Oxidative Cleavage of Styrenes Catalyzed by a PdIIComplex of an 8-Hydroxyquinolinonate Ligand

Article abstract of DOI:10.1002/ejic.201600951

Synthesis and catalytic activity of palladium complexes of 8-(pyridin-2-ylmethoxy)quinolinone (3) were investigated. Complexation of 3 with [Pd(CH₃CN)₂Cl₂] gave the dimeric palladium complex [{N,N-(3)PdCl}₂] (4). The structure of 4 was characterized by¹H/¹³C NMR spectroscopy and mass spectrometry, and further confirmed by X-ray crystallography. Complex 4 acts as a catalyst for oxidative cleavage of C=C bonds, and it catalyzes the hydration of styrenes in an anti-Markovnikov Wacker manner to give terminal aldehydes as the initial product, which then undergo C–C cleavage to yield the corresponding benzaldehydes. Systematic studies of this catalysis were performed.

Full text of DOI:10.1002/ejic.201600951

DOI: 10.1002/ejic.201600951
Full Paper
Palladium Catalysts
Oxidative Cleavage of Styrenes Catalyzed by a PdII Complex of
an 8-Hydroxyquinolinonate Ligand
Hong-Ming Jhong,[a] Yi-Hung Liu,[a] Shie-Ming Peng,[a] and Shiuh-Tzung Liu*[a]
Abstract: Synthesis and catalytic activity of palladium com- 4 acts as a catalyst for oxidative cleavage of C=C bonds, and it
plexes of 8-(pyridin-2-ylmethoxy)quinolinone (3) were investi- catalyzes the hydration of styrenes in an anti-Markovnikov
gated. Complexation of 3 with [Pd(CH3CN)2Cl2] gave the dimeric Wacker manner to give terminal aldehydes as the initial prod-
palladium complex [{N,N-(3)PdCl}2] (4). The structure of 4 was uct, which then undergo C–C cleavage to yield the correspond-
characterized by 1H/13C NMR spectroscopy and mass spectrom- ing benzaldehydes. Systematic studies of this catalysis were per-
etry, and further confirmed by X-ray crystallography. Complex formed.
Introduction
Pyridonate, the anionic form of the tautomers of 2-hydroxypyr-
idine and 2-pyridone, can behave as a monodentate, a 1,3-
bridging, or a chelating ligand through nitrogen and/or oxygen
donors (Scheme 1).[1–6] An attractive feature of this bifunctional
donor is its potential to act as a proton-responsive ligand for
the facilitation of proton-transfer catalysis.[6] The work of Rauch-
fuss and co-workers on the dehydrogenation of alcohols cata-
Scheme 2. Tautomers of 8-hydroxycarbostyril and ligand 3.
lyzed by [Cp*Ir(κ2-2-pyridonate)Cl] is an excellent illustration of
the use of this type of ligand in catalysis.[5]
Results and Discussion
Preparation of Palladium Complexes
Scheme 3 outlines the synthetic approach leading to the de-
sired ligand 3. Compound 1 was prepared according to a litera-
ture procedure.[9] Starting from 8-hydroxyquinoline (2), oxid-
Scheme 1. Coordination modes of pyridonate.
ation and acetylation followed by basic hydrolysis provided 1
in 65 % yield of isolated product. Nucleophilic substitution of
2-(bromomethyl)pyridine with 1 under basic conditions in
8-Hydroxycarbostyril (1) is the tautomer of 2,8-dihydroxy-
acetonitrile gave 3 in 82 % yield, which was characterized by IR
quinoline, similar to 2-pyridone. Claramunt and co-workers
and NMR spectroscopy and mass spectrometry. The carbonyl
found that 8-hydroxycarbostyril exists as the 8-hydroxyquinolin-
2(1H)-one (keto) form, rather than the hydroxy form 1a, in both
solution and the solid state (Scheme 2).[7] It occurred to us that
the 8-hydroxy functionality in 1 providing an extra donor for
the formation of a five-membered ring by O(phenoxy)···N chela-
tion of metal ions should make its coordination modes more
versatile. However, few reports address metal complexes based
on 1 or related derivatives.[8,9] In this work, we studied the prep-
aration of palladium complexes based on potentially tridentate
3 and their catalytic activity in the oxidation of styrenes.
stretching band at 1682 cm–1 in the IR spectrum clearly demon-
strates the existence of amido functionality. The H NMR spec-
trum of 3 in CDCl3 shows a downfield signal at δ = 10.02 ppm
corresponding to C(O)NH. The 13C NMR chemical shift of the
carbonyl carbon atom at δ = 162.6 ppm is in agreement with
1
[a] Department of Chemistry, National Taiwan University,
1, Sec. 4, Roosevelt Road, Taipei 106, Taiwan
E-mail: stliu@ntu.edu.tw
http://www.ntu.edu.tw/
Scheme 3. Preparation of ligand 3.
