Ruthenium meso-tetrakis(2,6-dichlorophenyl)porphyrin complex immobilized in mesoporous MCM-41 as a heterogeneous catalyst for selective alkene epoxidations

Article abstract of DOI:10.1021/jo981003l

A ruthenium complex of meso-tetrakis(2,6-dichlorophenyl)porphyrin, [Ru^II(TDCPP)(CO)(EtOH)], is immobilized into mesoporous MCM-41 molecular sieves; the supported Ru catalyst can effect highly selective heterogeneous alkene epoxidations using 2,6-dichloropyridine TV-oxide as terminal oxidant. Aromatic and aliphatic alkenes can be efficiently converted to their epoxides in good yields and selectivities, and cis-alkenes such as czs-stilbene, cis-β-methylstyrene, and cis-β-deuteriostyrene are epoxidized stereospecifically. Oxidation of cycloalkenes, e.g., norbornene and cyclooctene, can be carried out effectively using the heterogeneous Ru-catalyzed reaction while these alkenes are unreactive in the zeolite-based titanium silicate (TS-l)-catalyzed conditions (Murugavel, R.; Roesky, H. W. Angew. Chem., Int. Ed. Engl. 1997, 36, 477). On the other hand, the Ru/M-41(m) catalyst displays size selectivity in the (+)-limonene oxidation where the terminal C=C bond (vs internal trisubstituted C=C bond) becomes more readily oxidized. Bulky 3,4,6-tri-O-benzyl-D-glucal has failed to react under the heterogeneous Ru-catalyzed conditions, whereas the smaller acetyl derivative is converted to a 3:1 mixture of α- and β-glycal epoxides. The Ru/M-41(m) catalyst can be used repeatedly, and 67% of its initial activity is retained after 11 691 turnovers (three runs). The loss of activity is attributed to catalyst leaching and/or deactivation. On the basis of Hammett correlation (ρ⁺ = -0.72, R = 0.997) and product studies (cyclohexene and crs-alkenes as the substrates), a reactive dioxorutheniumCVI) porphyrin intermediate is not favored. An oxoruthenium(V) complex or oxoruthenium(IV) porphyrin cation radical could be the key intermediate for this highly selective epoxidation reaction.

Full text of DOI:10.1021/jo981003l

7
364
J . Org. Chem. 1998, 63, 7364-7369
Ru th en iu m m eso-Tetr a k is(2,6-d ich lor op h en yl)p or p h yr in Com p lex
Im m obilized in Mesop or ou s MCM-41 a s a Heter ogen eou s Ca ta lyst
for Selective Alk en e Ep oxid a tion s
Chun-J ing Liu, Wing-Yiu Yu, Shou-Gui Li, and Chi-Ming Che*
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong
Received May 26, 1998
II
A ruthenium complex of meso-tetrakis(2,6-dichlorophenyl)porphyrin, [Ru (TDCPP)(CO)(EtOH)],
is immobilized into mesoporous MCM-41 molecular sieves; the supported Ru catalyst can effect
highly selective heterogeneous alkene epoxidations using 2,6-dichloropyridine N-oxide as terminal
oxidant. Aromatic and aliphatic alkenes can be efficiently converted to their epoxides in good yields
and selectivities, and cis-alkenes such as cis-stilbene, cis-â-methylstyrene, and cis-â-deuteriostyrene
are epoxidized stereospecifically. Oxidation of cycloalkenes, e.g., norbornene and cyclooctene, can
be carried out effectively using the heterogeneous Ru-catalyzed reaction while these alkenes are
unreactive in the zeolite-based titanium silicate (TS-1)-catalyzed conditions (Murugavel, R.; Roesky,
H. W. Angew. Chem., Int. Ed. Engl. 1997, 36, 477). On the other hand, the Ru/M-41(m) catalyst
displays size selectivity in the (+)-limonene oxidation where the terminal CdC bond (vs internal
trisubstituted CdC bond) becomes more readily oxidized. Bulky 3,4,6-tri-O-benzyl-D-glucal has
failed to react under the heterogeneous Ru-catalyzed conditions, whereas the smaller acetyl
derivative is converted to a 3:1 mixture of R- and â-glycal epoxides. The Ru/M-41(m) catalyst can
be used repeatedly, and 67% of its initial activity is retained after 11 691 turnovers (three runs).
The loss of activity is attributed to catalyst leaching and/or deactivation. On the basis of Hammett
+
correlation (F ) -0.72, R ) 0.997) and product studies (cyclohexene and cis-alkenes as the
substrates), a reactive dioxoruthenium(VI) porphyrin intermediate is not favored. An oxoruthenium-
(V) complex or oxoruthenium(IV) porphyrin cation radical could be the key intermediate for this
highly selective epoxidation reaction.
In tr od u ction
few heterogeneous catalysts are able to attain high
catalytic turnovers (>1000) and product selectivity.
Notably, the zeolite-based titanium silicate (TS-1) can
bring about selective heterogeneous epoxidation by aque-
Epoxidation is one of the most important CdC bond
functionalization methods, and epoxides are valuable
2 2
ous H O ; its catalytic activity is limited by the small pore
intermediates for laboratory syntheses as well as chemi-
cal manufacturing.1 Although the reaction is typically
diameter (ca. 7 Å) of the zeolite support, and the oxidation
of cycloalkenes (size > 7 Å) is ineffective.7
carried out by peracid oxidation in a laboratory-scale
reactor with remarkable selectivities,2 the pursuit of
The use of ruthenium porphyrin complexes for epoxi-
8
9
dations, and recently aziridinations, has been exten-
highly efficient and selective catalytic methods that are
simple and safe to operate on a large scale remains an
area of current interest. Transition metal catalysts, for
sively studied by us and others.10 Indeed, this class of
complexes, among other metalloporphyrins, display
promising selectivities toward organic oxidations using
1
1
instance, Mo/W(VI),3 Re(VII), and Ru(III) complexes,
,4
5
6
1
2
dioxygen, as well as other mild oxidizing reagents such
are known to effect homogeneous selective alkene epoxi-
dations by employing the more tractable and economical
1
3
14
as N
2
O
2
and 2,6-dichloropyridine N-oxide (Cl pyNO).
