A typical reaction was carried out as follows: To a
2
By nonsupported Pd(OAc) combined with NPMoV, 2 was
8wt%
13wt%
suspended solution of [
mg) were added ethyl acrylate (1) (2 mmol) and CH
6 mg), and the mixture was stirred under dioxygen
Pd(OAc)
2
-
NPMoV/C] (50
obtained in moderate selectivity (71%) at 97% conversion
(entry 7). The benefit in the reaction using heterogeneous
catalytic system is that the catalyst is easily separable by
filtration from the reaction mixture. The acetalization of 1
was examined by use of the catalyst recovered from run 1.
3 3
SO H
(
atmosphere (1 atm) at 50 °C for 20 h (standard conditions).
Products were isolated by column chromatography on silica
gel with hexane/ethyl acetate eluent.
8wt%
13wt%
It was found that the recovered [ Pd(OAc)
2
-
NPMoV]
catalyst was considerably deactivated (entry 8). From the
3 3
The acetalization of 1 in ethanol acidified with CH SO H
or p-toluenesulfonic acid under standard conditions gave
ethyl 3,3-diethoxypropionate in good yield (83%) (Table 1,
ICP analysis, leaching of the metal species (Pd ) 0.01 ×
-
2
-2
1
)
0
mmol (0.6%), Mo ) 0.01 × 10 mmol (0.7%), and V
-2
1.53 × 10 mmol (55%)) from the catalyst was observed.
In particular, a large amount of vanadium ion was found to
be leached into the solution. Thus, from recovered catalyst,
reoxidation to Pd(II) of the Pd(0) reduced during the reaction
was difficult. Hence, when a small amount of NPMoV was
Table 1. Acetalization of Ethyl Acrylate (1) to Ethyl
,3-Diethoxypropionate (2)a
3
8wt%
13wt%
added to the recovered catalyst [ Pd(OAc)
2
-
NPMoV/
C], the acetalization was found to proceed in quantitative
yield (>99%) after 5 h (entry 9). It is interesting to note
that the acetalization of 1 was performed by the use of
8
wt%
[
[
(
2
Pd(OAc) /C] combined with NPMoV in place of the
entry
acid
activated carbon
convn (%) yield (%)
8
wt%
13wt%
Pd(II)-
NPMoV/C] to give 2 in quantitative yield
1
CH3SO3H
Kurare BP-25 (I)
I
I
Darco
Shirasagi
Kurare coal GLC
I
I
I
I
89
15
83
9
entry 10).
2b
3c
4d
5e
6f
7g
8h
9i
p-TsOH
84
56
37
63
97
60
>99
99
84
11
34
54
71
35
>99
99
Table 2 shows the acetalization of 1 by [Pd(OAc)
2
/C] and
[Pd(0)/C] in the presence of NPMoV. 1 was smoothly
acetalized by [
10
CH3SO3H
CH3SO3H
CH3SO3H
CH3SO3H
CH3SO3H
CH3SO3H
CH3SO3H
8wt%
Pd(OAc)
2
/C] and NPMoV in EtOH at 50
°
C for 8 h to form 2 in good yield (92%). The combined
10wt%
catalyst of [
Pd(0)/C] with NPMoV was found to catalyze
efficiently the acetalization of 1 to 2. This fact shows that
Pd(0)/C is readily oxidized to Pd(II)/C by NPMoV under
dioxygen.
0j
1
a
1
(2 mmol) was allowed to react with O2 (1 atm) in the presence of
catalyst (50 mg) and CH3SO3H (6 mg) in EtOH (3 mL) at 50 °C for 20 h.
b
c
In the absence of CH3SO3H. p-Toluenesulfonic acid (11 mg) was used
d
e
instead of CH3SO3H. Darco was used as activated carbon. Shirasagi was
f
used as activated carbon. Kurare coal GLC was used as activated carbon.
Table 2. Recycles of Pd(OAc)
2
/C and Pd(0)/C Combined with
g
Pd(OAc)2 (4.1 mg) and NPMoV (6.3 mg) were used instead of
NPMoV/C]. Recovered catalyst from run 1 was used.