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that of pyridone. The ESI-HR mass spectrum of 3 shows a peak gands. The bite angles of 89.7(1) and 86.1(1) in 4 both slightly
at m/z = 253.0976 [M + H], consistent with the proposed molec- deviate from 90°, presumably due to the larger chelate ring.
ular formula.
The C(15)–O(2) [1.287(4) Å] and C(30)–O(4) [1.284(4) Å] bond
lengths are typical for C=O bonds and are comparable to those
of palladium pyridonate complexes.[8a,11] The ether oxygen do-
nor in 4 is seated at the apical position of the coordination
plane [O(1)–Pd(1)–N(1) 74.9(1)° and O(1)–Pd(1)–N(2) 73.2(1)°] at
a distance of 2.632(3) Å, which is shorter than the sum of the
van der Waals radii (3.15 Å) and indicates some interaction with
the Pd center. The Pd centers in 4 are separated by a distance
of 3.011 Å, which is slightly longer than that for a metal–metal
interaction.[12]
Ligand substitution of [Pd(CH3CN)2Cl2] with 3 in a 1:1 molar
ratio in dichloromethane at ambient temperature yielded an
orange precipitate, which was collected by filtration to give air-
stable palladium complex 4 in 78 % yield (Scheme 4). This com-
plex is insoluble in most organic solvents and slightly soluble
in acetonitrile and methanol. The dimeric structure of 4 was
determined by XRD (see below). In solution, the complex exists
in monomeric form 5, as evidenced by mass determination: the
ESI-HR mass spectrum of 4 in acetonitrile matrix shows a peak
1
at m/z = 398.0221 [5 – Cl]. The H NMR spectrum of 4 shows a
Table 1. Selected bond lengths [Å] and angles [°] for 4.
downfield resonance at δ = 8.93 ppm (d, J = 3 Hz), which is
assigned to the C-6′ hydrogen atom of the pyridine ring. The
coordination chemical shift of this hydrogen signal of Δδ =
0.26 ppm indicates coordination of the pyridinyl nitrogen atom
to the metal center. The signal of the carbonyl carbon atom (C-
2) of 4 in the 13C NMR spectrum in CD3CN appears at δ = 172.5,
comparable to those of pyridonates.[10]
Pd(1)–N(1)
Pd(1)–N(2)
Pd(1)–Cl(1)
Pd(1)–O(4)
Pd(1)···O(1)
C(15)-O(2)
N(1)–Pd(1)–N(2)
N(1)–Pd(1)–O(4)
N(2)–Pd(1)–Cl(1)
N(1)–Pd(1)–Cl(1)
2.010(3)
2.049(3)
2.305(1)
2.014(2)
2.632(3)
1.287(4)
89.7(1)
170.2(1)
178.16(9)
88.51(9)
Pd(2)–N(3)
Pd(2)–N(4)
Pd(2)–Cl(2)
Pd(2)–O(2)
Pd(2)···O(3)
C(30)–O(4)
N(3)–Pd(2)–N(4)
N(3)–Pd(2)–O(2)
N(4)–Pd(2)–Cl(2)
N(3)–Pd(2)–Cl(2)
2.003(3)
2.045(3)
2.320(1)
2.029(2)
2.642(3)
1.284(4)
86.1(1)
168.4(1)
174.76(9)
88.71(9)
Catalytic Oxidative Cleavage of Styrenes
Palladium-catalyzed oxidation of 1-alkenes to yield ketones,
known as the Wacker process, is well documented.[13] However,
a few reports show that palladium complexes can also catalyze
Scheme 4. Preparation of palladium complex 4.
