electron microscope images were
obtained by a JEOL 2100 TEM (200 kV). 1H-NMR and
13C-NMR spectra were recorded on a Varian 200 MHz or Bruker
Avance DPX 400 MHz spectrometer.
1
the H-NMR and 13C-NMR spectra and spectral data are given
in ESI.†
General procedure for acetylation of 1,2-cis-diols
All the cis-diols were also characterized after their transformation
into corresponding acetates. The cis-diols were dissolved in
20.0 mL of methylene chloride and then 2.0 mL of pyridine and
2.5 equiv of acetic anhydrate were added into the solution. The
resulting mixture was stirred over night (approximately 12 h) at
room temperature. Next, the solution was poured into icy HCl
(20.0 mL of 2.0 N) and stirred for 5 min. The solution was
extracted with ethyl acetate (3 × 30 mL) and the ethyl acetate
phases were collected in a glass bottle and were then washed
with saturated NaHCO3 solution (2 × 30 mL). Next, the resulting
solution was washed with water to remove the inorganic impuri-
ties. Finally, the organic layer was dried over anhydrous Na2SO4
and the solvent evaporated in a vacuum, the crude products were
purified by silica-gel column chromatography. The acetylated
products were obtained in high chemical yields (68–83%).
General procedure for the preparation of zeolite-confined
osmium(0) nanoclusters
The zeolite-Os0 were prepared by our established procedure
reported elsewhere.20 In a typical procedure, osmium(III) cations
were introduced into the zeolite-Y by ion exchange of 1.0 g
zeolite-Y in 100.0 mL aqueous solution of 77.0 mg OsCl3·3H2O
for 72 h at room temperature. The opaque-brown supernatant sol-
ution became colorless after 72 h, indicating complete ion
exchange. Next, the sample was filtered by suction filtration,
washed three times with 20 mL of deionized water and dried
under vacuum at room temperature. Osmium content of the
Os3+-exchanged zeolite-Y sample was found as 2.1 wt% by
ICP-OES analysis corresponding to 64% of osmium used for the
ion exchange (Os2.5Na48.5[(AlO2)56(SiO2)140]·250H2O). Then,
0.4 g Os3+-exchanged zeolite-Y was added into 100.0 mL of
Computational methods
For each molecule considered in this research, theoretical mol-
ecular volume computations were carried out using a Gaussian
03 program package.21 Density functional theory (DFT) was
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Table 1 cis-Dihydroxylation of various olefins catalyzed by zeolite-
employed for this purpose. Geometry optimizations, harmonic
vibrational frequencies, molecular volumes are computed with
the B3LYP functional.22,23 In all computations Pople’s polarized
double-ζ split valence basis set, 6-31G(d), is employed.24–26
Os0 in the presence of H2O2 as a co-oxidanta
Results and discussion
Time
(h)
Yieldb
(%)
TON/
TOFc
Traditional cis-dihydroxylations of olefins are generally per-
formed in the presence of osmium tetraoxide (OsO4) as a homo-
geneous catalyst and N-methyl-morpholine oxide (NMO) as a
co-oxidant in various hazardous organic solvents, such as
toluene and tetrahydrofurane.3b Besides the traditional OsO4–
NMO method, an alternative route including ruthenium(III)
chloride (RuCl3) as a homogeneous catalyst, sodium periodide
(NaIO4) as a co-oxidant and H2SO4 as a solvent was used in the
dihydroxylation of olefins.27 Many hazardous chemicals and
irreversible homogenous catalyst systems are used in all proto-
cols mentioned above. Moreover, the use of OsO4 and RuCl3 as
homogeneous catalysts in the olefin dihydroxylation reactions is
not practical due to their high costs. In this regard, we report
herein a novel, practical, eco-friendly and industrially applicable
method for the synthesis of various 1,2-cis-diols via dihydroxy-
lation of olefins in the presence of zeolite-Os0 and hydrogen per-
oxide. In our protocol, zeolite-Os0 is used as a reusable
heterogeneous catalyst and H2O2 serves as an environmentally
benign co-oxidant. All catalytic olefin dihydroxylation reactions
are performed in aqueous acetone at room temperature.