Previously, we prepared a heterogeneous oxidation cata-
oxidants such as dilute H
2 2
O and tert-butyl hydroperoxide
(
TBHP). Apparently, heterogeneous catalysis can offer
(
7) Murugavel, R.; Roesky, H. W. Angew. Chem., Int. Ed. Engl. 1997,
36, 477 and references therein.
8) (a) Leung, W.-H.; Che, C.-M. J . Am. Chem. Soc. 1989, 111, 8812.
b) Ho, C.; Leung, W.-H.; Che, C.-M. J . Chem. Soc., Dalton Trans. 1991,
933. (c) Liu, C.-J .; Yu, W.-Y.; Peng, S.-M.; Mak, T. C. W.; Che, C.-M.
additional advantages over its homogeneous counterpart
for easy catalyst recycling and product isolation; however,
(
(
2
(
1) Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.; Ger-
J . Chem. Soc., Dalton Trans. 1998, 1805.
(9) Au, S.-M.; Fung, W.-H.; Cheng, M.-C.; Che, C.-M. Peng, S.-M. J .
Chem. Soc., Chem. Commun. 1997, 1655.
hartz, W., Yamamoto, Y. S., Kandy, L., Rounsaville, J . F., Schulz, G.,
Eds.; Verlag Chemie: Weinheim, 1987; Vol. A9, p 531.
(
2) Rao, A. S. In Comprehensive Organic Synthesis; Trost, B. M.,
Fleming, I., Ley, S. V., Eds.; Pergamon: Oxford, 1991; Vol. 7, p 357.
3) Catalytic Oxidations with Hydrogen Peroxide as Oxidant; Strukul,
G., Ed.; Kluwer: Dordrecht, The Netherlands, 1992.
4) Under phase-transfer conditions, for some selected examples: (a)
(10) (a) Mlodnika, T.; J ames, B. R. In Metalloporphyrin Catalyzed
Oxidations; Montanari, F., Casella, L., Eds.; Kluwer: Dordrecht, The
Netherlands, 1994; p 121. (b) Gross, Z.; Ini, S.; Kapon, M.; Cohen, S.
Tetrahedron Lett. 1996, 37, 7325. (c) Gross, Z.; Ini, S. J . Org. Chem.
1997, 62, 5514.
(
(
Venturello, C.; Alneri, E.; Ricci, M. J . Org. Chem. 1983, 48, 3831. (b)
Venturello, C.; D’Aloisio, R. J . Org. Chem. 1988, 53, 1553. (c) Sato, K.;
Aoki, M.; Ogawa, M.; Hashimoto, T.; Noyori, R. J . Org. Chem. 1996,
(11) (a) Metalloporphyrin in Catalytic Oxidations; Sheldon, R. A.,
Ed.; M. Dekker: New York, 1994. (b) Metalloporphyrin Catalyzed
Oxidations; Montanari, F., Casella, L., Eds.; Kluwer: Dordrecht, The
Nethrelands, 1994.
(12) (a) Groves, J . T.; Quinn, R. J . Am. Chem. Soc. 1985, 107, 5790.
(b) Lai, T.-S.; Zhang, R.; Kwong, H.-L.; Cheung K.-K.; Che, C.-M. J .
Chem. Soc., Chem. Commun. 1998, 1583.
6
1, 8310.
5) Rudolph, J .; Reddy, K. L.; Chiang, J . P.; Sharpless, K. B. J . Am.
Chem. Soc. 1997, 119, 6189.
6) Cheng, W.-C.; Fung, W.-H.; Che, C.-M. J . Mol. Catal. A 1996,
13, 311.
(
(
1
(13) Groves, J . T.; Roman, J . S. J . Am. Chem. Soc. 1995, 117, 5594.
S0022-3263(98)01003-2 CCC: $15.00 © 1998 American Chemical Society
Published on Web 09/19/1998

Catalyst for Selective Alkene Epoxidations
J . Org. Chem., Vol. 63, No. 21, 1998 7365
Sch em e 1. En ca p su la tion of m eso-Tetr a k is-
2,6-d ich lor op h en yl)p or p h yr in a tor u th en iu m (II)
Ca r bon yl Com p lex in to Mesop or ou s MC-41a
(
F igu r e 1. meso-Tetrakis(2,6-dichlorophenyl)porphyrin (TD-
CPP).
lyst by grafting a meso-tetrakis(4-chlorophenyl)porphy-
rinatoruthenium(II) carbonyl complex into mesoporous
a
Key: (a) modification of MCM-41 channel wall: APTES/
siliceous molecular sieves MCM-41 modified with (3-
II
CHCl3; (b) encapsulation of ruthenium porphyrin complex: [Ru -
TDCPP)(CO)(EtOH)]/CH2Cl2.
aminopropyl)triethoxysilane (APTES).1
5,16
The immobi-
(
lized ruthenium complex exhibits enhanced stability in
the oxidizing medium, as well as good catalytic activity.
The MCM-41 solid support consists of an ordered array
of hexagonal channels with an average pore diameter of
adsorption was proven to be unsuccessful, the ruthenium
complex was effectively immobilized after modification
of the channel surface with (3-aminopropyl)triethoxysi-
1
9
3
6 Å, which permits a lower diffusional resistance (cf.
lane (APTES) (see the Experimental Section).