Orange crystals of 4 formed on slow diffusion of diethyl the oxidative cleavage of olefins.[14] In this regard, two major
ether into a solution of 4 in acetonitrile at ambient temperature. reaction pathways have been proposed for this oxidation
The molecular structure of 4 is shown in Figure 1, and selected (Scheme 5). One involves metal-promoted peroxide formation
bond lengths and angles are summarized in Table 1. The carb- followed by oxidative cleavage.[14d] The other one proceeds by
onyl oxygen atom acts as a bridging donor to form a dimeric anti-Markovnikov Wacker oxidation[15] and then further oxid-
structure in the solid state. The Pd–O distances lie in the range ation resulting in C–C bond cleavage. Hence, the activity of
from 2.014(2) to 2.029(2) Å, which is typical for this kind of complex 4 toward olefin oxidation was investigated.
coordination. The geometry around the two palladium atoms
is slightly distorted square-planar with each metal center coor-
dinated by amido nitrogen, pyridinyl nitrogen, and carbonyl
oxygen atoms, as well as a chlorido ligand. The Pd–Cl and
Pd–N bond lengths and coordination angles (Table 1) are simi-
lar to those reported for related PdII complexes with N^N li-
Scheme 5. Metal-catalyzed oxidation of alkenes.
We chose the oxidation of styrene as the model reaction to
establish the optimized catalytic conditions. In a typical experi-
ment, a mixture of styrene, complex 4, CuCl2, and water in
propan-2-ol was heated for 16 h (Scheme 6). After the reaction,
the mixture was filtered through Celite to remove metal salts,
and the filtrate was dried and concentrated. The organic prod-
ucts were analyzed by 1H NMR spectroscopy (Table 2). First, we
Figure 1. ORTEP of 4 (30 % probability ellipsoids).
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screened the effect of an oxygen atmosphere.
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Table 2. Oxidation of styrene under various conditions.[a]
Entry
Reaction conditions/additive
T
[°C]
Conv.
[%]
Yield [%]
I
II
III
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
–
air instead of O2
N2 (1 atm) instead of O2
no CuCl2
50
50
50
50
50
20
40
60
50
50
60
50
50
50
50
50
50
50
50
50
50
100
5
64
trace
–
<3
–
11
trace
–
–
–
14
6
11
–
25
trace
trace
–
<5
ca. 3
ca. 3
100
100
100
100
100
80
76
50
100
30
73
no complex 4
–
–
–
–
21
56
45
75
76
55
40
40
7
20
47
34
–
64
38
44
25
20
14
26
10
90
6
16
8
–
–
–
tBuOH as solvent
phthalimide[b]
–
phthalimide[b]
trace
–
–
–
3
5
5
–
–
phthalimide[b]/MeOH as solvent
phthalimide[b]/acetone as solvent
phthalimide[b]/water as solvent
acetic acid (10 mol-%)
acetic acid (1 mol-%)
CsCO3 (1 mol-%)
CsCO3 (100 mol-%)
pyridine[b]
49
0
0
9
–
–
40
[b]
PPh3
–
6
without added water
70
18
[a] Reaction conditions: styrene (0.64 mmol), 4 (4 mol-%), CuCl2 (8 mol-%), H2O (0.7 mmol) in propan-2-ol (0.5 mL), O2 (1 atm), 16 h. Conversions and yields
were determined by 1H NMR integration. [b] 0.05 mmol, 8 mol-%.
To verify the activity of complex 4, the product distributions
of the oxidation of styrene catalyzed by various palladium com-
plexes and copper salts in the presence of phthalimide (8 mol-
%) and H2O (1.1 equiv.) in propan-2-ol (0.5 mL) at 50 °C were
Scheme 6. Oxidation of styrene.
investigated (Table 3). Clearly, complex 4 shows better activity
in this oxidation. Other types of palladium complexes exhibited
poor activity (Table 3, Entries 2–10). For comparison, the activity
Under the catalytic conditions, the oxidation of styrene gave
a mixture of the three possible products (Table 2, Entry 1). The
control experiments showed that an oxygen atmosphere, CuCl2,
and complex 4 were essential for complete conversion of the
substrate (Entries 2–5). Without CuCl2, colloidal metal or a zero-
valent complex was formed, and this indicates the necessity
of a re-oxidant for Pd0 in the catalysis.[14c] The best reaction
temperature for this catalytic oxidative cleavage is 50 °C. Lower
or higher temperatures led to greater production of aceto-
phenone (Table 2, Entries 6–8). The use of tBuOH as the solvent
in the oxidation of styrene leads to an increased production of
ꢀ-phenylacetaldehyde.[15b] When tBuOH was used in our study,
it provided a better selectivity in this oxidation as compared to
that in iPrOH (Table 2, Entry 9). We found that the selectivity
can be modified by the addition of phthalimide (8 mol-%) to
the catalytic system in iPrOH (Table 2, Entry 10). However, this
additive is not applicable to the other solvents such as MeOH
or acetone. Interestingly, the selectivity was totally reversed by
carrying out the reaction in water (Table 2, Entry 14), that is, the
reaction proceeded as a typical Wacker process (Scheme 5a).
