An efficient trifunctional benzhydrazone ligated Pd(II) complex for Heck reaction of aryl bromides

Article abstract of DOI:10.1016/j.tetlet.2015.05.016

Synthesis of air and thermally stable trifunctional O, N, and O-terdentate benzhydrazone Pd(II) complex is reported and characterized by elemental analysis, spectral methods, and single crystal X-ray crystallographic technique. The new and reusable homogeneous catalyst has been effectively applied to the Mizoroki-Heck reaction of activated and deactivated aryl bromides with various olefins under mild reaction conditions in good to excellent yields up to 99%.

Full text of DOI:10.1016/j.tetlet.2015.05.016

Accepted Manuscript
An efficient trifunctional benzhydrazone ligated Pd(II) complex for Heck reac-
tion of aryl bromides
Subramanian Muthumari, Nanjan Mohan, Rengan Ramesh
PII:
S0040-4039(15)00818-7
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.05.016
TETL 46288
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
16 March 2015
20 April 2015
6 May 2015
Please cite this article as: Muthumari, S., Mohan, N., Ramesh, R., An efficient trifunctional benzhydrazone ligated
Pd(II) complex for Heck reaction of aryl bromides, Tetrahedron Letters (2015), doi: http://dx.doi.org/10.1016/
j.tetlet.2015.05.016
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An efficient trifunctional benzhydrazone ligated Pd(II) complex for Heck reaction of aryl bromides
Leave this area blank for abstract info.
Subramanian Muthumari, Nanjan Mohan and Rengan Ramesh
DMF, K2CO3
100oC, 10 h, N2
Br
R1
R1
R
R

1
Tetrahedron Letters
journal homepage: www.elsevier.com
An efficient trifunctional benzhydrazone ligated Pd(II) complex for Heck reaction of
aryl bromides
Subramanian Muthumari, Nanjan Mohan and Rengan Ramesh
School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, Tamil Nadu, India
ARTICLE INFO
ABSTRACT
Synthesis of air and thermally stable trifunctional O, N and O- terdentate
Article history:
Received
R\eceived in revised form
Accepted
benzhydrazone Pd(II) complex is reported and characterized by elemental analysis, spectral
methods and single crystal X-ray crystallographic technique. The new and reusable
homogeneous catalyst has been effectively applied to the Mizoroki- Heck reaction of activated
and deactivated aryl bromides with various olefins under mild reaction conditions in good to
excellent yields up to 99%.
Available online
2015 Elsevier Ltd. All rights reserved.
Keywords:
Benzhydrazone ligand
Palladium(II)complex
Crystal structure
Mizoroki-Heck reaction
Palladium catalyzed carbon- carbon bond forming
catalyst for Heck cross-coupling of aryl halides with butyl
acrylate at 145oC for 24 h.11 Furthermore, Mizoroki-Heck
reaction of activated and deactivated aryl halides with acrylates
or ally phenyl ethers using palladium hydrazone complexes under
argon atmosphere for 24 h with the formation of byproduct have
been reported by Takashi Mino et al13. It is worth noting that
palladium-phosphine complexes have been employed as an
effective catalyst in controlling the reactivity and selectivity in
the Mizoroki-Heck reaction because of its bulkiness and electron
donating properties, hence efforts to make well dsigned
phosphine- based ligands have been increased.14 The main role of
tertiary monophosphine ligands is to stabilize the palladium by
entering the catalytic cycle and consequently prevent the
formation of inactive palladium black. Recently the use of
sterically demanding, electron-rich tertiary phosphines as catalyst
modifiers in the Mizoroki-Heck reactions was reported.15
Thermally robust catalysts, phosphine-based palladium
complexes and palladacycle complexes have been prepared and
extensively employed not only with iodo arenes but also with the
less reactive bromo and chloro arenes.16 However, reported
synthetic methods have some drawbacks such as expensive
reagents, loss of activity, high temperature, long reaction time
and stoichiometric catalyst loading, formation of byproducts and
even moderate yields, makes it necessary to develop a sustainable
strategy.
