Palladium-catalyzed oxidation of phenyl-substituted alkenes using molecular oxygen as the sole oxidant

Article abstract of DOI:10.1002/adsc.200900294

The palladium-catalyzed aerobic oxidation of styrene and 2-vinylnaphthalene in dimethylacetamide/water or dimethylformamide/water solutions under mild conditions has been developed, in which palladium(II) chloride is used in the absence of cocatalysts or special stabilizing ligands as the sole and recyclable catalyst. The corresponding methyl ketones have been obtained in good to excellent yields with low catalyst loadings (0.2-5 mol% ) and high turnover numbers (up to ca. 1000 to palladium). This simple and efficient catalytic method represents an ecologically benign and economically attractive synthetic pathway to industrially important compounds used in the manufacture of various polymers and drugs.

Full text of DOI:10.1002/adsc.200900294

FULL PAPERS
DOI: 10.1002/adsc.200900294
Palladium-Catalyzed Oxidation of Phenyl-Substituted Alkenes
using Molecular Oxygen as the Sole Oxidant
a
a
a,
Aline C. Bueno, ꢀgatha O. de Souza, and Elena V. Gusevskaya *
a
Departamento de Quꢁmica, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte, MG, Brazil
Fax : (+55)-31-3409-5700; phone: (+55)-31-3499-5741; e-mail: elena@ufmg.br
Received: April 23, 2009; Revised: July 29, 2009; Published online: September 23, 2009
Abstract: The palladium-catalyzed aerobic oxidation turnover numbers (up to ca. 1000 to palladium). This
of styrene and 2-vinylnaphthalene in dimethylacet- simple and efficient catalytic method represents an
ACHTUNGTRENNUNGa mide/water or dimethylformamide/water solutions ecologically benign and economically attractive syn-
under mild conditions has been developed, in which thetic pathway to industrially important compounds
palladium(II) chloride is used in the absence of co- used in the manufacture of various polymers and
catalysts or special stabilizing ligands as the sole and drugs.
recyclable catalyst. The corresponding methyl ke-
tones have been obtained in good to excellent yields Keywords: oxidation; oxygen; palladium; styrenes;
with low catalyst loadings (0.2–5 mol%) and high 2-vinylnaphthalenes
Introduction
oxygen-coupled Wacker oxidation of a number of ter-
minal alkenes. The system allows for an efficient
[
15]
Palladium complexes are remarkably versatile re- catalytic turnover without the need for additional co-
agents for the selective oxidation of organic mole- catalysts or special ligands under relatively mild con-
cules. The important advantage of these reactions is ditions (808C, 6 atm of oxygen). Sigman and co-work-
the possibility to involve molecular oxygen as a final ers have achieved a further improvement in the prac-
oxidant, which is usually achieved by the re-oxidation ticality of this reaction showing that the use of (À)-
of reduced palladium species with reversible co-oxi- sparteine as a ligand on palladium allows one to per-
dants, CuCl being the most convenient one (Wacker form the reaction in a DMA/H O solvent system
2
2
[1]
[14]
catalyst). However, the Wacker process requires under balloon pressure of oxygen.
large amounts of CuCl , chloride ions, and acid to
Inspired by these disclosures, we have decided to
2
maintain the catalytic cycle; therefore, the system is extend their applications to phenyl-substituted al-
highly corrosive and often causes the formation of kenes, i.e., styrene and 2-vinylnaphthalene. These al-
chlorinated side products. To overcome these prob- kenes are usually problematic substrates for oxidation
lems, much effort has been devoted to the develop- due to a tendency toward oxidative cleavage to the
ment of alternative halide-free co-catalysts, such as corresponding aldehydes and polymerization.
Cu
none;
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(OAc) , heteropoly acids, nitrates, and benzoqui-
The palladium-catalyzed oxidation of styrene to
2
[2–8]
as well as, more recently, palladium catalysts acetophenone has been investigated by various re-
[
8–14]
modulated by special ligands.
