Copper-free Sonogashira reactions catalyzed by a palladium(II) complex bearing pyrenealdehyde thiosemicarbazonate under ambient conditions

Article abstract of DOI:10.1016/j.tetlet.2015.07.076

Convenient synthesis of a new square-planar mononuclear palladium(II) complex bearing N,S-donor 1-pyrenaldehyde 4-methyl-3-thiosemicarbazonate (L^-) as ligand is described. The identity of the complex has been established as [Pd(L)Cl(PPh₃

Full text of DOI:10.1016/j.tetlet.2015.07.076

Accepted Manuscript
Copper-free Sonogashira reactions catalyzed by a palladium(II) complex bear-
ing pyrenealdehyde thiosemicarbazonate under ambient conditions
Rupesh Narayana Prabhu, Samudranil Pal
PII:
S0040-4039(15)01240-X
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.07.076
TETL 46565
Reference:
Tetrahedron Letters
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26 July 2015
Please cite this article as: Prabhu, R.N., Pal, S., Copper-free Sonogashira reactions catalyzed by a palladium(II)
complex bearing pyrenealdehyde thiosemicarbazonate under ambient conditions, Tetrahedron Letters (2015), doi:
http://dx.doi.org/10.1016/j.tetlet.2015.07.076
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Tetrahedron Letters
Copper-free Sonogashira reactions catalyzed by a palladium(II) complex bearing
pyrenealdehyde thiosemicarbazonate under ambient conditions
Rupesh Narayana Prabhu,§ Samudranil Pal
School of Chemistry, University of Hyderabad, Hyderabad 500 046, India
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Convenient synthesis of a new square-planar mononuclear palladium(II) complex bearing N,S-
donor 1-pyrenaldehyde 4-methyl-3-thiosemicarbazonate (L) as ligand is described. The identity
of the complex has been established as [Pd(L)Cl(PPh3)] by elemental analysis, ESI-MS, FT-IR,
NMR and single crystal X-ray crystallographic measurements. The complex has been found to
be an active and efficient homogeneous catalyst for the copper free Sonogashira coupling
reactions of phenylacetylene with various aryl halides (bromides and chlorides) at room
temperature under aerobic conditions.
Available online
Keywords:
Thiosemicarbazonate
Palladium(II) complex
Crystal structure
2015 Elsevier Ltd. All rights reserved.
Sonogashira reaction
———
§ Present address: Department of Chemistry, Srimad Andavan Arts And Science College, Tiruchirappalli 620 005, India
 Corresponding author. Tel.: +91 40 2313 4829; fax: +91 40 2301 2460; e-mail: spal@uohyd.ac.in (S. Pal)