Entry Olefin (a)
Product (b)
1
70
96
96
48
98
98
30
70
196/2.8
196/2.1
60/0.6
2
3
4
140/2.9
5
52
87
174/3.4
6
7
8
65
70
55
86
90
98
172/2.6
180/2.6
196/3.6
The zeolite-Os0 catalyst was prepared by the ion-exchange of
Os3+ ions with the extra framework Na+ ions of zeolite-Y fol-
lowed by reduction of the Os3+ ions in solution according to our
reported procedure and well-characterized by using various
advanced techniques.20 The osmium content of zeolite-Os0 was
determined as 2.1 wt% Os by inductively coupled plasma
optical emission spectroscopy (ICP-OES) and all osmium load-
ings in the catalytic olefin dihydroxylation reactions were calcu-
lated according to the ICP-OES results. Zeolite-Os0 was found
to be a highly efficient and selective catalyst for the dihydroxyla-
tion of a wide range of olefins. In all of the olefins surveyed, the
catalytic dihydroxylation reaction proceeds smoothly and the
corresponding 1,2-cis-diols are obtained in excellent chemical
yield under the optimized conditions (Table 1, entries 1–12). For
instance, the dihydroxylation of styrene (1a) gives styrene-cis-
diol (1b) in 98% chemical yield under the optimized conditions
(Table 1, entry 1). Our catalysis protocol can also be applied to
other families of olefins including α,β-unsaturated cyclic carbo-
nyl systems with or without different –R groups, which have a
large tendency to undergo 1,4-conjugate additions. For example,
the dihydroxylation of cyclopentenone (8a) and cyclohexzenone
(9a) were converted to corresponding cis-diols (8b) and (9b),
respectively, with an excellent chemical yield of 98% (Table 1,
entries 8 and 9). The dihydroxylation of cyclopentenone (8a)
was also performed by using traditional dihydroxylation method
including OsO4–NMO system but no desired 1,2-cis-diol
product was obtained even by repeating catalytic experiments
several times. Dihydroxylation of other olefins were examined
by using the zeolite-Os0 catalysis protocol, and the results
depicted in Table 1. It is worth mentioning that our catalyst
showed better performance in terms of chemical yields of the
9
35
65
98
98
196/5.6
196/3.0
10
11
12
96
96
25
25
50/0.5
50/0.5
Turn over frequency (TOF, h−1) = TON/time.a All reactions were
performed in aqueous acetone (20.0 mL, acetone/water = 9 : 1 v/v) using
zeolite-Os0 catalyst (100.0 mg, 2.1 wt% Os), olefin (2 mmol), H2O2 (2
equiv) at room temperature. All the cis-diols were also characterized
after their transformation into corresponding acetates. b Isolated yield.
c Turn over number (TON) = moles of substrate converted per mole
of Os.
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product and TON values as compared to the mesoporous28a or
mesoporous–transition metal hybrid materials28b tested as cata-
lysts in the dihydroxylation of olefins under similar reaction
conditions.
the cages of zeolite-Y. A similar theoretical explanation was also
valid for the low yield dihydroxylation of cyclohex-2-en-1-yl-
benzoate (12a) having calculated the Lennard–Jones kinetic
diameter of 7.9 Å. In light of the theoretical study results, it can
be concluded that the present catalytic dihydroxylation method
is not applicable for the molecules having a Lennard–Jones
kinetic diameter larger than the aperture size of the zeolite-Y
cages (7.4 Å).
To examine the effect of temperature on our protocol, we have
performed the catalytic dihydroxylation of (1a), (7a) and (9a) at
different temperatures (35 and 50 °C) under the optimized con-
ditions. The 1H-NMR spectra of the products obtained by cataly-
tic dihydroxylation of olefins at different temperature (see ESI†)
showed that the higher oxidation of olefins took place and corre-
sponding aldehydes were formed as by-products at higher reac-
tion temperatures. Additionally, the ratio of corresponding
aldehydes to desired product of 1,2-cis-diols increases by
increasing reaction temperature. For example, the dihydroxyla-
tion of cyclooctene (Table 1, entry 7) gives the corresponding
aldehydes and 1,2-cis-diol with an integration ratio of 3/7 (alde-
hyde/1,2-cis-diol) at 35 °C and the ratio increases to 1/1 at
50 °C. We can conclude by these results that the selectivity of
our catalyst decreases by increasing temperature while it works
well at room temperature, which is more important for green
chemistry applications.