The
nanoporous zeolite support) for the reactant molecules
to access the metal active sites located within the
channels.17 Herein is described a heterogeneous catalytic
system for highly selective alkene epoxidations based on
a supported ruthenium meso-tetrakis(2,6-dichlorophen-
amino groups of the surface-bound APTES should serve
as anchoring sites for the ruthenium complex via axial
coordination (Scheme 1). This can be implicated by the
IV
failure to graft the coordinatively saturated [Ru (TD-
-
20
CPP)(pz) ] complex (pz ) pyrazolate) into the surface-
2
II
yl)porphyrin, [Ru (TDCPP)(CO)(EtOH)], complex on me-
modified MCM-41 [M-41(m)].
soporous MCM-41 molecular sieves using Cl
terminal oxidant (Figure 1).
2
pyNO as
Encapsulation of the Ru complex was undertaken by
vigorous stirring of M-41(m) (400 mg) in the methylene
chloride solution of [Ru (TDCPP)(CO)(EtOH)] (10 mL,
II
0
.17-0.28 mM) at ambient temperature for 1 h. The
Resu lts a n d Discu ssion
supported catalyst, designated as Ru/M-41(m), was iso-
lated by filtration, washed thoroughly with methylene
chloride, and then dried in a vacuum. Its solid diffuse-
reflectance UV/vis and FT Raman spectra resemble to
those of the unbound ruthenium complex, indicating that
the molecule remains intact during the process of encap-
The parent MCM-41 was prepared by the literature
method18 using C16
Br (RBr) as the template
according to the following molar composition: 1.0 SiO
.37 RBr/0.23 Na O/83.3 H O. The solid product was
washed with deionized water at ambient temperature
and then calcined in air at 550 °C for 3-5 h to afford a
white powder product. The X-ray powder diffraction
H
3 3
33N(CH )
2
/
0
2
2
II
sulation (Table 1). To estimate the loading of [Ru -
TDCPP)(CO)] in Ru/M-41(m), the catalyst sample was
(
(XRD) pattern of this material shows a very intense
first dissolved into an aqueous 3% HF solution, followed
by exhaustive extraction with methylene chloride. The
ruthenium content in the combined organic extracts can
then be evaluated using UV/vis spectrophotometry by
absorption at 2θ ≈ 2.25°, the d spacing of the peak )
II
3
9.74 Å. Although direct encapsulation of the [Ru -
(TDCPP)(CO)(EtOH)] complex into MCM-41 by surface
monitoring the absorbance of the Soret band at λmax
409 nm (Table 2, see the Experimental Section for
detailed procedures).
)
(
14) (a) Higuchi, T.; Ohtake, H.; Hirobe, M. Tetrahedron Lett. 1989,
3
1
0, 6545. (b) Higuchi, T.; Ohtake, H.; Hirobe, M. Tetrahedron Lett.
991, 32, 7435. (c) Ohtake, H.; Higuchi, T.; Hirobe, M. Tetrahedron
Lett. 1992, 33, 2521.
15) Liu, C.-J .; Li, S.-G.; Pang, W.-Q.; Che, C.-M. J . Chem. Soc.,
Chem. Commun. 1997, 65.
16) For a review of supported metalloporphyrin catalysts, see: (a)
Figure 2 depicts the XRD pattern of 8.3 wt % Ru/M-
41(m), which shows the characteristic peaks of MCM-41
(
II
without any peaks arisen from [Ru (TDCPP)(CO)-
(
Lindsay-Smith, J . R. In Metalloporphyrin in Catalytic Oxidations:
Sheldon, R. A., Ed.; M. Dekker: New York, 1994; p 325. (b) Laszlo, P.
In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Ley,
S. V., Eds.; Pergamon: Oxford, 1991; Vol. 7, p 839.
(EtOH)]. This suggests that the solid support is struc-
turally unchanged and the ruthenium porphyrin should
be dispersed molecularly within the channels. The
regular porosity of the MCM-41 is preserved during the
transformations based on the XRD analyses (2θ ) 2.25°,
d spacing ) 39.75 Å) and nitrogen sorption studies (see
Table 2).
Aromatic and aliphatic alkenes (1 mmol) react with Cl -
2
pyNO (1.1 mmol) to produce epoxides selectively in good
yields using 50 mg of 0.4 wt % Ru/M-41(m) as catalyst
(17) Selected examples of transition-metal catalysts immobilized in
MCM-41 used for hydrocarbon oxidations: [Mn] (a) Sutra, P.; Brunel,
D. J . Chem. Soc., Chem. Commun. 1996, 2485. (b) Yonemitsu, M.;
Tanaka, Y.; Iwamoto, M. Chem. Mater. 1997, 9, 2679. (c) Burch, R.;
Cruise, N.; Gleeson, D.; Tsang, S. C. J . Chem. Soc., Chem. Commun.
1
996, 951. [V] (d) Reddy, K. M.; Moudrakovski, I.; Sayari, A. J . Chem.
Soc., Chem. Commun. 1994, 1059. [Fe] (e) Echchahed, B.; Moen, A.;
Nicholson, D.; Bonneviot, L. Chem. Mater. 1997, 9, 1716. [Ti] (f) Corma,
A.; Navarro, M. T.; Pariente, J . P. J . Chem. Soc., Chem. Commun. 1994,
1
47. (g) Krijnen, S.; Abbenhuis, H. C. L.; Hanssen, R. W. J . M.; Van
Hoff, J . H. C.; Van Santen, R. A. Angew. Chem., Int. Ed. Engl. 1998,
7, 356.
