A silica gel-supported ruthenium complex of 1,4,7-Trimethyl-1,4,7-triazacyclononane as recyclable catalyst for chemoselective oxidation of alcohols and alkenes by tert-butyl hydroperoxide

Article abstract of DOI:10.1021/jo0204404

A silica gel-immobilized [(Me₃tacn)Ru^III(CF₃ COO)₂(H₂O)]CF₃CO₂ complex (1-SiO₂, Me₃tacn = 1,4,7- trimethyl-1,4,7-triazacyclononane) was prepared by simple impregnation, and the catalyst was characterized by powdered X-ray diffraction, nitrogen adsorption/desorption, Raman, and diffuse reflectance UV-vis spectroscopies. The supported Ru catalyst can effect facile oxidation of alcohols by tert-butyl hydroperoxide (TBHP). Primary and secondary benzyl, allylic, and propargylic alcohols were transformed to their corresponding aldehydes and ketones in excellent yields; no oxidation of the C=C and C≡C bonds was observed for the allylic and propargylic alcohol oxidations. Likewise alkene epoxidation by TBHP can be achieved by 1-SiO₂; cycloalkenes such as norbornene and cyclooctene were oxidized to their exo-epoxides exclusively in excellent yields (>95%). The 1-SiO₂ catalyst can be recycled and reused for consecutive alcohol and alkene oxidations without significant loss of catalytic activity and selectivity; over 9000 turnovers have been attained for the oxidation of 1-phenyl-1-propanol to 1-phenyl-1-propanone. 4-Substituted phenols were oxidized by the "1 + TBHP" protocol to give exclusively ruthenium-catecholate complexes, which were characterized by UV-vis and ESI-MS spectroscopies. No (tert-butyldioxy)cyclohexadienone and other radical coupling/overoxidation products were produced using the "1 + TBHP" protocol. The formation of ruthenium-catecholate is proposed to proceed via ortho-hydroxylation of phenol.

Full text of DOI:10.1021/jo0204404

A Silica Gel-Su p p or ted Ru th en iu m Com p lex of
1
,4,7-Tr im eth yl-1,4,7-tr ia za cyclon on a n e a s Recycla ble Ca ta lyst for
Ch em oselective Oxid a tion of Alcoh ols a n d Alk en es by ter t-Bu tyl
Hyd r op er oxid e
Wai-Hung Cheung, Wing-Yiu Yu,* Wing-Ping Yip, Nian-Yong Zhu, and Chi-Ming Che*
Department of Chemistry and Open Laboratory of Chemical Biology of the Institute of Molecular
Technology for Drug Discovery and Synthesis, The University of Hong Kong, Pokfulam Road, Hong Kong
cmche@hku.hk
Received J uly 1, 2002
A silica gel-immobilized [(Me
3
tacn)RuIII(CF
3
COO)
2
(H
2
O)]CF
3
CO
2
2 3
complex (1-SiO , Me tacn ) 1,4,7-
trimethyl-1,4,7-triazacyclononane) was prepared by simple impregnation, and the catalyst was
characterized by powdered X-ray diffraction, nitrogen adsorption/desorption, Raman, and diffuse
reflectance UV-vis spectroscopies. The supported Ru catalyst can effect facile oxidation of alcohols
by tert-butyl hydroperoxide (TBHP). Primary and secondary benzyl, allylic, and propargylic alcohols
were transformed to their corresponding aldehydes and ketones in excellent yields; no oxidation of
the CdC and CtC bonds was observed for the allylic and propargylic alcohol oxidations. Likewise
alkene epoxidation by TBHP can be achieved by 1-SiO
2
; cycloalkenes such as norbornene and
cyclooctene were oxidized to their exo-epoxides exclusively in excellent yields (>95%). The 1-SiO
2
catalyst can be recycled and reused for consecutive alcohol and alkene oxidations without significant
loss of catalytic activity and selectivity; over 9000 turnovers have been attained for the oxidation
of 1-phenyl-1-propanol to 1-phenyl-1-propanone. 4-Substituted phenols were oxidized by the “1 +
TBHP” protocol to give exclusively ruthenium-catecholate complexes, which were characterized
by UV-vis and ESI-MS spectroscopies. No (tert-butyldioxy)cyclohexadienone and other radical
coupling/overoxidation products were produced using the “1 + TBHP” protocol. The formation of
ruthenium-catecholate is proposed to proceed via ortho-hydroxylation of phenol.
In tr od u ction
such as molecular oxygen, aqueous hydrogen peroxide,
4
and tert-butyl hydroperoxide (TBHP) is highly desirable.
Chemoselective alcohol and alkene oxidations have a
paramount importance in organic synthesis because their
reaction products (i.e., aldehydes/ketones and epoxides)
are key building blocks for construction of complex
organic molecules.1 Common laboratory-scale alcohol
and alkene oxidations usually employ a stoichiometric
5
Advances have been made by employing Mo/W(VI), Re-
6
7
8
9a-f
(
VII), Mn(II), Fe(II), or Ru
neous catalysts for organic oxidations using dilute H
TBHP as terminal oxidant. Recently promising results
complexes as homoge-
2
2
O /
,2
have been obtained with Ru,9
g-m
Pd(II), and Cu(I)
10
11
amount of toxic and reactive oxidants such as chromium-
(
4) (a) Applications of Hydrogen Peroxide and Derivatives; J ones,
3
(
VI) oxide, hypervalent iodine reagents (Dess-Martin),
C. W.; The Royal Society of Chemistry: Cambridge, 1999. (b) Catalytic
Oxidations with Hydrogen Peroxide as Oxidant; Strukul, G., Ed.;
Kluwer: Dordrecht, The Netherland, 1992.
2b
and peracids that pose severe safety and environmental
hazards in large-scale industrial reactions. In view of the
growing environmental concerns, development of envi-
ronmental-friendly and operationally safe catalytic sys-
tems for organic oxidations using more tractable reagents
(5) Selected examples for Mo(VI)/W(VI)-catalyzed organic oxidations,
see: (a) J ost, C.; Wahl, G.; Kleinhenz, D.; Sundermeyer, J . In Peroxide
Chemistry; Adam, W. Ed.; Wiley-VCH: Weinheim.; 2000, p 341. (b)
Sato, K.; Aoki, M.; Takagi, J .; Noyori, R. J . Am. Chem. Soc. 1997, 119,
1
2386 and references therein. (c) Sato, K.; Aoki, M.; Ogawa, M.;
Hashimoto, T.; Noyori, R. J . Org. Chem. 1996, 61, 8310. (d) Bailey, A.
J .; Griffith, W. P.; Parkin, B. C. J . Chem. Soc., Dalton Trans. 1995,
1833. (e) Venturello, C.; D’Aloisio, R. J . Org. Chem. 1988, 53, 1553. (f)
Bortolini, O.; Conte, V.; Di Furia, F.; Modena, G. J . Org. Chem. 1986,
51, 2661. For alkene epoxidations, see: (g) Venturello, C.; Alneri, E.;
Ricci, M. J . Org. Chem. 1983, 48, 3831.
