Pd(II)-catalyzed acetalization of terminal olefins with electron-withdrawing groups in supercritical carbon dioxide: selective control and mechanism

Article abstract of DOI:10.1016/j.tet.2006.08.041

Pd(II)-catalyzed acetalization of terminal olefins with electron-withdrawing groups was carried out smoothly in supercritical carbon dioxide under oxygen atmosphere when polystyrene-supported benzoquinone (PS-BQ) or Cu^II (Cu^I) chloride was employed as cocatalyst. The higher selectivity was achieved, without any chlorinated by-product detected, when using PS-BQ instead of Cu^II (or Cu^I) chloride. PS-BQ could be recycled with excellent catalytic activity remaining after each simple filtration. Chlorine ion was demonstrated to be a promoter. The different acetalization mechanisms were revealed by the subtle relationship of chlorine ion and benzoquinone (BQ) to the catalytic activity of PdCl₂/PS-BQ, Pd^II-CuCl₂ or Pd(OAc)₂/PS-BQ.

Full text of DOI:10.1016/j.tet.2006.08.041

Tetrahedron 62 (2006) 9846–9854
Pd(II)-catalyzed acetalization of terminal olefins with
electron-withdrawing groups in supercritical carbon dioxide:
selective control and mechanism
a
a
Zhao-Yang Wang,a,b Huan-Feng Jiang,a, Xiao-Yue Ouyang, Chao-Rong Qi
*
and Shao-Rong Yanga
aCollege of Chemistry, South China University of Technology, Guangzhou 510640, PR China
bDepartment of Chemistry, South China Normal University, Guangzhou 510006, PR China
Received 5 May 2006; revised 8 August 2006; accepted 11 August 2006
Available online 1 September 2006
Abstract—Pd(II)-catalyzed acetalization of terminal olefins with electron-withdrawing groups was carried out smoothly in supercritical car-
bon dioxide under oxygen atmosphere when polystyrene-supported benzoquinone (PS-BQ) or CuII (CuI) chloride was employed as cocatalyst.
The higher selectivity was achieved, without any chlorinated by-product detected, when using PS-BQ instead of CuII (or CuI) chloride. PS-BQ
could be recycled with excellent catalytic activity remaining after each simple filtration. Chlorine ion was demonstrated to be a promoter. The
different acetalization mechanisms were revealed by the subtle relationship of chlorine ion and benzoquinone (BQ) to the catalytic activity of
PdCl2/PS-BQ, PdII-CuCl2 or Pd(OAc)2/PS-BQ.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
a proton scavenger to prevent the formation of the Michael
adduct product when vinyl ketone (CH2]CH–CO–R) was
used as the substrate (Scheme 1).2a
Since Wacker process was discovered and industrialized,
many further researches and applications were developed.1
When terminal olefins with electron-withdrawing groups,
such as acrylate esters (CH2]CH–COOR), acrylonitrile
(CH2]CH–CN), and methyl vinyl ketone (CH2]CH–
CO–CH3), were treated with alcohols, the reaction afforded
3,3-dialkoxy acetals, which are important intermediates for
organic synthesis.2 For examples, alkyl 3,3-dialkoxy-propa-
noates, b-ketoacetals, and b-cyanoacetal were used for the
synthesis of coumarins, porphyrins, spermine metabolites,
loganin,3 and other practical drugs or dyes.4 Among the
methods to prepare these acetals, the simplest one was the
PdII salts catalyzed reaction of terminal olefins with elec-
tron-withdrawing groups and alcohols.
CH3OH
CH3OOC
OMe
OMe
CH3OOC
1
1
PdII- CuI- O2
HMPA, 92%
2
3
2
3
RCO
RCO
RCO
R'OH
PdII- CuI- O2
OR'
OR'
+
OR'
Scheme 1.
In 1999, we first reported our investigation on the PdII-
catalyzed acetalization of terminal olefins with electron-
withdrawing groups in supercritical carbon dioxide (scCO2),
and found that when the reaction solvent was replaced with
scCO2, the accelerator HMPA was not necessary.5 The fur-
ther experimental results showed that polystyrene-supported
benzoquinone (PS-BQ), its structure, and synthetic route
(Scheme 2), was a successful substitute for the cocatalyst
CuCl2 (or CuCl).6
In order to improve the yield and selectivity, additives were
added in the reaction. For example, hexamethylphosphoric
triamide (HMPA) was an effective accelerator in the acetal-
ization of methyl acrylate with methanol (Scheme 1),2b but
HMPA is expensive, toxic, and harmful to environment.
Other additives, i.e., triethyl amine (NEt3) or K2CO3, espe-
cially dibasic sodium phosphate (Na2HPO4), were used as
It is well-known that organic reactions in scCO2 is one of
the important and attractive subjects of research in green
chemistry.7 First, carbon dioxide (CO2) is inexpensive,
non-flammable, nontoxic, and chemically inert under
many conditions. Secondly, scCO2 is not only a substitute
Keywords: Acetalization; Benzoquinone; Palladium; Supercritical carbon
dioxide; Wacker oxidation.
* Corresponding author. Tel.: +8620 22236518; fax: +8620 87112906;
e-mail: jianghf@scut.edu.cn
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.08.041

Z.-Y. Wang et al. / Tetrahedron 62 (2006) 9846–9854
9847
O
O
CH2CH
CH2CH
OH
n
n CH3OCH2Cl
hydroquinone
ZnCl2
H2O2
P
CH2
CH2
P
(
Cl )
CH2
P
CH2Cl
chloro-ball
OH
main sketch of PS
(white-ball)
benzoquinone-ball
(PS-BQ)
hydroquinone-ball
Scheme 2.
for volatile and toxic organic solvents, but also possesses
some special properties. For examples, its hybrid property
of both liquid and gas is very advantageous to some reactions
involving gaseous reagents; controlling of the CO2 density
by the variation of the temperature and pressure enables
the solvent properties to be ‘tuned’ to reagents; separating
of CO2 from the reaction mixture is energy-efficient and
simple.8 Although PdII-catalyzed acetalization5,6,9 and other
reactions took place in scCO2,7,10 it is a puzzle how to effec-
tively control such a complicated reaction system.
olefins with electron-withdrawing groups in scCO2 are
summarized as the following:
(1) Sufficient high system pressure made the reaction go
smoothly at room temperature (20–27 ꢀC) with satisfac-
tory results.5,6
(2) The reaction time could be shortened from 24 h to 6 h
under higher total system pressure, and the excellent yield
and reaction selectivity were obtained (Table 1, entry 2).