Removal of water from the catalytic system decreased the con-
version of substrate under the standard catalytic conditions
(Table 2, Entry 21). Notably, this reaction catalyzed by 4 was
dramatically affected by the presence of acids or bases, which
led to either a lower or even no conversion of the substrate
(Table 2, Entries 15–20).
Table 3. Influence of metal complexes on the catalysis.[a]
Entry
Pd source[b]
Cu source
Conv.
[%]
Yield [%]
I
II
III
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
4
PdCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl
100
67
42
52
79
20
0
0
0
0
58
63
100
100
92
27
90
69
75
47
–
16
65
9
–
–
–
–
35
42
70
58
70
16
60
41
–
–
–
10
–
–
–
–
–
–
12
–
5
20
–
–
25
15
40
26
14
10
–
–
–
–
10
18
23
21
17
10
27
23
Pd(OAc)2
[Pd2(dba)3]
[Pd(CH3CN)2Cl2]
dppe[c]/PdCl2
bipy[c]/PdCl2
py[c] + PdCl2
DMAP/PdCl2
PPh3[c]/PdCl2
3[c] + PdCl2
6
4
4
4
4
CuBr
CuBr2
Cu(glycinate)2
CuCl2
4 (3 mol-%)
4 (2 mol-%)
–
–
CuCl2
[a] Reaction conditions: styrene (0.64 mmol), Pd complex (4 mol-%), Cu salt
(8 mol-%), H2O (0.7 mmol) in propan-2-ol (0.5 mL), O2 (1 atm), 16 h. Conver-
sion and yields were determined by 1H NMR integration. [b] bipy = 2,2′-
bipyridine, dba = trans,trans-dibenzylideneacetone, DMAP = 4-(dimethyl-
amino)pyridine, dppe = 1,2-bis(diphenylphosphino)ethane. [c] 0.05 mmol,
8 mol-%.
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of palladium complex 6[16] (Scheme 7) was examined, and its readily transformed into PhCHO under the catalytic conditions,
reactivity was poorer than that of 4 (Table 3, Entry 12), which consistent with the above observation. This in turn suggests
indicates that the carbonyl functionality may play some role in the efficiency of the oxidative cleavage is governed by the se-
the catalysis. PdCl2/2 also showed some activity in this oxid- lectivity of anti-Markovnikov versus Markovnikov Wacker oxid-
ation, but not as good as that of complex 4. Re-oxidation of ation.
Pd0 by CuII (generated in situ from CuI and O2) is quite common
To further validate the reaction pathway, both styrene oxide
in the Wacker reaction. In the current study, various copper salts
were suitable for this role in the oxidative cleavage of styrene,
except Cu(glycinate)2 (Table 3, Entries 13–16). A lower loading
of palladium catalyst decreases the conversion of the substrate,
that is, yields of products are lower (Table 3, Entries 17–18).
and 1-phenyl-1,2-ethanediol were examined as substrates un-
der the standard catalytic conditions. These reactions yielded
trace amounts of benzaldehyde and phenylacetaldehyde with
recovery of most of the starting substrates. The results clearly
rule out the possible pathway involving metal-catalyzed hydro-
peroxylation or hydroxylation followed by cleavage.
The scope of the reaction with respect to styrene derivatives
was investigated (Table 4). Under the optimized catalytic condi-
tions, various substituted styrenes underwent oxidative cleav-
age to yield the corresponding benzaldehydes in good yields,
except p-aminostyrene (Table 4, Entry 10). Presumably due to
the higher reactivity, the substrate p-aminostyrene yielded a
mixture of solid products. The palladium catalyst system is inac-
tive toward 4-vinylpyridine and 1-vinylimidazole (Table 4, En-
tries 11 and 12). In addition, the inactivity of α-methylstyrene
and ꢀ-methylstyrene toward this oxidation indicated that the
catalyst system is quite sensitive to substituent effects. Other
aliphatic substituted olefins such as 1-hexene and cyclohexane
were also inactive toward this catalytic system, with recovery of
the substrates.