In the latter sys- search groups due to, in particular, the practical im-
tems, oxidatively robust (usually, nitrogen-containing) portance of this compound. Acetophenone is a valua-
ligands are used to stabilize reduced palladium and ble intermediate for the manufacture of a variety of
promote its regeneration directly by molecular resins, pharmaceuticals, pesticides, and perfumery
oxygen avoiding the use of corrosive additives. A few products and is industrially produced by the Friedel–
examples of the ligand-modulated palladium-cata- Crafts acetylation of benzene with acetic aldehyde/
[
16]
lyzed oxidations of terminal alkenes into methyl ke- acetyl chloride. Most of reported examples of the
tones using molecular oxygen as the sole oxidant have palladium-catalyzed oxidation of styrene involve hy-
[9–11,14]
been published.
Recently, Kaneda and co-workers have discovered dants
that dimethylacetamide (DMA) as a solvent serves to oxidant – molecular oxygen.
drogen peroxide or peroxo compounds as oxi-
[
17–24]
and only few works use the most attractive
[
25–30]
To the best of our
stabilize a palladium catalyst preventing its precipita- knowledge, all reported aerobic palladium-catalyzed
tion into inactive metal and to promote a direct di- oxidations of styrene utilize the conventional Wacker
Adv. Synth. Catal. 2009, 351, 2491 – 2495
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2491

Aline C. Bueno et al.
FULL PAPERS
co-catalyst – CuCl – to mediate the re-oxidation of
2
[25–28]
Pd(0).
One exception is a recently published at-
tempt to oxidize styrene with PdCl as the sole cata-
2
lyst in an scCO /MeOH solvent system, which gave
2
[
30]
only a 12.4% yield of acetophenone.
Sigman and
co-workers observed that an N-heterocyclic carbene/
Pd(II) complex converted styrene to acetophenone
under aerobic conditions; however, it was found that
the reaction proceeded due to the in situ production
of a peroxide via co-oxidation of THF – the only
Scheme 1. Oxidation of styrene into acetophenone.
[
22]
competent reaction solvent.
Styrene readily reacts with PdCl in DMA solutions
2
Although 2-acetonaphthones are also industrially in the presence of water. A virtually complete conver-
important drug and polymer intermediates, the data sion has been attained for 5 h at 508C and 10 atm re-
on the palladium-catalyzed oxidation of 2-vinylnaph- sulting in ca. 90% yield of acetophenone (Scheme 1,
[
31]
thalenes are really scarce.
Table 1, run 1). The reaction is catalytic in palladium
In the present paper, we report a simple and effi- and shows a dioxygen-coupled turnover number
cient palladium-catalyzed direct dioxygen-coupled ox- (TON) of 20, with no palladium mirror being ob-
idation of styrene and 2-vinylnaphthalene into corre- served on the walls of the reaction vessel after the re-
sponding methyl ketones. The reactions do not need action. Thus, DMA as a coordinating solvent success-
additional co-oxidants or special ligands and proceed fully prevents zerovalent palladium species, Pd(0)Ln,
under mild aerobic conditions. The use of PdCl as from clustering into inactive bulk metal; so that their
2
the sole and recyclable catalyst and inexpensive high oxidation by molecular oxygen occurs faster then ag-
boiling amidic solvents as well as molecular oxygen as gregation.
the final oxidant is a significant practical advantage of
the process.
The increase in water content from 15 to 20 vol%
significantly accelerates the reaction, which can be
completed under these conditions for less than 2 h
showing the average turnover frequency (TOF) of
À1
Results and Discussion
12.7 h (Table 1, run 2). This value is considerably
greater that those usually reported for conventional
[
15]
We have studied the oxidation of styrene and 2-vinyl- Wacker oxidations using co-catalysts. The enhanc-
naphthalene by molecular oxygen using PdCl as the ing effect of water has also been observed in the
2
sole catalyst in dimethylacetamide (DMA) or dime- Pd[(À)-sparteine]Cl /DMA system for the oxidation
2
[
14]
thylformamide (DMF) solutions containing 10– of other terminal alkenes.