*
(
To whom correspondence should be addressed.
1) (a) Lee, T. V. In Comprehensive Organic Synthesis; Trost, B. M.,
Fleming, I., Ley, S. V., Eds.; Pergamon: Oxford, 1991; Vol. 7, p 291.
b) Ley, S. V.; Madin, A. In Comprehensive Organic Synthesis; Trost,
(
B. M., Fleming, I., Levy, S. V., Eds.; Pergamon: Oxford, 1991; Vol. 7,
p 251. (c) Mijs, W. J .; DeJ onge, C. R. H. I. Organic Synthesis with Metal
Compounds; Plenum: New York, 1986. (d) Sheldon, R. A.; Kochi, J .
K. In Metal-Catalyzed Oxidation of Organic Compounds; Academic
Press: New York, 1981.
3 3
(6) For CH ReO -catalyzed organic oxidations, see: (a) Kuhn, F. E.;
Herrmann, W. A. In Metal-Oxo and Metal-Peroxo Species in Catalytic
Oxidations; Meunier, B. Ed.; Springer-Verlag: Berlin, 2000; p 213. (b)
Saladino, R.; Neri, V.; Pelliccia, A. R.; Caminiti, R.; Sadun, C. J . Org.
Chem. 2002, 67, 1323. (c) Zauche, T. H.; Espenson, J . H. Inorg. Chem.
1998, 37, 6827. (d) Rudolph, J .; Reddy, K. L.; Chiang, J . P.; Sharpless,
K. B. J . Am. Chem. Soc. 1997, 119, 6189. (e) Murray, R. W.; Iyanar,
K.; Chen, J .-X.; Wearing, J . T. Tetrahedron Lett. 1995, 36(36), 6415.
(f) Herrmann, W. A.; Fischer, R. W.; Marz, D. W. Angew. Chem., Int.
Ed. Engl. 1991, 30, 1638.
(
2) (a) Rao, A. S. In Comprehensive Organic Synthesis; Trost, B. M.,
Fleming, I., Ley, S. V., Eds.; Pergamon: Oxford, 1991; Vol. 7, p 357.
b) Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.; Gerhartz,
(
W., Yamamoto, Y. S., Kandy, L., Rounsaville, J . F., Schulz, G., Eds.;
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1
0.1021/jo0204404 CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/10/2002
7
716
J . Org. Chem. 2002, 67, 7716-7723

Recyclable Catalyst for Chemoselective Oxidation
complexes as homogeneous catalysts for aerobic alcohol
oxidations.
epoxidations by 2,6-dichloropyridine N-oxide with up to
11000 turnovers being attained;1
6g,h
however, leaching/
Ruthenium complexes are versatile catalysts for or-
deactivation upon successive uses of the catalyst leading
to gradual decline in catalytic activity and product
selectivity remains a major drawback.
ganic synthesis.1
2-15
Developing recyclable heterogeneous
ruthenium catalysts for organic oxidations would be an
attractive strategy to achieve easy product separation
and catalyst recycling. However, few heterogeneous Ru
catalysts for organic oxidations are known in the litera-
ture; notable examples include Ru-grafted hydroxy-
Oxidation chemistry of aqua-ruthenium complexes
containing macrocyclic tertiary amine ligands has been
extensively investigated,17 and their high-valent oxo-
1
8,19
ruthenium derivatives are powerful oxidants.
apatites1 and hydrotalcites
6a
16b,c
for aerobic alcohol oxida-
III
3 3 2 2
Previously we described that [(Me tacn)Ru (CF CO ) -
1
6d
tions and supported perruthenate(VII) catalysts based
on cross-linked polymer and mesoporous molecular sieves
2 3 2
(H O)]CF CO (1) could be oxidized to a reactive cis-
VI
dioxoruthenium(VI) complex, cis-[(Me tacn)Ru O (CF -
3
2
3
(
MCM-41) for alcohol oxidations using morpholine N-
2 4
CO )]ClO , which had been structurally characterized by
1
6e
18b
oxide as oxidant. These reported heterogeneous cata-
lysts, however, exhibit modest product turnover values
X-ray crystal analysis.
Our earlier studies revealed
that 1 is an effective and robust catalyst for homogeneous
alcohol and alkene oxidations using TBHP as terminal
oxidant, and more than 6000 product turnovers were
attained.9 Compared to dilute H O , TBHP is less
(
<1000). Previously we prepared a heterogeneous ruthe-
nium porphyrin catalyst supported on MCM-41. This
Ru-MCM-41 catalyst can effect highly efficient alkene
c
2
2
expensive and safer to handle in large quantities; its
reaction byproduct, tert-butyl alcohol, is environmentally
benign and easily removed.20 In view of the robustness
of 1 in homogeneous oxidation catalysis, we anticipate
that an effective heterogeneous Ru catalyst could be
developed by grafting 1 onto inert solid support.
As our ongoing effort to develop oxidative robust
heterogeneous Ru catalysts based on molecularly defined
ruthenium complexes, we set forth to prepare a new
(
2 2
7) For Mn-catalyzed organic oxidations using H O , see for ex-
ample: (a) Brinksma, J .; Hage, R.; Kerschner, J .; Feringa, B. L. Chem.
Commun. 2000, 537. (b) Lane, B. S.; Burgess, K. J . Am. Chem. Soc.
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0, 7965. (d) Zondervan, C.; Hage, R.; Feringa, B. L. Chem. Commun.
997, 419. Selected examples for Mn-catalyzed alkene epoxidations,
2
4
1
see: (e) De Vos, D.; Bein, T. Chem. Commun. 1996, 917.
8) For Fe-catalyzed organic oxidations using H : (a) Gupta, S.
(
2 2
O
S.; Stadler, M.; Noser, C. A.; Ghosh, A.; Steinhoff, B.; Lenoir, D.;
Horwitz, C. P.; Schramm, K.-W.; Collins, T. J . Science 2002, 296, 326.
(
1
b) White, M. C.; Doyle, A. G.; J acobsen, E. N. J . Am. Chem. Soc. 2001,
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ruthenium-silica catalyst (1-SiO
2
) by simple impregna-
(
Komiya, N. In Biomimetic Oxidations Catalyzed by Transition Metal
Complexes; Meunier, B., Ed.; Imperial College Press: London, 2000; p
63. With H O as oxidant, see for examples: (b) Goldstein, A. S.; Beer,
2 2
R. H.; Drago, R. S. J . Am. Chem. Soc. 1994, 116, 2424. With TBHP as
oxidant, see for examples: (c) Fung, W.-H.; Yu, W.-Y.; Che, C.-M. J .
Org. Chem. 1998, 63, 2873. (d) Cheng, W.-C.; Fung, W.-H.; Che, C.-M.
J . Mol. Catal. (A) 1996, 113, 311. (e) Murahashi, S. I.; Naota, T.
Synthesis 1993, 433. (f) Tanaka, M.; Kobayashi, T.; Sakakura, T.
tion of 1 onto commercial chromatographic silica gel.
