[RuCl2(p-cymene)]2 on carbon: An efficient, selective, reusable, and environmentally versatile heterogeneous catalyst

Article abstract of DOI:10.1021/ol0260977

(Matrix Presented) A heterogeneous ruthenium catalyst, easily prepared by adsorption of [RuCl₂(p-cymene)]₂ on activated carbon, exhibited a highly efficient and selective catalytic activity in various environmentally attractive transformations such as aerobic oxidation, hydrolytic oxidation, and dehydration processes with excellent recyclability.

Full text of DOI:10.1021/ol0260977

ORGANIC
LETTERS
2002
Vol. 4, No. 14
2369-2371
[RuCl2(p-cymene)]2 on Carbon: An
Efficient, Selective, Reusable, and
Environmentally Versatile
Heterogeneous Catalyst
,‡
Eunjung Choi,† Chongmok Lee,† Youngim Na,‡ and Sukbok Chang*
Department of Chemistry, Ewha Womans UniVersity, Seoul 120-750, Korea, and
Department of Chemistry, Korea AdVanced Institute of Science and Technology
(KAIST), Daejeon 305-701, Korea
sbchang@mail.kaist.ac.kr
Received April 29, 2002
ABSTRACT
A heterogeneous ruthenium catalyst, easily prepared by adsorption of [RuCl2(p-cymene)]2 on activated carbon, exhibited a highly efficient and
selective catalytic activity in various environmentally attractive transformations such as aerobic oxidation, hydrolytic oxidation, and dehydration
processes with excellent recyclability.
Heterogeneous catalysis has been employed as an important
chemical technology because of the inherent operational
advantages such as ease of handling, separation, and recovery
for the reuse of insoluble catalysts.1 However, certain
limitations of heterogeneous catalysis are frequently encoun-
tered, such as gradual leaching of transition metals from the
solid supports and lower efficiency or selectivity compared
to that of homogeneous systems.2 To improve these proper-
ties, a variety of polymer-supported or microencapsulated
heterogeneous catalysts have been devised in recent years.3
During the course of our studies on catalytic reactions,4 we
have found that a dimeric Ru complex, [RuCl2(p-cymene)]2
(1), displays a surprisingly diverse and highly selective
catalytic activity in certain transformations such as hydro-
silylation of alkynes,4a aerobic oxidation of alcohols,4b,c
hydrolytic oxidation of silanes to silanols,4d 1,4-addition of
terminal alkynes to conjugated enones,4e and dehydration of
aldoximes to nitriles.4f Given its wide utility as an efficient
catalyst in various other reactions,5 a heterogeneous version
of the ruthenium catalyst with practical aspects would be
highly desirable. Herein, we describe the preparation of a
† Ewha Womans University.
‡ Korea Advanced Institute of Science and Technology (KAIST).
(1) For reviews, see: (a) Smith, K. Solid Supports and Catalysts in
Organic Synthesis; Ellis Hornood: Chichester, 1992. (b) Keim, W.;
Driessen-Ho¨lscher B. In Handbook of Heterogeneous Catalysis; Ertl, G.,
Kno¨zinger, H., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, 1997; Vol. 1,
p 231. (c) Basset, J. M.; Candy, J. P.; Santini, C. C. In Transition Metals
for Organic Synthesis; Beller, M., Bolm, C., Eds.; Wiley-VCH: Weinheim,
1998; Vol. 2, p 387.
(3) For some selected examples, see: (a) Annis, D. A.; Jacobsen, E. N.
J. Am. Chem. Soc. 1999, 121, 4147. (b) Kobayashi, S.; Endo, M.; Nagayama,
S. J. Am. Chem. Soc. 1999, 121, 11229. (c) Son, S. U.; Lee, S. I.; Chung,
Y. K. Angew. Chem., Int. Ed. 2000, 39, 4158. (d) Yu, X.-Q.; Huang, J.-S.;
Yu, W.-Y.; Che, C.-M. J. Am. Chem. Soc. 2000, 122, 5337. (e) Djakovitch,
L.; Koehler, K. J. Am. Chem. Soc. 2001, 123, 5990.
(4) (a) Na, Y.; Chang, S. Org. Lett. 2000, 2, 1887. (b) Lee, M.; Chang,
S. Tetrahedron Lett. 2000, 41, 7507. (c) Chang, S.; Lee, M.; Ko, S.; Lee,
P. H. Synth. Commun. 2002, 32, 1279. (d) Lee, M.; Ko, S.; Chang, S. J.
Am. Chem. Soc. 2000, 122, 12011. (e) Chang, S.; Na, Y.; Choi, E.; Kim,
S. Org. Lett. 2001, 3, 2089. (f) Yang, S. H.; Chang, S. Org. Lett. 2001, 3,
4209. (g) Ko, S.; Na, Y.; Chang, S. J. Am. Chem. Soc. 2002, 124, 750.
(2) (a) Blum, J.; Rosenfeld, A.; Polak, N.; Israelson, O.; Schumann, H.;
Avnir, D. J. Mol. Catal. 1996, 107, 217. (b) Herrmann, W. A.; Cornils, B.
Angew. Chem., Int. Ed. Engl. 1997, 36, 1048. (c) Thomas, J. M. Angew.
Chem., Int. Ed. 1999, 38, 3588.
10.1021/ol0260977 CCC: $22.00 © 2002 American Chemical Society
Published on Web 06/19/2002