(3) It is possible to reduce the amount of methanol to
accomplish the present reaction (Table 1, entry 2).
(4) The suitable oxygen pressurewas 0.5 MPa. Once oxygen
pressure was below 0.5 MPa, the conversion, yield, and
selectivity were declined, even under longer reaction
time and higher system pressure (Table 1, entries 3–5).
In this work, we further investigate PdII-catalyzed acetaliza-
tion of terminal olefins with electron-withdrawing groups in
scCO2. Our purposes here are to present detailed knowledge
of catalysts and cocatalysts, reaction rate, and selectivity,
solvent effects and to provide theoretical explanations about
the selectivity control and the reaction mechanism based on
the systematic comparison. We wish to report the conclusive
discussion in this paper.11
2.2. The influences of different cocatalysts on the
PdII-catalyzed acetalization of methyl acrylate
It has been reported that, as cocatalyst in the acetalization of
methyl acrylate 1a, CuCl2 was inferior to CuCl for its liabil-
ity to produce chlorinated by-product 5a.2a,12 When the
reaction was carried out in scCO2, CuCl2 gave similar or
better results than CuCl did,5 and 5a was usually produced
with yield from 0.3% to 1.6%.
2. Results and discussion
2.1. The conditions for PdII-catalyzed acetalization
of terminal olefins with electron-withdrawing groups
in scCO2
When polymeric cocatalyst PS-BQ was employed, the result
is satisfactory as expected without any chlorinated by-prod-
ucts detected (Table 1, entries 1–5). At the same time, the
yield and selectivity of the major product were almost the
same as that in CuCl2 (or CuCl).6 And the best catalyst
dosage is leading catalyst PdCl2 0.15 mmol (3 mol % of
methyl acrylate 1a) and PS-BQ 2 mmol (Table 1, entry 1).
Based on previous reports,5,6 the conditions for PdCl2/PS-
BQ-catalyzed acetalization of methyl acrylate 1a in scCO2,
especially the effect of oxygen pressure, were further inves-
tigated in Table 1. Using methyl acrylate 1a as the typical
substrate, and methanol as the typical alcohol, the appropri-
ate conditions for PdII-catalyzed acetalization of terminal
a
Table 1. PdCl2-catalyzed acetalization of 1a with MeOH using PS-BQ as cocatalyst in scCO2
CH3OH, O2
CH3OOC
CH3OOC
OMe
OMe
CH3OOC
CH3OOC
CH3OOC
+
+
+
PdII, cocatalyst
OMe
Cl
OMe
1a
2a
3a
4a
5a
Entry
Cocatalyst
(mmol)
Temp
(ꢀC)
T (h)
Pb
(MPa)
Mol ratioc
Conv.
(%)
Yieldd (%)
Select. for
Yield of
2ae (%)
5af (%)
2a
3a
4a
1g
2
3
4
PS-BQ (2)
PS-BQ (2)
PS-BQ (2)
PS-BQ (2)
PS-BQ (2)
50
50
50
50
50
24
6
24
47
24
0.5/11
0.5/15
0.4/12
0.2/14
0.02/10
4.94:1
3.50:1
4.94:1
4.94:1
4.94:1
42.7
100
99.5
98.5
49.5
14.4
96.8
65.5
82.6
35.4
0
0
0
0
0
2.1
0
14.1
0
87.1
100
82.3
100
90.4
0
0
0
0
0
5
5.0
a
Reaction conditions: 5 mmol methyl acrylate 1a and 0.15 mmol (3 mol %) PdCl2.
O2 pressure/total system pressure.
Methanol/methyl acrylate.
b
c
d
e
f
By GC spectrum.
Selectivity¼[2a/(2a+3a+4a)]ꢁ100.
When CuCl2 (or CuCl) was used as cocatalyst, the chlorinated adduct 5a usually was obtained with yield from 0.3% to 1.6%.
PdCl2 0.5 mol %.
g

9848
Z.-Y. Wang et al. / Tetrahedron 62 (2006) 9846–9854
a
Table 2. Recyclability of PS-BQ in PdCl2-catalyzed acetalization of 1a with MeOH in scCO2
CH3OOC
CH3OOC
CH3OH, O2
PdII, PS-BQ
CH3OOC
CH3OOC
COOCH3
COOCH3
OMe
OMe
CH3OOC
+
+
+
OMe
OMe
1a
2a
3a
4a
6
Entry
Recycle times
T (h)
Pb
(MPa)
Mol ratioc
Conv.
(%)
Yieldd (%)
Select. for
Yield of
2ae (%)
6d (%)
2a
3a
4a
1
2
3
4
5
Fresh
24
24
24
24
24
0.5/10
0.5/13
0.5/12
0.5/14
0.5/18
4.94:1
4.94:1
4.94:1
4.94:1
4.94:1
99.8
100
99.8
100
100
82.8
88.7
84.3
80.3
76.7
1.3
0
0
0
0
4.8
7.8
7.0
8.4
1.7
93.1
91.9
92.3
90.5
97.8
3.5
1.3
6.9
11.2
21.6
1
2
3
4
a
Reaction conditions: 5 mmol methyl acrylate 1a, 0.15 mmol (3 mol %) PdCl2, and 2 mmol PS-BQ. The reaction was carried out at 50 ꢀC.
O2 pressure/system pressure.
Methanol/methyl acrylate.
By GC spectrum.
b
c
d
e
Selectivity¼[2a/(2a+3a+4a)]ꢁ100.