2
-1
Because of its high surface area (5-800 m g ) and high
porosity, silica gel is a common sorbent for chromato-
graphic separation of organic compounds. Besides, silica
gel has found increasing applications in organic synthe-
sis;21 examples include cyclization of ketene acetals to
5
2
2
Angew. Chem., Intl. Ed. Engl. 1984, 23, 518. With O
2
as oxidant, see
form polyfunctional bicyclo[2.2.2]octanones and Kno-
for examples: (g) Dijksman, A.; Arends, I. W. C. E.; Sheldon, R. A.
Chem. Commun. 1999, 1591. (h) Lai, T.-S.; Zhang, R.; Cheung, K.-K.;
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Takezawa, E.; Sakaguchi, S.; Ishii, Y. Tetrahedron Lett. 1998, 39, 5557.
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see: (a) Yamaguchi, K.; Mori, K.; Mizugaki, T.; Ebitani, K.; Kaneda,
K. J . Am. Chem. Soc. 2000, 122, 7144. (b) Matsushita, T.; Ebitani, K.;
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T.; Matsushita, T.; Ebitani, K. J . Org. Chem. 1998, 63, 1750. (d) Dalal.
M. K.; Upadhyay, M. J .; Ram, R. N. J . Mol. Catal. (A) 1999, 142, 325.
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S.; Thomas, A. W. Chem. Commun. 1999, 1907. (f) Hinzen, B.; Lenz,
R.; Ley, S. V. Synthesis 1998, 977. (g) Liu, C.-J .; Yu, W.-Y.; Li, S.-G.;
Che, C.-M. J . Org. Chem. 1998, 63, 7364. (h) Liu, C.-J .; Li, S.-G.; Pang,
W.-Q.; Che, C.-M. J . Chem. Soc., Chem. Commun. 1997, 65. (i) Hinzen,
B.; Ley, S. V. J . Chem. Soc., Perkin Trans. 1 1997, 1907.
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1999, 71, 281. (b) Che, C.-M.; Yam, V. W.-W. Adv. Transition Metal
Coord. Chem. 1996, 1, 209. (c) Che, C.-M.; Yam, V. W.-W. Adv. Inorg.
Chem. 1992, 39, 233.
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T.-C. J . Chem. Soc., Dalton Trans. 1991, 1259.
(
j) Mark oˆ , I. E.; Giles, P. R.; Tsukazaki, M.; Chell e´ -Regnaut, I.; Urch,
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(
10) For Pd-catalyzed aerobic alcohol oxidations: (a) Brink, G.-J .
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6
4, 6750. (c) Nishimura, T.; Onoue, T.; Ohe, K.; Uemura, S. Tetrahe-
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(
Giles, P. R.; Tsukazaki, M.; Chelle-Regnaut, I.; Gautier, A.; Brown, S.
M.; Urch, C. J . J . Org. Chem. 1999, 64, 2433. (b) Mark oˆ , I. E.; Gautier,
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(
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9
(
(
13) Groves, J . T.; Roman, J . S. J . Am. Chem. Soc. 1995, 117, 5594.
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1
1
(
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(
1
1
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J . Org. Chem, Vol. 67, No. 22, 2002 7717

Cheung et al.
adopts a distorted octahedral geometry with a coordi-
nated H
2
O molecule. The measured Ru-O(6) distance is
2
.111(2) Å, which is comparable to the reported values
II
+
found in [(Me
3 2
tacn)Ru (bpy)(H O)] [bpy ) 2,2’-bipyri-
2
4
III
3+
dine, 2.169(3) Å], [Ru (H
2
O)
6
]
[2.016(4) - 2.037(5)
Å],25 and [Ru (terpy)(tmea)(H
II
O)] [2.151(7) Å; terpy )
2+
2
terpyridine, tmea ) N,N,N′,N′-tetramethyl-1,2-ethanedi-
amine].2
6
1
Two η -bound trifluoroacetate ligands coordinate to the
ruthenium atom in a cis configuration. The respective
Ru-O(1) and Ru-O(3) distances are 2.078(2) and 2.028-
(2) Å, which are comparable to the corresponding values
tacn)RuIII(CF
F IGURE 1. A perspective view of [(Me
3
3
CO
2 2
) -
+
(
H O)] cation. Bond distances (Å): Ru-O(1) ) 2.078(2), Ru-
2
O(3) ) 2.028(2), Ru-O(6) ) 2.111(2), Ru-N(1) ) 2.080(3), Ru-
N(2) ) 2.090(3), Ru-N(3) ) 2.086(3). Bond angles (deg): O(1)-
Ru-O(3) ) 92.54(9), O(1)-Ru-O(6) ) 91.48(9), O(3)-Ru-O(6)
IV
+
observed in [(Me
(
3
8f
tacn)(CF
3
CO
6) Å] complex.1 The Me
tacn ligand coordinates facially
3
2 2 3 2
)Ru OC (SiMe ) O] [2.075-
)
90.89(8), O(1)-Ru-N(1) ) 89.4(1), O(1)-Ru-N(2) ) 92.9-
to the Ru atom with the respective Ru-N(1), Ru-N(2)
and Ru-N(3) distances being 2.080(3), 2.090(3), and
.086(3) Å, which are close to the corresponding values
(1), O(1)-Ru-N(3) ) 173.3(9).
2
evenagel condensation.23 In this work, 1-SiO
is an
II
2+
3 2
for [(Me tacn)Ru (bpy)(OH )] complexes [2.154(4), 2.087-
2
2
4
effective catalyst for heterogeneous alcohol and alkene
oxidations by TBHP, and the catalyst is readily recycled
for successive reactions without showing significant
deterioration of chemoselectivity and catalytic activity.
Up to 9000 product turnovers have been achieved for the
(4), and 2.149(5) Å].
P r ep a r a tion of th e Silica Gel-Su p p or ted Ru th e-
n iu m Ca ta lyst. Immobilization of ruthenium catalyst
was undertaken by stirring an ethanolic solution of 1
with chromatographic grade commercial silica gel at room
temperature. Removal of the solvent by rotary evapora-
tion at 40 °C afforded the heterogeneous Ru catalyst,
1
1
-SiO
2
-catalyzed oxidation of 1-phenyl-1-propanol to
-phenyl-1-propanone under mild conditions.
designated as 1-SiO
2
(1%w/w), as a pale yellow solid.
showed the νSi-O
The FT-Raman analysis of 1-SiO
2
Resu lts a n d Discu ssion
-
1
stretches at 795 and 877 cm and νCH stretches at 2900
P r ep a r a tion a n d Ch a r a cter iza tion of [(Me
3
ta cn )-
-1
3
and 2979 cm (characteristic absorption of the Me tacn
III
Ru (CF
3
CO
2
)
2
(H
2
O)]CF
3
CO
2
(1). Following our re-
ligand). To evaluate the Ru content, the supported
catalyst was treated with concentrated HCl, followed by
inductively coupled plasma (ICP) analysis. The Ru
content was determined to be 0.94% w/w, which is close
to the expected value of 1%w/w (see Experimental Section
for detail). Both the X-ray powder diffraction (XRD)
9
c
III
ported procedure, [(Me
3
tacn)Ru Cl ] was gently re-
3
fluxed with silver trifluoromethanesulfonate in 0.2 M
trifluoroacetic acid. After removal of the insoluble AgCl,
slow evaporation of the pale red solution by heating at
or below 70 °C gave 1 as a pale yellow microcrystalline
solid (see Experimental Section for details). Attempt to
speed up the evaporation process by heating at above 70
2
patterns of 1-SiO and silica gel show a broad and
featureless absorption peak at ca. 2θ ) 24°, suggesting
that complex 1 should be uniformly dispersed over the
silica gel. The surface area of unloaded silica gel was
°
C led to a white powder, instead of a yellow crystalline
solid. We found that this white powder sample gave poor
catalytic activity and product selectivity for epoxidation
of styrene by TBHP. As a result, only the yellow micro-
crystalline solid of 1 was employed for preparation of
supported catalyst.
