Keggin-type polyacid clusters on apatite: Characteristic catalytic activities in solvent-free oxidation

Article abstract of DOI:10.1016/S0040-4039(02)02033-6

We found that Keggin-type phosphometalates are effective catalysts for solvent-free oxidation with urea-H₂O₂ by dispersing on fluorapatite solid phase. In the solid phase system the phosphomolybdate (NH₄)₃PMo₁₂O₄₀ was more effective than the phosphotungstate (NH₄)₃PW₁₂O₄₀, whereas the latter was much superior to the former in the liquid-phase reaction with aqueous H₂O₂. In situ formation of novel peroxo-type species from (NH₄)₃PMo₁₂O₄₀/FAp and urea-H₂O₂, which may lead to the high catalytic activity in the solid phase system, was observed by ³¹P solid-state NMR.

Full text of DOI:10.1016/S0040-4039(02)02033-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 8231–8234
Keggin-type polyacid clusters on apatite: characteristic catalytic
activities in solvent-free oxidation
Junko Ichihara,a,* Shunro Yamaguchi,a Takuya Nomoto,a Hirokazu Nakayama,b Katsuma Iteya,c
Nozomu Naitohc and Yoh Sasakic,*
aInstitute of Scientific and Industrial Research, Osaka University, Mihogaoka, Ibaraki, Osaka 567-0047, Japan
bKobe Pharmaceutical University, Motoyama-kitamachi, Higashinada, Kobe, Hyogo 658-8558, Japan
cFaculty of Science and Engineering, Kinki University, Kowakae, Higashiosaka, Osaka 577-8502, Japan
Received 7 August 2002; revised 17 September 2002; accepted 18 September 2002
Abstract—We found that Keggin-type phosphometalates are effective catalysts for solvent-free oxidation with urea–H2O2 by
dispersing on fluorapatite solid phase. In the solid phase system the phosphomolybdate (NH4)3PMo12O40 was more effective than
the phosphotungstate (NH4)3PW12O40, whereas the latter was much superior to the former in the liquid-phase reaction with
aqueous H2O2. In situ formation of novel peroxo-type species from (NH4)3PMo12O40/FAp and urea–H2O2, which may lead to the
high catalytic activity in the solid phase system, was observed by 31P solid-state NMR. © 2002 Published by Elsevier Science Ltd.
A solid phase-assisted reaction system without organic
solvent has been attracted as an environmentally benign
catalyzed organic reaction system.1,2 We have devel-
oped new catalytic reaction systems using apatite as a
solid phase. Apatites, Ca10(PO4)6X2, which form the
mineral component of bones and teeth, are handled as
a harmless solid to environment.3 Importantly, they
have both ion exchange ability and organic compound-
affinity. Recently, we have found that apatites are
effective as a solid disperse phase for assisting tungstic
acid (H2WO4)-catalyzed epoxidation of alkenes and
allylic alcohols with solid urea–H2O2 without solvent.4
In homogeneous or heterogeneous liquid-phase oxida-
tion with aqueous H2O2–organic solvent, it has been so
far recognized that Keggin-type heteropolytungstic
acid, PW12O403−, is the most effective catalyst among a
variety of polyoxometalates, and that its active species
is phosphoperoxotungstate [PW4O8(O2)8]3− (PW4),
formed via degradation by aqueous H2O2.5–12 In the
aqueous H2O2 solution, however, the PW4-catalyzed
epoxidation reaction does not efficiently proceed with-
out organic solvent and a phase transfer catalyst. We
thought that the reaction of the heteropoly acid with
urea–H2O2 on apatite solid phase must proceed in a
different way to form another active species. We now
report that the combinations of solid urea–H2O2 and
Keggin-type heteropoly-tungstates or molybdates on
apatites have their characteristic activities for solvent-
free organic oxidation reactions.
The salts of Keggin type-heteropolyacids (M3PW12O40,
M4SiW12O40, and M3PMo12O40) and the salts of iso-
polyacids (M10H2W12O42, M6Mo7O24) were used as
solid catalysts. Hydroxyapatite (HAp, X=OH) or
fluorapatite (FAp, X=F) was used as a solid disperse
phase.13 The poly-tungstates or molybdates on apatites
were not previously prepared by impregnating in solu-
tion but simply mixed in powders.4 Activities of the
solid catalysts were examined based on the epoxidation
of cyclooctene with urea–H2O2 as follows. A solid
catalyst (0.01 mmol, 1.0 mol%), apatite (1.0 g), and
solid urea–H2O2 (2.5 mmol) were simply mixed, and the
solid mixture was permeated by liquid cyclooctene (1.0
mmol). Then the mixture was left without stirring at
room temperature. The reaction was followed by gas
chromatography using the internal standard method.
The catalytic activities in the epoxidation of
cyclooctene at rt were summarized in Fig. 1. All the
polyacids except for H4SiW12O40 showed positive activ-
ities for the epoxidation in the presence of HAp or
FAp. The catalytic activities in the solid-phase system
were not greatly affected by using either the heteropoly-
or isopoly-acids and either tungstates or molybdates,
and also by using either HAp or FAp solid phase.
Thus, the employed polyacids on apatites were found to
Keywords: epoxidation; phosphomolybdates; fluorapatite; urea–
H2O2; environmentally benign catalyst; peroxo species.
* Corresponding authors. Fax: +81-6-6879-8469; e-mail: ichihara@
sanken.osaka-u.ac.jp; sasaki@meta.kindai.ac.jp
0040-4039/02/$ - see front matter © 2002 Published by Elsevier Science Ltd.
PII: S0040-4039(02)02033-6