When PS-BQ was used as cocatalyst, its reusability was
trimerization of ethyl propiolate in scCO210i or organic sol-
vent.15 Acrylate ester is cheaper and more abundant than
propiolate ester. Undoubtedly, it is very valuable to synthe-
size 6 via the trimerization of methyl acrylate 2a and to
make the reaction mechanism clear. The further research is
still in progress.
investigated as shown in Table 2. In each cycle, PS-BQ
was separated easily through simple filtration, then washed
with acetone, and dried in vacuo before use for the next
experiment. After several runs, the selectivity for 2a was still
excellent though the yield of 2a decreased slightly (Table 2,
entries 2–5). Therefore, it is obvious that PS-BQ can be
recycled after simple filtration with excellent recovery, as
well as excellent catalytic activity.
2.3. The influences of different PdII catalysts
on the acetalization of methyl acrylate
To our surprise, trimethyl benzenetricarboxylate 6 was
detected when PS-BQ was used as cocatalyst (Table 2,
entry 1). It is very interesting that the yield of 6 increased
with the cycle times of PS-BQ. As one of the useful multi-
functional methyl benzoate derivatives,13 6 was usually pre-
pared from propiolate esters,14 including the PdII-catalyzed
PdCl2-catalyzed acetalization of methyl acrylate 1a with
methanol was slightly faster than that of PdCl2(MeCN)2
when copper salt was employed as cocatalyst.5 In order to
eradicate the corrosion of reactor by chlorine ion,
Pd(OAc)2/PS-BQ catalytic system was further investigated,
and the results are shown in Table 3.
a
Table 3. Acetalization of 1a with MeOH using Pd(OAc)2 as catalyst in scCO2 or compressed CO2
Entry
Catalyst
(mol %)
Temp
(ꢀC)
T (h)
Pb
(MPa)
Mol ratioc
Conv.
(%)
Yieldd (%)
Select. for
Yield of
2ae (%)
5a/6d (%)
2a
3a
4a
1
2
3
4
5
6
7
8
PdCl2 (3)
50
50
50
50
50
50
50
50
50
50
50
50
40
50
50
50
rti
6
24
24
24
24
24
22
24
24
24
24
24
24
12
12
6
0.5/15
0.2/13
0.4/15
0.4/3
0.5/13
0.5/11
0.5/6
0.5/11
0.5/3
0.5/15
0.5/8
0.5/6
0.5/14
0.4/15
0.5/10
0.4/11
0.5/6
3.50:1
4.94:1
4.94:1
4.94:1
4.94:1
2.47:1
9.88:1
4.94:1
4.94:1
4.94:1
4.94:1
4.94:1
4.94:1
4.94:1
4.94:1
4.94:1
4.94:1
100
96.8
77.4
62.4
76.9
78.2
71.1
54.4
71.5
74.3
41.4
68.0
76.9
69.0
72.6
80.1
75.2
73.3
0
0
1.1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
100
0/0
0/0
0/0
0/0
0/0
0/0
0/0
0/0
0/0
0/37.8h
0/0
0/0
0/0
0/0
0/0
0/0
0/0
Pd(OAc)2 (3)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
Pd(OAc)2 (4)f
Pd(OAc)2 (3)
Pd(OAc)2 (2)
Pd(OAc)2 (4.5)
Pd(OAc)2 (3)g
Pd(OAc)2 (3)f
Pd(OAc)2 (4)f
Pd(OAc)2 (3)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
98.2
98.5
98.5
99.4
95.1
94.6
96.2
98.5
91.4
93.7
98.7
97.7
88.7
88.8
83.1
83.3
10.6
22.9
2.1
15.0
13.9
29.7
23.5
20.0
0
16.5
12.1
25.4
9.7
6.9
3.2
88.0
72.2
97.3
83.9
83.6
64.7
75.3
78.8
100
80.5
86.4
73.1
88.2
92.1
95.9
94.9
9
10
11
12
13
14
15
16
17
6
3.9
a
Reaction conditions: 5 mmol methyl acrylate 1a and 2 mmol (40 mol %) PS-BQ.
O2 pressure/total system pressure.
Methanol/methyl acrylate.
b
c
d
e
f
By GC spectrum.
Selectivity¼[2a/(2a+3a+4a)]ꢁ100.
PS-BQ 2.67 mmol (53.3 mol %).
PS-BQ 1.33 mmol (26.7 mol %).
g
h
i
By-product 6 was seldom obtained in other case.
Room temperature 20–27 ꢀC.

Z.-Y. Wang et al. / Tetrahedron 62 (2006) 9846–9854
9849
No adduct 5a was detected in Pd(OAc)2/PS-BQ catalytic
system because of the absence of chlorine ion, but the cata-
lytic activity was obviously lower than that of PdCl2/PS-BQ
catalytic system (Table 3, entry 1). In order to raise the yield
of 2a, the conditions for Pd(OAc)2/PS-BQ-catalyzed acetal-
ization of 1a with MeOH were optimized.
though the conversion of methyl acrylate 1a was slightly
lowered (Table 3, entries 14–17).
2.4. Influences of different PdII catalyst system on
acetalization of acrylate esters and acrylonitrile
By using different cocatalysts, the PdII-catalyzed acetaliza-
tion of different types of substrates with methanol or ethanol
was investigated as shown in Table 4.
Higher oxygen pressure was favor to the reaction catalyzed
by Pd(OAc)2/PS-BQ (Table 3, entries 2–5). As Pd(OAc)2
had good solubility in methanol, the system pressure showed
no remarkable effect on the reaction.
It is obvious that, in different PdII catalyst system, all of the
acrylate esters and acrylonitrile gave the expected acetal 2
as major product, and the chlorinated by-product 5 was pro-
duced only when copper salt was used as cocatalyst (Table 4,
entries 1, 3, 4, and 10). Although substrates were different,
the yield and selectivity of 2 usually had a corresponding
increment in most case when PS-BQ was employed as
cocatalyst instead of CuCl2 (CuCl).
Excess methanol might promote the dissolution of Pd(OAc)2
in scCO2 more effectively than that of PdCl2, and the
decrease of methanol amount slightly reduced the yield and
selectivity of 2a (Table 3, entry 6). However, higher molar
ratio of methanol was disadvantageous to the reaction
(Table 3, entry 7).