2
-1
determined to be 514.46 m g by nitrogen adsorption
isotherm (BET) measurement, whereas the overall sur-
2
-1
face area of 1-SiO
consistent with surface immobilization of the Ru complex.
The solid diffuse-reflectance UV-vis spectrum of 1-SiO
2
was reduced to 483.88 m
g
The infrared spectrum of the yellow crystalline Ru
2
-
1
complex displays a νCdO stretch at 1713 cm ; its FAB
shows an intense absorption band at λmax ) 320 nm,
which is absent in unloaded silica gel sample (see
Supporting Information). This absorption band is sig-
mass spectrum exhibits two prominent ion cluster peaks
at m/z ) 499 and 385 assignable to [(Me
3
tacn)Ru(CF
COO)] ions, respectively.
Complex 1 is insoluble in n-hexane, diethyl ether, CH
, CHCl , and CH CN but slowly dissolves in methanol
or ethanol to give a pale yellow solution. A methanolic
solution of 1 shows an intense absorption band at λmax
3
-
+
+
COO)
2 3 3
] and [(Me tacn)Ru(CF
3
nificantly blue-shifted from the absorption band of [(Me -
2
-
+
tacn)Ru(CF
that [(Tet-Me
3
COO)(OMe)] . Our previous work showed
Cl
2
3
3
III
+
6
3 2 6
)Ru (OSiMe ) ] (Tet-Me ) N,N,N’,N’-3,6-
diazaoctane-3,6-dimethyl-1,8-diamine) displays an in-
tense ligand-to-metal charge transfer (LMCT) absorption
)
3
-1
-1
3
92 nm (ꢀmax ) 2290 dm mol cm ); its ESI-MS spectrum
27
band at λmax ) 340 nm. We tentatively assign the 320
is featured by a prominent ion species at m/z ) 417 that
matches with the [(Me tacn)Ru(CF COO)(OMe)] formu-
3 3
nm absorption band of 1-SiO
2
to a similar LMCT
+
transition. The importance of polar Si-OH groups for
immobilization of 1 is reflected by the following observa-
lation. The 392 nm absorption is assigned to ligand-to-
metal charge-transfer transition in nature.
The molecular structure of 1 has been determined by
X-ray crystallography (see Supporting Information for
details). As shown in Figure 1, the ruthenium atom
(
24) Cheng, W.-C.; Yu, W.-Y.; Cheung, K.-K.; Che, C.-M. J . Chem.
Soc., Dalton Trans. 1994, 57.
25) Bernhard, P.; B u¨ rgi, H.-B.; Hauser, J .; Lehmann, H.; Ludi, A.
Inorg. Chem. 1982, 21, 3936.
26) Grover, N.; Gupta, N.; Singh, P.; Thorp, H. H. Inorg. Chem.
(
(
(
22) Schinzer, D.; Kalesse, M.; Kabbara, J . Tetrahedron Lett. 1988,
1992, 31, 2014.
2
9, 5241.
(27) Chiu, W.-H.; Che, C.-M.; Mak, T. C.-W. Polyhedron 1996, 15,
(23) Brillon, D.; Sauve, G. J . Org. Chem. 1992, 57, 1838.
1129.
7
718 J . Org. Chem., Vol. 67, No. 22, 2002

Recyclable Catalyst for Chemoselective Oxidation
TABLE 1. Heter ogen eou s 1-SiO2-Ca ta lyzed Alcoh ol Oxid a tion by ter t-Bu tyl Hyd r op er oxid e
Reaction conditions: A mixture of alcohol (1 mmol), 1-SiO2 (600 mg) and TBHP (2.3 mmol) in n-hexane (5 mL) was stirred in a
screw-capped vessel at 45 °C for 12-16 h. After centrifugation, aliquots were taken from the reaction mixture for capillary GC analysis.
a
b
Yields were determined by capillary GC analysis based on % alcohol conversion. In CH2Cl2.
tion: when 1-SiO
2
(0.1 g) was washed with ethanol (3
produced aldehydes/acids as side-products.28 In this work,
×
30 mL), no leaching of the ruthenium complex was
no benzaldehyde formation was observed for the 1-SiO
catalyzed hydrobenzoin/benzoin oxidation.
2
observed. However, when the silica surface was modified
with hydrophobic triphenylsilyl groups, the resultant
silica was ineffective for complex immobilization, and 1
can be easily washed off with ethanol. As noted in
previous section, to release the Ru complex from silica
Benzyl alcohol and its substituted derivatives (entry
6-8) are oxidized effectively to their corresponding
benzaldehydes by the “1-SiO + TBHP” protocol. Unlike
2
the 1-phenyl-1-propanol oxidation discussed in earlier
section, 1.2 equiv of TBHP was required to achieve 81%
conversion with 86% yield of benzaldehyde in the benzyl
alcohol oxidation. When 2.3 equiv of TBHP was em-
ployed, a significant amount of benzoic acid (25%) was
formed and benzaldehyde was obtained in only 57% yield.
It should be noted that silica gel alone (i.e., without the
immobilized Ru-catalyst) is ineffective toward oxidation
of alcohols by TBHP; all the starting alcohol was fully
recovered.
support would require treatment of 1-SiO
trated HCl. We propose that grafting of 1 onto silica may
involve reaction of [(Me tacn)Ru (CF CO )(H O)] with
3 3 2 2
the surface Si-OH groups to form Ru-OSi linkages.
2
with concen-
III
+
Heter ogen eou s Catalytic Alcoh ol Oxidation s. Oxi-
d a tion of Ben zylic Alcoh ols. In the presence of 1-SiO
1%w/w, 600 mg), 1-phenylethanol (1 mmol) was oxidized
2
(
by TBHP (2.3 mmol) in n-hexane (5 mL) to afford
acetophenone in 94% yield (99% conversion; Table 1,
entry 1). Previously, CH Cl was found to be the best
2 2
solvent for the homogeneous 1-catalyzed alcohol oxida-
tions (80% conversion and 93% yield). However, using
2 2
CH Cl as solvent for the heterogeneous 1-phenylethanol
oxidation resulted in slightly reduced substrate conver-
sion (81%) and lower product yield (93%), when compared
to the results obtained with n-hexane as solvent.
