Highly efficient deoxygenation of sulfoxides using hydroxyapatite-supported ruthenium nanoparticles

Article abstract of DOI:10.1246/cl.131077

We report the first example of the deoxygenation of sulfoxides using heterogeneous catalysts with alcohols as environmentally friendly reducing reagents. Hydroxyapatitesupported Ru nanoparticles (RuNPs/HAP) act as a highly efficient and reusable heterogeneous catalyst for deoxygenation of sulfoxides using alcohols as reductants. The catalytic activity of Ru nanoparticles is outstanding compared to other metal nanoparticles such as Pt, Pd, Rh, and Au nanoparticles. RuNPs/HAP can also catalyze the selective deoxygenation of various sulfoxides, giving the corresponding sulfides in excellent yields.

Full text of DOI:10.1246/cl.131077

CL-131077
Received: November 15, 2013 | Accepted: December 2, 2013 | Web Released: December 6, 2013
Highly Efficient Deoxygenation of Sulfoxides
Using Hydroxyapatite-supported Ruthenium Nanoparticles
Yusuke Takahashi,1 Takato Mitsudome,1 Tomoo Mizugaki,1 Koichiro Jitsukawa,1 and Kiyotomi Kaneda*1,2
1Department of Materials Engineering Science, Graduate School of Engineering Science,
Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531
2Research Center for Solar Energy Chemistry, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531
(E-mail: kaneda@cheng.es.osaka-u.ac.jp)
We report the first example of the deoxygenation of
sulfoxides using heterogeneous catalysts with alcohols as
environmentally friendly reducing reagents. Hydroxyapatite-
supported Ru nanoparticles (RuNPs/HAP) act as a highly
efficient and reusable heterogeneous catalyst for deoxygenation
of sulfoxides using alcohols as reductants. The catalytic activity
of Ru nanoparticles is outstanding compared to other metal
nanoparticles such as Pt, Pd, Rh, and Au nanoparticles. RuNPs/
HAP can also catalyze the selective deoxygenation of various
sulfoxides, giving the corresponding sulfides in excellent yields.
friendly catalyst systems for the deoxygenation of sulfoxides.
Herein, we report an alternative green methodology to the
catalytic deoxygenation of sulfoxides using alcohols as reducing
reagents. HAP-supported Ru nanoparticles (RuNPs/HAP) act as
a highly efficient and reusable heterogeneous catalyst for the
deoxygenation of sulfoxides. RuNPs/HAP can also catalyze
the selective deoxygenation of various sulfoxides, giving the
corresponding sulfides in excellent yields. The solid RuNPs/
HAP catalyst is easily separable from the products and can be
reused. To the best of our knowledge, this is the first report of the
catalytic deoxygenation of sulfoxides using heterogeneous
catalysts with alcohols as reagents.
Deoxygenation of sulfoxides to sulfides is a fundamental
and significant reaction in both organic synthesis1 and bio-
chemistry.2 For example, in the asymmetric synthesis of
carbinols, chiral sulfoxides are introduced to carbonyl com-
pounds as chiral auxiliaries. After the asymmetric transforma-
tions of the carbonyl groups to carbinols, the sulfoxides are
removed from the parent molecules by deoxygenation followed
by desulfidation.3 Conventionally, stoichiometric deoxygenation
of sulfoxides has been carried out by excess amounts of reducing
reagents such as metal hydrides,4 halogens,5 hydrogen halides,6
thiols,7 and phosphines.8 However, these reactions have fatal
drawbacks such as the use of highly toxic reagents, the
production of large amounts of waste, and low yields of sulfides
caused by side reactions such as reductive chlorination.5,6 To
date, many efforts have been devoted to the replacement of