supported ruthenium catalyst 1 and its versatile uses for some
environmentally attractive reactions.
were readily oxidized under 1 atm of O2 to the corresponding
carbonyl compounds in high yields in the presence of Cs2CO3
(5 mol %) (Table 1).
The heterogeneous catalyst was readily prepared by the
treatment of [RuCl2(p-cymene)]2 (1) with activated carbon
(KB, 100 mesh) in benzene.6 The adsorbed amounts of
ruthenium was determined by elemental analysis to be in
the range of 2.15-2.65 wt % to C.7 Although the Ru complex
1 was also easily adsorbed on other types of carbon to a
similar extent,8 supported catalysts thus obtained showed
lower activities in the examined reactions compared to that
adsorbed on the charcoal KB (100 mesh). A transmission
electron microscopy (TEM) image of 1 immobilized on the
activated carbon (KB, 100 mesh) shown in Figure 1 reveals
that ruthenium atoms are not uniformly distributed in the
carbon atoms.9
Table 1. Catalytic Aerobic Oxidation of Various Alcoholic
Substratesa
a A solution of alcohol (1.0 mmol) in toluene (4 mL) was heated to 110
°C in the presence of 1/C (2.2 wt %, 300 mg) and Cs2CO3 (5 mol %) under
1 atm of O2. b Isolated yield after column chromatography.
Likewise in reactions using the soluble catalyst 1, over-
oxidized side products such as carboxylic acids or esters were
not observed under the heterogeneous conditions. The solid-
supported catalyst (1/C) was recovered quantitatively by a
simple filtration, and the filtered catalyst along with the
replenished Cs2CO3 (3 mol % each run) could be reused for
the next cycles with minimal loss of activity up to nine cycles
(Table 2).11 It should be noted that leaching of Ru metal
Figure 1. Transmission electron microscopy (TEM) of 1/C.
On the basis of our earlier reports on the aerobic oxidation
of alcohols by the soluble catalyst [RuCl2(p-cymene)]2 (1),4b,c
we first examined the catalytic activity of 1/C thus prepared
for the aerobic oxidation. Using the heterogeneous catalyst
1/C (2.2 wt %), various substrates such as benzylic, allylic,
and R-carbonyl alcohols (6.5 mol % of ruthenium atom)10
Table 2. Reusability of a Heterogeneous Catalyst 1/C for the
Aerobic Oxidation of 4-Bromobenzyl Alcohola
(5) For some recent examples, see: (a) Karlsson, U.; Wang, G.-Z.;
Ba¨ckvall, J.-E. J. Org. Chem. 1994, 59, 1196. (b) Gendre, P. L.;
Offenbecher, M.; Bruneau, C.; Dixneuf, P. H. Tetrahedron: Asymmetry
1998, 9, 2279. (d) Lee, D.; Huh, E. A.; Kim, M.-J.; Jung, H. M.; Koh, J.
H.; Park, J. Org. Lett. 2000, 2, 2377. (e) Trost, B. M.; Doherty, G. A. J.
Am. Chem. Soc. 2000, 122, 3801. (f) Iwasa, S.; Takezawa, F.; Tuchiya, Y.;
Nishiyama, H. Chem. Commun. 2001, 59.
(6) Procedure for Preparation of Supported Catalyst 1/C. To a stirred
suspension of charcoal (KB, 100 mesh, surface area 1500 m2/g, 1.25 g) in
benzene (37 mL) was added the ruthenium complex 1 (250 mg), and the
mixture was stirred for 14 h at room temperature. The mixture was filtered
and washed with DMF, methanol, and hot benzene, and the filtered solid
was dried under vacuum at 50-60 °C for 24 h.
(7) The adsorbed amounts of Ru on C were lower when other solvents
were used (e.g., 0.6 wt % to C prepared in CH3CN, 1.4 wt % in CH2Cl2,
1.2 wt % in 1,4-dioxane, and 2.1 wt % in THF).
run
1
2
3
4
5
6
7
8
9
time (h)
yield (%)c
6
89
7
92
8
97
8
92
8
94
8
89
8
93
8
85
9
81
a A solution of alcohol (1.0 mmol) in toluene (3.3 mL) was reacted under
1 atm of O2 in the presence of 1/C (initially 2.2 wt %, 300 mg) at 110 °C.
b Additional amounts of Cs2CO3 (3 mol %) were added in each cycle from
the second run. c Indicates isolated yield after chromatography.
from the solid support was almost negligible over the
recycles. For example, when 1/C of 2.2 wt % Ru to C was
employed as an initial catalyst in the aerobic oxidation, the
Ru content on C was determined to be 1.5 wt % after nine
(8) Recently, a supported catalyst of RuCl2(PPh3)3/C was prepared and
used for the oxidative cleavage of vicinal diols; see: Takezawa, E.;
Sakaguchi, S.; Ishii, Y. Org. Lett. 1999, 1, 713.
(9) Although further studies have to be carried out to elucidate what
type of interactions the adsorbed ruthenium maintains, it is assumed that
the cleaved ruthenium monomer replaces cymene ligand with a solvent that
(10) It is based on the elemental analysis of the prepared solid-supported
complex.
1
was observed by H NMR of filtrate after the adsorption procedure.
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Org. Lett., Vol. 4, No. 14, 2002

