Angewandte
Chemie
TPA [{WO(O ) } (m-O)] reacts with 0.5 equivalents of an acid
to form a new peroxotungstate, which would catalyze the
epoxidation.
oxidation of triphenylphosphine (1 mmol) with 1 (10 mmol)
gave 81 mmol of triphenylphosphine oxide, thus showing that
2
2 2 2
[
19]
1 has 8 equivalents of active oxygen species. On the other
hand, stoichiometric epoxidation of cyclooctene (1 mmol)
with 1 (20 mmol) at 293 K for 12 h produced 58 mmol of the
corresponding epoxide. All peroxo groups did thus not react
with cyclooctene, in contrast with triphenylphosphine, and
such stoichiometry has also been observed for stoichiometric
A TPA salt of a novel tetranuclear peroxotungstate (1)
was synthesized by the reaction of TPA [{WO(O ) } (m-O)]
2
2 2 2
with HNO . Single crystals of 1 suitable for X-ray structure
3
analysis were successfully obtained from the reaction solution
[
15]
by vapor diffusion of diethyl ether. The molecular structure
of the anionic part of 1 is shown in Figure 2. This part consists
epoxidation of (R)-(+)-limonene with the di- and tetranu-
nÀ
clear peroxotungstates with XO
etc.).
Epoxidation of cyclooctene with H O was carried out
ligands (X = S, As, P,
4
[
7]
2
2
under various conditions (Table S5). The 1-catalyzed epox-
idation efficiently proceeded to give 1,2-epoxycyclooctane in
8
6% yield with only one equivalent of H O with respect to
2 2
À1
cyclooctene. The reaction rate ((1.31 Æ 0.10) mm min ) of
À1
1
was almost the same as that ((1.42 Æ 0.18) mm min ) of
a mixture of TPA [{WO(O ) } (m-O)] and 0.5 equivalents of
2
2 2 2
[20]
HClO4. Other oxidants were not effective for the present
epoxidation. Among the solvents tested, acetonitrile was the
most effective.
The catalytic activity of 1 was compared with those of di-
nÀ
and tetranuclear peroxotungstates with XO4 ligands (X =
Se, S, As, P, and Si) for epoxidation of cyclooctene with H O
2
2
(
Figure S4). The turnover frequency (TOF (determined by
À1
the initial rate), 314 h ) of 1 was much larger than those
À1
(< 0.1–63 h ) of the other peroxotungstates. In order to
Figure 2. Molecular structure of the anionic part of 1. Therman
elliposoids set at 50% probability.
investigate the high catalytic activity of 1, the structural data
were compared. It has been reported that the reaction rates
for di- and tetranuclear peroxotungstates increase as the WÀ
nÀ
O bond lengths in peroxotungstates with the XO4 ligands
2
À
of two [{WO(O ) } (m-O)] units that form the tetranuclear
(XOÀW) increases, and that epoxidation efficiently proceeds
2
2 2
nÀ
tungsten–oxygen cluster. The tungsten atoms have two differ-
using peroxotungstates with weakly coordinated XO
4
2
1
2
[7d]
ently bound peroxo ligands, namely h -O and m-h :h -O . The
ligands (i.e., longer XOÀW bond lengths). Such a push–
2
2
OÀO (1.49–1.54 ꢀ), WÀO(peroxo) (1.90–2.04 ꢀ), WÀOW
pull ligand effect modifies the Lewis acidity of tungsten atoms
(
1.95–2.03 ꢀ), and W=O (1.69–1.76 ꢀ) bond lengths are
and the activity of peroxo ligands. The WÀO bond lengths of
[
5–9]
typical for peroxotungstates.
The existence of three TPA
1 (W2–O22 = 2.03 ꢀ) were longer than those (1.94–2.01 ꢀ) of
the other peroxotungstates (Figure S5), in agreement with the
highest catalytic activity of 1. Thus, the proton in 1 plays an
important role in increasing the Lewis acidity of the tungsten
atoms.
cations per anion implies that the anion charge is À3. The
bond valence sum (BVS) values of tungsten (6.26–6.50) atoms
in 1 indicate the valence of + 6. The distance between O21
and O22 was 2.43 ꢀ, thus suggesting the formation of
[
16]
a hydrogen-bonding network.
The BVS values of O21
The scope of this catalytic system for oxidation of various
kinds of alkenes, sulfides, amines, and a silane was inves-
tigated (Table 1). Cyclic and internal alkenes could selectively
be transformed to the corresponding epoxides in high yields
by using an almost-stoichiometric amount of H O (1.0–
and O22 were 1.75 and 1.54, respectively. These results show
[17]
that one proton is disordered over O21 and O22. The IR
and Raman bands in the ranges 980–960, 860–840, and 580–
À1
5
10 cm are assignable to n(W=O), n(OÀO), and n(W(O )),
2
2
2
respectively, and these band positions are close to those of
peroxotungstates.
1.5 equivalents; Table 1, entries 1–7). For epoxidation of cis-
and trans-2-octenes, the configurations around the C=C
[5–9]
The X-ray, elemental analysis, IR, and
Raman data show that the formula of 1 is TPA [H-
moieties were retained in the corresponding epoxides, thus
suggesting that free-radical intermediates are not involved in
this reaction. Epoxidation of dicyclopentadiene gave the
corresponding diepoxide in 93% yield (Table 1, entry 8).
1-Octene was also efficiently oxidized to the corresponding
epoxide (Table 1, entry 9). Thioanisole and methyl n-octyl
sulfide could selectively be transformed to the corresponding
sulfoxides in high yields (Table 1, entries 10 and 11). Di-n-
butylamine and aniline were oxidized to the corresponding
nitrone and nitroso compounds in 84 and 85% yields,
respectively (Table 1, entries 12 and 13). Triethylsilane was
3
1
{
W O (O ) (m-O)} ]. It has been reported that H NMR
2
2
2
4
2
signals of protons with strong intramolecular OÀH···O hydro-
gen bonds (d(OÀO) = 2.41–2.55 ꢀ) appear in the range 19.0–
[
16,18]
1
4.9 ppm.
Therefore, the signal at 15.8 ppm is assignable
to the proton of the intramolecular OÀH···O hydrogen bond
1
83
(
Figure S2(a) in the Supporting Information). The W NMR
spectrum of 1 in CD CN showed one signal at À566 ppm
3
(
Dn =1.6 Hz; Figure S2(b)), thus suggesting that 1 is a single
1/2
species. These results show that the solid-state structure of the
anionic part of 1 is maintained in solution. Stoichiometric
Angew. Chem. Int. Ed. 2012, 51, 4662 –4665
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Ishimoto, Ryo