Oxid a tion of Allylic a n d P r op a r gylic Alcoh ols.
Allylic alcohols such as 2-cyclohexenol, geraniol, and
cinnamyl alcohol (entries 9, 10, and 11) were selectively
oxidized to the corresponding ketones/aldehydes using
1
2
-SiO (600 mg) and TBHP (2.3 equiv) in n-hexane; no
oxidation of CdC bond was observed. For the geraniol
and cinnamyl alcohol oxidations, geranial and cinnamyl
aldehyde were formed in 87-89% yields (entries 10-11)
without overoxidation to carboxylic acids. Furthermore,
no isomerization of the allylic CdC bonds was observed.
We previously showed that the “1 + TBHP” protocol can
effectively epoxidize unfunctionalized alkenes under
homogeneous conditions.9d Yet, in this work, we did not
encounter any epoxidation of the isolated trisubstituted
Likewise, 1-phenyl-1-propanol was converted to its
ketone product in 99% yield (93% conversion) under the
1
2
-SiO catalyzed conditions (entry 3). For oxidation of
hydrobenzoin, the reaction required 16 h to achieve 95%
conversion and benzoin was formed in 89% yield (entry
4
), albeit with minor formation of benzil (7%). We found
that further oxidation of benzoin to benzil (entry 5, 67%
conversion and 99% yield) required a longer reaction time
2
CdC bond for the 1-SiO -catalyzed geraniol oxidation
(entry 10), and no 6,7-epoxy allylic alcohols/aldehydes
(72 h) and 3 equiv of TBHP. It is known that the
tungsten-catalyzed oxidation of 1,2-diols using H
2
O
2
(28) Krohn. K.; Vinke, I.; Adam, H. J . Org. Chem. 1996, 61, 1467.
J . Org. Chem, Vol. 67, No. 22, 2002 7719

Cheung et al.
were detected based on 1H NMR analysis of the crude
product mixture.
TABLE 2. Ca ta lyst Recycla bility Stu d ies in n -Hexa n e
a n d Dich lor om eth a n e
In contrast to the Swern oxidation that is ineffective
for oxidation of propargylic alcohols,29 the “1-SiO
+
2
TBHP” protocol can also oxidize 1-octyn-3-ol and 1-phen-
yl-1-propyn-3-ol to their corresponding carbonyl products
in ca. 90% yields (entries 12 and 13). As reported earlier,
n-hexane
CH2Cl2
reaction run % conversion % yield % conversion % yield
VI
+
3 2 3 2
cis-[(Me tacn)Ru O (CF CO )] complex readily reacts
2
0b
1
2
3
4
5
6
7
8
93
94
95
93
96
93
95
95
99
99
99
99
99
99
99
99
76
59
62
52
58
58
54
57
97
84
80
97
97
97
97
85
with internal alkynes to give 1,2-diketones. However,
in this work, formation of 1,2-diketones was not observed
in the catalytic alkynol oxidations.
Oxid a tion of Alip h a tic Secon d a r y Alcoh ols. With
1
-SiO
aliphatic cyclic alcohols (entries 14-18) were converted
to their ketones in ca. 80% yields. The 1-SiO catalyzed
2
(600 mg) and TBHP (2.3 mmol) in n-hexanes,
2
Reaction conditions: A mixture containing 1-phenyl-1-propanol
1 mmol), 1-SiO2 (600 mg), and TBHP (2.3 equiv) in either
oxidation of substituted cyclohexanols can be significantly
influenced by bulky substituents. For example, oxidation
of 4-cis/trans-tert-butylcyclohexanol (entries 14 and 15)
can proceed to >70% conversion in 12 h, affording the
corresponding ketone product in 99% yield. Yet, the
analogous reaction with menthol (entry 16) achieved only
(
n-hexane or CH2Cl2 was stirred at 40-45 °C for 14 h. After
centrifugation, aliquots were taken from the supernatant liquid
for product identification and quantitation using capillary GC. To
recycle the supported catalyst, the supernatant liquid was removed
from the reaction vessel, followed by washing the catalyst with
fresh CH2Cl2 (3 × 10 mL). The reaction vessel containing 1-SiO2
was recharged with fresh alcohol and TBHP for subsequent
consecutive reactions.
1
1% conversion under the same reaction conditions. In
contrast, the homogeneous 1-catalyzed menthol oxidation
was reported to attain 59% substrate conversion with
9
2% yield of menthone under the same reaction condi-
h. Subsequent consecutive run gave similar results (100%
9
c
tions. Assuming that the active Ru species is bound to
silica surface, such reactivity difference of the heteroge-
neous system versus the homogeneous one is attributed
to increased steric interaction of menthol with the silica
gel surface (see also trans-stilbene oxidation in later
sections).
alcohol consumption; 99% ketone yields, turnovers )
5
129), and up to 9495 turnovers were achieved for two
successive reactions.
When CH Cl was used as solvent, lower alcohol
consumption was observed for the 1-SiO catalyzed
-phenyl-1-propanol oxidation (see Table 2). The ICP
2
2
2
1
Analogous to the homogeneous reactions,9c oxidation
analysis of the recycled catalyst (after the eighth reaction
run) revealed the Ru content being 0.92% w/w, which is
of primary alcohols such as 1-octanol by the “1-SiO +
2
TBHP” protocol was also futile. No formation of aldehyde
was detected and the starting alcohol was fully recovered
after stirring for 24 h at 45 °C.
comparable to the initial value of 0.94%w/w. Using CH
2
-
Cl as solvent, we noticed that the supported catalyst
2
gradually developed a pink red color upon consecutive
reaction runs. The solid diffuse-reflectance UV-vis spec-
trum showed an intense absorption band at λmax ) 546
nm, which is conspicuously absent in the corresponding
2
Recyclin g of th e 1-SiO Ca ta lyst a n d Lea ch in g
Stu d ies. The silica gel-supported Ru catalyst has been
subjected to eight successive reuses under the reaction
conditions: 1-phenyl-1-propanol (1 mmol), 1-SiO
mg), and TBHP (2.3 mmol) in n-hexane at 40-45 °C for
4 h. After centrifugation of the reaction mixture, the
supernatant solution was decanted to separate the
2
catalyst, which was washed several times with CH Cl .
