Nickel(II) thiosemicarbazone complex catalyzed Mizoroki-Heck reaction

Article abstract of DOI:10.1016/j.tetlet.2013.08.018

Convenient synthesis of a new square planar nickel(II) naphthaldehyde thiosemicarbazone complex has been described. The composition of the complex has been established by elemental analysis, spectral methods, and single crystal X-ray crystallography. The

Full text of DOI:10.1016/j.tetlet.2013.08.018

Accepted Manuscript
Nickel(II) thiosemicarbazone complex catalyzed Mizoroki-Heck reaction
Pandimuni Kalpaga Suganthy, Rupesh Narayana Prabhu, Venugopal
Shamugham Sreedevi
PII:
S0040-4039(13)01359-2
DOI:
http://dx.doi.org/10.1016/j.tetlet.2013.08.018
TETL 43379
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
24 May 2013
30 July 2013
2 August 2013
Please cite this article as: Suganthy, P.K., Prabhu, R.N., Sreedevi, V.S., Nickel(II) thiosemicarbazone complex
catalyzed Mizoroki-Heck reaction, Tetrahedron Letters (2013), doi: http://dx.doi.org/10.1016/j.tetlet.2013.08.018
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1
Tetrahedron Letters
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Nickel(II) thiosemicarbazone complex catalyzed Mizoroki-Heck reaction
Pandimuni Kalpaga Suganthy,a Rupesh Narayana Prabhu,b,† Venugopal Shamugham Sreedevia,
a
Department of Chemistry, Periyar E.V.R. College, Tiruchirappalli 620 023, Tamil Nadu, India
b School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, Tamil Nadu, India
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Convenient synthesis of a new square planar nickel(II) naphthaldehyde thiosemicarbazone
complex has been described. The composition of the complex has been established by elemental
analysis, spectral methods and single crystal X–ray crystallography. The new complex acts as an
active homogeneous catalyst for the Mizoroki-Heck reaction of electron deficient (activating)
and electron rich (deactivating) aryl bromides with various olefins under optimized conditions.
Available online
2009 Elsevier Ltd. All rights reserved.
Keywords:
Thiosemicarbazone ligand
Nickel(II) complex
Crystal structure
Mizoroki-Heck reaction
The chemistry of Schiff base complexes containing transition
metal ions is of significant importance due to their potentially
in the exploration of less costly transition metals as alternatives
to palladium systems for cross-coupling reactions while still
preserving high levels of efficiency and functional group
tolerance.
beneficial
catalytic
and
biological
properties.1
Thiosemicarbazones form an important class of Schiff base
ligand containing nitrogen and sulphur donor atoms. They exhibit
thione-thiol tautomerism in solution owing to the presence of the
–NH–C=S functional group. Depending on the reaction
conditions, basicity of the medium or the metal ions, these
ligands coordinate to a metal centre via the azomethine nitrogen
either in the neutral thione form or in the anionic thiolate form, as
bidentate ligands forming five membered chelate rings.2 The
ligand can also bind to the metal ion via the hydrazinic nitrogen
and the thiolate sulphur as monoanionic bidentate N,S donor
forming a four membered chelate ring.3
Nickel appears the most promising for the replacement of
palladium as catalyst for carbon-carbon cross-coupling reactions8
due to its relative low cost and ability to readily undergo
oxidative addition reactions with C-X bonds. However relatively
little effort has been dedicated to the development of nickel
complexes as catalysts for the Mizoroki-Heck reaction. The
earlier attempts to use the Ni(II) complexes, such as
[NiCl2(PPh3)2], to catalyze the coupling between activated olefins
and aryl halides in the presence of additives like zinc dust gave a
mixture of addition and substitution products, resulting in poor
selectivity.9 However, in subsequent years, nickel complexes
containing various ligands such as pincer,10 phosphine,11
carbene12 etc., were developed as effective catalysts for
Mizoroki-Heck coupling reactions, although so far no nickel
catalyst has been able to rival palladium for the Mizoroki-Heck
coupling reaction.
Transition-metal catalyzed cross-coupling of aryl halides and
olefins provides a simple and efficient route for the synthesis of
substituted molecules which are of significant importance as
intermediates in pharmaceutical industry, as UV absorbers, as
antioxidants or as molecular devices.4 Since its development in
the early 1970s,5 palladium catalyzed Mizoroki-Heck reaction
has become a powerful tool for the arylation of olefins.6 Unlike
other C–C coupling reactions, the Mizoroki-Heck reaction has
the ability to tolerate a variety of functional groups (such as
unprotected amino, hydroxyl, aldehyde, ketone, carboxyl or ester
groups) thus avoiding the need for protection and deprotection of
functional groups during the organic transformations.7 Though
palladium complexes show high catalytic activities, one obvious
Though there are a number of reports on the synthesis,
characterization, catalytic and biological applications of
nickel(II) thiosemicarbazone complexes,13 the use of these
complexes as catalyst in the Mizoroki-Heck reaction is not
explored in the literature. In this paper we describe the synthesis
and structural characterization of air- and moisture-stable square-
planar nickel(II) thiosemicarbazone complex, 1, that can catalyze
the Mizoroki-Heck reaction of aryl bromides with various
d—ra—wba—ck is the prohibitive cost of palladium which has resulted
 Corresponding author. Tel.: +91–431–242–0079; fax: +91–431–2332010; e-mail: sridevi.evr@gmail.com
† Present address: School of Chemistry, University of Hyderabad, Hyderabad – 500 046, Andhra Pradesh, India