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J. Ichihara et al. / Tetrahedron Letters 43 (2002) 8231–8234
Figure 1. Catalytic activities of polyoxometalates on apatites in the solvent-free epoxidation of cyclooctene with urea–H2O2.
The order of catalytic activities in the liquid-phase
system:
catalyze the epoxidation of CꢀC double bond without
solvent, whereas in the absence of apatite the catalytic
reaction proceeded slightly.
3−
3−
4−
PW12O40 ꢀ PMo12O40 > SiW12O40
Among them (NH4)3PMo12O40 on FAp had the highest
activity for the solvent-free reaction. As shown in Table
1, (NH4)3PMo12O40 on FAp catalyzed the solvent-free
oxidations of some alkenes, sulfides and sulfoxides with
urea–H2O2.14 The selective oxidation of aryl methyl
sulfide to the corresponding sulfoxide was accomplished
by using a small excess of urea–H2O2. The reaction of
cyclic alkenes afforded the epoxides, and acyclic cis-alke-
nes were oxidized to give the diols. trans-Alkene slightly
oxidized under the same reaction conditions to form the
epoxide. Our system of phosphomolybdate without
organic solvent and a phase transfer catalyst provided
comparable catalytic activity to the phase-transfer cata-
lyzed system using aq. H2O2–chloroform biphase.15
The higher activity of (NH4)3PMo12O40/FAp than that
of (NH4)3PW12O40/FAp in our reaction system might be
attributed to the formation of a new active species or
stability of the known active species (PM4 type) on
apatite solid phase. In order to search the catalytic active
species, the reactions of (NH4)3PMo12O40/FAp and
(NH4)3PW12O40/FAp with urea–H2O2 were directly fol-
lowed by 31P solid-state NMR. The reaction was carried
out under the following conditions: catalyst/FAp or
CaF2/urea–H2O2=0.10 mmol/0.19 mmol/1.0 mmol) at rt
for 3 days. NMR spectra behavior of catalysts with CaF2
3−
was similar to that with FAp, while the PO4 signal of
FAp overlapped in the latter case. Fig. 2 shows the NMR
spectra in the reaction of (NH4)3PMo12O40/CaF2 by
urea–H2O2. In the case of (NH4)3PMo12O40 new signals
together with the parent one were observed immediately
after mixing the solid powders. NMR spectra after 24 h
(Fig. 2b) clearly demonstrated that (NH4)3PMo12O40
reacted with H2O2 on solid phase to form several new
species. These species were different from PMo4-type
species (l ꢁ7.5 ppm).11 In this case the presence of the
peroxo group (MꢁOꢁOꢁ) was confirmed from ATR
method of FT-IR. In contrast to (NH4)3PMo12O40, in the
case of (NH4)3PW12O40 NMR spectra did not have any
variation for 3 days. These results showed that the
activation of (NH4)3PMo12O40/FAp to the new peroxo
species by urea–H2O2 proceeded faster than that of
(NH4)3PW12O40/FAp. The faster formation of the active
species may lead to the higher catalytic activity of
(NH4)3PMo12O40/FAp in the solid-phase reaction sys-
tem.
Remarkably, the phosphomolybdates, PMo12O403−, were
3−
more effective than the phosphotungstates, PW12O40
,
in the solid phase reaction system. In liquid-phase
reactions, in contrast, it has been reported that the
phosphotungstates are much more effective for oxidation
of organic compounds than the phosphomolybdates.6,8,15
For their active peroxo-type species, it has been reported
by J.-M. Bregeault that the peroxo-type tungsten cata-
lysts (PW4) prepared previously are about 30 times more
active than the molybdenum analog (PMo4) in water–
chloroform biphase.10,11 The catalytic activities orders in
the two reaction systems are different as described below.
The order of catalytic activities in the solid phase system:
4−
PMo12O403−/apatite > PW12O403−/apatite ꢀ SiW12O40
apatite
/

J. Ichihara et al. / Tetrahedron Letters 43 (2002) 8231–8234
8233
Table 1. Solvent-free oxidation using (NH4)3PMo12O40/FApa
Although the activation process of (NH4)3PW12O40/
FAp was not confirmed from solid state NMR,
(NH4)3PW12O40/FAp showed positive activity in
our solid phase system. This might be promoted by
another peroxo-type species without significant
degradation. The further studies on the line are under
way.
In our system, Keggin-type structure of (NH4)3-
PMo12O40 was an effective catalyst for the solvent-free
epoxidation with urea–H2O2 by dispersing on FAp
solid phase. FAp disperse phase can play an important
role to assist the formation of novel active species from
(NH4)3PMo12O40 and the species-catalyzed organic oxi-
dation.

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14. The reactions were carried out under the conditions
described in Table 1. The solid reaction mixture was
extracted with pentane. Conversions were determined by
GC or 1H NMR analysis of the pentane solution. In most
cases, after evaporating the solvent, the mixture of the
starting materials and the described products were
obtained without loss of weight. The yields of the prod-
Figure 2. 31P solid-state NMR spectra of (NH4)3PMo12O40
(a) and the solid mixture of (NH4)3PMo12O40/CaF2/urea–
H2O2 after 24 h (b).
1
ucts were estimated without separation by GC and/or H
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