Scheme 7. Structure of complex 6.
To elucidate more details of the reaction pathway, oxidation
of styrene catalyzed by 4 under standard catalytic conditions
was monitored to analyze the product distribution (Figure 2).
All three products were gradually formed in the beginning of
Instead of the product of oxidative cleavage, treatment of
the reaction. When the substrate was completely consumed (ca. phenylacetylene under the catalytic conditions described above
6 h), the yields of these three products reached the maximum. yielded 1,4-diphenylbutadiyne as the major product, which in-
Then, phenylacetaldehyde was slowly converted into benzalde- dicates that the homocoupling reaction is dominant over the
hyde, whereas the amount of acetophenone remained without Wacker process. However, internal alkynes gave 1,2-diones as
significant change. This suggests that benzaldehyde comes reaction products (Scheme 8). In several attempts, we found
from the C–C bond cleavage of phenylacetaldehyde. Further- that it gave the optimal yield of products by carrying out this
more, in a separate run, we found that phenylacetaldehyde was catalytic reaction in dioxane/water. Typically, a mixture of di-
Figure 2. Product yields along the reaction time catalyzed by 4.
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1
given in ppm relative to TMS for H and 13C NMR data. IR spectra
were measured on KBr pellets. Complex 6 was prepared according
to a reported method.[16]
Table 4. Reaction scope.[a]
Entry
ArCH=CH2 → ArCHO
Ar
Yield[b]
1
2
3
4
5
6
7
8
9
C6H5
p-BrC6H4
p-ClC6H4
m-ClC6H4
p-CH3C6H4
p-CH3OC6H4
p-NCC6H4
p-CH3COC6H4
p-HOC6H4
p-H2NC6H4
4-pyridinyl
1-imidazolyl
69 %
62 %
76 %
65 %
80 %
85 %
57 %
55 %
47 %
0 %
8-Hydroxyquinolin-2(1H)-one (1): m-Chloroperbenzoic acid (6.0 g,
34.8 mmol) was slowly added to a solution of quinolin-8-ol (5.0 g,
34.5 mmol) in CHCl3 (100 mL). The solution turned orange and gave
3-chlorobenzoic acid as a white precipitate. After removal of solids,
the solution was washed with 2 % aqueous NH4OH solution and
concentrated. The residue was recrystallized to yield 8-hydroxyquin-
oline 1-oxide (5.24 g, 95 %). This compound (1.11 g, 6.89 mmol) in
acetic anhydride (10 mL) was heated to reflux for 5 h. The reaction
mixture was slowly added to an ice-cold aqueous ammonia solu-
tion, and the solution was adjusted to pH 8. A bright yellow solid
(1.09 g, 78 %) was obtained, which was identified to be 2-oxo-1,2-
dihydroquinolin-8-yl acetate. Without further purification, a solution
of this acetate (1.0 g, 4.92 mmol) in methanol (30 mL) was treated
with K2CO3 (747 mg, 5.41 mmol) at ambient temperature for 1 h.
After removal of methanol, the residue was dissolved in water. On
addition of 10 % HCl solution, the desired product precipitated as
a white solid (0.7 g, 88 %). 1H NMR (400 MHz, [D6]DMSO): δ = 10.52
(br. s, 1 H), 7.83 (d, J = 9.2 Hz, 1 H), 7.08 (t, J = 4.4 Hz, 1 H), 6.98 (d,
J = 4.5 Hz, 2 H), 6.50 (d, J = 9.6 Hz, 1 H) ppm. 13C NMR (100 MHz):
δ = 161.7, 143.9, 140.6, 128.2, 122.1, 122.0, 120.1, 118.2, 114.7 ppm.
These spectral data are consistent with those reported.[9]
10
11
12
n.r.[c]
n.r.