However, it should be
20 vol% of water. In most of the runs, elevated pres- mentioned that increasing the water concentration
sures of oxygen are used in order to ensure efficient must be equilibrated with the miscibility of the sub-
capture of reduce palladium and prevent metal pre- strates.
cipitation. Some experiments were performed at at-
The reaction with styrene (and with 2-vinylnaphtha-
mospheric pressure. The reactions with each substrate lene, as it will be shown below) also occurs smoothly
resulted in corresponding methyl ketone as a major in another amidic solvent, DMF, although at a slightly
product in 85–95% yields in most of the runs. The GC lower rate (Table 1, run 3 vs. run 1). This result is re-
mass balance was based on the substrate charged markable in light of a previously reported observation
using bornyl acetate as the internal standard. The dif- that 1-decene gives only a trace yield of the corre-
ference in mass balance in the Tables is usually less sponding methyl ketone in DMF under similar condi-
[
15]
than 5% and corresponds to unidentified high boiling tions. Thus, DMF also acts as an efficient solvent to
products. Thus, a high stability of these delicate sub- promote the regeneration of the palladium species at
strates toward polymerization under the conditions the oxidation of styrene without the need of co-cata-
used for oxidation is especially noteworthy.
lysts or special ligands, making more flexible the
The main minor products were benzaldehyde and choice of solvent for practical purposes. Selectivity for
-naphthalenecarboxaldehyde formed due to the oxi- acetophenone in DMF is slightly lower then in DMA
2
dative cleavage of carbon-carbon double bonds in sty- (83 vs. 92%), but the reaction variables have not been
rene and 2-vinylnaphthene, respectively. The cleavage optimized yet.
reaction seems to occur through a radical auto-oxida-
It is also important that the catalytic system effi-
tion mechanism, as it can be suppressed, at least in ciently operates at room temperature (Table 1, runs
the case of the conventional Wacker (PdCl /CuCl - 4–7). The reaction rate decreases, as expected; how-
2
2
catalyzed) oxidation of styrene, by adding radical in-
hibitors to the system.
ACHTUNGTRENNGNUe ver, the effect seems to be partly compensated by
the increased solubility of molecular oxygen at lower
temperature. Really, the initial reaction rate depends
[
20]
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Adv. Synth. Catal. 2009, 351, 2491 – 2495

Palladium-Catalyzed Oxidation of Phenyl-Substituted Alkenes
FULL PAPERS
[a]
Table 1. Palladium-catalyzed oxidation of styrene by molecular oxygen in dimethylacetamide.
[
b]
[c]
Run H O
Temp.
[ C]
Pressure
[atm]
Time
[h]
Conversion
[%]
Rate
TON
Selectivity (isolated yield)
[%]
2
o
À2
À1
[vol%]
[10 Mh ]
acetophenone benzaldehyde
1
2
3
4
5
6
7
8
9
1
15
20
15
15
15
20
15
20
15
15
50
50
50
25
25
25
25
50
60
50
10
10
10
10
5
10
10
10
10
1
5
1.5
8
8
9
7
32
5
24
6
1
99
95
96
95
97
96
95
97
98
33
40
95
98
7.0
12.6
4.6
3.0
2.2
4.0
1.7
20.0
12.0
–
20
19
19
19
19
19
95
97
490
–
92
91 (76)
83
90
90
93
94
91 (74)
80
–
6
7
11
5
7
5
4
5
16
–
[
d]
[
[
[
e]
e]
f]
[
g]
0
6
1.3
4.0
10.0
8
19
490
96
82 (65)
80
4
18
12
1
1
1
2
15
15
50
60
1
10
16
28
[
h]
[
a]
Conditions: [styrene]=0.20M; [PdCl ]=0.01M; gas phase – O ; conversion and selectivity were determined by GC and
2
2
based on the consumed styrene.
[
[
[
[
[
[
[
b]
Initial rate of the substrate conversion.
TON – moles of the substrate converted/moles of Pd.
DMF was used as the solvent.
c]
d]
e]
f]
[
[
[
PdCl ]=0.0025M; [styrene]=0.25M.
2
PdCl ]=0.0020M; [styrene]=1.0M.
2
g]
h]
(À)-sparteine]=0.01M.