18) Beck, J . S.; Vartuli, J . C.; Roth, W. J .; Leonowicz, M. E.; Kresge,
(substrate/oxidant/Ru ratio ) 5245:5769:1) in methylene
3
(
(19) Earliest examples for the APTES modification of oxide sur-
C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.;
McCullen, S. B.; Higgins, J . B.; Schlenker, J . L. J . Am. Chem. Soc.
face: (a) Moses, P. R.; Wier, L.; Murray, R. W. Anal. Chem. 1975, 47,
1882. (b) Haller, I. J . Am. Chem. Soc. 1978, 100, 8050.
1
992, 114, 10834.
(20) Liu, C.-J . Ph.D. Thesis, The University of Hong Kong, 1998.

7
366 J . Org. Chem., Vol. 63, No. 21, 1998
Liu et al.
Ta ble 1. Ch a r a cter iza tion Da ta (UV/vis, F T-Ra m a n , a n d XRD) of MCM-41, M-41(m ), a n d Ru /M-41(m )
solid diffuse-reflectance
XRD 2θ(deg), pore sizeg
entry
compd
UV/vis (λmax/nm)
FT-IR (cm-1)
FT-Raman (cm-1
)
d-spacing (Å)
(Å)
1
2
MCM-41
M-41(m)
3410-3100 (br)a
2.25, 39.74
2.25, 39.74
38
36
b
3510-3250 (br),
c
3
000-2780 (br)
e,f
II
409, 529, 558d
410, 531, 560
3
4
[Ru (TDCPP)(CO)(EtOH)]
1948
2251, 2070, 1552, 1490, 1363,
232, 339, 287, 220
2241, 2063, 1552, 1490, 1363, 2.25, 39.74
232, 393, 287, 220
1
8.3 wt % Ru/M-41(m)
1947f
35
1
a
br) broad peak, νOH. b νNH. c νCH2. d In CH2Cl2. e KBr. f νCO. g Determined by N2 adsorption isotherm.
(
Ta ble 2. P r ep a r a tion of th e MCM-41 Su p p or ted
Ta ble 3. Heter ogen eou s Alk en e Ep oxid a tion s Ca ta lyzed
by a MCM-41-Su p p or ted Ru th en iu m P or p h yr in Com p lex
a
Ru th en iu m P or p h yr in Ca ta lysts
wt of M-41(m)
[
Ru]
o
/
used for
[Ru]
e
/
Ru content/
e
V /mL wt %
entry mM
V
o
/mL encapsulation/mg mM
1
2
3
4
0.48
0.29
0.16
20.0
20.0
10.0
100
100
400
400
7.91 100.0
4.76 100.0
8.3
5.0
0.4
0.1
1.52
0.95
25.0
10.0
0.042 10.0
a
Ru]o ) concentration of the stock CH2Cl2 solution of [RuII-
[
(
TDCPP)(CO)(EtOH)] used for immobilization into M-41(m). Vo )
volume of the Ru stock solution used for immobilization into
M-41(m). [Ru]e ) concentration of the Ru complex in CH2Cl2 after
dissolution of 10.0 mg of Ru/M-41(m) into 3% HF. Ve ) final
volume of CH2Cl2 solution used for UV/vis spectrometry.
Reaction conditions: alkene (1 mmol), Cl2pyNO (1.1 mmol), 0.4
II
F igu r e 2. XRD patterns of (a) [Ru (TDCPP)(CO)(EtOH)], (b)
wt % Ru/M-41(m) (0.195 µmol of Ru), HCl (∼0.3 mmol), CH2Cl2 (5
II
a 8:92 (w/w) physical mixture of [Ru (TDCPP)(CO)(EtOH)] and
mL), 40 °C under an Ar atmosphere. a Yields are based on the
MCM-41(m), (c) 8.3 wt % Ru/M-41(m), and (d) MCM-41(m).
amount of substrates consumed; products were identified and
1
quantified by either GLC or H NMR using the internal standard
method unless otherwise stated. b Trace amounts of benzaldehyde
chloride (5 mL) containing a small quantity of hydro-
c
and phenylacetaldehyde were also detected. Trace amount of
chloric acid (0.03 mL of 36% HCl, ∼0.3 mmol). The CH
Cl /HCl mixture was predried by anhydrous sodium
sulfate before use, and the suspension was stirred at 40
C under an argon atmosphere for 18-30 h. The results
for the catalytic oxidation of some representative alkenes
are summarized in Table 3.
2
-
benzaldehyde was detected. d Isolated yield. e 2-Cyclohexen-1-ol
2
f
(
14%) and 2-cyclohexen-1-one (11%) were formed. Glycal (0.5
mmol), Cl2pyNO (0.62 mmol), 0.4 wt % Ru/M-41(m) (0.195 µmol
°
of Ru), HCl (∼0.3 mmol), CH Cl (5 mL), 40 °C under an Ar
2
2
atmosphere. g Overall isolated yield after epoxidation and metha-
h
nolysis; ratio of R:â-epoxides ) 3:1. Turnover frequencies (TOF)
were determined by monitoring the reactions using GC within the
first 2 h of the reactions. n.d.: not determined.
Styrene was oxidized to styrene oxide in 91% yield
(
Table 3, entry 1) under the heterogeneous Ru-catalyzed
conditions, and only traces of undesirable benzaldehyde
CdC bond cleavage) and phenylacetaldehyde (rear-
(
1 mmol), Cl
mg of 0.4 wt % Ru/M-41(m) in CH
2
pyNO (1.1 mmol), HCl (∼0.3 mmol), and 50
Cl at 40 °C, cis-â-
(
2
2
rangement) were detected. Turnovers of 4550 for the
epoxide production were attained in 24 h. In addition,
the oxidation of cis-stilbene (entry 2) and cis-â-methyl-
styrene (entry 3) took place stereospecifically with full
retention of configuration of the starting alkenes, and the
cis-epoxides were formed in >90% yields. The concert-
edness of the reaction was further tested by employing
cis-â-deuteriostyrene (entry 4) as a mechanistic probe.21
Applying the standard reaction conditions, i.e., alkene
deuteriostyrene oxide was produced exclusively in 93%
isolated yield, and yet the trans-epoxide was not detected
1
using H NMR spectroscopy. Oxidation of trans-stilbene
is completely ineffective using this Ru-catalyzed reaction.
