factor triggering the shield effect. Also as is well known,
hydrocarbons acting as a Brønsted acid (proton donor) are
stabilized by solvation of a basic solvent via hydrogen bonding.12
MeCN, of weak basicity, may stabilize CHE and, hence, suppress
the proton transfer from CHE to OL2 , while keeping OL2 active.
Use of MeCN-d3 (DN = 14.0), with similar DN to MeCN, shows
a similar ESR spectrum (Fig. S6{). These findings indicate that the
‘‘weak’’ basicity of MeCN may be the strong factor triggering the
shield effect. Neither nonpolar hydrocarbon13 nor protic alcohol14
is suitable; therefore, MeCN is the best solvent for this effect. The
mechanism of the MeCN-assisted selective epoxidation can
therefore be summarized in Scheme 1, where route A is preferred
because routes B and C are suppressed by the shield effect.
Photocatalytic epoxidation of various cyclic and linear olefins
on T-S(0.9) using an L/S system also proceeds with excellent
selectivity (> 98%, Table 1),15 whereas the use of T-S(0.9) in a G/S
system and bulk TiO2 in an L/S system shows much lower
selectivity. The epoxide selectivities presented here are the highest
values among those obtained in the photocatalytic systems
reported to date.3–5
‘‘Fundamental Science and Technology of Photofunctional
Interfaces (417)’’ (Nos. 15033244 and 17029037) from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan (MEXT).
Notes and references
{ T-S(x) preparation: a mixture of TEOS (10.4 g) and tetramethylammo-
nium hydroxide (9.1 g) was added slowly to a solution of cetyltrimethy-
lammonium bromide (10.9 g) dissolved in water (32.7 g) with stirring at
278 K and stirred for 0.5 h. Tetrabutyl orthotitanate (0.17 g for x = 0.9;
0.35 for x = 2.2; 0.71 for x = 5.4) dissolved in i-PrOH (40 mL) was added
slowly to the mixture and stirred for 0.5 h. Water (13.7 g) was added and
stirred for 4 h at 358 K. The resulting mixture was transferred into a
stainless steel autoclave and heated at 373 K for 10 days under static
conditions. The solid obtained was recovered by filtration, washed with
water, dried at 333 K for 10 h, and calcined at 813 K for 6 h under air flow,
affording white powders.
1 (a) Oxidations in Organic Chemistry, ed. M. Hudlucky, American
Chemical Society, Washington, DC, 1990; (b) Metal-Catalyzed
Oxidations of Organic Compounds, ed. R. A. Sheldon and J. K.
Kochi, Academic Press, New York, 1981; (c) Encyclopedia of Catalysis,
vol. 3, ed. I. T. Horvath, John Wiley & Sons, Hoboken, 2002.
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Chem. Rev., 2003, 103, 2457; (c) X. Zuwei, Z. Ning, S. Yu and
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A. W. Gal, Angew. Chem., Int. Ed., 2004, 43, 4142.
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(b) T. Tanaka, H. Nojima, H. Yoshida, H. Nakagawa, T. Funabiki and
S. Yoshida, Catal. Today, 1993, 16, 297; (c) C. Murata, H. Yoshida and
T. Hattori, Chem. Commun., 2001, 2412.
4 (a) M. A. Fox and C. Chen, J. Am. Chem. Soc., 1981, 103, 6757; (b)
Y. Kanno, T. Oguchi, H. Sakuragi and K. Tokumaru, Tetrahedron
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J. Catal., 1998, 176, 76; (d) T. Ohno, T. Kigoshi, K. Nakabeya and
M. Matsumura, Chem. Lett., 1998, 877.
Another notable feature of the present photocatalytic system is
the retention of the configuration around the CLC moiety of
olefins in the resulting epoxide. Reactions of cis- and trans-2-
hexene on T-S(0.9) in an L/S system (runs 21, 24) afford
respectively cis- and trans-2,3-epoxyhexane with > 99% selectivity.
Reactions of these olefins on T-S(0.9) without MeCN (runs 22, 25)
yield ca. 40% byproducts formed via route B or C, but do not yield
2
?
isomerized epoxides. This means that O3 is a potential oxidant
for stereoretentive olefin epoxidation, although this interesting fact
has never been reported until now. The results imply that the
MeCN-driven shield effect suppresses the undesirable side
reactions occurring on Ti-O4 and maximizes the inherent
5 (a) C. Murata, H. Yoshida and T. Hattori, Chem. Commun., 1999, 1551;
(b) C. Murata, H. Yoshida, J. Kumagai and T. Hattori, J. Phys. Chem.