The good catalyst loading for Pd(OAc)2 was 3 mol % of sub-
strate methyl acrylate 1a (Table 3, entries 6, 8, and 9), and for
PS-BQ was 40 mol % (Table 3, entries 6, 10, and 11). When
both of the loading of Pd(OAc)2 and PS-BQ were increased
by 33%, the yield and selectivity of 2a were slightly raised,
but the best yield was still less than 80% (Table 3, entry 12).
The conversion, yield, and selectivity of acetalization were
affected by the hindrance of alkyl group (R0 and R) in
CH2]CH–COOR0 (acrylate esters, 1a, 1b, and 1c) and
ROH, when either PS-BQ6 or copper salt was used as cocat-
alyst. Because of the coordination of CN with PdII,2a higher
oxygen pressure and temperature were necessary for the
acetalization of acrylonitrile 1d (Table 4, entry 10).
The appropriate reaction temperature was 50 ꢀC, and the
lower temperature was slightly disadvantageous to the reac-
tion (Table 3, entry 13).
According to the classical Wacker oxidation mechanism, it
is inevitable to produce acidic substance (e.g., HCl) in the re-
action,2a,16 and the transesterification between alcohol and
acrylate ester may occur subsequently in acidic condition.
In order to avoid the transesterification, the acetalization of
Shortening the reaction time was feasible because the varia-
tion of the yield and selectivity of 2a was not remarkable,
a
Table 4. PdII-catalyzed acetalizations of acrylate esters and acrylonitrile with alcohols using different cocatalyst in scCO2
ROH, O2
X
X
X
X
OR
OR
X
+
+
+
PdII, cocatalyst
Cl
OR
OR
1
2
3
4
5
2c, X = COOCH2CH2CH2CH3
R = Me
2c', X =COOCH2CH2CH2CH3
R = Et
1a, X = COOMe
1b, X = COOEt
1c, X = COOCH2CH2CH2CH3
1d, X = CN
2a, X = COOMe, R = Me
2d, X = CN, R = Me
2b, X = COOEt, R = Me
2b', X = COOEt, R = Et
Entry
Cocatalyst
(mmol)
Pb
(MPa)
Mol ratio of substrate
to alcohol
Conv.
(%)
Yieldc (%)
Select. for
Yieldc of
5 (%)
2d (%)
2
3
4
1e
2
CuCl2 (4)
PS-BQ (2)
CuCl2 (4)
CuCl (4)
PS-BQ (2)
PS-BQ (2)
PS-BQ (2)
PS-BQ (2)
PS-BQ (2)
CuCl2 (4)
PS-BQ (2)
0.5/16
0.5/10
1.0/12
1.0/13
0.5/8
4.94:1 (Methanol to 1a)
4.94:1 (Methanol to 1a)
3.44:1 (Ethanol to 1b)
3.44:1 (Ethanol to 1b)
3.44:1 (Ethanol to 1b)
4.94:1 (Methanol to 1b)
4.94:1 (Methanol to 1b)
4.94:1 (Methanol to 1c)
3.44:1 (Ethanol to 1c)
4.94:1 (Methanol to 1d)
4.94:1 (Methanol to 1d)
99.8
99.8
89.0
90.0
100
99.8
100
78.9
28.8
99.8
88.1
1.7
0.5
4.8
5.6
0
0
0
0
4.0
3.1
94.0
96.4
79.3
86.7
82.0
88.1
90.3
98.6
87.9
88.8
100
1.6
0
1.8
1.4
0
0
0
0
0
95.5
68.0
75.8
77.6
74.2
84.5
77.8
23.9
87.5
79.1
3f
4g
5
13.0
6.0
17.0
10.0
9.1
1.1
2.9
0.3
0
6h
7
0.5/9
0.5/10
0.5/8
0.5/8
1.0/13
0.5/9
8
9
0.4
10.7
0
10i
11
0.5
0
100
a
Reaction conditions: 5 mmol substrate 1 and 0.15 mmol (3 mol %) PdCl2. The reaction was carried out at 50 ꢀC for 12 h.
O2 pressure/total system pressure.
By GC spectrum.
b
c
d
e
f
Selectivity¼[2/(2+3+4)]ꢁ100.
Temperature 40 ꢀC, and time 24 h.
Temperature 40 ꢀC.
g
h
i
PdCl2(MeCN)2 3 mol %, and temperature 40 ꢀC.
Time 11 h.
Temperature 57 ꢀC.

9850
Z.-Y. Wang et al. / Tetrahedron 62 (2006) 9846–9854
methyl acrylate 1a was usually with methanol, and ethanol
to ethyl acrylate 1b, especially when CuCl2 was used as
cocatalyst.2a,12
(Table 5, entries 1–3), the use of CuCl instead of CuCl2
(Table 5, entries 2, 4, and 5), and the addition of Na2HPO4
as a proton scavenger according to the literature2 (Table 5,
entries 4 and 5). The maximum of the total yield was 72.1%.
But in our experiments, when PS-BQ was used as cocatalyst
in scCO2, the transesterification did not occur in the acetal-
ization of ethyl acrylate 1b with methanol, and ethyl 3,3-
dimethoxypropanoate (2b, X¼COOC2H5, R¼CH3) was
detected as the sole acetal product (Table 4, entries 6 and
7). It indicated that the prevention of transesterification
was attributed to the replacement of CuCl2 with PS-BQ.6
This interesting phenomenon strongly suggests that some
transformations in PdII/PS-BQ-catalyzed acetalization
might be different from that in PdII–CuCl2 catalytic system.
When PS-BQ was employed as cocatalyst instead of copper
salt, the PdII-catalyzed acetalization proceeded smoothly at
mild conditions, and the yield of anticipant acetal products
was 80.0% in absence of Na2HPO4 (Table 5, entries 5 and
6). Therefore, using of PS-BQ as cocatalyst was the best one
to increase the acetal yield in the acetalization of acrolein 7.
