Solvent-free epoxidation using a tungstic acid catalyst on fluoroapatite

Article abstract of DOI:10.1016/S0040-4039(00)02040-2

Tungstic acid dispersed on fluoroapatite solid phase (H₂WO₄/FAp) catalyzed the epoxidation of cycloalkenes and allylic alcohols with a solid urea-hydrogen peroxide complex (urea-H₂O₂) without a solvent.

Full text of DOI:10.1016/S0040-4039(00)02040-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 695–697
Solvent-free epoxidation using a tungstic acid catalyst on
fluoroapatite
Junko Ichihara*
Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
Received 10 October 2000; revised 7 November 2000; accepted 10 November 2000
Abstract—Tungstic acid dispersed on fluoroapatite solid phase (H2WO4/FAp) catalyzed the epoxidation of cycloalkenes and
allylic alcohols with a solid urea–hydrogen peroxide complex (urea–H2O2) without a solvent. © 2001 Elsevier Science Ltd. All
rights reserved.
Recently, a number of novel solid catalysts and solid-
supported catalysts have been developed for basic and
important oxidation reactions of organic compounds.1,2
And use of oxygen gas or aqueous hydrogen peroxide
solution has been desired as the oxygen donor which
has economic and environmental advantages. In most
cases, the reactions are carried out in a heterogeneous
system with liquid phase, and solvent-free reactions
using solid phase have been rarely reported. We have
constructed a new type of solid-phase-assisted epoxida-
tion reaction system using solid tungstic acid (H2WO4)
and a solid urea–hydrogen peroxide complex (urea–
H2O2).3–5 As the appropriate solid phase, we selected
apatites, a biomaterial, because of their ion exchange
ability and affinity for organic compounds. We now
report that the combination of fluoroapatite (FAp) and
the tungstic acid catalyst with solid urea–H2O2 is an
effective reaction system for promoting epoxidation
reaction on solid phase.
smoothly proceeded to afford epoxycyclooctane in 90%
yield. When tungstic acid was used alone, in contrast,
the reaction sluggishly proceeded, as shown in Fig. 1.
Since no reaction occurred with the use of only FAp, it
was found that FAp had no catalytic activity for the
oxidation with urea–H2O2.
The catalytic activity of H2WO4/FAp increased with
increasing the FAp amount from 0.25 g to 1.0 g per 1
mmol of H2WO4, although the amount had an upper
limit. Thus, for enhancing the catalytic activity it is
important that the tungstic acid powder is more widely
dispersed on the FAp solid phase. After mixing
H2WO4/FAp with urea–H2O2 for 1 day at rt, dispersion
of the catalyst on the FAp surface was observed by
scanning electron microscopy.
A heterogeneous epoxidation of cyclooctene using the
tungstic acid catalyst on fluoroapatite (H2WO4/FAp,
0.20 mmol/g, 10 mol%) with urea–H2O2 was carried out
as follows. To a solid mixture of FAp powder (0.50 g)6
with urea–H2O2 powder (0.235 g, 2.50 mmol) was
added tungstic acid powder (0.025 g, 0.10 mmol) in a
test tube with a screw-cap, and mixed sufficiently. The
solid mixture was permeated by a cyclooctene liquid
(0.110 g, 1.0 mmol), and the mixture was left without
stirring at room temperature. After 48 h the reaction
Keywords: epoxidation; tungstic acid; fluoroapatite; solvent-free reac-
tion.
Fig. 1. Tungstate-catalyzed epoxidation of cyclooctene with
urea–H2O2 at rt. (ꢀ) H2WO4/FAP; (ꢁ) H2WO4; (ꢂ) FAp.
* Corresponding author. Fax: +81-6-6879-8469; e-mail: ichihara@
sanken.osakau.ac.jp
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)02040-2

696
J. Ichihara / Tetrahedron Letters 42 (2001) 695–697
Table 1. Solvent-free epoxidation using the H2WO4/FAp catalysta
Yield/%b
62
erythro: threod
Substrate
Time
26 h
Product
O
O
90c
8 h
O
70c
71
4 d
4 h
O
O
O
OH
OH
OH
OH
OH
OH
48d,e
90
30 h
6 h
7 d
8 h
77 : 23
63 : 37
O
78c,f
OH
OH
O
99
OH
OH
O
O
86c,g
74
73 : 23
67 : 33
72 h
30 h
OH
OH
OH
OH
49d,h
3.5 h
O
OH
OH
a
Substrate/H2WO4/FAp/urea·H2O2=0.5 mmol/0.05 mmol/0.25g/1.25 mmol at rt.
b Isolated yield unless otherwise noted.
c G.l.c. yield.
d Determined by 1H NMR.
e The corresponding enone (8%) was also formed.
f Starting materials (22%) remained.
g The corresponding enone (10%) was also formed.
h
The corresponding enone (34%) was also formed.
As well as FAp, such solids as silica gel, calcium
fluoride, porous aluminum fluoride, and hydroxyap-
atite, etc., accelerated the epoxidation reaction. How-
ever, FAp was the most effective as a solid phase
among them. Compared with the other solids, FAp is
considered to assist the tungstic acid-catalyzed reactions
as solid media moderately interacting with both the
inorganic catalyst and alkenes. As expected, the surface
phosphate ion of FAp crystal might be exchanged with
the tungstate ion partially, when the tungstic acid cata-
lyst was activated to peroxo-type by solid-phase reac-
tion with solid urea–H2O2.
The epoxidations of some cycloalkenes and allylic alco-
hols using H2WO4/FAp and urea–H2O2 are summa-
rized in Table 1. The epoxidation of the CꢀC double
bonds of these compounds proceeded on and/or in
solid phase at room temperature to afford the corre-
sponding epoxides in good yields.
Our method using anhydrous hydrogen peroxide was
performed under neutral conditions without a solvent
and, thus, was preferable for the preparation of acid-
sensitive or water-miscible epoxy-compounds consisting
of 4 or 5 carbons. The epoxides were easily isolated via

