Synthesis, characterization of palladium hydroxysalen complex and its application in the coupling reaction of arylboronic acids: Mizoroki-Heck type reaction and decarboxylative couplings

Article abstract of DOI:10.1016/j.inoche.2012.05.013

(Salen-OH)Pd (1, salen-OHN,N′-bis(3,5-di-hydroxysalicylidene)- ethylenediamine) was prepared by a simple one step reaction and fully characterized by ¹H and ¹³C NMR, IR spectroscopy, and X-ray crystallography. This palladium complex showed good activities as a catalyst in the Mizoroki-Heck-type reaction and the decarboxylative coupling reaction. In the Mizoroki-Heck type reaction, arylboronic acids and alkenes were reacted at 90°C for 3 h in the presence of 2.0 mol% of the palladium complex 1 and AgOAc to give the desired coupled product in good yields. In the decarboxylative coupling reactions, the desired coupled products were obtained in good yields when 0.5 mol% of the palladium complex was employed at room temperature.

Full text of DOI:10.1016/j.inoche.2012.05.013

Inorganic Chemistry Communications 23 (2012) 1–5
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, characterization of palladium hydroxysalen complex and its application in
the coupling reaction of arylboronic acids: Mizoroki–Heck type reaction and
decarboxylative couplings
⁎
Yumi Heo, Yi Young Kang, Thiruvengadam Palani, Junseong Lee , Sunwoo Lee ⁎⁎
Department of Chemistry, Chonnam National University, 300 Yongbong-dong, Buk-gu, Gwangju 500‐757, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
(Salen-OH)Pd (1, salen-OH_N,N′-bis(3,5-di-hydroxysalicylidene)-ethylenediamine) was prepared by a
simple one step reaction and fully characterized by 1H and 13C NMR, IR spectroscopy, and X-ray crystallog-
raphy. This palladium complex showed good activities as a catalyst in the Mizoroki–Heck-type reaction and
the decarboxylative coupling reaction. In the Mizoroki–Heck type reaction, arylboronic acids and alkenes
were reacted at 90 °C for 3 h in the presence of 2.0 mol% of the palladium complex 1 and AgOAc to give
the desired coupled product in good yields. In the decarboxylative coupling reactions, the desired coupled
products were obtained in good yields when 0.5 mol% of the palladium complex was employed at room
temperature.
Received 22 February 2012
Accepted 4 May 2012
Available online 15 May 2012
Keywords:
Palladium
Arylboronic acid
Mizoroki–Heck
Decarboxylative coupling
Hydroxysalen
© 2012 Elsevier B.V. All rights reserved.
Tetradentate N,N′-ethylenebis(salicylimine) (salen) is a popular
ligand system, which provides a square planar or octahedral geome-
try around the metal center. Since it can be prepared simply in a
one step reaction from inexpensive starting materials, transition
metal complexes having salen ligands have been widely studied and
developed as catalysts for use in many reactions, such as asymmetric
synthesis and polymerization for a long time [1–8]. Although several
palladium salen complexes have been reported, they have only low
activities as catalysts [9–13]. Thus, the design of novel palladium
salen complexes which show high catalytic activities is becoming an
important challenge. Inspired by the fact that there are only limited
examples of palladium salen coupling reaction catalysts, we designed
a new palladium salen complex having a hydroxyl group in the salen
ligand. Hydroxyl group can improve the solubility of complexes in
polar solvents which used in common coupling reactions. Moreover,
it can immobilize the catalysts onto the heterogeneous materials
such as single-walled carbon nanotubes [14].
of a palladium catalyst [16,17]. However, the coupling reaction with
an arylboronic acid and alkene to afford the Mizoroki–Heck-type
product has been less studied than the original Mizoroki–Heck reac-
tion, which is the coupling of aryl halides and alkenes [18–23].
Recently, we reported the decarboxylative coupling reaction of
alkynyl carboxylic acids and aryl halides using a palladium catalyst
for the first time [24,25]. This methodology afforded a variety of sym-
metric and unsymmetric diaryl alkynes with good yields [26]. Since
our first report, a variety of decarboxylative coupling reactions have
been developed including the coupling reaction of arylboronic acids
and alkynyl carboxylic acids [27–35].