The heterogeneous oxidation of cyclohexene (entry 5)
proceeded smoothly to form cyclohexene oxide (61%) as
the major product, though 2-cyclohexen-1-ol (14%) and
-cyclohexen-1-one (11%) were also generated by allylic
C-H oxidation. Bulkier cycloalkenes such as norbornene
entry 6) and cyclooctene (entry 7) can also be effectively
2
(
(
21) (a) Palucki, M.; Pospisil, P. J .; Zhang, W.; J acobsen, E. N. J .
Am. Chem. Soc. 1994, 116, 9333. (b) Fung, W.-H.; Yu, W.-Y.; Che, C.-
M. J . Org. Chem., in press.
converted to exo-epoxynorbornane (94%) and cyclooctene
oxide (91%), respectively, while these two cycloalkenes

Catalyst for Selective Alkene Epoxidations
J . Org. Chem., Vol. 63, No. 21, 1998 7367
Sch em e 2. Ru -Ca ta lyzed (+)-Lim on en e Oxid a tion
Sch em e 3. Ru -Ca ta lyzed Glyca l Oxid a tion sa
are poor substrates for the TS-1-catalyzed epoxidation.7
Terminal alkenes are usually the least reactive, but when
1
2
-octene (entry 8) was treated with Cl pyNO under the
catalytic conditions, 1-octene oxide was obtained in 92%
yield after 24 h of reaction (entry 8).
The oxidation of (+)-limonene, which contains an
internal trisubstituted and an isolated terminal CdC
bond, afforded the 8,9-epoxide as the major product (8,9-/
1
,2-epoxide ) 1.6:1) (entry 9); i.e., the terminal CdC bond
reacts preferentially. It should be noted that the reaction
a
II
with the electron-rich trisubstituted CdC bond prevails
Key: (a) homogeneous conditions: [Ru (TDCPP)(CO)(EtOH)],
Cl2pyNO, HCl/CH2Cl2; (b) heterogeneous conditions: [Ru/M-
41(m)], Cl2pyNO, HCl/CH2Cl2.
when m-chloroperoxybenzoic acid is the oxidant (1,2-/8,9-
epoxide ) 8.9:1).22 Moreover, when (+)-limonene was
II
subjected to a homogeneous condition using [Ru (TDCP-
methyl 3,4,6-tri-O-acetyl-â-D-glucopyranoside (δ
ppm, s, 3H, OCH ) was obtained, and the isomeric
R-glucopyranoside (δ ) 3.44 ppm, s, 3H, OCH ) was not
detected. This indicates that the epoxidation is directed
selectively to the R-face, yet a previous report had already
showed that the dioxirane oxidation of 3,4,6-tri-O-acetyl-
H
) 3.58
P)(CO)(EtOH)] as the catalyst and 1 equiv of Cl
the 1,2-epoxides were also produced predominantly (1,2-/
,9-epoxide ) 3.4:1) (Scheme 2). The apparent selectivity
2
pyNO;
3
H
3
8
reversal is in good agreement with the theory that the
Ru-catalyzed epoxidation reaction should be largely
heterogeneous. Since the oxidation has to take place
within the MCM-41 channel, the hindered trisubstituted
CdC bond should become more inaccessible to the
entrapped active metal centers than the terminal one.
On the other hand, if a reactive oxoruthenium inter-
mediate is assumed, the facial selectivity for the oxidation
of the trisubstituted CdC bond of (+)-limonene in both
the homogeneous and heterogeneous oxidations (respec-
tive cis/trans-1,2-epoxide ratio ) 2.9:1 and 2.7:1) can be
understood on the basis of the “side-on approach” model
proposed by Groves,2 since the facial approach leading
to the trans-1,2-epoxide should be more hindered because
of the steric repulsion between the vinyl groups of the
alkene and the chloro substituents at the meso-phenyl
rings of the ruthenium complex. On a contrary, the
m-chloroperoxybenzoic acid-mediated reaction is featured
by an equimolar production of the cis- and trans-ep-
oxides.22
D-glucal should result in a mixture of R- and â-glycal
epoxides.
23a
Likewise, the benzyl-substituted glucal de-
rivative can also be transformed selectively to its R-glycal
epoxide by adopting the same protocol (Scheme 3). As
opposed to the homogeneous reactions, the Ru/M-41(m)-
catalyzed oxidation of 3,4,6-tri-O-acetyl-D-glucal fur-
nished a 3:1 mixture of R- and â-glycal epoxides based
on H NMR analysis of the methanolysis products (55%
overall isolated yield). It is noteworthy that the larger
,4,6-tri-O-benzyl-D-glucal did not react effectively under
the heterogeneous conditions, and the alkene remained
largely unreacted and was recovered after 3 days of
continuous reaction.
The Ru/M-41(m) catalyst has been reused successively
three times under the typical reaction conditions: styrene
1 mmol), oxidant (1.1 mmol), and HCl (∼0.3 mmol) in
CH Cl (5 mL) at 40 °C for 24 h, except that 100 mg of
.4 wt % Ru/M-41 (m) was used. The solid catalyst was
recycled by filtration after each reaction and washed
thoroughly with methylene chloride before reuse. The
Ru/M-41(m) catalyst still retains ca. 67% of its initial
activity after a total of 11 691 turnovers: first run,
1
3
2a
(
2
2
0
1
,2-Anhydrosugars (glycal epoxides) are important and
versatile intermediates for carbohydrates syntheses; the
compounds were hitherto derived directly from glycal
2
3
using dioxirane as the active oxidant.