these stoichiometric reactions with catalytic ones; consequently,
many catalytic systems have been proposed.9-12 These
catalytic systems, however, often suffer from low catalytic
activity,9d,9e,10a,10e,11,12 harsh reaction conditions,9e,9h,12a,12b and
difficulties in work-up procedures, such as separation of the
catalysts from the reaction mixtures and reuse of the cata-
lysts.9a-9f,10,11 Therefore, the development of highly efficient
heterogeneous catalysts for the deoxygenation of sulfoxides
under mild reaction conditions is still desired.
Hydroxyapatites (HAPs), which are a phosphate mineral
species found in teeth and bones, are of considerable interest due
to their potential usefulness as biomaterials,13 adsorbents,14 and
ion exchangers.15 We previously reported the benefits of utiliz-
ing HAP as a support, leading to high-performance heteroge-
neous catalysts for various organic syntheses.16 Recently, we
reported that HAP-supported Au nanoparticles (AuNPs/HAP)
acted as an efficient heterogeneous catalyst for the deoxygena-
tion of sulfoxides to sulfides using silanes.17 AuNPs/HAP
showed significantly higher catalytic activity than both homo-
geneous metal complexes and other heterogeneous catalysts.
From the environmental and practical synthetic point of
view, our next target is the design of more environmentally
RuNPs/HAP was synthesized as follows. 1.0 g of HAP was
stirred in 100 mL of an aqueous solution of RuCl3 (5 mM) at
room temperature for 12 h. The resulting dark brown solid was
filtered, washed with deionized water, and dried at 110 °C. The
obtained solid was treated with H2 (1 atm) at 110 °C for 1 h,
affording RuNPs/HAP. The Ru loading on RuNPs/HAP was
estimated to be 4.76 wt % by elemental analysis. The Ru K-edge
X-ray absorption near-edge structure (XANES) spectrum of
RuNPs/HAP showed that the shape of the spectrum was very
similar to that of Ru foil, and the absorption edge was assigned
to the formation of Ru0 species.18 The Fourier transform of the
k3-weighted Ru K-edge extended X-ray absorption fine structure
(EXAFS) of RuNPs/HAP revealed a peak around 2.4 ¡
attributed to Ru-Ru bonds. The peak magnitude for RuNPs/
HAP was much lower than that for the Ru foil, showing the
formation of Ru NPs in RuNPs/HAP. The XRD peak positions
of RuNPs/HAP were similar to those of HAP, indicating that the
Ru NPs were located on the surface of HAP. Transmission
electron microscopy (TEM) confirmed that highly dispersed Ru
NPs with a mean diameter of 3.0 nm were formed on the surface
of HAP.
We first investigated the catalytic potential of various metal
NPs on HAP in the deoxygenation of diphenyl sulfoxide (1) as
a model substrate using isopropanol as a reducing reagent
in toluene solvent at 110 °C under Ar atmosphere (Table 1).
Among them, Ru nanoparticles showed unique and high
catalytic activity, affording the corresponding diphenyl sulfide
(2) in >99% yield within 1 h (Entry 1).19 In sharp contrast with
the high efficiency of RuNPs/HAP, other metal nanoparticles
such as Rh, Pt, and Pd did not exhibit any catalytic activity
(Entries 2-4). Although Au NPs on HAP are effective in
deoxygenation using silanes as we reported previously,17
AuNPs/HAP exhibited no activity under the present reaction
conditions (Entry 5). Further optimization with RuNPs/HAP
showed that toluene was the best solvent (Entries 6-9), and other
alcohols such as ethanol and benzyl alcohol were also applicable
(Entries 10 and 11). Notably, even at lower temperature (40 °C),
420 | Chem. Lett. 2014, 43, 420–422 | doi:10.1246/cl.131077
© 2014 The Chemical Society of Japan