When the acetalization of methyl vinyl ketone (CH2]CH–
CO–CH3, 12) with methanol was catalyzed by PdCl2/copper
salt, there were many methods to improve the selectivity of
acetal 13,9 including replacement of CuCl2 with CuCl (the
suitable amount of CuCl was 3 mmol), adding Na2HPO4
as a proton scavenger,2 raising O2 pressure and CO2 pres-
sure. When the conditions were optimal, the maximum
selectivity for 13 was 86.7% (Table 6, entry 1), which
was 1.6% higher only than that catalyzed by PdCl2/PS-BQ
catalytic system6 (Table 6, entry 2).
2.5. Application of different PdII catalyst system
in the acetalization of other substrates
Investigation of PdII-catalyzed acetalization of other sub-
strates showed that acrylic acid (CH2]CH–COOH) and
acrylamide (CH2]CH–CONH2) did not give the desired
products whether PS-BQ6 or CuCl2 (CuCl) was used as
cocatalyst, and the reason is not clear yet.
In traditional methods, basic additives, such as NEt3 or
K2CO3, or Na2HPO4, were used to keep the non-anticipant
Michael adduct minimum.2a,12 Even in scCO2, the basic
additives including Na2HPO4 or OH type anion resin were
needed to improve the selectivity of the anticipant acetal.9
However, in PS-BQ/PdII catalytic system, good selectivity
could be achieved in the acetalization of methyl vinyl ketone
12 without the addition of any additives (Table 6). This indi-
cates that different cocatalysts in PdII-catalyzed acetaliza-
tion of terminal olefins with electron-withdrawing groups
go through different mechanisms.
For methyl methacrylate [CH2]C(CH3)–COOCH3], as
Hosokawa’s report, though the steric hindrance, its PdII-
catalyzed acetalization with 2,4-pentanediol still produced
acetal with a lower yield (25%).2a However, the acetalization
of methyl methacrylate with methanol did not give the ex-
pected acetal in our reaction system, even when the reaction
temperature applied was as high as that of acrylonitrile 1d.
This difference may have a touch with the alcohol. 2,4-Pen-
tanediol yielded a cyclic acetal, which was more stable than
that produced by methanol.
Acrolein (CH2]CH–CHO, 7) was easily acetalized when
PS-BQ6 or CuCl2 (CuCl) was used as cocatalyst. After the
substrate was totally converted, the acetalization of acrolein
gave a mixture of anticipant acetal products (8 and 10) and
Michael addition products (9 and 11) in different ratios,
and the result is summarized in Table 5.
2.6. Mechanism of the PdII-catalyzed acetalization
with different cocatalyst in scCO2
Based on the above results, the mechanism of the PdII-cata-
lyzed acetalization using copper salt as cocatalyst in scCO2
is suggested in Scheme 3.
There were three methods to improve the ratio of acetals (the
sum of the yield of 8, 10, and 11) when copper salt was used
as cocatalyst, including the increase of oxygen pressure
In this mechanism, the key step is the production of inter-
mediate 4a,2a,16 which was frequently detected in our exper-
iments. At the same time, by-product 5a was often found
a
Table 5. PdII-catalyzed acetalization of acrolein with MeOH using different cocatalyst in scCO2
OMe
OMe
OMe
MeO
MeO
CH3OH, O2
CHO
CHO
CHO
+
+
+
PdII
cocatalyst
MeO
9
OMe
MeO
MeO
MeO
11
7
8
10
Entry
Cocatalyst (mmol)
Time (h)
Pb (MPa)
Yieldc (%)
8
9
10
11
8+10+11
1
2
3
4
5
6
CuCl2 (0.6)
CuCl2 (0.6)
CuCl2 (0.6)
CuCl (0.6)
CuCl (0.6)d
PS-BQ (2)
24
24
24
24
24
12
0.2/12
0.5/12
1.0/12
0.5/12
0.5/12
0.5/9
0
62.5
56.8
26.3
34.2
26.2
3.9
2.1
3.2
6.7
3.4
17.5
46.7
12.7
24.4
48.9
42.7
54.6
12.9
14.8
27.6
55.6
46.1
72.1
80.0
0
0
0
0
20.4
a
Reaction conditions: 5 mmol acrolein 7, 0.15 mmol (3 mol %) PdCl2, and 24.7 mmol methanol. The reaction was carried out at 50 ꢀC.
O2 pressure/total system pressure.
By GC spectrum.
Na2HPO4 0.3 mmol as additive.
b
c
d

Z.-Y. Wang et al. / Tetrahedron 62 (2006) 9846–9854
9851
a
Table 6. PdCl2-catalyzed acetalization of methyl vinyl ketone 12 with MeOH using different cocatalyst in scCO2
MeO
CH3OH, O2
COCH3
COCH3
COCH3
+
PdCl2
cocatalyst
MeO
MeO
12
13
14
Entry
Cocatalyst
(mmol)
Additive
(mmol)
Pb
(MPa)
Conv.
(%)
Yieldc (%)
Select. for
13d
13
14
1e
2f
CuCl (3)
PS-BQ (2)
Na2HPO4 (0.5)
No addition
0.5/12
0.5/8
100
100
83.2
76.8
12.8
13.4
86.7
85.1
a
Reaction conditions: 5 mmol methyl vinyl ketone 12 and 50 ꢀC, 12 h.
O2 pressure/total system pressure.
By GC spectrum.
b
c
d
e
f
Selectivity¼[13/(13+14)]ꢁ100.
PdCl2 0.5 mmol (10 mol %), and mol ratio of methanol to 12 was 9.88:1.
PdCl2 0.15 mmol (3 mol %), and mol ratio of methanol to 12 was 4.94:1.