3
Treatment of
pyNO (0.62
,4,6-tri-O-acetyl-D-glucal (0.5 mmol) with Cl
2
epoxide yield ) 91%, turnovers ) 4550, turnover rate )
II
mmol), HCl (∼0.3 mmol), and a catalytic quantity of [Ru -
-1
2
4
09 h ; second run, epoxide yield ) 88%, turnovers )
(
TDCPP)(CO)(EtOH)] (1.91 µmol) in methylene chloride
-1
085, turnover rate ) 187 h ; and third run, epoxide
(3 mL) under homogeneous conditions afforded the cor-
-1
yield ) 76%, turnovers ) 3056, turnover rate ) 171 h
.
responding glycal epoxide, and subsequent methanolysis
occurred with an inversion of configuration (77% overall
No induction period was observed for all reactions, and
the turnover rates were obtained by monitoring the
epoxide production rate during the first 2 h of reaction
using gas chromatography. In addition, when sample
was taken from the recovered Ru/M-41(m) after the first
reaction run for analysis of the Ru content; an ap-
proximately 5% loss of the Ru catalyst was registered,
and the decrease of catalytic activity is therefore at-
tributed to leaching and/or catalyst deactivation.
1
isolated yield). On the basis of H NMR analysis, only
(
22) (a) Groves, J . T.; Nemo, T. E. J . Am. Chem. Soc. 1983, 105,
786. (b) Battioni, P.; Renaud, J . P.; Bartoli, J . F.; Reina-Artiles, M.;
Fort, M.; Mansuy, D. J . Am. Chem. Soc. 1988, 110, 8462.
23) (a) Halcomb, R. L.; Danishefsky, S. J . J . Am. Chem. Soc. 1989,
11, 6661. (b) Danishefsky, S. J .; Bilodeau, M. T. Angew. Chem., Int.
5
(
1
Ed. Engl. 1996, 35, 1380. (c) Yang, D.; Wong, M.-K.; Yip, Y.-C. J . Org.
Chem. 1995, 60, 3887.

7
368 J . Org. Chem., Vol. 63, No. 21, 1998
Liu et al.
patterns (catalytic vs stoichiometric reactions) are in-
consistent with a dioxoruthenium(VI) porphyrin inter-
mediate. When Ru/M-41(m) was recovered by filtration
after the catalytic reaction, ESR analysis of the catalyst
sample revealed a signal (g
|
) 2.23, g
⊥
)2.11) of a
paramagnetic species with one unpaired electron, likely
to be a ruthenium(III) complex. Since 2,6-dichloropyri-
dine N-oxide can be regarded as a two-electron oxygen
atom donor, we speculate that an oxoruthenium(V)
species may have been generated as the key intermediate
for this reaction. Note that oxoruthenium(V) species, e.g.,
V
-
[
Ru (O)(N O)]ClO [N O ) [2-hydroxy-2-(2-pyridyl)eth-
4 4 4
28
yl]bis[2-(2-pyridyl)ethyl]amine anion], can be powerful
oxidants capable of oxidizing an unactivated C-H bond
and an oxoruthenium(V) intermediate has also been
proposed by Groves and co-workers in the ruthenium
+
29
F igu r e 3. Hammett correlation studies (log krel vs σ ) for the
2
porphyrin-catalyzed alkane oxidations by Cl pyNO.
IV
Ru/M-41(m)-catalyzed epoxidation of substituted styrenes by
However, the involvement of a reactive Ru -oxoporphy-
Cl
2
pyNO.
30
rin cation radical cannot be excluded.
Ta ble 4. Listin g of log kr el a n d σ+ Va lu es for th e
Heter ogen eou s Ru -Ca ta lyzed Alk en e Ep oxid a tion s
Con clu sion
entry
Y
log krel
σ+
A ruthenium complex of meso-tetrakis(2,6-dichloro-
phenyl)porphyrin, [Ru (TDCPP)(CO)(EtOH)], was im-
1
2
3
4
5
6
4-MeO
4-Me
4-F
0.61
0.20
0.03
0.00
-0.03
-0.48
-0.78
-0.31
-0.05
0.00
+0.11
+0.71
II
mobilized in mesoporous molecular sieves MCM-41; the
supported Ru catalyst could bring about heterogeneous
selective alkene epoxidations using 2,6-dichloropyridine
N-oxide as the oxidant. This catalytic system is also
effective toward norbornene and cyclooctene oxidations,
while these transformations are unsuccessful when the
zeolite-based TS-1-catalyzed conditions are applied. Nev-
ertheless, the MCM-41 supported catalyst does exhibit
size selectivity in the catalytic (+)-limonene oxidation
where the terminal CdC bond (vs the internal trisubsti-
tuted CdC bond) reacts more readily. Bulky 3,4,6-tri-
O-benzyl-D-glucal has failed to react under the hetero-
geneous Ru-catalyzed conditions, whereas the smaller
acetyl derivative is converted to a 3:1 mixture of R- and
â-glycal epoxides. The Ru/M-41(m) can be reused, and
over 11 000 turnovers are easily attained. The loss of
activity after prolonged/repeated usage is attributed to
catalyst leaching and/or deactivation, which demands
further improvement. Irrespective to this, the Ru/M-41-
H
4-Cl
3-NO2
The effect of para and meta substituents on the
catalytic styrene oxidation has also been examined. The
relative rates for the catalytic oxidation of several
substituted styrenes (Y-styrene, Y ) 4-MeO, 4-Me, 4-F,
2
H, 4-Cl, and 3-NO ), krel, were evaluated by monitoring
the reactions with gas chromatography (see Experimental
Section). Figure 3 depicts a linear correlation (R ) 0.997)
of log krel [krel ) k(Y-styrene)/k(styrene)] vs substituent
+
+
constants σ . The slope (F ) of the plot is -0.72, which
+
is much smaller than those (F ) -1.2 to -2.1) reported
2
4
IV
•+
for the styrene oxidations by peracids, [Fe (TMP )O],
V
(
(
(
TMP ) tetramesitylporphyrin),25 [(Br
8
TPP)Cr (O)(X)]
2
6
VI
Br
8
TPP ) octabromotetraphenylporphyrin), and [Ru -
2
2
+
27
N
4
)O
]
4
(N ) macrocyclic tertiary tetramine) com-
plexes (Table 4).
