Ruthenium nanoparticles supported on hydroxyapatite as an efficient and recyclable catalyst for cis-dihydroxylation and oxidative cleavage of alkenes

Article abstract of DOI:10.1002/anie.200453703

Impregnation of hydroxyapatite with colloidal ruthenium results in the formation of a catalyst that effects cis-dihydroxylation and oxidative cleavage of alkenes to their respective cis-1,2-diols and carbonyl products in good to excellent yields (see scheme). The supported ruthenium catalyst can be easily recycled and reused for consecutive reaction runs without significant deterioration of the catalytic activities. R¹, R² = H, alkyl, aryl.

Full text of DOI:10.1002/anie.200453703

Angewandte
Chemie
Supported Catalysts
Ruthenium Nanoparticles Supported on
Hydroxyapatite as an Efficient and Recyclable
Catalyst for cis-Dihydroxylation and Oxidative
Cleavage of Alkenes**
Chi-Ming Ho, Wing-Yiu Yu,* and Chi-Ming Che*
The development of osmium-catalyzed alkene dihydroxyla-
tion and its asymmetric variants represents an important
milestone in modern organic synthesis.[1] While this technol-
ogy has been widely applied to the synthesis of pharmaceut-
icals and fine chemicals,the high cost and possible product
contamination by toxic osmium metal have restricted its use
for large-scale reactions in industry. Currently,there is
considerable interest in the search for alternative metal
catalysts for alkene dihydroxylations.[2] Ruthenium and its
complexes,being isoelectronic to its osmium counterparts,are
well-known for their catalytic activities for oxidative trans-
formations.[3,4] And indeed,RuCl 3·xH2O has been demon-
strated to catalyze cis-dihydroxylation[5] and oxidative cleav-
age[6] of alkenes.
Application of nanosized metal particles as catalysts for
organic transformations is an area of growing interest.[7,9a]
Because of its high surface area and high density of active
sites,nanosized metal particles exhibit superior catalytic
activities compared with the corresponding bulk materials.
Herein,we report that ruthenium nanoparticles immobilized
on hydroxyapatite[8] are an effective catalyst for cis-dihydroxyl-
ation and oxidative cleavage of alkenes. With ethyl trans-
cinnamate as substrate,the supported catalyst has been
recycled for successive cis-dihydroxylation reactions without
significant deterioration of catalytic activities. Prior to this
work,there are no precedent examples in the literature of
recyclable ruthenium catalysts for cis-dihydroxylation and
oxidative cleavage of alkenes.[9,10]
One approach to obtain supported catalysts with well-
defined metal particles is to immobilize metal colloids onto
solid supports.[7] Based on the FiØvetꢀs protocol,[11a] stable
ruthenium colloids can be prepared by the reduction of
RuCl3·xH2O (0.32 mmol) in 1,2-propanediol (100 mL) in the
presence of sodium acetate (1 mmol) at 1508C. Reduction of
[*] Dr. C.-M. Ho, Dr. W.-Y. Yu, Prof. Dr. C.-M. Che
Department of Chemistry and
Open Laboratory of Chemical Biology
Institute of Molecular Technology for Drug Discovery and Synthesis
The University of Hong Kong
Pokfulam Road, Hong Kong (China)
Fax: (+852)2857-1586
E-mail: cmche@hku.hk
[**] This work is supported by the Areas of Excellence Scheme
established under the University Grants Committee of the Hong
Kong Special Administrative Region, China (AoE/P-10/01), The
University of Hong Kong (University Development Fund), and the
Hong Kong Rearch Grants Council (HKU7384/02P).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2004, 43, 3303 –3307
DOI: 10.1002/anie.200453703
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3303

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the Ru3+ salt was considered complete upon the disappear-
Table 1: Nano-RuHAP-catalyzed cis-dihydroxylation of ethyl trans-cinna-
mate.[a]
ance of the band at about 400 nm in the corresponding UV/
Vis spectrum (see Supporting Information). The ruthenium
colloid (3.2 mm) prepared in this manner exhibited excellent
stability against agglomeration since no precipitation of metal
powder was evident upon standing the colloidal solution at
room temperature for 7 days. Successful immobilization of
the ruthenium nanoparticles was achieved by treating the
colloidal solution with calcium hydroxyapatite (0.4 g) and
H2O (50 mL) for 24 h. [12] Ruthenium nanoparticles that were
grafted onto hydroxyapatite (nano-RuHAP) were obtained
as a dark-grey solid after centrifugation,washing with
deionized water,and drying under vacuum at 110 8C over-
night. By means of inductively coupled plasma mass spec-
trometry ICPMS,the Ru content of nano-RuHAP was
determined to be 5 wt% (0.5 mmolgÀ1) and the Ca:P molar
ratio was 1.66; the latter agrees with the reported value of
1.67,[10d] thus indicating no significant exchange of the
constituent Ca2+ ions. Attempts to immobilize the ruthenium
nanoparticles on some common inorganic supports such as
silica gel and alumina were not successful,since the resulting
materials contained < 0.1 wt% Ru based on ICP analysis.