It is well known that the active catalytic species in the coupling re-
action with arylboronic acids as electrophiles is palladium (II), which
is different from the coupling reaction with aryl halides in which pal-
ladium (0) is the active species [36]. Therefore, N,O-tetradentated
palladium complexes might be suitable catalysts in the coupling reac-
tion of an arylboronic acids. Although, a variety of new methods for
the coupling reaction with arylboronic acids have been attempted, in-
cluding the employment of nitrogen or phosphine-based ligand sys-
tems [37–39], to the best of our knowledge, there is no example of
the employment of a palladium salen catalyst in the coupling reaction
of arylboronic acids. In addition, a low catalyst loading is needed in
the coupling reaction of arylboronic acids, because most of the previ-
ously reported methods required a high palladium loading. Herein,
we report the synthesis of a palladium salen complex and its applica-
tion as a catalyst in the Mizoroki–Heck type and the decarboxylative
coupling reactions.
Coupling reactions with arylboronic acids have been widely devel-
oped in the field of palladium-catalyzed coupling reactions, due to the
advantages of these compounds, namely their stability to moisture
and air, easy availability and low toxicity [15]. One of the most well
studied coupling reactions is the Suzuki coupling, which is the cou-
pling reaction of arylboronic acids and aryl halides in the presence
⁎
Corresponding author. Tel.: +82 62 530 3371; fax: +82 62 530 3389.
⁎⁎ Corresponding author. Tel.: +82 62 530 3385; fax: +82 62 530 3389.
E-mail addresses: leespy@chonnam.ac.kr (J. Lee), sunwoo@chonnam.ac.kr (S. Lee).
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2012.05.013

2
Y. Heo et al. / Inorganic Chemistry Communications 23 (2012) 1–5
{1H} NMR spectra of compound 1 were recorded and compared with
those of the corresponding ligands salen-OH. The EI/HR-MS peaks of 1
are attributable to the loss of H, [1−H]− (m/z : calculated=400.9916;
observed=400.9987) [41].
Complex 1 in the solid state was stable for more than a week in air
according to 1H NMR spectroscopy. It is insoluble in most solvents;
however, it is soluble in a 4:1 mixture of water and acetone and
polar aprotic solvents, such as DMF and DMSO.
Scheme 1. Synthesis of (salen-OH) palladium complex 1.
Solid state molecular structures of compound 1 was characterized
by the single crystal X-ray diffraction method [42]. The suitable single
crystal for X-ray crystallography of complex 1 was obtained from a
solution of 4:1 mixture of water and acetone at 25 °C. Molecular
structure of complex 1 with selected bond lengths and angles in the
figure caption is shown in Fig. 1, which was drawn by ortep3. The
complex 1 crystallized in space group P21/n, and form a distorted
square planar geometry around the palladium metal center. The
bond lengths of Pd\OLig (1.947(2) Å and 1.950(2) Å) and Pd\NLig
(1.9903(17) Å and 1.9941(17) Å) fall in the range for other known
complexes [12].
In order to evaluate the ability of this palladium complex as a cat-
alyst, we first employed it in the coupling reaction of organoboronic
acids with alkenes, namely the Mizoroki–Heck type reaction. As an
appropriate test reaction, phenyl boronic acid and n-butyl acrylate
were employed as the substrates. The results are summarized in
Table 1. It is well known that an oxidant is required in the
Mizoroki–Heck type coupling reaction of arylboronic acids. We first
tested a variety of oxidants or bases. Among the oxidants and bases
screened, AgOAc showed the best result. The other silver cations
afforded very low yields (entries 2 and 3). Although copper (II) com-
plexes are known to act as an oxidant, only Cu(OAc)2 produced the
desired product in moderate yield (entry 4). Mild inorganic bases in-
cluding Na2CO3 showed almost no activity in this coupling reaction
(entries 7–9). Among the tested solvents, amide type solvents such
as NMP, DMAc and DMF afforded the desired product in good yields.