Table 1. Deoxygenation of diphenyl sulfoxide using various
Table 2. Deoxygenation of various sulfoxides using RuNPs/
HAP and i-PrOHa
supported metal nanoparticle catalystsa
O
S
O
S
RuNPs/HAP (5 mol%)
Catalyst (5 mol%)
S
S
R1
R2
R1
R2
toluene (5 mL), i-PrOH (0.5 mL), Ar, 110 °C
Solvent, i-PrOH,
Ar, 110 °C, 1 h
1
2
Time Conv.b Yieldb
Entry
Substrate
Product
Conv. of Yield of
1b/%
/h
/%
/%
Entry Catalyst
Solvent
2b/%
1
1
3
3
3
>99
>99
>99
>99
>99
>99
>99
>99
O
S
2c
3d
4e
1
2
3
4
5
6
7
8
9
RuNPs/HAP toluene
RhNPs/HAP toluene
>99
1
<1
<1
<1
84
83
21
12
>99
1
<1
<1
<1
84
83
21
12
S
PtNPs/HAP toluene
PdNPs/HAP toluene
AuNPs/HAP toluene
RuNPs/HAP trifluorotoluene
RuNPs/HAP n-heptane
RuNPs/HAP 1,4-dioxane
RuNPs/HAP AcOEt
RuNPs/HAP toluene
RuNPs/HAP toluene
RuNPs/HAP toluene
O
S
S
5
1
>99
>99
Cl
Cl Cl
Cl
O
S
S
S
6
7
2
2
>99
>99
>99
>99
Br
Br
O
S
10c
11d
12e
>99
>99
>99
>99
>99
>99
O
O
O
S
8
9
1
1
>99
>99
>99
>99
S
aReaction conditions: catalyst (0.05 g), 1 (0.5 mmol), solvent
(5 mL), and i-PrOH (10 mmol). Determined by GC using an
internal standard. cEtOH (10 mmol) was used instead of
O
S
b
S
aReaction conditions: RuNPs/HAP (0.050 g), sulfoxide (1.0
mmol). bDetermined by GC using an internal standard. cReuse
1. Reuse 2. Reuse 3.
d
e
i-PrOH. Benzyl alcohol (10 mmol) for 1 h. Catalyst (0.1 g),
40 °C, 12 h.
d
e
RuNPs/HAP gave a quantitative yield of 2 (Entry 12). These
results revealed that RuNPs/HAP provided distinct catalytic
activity in the deoxygenation of 1 using alcohol reagents.
Using the optimized reaction conditions, RuNPs/HAP was
also capable of having high catalytic activity for the selective
deoxygenation of diverse sulfoxides to sulfides (Table 2).
Various aromatic sulfoxides were efficiently converted to the
corresponding sulfides in excellent yields (Entries 1 and 5-7).
Interestingly, the RuNPs/HAP catalyst showed excellent che-
moselectivity for the deoxygenations; reducible functional
groups such as halogens and carbonyl remained intact during
the deoxygenation (Entries 5-7). Aliphatic sulfoxides could also
be successfully deoxygenated, affording the desired sulfides
(Entries 8 and 9).
One of the advantages of this catalyst system is its
reusability. After the reaction, the RuNPs/HAP was easily
recoverable by simple filtration and reusable (Entries 2-4). TEM
analysis on the used RuNPs/HAP catalyst revealed that the size
of Ru NPs on HAP did not significantly change from the fresh
catalyst, nor were aggregated Ru species observed. These
observations are consistent with the high durability of RuNPs/
HAP during the recycling experiments. Furthermore, this
catalyst system was also applicable under scale-up conditions.
For example, in the 10 mmol-scale reaction of 1, 2 was obtained
in 90% isolated yield by simple filtration of the catalyst and
did not promote the present deoxygenation. From these results,
the excellent catalytic activity of RuNPs/HAP in the deoxyge-
nation of sulfoxides under mild reaction conditions may be
attributed to the high durability of Ru NPs against the poisoning
of sulfoxides and the sulfide product. In fact, the reduction
reactions of 2-octanone to 2-octanol occurred using metal
nanoparticles of Ru, Pt, Pd, and Rh tested in Table 1 in the
presence of isopropanol. However, these reduction reactions
were suddenly terminated by the addition of sulfide except for
the reaction using RuNPs/HAP.18
In conclusion, the highly efficient catalytic deoxygenation
of sulfoxides catalyzed by heterogeneous catalysts using
alcohols as environmentally friendly reductants was demon-
strated. HAP-supported Ru nanoparticles can promote the
selective hydrogenation of various sulfoxides, giving the
corresponding sulfides in excellent yields. The present reaction
system exhibits functional group tolerances. Furthermore,
RuNPs/HAP is separable, reusable, and applicable to scale-up
conditions with high catalytic activity.
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O
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S
ð1Þ
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© 2014 The Chemical Society of Japan