NPMoV (6.3 mg) was added to the recovered catalyst from run 1. The
NPMoV in the Acetalization of 1 to 2a
8
wt%
13wt%
h
[
Pd(OAc)2-
i
[8wt%
[10wt%Pd(0)/C]b
Pd(OAc)2/C]
j
reaction was carried out at 50 °C for 5 h. The reaction was carried out by
using [
8
wt%
Pd(OAc)2/C] (50 mg) and NPMoV (6.3 mg).
no. of recovery convn (%) yield (%) convn (%) yield (%)
0
1
2
3
4
5
92
85
85
86
49
33
92
85
85
86
48
22
>99
>99
>99
92
89
89
95
92
92
92
89
69
entries 1 and 3). However, a very low yield of 2 was obtained
in the absence of CH SO H (entry 2). Similar effects by acids
3
3
have been observed in the NPMoV-catalyzed oxidation of
9
isophorone. Among the activated carbons examined, Kurare
BP-25 (I) was found to give the best results (entries 4-6).
a
1
(3 mmol) was allowed to react with O2 (1 atm) in the presence of
catalyst (50 mg) and NPMoV (6.3 mg) acidified by CH3SO3H (6 mg) in
EtOH (3 mL) at 50 °C for 8 h. b 12 h.
(
7) Activated carbons available from a commercial source: Kurare BP-
2
2
2
2
5 (2420 cm /g), Kurare coal GLC (1570 cm /g), Shirasagi (1200 cm /g),
and Darco (650 cm /g). Molybdovanadophosphate (NPMoV) was prepared
according to the literature procedure. To a solution of NaVO2 (7.32 g, 60
2
8
Table 3 shows the acetalization of various olefins by the
[8wt%Pd(OAc)
/C-NPMoV] system. The reaction of methyl
acrylate and 1 in methanol gave the corresponding acetals
in almost quantitative yields (entries 1 and 2). The acetal-
mmol) in water (38 mL) was added Na2MoO4‚5H2O (18.22 g, 34 mmol)
in water (12 mL). To the resulting solution was added 85% H3PO4 (7.6 g,
2
6
6 mmol) in water (10 mL), and the mixture was heated to 95 °C under
stirring for 1 h. After cooling to 0 °C, a saturated aqueous ammonium
chloride (150 mL) was added to the solution to give NPMoV which was
purified by recrystallization from water and dried in vacuo with heating at
about 90 °C. The preparation of [Pd(OAc)2-NPMoV/C] is as follows: Pd-
(
(
OAc)2 (268 mg) was dissolved in excess acetone and then activated carbon
3 g) was added. After stirring overnight at room temperature, [Pd(OAc)2/
(8) (a) Yokota, T.; Fujibayashi, S.; Nishiyama, Y.; Sakaguchi, S.; Ishii,
Y. J. Mol. Catal. A: Chem. 1996, 114, 113. (b) Yokota, T.; Sakurai, Y.;
Sakaguchi, S.; Ishii, Y. Tetrahedron Lett. 1997, 38, 3923. (c) Sakurai, Y.;
Sakaguchi, S.; Ishii, Y. Tetrahedron Lett. 1999, 40, 1701.
C] was obtained in quantitative yield. To the suspended water of the [Pd-
OAc)2/C] (3.27 g) was added NPMoV (471 mg), and the resulting solution
(
was vigorously stirred for 3 h at room temperature. [Pd(OAc)2-NPMoV/
C] was filtered off, washed with water, and dried in vacuo with heating at
about 90 °C to give [Pd(OAc)2-NPMoV/C] in almost quantitative yield.
(9) Hanyu, A.; Sakurai, Y.; Sakaguchi, S.; Fujibayashi, S.; Ishii, Y.
Tetrahedron Lett. 1997, 38, 5659.
(10) [1
0wt%
Pd(0)/C] catalyst was purchased from N.E.CHEMCAT.
524
Org. Lett., Vol. 2, No. 4, 2000