organic transformations serve as useful tools in synthetic organic
chemistry.1,2 Especially, the study of Mizoroki-Heck reactions
are receiving considerable attention due to its wide range of
applications in natural product synthesis, pharmaceuticals,
herbicides, material sciences, molecular wires, liquid crystals and
bioactive compounds.3It has the ability to tolerate a variety of
functional groups such as such as unprotected amino, hydroxyl,
aldehyde, ketone, carboxyl, ester, cyano or nitro groups in
organic transformations.4 A very high turnover number could
already be observed for the Heck reaction of iodobenzene with
styrene,5 while the search for effective catalysts for the coupling
of bromo- and chloroarenes with alkenes is ongoing. The
development of catalysts with high turnover number for the
carbon–carbon coupling of alkenes with bromo- and chloroarenes
is highly important, since these are interesting substrates for
industrial applications.6 A number of articles on the coupling
reaction of aryl bromides and aryl chlorides has been reported by
using palladium complexes containing different ligands.7
Recently, Zhang et al have reported a novel D-
glucosamine based palladium catalyzed Mizoroki- Heck for aryl
bromides with good catalytic activity in 12 h.8 Gupta and co-
workers have described the use of palladium complexes
containing amide based macrocycle as active catalysts for Heck
reaction using 5 mol % of catalyst loading at 120oC along with
the formation of py-product.9 Further, furan carbothioamide
based palladacycle was reported as valuable catalytic precursor
for Heck reaction under Jeffery conditions at 120oC for 15
Generally, hydrazones have been widely investigated in
several research fields because of their chelating capability and
their catalytic and biological applications. Due to the excellent
h,10.Palladium γ-diimine complex has been reported as effective
———
Corresponding author. Tel.: +91 431 2407053; fax: +91 431 2407045/2407020; E-mail address: ramesh_bdu@yahoo.com

2
Tetrahedron
complexation ability towards transition metals, ease of
to the palladium (II) ion (Fig. S1, Supplementary material).
Multiplets observed in the region δ 8.2-7.0 ppm in the complex
have been assigned to the aromatic protons of PPh3and benzoyl
hydrazone ligands. The signal due to the azomethine proton
appears in the region δ 9.33 ppm. The position of the azomethine
signal in the complex is slightly downfield in comparison with
that of the free ligand, suggesting deshielding of the azomethine
proton due to its coordination to palladium. Absence of two sharp
peaks of -OH and -NH in the region δ12.7 ppm and δ12.2 ppm
supporting coordination of the ligand to the Pd(II) ion via the
phenolate oxygen. The singlet due to the -NH proton of the free
ligand in the region δ12.2 ppm is absent in the complex, further
supporting enolization and coordination of the imidolate oxygen
to the Pd(II) ion. Therefore, the 1H NMR spectrum of the
complex confirms the tridentate coordination mode of the
benzoyl hydrazone ligand to Pd(II) ion and also indicates the
presence of PPh3 group. The 13C NMR of the Pd(II) complex
showed resonance in the expected region (Fig. S2,
Supplementary material). The complex (δ142 ppm) revealed a
downfield shift of the azomethine carbon (=C=N-) relative to the
free ligands (δ145 ppm) indicating coordination of the
azomethine nitrogen to the metal centre. Also, the signal assigned
to the amide carbon (-NH-C=O), which moves upfield from δ168
ppm in the free ligand to δ162ppm in the Pd(II) complex, results
from the reduced bond order (-N=C-O) upon coordination.
synthesis and air stability, studies of hydrazone based ligands
became attractive.17 Depending on pH it can exist in two different
forms such as neutral and anionic.18Only few reports are
available for Mizoroki-Heck reaction catalyzed by employing
palladium hydrazone complexes. In continuation of our19 research
on synthesis and catalytic applications of Ru and Pd complexes19,
herein, we describe the synthesis and characterization of new air
stable palladium(II) benzhydrazone complex incorporated with
chloride and triphenylphosphine as ancillary ligands and its
catalytic performance in the Heck reaction of electronically
activated, deactivated and sterically hindered aryl bromides with
different olefins .
The benzoylhydrazone ligand was conveniently
prepared in excellent yield (92 %) by the condensation of 2-
hydroxy-1-naphthaldehyde with p-chlorobenzhydrazide in
equimolar ratio in ethanol under reflux. Further the ligand reacts
with [PdCl2(PPh3)2] in 1: 1 molar ratio in DMF/ ethanol and Et3N
for 2 h at reflux temperature to give the new Pd(II) complex of
the formula [Pd2(L)Cl2(PPh3)2] (1) in 85% yield (Scheme 1). The
oxidation state of palladium remains unchanged during the
formation of the complex. The complex is air stable in both the
solid and liquid states at room temperature and is non-
hygroscopic. The complex is readily soluble in polar and non
polar solvents such as dichloromethane, chloroform,
dimethylformamide and dimethyl sulfoxide producing intense
orange colored solutions. The elemental analysis data of the
complex (Calcd: C, 61.18; H, 3.93; N, 4.20. Found: C, 61.17; H,
3.95; N, 4.18) are in good agreement with the molecular structure
proposed.