2
Tetrahedron Letters
The Sonogashira cross-coupling reaction between an aryl
halide and a terminal alkyne has emerged as one of the most
widely used carbon-carbon bond forming reactions in organic
chemistry. The reaction has been utilized as an efficient
methodology to synthesize dendrimers,1 conjugated oligomers
and polymers,2 substituted alkynes,3 intermediates in natural
products, pharmaceuticals,4 optical materials,5 etc. Since its
discovery in 1975,6 the original procedure has been modified
from time to time by varying the palladium-source, ligand,
solvent, base, additive, reaction temperature and catalyst loading
to expand the scope of the reaction.7 The most common catalytic
systems used for this coupling reaction include [PdCl2(PPh3)2],
PdCl2/PPh3, and [Pd(PPh3)4] together with copper(I) salt as the
co-catalyst. However, if the reactions are not performed under
oxygen free conditions copper salts can induce Glaser-Hay type
homocoupling of terminal alkynes to yield diynes, which are
difficult to separate from the desired products due to their similar
chromatographic mobility8 and in addition, potentially explosive
copper acetylides can also be formed. Hence development of
copper-free systems has received considerable attention.7,9
Significant progresses in this direction have been made using
palladium nanoparticles,10 palladacycles11 and a variety of
palladium complexes with Schiff bases,12 pincers,13 carbenes14
and N-15 and P-donor16 ligands.
Scheme 1. Synthesis of [Pd(L)Cl(PPh3)] (1).
In the infrared spectra, the lowering of the C=N stretching
frequency of 1 (1577 cm–1) compared to that of the free HL (1600
cm1) indicates coordination of the azomethine-N to the metal
centre in 1. The thioamide NH (3150 cm–1) and C=S (833 cm1)
stretches displayed by free HL are not observed in the spectrum
of 1. The nonappearance of these bands suggests deprotonation
of the thioamide moiety and subsequent coordination of the
thioamidate-S to the metal centre in 1.19a-c,20d,23
In the 1H NMR spectra, the azomethine proton of L in 1 and
that of the free HL resonates as a singlet at  10.22 and 9.23 ppm,
respectively. The downfield shift of the signal in the complex
suggests deshielding of the CH=N proton due to coordination
of the N-atom to the palladium(II) centre. In case of 1, the
absence of the singlet that appeared at  11.5 ppm for the
thioamide proton of the free HL supports the deprotonation of
thioamide and thioamidate-S coordination to the palladium(II)
centre.19a-c,20b,24 The chemical shifts of the other protons of the
coordinated PPh3 and thiosemicarbazonate ligands are
unexceptional. In the 13C NMR spectrum of 1, a downfield shift
of the azomethine-C ( 153.0 ppm) relative to that of the free HL
( 140.1 ppm) substantiates the coordination of the azomethine-N
to the metal centre. The signal due to the thioamide-C in free HL
( 178.1 ppm) moves upfield in 1 ( 166.5 ppm) indicating
Thiosemicarbazones containing N- and S-donor atoms act as
versatile ligands for transition metal ions and exhibit a wide
range of coordination modes in their metal complexes.17 Though
there are plenty of reports related to the biological applications of
thiosemicarbazones and their metal complexes,18 research into
their role in catalysis is still a relatively new area of interest. In
recent years, some palladium(II) thiosemicarbazone complexes
have been employed as effective catalysts for carbon-carbon and
carbon-heteroatom coupling reactions.19 Though there are a few
reports on the use of such complexes as catalysts for Sonagashira
cross-coupling reactions under aerobic conditions,20 there is
ample scope for developing new air-stable palladium(II)
thiosemicarbazonate complexes that can act as efficient catalysts
for the room temperature copper-free Sonogashira reaction of
aryl bromides and the more challenging aryl chlorides in air. In
continuation of our research on transition metal complexes with
Schiff bases derived from acid- and thioacid-hydrazides and
coordination of the thiolate-S to the palladium(II) ion.19a-c,25
A
singlet resonance observed at  28.0 ppm in the 31P NMR
spectrum of 1 is in agreement with the existence of one PPh3
ligand.19b,26
thiosemicarbazide,21 herein we report
a
new air-stable
palladium(II) complex with 1-pyrenaldehyde 4-methyl-3-
thiosemicarbazone and its application as catalyst in the copper-
free, homogeneous Sonogashira cross-coupling reactions of
phenylacetylene with aryl bromides and also with aryl chlorides
at room temperature in air.
Condensation of 4-methyl-3-thiosemicarbazide with 1-
pyrenaldehyde in acidic ethanol affords the Schiff base 1-
pyrenaldehyde 4-methyl-3-thiosemicarbazone (HL, where H
represents the dissociable thioamide proton).21a,22 The square-
planar palladium(II) complex, [Pd(L)Cl(PPh3)] (1), was
synthesized in ~67% yield by refluxing equimolar amounts of
[Pd(PPh3)2Cl2] and HL in toluene in presence of triethylamine
(Scheme 1). The elemental analysis (Calc.: C, 61.67; H, 4.06; N,
5.83%. Found: C, 61.56; H, 4.12; N, 5.75%) and ESI-MS
spectroscopic (m/z {M+H}+ Calc.: 720.0622; Found: 720.0619)
data of 1 are in good agreement with the proposed molecular
formula of 1. The complex is non-hygroscopic and air stable in
both solid and liquid states at room temperature. It is soluble in
organic solvents such as chloroform, dichloromethane, toluene,
acetonitrile, methanol, dimethyl sulphoxide, dimethylformamide,
etc., producing intense orange coloured solution.
Figure 1. ORTEP of 1 at the 50% probability level. Some selected atoms are
labeled for clarity. Selected bond lengths (Å) and angles (º): PdN(1)
2.0983(16), PdS 2.2584(6), PdP 2.2703(5), PdCl 2.3472(6), N(1)C(17)
1.292(3), N(1)N(2) 1.391(2), N(2)C(18) 1.290(3), C(18)S 1.758(2),
C(18)N(3) 1.349(3), N(1)PdS 82.61(5), N(1)PdCl 94.09(5), SPdP
91.863(19), PPdCl 91.029(19), N(1)PdP 173.79(5), SPdCl 171.95(2).
The molecular structure of 1 has been determined by single
crystal X-ray diffraction to confirm the coordination mode of the
ligand and the geometry of the complex. The ORTEP view of 1
is shown in Figure 1. In 1, the metal centre is tetracoordinated.
The five-membered chelate ring forming thiosemicarbazonate
(L) coordinates to the metal centre via the azomethine-N and the
thioamidate-S atoms and the remaining two coordination sites are
occupied by the P-atom of the PPh3 and a chloride ion.
Consequently the palladium(II) centre resides in a NSClP
coordination environment which is distorted from the ideal