2
Tetrahedron Letters
olefins. The complex has been characterized by analytical and
The fact, that nickel complexes are becoming increasingly
popular as catalyst in bringing about C–C cross coupling
reactions of different types,18 has led us to explore catalytic
efficiency of complex 1 in the Mizoroki-Heck reaction. The
initial test for the efficacy of the complex in this cross-coupling
reaction involved optimization of the base. The cross-coupling
reaction between 4-bromoacetophenone (substrate) with t-butyl
acrylate with 1 as catalyst in DMF solvent at 110 C was initially
examined (Table 1). A controlled experiment indicated that no
cross-coupling product was observed in the absence of the
catalyst or base. When inorganic bases like K2CO3 or Na2CO3
were used (entries 1,2), the isolated yields were excellent. The
yield of the cross-coupled product was considerably reduced
when inorganic acetates or organic bases were used (entries 3-7).
Thus, K2CO3 has a pronounced effect on the product yield.
spectral methods. The structure of the complex 1 has been probed
with the help of single crystal X-ray diffraction analysis.
The naphthaldehyde methylthiosemicarbazone ligand (HL)
was prepared by the condensation of 1-naphthaldehyde and N-
methyl thiosemicarbazide.14 Treatment of (CH3COO)2Ni4H2O
with 2 equivalents of the ligand in ethanol-dichloromethane at
room temperature resulted in the formation of the new square
planar nickel(II) thiosemicarbazone complex (1) of the formula,
[Ni(L)2] (Scheme 1).
CH3
HN
N
H
S
N
Ni(OAc)2..4H2O
N
S
H
2
HN
Ni
EtOH-CH2Cl2
RT, 3 h
NH
H
H3C
N
S
N
(1)
Table 1. Optimization of reaction conditionsa
NH
H3C
O
Scheme 1. Synthesis of nickel(II) thiosemicarbazone complex
O
Complex 1
DMF, Base
tBu
Br
tBu
O
+
O
110 oC, 24 h
O
O
In the FT-IR spectrum, the C=S and N–H of the –N–NH–C=S
group at 845 cm-1 and 3210 cm-1 respectively in the free ligand
disappeared in the complex suggesting enolization and
subsequent coordination through the thiolate sulfur to the Ni(II)
ion. The complex displays C=N stretch at 1591 cm1 which is at a
lower frequency than that of the free ligand (1626 cm1)
indicating coordination of azomethine nitrogen to Ni(II) ion.15 In
Entry
Base
Yieldb (%)
TONc
1
2
3
4
5
6
7
K2CO3
98
98
Na2CO3
96
96
CH3COOK
CH3COONa
Et3N
35
35
1
the H-NMR spectrum of the complex, the signals observed in
31
31
the region  8.3-6.9 ppm have been assigned to the aromatic
protons of the coordinated ligand. The singlet due to azomethine
proton ( 8.7 ppm) in the complex is slightly downfield
compared to the free ligand ( 8.0 ppm), suggesting deshielding
upon coordination to Ni(II) ion. The singlet that appeared for the
N–NH–C=S protons of the free ligand in the region  11.6 ppm is
absent in the complex, supporting enolization and coordination of
the thiolate sulphur to the Ni(II) ion.16
<10
<10
<10
<10
<10
<10
Pyridine
Pyrrolidine
a
Reaction conditions: 4-bromoacetophenone (5 mmol), t-butyl acrylate (10
mmol), base (6 mmol), complex 1 (1 mol%), DMF (5 mL) at 110 C for 24 h
under N2 atmosphere.
b Isolated yield after column chromatography based on 4-bromoacetophenone
(average of two runs).
The molecular structure of the complex 1 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 1 is shown in Figure 1. The nickel
ion is tetra coordinated in a square planar geometry by two ligand
molecules acting as monoanionic bidentate NS-donor forming
two five-membered rings. The ligand is in trans position with
respect to the C-N bond and the complex is centrosymmetric
around the nickel centre. The bond lengths and bond angles are in
good agreement with reported data on related Ni(II)
thiosemicarbazone complexes.17
c TON = Turnover number = ratio of moles of product formed to moles of
catalyst used.
Table 2. Effect of catalyst loadinga
O
O
tBu
Br
Complex 1
tBu
O
+
DMF, K2CO3
110 oC, 24 h
O
O
O
Entry
Catalyst (mol%)
Yieldb (%)
TONc
1
2
3
4
1.0
98
93
75
54
98
0.5
186
300
432
0.25
0.125
a
Reaction conditions: 4-bromoacetophenone (5 mmol), t-butyl acrylate (10
mmol), K2CO3 (6 mmol), DMF (5 mL) at 100 °C for 24 h under N2
atmosphere.
b Isolated yield after column chromatography based on 4-bromoacetophenone
(average of two runs).
c
TON = Turnover number = ratio of moles of product formed to moles of
catalyst used.
Figure 1. ORTEP view of 1 at 30% probability.