Single crystals of the complex 1, was obtained by the slow
evaporation of DMF/Ethanol at room temperature. The molecular
structure of the complex has been determined by single crystal X-
ray diffraction to confirm the coordination mode of the ligand
and geometry of the complex. The ORTEP view of complex is
shown in Figure 1. The complex crystallized in P 1 space group.
The ligand coordinates with Pd (II) ion via the phenolic oxygen,
azomethine nitrogen and the imidolate oxygen forming five and
six membered chelate rings. One PPh3 group trans to the
azomethine nitrogen, and also coordinate with the Pd(II) ion to
form a ONPO square plane. The complex is having a distorted
square planar geometry as reflected in all the bond parameters
around Pd(II) ion. The bond lengths and bond angles are in good
agreement with the reported Pd(II) hydrazone complexes. The
bond angles around palldium(II) ion are O(1)-Pd(1)-O(2)
PPh3
OH
O
OH
O
O
Pd
N
O
DMF/Ethanol
H2N
Ethanol
O
Cl
N
H
N
PdCl2(PPh3
)
+
2
N
H
Et3N, Reflux, 2 h
N
Reflux, 3 h
Cl
Cl
(1)
Scheme 1. Synthesis of Pd(II)benzhydrazone complex (1)
The FT-IR spectrum of the ligand showed a medium to
strong band in the regions 3439 cm-1 and 3175 cm-1 which are
characteristic of the O-H and N-H functional groups respectively
and the free ligand also display νN=N and νc=O absorptions in the
region 1653 cm-1. The disappearance of the strong band due of
νO-H in the complex indicates the deprotonation of phenolic
oxygen prior to coordination to the palladium (II) ion. The
absence of νN=N and νC=O stretching vibrations in the complex
indicating that the ligand undergo tautomerization and
subsequent coordination of the imidolate oxygen to the palladium
(II) ion. The νC=N absorption is observed at a lower frequency in
the complex 1597 cm-1 indicating the coordination of azomethine
nitrogen to Pd(II) ion. This is also supported by the appearance
of new bands in the region 1245 cm-1 and 1597 cm-1 which may
be attributed to the C-O and C=N-N=C fragments respectively of
the coordinated ligand . In addition, other characteristic band due
to palladium bound PPh3 in the region 1428 cm-1 are also present
in the spectrum of the complex. The absorption spectrum of the
complex in chloroform at room temperature showed three bands
in the region 256-432 nm. The high intensity bands in the region
256-330 nm were assignable to ligand-centred (LC) transitions
and have been designated as π-π* and n-π* transitions. In the
complex the lowest energy bands observed in the region 432 nm
were attributed to the Pd(dπ) →L(π*) metal to ligand charge
transfer (MLCT) transitions.
=173.35(6)o,
O(1)-Pd(1)-P(1)=89.79(5)o,
N(1)-Pd(1)-O(2)
=88.34(7)o, O(2)-Pd(1)- P(1)= 96.85(5)o and N(1)-Pd(1)-
P(1)=176.20(5)o, and bond lengths of 1.9665(16) Å for Pd(1)-
O(1), 1.9812(16) Å for Pd(1)-O(2), 2.2963(9) Å for Pd(1)-
N(1)and 1.9752(18) Å for Pd(1)-N(1). The bond lengths and
bond angles are in good agreement with reported data on related
square planar palladium benzoylhydrazone complexes.20
1
The H NMR spectrum of the complex 1 was recorded
in CDCl3 to confirm the binding of the benzoyl hydrazone ligand
Figure 1: ORTEP diagram of complex 1 with 30%
probability.

3
To evaluate the catalytic efficiency of the complex (1) in Heck
reaction, the model coupling reaction between 4-
bromoacetophenone and t-butyl acrylate was conducted under the
optimal reaction conditions including, bases, solvents and
catalyst loading. In a typical reaction, 4-bromoacetophenone (1.0
equiv), t-butyl acrylate (2.0 equiv) and base K2CO3 (2.0 equiv)
are taken in the presence of catalyst (1.0 mol %) under open air at
room temperature and 46% isolated yield was obtained (Table 1,
and the results are summarized in table 2. The reaction of 4-
bromoacetophenone with t-butyl acrylate proceeded with
quantitative yield when 1.0 or 0.5 mol % of catalyst used (Table
2, entries 1 & 2). Even when 0.2 mol % of the catalyst used
(entry 3), excellent isolated yield of the cross coupling product is
obtained. Further, it was observed that even under very low
catalyst loading of 0.1, 0.01 and 0.001% (entries 4, 5 & 6), the
reaction the reactions underwent with a drop in isolated yields.