The powder X-ray diffraction (XRD) pattern [Philips
PW1830 (2 kW),Cu Ka,40 kV,40 mA,25 8C] of nano-RuHAP
showed prominent peaks at 2q = 25.908,31.85 8,32.25 8,32.95 8
and 49.558,which are indexed to the [002],[211],[112],[300],
and [213] reflections of the hexagonal crystal system of
hydroxyapatite (JCPDS card no: 09-0432),respectively (see
Supporting Information). It should be noted that free
hydroxyapatite (i.e.,without Ru particles) exhibits an
almost identical XRD pattern,thus indicating that the
crystallinity of hydroxyapatite was retained after immobiliza-
tion of ruthenium nanoparticles. The TEM image of nano-
RuHAP shows randomly dispersed particles,and energy-
dispersive X-ray (EDX) and selected-area electron diffrac-
tion (SAED) analyses confirmed that the particles corre-
sponded to ruthenium with an average diameter of about
4 nm. X-ray photoelectron spectroscopy (XPS) analysis of the
freshly prepared nano-RuHAP catalyst revealed the binding
energy of Ru 3p3/2 at 462.7 eV (see Supporting Information).
This value is slightly larger than the reported value (462 eV)
for a bulk ruthenium metal.[13] There is a small increase in the
surface area of nano-RuHAP (25 m2 gÀ1) compared to its
parent hydroxyapatite (17 m2 gÀ1) as determined by the N2
adsorption isotherms. This is accounted for by the physical
adsorption of ruthenium nanoparticles on the solid support.
The nano-RuHAP catalyst was tested for cis-dihydrox-
ylation of ethyl trans-cinnamate.[5a] In the presence of nano-
RuHAP (1 mol% Ru),ethyl trans-cinnamate (2 mmol) was
treated with NaIO4 (3.0 mmol) and H2SO4 (20 mol%) in
EtOAc/MeCN/H2O (3:3:1 v/v) at 08C for 30 min with
complete substrate consumption. After workup and chroma-
tographic purification,( Æ )-threo-ethyl 2,3-dihydroxy-3-phe-
nylpropanoate was isolated in 70% yield,and benzaldehyde
(18%) and benzoic acid (6%) were obtained as minor
products (Table 1,entry 1). By centrifugation,the supported
ruthenium catalyst was recovered and was used for the
second,third,and fourth runs under identical reaction
conditions. No apparent loss of catalytic activity was
Entry
Catalyst
Reaction run
Yield [%][b]
1
2
3
4
nano-RuHAP
first
second
third
70
80
82
66
67
80
50
fourth
5[c]
6
nano-RuHAP
RuHAP[d]
first
second
7[e]
[a] Reaction conditions: ethyl trans-cinnamate (2 mmol), nano-RuHAP
(40 mg, 1 mol%), NaIO4 (1.5 equiv) and H2SO4 (20 mol%) in EtOAc-
MeCN-H2O 3:3:1 at 08C, time=30 min. [b] Yield of isolated product
based on complete alkene consumption. [c] Ethyl trans-cinnamate
(10 mmol). [d] Kaneda’s catalyst, see reference [12]. [e] Conversion=
80%.
observed,and the product, cis-diol,was isolated in 80,82,
and 66% yields (entries 2–4,respectively). A total product
turnover of 298 was attained over four consecutive runs. No
alkene conversion to cis-diol or any oxidized products was
detected in the absence of the nano-RuHAP catalyst. It is
noteworthy that when the ethyl trans-cinnamate (2 mmol)
oxidation was performed by using commercially available
ruthenium metal powder (1 mol% Ru) as catalyst [reaction
conditions: NaIO4 (1.5 equiv) and H2SO4 (20 mol%) in an
EtOAc/MeCN/H2O mixture (3:3:1)],neither cis-1,2-diol nor
benzaldehyde was obtained,and the starting alkene was
recovered quantitatively.
The nano-RuHAP-catalyzed protocol is equally effective
on a larger scale. For example,the reaction of ethyl trans-
cinnamate (10 mmol) in the presence of nano-RuHAP (0.2
mol%) gave the corresponding cis-1,2-diol in 67% isolated
yield (product turnover= 330; Table 1,entry 5).
During our catalyst recycling study,we observed about
20–30% weight loss of the nano-RuHAP catalyst after each
reaction run. The observed weight loss was attributed to slow
dissolution of the HAP support in the acidic reaction medium.