3
In order to optimize the effect of catalyst loading, different
catalyst:substrate ratios were tested in the cross-coupling reaction
of 4-bromoacetophenone with t-butyl acrylate to form 4-acetyl-
trans-cinnamic acid tert-butyl ester (Table 2). The reaction
proceeds well when 1.0 or 0.5 mol% of the catalyst is used
(entries 1,2). Upon changing the amount of catalyst from 0.25%
to 0.125 mol% of the catalyst, the reaction proceeds smoothly
accompanied by a drop in the isolated yield (entries 3-5). The
isolated yield is good with appreciable TON when 0.5 mol% of
the catalyst is used and it was concluded that this
catalyst:substrate ratio is the best suitable for the coupling
reaction.
electron deficient aryl bromide (4-bromo acetophenone) could
effectively couple with the olefins providing the corresponding
products in excellent yield after 24 h. The non-activated electron
neutral substrate (bromo benzene) and deactivated electron rich
substrate (4-bromo toluene) gave moderate amount of products
when coupling with the olefins.
The ultimate objective in designing Mizoroki-Heck coupling
catalyst is to form active species that can catalyze the coupling of
the more difficult aryl chlorides. No catalytic activity was
observed in the coupling of 4-chloroacetophenone with t-butyl
acrylate using complex 1 in DMF/ K2CO3 even after 24 h at
elevated temperatures of 150 C. This trend is consistent with the
increasing strength of carbon-halide bonds from aryl bromide to
aryl chloride.19 In terms of TON, the catalytic efficiency of these
nickel(II) thiosemicarbazone complexes are superior to the
previously reported nickel(II) complexes20 used as catalysts for
the Mizoroki-Heck reaction.
Table 3. Mizoroki-Heck reaction of aryl bromides with
olefinsa
Complex 1
(0.5 mol%)
R2
Br
R2
+
DMF, K2CO3
110 0C, 24 h
R1
R1
In conclusion, nickel(II) complex bearing thiosemicarbazone
ligand has been synthesized and characterized. The single crystal
X-ray evidenced the N and S coordination mode of the ligand
and a distorted square planar geometry around each Ni(II) ion.
Furthermore, the nickel(II) thiosemicarbazone complex was
found to be highly efficient homogeneous catalyst for the
Mizoroki-Heck reaction of aryl bromides with various olefinic
substrates.
Entry
-R1
-R2
Yieldb (%)
TONc
1
-COCH3
-H
-COOtBu
-COOtBu
-COOtBu
-COOCH3
-COOCH3
-COOCH3
-C6H5
93
85
76
94
86
77
80
73
67
77
69
60
87
80
72
186
170
152
188
172
154
160
146
134
154
138
120
174
160
144
2
3
-CH3
-COCH3
-H
4
Supplementary Material
5
6
-CH3
-COCH3
-H
Crystallographic data for the structural analysis have been
deposited with Cambridge crystallographic center, CCDC No.
892892. Copies of this information may be obtained free of
charge from The Director, CCDC, 12 Union roads, Cambridge
CB2 1EZ, UK (email: deposit@ccdc.cam.ac.uk). The summary
of the data collection and refinement parameters for the complex;
Selected bond parameters; Experimental procedures; 1H-NMR
spectrum of the complex 1; H-NMR data for all the Mizoroki-
Heck coupling products. Supplementary data associated with this
article can be found, in the online version.
7
8
-C6H5
9
-CH3
-COCH3
-H
-C6H5
10
11
12
13
14
15
-C6H4-4-CH3
-C6H4-4-CH3
-C6H4-4-CH3
-C6H4-4-Cl
-C6H4-4-Cl
-C6H4-4-Cl
1
-CH3
-COCH3
-H
-CH3
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*Revised Manuscript
Graphical Abstract
Square planar nickel(II) thiosemicarbazone complex was synthesized and characterized. The new complex acts as an
effective homogeneous catalyst for the Mizoroki-Heck reaction of aryl bromides with various olefins. The effects of base and
catalyst loading on the catalytic activity of the complex were also investigated.
Leave this area blank for abstract info.
Nickel(II) thiosemicarbazone complex catalyzed Mizoroki-Heck reaction
Pandimuni Kalpaga Suganthy, Rupesh Narayana Prabhu, Venugopal Shamugham Sreedevia,*
H3C NH
N
S
N
Ni
N
N
S
HN CH3
R2
Br
R2
+
DMF, K2CO3
110 0C, 24 h
R1
R1