Further the reaction can also be conducted with an ultra-low
catalyst loading of 0.0001 mol% (entry 7) with high turnover
number. Since the isolated yield are good with appreciable TON
when 0.2 mol % of catalyst is used, it was concluded that 1:
500(C: S) ratio is the suitable condition for further coupling
reactions.
0
entry 1). When reaction temperature was raised to 100 C, the
desired product was obtained in 63% yield (Table 1, entry 2).
Next we have conducted the same reaction in nitrogen
atmosphere. To our delight, the reaction furnished excellent yield
in 10 h (Table 1, entry 3). In the absence of base no reaction was
occurred, indicates base is necessary for the reaction. In order to
find a suitable base, we screened several inorganic and organic
bases such as K2CO3, Na2CO3, KOH, Et3N and pyridine. Among
all the bases, inorganic bases gave better yield than organic bases
due to its high basic nature. In inorganic bases, K2CO3 furnished
highest yield of the coupled product (Table 1, entry 3-8). After
much experimentation on optimizing the solvent, it was found
that the reaction rate and yield was significantly enhanced by
using polar solvents like N,N-dimethylformamide and
dimethylsulfoxide (Table 1, entries 3 & 9). The ethereal dioxane
solvent or non polar toluene solvent was not effective for
coupling reaction (Table 1, entries 9 - 12). Hence, DMF was
preferred for further study.
Table 2
Effect of catalyst loadinga
O
Br
O
tBu
Complex 1
tBu
O
DMF, K2CO3
100cC, 10 h, N2
O
O
O
Entry
Catalyst (mol %)
Yieldb
TONc
1
2
1.0
0.5
96
94
96
188
3
4
0.2
0.1
93
69
465
6900
Table 1
Optimization of reaction conditionsa
5
6
0.01
44
32
4400
O
Br
O
0.001
32000
tBu
Complex 1
tBu
O
DMF,
100cC, 10 h, N2
O
O
O
7
0.0001
20
180000
Entry
Solvent
Base
K2CO3
Yield(%)b
TONc
46
aReaction condition: 4-Bromoaceophenone (1mmol), t-butyl acrylate
(2 mmol), base (2 mmol), solvent (2 mL) at 100oC for 10 h under N2
atmosphere.
1
2
DMF
46d
63e
94
--
DMF
DMF
DMF
DMF
DMF
DMF
DMF
K2CO3
K2CO3
--
63
94
--
bIsolated yield after column chromatography based on 4-
bromoacetophenone (average of two runs).
3
cTON mol of product/ mol of catalyst.
4
5
Na2CO3
KOH
89
81
56
50
85
46
69
67
89
81
56
50
85
46
69
67
The scope of the catalyst is explored further in the
coupling reaction of activated and deactivated aryl bromides with
different olefins such as t-butyl acrylate, methyl acrylate, styrene,
4-methyl styrene and 4-chlorostyrene under the optimized
condition. It has been observed from NMR spectra that only trans
product were obtained selectively in all the cases (Fig. S7- S27,
Supplementary material). In general, for a particular aryl
bromide, the cross coupled products were obtained in excellent
isolated yields within the range of 85% - 93 % with t-butyl
acrylate (entries 1-4). Similarly the catalyst works well with the
coupling of substituted aryl bromides and methyl acrylate (entries
5-8) and the maximum of 85% yield was obtained. The
corresponding reactions with styrene as the olelifinic partner
gave slightly lower yields (entries 9-12). Further the electron
donating methyl substituent on styrene offered better yield
(entries 13-16) when compared to that of styrene, where electron
withdrawing chloro substituent on styrene gave slightly lower
yield (entries 17-20). It was also observed that for a particular
olefin, activated electron deficient aryl bromide, 4-bromo
acetophenone could effectively coupled with the olefins
providing the corresponding coupled products in excellent yield
after 10 h. The non-activated electron neutral 4-bromo benzene
and deactivated electron rich 4-bromo toluene and 4-
bromoanisole offered moderate yields when coupled with the
olefins. All these observations indicated that the electronic effects
of substituents on the aryl bromides has great influence on the
6
7
Et3N
8
Pyridine
K2CO3
K2CO3
K2CO3
K2CO3
9
DMSO
H2O
10
11
12
Dioxane
Toluene
aReaction condition: 4-Bromoaceophenone (1mmol), t-butyl acrylate (2
mmol), base (2 mmol), solvent (2 mL) at 100oC for 10 h under N2
atmosphere.
bIsolated yield after column chromatography based on 4-
bromoacetophenone (average of two runs).
cTONmol of product/ mol of catalyst.
dreaction in open air at r.t 36 h.
ereaction in open air at 100oC for 36 h.