After run 9, the products were separated by extraction with n-heptane, the reactor was recharged with fresh styrene
(1.0M) and the reaction was allowed to proceed further. TON is given for the second reaction cycle.
on the oxygen pressure showing a positive order acetophenone and benzaldehyde accounted for near
Table 1, runs 4 and 5), but even at 5 atm no palladi- 95% of the mass balance. Thus, the substrate is quite
(
um precipitation has been observed and the reaction tolerant to polymerization under the conditions used.
À1
has been completed for 9 h. These observations sug- The average TOF of 16.6 h was obtained in this re-
gest that the step which determines the rate of the action without the precipitation of palladium metal,
whole process is the re-oxidation of the reduced palla- although the selectivity for acetophenone decreased
dium species by molecular oxygen. The accelerating to 80% at the expense of benzaldehyde (16%).
effect of water is also pronounced at room tempera-
The reaction also operates at ambient pressure of
ture and the catalyst is still stable at 20 vol% of water oxygen in the presence of (À)-sparteine (1 equiv. to
[14]
in DMA (Table 1, run 4 vs. run 6).
Pd) used to stabilize reduced palladium species
To improve the catalyst efficiency in terms of (Table 1, run 10). No precipitation of palladium metal
TONs, the substrate/catalyst ratio was raised to 100 occurs, but the reaction is slow and becomes stagnat-
by decreasing the PdCl concentration (Table 1, run ed at near 40% conversion (TON=8). Finally, the re-
2
7). We were cautious in increasing the concentration action was performed at 1 atm of oxygen in the ab-
of the substrate in order to avoid its polymerization. sence of the auxiliary ligand to verify if the coordinat-
The reaction was virtually completed even at room ing ability of DMA is sufficient to keep palladium in
temperature, albeit at a rather low rate, and showed the solution. To our surprise, the reaction was even
an excellent selectivity of 94% for acetophenone. much faster than that in the presence of (À)-sparteine
Thus, a dioxygen-coupled TON in this run reached and reached a near complete conversion (TON=19)
the value of 95 with no precipitation of the palladium without the precipitation of palladium (Table 1, run
metal being observed. A TON of ca. 100 can be at- 11). Thus, the catalytic system in DMA is quite stable
tained much faster at higher temperature and higher and efficient for the oxidation of styrene even at am-
water content without a significant polymerization of bient pressure of oxygen without the use of any auxil-
the substrate (Table 1, run 8). Moreover, a further in- iary stabilizing ligand on palladium.
crease in the substrate concentration allowed attain-
In a further attempt to improve the practicality of
ing TON of near 500 (Table 1, run 9). Although large this reaction, the catalyst after run 9 was re-used. The
amounts of styrene (1.0M) were charged in one por- products were separated from the catalytic system by
tion at the beginning of this reaction, no significant extraction with n-heptane upon the completion of the
amounts of high boiling products were observed as reaction. The residual DMA/H O solution of PdCl2
2
Adv. Synth. Catal. 2009, 351, 2491 – 2495
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asc.wiley-vch.de
2493

Aline C. Bueno et al.
FULL PAPERS
[a]
Table 2. Palladium catalyzed oxidation of 2-vinylnaphthalene by molecular oxygen.
[
b]
[c]
Run Solvent
vol%]
Temp.
[ C]
Pressure
[atm]
Time
[h]
Conversion
[%]
Rate
TON
Selectivity (isolated yield)
[%]
o
À2
À1
[
[10 Mh ]
acetonaphthone aldehyde
1
2
3
4
DMA/H O
80
60
80
80
10
10
10
10
1
98
98
50
80
17.2
5.0
20
78
10
16
87 (75)
87
13
10
15
10
2
(15)
[
d]
DMA/H O
12
2
2
(10)
DMF/H O
4.0
85
2
(15)
DMF/H O
5
9.0
80
2
(20)
[
a]
Conditions: [2-vinylnaphthalene]=0.10M; [PdCl ]=0.005M; gas phase – O ; conversion and selectivity were determined
2
2
by GC and based on the consumed styrene; aldehyde – 2-naphthalenecarboxaldehyde.
Initial rate of the substrate conversion.
TON – moles of the substrate converted/moles of Pd.
[
[
[
b]
c]
d]
[
PdCl ]=0.0025M; [2-vinylnaphthalene]=0.20M.