B, 2003, 107, 4364.
2
?
stereoretentivity of the O3 radical for olefin epoxidation.
The photostability of MeCN in the present catalytic system
must be checked. Photoirradiation of T-S(0.9) in O2-saturated
MeCN without olefin, in a similar manner to the photocatalytic
reaction, reveals that the quantity of CO2 formed is , 0.1 mmol
(12 h), which is less than 1% of the olefin converted (Table 1,
run 2). In contrast, use of bulk TiO2 gives 2.4 mmol CO2
formation.16 The results indicate that the MeCN stability on
T-S(0.9) is much higher than that on bulk TiO2.
6 K. A. Koyano and T. Tatsumi, Microporous Mater., 1997, 10, 259.
7 (a) S. Bordiga, S. Coluccia, C. Lamberti, L. Marchese, A. Zecchina,
F. Boscherini, F. Buffa, F. Genoni, G. Leofanti, G. Petrini and G. Vlaic,
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M. A. Banares and I. E. Wachs, J. Phys. Chem. B, 1998, 102, 5653.
8 Y. Shiraishi, N. Saito and T. Hirai, J. Am. Chem. Soc., 2005, 127, 8304.
9 TON = (amount of epoxide formed)/(amount of Ti on catalyst).
10 S. Ohnishi and I. Nitta, J. Chem. Phys., 1963, 39, 2848.
11 (a) Non-aqueous Solvents, ed. J. R. Chipperfield, Oxford University
Press Inc., New York, 1999; (b) Y. Misono, L. Limantara, Y. Koyama
and K. Itoh, J. Phys. Chem., 1996, 100, 2422.
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Soc., 1974, 96, 3875; (b) C. V. Krishnan and H. L. Friedman, J. Phys.
Chem., 1971, 75, 3598.
In conclusion, we found that Ti-containing silica with Ti-O4
species catalyzes highly selective photocatalytic olefin epoxidation
with O2 and a simple addition of MeCN. This is achieved via
MeCN-assisted selective suppression of side reactions, the so-
labeled ‘‘shield effect’’. The process has several problems: i) low
olefin conversion (, 12%: Table 1) and ii) requirement of UV light
(, 300 nm) for catalyst activation. The latter suggests that the
process is inapplicable to aryl olefins, which absorb light with
wavelength longer than 300 nm. However, the process shows the
highest epoxide selectivity of aliphatic olefins among those which
have been proposed so far and promotes stereoretentive olefin
epoxidation, where stable MeCN may be reusable for further
reaction. The basic concept presented here will contribute to the
development of a more efficient photocatalytic system for selective
epoxide synthesis in an economically- and environmentally-
friendly way.
13 Photocatalytic CHE oxidation by T-S(0.9) with n-hexane (DN = 0)
shows only , 1% conversion. This is because nonpolar n-hexane
destabilizes the highly polarized [Ti3+–OL2]*. K. Ikeue, H. Yamashita,
M. Anpo and T. Takewaki, J. Phys. Chem. B, 2001, 105, 8350.
14 Photocatalytic CHE oxidation by T-S(0.9) with MeOH (DN = 25)
shows 11% conversion, which is comparable to that obtained with
?
MeCN, but epoxide selectivity is only 39%. This is because CH2OH
radical, formed by proton transfer from MeOH to OL2, causes
nonselective olefin oxidation: O. I. Micic, Y. Zhang, K. R. Cromack,
A. D. Trifunac and M. C. Thurnauer, J. Phys. Chem., 1993, 97, 13284.
15 Much higher epoxide selectivities of these olefins than that of CHE are
attributable to the lower activity of the allylic position of the olefins:
D. E. Van Sickle, F. R. Mayo and R. M. Arluck, J. Am. Chem. Soc.,
1965, 87, 4824.
16 (a) J. Zhuang, C. N. Rusu and J. T. Yates, Jr., J. Phys. Chem. B, 1999,
103, 6957; (b) N. N. Lichtin and M. Avudaithai, Environ. Sci. Technol.,
1996, 30, 2014.
We are grateful for financial support through a Grant-in-Aid
for Scientific Research (No. 15360430) and on Priority Areas
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 5977–5979 | 5979