[a] Olefin (0.64 mmol), 4 (4 mol-%), CuCl2 (8 mol-%), phthalimide (8 mol-%),
H2O (0.7 mmol) in IPA (isopropyl alcohol) (0.5 mL), O2 (1 atm), 16 h. [b] Yields
of isolated products. [c] No reaction.
arylacetylene, complex 4 (3 mol-%), and CuCl2 (10 mol-%) in
dioxane/water (5:1) was heated at 85 °C for 30 h. The desired
product was obtained by extraction. In all instances, the diones
were obtained in good yields (Table 5).
8-(Pyridin-2-ylmethoxy)quinolin-2(1H)-one (3): A mixture of 2
(500 mg, 3.10 mmol), 2-(bromomethyl)pyridine hydrobromide
(860 mg, 3.43 mmol), and K2CO3 (775 mg, 5.6 mmol) was loaded
into a round-bottom flask equipped with a reflux condenser. CH3CN
(30 mL) was added under anhydrous conditions. The solution was
heated at reflux temperature for 10 h. After cooling, water (20 mL)
and ethyl acetate (40 mL) were added. The organic portion was
collected, dried with MgSO4, and concentrated. The residue was
subjected to chromatography on silica gel with hexane/ethyl acet-
ate as eluent. The desired product was obtained as a light pink
solid (642 mg, 82 %). 1H NMR (400 MHz, CDCl3): δ = 10.02 (br., NH),
8.67 (d, J = 4.8 Hz, 1 H, 10-H), 7.78 (d, J = 8 Hz, 1 H, 2-H), 7.77 (td,
J = 8, 1.6 Hz, 1 H, 8-H), 7.58 (d, J = 8 Hz, 1 H, 7-H), 7.31 (t, J = 6 Hz,
1 H, 9-H), 7.21 (dd, J = 8, 1.2 Hz, 1 H, 3-H), 7.13 (t, J = 8 Hz,1 H, 4-
H), 7.09 (dd, J = 8, 1.2 Hz, 1 H, 5-H), 6.71 (d, J = 8 Hz, 1 H, 1-H), 5.37
(s, 2 H, 6-H) ppm. 13C NMR (100 MHz): δ = 162.6, 155.8, 149.4, 144.6,
140.8, 137.3, 128.8, 123.4, 122.5, 122.5, 122.0, 120.5, 120.4, 112.3,
Scheme 8. Oxidation of internal alkynes to diones.
Table 5. Oxidation of diarylacetylenes catalyzed by 4/CuCl2.[a]
R =
H
o-Me
p-Me
p-OMe
p-Cl
Yield
90 %
76 %
83 %
72 %
65 %
[a] Reaction conditions: diarylacetylene (1 mmol), 4 (3 mol-%), CuCl2 (10 mol-
%) in H2O/dioxane (1:5; 0.5 mL), O2 (1 atm) for 30 h. Yields of isolated prod-
ucts.
Conclusion
71.8 ppm. IR: ν(CO) = 1682 cm–1. C15H12N2O2·0.5H2O (261.28): calcd.
˜
The 8-hydroxyquinolinonate ligand was found to have unique C 68.95, H 5.02, N 10.72; found C 69.09, H 5.10, N 10.46.
coordination chemistry with PdII, and crystallographic charac-
terization of the coordination modes showed that the carbonyl
group acts as a bridging donor for the construction of a dimeric
Preparation of Pd Complex 4: A mixture of 3 (100 mg, 0.48 mmol)
and [Pd(CH3CN)2Cl2] (122.8 mg, 0.48 mmol) in CH2Cl2 (10 mL) was
heated to reflux for 4 h. After the reaction, diethyl ether was slowly
structure. Complex 4 was shown to be an effective catalyst for
added to the reaction mixture to give a precipitate. Filtration fol-
the oxidative cleavage of styrenes to yield the corresponding
lowed by washing with diethyl ether yielded the desired complex
benzaldehydes. An anti-Markovnikov Wacker process followed
by C–C cleavage is operative in this catalysis. Notably, the sub-
strate scope of this catalysis is limited to styrene derivatives.