We found that aromatic solvents such as toluene and p-xylene and
Fig. 1. Molecular drawing of compound 1 and atom labeling. (H atoms were omitted for
clarity) Selected bond distances (A°): Pd\N1 1.947(2), Pd\N2=1.950(2), Pd\O1=
1.9903(17), Pd\O2=1.9941(17). Selected bond angle (°): N1\Pd\N2 84.02(10),
N1\Pd\O1 178.38(9), N2\Pd\O1 94.44(8), N1\Pd\O2 94.38(9). N2\Pd\O2
178.24(8), O1\Pd\O2 87.17(7).
The salen-OH ligand was prepared by the procedure reported in a
previous article [40]. (Salen-OH)Pd (1, salen-OH_N,N′-bis(3,5-di-
hydroxysalicylidene)-ethylenediamine) was prepared by the reaction
of the salen-OH ligand with PdCl2(CH3CN)2 in ethanol for 12 h at
50 °C (Scheme 1). Compound 1 was fully characterized by 1H and 13C
{1H} NMR, IR spectroscopy, and EI/HR-MS analysis. The 1H and 13C
Table 1
Optimization of the Mizoroki–Heck-type reaction of phenylboronic acid and n-butyl acrylate using palladium complexa.
Entry
Additive
Solvent
Temp (°C)
Yield (%)
1
2
3
4
5
6
7
8
AgOAc
Ag2CO3
Ag2O
Cu(OAc)2
CuCl2
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
120
95
8
5
68
5
14
3
Trace
Trace
88
92
Trace
7
CuBr2
Na2CO3
K2CO3
Cs2CO3
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
9
NMP
DMF
10
11
12
13
14
15
16
17
18b
19c
DMAc
p-Xylene
Toluene
Diglyme
monoglyme
Dioxane
EtOH
8
Trace
Trace
88
19
67
DMSO
NMP
a
Reaction conditions: ArB(OH)2 (0.33 mmol), alkene (0.3 mmol), Pd complex (0.006 mmol), AgOAc (0.45 mmol), and NMP (1.5 mL) were stirred at 90 °C for 6 h.
Determined by gas chromatography (GC) analysis with internal standard.
Pd complex (0.003 mmol) was employed.
b
c

Y. Heo et al. / Inorganic Chemistry Communications 23 (2012) 1–5
3
Table 2
ether type solvents such as monoglyme, diglyme and dioxane are not
The Mizoroki–Heck-type reactions of arylboronic acids and alkenesa.
suitable solvents in this coupling reaction. Among the polar solvents,
EtOH showed good yields, but DMSO did not. When the amount of
catalyst was decreased to 1.0 mol% at 120 °C, the desired product
was obtained with 67% yield.
The best reaction conditions for the Mizoroki–Heck type coupling
reaction is 2.0 mol% of the palladium catalyst, AgOAc as the oxidant
and NMP as the solvent. Under these conditions, we did not find any
byproducts derived from the homocoupling of phenyl boronic acid
or the Michael-type oxidative addition reaction. In addition, this palla-
dium complex showed higher turnover numbers than the ligand free
palladium catalyst [23].
We evaluated the optimized reaction conditions for the coupling re-
actions with different arylboronic acids and alkenes, as presented in
Table 2 [43]. The Mizoroki–Heck-type coupling products were afforded
as the major products in all cases. The coupling of phenyl boronic acids
and alkenes showed good yields (entries 1–5). Arylboronic acids
bearing an electron donating group at the para position produced the
desired products with good yields (entries 6–10). Ortho-tolyl boronic
acid gave lower yields than meta-tolyl boronic acid due to its steric hin-
drance (entries 11–14). Arylboronic acids having electron withdrawing
groups such as acetyl and phenyl also afforded the corresponding
coupled products in good yields (entries 15–18). We found that styrene
showed a higher coupled yield than the other alkenes.