CH3OOC
PdCl2
CH3OH
CH3OOC
5a
Cl
CH3OOC
1a
OMe
HCl
HCl
PdCl2
CH3OOC
O2
3a
CuCl
CH3OOC
Cl-Pd
PdCl2
OMe
CH3OOC
OMe
H-Pd-Cl
CH3OOC
PdCl2
H2O
Pd
CuCl2
H-Pd-Cl
HCl
CH3OOC
OMe
CH3OH
HCl
OMe
4a
PdCl2
CH3OOC
2a
CH3OOC
Cl-Pd
OMe
OMe
OMe
OMe
HCl
Scheme 3.
when CuCl2 or CuCl was used as cocatalyst because of the
production of HCl.
of PS-BQ was different from that of PdII–CuCl2 system too
(Scheme 5). In the absence of chlorine ion, the catalytic
activity of Pd(OAc)2/PS-BQ was lower than that of PdCl2/
PS-BQ catalytic system. These explanation were further
demonstrated by the influences of additives on Pd(OAc)2/
PS-BQ-catalyzed acetalization (Table 7).
WhenPS-BQwasusedascocatalyst,thebasiccyclewassimi-
lar to the PdII–CuCl2 system, but benzoquinone (BQ) might
absorb HCl via Michael addition17 in situ, and was converted
to chlorohydroquinone (CHQ). CHQ took part in hydroge-
nolysis to give PdCl2 and products, and then regenerated
BQ (Scheme 4). Therefore, by-product 5a was not detected.
OH
P
Pd(OAc)2
O2
O
OH
OH
OH
PdCl2
HCl
P
(or 3a, or
2a
)
P
O
HOAc
O
O
Pd
(BQ)
CH3OOC
OMe
OH
H-Pd-OAc
H2O
P
P
Cl
Cl-Pd
CH3OOC
OMe
OMe
)
(or
Pd
, or BQ+
OH
(CHQ)
Scheme 5.
Cl-Pd
It is believed that the formation of PdCl24ꢂ from PdCl2 may
enhance the activity of PdCl2.16 In order to improve the ac-
tivity of Pd(OAc)2/PS-BQ and increase the yield and selec-
tivity of 2a, especially the yield, we tried to add anhydrous
NaOAc to enhance the activity of Pd(OAc)2, but the result
Scheme 4.
The mechanism of Pd(OAc)2/PS-BQ catalytic system is not
similar with that of PdCl2/PS-BQ system because no cycle of
HCl absorption exists. And the cycle involving the oxidation

9852
Z.-Y. Wang et al. / Tetrahedron 62 (2006) 9846–9854
a
Table 7. Pd(OAc)2/PS-BQ-catalyzed acetalization of 1a with MeOH using different additives in scCO2 or compressed CO2
Entry
Catalyst
(mol %)
Additive
(mol %)
Pb
(MPa)
Conv.
(%)
Yieldc (%)
Select. for
2ad (%)
2a
3a
4a
1
2
3
Pd(OAc)2 (3)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
Ne
0.2/13
0.4/3
0.5/13
0.5/16
0.5/6
0.5/8
0.5/16
0.5/6
0.5/10
0.5/13
0.5/5
98.2
98.5
99.4
92.4
89.5
98.2
99.7
98.1
94.7
99.8
99.7
99.8
77.4
76.9
78.2
25.5
16.0
67.6
75.6
66.5
58.3
74.8
88.4
90.8
0
0
0
0
0
0
0
0
0
0
0
0
10.6
2.1
88.0
97.3
83.9
30.9
20.5
75.1
79.5
73.9
71.2
84.4
92.9
94.8
Ne
Ne
15.0
57.0
61.9
22.4
19.5
23.5
23.6
13.8
6.8
4
NaOAc (3)
NaOAc (3)
HOAc (3)
HOAc (15)
HOAc (30)
HOAc (90)
NaCl (3)
NaCl (6)
LiCl (3)
5f
6
7
8
9
10
11
12
0.5/20
5.0
a
Reaction conditions: 5 mmol methyl acrylate 1a, 2 mmol (40 mol %) PS-BQ, and 24.7 mmol methanol. The reaction was carried out at 50 ꢀC for 24 h.
O2 pressure/total system pressure.
By GC spectrum.
b
c
d
e
f
Selectivity¼[2a/(2a+3a+4a)]ꢁ100.
No additive.
Room temperature 20–27 ꢀC, 6 h.
was not satisfactory. Contrarily, the yield and selectivity
withdrawing groups, i.e., acrylate esters, acrylonitrile,
methyl vinyl ketone, and acrolein, could be carried out
smoothly under oxygen atmosphere. The effect of catalysts,
cocatalysts, alcohols, system temperature, reaction time, and
reactant molar ratio, is systematically investigated and
summarized in this paper.
were greatly decreased (Table 7, entries 4 and 5). This can
be ascribed to the oxidation of reactant methanol by
Pd(OAc)2/NaOAc,18 as the consumption of alcohol is disad-
vantageous to the next conversion of the intermediate 4a into
the expected product 2a.
Using HOAc instead of NaOAc as the additive, the yield and
selectivity were not satisfied neither. Increasing the amount
of acidic HOAc was not workable too (Table 7, entries 6–9).
Backvall19 reported work on the application and mechanism
of Pd(OAc)2/BQ catalytic system with HOAc or NaOAc,
and only PdCl24ꢂ complex structure was mentioned. There-
fore, it may be very difficult for Pd(OAc)2 to form
Pd(OAc)24ꢂ complex structure,16 and our experiment results
were consistent with it.
Cocatalyst PS-BQ could be recycled with excellent catalytic
activity, and without any chlorinated by-product detected.
The catalytic activity was slightly decreased when Pd(OAc)2
was used as leading catalyst instead of PdCl2. While the
good yield and selectivity was achieved when the chlorine
ion additive, i.e., NaCl or LiCl, was added. For different ace-
talization mechanism, PdCl2/PS-BQ was a better catalytic
system with higher selectivity than PdII/CuII (CuI) system.
Especially for acrolein, and methyl vinyl ketone, basic addi-
tives were not necessary. In the Wacker oxidation cycle
catalyzed by PdCl2/PS-BQ, BQ was proposed to be initially
converted to CHQ by the addition reaction with HCl and
subsequently regenerated by the hydrogenolysis to give
PdCl2 and products.