4 as a dark yellow solid (123 mg, 79 %): 1H NMR (800 MHz, CD3CN):
δ = 8.93 (d, J = 2.9 Hz, 1 H), 7.92 (t, J = 3.84 Hz, 1 H), 7.84 (d, J =
4.8 Hz, 1 H), 7.64 (d, J = 4 Hz, 1 H), 7.45 (t, J = 3 Hz, 1 H), 7.29 (d,
Furthermore, complex 4 also acted as a good catalyst for the J = 4 Hz, 1 H), 7.26 (d, J = 4 Hz,1 H), 7.13 (t, J = 4 Hz, 1 H), 6.56 (d,
J = 4.8 Hz, 1 H), 6.14 (s, 2 H) ppm. 13C NMR (200 MHz, CD3CN): δ =
172.5, 158.5, 154.2, 144.7, 141.3, 130.1, 126.5, 126.1, 124.1, 123.7,
122.9, 121.8, 121.4, 113.1, 71.5 ppm. C30H22Cl2N4O4Pd2·2CH3CN
oxidation of diarylacetylenes to the corresponding 1,2-diaryl-
ethanedione.
(868.37): calcd. C 47.03, H 3.25, N 9.68; found C 46.83, H 3.01, N
9.46.
Experimental Section
General Information: Chemicals and solvents were of analytical
grade and were used without further purification. NMR spectra
were recorded with a 400 MHz spectrometer. Chemical shifts are
Crystallography: A crystal of 4 was obtained by recrystallization
from diethyl ether/acetonitrile. Cell parameters were determined
with a Siemens SMART CCD diffractometer. The structure was
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solved by using the SHELXS-97 program[17a] and refined by using
the SHELXL-97 program[17b] by full-matrix, least-squares methods
on F2 values. Crystal data: C34H28Cl2N6O4Pd2 (868.32), monoclinic,
P21/c, a = 11.7697(4), b = 11.1552(3), c = 25.4893(7) Å, ꢀ = 92.353(3),
V = 3343.75(17) Å3, Z = 4, ρcalcd. = 1.725 Mg/m3, F(000) = 1728,
crystal size: 0.20 × 0.15 × 0.10 mm, 3.02–27.49°, 14317 reflections
collected, 7312 reflections [R(int) = 0.0360], final R indices [I > 2σ(I)]:
R1 = 0.0411, wR2 = 0.0696, final R indices (all data): R1 = 0.0649,
wR2 = 0.0821, goodness of fit on F2 = 1.095. CCDC 1493123 (for 4)
contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre.
ppm. 13C NMR (100 MHz): δ = 194.9, 194.5, 146.4, 134.9, 133.2,
130.7, 130.2, 130.0, 129.9, 129.1, 22.1 ppm.
1-Phenyl-2-p-tolylethane-1,2-dione:[22] 1H NMR (400 MHz, CDCl3):
δ = 7.97 (d, J = 7.5 Hz, 2 H), 7.87 (d, J = 7.5 Hz, 2 H) 7.65 (t, J =
5.8 Hz, 1 H), 7.51 (t, J = 7.5 Hz, 2 H), 7.31 (d, J = 7.5 Hz, 2 H), 2.44
(s, 3 H) ppm. 13C NMR (CDCl3, 100 MHz): δ = 194.9, 194.5, 146.4,
134.9, 133.2, 130.7, 130.2, 130.0, 129.9, 129.1, 22.1 ppm.
1-(4-Methoxyphenyl)-2-phenylethane-1,2-dione:[23] 1H NMR
(400 MHz, CDCl3): δ = 7.96 (m, 4 H), 7.64 (tt, J = 7.5, 1.2 Hz, 1 H),
7.50 (t, J = 7.5 Hz, 2 H), 6.98 (d, J = 8 Hz, 2 H), 3.88 (s, 3 H) ppm.
13C NMR (100 MHz): δ = 195.0, 193.3, 165.1, 134.9, 133.3, 132.5,
130.0, 129.1, 126.2, 114.5, 55.8 ppm.
Typical Catalysis Procedure: Styrene substrate (0.25 mmol), Ru
complex (1 mol-%), and NaIO4 (1.025 mmol) in water (1 mL) were
loaded into a reaction vessel with a stirring bar. The mixture was
heated at 40 °C for 18 h. After the reaction, brine (4 mL) was added
to reaction mixture, which was then extracted with diethyl ether
(3 × 5 mL). The combined organic extracts were dried and concen-
trated. The residue was analyzed by NMR spectroscopy. Purification
by recrystallization provided the desired compound in pure form.
The spectral data of the organic products are essentially identical
to those reported.