Further to optimize the catalyst loading, it is preferred
to use small amount of catalyst and still achieve high TON in the
Mizoroki-Heck cross coupling reactions. Recently, it has been
reported that low palladium catalyst loading is required in some
cases because the palladium black formation at high catalyst
loading inhibits the catalytic reaction.21To our pleasure the
reaction worked well with different catalyst: substrate (C/S ratio)

4
Tetrahedron
reaction rate and thus the electron withdrawing substituents
Table 4
were found to be more favorable for the coupling reactions. It is
our delight to mention that the present catalyst (1) has shown
better catalytic efficiency than palladium(II) thioamide
complex.22
Recycling of catalysta
Br
Ph
Complex 1
DMF, K2CO3
100cC, 10 h, N2
Ph
O
O
Table 3. Heck reaction of aryl bromides and alkenesa
Catalyst recycle Yieldb(%)
TONc
Br
Entry
R1
Complex 1
R1
R
DMF, K2CO3
100cC, 10 h, N2
R
1
2
3
4
5
1
2
3
4
5
75
71
62
20
15
375
350
310
100
75
Entry
1
-R
-R1
Yieldb(%)
93
TONc
465
450
435
425
420
410
400
365
405
440
380
375
435
415
430
385
410
395
360
350
-COCH3
-H
-COOtBu
-COOtBu
-COOtBu
-COOtBu
-COOCH3
-COOCH3
-COOCH3
-COOCH3
-C6H5
2
90
3
-CH3
87
4
-OCH3
-COCH3
-H
85
5
84
6
82
7
-CH3
80
a Reaction conditions: 4-bromoacetophenone (1 mmol), styrene
(2 mmol), K2CO3 (2 mmol), DMF (2 mL) at 1000 C for 10 h under N2
atmosphere.
b Isolated yield after column chromatography based on 4-
bromoacetophenone.
8
-OCH3
-COCH3
-H
73
9
81
10
11
12
13
14
15
16
17
18
19
20
-C6H5
88
-CH3
-C6H5
76
-OCH3
-COCH3
-H
-C6H5
75
c TON = Turnover number = ratio of moles of product formed to moles of
catalyst used.
-C6H4-4-CH3
-C6H4-4-CH3
-C6H4-4-CH3
-C6H4-4-CH3
-C6H4-4-Cl
-C6H4-4-Cl
-C6H4-4-Cl
-C6H4-4-Cl
87
83
In conclusion, a new air stable Pd(II) complex bearing
terdentate benzhydrazone ligand has been synthesized and
characterized. Single crystal X-ray diffraction studies of the
complex (1) evidenced the O, N and O co-ordination mode of the
ligand to palladium and reveals the presence of a distorted square
planar geometry around Pd(II) ion. The catalyst was found to be
an active and recyclable catalyst for the Mizoroki- Heck reaction
in good to excellent yields in relatively short time without any
additives.
-CH3
86
-OCH3
-COCH3
-H
77
80
79
-CH3
72
-OCH3
70
a Reaction conditions: aryl bromide (1 mmol), olefin
(2 mmol), K2CO3 (2 mmol), complex 1 (0.2 mol %), DMF (2 mL) at
1000 C for 10 h under N2 atmosphere.
Isolated yield after column chromatography based on aryl bromide
Acknowledgments
b
(average of two runs).
TON = Turnover number = ratio of moles of product formed to
moles of catalyst used.
We sincerely thank the UGC, New Delhi, India, for financial
support [F. 39/ 787/2010 (SR)], also UGC-RFSMS (fellowship to
S. M.), New Delhi, India, is gratefully acknowledged. We
express sincere thanks to DST-FIST, India for the use of Bruker
400 MHz spectrometer at the School of Chemistry,
Bharathidasan University, Tiruchirappalli.
c
Further, to show the recycling ability of our new
catalyst, the reaction of styrene with 4-bromoacetophenone was
conducted under optimal reaction condition. After the first use,
the catalyst was recovered by column chromatography and the
recovered catalyst was successfully reused in the subsequent
three cycles with consistent activity with 75 % ,71 % and 62 %
isolated yields. After that, a gradual loss of activity was observed
with trace amount of desired product in fourth and fifth cycles
(Table 4, entries 4& 5). Further, a key advantage of the ligand
and the complex over some others is that the reaction can
typically be performed in standard laboratory glassware without
any dry-box technique.
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