square-planar geometry as manifested in the bond parameters
around it. The N(2)C(18) and C(18)S bond lengths of 1.290(3)
and 1.758(2) Å, respectively are consistent with the
deprotonation of the thioamide functionality in L. All the metal
centred bond lengths and bond angles in 1 are consistent with
those found in structurally characterized related square-planar
palladium(II) complexes containing thiosemicarbazonate
ligands.19a-c,20b,2426
achieve maximum conversion, the yield of the target product was
verified with different catalyst loadings (Table 2). Good isolated
yields of diphenylacetylene were obtained when 1.0 or 0.5 mol %
of the catalyst (entries 1 and 2) was used. The yield significantly
dropped with catalyst loading of 0.2 or 0.1 mol % (entries 3 and
4). For further room temperature reactions with various
substrates, DMF as solvent, Et3N as base and catalyst loading of
0.5 mol % were used as optimized conditions.
Table 1
Table 2
Optimization of reaction conditionsa
Effect of catalyst loadinga
Entry
Solvent
Toluene
p-Xylene
Acetone
THF
Base
Yieldb (%)
<20
<20
<20
<20
57
Entry
mol % of 1
Yieldb (%)
1
Et3N
1
2
3
4
1.0
0.5
0.2
0.1
85
81
56
23
2
Et3N
3
Et3N
4
Et3N
a Bromobenzene (1.0 mmol), phenylacetylene (1.5 mmol), DMF (5 ml),
Et3N (3.0 mmol).
5
MeCN
DMSO
DMF
Et3N
6
Et3N
77
b Isolated yield after column chromatography.
7
Et3N
85
Table 3
8
DMF
NaOAc
KOH
NaHCO3
K2CO3
30
Sonogashira reaction of aryl bromides with phenylacetylenea
9
DMF
<20
46
10
11
DMF
DMF
58
a
Bromobenzene (1.0 mmol), phenylacetylene (1.5 mmol), 1 (1 mol%),
solvent (5 ml), base (3.0 mmol), room temperature, 12 h.
b Isolated yield after column chromatography.
Entry
1
Yieldb (%)
99
TONc
198
Product
NO2
COCH3
CHO
H
Among the various cross-coupling reactions, Sonogashira
reaction of aryl halides with phenylacetylenes has emerged as a
practical and efficient method for the preparation of diaryl
acetylenes. It is very apparent from the available literature reports
that the reaction conditions such as solvent, base and the amount
of catalyst loading influence the yield of product formed, though
there is no well-defined rule that a specific solvent or a certain
base can be used to attain highest efficiency. Thus to determine
the effectiveness of 1 as catalyst in the Sonogashira cross-
coupling reaction optimization of the reaction condition was
performed. As a model system, the reaction of bromobenzene
with phenylacetylene in the presence of 1 as catalyst under
various reaction conditions was examined at room temperature
under aerobic conditions (Table 1). Various solvents were
screened in the beginning (entries 1-7). As can be inferred from
the scrutiny of the obtained results, the reaction proceeded
relatively well in polar solvents when compared with non-polar
hydrocarbons and DMF emerged as the solvent of choice with
excellent isolated yield of diphenylacetylene at room
temperature. Significant sensitivity to base in DMF was also
noted. Among the various bases screened (entries 7-11), Et3N
gave excellent results at room temperature. Other bases such as
NaOAc, KOH, NaHCO3 or K2CO3 were less active or inactive
under the studied conditions. When the coupling reaction was
carried out at 50 C, the yield of diphenylacetylene was slightly
higher (90%, 10 h). However, at elevated temperatures in
addition to the desired product, the undesired biphenyl (10% at
80 C, 10 h; 15% at 110 ˚C, 8 h) was also obtained due to the
homocoupling of bromobenzene, thereby decreasing the yield of
the diphenylacetylene. Hence, the coupling reactions were
performed at room temperature. It may also be noted that
reactions those proceed at room temperature have significant
practical advantages relative to those that require elevated
temperatures. In addition, the formation of the inactive
palladium-black was not observed during the reaction. Controlled
experiments showed that the absence of the catalyst or the base
resulted in no detectable cross-coupling product. In order to
2
3
4
5
6
7
8
9
92
90
81
77
74
72
67
88
184
180
162
154
148
144
134
176
CH3
OCH3
OH
NH2
CHO
10
73
146
OCH3
11
12
78
68
156
136
OHC
H3CO
a Aryl bromide (1.0 mmol), phenylacetylene (1.5 mmol), 1 (0.5 mol %), DMF
(5 ml), Et3N (3.0 mmol).
b Isolated yield after column chromatography.
c TON = ratio of moles of product formed to moles of catalyst used.
By using the above mentioned optimized reaction conditions
the copper free room temperature Sonogashira coupling reactions
of phenylacetylene with activated (electron-withdrawing