1-(4-Chlorophenyl)-2-phenylethane-1,2-dione:[22]
(400 MHz, CDCl3): δ = 7.98–7.91 (m, 4 H), 7.68 (tt, J = 8, 1.2 Hz, 1
H), 7.55–7.47 (m, 4 H) ppm. 13C NMR (100 MHz): δ = 194.1, 193.2,
141.7, 135.2, 132.9, 131.5, 131.4, 130.1, 129.6, 129.2 ppm.
1H
NMR
Acknowledgments
We thank the Ministry of Science and Technology, Taiwan for
the financial support (MOST103-2113-M-002-002-MY3).
Benzaldehyde:[18] 1H NMR (400 MHz, CDCl3): δ = 10.03 (s, 1 H), 7.89
(d, J = 8 Hz, 2 H), 7.64 (t, J = 8 Hz, 1 H), 7.54 (t, J = 8 Hz, 2 H) ppm.
13C NMR (100 MHz): δ = 192.5, 136.5, 134.6, 129.9, 129.1 ppm.
Keywords: N,O ligands · Palladium · Oxidation · Synthetic
methods · Homogeneous catalysis
p-Bromobenzaldehyde:[18] 1H NMR (400 MHz, CDCl3): δ = 9.97 (s,
1 H), 7.74 (d, J = 8 Hz, 2 H), 7.68 (d, J = 8 Hz, 2 H) ppm. 13C NMR
(100 MHz): δ = 191.2, 135.2, 132.6, 131.1, 129.9 ppm.
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p-Chlorobenzaldehyde:[18] 1H NMR (400 MHz, CDCl3): δ = 9.98 (s,
1 H), 7.82 (dt, J = 8, 2.2 Hz, 2 H), 7.51 (dt, J = 8, 2.2 Hz, 2 H) ppm.
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m-Chlorobenzaldehyde:[19] 1H NMR (400 MHz, CDCl3): δ = 9.93 (s,
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1 H), 7.78 (d, J = 8 Hz, 2 H), 7.33 (d, J = 8 Hz, 2 H), 2.44 (s, 3 H) ppm.
13C NMR (100 MHz): δ = 192.2, 145.7, 134.3, 130.0, 139.8. 22.0 ppm.
p-Methoxybenzaldehyde:[18] 1H NMR (400 MHz, CDCl3): δ = 9.88
(s, 1 H), 7.83 (d, J = 8 Hz, 2 H), 7.00 (d, J = 8 Hz, 2 H), 3.88 (s, 3 H)
ppm. 13C NMR (100 MHz): δ = 191.0, 164.7, 132.1, 130.1, 114.4,
55.7 ppm.
p-Cyanobenzaldehyde:[18] 1H NMR (400 MHz, CDCl3): δ = 10.10 (s,
1 H), 7.99 (d, J = 8 Hz, 2 H), 7.85 (d, J = 8 Hz, 2 H) ppm. 13C NMR
(100 MHz): δ = 190.8, 139.0, 133.1, 130.1, 117.9, 117.8 ppm.
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Benzil:[22] 1H NMR (400 MHz, CDCl3): δ = 7.97 (d, J = 7.5 Hz, 2 H),
7.65 (t, J = 7.5 Hz, 2 H), 7.50 (t, J = 7.5 Hz, 4 H) ppm. 13C NMR
(100 MHz): δ = 194.7, 135.0, 133.0, 130.0, 129.1 ppm.
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δ = 7.98 (dd, J = 8, 1.2 Hz, 2 H), 7.66 (t, J = 1.2 Hz, 2 H), 7.55–7.48
(m, 3 H), 7.35 (d, J = 8 Hz, 1 H), 7.27 (t, J = 8 Hz, 1 H), 2.71 (s, 3 H)
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Received: August 1, 2016
Published Online: ■
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Palladium Catalysts
A dimeric PdII complex with an 8-(pyr-
idin-2-ylmethoxy)hydroxyquinolinon-
ate ligand was prepared and charac-
terized. This complex is a good catalyst
for the oxidative cleavage of styrenes
in the presence of CuCl2 as cocatalyst.
H.-M. Jhong, Y.-H. Liu, S.-M. Peng,
S.-T. Liu* ................................................ 1–8
Oxidative Cleavage of Styrenes Cat-
alyzed by a PdII Complex of an 8-
Hydroxyquinolinonate Ligand
DOI: 10.1002/ejic.201600951
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