J. Ichihara / Tetrahedron Letters 42 (2001) 695–697
697
direct molecular distillation under vacuum from the
reaction mixtures or extraction by n-hexane. Since
tungstic acid-catalyzed epoxidations have been so far
carried out with aqueous hydrogen peroxide under
acidic conditions,3,7,8 the formation of ring-opening
products and difficulties of isolation are apt to occur.
An improved method using a buffered solvent has been
reported.3 As regards diastereoselectivity for epoxida-
tions of secondary allyl alcohols the solid-phase reac-
tion showed similar preference but higher selectivity
than those of aqueous liquid-phase reactions.3,7,8
Guerzo, A. D.; Pierard, F.; Mesmaeker, A. K.-D. J. Chem.
Soc., Chem. Commun. 1998, 267–268; Sels, B. F.; De Vos,
D. E.: Jacobs, P. A. Tetrahedron Lett. 1996, 47, 8557–
8560.
3. Tungstic acid is a well-known, conventional catalyst for
H2O2-epoxidation of CꢀC double bonds in aqueous solu-
tion among numerous polyoxometalate catalysts. Prat D.;
Lett, R. Tetrahedron Lett. 1986, 27, 707–710; Prat, D.;
Delpech, B.; Lett, R. Tetrahedron Lett. 1986, 27, 711–714;
Stevens, H. C.; Kaman, A. J. J. Am. Chem. Soc. 1965, 87,
734–737.
4. Urea–H2O2 is crystalline powder prepared by recrystalliza-
tion of urea from aqueous hydrogen peroxide. It was
purchased by Aldrich Chemical Co. and used.
5. In most cases, urea–H2O2 have been used under solid–liq-
uid heterogeneous conditions with an organic solvent; D.
Cosgrove, W, Jones, J. Chem. Soc., Chem. Commun. 1994,
2255–2256; Adam, W.; Mitchell, C. M. Angew. Chem., Int.
Ed. Engl. 1996, 35, 533–535; Adam, W.; Kumar, R.;
Reddy, T. R; Renz, M. Angew. Chem., Int. Ed. Engl. 1996,
35, 880–882.
In conclusion, not by use of solid catalysts which are
rigidly and chemically bonded or impregnated to the
solid surface but by both use of weak interaction of the
catalyst with the solid surface and dispersion by activat-
ing the catalyst, we have succeeded in the tungstic
acid-catalyzed epoxidation on the FAp solid phase
without a solvent under mild conditions. The solvent-
free organic reaction system using solid phase should be
an attractive method for an environmentally friendly-
catalyzed organic reaction system. Further studies
along this line with other tungstates and molybdates
catalysts and the possibilities of the method are in
progress.
6. Fluoroapatite Ca10(PO4)6F2 was presented by Sekisui
Kasei Co., Ltd.
7. Venturello, C.; D’Aloiso, R. J. Org. Chem. 1988, 53,
1553–1557; Venturello, C.; D’Aloiso, R.; Bart, J. C.; Ricci,
M. J. Mol. Catal. 1985, 32, 107–110; Ishii, Y.; Yamawaki,
K.; Yoshida, T.; Ura, T.; Ogawa, M. J. Org. Chem. 1987,
52, 1868–1870; Aubry, C.; Chottard, G.; Platzer, N.;
Bregeault, J.-M.; Thouvenot, R.; Chauveau, F.; Huet, C.;
Ledon, H. Inorg. Chem. 1991, 30, 4409–4415; Dengel, C.;
Griffith, W. P.; Parkin, B. C. J. Chem. Soc., Dalton Trans.
1993, 2683–2688.
8. Prandi, J.; Kagan, H.B.; Mimoun, H. Tetrahedron Lett.
1986, 27, 2617–2620; Campbell, N. J.; Dengel, A. C.;
Edwards, C. J.; Griffith, W. P. J. Chem. Soc., Dalton
Trans. 1989, 1203–1208; Ichihara, J.; Yamaguchi, S. Phos-
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