After centrifugation,the supernatant solution was subject to
ICPMS,and the Ru content in solution was measured to be
< 5 ppm. Both recovered and freshly prepared catalysts
exhibited similar TEM images (i.e.,metal particles with an
average diameter ca. 4 nm). By XPS analysis,the binding
energy of Ru 3p3/2 of the recovered catalyst was found to be
463.3 eV,which is different from the 462.7 eV value found for
the freshly prepared catalyst sample (see Supporting Infor-
mation). Similar SAED patterns of the ruthenium particles
were obtained for the recovered and the freshly prepared
catalysts,thus revealing that the crystal packing structure of
the metal particles did not exhibit any significant changes
after the oxidation reaction (see Supporting Information).
Previously Kaneda and co-workers reported that an Ru3+-
exchanged hydroxyapatite,prepared by treating hydroxyapa-
tite with methanolic RuCl3,is an efficient heterogeneous
catalyst for aerobic alcohol oxidations.[10d] In the work
presented herein,we found that Kanedaꢀs catalyst is also
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effective for dihydroxylation of ethyl trans-cinnamate under
the reaction conditions: alkene (2 mmol),Ru (1 mol%),
NaIO4 (3.0 mmol),H 2SO4 (20 mol%) in EtOAc-MeCN-H2O
(3:3:1) at 08C for 30 min,and ( Æ )-threo-ethyl 2,3-dihydroxy-
3-phenylpropanoate was isolated in 80% yield (Table 1,
entry 6). However,when the recovered Kanedaꢀs catalyst
(by centrifugation) was subjected to a second reaction run,a
mixture of cis-1,2-diol (50%) and benzaldehyde (28%) was
obtained (Table 1,entry 7).
With trans-stilbene as the model substrate,treatment of the
alkene (0.5 mmol) with NaIO4 (1 mmol) and nano-RuHAP
(4 mol% Ru) in a 1,2-dichloroethane-H2O (5:1 v/v) at room
temperature for 2 h gave benzaldehyde in 81% yield based on
capillary GC analysis (Table 3,entry 1). The catalyst was
recycled for repetitive trans-stilbene oxidations,and benzal-
dehyde was formed in comparable yields: 75% (2nd run),
71% (3rd run) and 72% (4th run). No significant weight loss
of the catalyst was observed under the reaction conditions for
=
Dihydroxylations of other alkenes were examined by
using the nano-RuHAP-catalyzed protocol,and the results
are depicted in Table 2; a family of a,b-unsaturated alkenes,
including ethyl crotonate,dimethyl fumarate,dimethyl mal-
eate,and isophorone can be converted into their correspond-
ing cis-12,-diols in 65,53,50,and 64% isolated yields,
respectively (Table 2,entries 2–5). The lower yields obtained
for the dihydroxylations of dimethyl fumarate and dimethyl
maleate are probably due to the high aqueous solubility of the
cis-diol products. Under the Ru-catalyzed conditions,oxida-
tion of 1-dodecene and styrene gave the respective dodecene
glycol (75%) and styrene glycol (85%) in good yields
(entries 6 and 7). Likewise,dihydroxylation of 12,-disubsti-
tuted alkenes such as 1,4-diacetoxy-cis-but-2-ene and phen-
anthrene afforded the corresponding cis-1,2-diols in 61 and
60% yields (entries 8 and 9). With 1-acetoxy-2-cyclohexene
as substrate,the Ru-catalyzed dihydroxylation furnished 1-
the oxidative C C cleavage reactions.
Oxidative cleavages of other stilbene derivatives such as
cis-stilbene and 1-methyl-1,2-diphenylethylene under the
nano-RuHAP catalyzed conditions also proceeded efficiently,
and the carbonyl products were produced in excellent yields
(> 85%,Table 3 entries 2 and 3). Styrene, a-methylstyrene,
and trans-b-methylstyrene were found to undergo oxidative
cleavage by using the “nano-RuHAP + NaIO4” protocol,
and benzaldehyde was obtained in 66–92% yield (entries 4–
6). When dihydronaphthalene was employed as substrate,the
corresponding dicarbonyl product was isolated in 86% yield
with 76% substrate conversion (entry 7). Likewise,cyclic
aliphatic alkenes such as norbornene,cyclohexene,1- tert-
butyldimethylsiloxy-2-cyclohexene,and cyclooctene were
=
found to undergo effective C C bond cleavage to afford the
corresponding dicarbonyl products in 51–87% yields
(Table 3,entries 8–11). Catalytic oxidative C C bond cleav-
=
acetoxy-2,3-cis-dihydroxycyclohexane
in
75%
yield
age was also achieved for linear aliphatic alkenes such as
methyl oleate and 1-dodecene,giving the corresponding
carbonyl products in about 80% yields,albeit with 10–20%
substrate conversion (entries 12 and 13).