The possible intermediary of a dioxoruthenium(VI)
complex has been assessed by Hammett correlation
2
studies on the stoichiometric [Ru (TDCPP)O ] epoxida-
tion of styrenes. The log krel vs σ plot for the stoichio-
metric reactions has established a nonlinear Hammett
relationship where both electron-donating and -with-
drawing substituents can mildly accelerate the epoxida-
(
2
m)-Cl pyNO system is highly active and yet selective,
simple, and safe to operate. We anticipate that the
present heterogeneous catalytic system would be of
potential application to practical organic synthesis.
VI
+
Exp er im en ta l Section
Ma ter ia ls. All solvents and alkenes substrates were puri-
fied by the standard procedures. Cyclohexene, cyclooctene,
and styrene were first washed with 10% sodium hydroxide and
then distilled over calcium hydride. Norbornene was purified
by sublimation, whereas cis-stilbene was passed through a
column of activated alumina before use. 2,6-Dichloropyridine
N-oxide was prepared by Rousseau’s method.31 Mesoporous
MCM-41 was prepared according to the literature procedures
2
0
tion reactions. Furthermore, the cyclohexene oxidation
VI
by [Ru (TDCPP)O
2
] is in favor of allylic C-H oxidation
rather than epoxidation: 2-cyclohexen-1-one/2-cyclohexen-
-ol/cyclohexene oxide ) 11:33:30. These findings had
also been encountered in other dioxoruthenium(VI) por-
phyrin systems.8 In addition, cis-â-deuteriostyrene re-
acts nonstereospecifically with the dioxoruthenium(VI)
complex to produce both cis- and trans-â-deuteriostyrene
oxide (cis/trans ) 87:13).20 These contrasting reactivity
2
0
1
b
18
by Beck and co-workers. Sodium silicate solution (27 wt %
SiO , 14 wt % NaOH), cetyltrimethylammonium bromide, and
2
(
28) Che, C.-M.; Ho, C.; Lau, T.-C. J . Chem. Soc., Dalton Trans.
(24) Ogata, Y.; Tabushi, I. J . Am. Chem. Soc. 1961, 83, 3440.
(25) Groves, J . T.; Watanabe, Y. J . Am. Chem. Soc. 1986, 108, 507.
(26) Garrison, J . M.; Ostovic, D.; Bruice, T. C. J . Am. Chem. Soc.
1991, 1259.
(29) Groves, J . T.; Bonchio, M.; Carofiglio, T.; Shalyaev, K. J . Am.
Chem. Soc. 1996, 118, 8961.
1
989, 111, 4960.
27) (a) Che, C.-M.; Li, C.-K.; Tang, W.-T.; Yu, W.-Y. J . Chem. Soc.,
(30) (a) Dolphin, D.; J ames, B. R.; Leung, T. Inorg. Chim. Acta 1983,
79, 25. (b) Leung, T.; J ames, B. R.; Dolphin, D. Inorg. Chim. Acta 1983,
79, 180.
(
Dalton Trans. 1992, 3153. (b) Cheng, W.-C.; Yu, W.-Y.; Li, C.-K.; Che,
C.-M. J . Org. Chem. 1995, 60, 6840.
(31) Rousseau, R. J .; Robins, R. K. J . Heterocycl. Chem. 1965, 193.

Catalyst for Selective Alkene Epoxidations
J . Org. Chem., Vol. 63, No. 21, 1998 7369
(3-aminopropyl)triethoxysilane (APTES) were obtained from
temperature, the solid catalyst was filtered, and the organic
Aldrich and used as received. meso-Tetrakis(2,6-dichlorophen-
products were analyzed and quantified by gas-liquid chro-
II
1
yl)porphyrin (H
plex were synthesized by the previously reported procedures.
2
TDCPP) and [Ru (TDCPP)(CO)(EtOH)] com-
matography and/or H NMR spectroscopy. The recovered Ru
3
2
catalyst may be reused two to four times. Controlled experi-
ments using metal free M-41(m) as catalyst under identical
reaction conditions did not afford any epoxides.
P h ysica l Mea su r em en ts. UV/vis data for the MCM-41
samples were obtained from a UV/vis/NIR integrating sphere
attachment, which employs the diffuse-reflectance technique.
X-ray powder diffraction (XRD) studies were performed using
monochromatized Cu KR radiation and recorded over the 2θ
range from 1.5° to 30° in steps of 0.05° with a count time of 1
s at each point. The tube voltage was 40 kV with a 30 mA
current. The pore sizes of the MCM-41 samples were obtained
from the total pore volumes and the BET surfaces assuming
cyclindrical pores using nitrogen adsorption isotherms. Gas
chromatography was performed on a SPB-5 capillary column
Hom ogen eou s Oxid a tion of Glu ca l. A mixture of 3,4,6-
tri-O-acetyl-D-glucal or 3,4,6-tri-O-benzyl-d-glucal (0.5 mmol),
II
Cl
2
pyNO (0.62 mmol), HCl (∼0.3 mmol), and [Ru (TDCPP-
)(CO)(EtOH)] (2 mg, 1.91 µmol) in CH
2
2
Cl (5 mL) was stirred
in a sealed vial under an argon atmosphere at room temper-
ature. The reaction was completed in 24 h as shown by TLC.