Next, we turned our attention to the decarboxylative coupling of
alkynyl carboxylic acids with arylboronic acids, which was originally
developed by the Loh group [33]. The employment of arylboronic
acids as a coupling partner afforded milder reaction conditions than
those of aryl halides. However, it required a higher catalyst loading
(5.0 mol%) and additional additive such as a molecular sieve. Therefore,
a more general and simple reaction method needs to be developed.
First, we applied the optimized conditions for the Mizoroki–Heck-type
coupling reaction to the decarboxylative coupling of phenyl propiolic
acid with phenyl boronic acid [44]. As shown in Table 3, the desired
product, diphenyl acetylene, was obtained with 18% yield in DMF at
90 °C in the presence of 2.0 mol% of the palladium complex (entry 1).
Among the tested silver complexes, Ag2O showed good yield in the
presence of KOAc in DMF solvent at 90 °C (entry 4). The employment
of AgOAc did not give a satisfactory result at 25 °C in either DMF or
CH2Cl2 (entries 5 and 6). The best result was obtained in the presence
of Ag2O and KOAc in CH2Cl2 at 25 °C, as reported by the Loh group
(entry 7). Decreasing the amount of the palladium complex to
1.0 mol% also gave the desired product with high yield at room temper-
ature without a molecular sieve (entry 8). We found that the palladium
complex showed higher catalytic activities in the decarboxylative cou-
pling of arylboronic acids and phenyl propiolic acid than those reported
by the Loh group.
Yield
(%)c
Entry
1
ArB(OH)2
Alkenes
Productsb
88
86
72
2
3
4
5
6
92
98
82
7
85
98
66
97
8
9
10
11
12
13
55
75
88
98
72
14
15
16
86
We evaluated the scope of the substrates in the decarboxylative
coupling of alkynyl carboxylic acids and arylboronic acids under these
optimized conditions. The results are summarized in Table 4. Phenyl
boronic acid coupled with phenyl propiolic acid, 2-octynoic acid, 3-
(4-methoxyphenyl)propiolic acid, and 3-(4-acetylphenyl)propiolic
acid to produce the corresponding arylated alkynes in 94%, 92%,
91% and 48% yields, respectively (entries 1–4). Sterically demanding
arylboronic acids gave low yields of the coupled product (entries 5
and 6). Arylboronic acids bearing electronic donating groups such
as methyl and methoxy showed moderate to good yields (entries
7–12). However, arylboronic acids having an acetyl group, which is
an electron withdrawing group, afforded the coupled product in
34% and 31% yields (entries 13 and 14). Heteroaromatic boronic
acids such as pyridine-4-yl boronic acid produced the desired
coupled products in 28% and 24% yields (entries 15 and 16). Unfortu-
nately, we could not obtain the desired coupled products from
thiophen-2-yl boronic acid (entries 17 and 18).
17
18
95
96
a Reaction conditions: ArB(OH)2 (3.3 mmol), alkene (3.0 mmol), palladium complex 1
(0.06 mmol), AgOAc (4.5 mmol), and NMP (15.0 mL) were stirred at 90 °C for 6 h.
b
All compounds were characterized by comparison using gas chromatography (GC)
analysis, 1H and 13C NMR spectra with authentic samples or literature data. cIsolated
yield.
reaction. This palladium complex was employed as a catalyst in
Mizoroki–Heck type and decarboxylative coupling reactions. It
showed high catalytic activities in both cases when arylboronic
acids were used the coupling substrate. In addition, it required a
In summary, we synthesized the air- and moisture-stable palladi-
um (II) complex 1 bearing a salen-OH ligand using a simple one step

4
Y. Heo et al. / Inorganic Chemistry Communications 23 (2012) 1–5
Table 3
Optimization of the decarboxylic coupling reaction of phenylboronic acid and phenylpropiolic acid using palladium complexa.