4
Tetrahedron Letters
substituents), non-activated (unsubstituted) and deactivated
expected to be the primary reason for the variation of their
reactivities.
(electron-donating substituents) aryl bromides were investigated
(Table 3). All the reactions were carried out under identical
conditions to allow comparison of the results. Notwithstanding
the variation of the electronic nature of the substituents, all the
aryl bromides coupled rather smoothly with phenylacetylene and
the corresponding internal alkynes were obtained in moderate to
excellent yields. It was observed that aryl bromides containing
electron-withdrawing groups (4-nitro, 4-acetyl and 4-formyl)
could effectively couple (entries 13) and provide the
corresponding products in excellent isolated yields in 12 h.
Bromobenzene (entry 4) and aryl bromides containing electron-
donating groups (4-methyl, 4-methoxy, 4-hydroxy and 4-amino)
(entries 58) gave moderate amount of the corresponding internal
alkynes. Coupling of meta-bromo benzaldehde (entry 9) is less
effective than that of the corresponding para substituted
derivative (entry 3). Notably, sterically hindered ortho-bromo
benzaldehde also participated in the reaction and the desired
product was obtained in satisfactory yield (entry 11). A similar
trend was observed in the case of ortho-, meta- and para-bromo
anisoles (entries 6, 10 and 12).
In conclusion, a mononuclear square-planar palladium(II)
complex of formula [Pd(L)Cl(PPh3)] (L = 1-pyrenaldehyde 4-
methyl-3-thiosemicarbazonate) has been synthesized and
characterized by elemental analysis and spectroscopic (ESI-MS,
FT-IR, NMR) measurements. Single crystal X-ray diffraction
analysis revealed the coordination of the thiosemicarbazonate
ligand through the azomethine-N and the thioamidate-S atoms
and a distorted square planar geometry around the metal centre.
The utility of the new complex as an effective catalyst for the
copper-free room temperature Sonogashira cross-coupling
reaction of activated, neutral and deactivated aryl bromides and
also chlorides with phenylacetylene under aerobic conditions has
been demonstrated.
Acknowledgments
Dr. R. N. Prabhu thanks the University Grants Commission
(UGC), New Delhi for the Dr. D. S. Kothari Postdoctoral
Fellowship (No. F. 13-897/2013(BSR)). We are grateful to the
Department of Science and Technology (DST), New Delhi and
the UGC for the facilities provided under the FIST and the CAS
programmes, respectively.
Table 4
Sonogashira reaction of aryl chlorides with phenylacetylenea
Supplementary Material
Entry Product
1
Yieldb (%)
90
TONc
90
Crystallographic data for 1 have been deposited with the
Cambridge Crystallographic Data Centre as supplementary
publication no. CCDC 1408691. Copies of the data can be
obtained, free of charge, on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK, (fax: +44-(0)1223-336033 or e-
mail: deposit@ccdc.cam.ac.uk). Other supplementary material
associated with this article includes experimental procedures; 1H,
NO2
COCH3
CHO
H
2
3
4
5
6
7
82
78
64
58
54
50
82
78
64
58
54
50
1
13C and 31P NMR spectra of the complex; H and 13C NMR data
for all the coupling products.
CH3
OCH3
OH
References and notes
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6
Tetrahedron Letters
Graphical Abstract
Leave this area blank for abstract info.
Copper-free Sonogashira reactions catalyzed by a palladium(II) complex
bearing pyrenealdehyde thiosemicarbazonate under ambient conditions
Rupesh Narayana Prabhu, Samudranil Pal*

Highlights
• A Pd(II) complex catalyst with precise structure
for Sonogashira reactions is reported.
• The catalyst was used at room temperature under
aerobic conditions.
• This catalytic process displays good functional
group tolerance and high reactivity.