The solvent was evaporated under high vacuum to dryness to
yield the 1,2-anhydro sugars as colorless oils. To these oily
compounds were added anhydrous methanol under nitrogen,
and the resulting solutions were stirred at room temperature.
The progress of methanolysis was monitored by TLC. After 1
h, methanol was removed by rotary evaporation, and the crude
products were purified by flash column chromatography.
(30 m) using nitrogen as the carrier gas, a flame ionization
detector, and an integrator.
P r oced u r e for th e P r ep a r a tion of M-41(m ) by Silyla -
tion of MCM-41. MCM-41 (500 mg) was suspended in a
chloroform solution (0.1 M, 50 mL) of (3-aminopropyl)triethox-
ysilane (APTES) at room temperature for 8-12 h with stirring.
The solid product was collected by filtration, washed with
chloroform, and then dried in a vacuum.
2
3a
Meth yl 3,4,6-tr i-O-a cetyl-â-D-glu cop yr a n osid e: white
crystalline solid; yield 60%; R (50% EtOAc in hexane) 0.54;
) δ 5.20 (t, J ) 9.4 Hz, 1H), 5.06 (t,
f
1
H NMR (300 MHz, CDCl
3
J ) 9.7 Hz, 1H), 4.27 (d, J ) 7.8 Hz, 1H), 4.11-4.19 (dd, J )
1
3
2.0, 2.1 Hz, 1H), 3.96-4.05 (dd, J ) 12.1, 6.0 Hz, 1H), 3.67-
Gen er a l P r oced u r e for th e P r ep a r a tion of Ru /M-41-
.75 (m, 1H), 3.58 (s, 3H), 3.50-3.55 (br t, 1H), 2.44 (br s, 1H),
(
m ) Ca ta lyst. MCM-41(m) (400 mg) was suspended with
+
II
1.25 (s, 9H); MS-EI m/z ) 289(M - OMe); HRMS calcd for
C
adequate stirring in a methylene chloride solution of [Ru -
TDCPP)(CO)(EtOH)] (0.17-0.28 mM, 10 mL) at room tem-
13
H
20
O
9
320.290, found 320.288.
(
2
3a
Meth yl 3,4,6-tr i-O-ben zyl-â-D-glu cop yr a n osid e: white
perature for 1 h. The resulting solid product was isolated by
filtration, washed with methylene chloride, and then dried in
a vacuum at room temperature for 1 day. Ru/M-41(m)
catalysts containing 0.1-8.3 wt % of [Ru(TDCPP)(CO)] were
prepared.
1
solid; yield 63%; R
MHz, CDCl
4
1
f
(50% EtOAc in hexane) 0.58; H NMR (300
) δ 7.42-7.10 (m, 15H), 4.98 (t, J ) 8.5 Hz, 1H),
3
.86-4.75 (m, 1H), 4.55-4.73 (m, 2H), 4.30 (d, J ) 7.8 Hz,
H), 3.83-3.62 (m, 2H), 3.49 (s, 3H), 3.55-3.43 (m, 1H).
Deter m in a tion of th e Rela tive Rea ctivities (kr el) for
Deter m in a tion of Ca ta lyst Loa d in g in Ru /M-41(m ).
Ru/M-41(m) (10.0 mg) was digested in a 3% aqueous HF
solution (10 mL). After 10 min of stirring, a clear red solution
resulted, and the ruthenium porphyrin was extracted with
methylene chloride (3 × 15 mL). The organic extracts were
combined, and solvent was removed by rotary evaporation to
dryness. The residue was redissolved in a small amount of
methylene chloride, transferred to a volumetric flask, and then
diluted to 10.00-100.0 mL with methylene chloride. The Ru
concentrations were determined by measuring the absorbance
th e Heter ogen eou s Ru /M-41(m )-Ca ta lyzed Oxid a tion of
Su bstitu ted Styr en es. To a CH Cl solution containing
styrene (1.0 mmol), substituted styrene (1.0 mmol), 1,4-
dichlorobenzene (1.0 mmol as internal standard), and Cl pyNO
0.5 mmol, 80 mg) was added 0.4 wt % Ru/M-41(m) (50 mg,
2
2
2
(
0
.195 µmol of Ru). The suspension was stirred for 12 h, and
the amount of styrenes before and after the reaction was
determined by GLC. The relative reactivities were determined
by the following equation:
of the Soret band (410 nm) with reference to a linear calibra-
krel ) k /k ) log(Y /Y )/log(H /H )
II
Y
H
f
i
f
i
tion curve [0.76-8.6 µM of [Ru (TDCPP)(CO)(EtOH)] in CH
2
2
-
Cl
, R ) 0.999] of absorbance vs concentration.
f i
where Y and Y are the final and initial quantities of the
substituted styrenes and H and H are the final and initial
f i
quantities of styrene.
Gen er a l P r oced u r e for th e Ru /M-41(m )-Ca ta lyzed Alk -
en e Ep oxid a tion s. A mixture of alkene (1.0 mmol), 2,6-
dichloropyridine N-oxide (1.1 mmol, 200 mg), HCl (∼0.3 mmol),
2
and 0.4 wt % [Ru/M-41(m)] (50 mg, 0.195 µmol of Ru) in CH -
Ack n ow led gm en t. We acknowledge support from
The University of Hong Kong and The Hong Kong
Research Grants Council. Dr. Wai-Hong Fung is sin-
cerely thanked for invaluable discussion.
Cl
2
(5 mL) was stirred in a sealed vial for 24 h at 40 °C under
an argon atmosphere. After the mixture was cooled to room
(32) Leung, W.-H.; Che, C.-M.; Yeung, C.-H.; Poon, C.-K. Polyhedron
1
993, 12(19), 2331.
J O981003L