The zeolite-Os0 catalyst was recovered from the reaction sol-
ution by simple centrifugation and re-used in the dihydroxylation
of various olefins (Table 2, entries 1–5). The recovered zeolite-
Os0 catalysts preserve their initial activity even after 5th catalytic
runs without any important loss in the chemical yield (the
decrease in the chemical yields is in the range of 3–7%). of dihy-
droxylation of selected alkenes to corresponding 1,2-cis-diols
It is noteworthy to discuss the intriguing results obtained by
catalytic dihydroxylation of trans-stilbene (2a) and cis-stilbene
(3a). The dihydroxylation of (2a) gives the major product (2b),
1,2-trans-diphenyl ethane-1,2-cis-diol, in a very high chemical
yield of 98% (Table 1, entry 2), while that of (3a) results in the
formation of the corresponding 1,2-cis-diol (3b) in a lower
chemical yield of 30% under the optimized reaction conditions
(Table 1, entry 3). Actually, this result provides compelling evi-
dence for the formation of osmium(0) nanoclusters mostly
within the cages of zeolite-Y, which has an aperture diameter of
7.4 Å.18 Additionally, the HRTEM image of zeolite-Os0 (see
ESI, Fig. S1†) also shows the osmium(0) nanoclusters within the
cages of zeolite-Y.
Fig. 1 TEM image of osmium(0) nanoclusters formed on the external
surface of zeolite-Y.
Table 2 Reusability of zeolite-Os0 catalyst in the dihydroxylation of
various olefinsa
Theoretical molecular volume computations using density
functional theory (DFT) was also carried on molecules (2a),
(3a), (10a), (11a) and (12a) for calculation of their Lennard–
Jones kinetic diameters to discuss their availability to enter into
zeolite cages, where the catalytic reaction takes place. Details of
the theoretical study performed on the molecules to calculate
their Lennard–Jones kinetic diameters can be seen in ESI.† The
Lennard–Jones kinetic diameter of trans-stilbene (2a) is calcu-
lated as 7.1 Å, which permits its entrance into cages of zeolite-Y,
while that of cis-stilbene (3a) is large (7.8 Å), which does not
permit its entrance into the cages of zeolite-Y. Additionally, the
low yield dihydroxylation of cis-stilbene (3a) indicates the exist-
ence of some osmium(0) nanoclusters on the external surface of
zeolite-Y, which are available for the large substrate molecules as
well. The transmission electron microscopy (TEM) image
(Fig. 1) confirms the existence of osmium(0) nanoclusters on the
external surface of zeolite as well as some in the cavities.
Similar results were also obtained for the catalytic dihydroxyla-
tion of cyclohexene derivatives (10a) and (11a). Cyclohex-2-en-
1-yl acetate (10a) has one acetate group perpendicular to plane,
while cyclohex-2-en-1,4-diacetate (11a) has two acetate groups
in 1,4 position. The theoretical calculations showed that com-
pound (11a) has a Lennard–Jones kinetic diameter of 7.6 Å,
which is large for its entrance into the cages of zeolite-Y, while
that of compound (10a) is 6.9 Å, which permits its entrance into
Yield (%)
1st run
Yield (%)
5th run
Entry Olefin (a)
Product (b)
1
98
98
90
95
91
85
2
3
4
5
98
98
92
91
a All reactions were performed in aqueous acetone (20.0 mL, acetone–
water = 9 : 1 v/v) using 100.0 mg zeolite-Os0 (2.1 wt% Os), 2 mmol
olefin (5 times), 2 equiv H2O2 (5 times) at room temperature.
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diols over the reusability test most probably stems from the loss
of some of the zeolite-Os0 catalyst during the centrifugation and
drying process. The ICP-OES analyses performed on the super-
natant of the catalytic reaction solutions revealed that a negli-
gible amount of osmium was detected (<1 ppm), which is clear
evidence of no leaching of the osmium into the reaction
solution.
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In conclusion, we have developed a facile recoverable, reusable
and bottleable catalyst system for the dihydroxylation of olefins
to 1,2-cis-diols. Zeolite-confined osmium(0) nanoclusters exhib-
ited excellent activity in the olefin dihydroxylation. The present
heterogeneous catalyst system is novel and provides many
advantages such as being eco-friendly and industrially applicable
over the homogenous one for the dihydroxylation of olefins. We
hope that the method described here might open a new perspec-
tive for the application of heterogeneous catalysts in the dihy-
droxylation of olefins for the synthesis of many important
compounds bearing diol groups in the context of green
chemistry.
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Acknowledgements
Partially support by TUBITAK (project no. 109T815) and the
Ataturk University Scientific Research Project Council (project
no. 2010/32).The Turkish Academy of Sciences is gratefully
acknowledged.
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