2
(600
2
spectrum of 1-SiO . This spectral feature resembles to
III
the visible absorption of [(Me
3
tacn)Ru (µ-O)(µ-CF
3 2 2
CO ) -
1
IV
+
Ru (Me
3
tacn)] at λmax ) 560 nm (Supporting Informa-
tion). We found that the dimeric ruthenium complex
exhibits negligible catalytic activity. Treatment of benzyl
30
2
The reaction vessel containing the catalyst was recharged
with 1-phenyl-1-propanol, TBHP, and n-hexane for an-
other consecutive reaction run. The combined superna-
tant and organic washings were collected for product
identification and quantitation. As shown in Table 2, the
alcohol (1 mmol) with TBHP (1.2 mmol) and [(Me
3
tacn)-
tacn)] (1 mol %) in CH
(5 mL) at room temperature failed to produce
III
IV
+
Ru (µ-O)(µ-CF
Cl
3
CO
2
)
2
Ru (Me
3
2
-
2
benzaldehyde as detected by capillary GC, and the
starting alcohol was completely recovered. Therefore, the
2
catalytic activity of 1-SiO remained largely unchanged
apparent diminished catalytic activity of 1-SiO
Cl is presumably due to formation of inactive dimeric
ruthenium complex.
2 2
in CH -
for eight successive runs, and each reaction run could
attain >90% alcohol consumption and 99% ketone yield
with a total of 690 turnovers being achieved. After eight
2
Heter ogen eou s Alk en e Ep oxid a tion . Oxid a tion of
Disu bstitu ted Alk en es. As reported earlier, 1 is an
effective catalyst for homogeneous epoxidation of alkenes
by TBHP. In this work, treatment of norbornene (1 mmol)
with TBHP (2.3 mmol, 4.92 M in 1,2-dichloroethane) and
2
consecutive reactions, the recovered 1-SiO was found
to contain 0.94%w/w of Ru based on ICP analysis. This
Ru content is essentially the same as the initial value,
indicative of no catalyst leaching during the reaction
cycles. By using less 1-SiO
of Ru), 1-phenyl-1-ethanol (10 mmol) reacted with TBHP
23 mmol) in n-hexane (20 mL) at 40 °C to afford ethyl
2
catalyst (120 mg, 1.9 µmol
1
2 2 2
-SiO (600 mg) in CH Cl furnished exo-epoxynorbor-
nane exclusively in 96% yield (Table 3). No endo-epoxides
(
and other rearranged products such as norcamphor and
cyclohexene carboxyaldehyde were detected in the 1-SiO -
2
phenyl ketone (84% substrate conversion, 99% yield,
turnovers ) 4366), albeit with longer reaction time of 72
(
30) Neubold, P.; Wieghardt, K. Nuber, B.; Weiss, J . Angew. Chem.,
(29) Tidwell, T. T. Synthesis 1990, 10, 857.
Int. Ed. Engl. 1988, 27, 933.
7
720 J . Org. Chem., Vol. 67, No. 22, 2002

Recyclable Catalyst for Chemoselective Oxidation
TABLE 3. Heter ogen eou s 1-SiO2 Ca ta lyzed Alk en e
Ep oxid a tion by TBHP
The heterogeneous oxidation of disubstituted aromatic
alkenes have been examined. With trans-â-methylstyrene
as substrate, the 1-SiO
2
catalyzed alkene epoxidation
proceeded effectively to afford trans-â-methylstyrene
oxide (71%) and benzaldehyde (8%), comparable to previ-
9
d
ous results for the analogous homogeneous reaction.
Our previous study showed that the 1-catalyzed trans-
stilbene oxidation afforded trans-stilbene oxide in 54%
yield with 62% alkene consumption.9d In this work, the
heterogeneous trans-stilbene oxidation [1-SiO
2
(600 mg),
2
Cl ]
alkene (1.0 mmol), and TBHP (2.3 mmol) in CH
2
attained only 19% substrate conversion in 14 h, though
trans-stilbene oxide was produced exclusively in 75%
yield. Analogous to the heterogeneous menthol oxidation
discussed in earlier section, we propose that the de-
creased reactivity of trans-stilbene could be related to
unfavorable steric interaction of the alkene with the silica
surface.
We have examined oxidation of methyl 9-cis-octade-
cenoate under the 1-SiO -catalyzed conditions. We found
2
that only cis-epoxide was selectively produced (67%
isolated yield) and no trans-epoxide was observed based
1
on H NMR analysis of the crude reaction mixture. This
high stereoselectivity compares well with a similar reac-
tion using dioxirane as oxidant,32 whereas the analogous
reaction with high valent RudO species resulted in
1
7, 18a-h
considerable formation of trans-epoxide products.
Reaction conditions: A mixture of alkene (1 mmol), 1-SiO2 (600
mg), and TBHP (2.3 mmol) in CH2Cl2 (5 mL) was stirred at room
temperature for 14 h. After centrifugation, aliquots were taken
from the supernatant liquid for capillary GC analysis with 1,4-
Oxid a tion of Ter m in a l Alk en es. Under the standard
reaction conditions, styrene (1 mmol) was effectively
oxidized to styrene oxide (89% yield) by TBHP (2.3 mmol)
a
dichlorobenzene as internal standard. Yields were determined
2 2 2
in the presence of 1-SiO (1%w/w, 600 mg) in CH Cl
based on %alkene conversion. Isolated yield. c Yield was deter-
b
1
(78% alkene consumption for 8h reaction time; see Table
3, entry 8); benzaldehyde (8%) was also formed. In this
case, no phenylacetaldehyde was detected. In attempt to
obtain complete alkene consumption, the reaction mix-
ture was left for prolonged stirring for an additional 6 h.
Subsequent capillary GC analysis showed that styrene
oxide was produced in 46% yield albeit with complete
styrene consumption. In contrast, prolonged reaction time
has no significant effect on epoxide yields for the analo-
gous norbornene and trans-â-methylstyrene oxidations.
mined by H NMR using 1,1-diphenylethene as internal standard.
d
Benzaldehyde (8%) was also detected.
catalyzed reaction. Changing the solvent to n-hexane,
benzene, or toluene as solvent produced almost identical
results.
Likewise, cyclooctene was effectively oxidized to its exo-
2
epoxide exclusively (96% yield) under the 1-SiO cata-
lyzed conditions. It is noteworthy that both norbornene
and cyclooctene are poor substrates for the TS-1 catalyzed
3
1
2
In view of the lower yield observed for the 1-SiO -
oxidation by H
2
O
2
.
With cyclohexene as substrate,
catalyzed styrene oxidation, the reaction was monitored
by capillary GC; the time courses for the respective
styrene consumption and styrene oxide formation are
represented by curves A and B of Figure 2. During 0-2
cyclohexene oxide was formed in 75% yield; 2-cyclohex-
enone was formed as the side-product in 14% yield. The
observed product profile for the heterogeneous cyclohex-
ene oxidation is comparable to that of the analogous
homogeneous 1-catalyzed reactions. It is known that
allylic C-H bond oxidation is usually prevailing for
-1
h (1st phase) more styrene was consumed (5.8% h ) than
-
1
the product epoxide being formed (1.7% h ) and the
benzaldehyde:styrene oxide ratio was found to be 61:14.
However, during 3-8 h (2nd phase) the styrene con-
1
7, 18a-h
cyclohexene oxidation by RudO complexes.
Previously we found that the homogeneous 1-catalyzed
+)-limonene oxidation gave 1,2-epoxide (54%) as major
-
1
sumption (18% h ) and styrene oxide formation (16%
(
-
1
h
) proceeded at similar rates, and the maximum
product and terminal 8,9-epoxide as minor product (16%
yield).9d Nevertheless, the heterogeneous 1-SiO
epoxide yield of 89% was obtained after 8 h of reaction.