When NaCl was used as an additive, PdCl2 could be pro-
duced by the ion exchange of Pd(OAc)2 and NaCl as shown:
2NaCl þ PdðOAcÞ2/2NaOAc þ PdCl2
Although the simultaneously produced NaOAc was disad-
vantageous to the acetalization because of the oxidation of
reactant methanol by Pd(OAc)2/NaOAc,18 the activation
property of chlorine ion could offset the side-effect caused
by NaOAc. Hence, 2a was still the major product, and its
yield and selectivity remained (Table 7, entries 1–3 and
10). When the loading of NaCl was increased to 1 equiv of
Pd(OAc)2, the result observed was similar with the one
from the catalytic system of PdCl2 (Table 7, entry 11).
Thus, the method for the synthesis of important organic
intermediates, i.e., alkyl 3,3-dialkoxypropanoates, b-keto-
acetals, and b-cyanoacetal, environment-friendly and high
selectivity, by the acetalization reaction catalyzed by
PdCl2/PS-BQ was successfully developed. And the recycling
of the precious metal palladium was feasible in a convenient
manner. More importantly, the mechanism of PdII/PS-BQ
catalytic system may be helpful to explain other novel
phenomena, e.g., the production of tri-substituted benzene
derivatives from acrylate esters, and methyl vinyl ketone.
When LiCl was used as the additive, though its molar
amount was equivalent to half of Pd(OAc)2, only the activa-
tion of chlorine ion existing, the aim to enhance the activity
of Pd(OAc)2 was realized as expected (Table 7, entry 12).
Thus, using chlorine ion and BQ to control the reaction
selectivity was achieved.
4. Experimental
4.1. General
1H NMR spectra were recorded on BRUKER DRX-400
spectrometer using CDCl3 as solvent and TMS as an internal
standard. GC analyses were performed on a GC-930 chro-
matograph (Shanghai Haixian Chromatograph Instrument
Ltd. Co.) with a flame ionization detector equipped with
3. Conclusions
Using scCO2 as an environment-friendly solvent, PdII-cata-
lyzed acetalization of terminal olefins with electron-

Z.-Y. Wang et al. / Tetrahedron 62 (2006) 9846–9854
9853
+
+
+
an OV-101 capillary column (internal diameter¼0.25 mm,
length¼30 m). Mass spectra were recorded on a Shimadzu
GCMS-QP5050A at an ionization voltage of 70 eVequipped
with a DB-WAX capillary column (internal diameter¼
0.25 mm, length¼30 m). IR spectra were recorded on Ana-
lect RFX-65A spectrometer.
C5H9O3 ], 103 (8) [C4H7O3 ], 89 (26) [C4H9O2 ], 75 (100)
[(CH3O)2CH+], 61 (26) [C2H5O2 ], 43 (16) [C2H3O+], 29
(50) [C2H5 ].
+
+
4.3.3. Ethyl 3,3-diethoxypropanoate (acetal 2b0). Oil; IR
(KBr, film): n 2980, 2933, 2893 cmꢂ1 (CH3, CH2, CH);
1738 cmꢂ1 (COO, very strong); 1378, 1449 cmꢂ1 (CH3,
CH2); 1064, 1120, 1195 cmꢂ1 (C–O–C in ether or ester
All acrylate esters, acrylonitrile, acetonitrile, acrylic acid,
acrylamide, methyl methacrylate, methanol, ethanol, palla-
dium chloride, acetone, and porous polystyrene resin, etc.,
were commercially purchased and used without further
purification. Acrolein was distilled before use. Methyl vinyl
ketone, PdCl2(MeCN)2, and CuCl were prepared according
to the literature.2b,9
1
structure, strong). H NMR (CDCl3, TMS, ppm): d 1.17
(6H, t, J¼7.2 Hz, OCH2CH3), 1.25 (3H, t, J¼3.6 Hz,
COOCH2CH3), 2.64 (2H, d, J¼5.6 Hz, CH2CH), 3.50–
3.55 (2H, m, OCH2CH3), 3.63–3.68 (2H, m, OCH2CH3),
4.14 (2H, q, J¼7.2 Hz, COOCH2CH3), 4.93 (1H, t,
J¼2.4 Hz, CH2CH). GC–MS: m/z 189 (2) [Mꢂ1,
C9H17O4 ], 161 (16) [M+ꢂCH2CH3, C7H13O4 ], 145
+
+
+
+
4.2. Preparation of cocatalyst PS-BQ
(46) [C7H13O3 ], 117 (38) [C5H9O3 ], 103 (100)
[(CH3CH2O)2CH+], 89 (38) [C3H5O3 ], 75 (70) [C3H7O2 ],
71 (71) [C3H3O2 ], 47 (75) [C2H5OH2 ], 29 (58) [C2H5 ].
+
+
+
+
+
The cocatalyst PS-BQ was prepared in three steps: chloro-
methylation of commercial polystyrene (PS) porous resin,
alkylation of hydroquinone by chloromethylated PS, and
oxidation of PS supported-hydroquinone by H2O2 (Scheme
2), and the composition of the product from each step was
successfully confirmed with IR and elemental analyses of
chlorine.6
4.3.4. n-Butyl 3,3-dimethoxypropanoate (acetal 2c). Oil;
IR (KBr, film): n 2961, 2876, 2838 cmꢂ1 (CH3, CH2, CH);
1737 cmꢂ1 (COO, very strong); 1461, 1402 cmꢂ1 (CH3,
CH2); 1070, 1122, 1192 cmꢂ1 (C–O–C in ether or ester
structure, strong); 740 cmꢂ1 (CH2CH2CH2, weak). 1H
NMR (CDCl3, TMS, ppm): d 0.92 (3H, t, J¼2.4 Hz,
OCH2CH2CH2CH3), 1.34–1.38 (2H, m, OCH2CH2CH2CH3),
1.57–1.64 (2H, m, OCH2CH2CH2CH3), 2.63 (2H, d,
J¼6.0 Hz, CH2CH), 3.34 (6H, s, OCH3), 4.09 (2H, t,
J¼3.2 Hz, OCH2CH2CH2CH3), 4.82 (1H, t, J¼6.0 Hz,
4.3. Typical procedure for the acetalization
The reaction was carried out in a HF-25 autoclave. Catalyst
PdCl2 (0.15 mmol, 3 mol %), PS-BQ (2 mmol), MeOH
(1 mL, 24.7 mmol),andmethylacrylate(5 mmol)wereadded
into a 25 mL autoclave in sequence. O2 and liquid CO2 were
pumped into the autoclave by a cooling pump to reach the de-
sired pressure, then the autoclavewas heated by oil bath under
magnetic stirring for the desired reaction time. After the reac-
tion finished, the autoclave was allowed to cool to ꢂ30 ꢀC.