2
was recharged with fresh styrene and the reaction was thalene into corresponding methyl ketones, under
allowed to proceed further (Table 1, run 12). The mild aerobic conditions. The use of PdCl as the sole
2
total TON obtained in two reaction cycles reached and recyclable catalyst and inexpensive high boiling
the value of 980. The reaction rate with the recharged solvents as well as molecular oxygen as the final oxi-
styrene varied only slightly and the selectivity for ace- dant is a significant practical advantage of the process.
tophenone was near 80%.
-Vinylnaphthalene can also be oxidized smoothly co-oxidant, often corrosive, which must be employed
It is also important that the reaction does not require
2
by molecular oxygen in DMA/H O solutions contain- in conventional palladium-catalyzed oxidations of al-
2
ing PdCl (Table 2, runs 1 and 2). An industrially im- kenes. This simple and efficient catalytic method rep-
2
portant compound, i.e., acetonaphthone, is obtained resents an attractive synthetic pathway to the industri-
in ca. 85% yield, with 2-naphthalenecarboxaldehyde ally important compounds used in the manufacture of
formed due to the oxidative cleavage of the carbon- various polymers and drugs. Further studies are tar-
carbon double bond being the only minor product geted towards the development of solid palladium
(
Scheme 2). The reaction is fast and shows the aver- catalysts resistant to leaching in polar solvents in
À1
age TOF of 20 h (Table 2, run 1). DMF also can be order to facilitate catalyst separation.
used as the solvent for the oxidation of 2-vinylnaph-
thalene; however, not with the same efficiency
(
Table 2, runs 3 and 4). Although no palladium mirror
is formed and selectivity for acetonaphthone is high
80–85%), the reaction becomes stagnated at incom-
Experimental Section
(
plete conversion of the substrate and the best yields
of acetonaphthone do not exceed 60–65%.
All reagents were purchased from commercial sources and
used as received, unless otherwise indicated. Styrene and 2-
vinylnaphthalene were distilled under reduced pressure
before use.
Conclusions
The reactions at atmospheric pressure were carried out in
a glass reactor under magnetic stirring and equipped with a
In summary, we have reported a highly selective condenser and followed by measuring dioxygen uptake and
method for the oxidation of styrene and 2-vinylnaph- by gas chromatography (GC).
Scheme 2. Oxidation of 2-vinylnaphthalene into acetonaphthone.
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Adv. Synth. Catal. 2009, 351, 2491 – 2495

Palladium-Catalyzed Oxidation of Phenyl-Substituted Alkenes
FULL PAPERS
The reactions at higher pressures were carried out in a
stainless steel magnetically stirred 100-mL reactor (auto-
clave) and followed by GC.
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E. V. Gusevskaya, Organometallics 2009, 28, 3186.
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In a typical run, the solution of the substrate (2.0–
[
2
0.0 mmol, 0.1–1.0M), PdCl (0.04–0.2 mmol, 0.002–0.01M)
2
R. A. Sheldon, Chem. Commun. 1998, 2359.
and bornyl acetate (internal standard, 2 mmol, 0.1M) in the
mixture of amidic solvent with water in indicated propor-
tions (20 mL) was transferred in the reactor. Concentrations
of the components are given in the Table 1 and Table 2. The
glass reactor was connected to a gas burette containing mo-
lecular oxygen to measure the gas uptake. The autoclave
was pressurized with dioxygen to the total pressure indicat-
ed in the Table 1 and Table 2 (1–10 atm). The reactors were
placed in an oil bath; then, the solutions were stirred at a
specified temperature (25–808C). At appropriate time inter-
vals, aliquots were taken via special sampling systems with-
out depressurization of the reactors and analyzed by GC
using a Shimadzu 17B instrument fitted with a Carbowax
[
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2
0 m capillary column and a flame ionization detector. After
carrying out the reaction and cooling to room temperature,
the excess of oxygen was slowly vented from the autoclave.
Products were identified by GC/MS (Shimadzu QP2010-
PLUS, 70 eV).
[
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Adv. Synth. Catal. 2009, 351, 2491 – 2495
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2495