From 8 h and beyond (3rd phase), a gradual decrease in
styrene oxide was found. This could be attributed to
decomposition of styrene oxide by the silica surface
acidity, presumably via epoxide ring opening reaction.
2
cata-
lyzed reaction afforded 1,2-epoxide as the only isolable
product (53% yield) with complete alkene consumption.
1
When the crude reaction mixture was examined by H
NMR spectroscopy, a complex mixture was observed with
the 1,2-epoxide being the only identifiable product. The
absence of terminal 8,9-epoxide could be due to epoxide
decomposition induced by the surface acidity of the silica
support (see later section).
Recyclin g of 1-SiO
2
Ca ta lyst a n d Lea ch in g Stu d -
ies. Similar to the alcohol oxidations described earlier,
(
32) For reviews of dioxirane chemistry, see: (a) Curci, R. In
Advances in Oxygenated Processes; Baumstark, A. L., Ed.; J AI Press:
Greenwich, 1990; Vol. 2, p 1. (b) Adam, W.; Curci, R.; Edwards, J . O.
Acc. Chem. Res. 1989, 22, 205. (c) Murray, R. W. Chem. Rev. 1989, 89,
1187.
(
31) Murugavel, R.; Roesky, H. W. Angew. Chem., Int. Ed. Engl.
1
997, 36, 477 and references therein.
J . Org. Chem, Vol. 67, No. 22, 2002 7721

Cheung et al.
TABLE 4. Ru th en iu m -Med ia ted Oxid a tion of P h en ols
by TBHP
entry
1
Y
UV-vis (nm)
ESI-MS (m/ z)
276 (ꢀ ) 4332 dm3 mol cm
-1
-1
-1
-1
-1
-1
-1
-1
-1
)
)
)
)
)
)
)
)
536
iPr
-1
5
76 (ꢀ ) 1993 dm3 mol cm
3
-1
2
3
4
Me
Ph
H
271 (ꢀ ) 5625 dm mol cm
508
584
494
-1
5
75 (ꢀ ) 1020 dm3 mol cm
3
-1
325 (ꢀ ) 1450 dm mol cm
-1
5
75 (ꢀ ) 1640 dm3 mol cm
3
-1
275 (ꢀ ) 6950 dm mol cm
-1
5
63 (ꢀ ) 3540 dm3 mol cm
2
F IGUR E 2. Time course for the 1-SiO catalyzed styrene
oxidation. Curve A: styrene consumption; Curve B: styrene
oxide formation.
blue color developed over the silica gel surface within
-10 min. On the basis of capillary GC analysis, no
5
formation of (tert-buthyldioxy)cyclohexadienone and/or
other radical coupling/overoxidation products was ob-
served with over 95% recovery of the starting phenol.
Similar results [i.e., no formation of (tert-butyldioxy)-
cyclohexadienone and other phenol oxidation products]
were observed when reacting 1 (1 equiv) with 4-isopro-
the supported Ru catalyst has been reused for four
successive reactions under the reaction conditions: nor-
bornene (1 mmol), 1-SiO (600 mg), TBHP (2.3 mmol)
2
in n-hexane (5 mL) for 12 h at room temperature. The
supported catalyst was recycled by centrifugation, fol-
lowed by removal of the supernatant solution and several
2 2
pylphenol (1.5 equiv) and TBHP (10 equiv) in CH Cl at
2 2
washings with CH Cl . A total of 385 turnovers was
room temperature. Instead, a dark blue species with a
characteristic absorption band λmax ) 576 nm was formed
spontaneously. ESI-MS analysis of the reaction mixture
revealed a prominent ion cluster peak at m/z ) 536,
attained for four consecutive reactions: first run, epoxide
yield ) 99%, alkene conversion ) 96%, turnovers ) 96;
second run, epoxide yield ) 99%, alkene conversion )
9
5%, turnovers ) 95; third run, epoxide yield ) 99%,
III
+
corresponding to [(Me
mulation. The same species can be detected by reacting
with 4-isopropylcatechol in CH Cl , and it showed
3 3 2 9 11 2
tacn)Ru (CF CO )(C H O )] for-
alkene conversion ) 97%, turnovers ) 97; fourth run,
epoxide yield ) 99%, alkene conversion ) 97%, turnovers
1
2
2
)
97. There is no apparent loss of catalytic activity over
identical UV-vis and ESI-MS spectral patterns. We
tentatively assign this species to be a ruthenium-
catecholate complex; attempt to isolate the complex gave
an intractable solid that rendered structural character-
ization difficult.
Oxidation of several phenol derivatives by the “1 +
TBHP” protocol has been examined, and Table 4 depicts
the relevant UV-vis and ESI-MS data. In this work,
formation of the ruthenium-catecholate complex can be
regarded as ortho-hydroxylation of phenols, analogous to
the copper-mediated reaction involving dicopper-peroxo
active intermediate.36
four successive catalyst recycling, and the Ru content of
the reused supported catalyst was found to be the same
as the initial value of 0.94%w/w determined by ICP. On
the basis of FT-Raman analysis, the Me
probably retained since the characteristic C-H stretches
2900 and 2979 cm-1) were still observed in the recovered
-SiO catalyst.
Oxid a tion of P h en ols. Oxidative transformation of
3
tacn ligand was
(
1
2
phenols to quinones has been a subject of extensive
studies due to its biological and synthetic importance.33
Transition metal catalyzed oxidation of phenols with
peroxides is known to proceed nonselectively with forma-
tion of various side-products from radical coupling and/
3
4
Exp er im en ta l Section
or overoxidation products. Murahashi and co-workers
II
had reported that [Ru (PPh
3
)
3
Cl
2
] can effect oxidation of
Ma ter ia ls. All the solvents and alcohols/alkenes substrates
were purified by standard procedure before uses. Cyclohexene,
cyclooctene, and styrene were first washed with 10% sodium
hydroxide and then distilled over calcium hydride. Norbornene
was purified by sublimation, whereas trans-stilbene was
purified by recrystallization before use. tert-Butyl hydroper-
oxide, obtained commercially as a 70% aqueous solution, was
pretreated by the procedure described by Sharpless and co-
workers using 1,2-dichloroethane and was later standardized
4
(
-substituted phenols and derivatives by TBHP to give
tert-butyldioxy)cyclohexadienones selectively.35
Given that the “1-SiO + TBHP” protocol is effective
2
for alcohol and alkene oxidations, we therefore examined
its activity toward phenol oxidation. However, when
4
-isopropylphenol was treated with 1-SiO
2
(600 mg) and
TBHP (2.3 equiv) in CH Cl at room conditions, a dark
2
2
1
21
by H NMR (4.92 M in 1,2-dichloroethane).