CO2 was vented and the surplus was extracted with n-hexane
or petroleum ether. The extract was filtrated and condensed
under reduced pressure. The product was purified by prepar-
ative TLC on silica gel using light petroleum ether/ethyl
acetate as eluent before the test of 1H NMR and IR.
+
CH2CH). GC–MS: m/z 189 (1) [Mꢂ1, C9H17O4 ], 159 (6)
[M+ꢂCH3O, C8H15O3 ], 117 (8) [M+ꢂC4H9O, C5H9O3 ],
+
+
+
+
103 (17) [C4H7O3 ], 85 (18) [C4H5O2 ], 75 (100)
[(CH3O)2CH+], 57 (18) [C4H9 ], 41 (24) [C3H5 ], 29 (31)
+
+
+
[C2H5 ].
4.3.5. 3,3-Dimethoxypropionitrile (acetal 2d). Oil; IR
(KBr, film): n 2936, 2845 cmꢂ1 (CH3, CH2, CH);
2255 cmꢂ1 (C^N, weak); 1455, 1417 cmꢂ1 (CH3, CH2);
1075, 1122 cmꢂ1 (C–O–C in ether structure, strong). H
1
NMR (CDCl3, TMS, ppm): d 2.65 (2H, d, J¼5.6 Hz,
CH2CH), 3.39 (6H, s, OCH3), 4.66 (1H, t, J¼5.6 Hz,
+
4.3.1. Methyl 3,3-dimethoxypropanoate (acetal 2a). Oil;
IR (KBr, film): n 2948, 2840 cmꢂ1 (CH3, CH2, CH);
1740 cmꢂ1 (COO, very strong); 1378, 1444 cmꢂ1 (CH3O);
1069, 1122, 1176 cmꢂ1 (C–O–C in ether or ester structure,
strong). 1H NMR (CDCl3, TMS, ppm): d 2.62 (2H, d,
J¼2.0 Hz, CH2), 3.33 (6H, s, OCH3), 3.65 (3H, s, OCH3),
CH2CH). GC–MS: m/z 114 (2) [Mꢂ1, C5H8NO2 ], 84
(92) [M+ꢂCH3O, C4H5NO+], 75 (100) [(CH3O)2CH+], 56
(46) [C3H4O+].
4.3.6. 4,4-Dimethoxy-2-butanone (acetal 13). Oil; IR
(KBr, filꢂm1): n 2937, 2839 cmꢂ1 (CH3, CH2, CH, strong);
(C]O, very strong); 1446 cmꢂ1 (CH3O);
+
4.80 (1H, s, CH). GC–MS: m/z 147 (2) [Mꢂ1, C6H11O4 ],
1718 cm
133 (8) [M+ꢂCH3, C5H9O4 ], 117 (29) [M+ꢂCH3O,
1362 cmꢂ1 (CH3CO); 1080, 1122, 1167, 1192 cmꢂ1
(C–O–C in ether structure, strong). 1H NMR (CDCl3,
TMS, ppm): d 2.16 (3H, s, CH3CO), 2.72 (2H, d,
J¼5.2 Hz, CHCH2COCH3), 3.34 (6H, s, OCH3), 4.77 (1H,
t, J¼5.6 Hz, CHCH2COCH3). GC–MS: m/z 132 (3) [M+,
+
+
+
+
C5H9O3 ], 101 (8) [C4H5O4 ], 85 (5) [C4H5O2 ], 75 (100)
[(CH3O)2CH+], 59 (25) [C2H3O2 ], 47 (25) [C2H7O+], 31
+
(16) [CH3O+].
C6H12O3 ], 117 (21) [M+ꢂCH3, C5H9O3 ], 101 (22)
+
+
4.3.2. Ethyl 3,3-dimethoxypropanoate (acetal 2b). Oil; IR
(KBr, film): n 2985, 2944, 2837 cmꢂ1 (CH3, CH2, CH);
1738 cmꢂ1 (COO, very strong); 1378, 1455 cmꢂ1 (CH3,
CH2); 1069, 1123, 1176 cmꢂ1 (C–O–C in ether or ester
[M+ꢂCH3O, C5H9O2 ], 85 (12) [CH3COCH2CO+,
+
C4H5O2 ], 75 (71) [(CH3O)2CH+], 59 (17) [CH3OCO+],
+
43 (100) [CH3CO+], 31 (10) [CH3O+].
1
structure, strong). H NMR (CDCl3, TMS, ppm): d 1.25
(3H, t, J¼3.2 Hz, OCH2CH3), 2.63 (2H, d, J¼6.0 Hz,
CH2CH), 3.35 (6H, s, OCH3), 4.15 (2H, q, J¼3.6 Hz,
OCH2CH3), 4.82 (1H, t, J¼6.0 Hz, CH2CH). GC–MS: m/z
Acknowledgements
161 (2) [Mꢂ1, C7H13O4 ], 147 (7) [M+ꢂCH3, C6H11O4 ],
The authors thank National Natural Science Foundation
of China (No. 20332030 and No. 20572027) and Young
+
+
131 (25) [M+ꢂCH3O, C6H11O3 ], 117 (13) [M+ꢂOCH2CH3,
+

9854
Z.-Y. Wang et al. / Tetrahedron 62 (2006) 9846–9854
Rev. 1999, 99, 475–493; (k) Jiang, H. F. Curr. Org. Chem.
2005, 9, 289–297.
Foundation of SCUT for the financial support of this work
and Dr. Jing-Mei Huang for helpful discussion.
9. Ouyang, X. Y.; Jiang, H. F.; Cheng, J. S.; Zhang, Q. J. Chin. J.
Chem. 2002, 20, 1326–1329.
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