Entry
Pd (mol%)
Base
Additiveb
Solvent
Temp.(°C)
Yield (%)c
1
2
3
4
5
6
7
8
2.0
2.0
2.0
2.0
2.0
2.0
2.0
1.0
AgOAc
Ag2CO3
Ag2O
–
DMF
DMF
DMF
DMF
90
90
90
90
25
25
25
25
18
7
2
31
0
34
98
97
–
–
Ag2O
KOAc
–
AgOAc
AgOAc
Ag2O
DMF
–
CH2Cl2
CH2Cl2
CH2Cl2
KOAc
KOAc
Ag2O
a
Reaction condition: PhB(OH)2 (0.33 mmol), phenylpropiolic acid (0.3 mmol), Pd complex 1 (0.006 mmol or 0.003 mmol), Ag complex (0.39 mmol), and solvent (1.5 mL) were
stirred for 12 h.
b
c
0.39 mmol was used.
Determined by gas chromatography (GC) analysis with internal standard.
Table 4
lower catalytic loading than the previously reported catalysts. To the
best of our knowledge, this is the first example in which a palladium
salen complex showed catalytic activities in coupling reactions.
The decarboxylative coupling reactions of arylboronic acids and alkynyl carboxylic acidsa.
Acknowledgment
This work was supported by National Research Foundation of
Korea Grant funded by the Korean Government (2010‐0003141).
Appendix A. Supplementary data
CCDC 867810 contains the supplementary crystallographic data
for 1. The data can be obtained free of charge via http://www.ccdc.
cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
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carboxylic acids and aryl halides, J. Org. Chem. 74 (2009) 1403–1406.
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[42] Crystal data for 1: Monoclinic, space group=P21/n, a=8.2695(4) Å, b=11.1772(6)
Å, c=19.0167(10) Å, α=γ=90.00 deg, β=98.370(3) deg, V=1738.99(16) Å3,
Z=4, F(000)=888, D=1.102 g m−3, R1=0.0297, wR2=0.0790 for 4662 reflec-
tions with I0>2 σ(I0). The data were collected at 293(2) K on a APEX-II diffractometer
with monochromated Mo Kα radiation (λ=0.71073 Å). The structure was solved by
the direct method and refined on F2 by full-matrix least squares using the SHELXTL-97
program.
[43] Typical experimental of Mizoroki-Heck type coupling reactions: ArB(OH)2 (3.3 mmol),
alkene (3.0 mmol), Pd complex 1 (24.3 mg, 0.06 mmol), and AgOAc (751.1 mg,
4.5 mmol) were combined with NMP (15.0 mL) and stirred at 90 °C for 6 h. The
reaction mixture was poured into 20 mL of saturated aqueous ammonium chloride
and extracted with Et2O (3×20 mL). The combined ether extracts were washed
with brine (60 mL), dried over MgSO4, and filtered. The solvent was removed
under vacuum. The crude product was purified by flash chromatography (15%
ethyl acetate in hexane).
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tions of aryl and vinyl halides and triflates with α, β-Ynoic acids using silver
oxide, Adv. Synth. Catal. 351 (2009) 2827–2832.
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Chem. 74 (2009) 7199–7202.
[44] Typical experimental of Decarboxylative coupling reactions: ArB(OH)2 (3.3 mmol),
alkynyl carboxylic acid (3.0 mmol), Pd complex 1 (12.2 mg, 0.03 mmol), Ag2O
(903.8 mg, 3.9 mmol) were combined with KOAc (382.8 mg, 3.9 mmol) in CH2Cl2
(15.0 mL) and stirred at room temperature for 12 h. The reaction mixture was
poured into 20 mL of saturated aqueous ammonium chloride and extracted with
Et2O (3×20 mL). The combined ether extracts were washed with brine (60 mL),
dried over MgSO4, and filtered. The solvent was removed under vacuum. The crude
product was purified by flash chromatography (15% ethyl acetate in hexane).
[29] S. Ranjit, Z. Duan, P. Zhang, X. Liu, Synthesis of vinyl sulfides by copper-catalyzed
decarboxylative C–S cross-coupling, Org. Lett. 12 (2010) 4134–4136.
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decarboxylative coupling, Org. Lett. 12 (2010) 2000–2003.
[31] W.-W. Zhang, X.-G. Zhang, J.-H. Li, Palladium-catalyzed decarboxylative coupling
of alkynyl carboxylic acids with benzyl halides or aryl halides, J. Org. Chem. 75
(2010) 5259–5264.