P r ep a r a tion of [(Me ta cn )Ru (CF CO
(1). A modified procedure was employed. [(Me
)
2
(H CO
2
O)]CF
3
2
(33) (a) Hudlicky, M. Oxidations in Organic Chemistry; American
3
3
2
Chemical Society: Washington, D. C. 1990; p 163. (b) Whiting D. A.
In Comprehensive Organic Chemistry; Stoddart, F., Ed.; Pergamon:
Oxford, 1979; Vol. 1, p 707. (c) Harborne, J . B. Biochemistry of Phenolic
Compounds; Academic Press: London, 1964. (d) Pridham, J . B. Enzyme
Chemistry of Phenolic Compounds; Pergamon: New York, 1963.
3
tacn)RuCl ] (1.0
3
g, 2.54 mmol) was heated at reflux with silver trifluo-
romethanesulfonate (1.92 g, 7.6 mmol) in trifluoroacetic acid
(
34) (a) Ingold, K. U. In Free Radicals; Kochi, J . K., Ed.; J ohn Wiley
Sons: New York, 1973; Vol. 1, p 37. (b) Taylor, W. I.; Battersby, A.
R. Oxidative Coupling of Phenols; Marcel Dekker: New York, 1967.
35) Murahashi, S.-I.; Naota, T.; Miyaguchi, N.; Noda, S. J . Am.
Chem. Soc. 1996, 118, 2509.
(36) Examples for phenol oxidation by peroxo-copper complexes to
form discrete catechol intermediates, see: (a) Mandal, S.; Macikenas,
D.; Protasiewicz, J . D.; Sayre, L. M. J . Org. Chem. 2000, 65, 4804. (b)
Berreau, L. M.; Mahapatra, S.; Halfen, J . A.; Houser, R. P.; Young, V.
G., J r.; Tolman, W. B. Angew. Chem., Int. Ed. 1999, 38, 207.
&
(
7
722 J . Org. Chem., Vol. 67, No. 22, 2002

Recyclable Catalyst for Chemoselective Oxidation
0.2 M, 125 mL) for 0.5 h. During this time, the starting
(
To obtain the products, the supernatant liquid was sepa-
rated, and the catalyst was washed with CH Cl
The combined supernatant liquid and washings were then
evaporated to dryness by rotary evaporation, and the residue
was loaded onto a silica gel column for chromatographic
purification.
ruthenium complex dissolved to give a pale red solution with
formation of AgCl precipitate. The reaction mixture was
filtered, and the volume was reduced slowly by evaporation
at 70 °C to ca. 20 mL. Any insoluble solid was removed by
filtration to give a pale red solution. The solution was left
standing at room temperature for several days to afford the
product complex as a yellow microcrystalline solid. Additional
crops of the product could be obtained by further slow
evaporation of the reaction mixture. Yield of the solid: 0.82 g
2
2
(3 × 10 mL).
Gen er a l P r oced u r e for th e 1-SiO -Ca ta lyzed Alk en e
2
Oxid a tion . To a screw-capped vessel was charged with a
mixture of alkene (1 mmol), 1-SiO2 (600 mg, 1%w/w), and
TBHP (2.3 mmol) in CH Cl /n-hexane (5 mL). After stirring
2
2
9
c
(
52%). Anal. Calcd for C15
H
23
F
9
N
3
O
7
Ru: C, 28.62; H, 3.68;
at room temperature for 14 h (except for styrene), the reaction
mixture was centrifuged, and the aliquots were taken from
the supernatant liquid after addition of internal standard (1,4-
dichlorobenzene) for capillary GC analysis. For trans-stilbene
oxidation, the supernatant liquid was separated, and the Ru
catalyst was washed repeatedly (3 × 10 mL) with CH Cl . The
N, 6.68. Found: C, 28.94; H, 3.53, N, 6.33. FAB-MS: m/z )
+
+
4
99 [M - H
IR: 1713 (νCdO).
P r ep a r a tion of th e Silica Gel-Su p p or ted Ru th en iu m
Ca ta lyst, 1-SiO . Silica gel (5.940 g, Merck Kieselgel, 230-
00 mesh ASTM, preheated at 110 °C for overnight to remove
2
O - CF
3
CO
2
], 385 [M - H
2
O - 2CF
3 2
CO ]. FT-
2
2
2
4
combined supernatant liquid and washings were evaporated
to dryness by rotary evaporator. After addition of 1,1-diphe-
nylethene as internal standard, the residue was dissolved in
CDCl3 for 1H NMR analysis. The amount of trans-stilbene
oxide was determined by comparing its integral ratio at (δH
3.8 ppm) with that at (δH 5.4 ppm) of 1,1-diphenylethene.
moisture) was added to an ethanolic solution (50 mL of
absolute ethanol) of 1 (60 mg, 10 µmol) with continuous
stirring at room temperature. The solvent was removed by
rotary evaporation at 40 °C to afford a pale yellow powder
solid, which was further dried in a vacuum at room temper-
ature for overnight. The solid was kept under a nitrogen
atmosphere to avoid contact with moisture.
Ack n ow led gm en t. 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 Uni-
versity of Hong Kong (Generic Drugs Research Pro-
gram), and the Hong Kong Research Grants Council
(HKU7092/98P). We acknowledge Drs. Stephen S.-Y.
Chui and Yunning Liu for experimental assistance and
discussion for the surface characterization studies.
2
Deter m in a tion of th e Ru Con ten t in 1-SiO . The silica-
supported Ru-catalyst (208.8 mg) was extracted with concen-
trated HCl (5 × 2 mL) in a screw-capped vessel to give a pale
yellow solution. The combined acid solutions containing the
Ru complex were evaporated by heating to ca. 1 mL, followed
by treatment with concentrated nitric acid (2 mL) to digest
the metal complex. Removal of the nitric acid by heating gave
a red-brown residue, which was dissolved in deionized water
(
3 × 1 mL) and transferred to a volumetric flask (5.0 mL) for
ICP analysis. The Ru concentration was determined by
measuring the atomic emission (240.2 nm) with reference to
a linear (R ) 0.99) calibration curve of (0.26-1.3 mM) 1
prepared in a manner identical to the sample preparation.
Su p p or tin g In for m a tion Ava ila ble: Detailed experi-
mental for the X-ray structure determination of complex 1,
crystal data and structure refinement, bond distances and
angles, atom coordinates and isotropic displacement param-
eters, solid diffuse reflectance UV spectra of silica gel, and
recovered 1-SiO with CH Cl as solvent for oxidation. This
2 2 2
material is available free of charge via the Internet at
http://pubs.acs.org.
Gen er a l P r oced u r e for th e 1-SiO
Oxid a tion . To a screw-capped vessel was charged with a
mixture of alcohol (1 mmol), 1-SiO (600 mg, 1%w/w), and
2
-Ca ta lyzed Alcoh ol
2
TBHP (2.3 mmol) in n-hexane (5 mL). After stirring at 45 °C
for 14 h, the reaction mixture was centrifuged, and the aliquots
were taken from the supernatant liquid for product identifica-
tion and quantitation using capillary GC analysis.
J O0204404
J . Org. Chem, Vol. 67, No. 22, 2002 7723