However, a,b-unsaturated alkenes such as chalcone are
poor substrates for the “nano-RuHAP + NaIO4” protocol,
and only 23% substrate conversion was attained with
benzaldehyde (16%) and benzoic acid (14%) being formed
after one day of reaction. In this work,we found that the use
(entry 10) with the anti-isomer being the major product
(anti:syn = 9:1). Yet,the catalytic dihydroxylation of trisub-
stituted alkenes such as (3-benzloxy)-1-methylbut-1-ene
appeared to be less effective,and the corresponding cis-1,2-
diol was obtained in only 45% yield (entry 11).
Further to alkene dihydroxylation,we examined the
catalytic activity of nano-RuHAP for oxidative cleavage of
alkenes,which is also a synthetically useful transformation. [14]
Table 2: Nano-RuHAP-catalyzed cis-dihydroxylation of alkenes.[a]
Entry Alkene
Product
Yield [%][b] Entry Alkene
Product
Yield [%][b]
75
1
70
65
53
6
7
8
2
3
85
61
4[c]
50
64
9[d]
60
5
10
11
75 (anti:syn=9:1)
45
[a] Reaction conditions: alkene (2 mmol), nano-RuHAP (40 mg, 1 mol%), NaIO4 (1.5 equiv) and H2SO4 (20 mol%) in EtOAc/MeCN/H2O (3:3:1 v/v)
at 08C, reaction time=30 min. [b] Isolated yield based on complete alkene consumption. [c] Reaction time=10 min. [d] Reaction time=40 min.
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Table 3: Nano-RuHAP catalyzed oxidative cleavage of alkenes.[a]
observed when using nano-RuHAP as
catalyst and oxone as oxidant (entries 2
and 3). Notably,oxidative cleavage of
Entry
Alkene
t [h]
Product
Yield [%][b]
1
2
2
2
PhCHO
PhCHO
81[c]
85[c]
ꢀ
C C bonds was also achieved using the
“nano-RuHAP + Oxone” protocol. For
example,when alkynes such as 1-phenyl-
1-propyne were subjected to the Ru-
catalyzed conditions,benzoic acid was
produced in 82% yield (entry 4). Results
for the catalytic oxidation of other
alkenes are given in the Supporting
Information.
3
3
PhCHO
85
PhCOMe
PhCHO
89
66
4
5
6
2
3
3
PhCOMe
PhCHO
92
90
In summary,a heterogeneous catalyst
was prepared by grafting ruthenium
nanoparticles onto hydroxyapatite. The
supported Ru nanoparticle catalyst exhib-
ited excellent activities for dihydroxyla-
tion and oxidative cleavage of alkenes,
and the catalyst can be recycled for
repeated uses. Considering that prepara-
tion of the supported catalyst involves
easy chemical manipulation and the lower
toxicity of ruthenium compared to OsO4,
the application of the ruthenium nano-
particles-based catalyst to other catalytic
transformations are being pursued.
7
3
86[d,e]
8
9
3
3
72[d]
51[d]
10
3
79[d,f]
87[d]
11
12
3
12
84[g]
79
82[h]
13
12
Received: January 9,2004
Revised: March 31,2004 [Z53703]
[a] Reaction conditions: alkene (0.5 mmol), nano-RuHAP (40 mg, 4 mol%), NaIO4 (1 mmol), in
ClCH2CH2Cl/H2O (5:1 v/v) at room temperature, and the products were identified and quantified by GC
analysis. [b] Yield based on complete alkene consumption. [c] Yield based on 2 equiv of benzaldehyde
formed. [d] Yield of isolated product. [e] 76% conversion. [f] 48% conversion. [g] 16% conversion.
[h] 12% conversion.
Keywords: alkenes · heterogeneous
.
catalysis · nanostructures · ruthenium ·
solid-phase synthesis
of a “nano-RuHAP + oxone” protocol[6a,15,16] would effect
oxidative cleavage of a,b-unsaturated alkenes (Table 4).
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nd
When chalcone (0.5 mmol) was treated with oxone
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buffered MeCN/H2O (1.5:1 v/v) at room temperature for
4 h,benzaldehyde and benzoic acid were produced in 67 and
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t [h]
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1
4
PhCHO
PhCO2H
67
38,980.
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72
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3
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NaHCO3 (7.7 equiv) in MeCN-H2O 1.5:1 at RT. [b] Yield of isolated product based on complete alkene
consumption. [c] Oxone (4 equiv) and NaHCO3 (12 equiv).
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