cycles. To demonstrate whether the observed catalysis is
heterogeneous, the catalyst was filtered off after ca. 50%
conversion at the same temperature of the reaction and the
filtrate was then allowed to react at 110 °C. No further
reaction proceeded from the filtrate even in the presence of
additional cesium salt, demonstrating that the present system
is heterogeneous.12
from the reusability of the heterogeneous catalyst as well as
the use of water as an environmentally benign oxidant.
Catalytic conversion of aldoximes to the corresponding
nitriles was another example of the successful application
of the solid 1/C catalyst (Table 4). Various aldoximes were
Table 4. Catalytic Dehydration of Various Aldoximes to
Nitrilesa
The hydrolytic oxidation of organosilanes with the het-
erogeneous ruthenium catalyst turned out also to be highly
efficient and selective for the formation of silanols (Table
3).4d Using 1/C as a catalyst (9.8 mol % of ruthenium atom),
Table 3. Hydrolytic Oxidation of Organosilanes to Silanolsa
a A solution of aldoxime (0.5 mmol) in CH3CN (5 mL) was heated to
80 °C in the presence of 1/C (200 mg) and molecular sieves 4 Å (2 wt
equiv to aldoxime). b Isolated yield. c No additional molecular sieves were
added during the recycles.
readily dehydrated under the heterogeneous conditions (4.4
mol % of ruthenium atom) at 80 °C in CH3CN to afford the
corresponding nitriles in high yields (entries 1-5) with
similar efficiency compared to that of the reaction of the
homogeneous conditions.4f Recovery and reuse of 1/C
catalyst was examined in the dehydration of 4-bromo-
benzaldoxime under the same conditions (entries 6-10). The
efficiency of the heterogeneous catalyst on the dehydration
was maintained up to four runs in this case, and then it took
slightly longer to get a complete conversion.
In conclusion, a heterogeneous ruthenium catalyst has been
readily prepared by a simple adsorption of [RuCl2(p-
cymene)]2 on active charcoal, and its utility has been
demonstrated in some environmentally attractive reactions
of the aerobic oxidation of alcohols, hydrolytic oxidation of
silanes, and dehydration of aldoximes with high efficiency,
selectivity, and reusability.
a Silane (1.0 mmol) was reacted with water (4 equiv) in the presence of
1/C (450 mg) in a solvent (10 mL, CH3CN in entries 1-4 and THF in
entries 5-10) at room temperature. b Isolated yield.
conversion of silanes to silanols proceeded smoothly at room
temperature upon addition of excess water (4 equiv) in
CH3CN. Under the heterogeneous conditions, silanols were
obtained in high yields with only negligible formation of
disiloxanes (<5%, entries 1-5). Recovery and reuse of the
heterogeneous catalyst in the hydrolytic oxidation was readily
achieved, as demonstrated in the reaction of diphenyl-
methylsilane in THF, although the reaction times were longer
over the cycles (entries 6-10). Of special note is that the
conversion of silanes to silanols is highly practical judged
(11) Addition of fresh Cs2CO3 in certain amounts (2-3 mol %) in each
cycle was important to maintain the catalytic activity of the filtered 1/C
high. For example, catalytic activity of the recycled 1/C was lost gradually
after the second cycle when fresh Cs2CO3 was not replenished.
(12) For references, see: (a) Arends, I. W. C. E.; Sheldon, R. A.; Wallau,
M.; Schuchardt, U. Angew. Chem., Int. Ed. Engl. 1997, 36, 1144. (b)
Sheldon, R. A.; Wallau,; Arends, I. W. C. E.; Schuchardt, U. Acc. Chem.
Res. 1998, 31, 485.
Acknowledgment. We thank the Center for Molecular
Design and Synthesis (CMDS) at KAIST for this work. E.C.
is a recipient of a fellowship of Research Institute for Basic
Science at Ewha Womans University.
OL0260977
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