Palladium chloride catalyzed photochemical Heck reaction

Article abstract of DOI:10.1139/cjc-2012-0377

PdCl₂ catalyzed carbon-carbon bond formation (Heck reaction) between substituted aryl halides and olefins was carried out without a ligand, under irradiation with UV-visible light. The results demonstrated that UV-visible light accelerated the rate of the reaction, leading to an excellent yield of corresponding products. The recovered palladium nanoparticles could be thermally recycled several times. PdCl₂ gave excellent conversion up to the fifth addition of substrate.

Full text of DOI:10.1139/cjc-2012-0377

348
ARTICLE
Palladium chloride catalyzed photochemical Heck reaction
Suresh B. Waghmode, Sudhir S. Arbuj, Bina N. Wani, and C.S. Gopinath
Abstract: PdCl2 catalyzed carbon–carbon bond formation (Heck reaction) between substituted aryl halides and olefins was
carried out without a ligand, under irradiation with UV–visible light. The results demonstrated that UV–visible light accelerated
the rate of the reaction, leading to an excellent yield of corresponding products. The recovered palladium nanoparticles could
be thermally recycled several times. PdCl2 gave excellent conversion up to the fifth addition of substrate.
Key words: C–C coupling, photochemical reaction, Heck reaction, iodobenzene.
Résumé : On a effectué les réactions de Heck, formation de liaisons carbone–carbone par réactions d'halogénures d'aryles
substitués et d'oléfines, catalysée par le PdCl2 sans ligand, sous irradiation par de la lumière UV–visible. Les résultats démontrent
que la lumière UV–visible accélère la vitesse de la réaction qui conduit a` d'excellents rendements des produits correspondants.
Les nanoparticules de palladium récupérées peuvent être recyclées a` plusieurs reprises d'une façon thermique. Le PdCl2 conduit
a` d'excellentes conversions, jusqu'a` la cinquième addition de substrat. [Traduit par la Rédaction]
Mots-clés : couplage C–C, réaction photochimique, réaction de Heck, iodobenzène.
the trans product was considerably low. Still there is a need for the
development of a new catalyst for the Heck reaction under UV–
Introduction
The Heck reaction,1 for the carbon–carbon (C–C) bond forma-
tion between aryl halides and olefins is an important reaction in
organic chemistry.2 It provides the simplest and most efficient
way to synthesize various compounds useful in the pharmaceuti-
cal and agrochemical industries.3 The Heck reaction tolerates var-
ious functionalities on both of the reactants. Traditionally, the
Heck reactions have been carried out using 1–5 mol% palladium
along with phosphine ligands in the presence of a suitable base
under thermal conditions. Most of the efforts have been directed
to enhance the catalytic activity of palladium by using homoge-
neous as well as heterogeneous reaction conditions.4 Various li-
gands based on phosphorous,5,6 nitrogen,7 sulfur,8 and carbene9
have been employed successfully. However, owing to their sensi-
tivity to the air and moisture, their toxicity, the expense, and their
unrecoverable and severe reaction conditions, a ligand free
Heck reaction has been developed.10–13 Many researchers have
studied the Heck reaction over a heterogeneous catalyst, such
as palladium, supported on various metal oxides,10a silica,10b
carbon,10c,10d zeolite,10e polyaniline,11 polyacrylamide,12 polyeth-
ylene glycol, and non-cross-linked polystyrene.13 The incorpora-
tion of palladium in zeolite and ordered mesoporous materials
enhances the catalytic activity and recyclability.10e,14 Recently,
simple palladium compounds such as PdCl2 or Pd(OAc)2, which
are without any fancy ligands, have been increasingly used for the
Heck reaction, owing to their low cost.15 To increase the effective-
ness of the catalyst, various techniques such as ultrasonication,16
microwaves,17 electrochemicals,18 and photocatalysis19 have also
been used. Photochemical reactions have reported Heck, Suzuki–
Miyaura, and Stille-type coupling reactions successively over
palladium acetate and phosphine ligands.19d,19e,19f Recently,
Chaudhary and Bedekar reported Pd(OAc)2 and 1-(␣-aminobenzyl)
2-naphthols, as ligands, catalyzed Mizoroki–Heck, Suzuki–
Miyaura, and Sonogashira reactions under sunlight. The time
taken for this reaction is very long (5–9 days19g) and selectivity for
visible light irradiation (UV–vis). UV–vis energy is abundant,
cheap, and the “greenest” source of energy. As we were involved
in a program aimed towards the development of new strategies
for C–C bond formation, we were encouraged to design a conve-
nient and effective route to C–C bond formation under UV–vis
without a ligand.20 Herein, a UV–vis irradiated Heck reaction with
various aryl iodides and olefins using PdCl2 as a catalyst is re-
vealed. Applicability of this methodology and optimization of re-
action parameter is studied.
Results and discussion
The reaction between iodobenzene and ethyl acrylate under
UV–vis was studied as a model system (Scheme 1). The reaction
parameters were systematically optimized using various solvents,
bases, and different palladium salts. In a photochemical Heck
reaction, 99% conversion of iodobenzene and 9:1 selectivity for
trans and cis isomers was observed in 3 h (Table 1; entry 1). The cis
product was observed due to the UV–vis induced isomerization of
alkenes (trans–cis).21 To confirm whether the reaction is photo-
chemical or thermal, the reaction was carried out without UV–vis
(thermally) at 45 °C by keeping all other parameters constant. It
gave only 12% conversion with 100% selectivity for the trans-ethyl
cinnamate (Table 1; entry 2). This result shows that UV–vis exerts
an influence on C–C bond formation.
Among the various solvents studied, dimethyl formamide
(DMF) and dimethyl acetamide (DMAc) gave excellent conversion
and selectivity for trans-ethyl cinnamate (Table 1; entries 1–2),
whereas DMSO and acetonitrile (ACN), as solvents, gave poor
yields (entries 5, 6). Under photochemical reaction conditions, it is
very likely that UV–vis helps towards the formation of species
intermediate between PdCl2, solvent, base, and substrate, which
accelerates the rate of C–C bond formation reaction. The forma-
tion of these intermediate species depends upon the nature of the
nucleofuge and the C–I bond strength. The order of reactivity is
Received 29 September 2012. Accepted 26 November 2012.
S.B. Waghmode and S.S. Arbuj. Department of Chemistry, University of Pune, Ganeshkhind, Pune-411007, India.
B.N. Wani. Chemistry Division, Bhabha Atomic Research Centre, Mumbai-400085, India.
C.S. Gopinath. Catalysis Division, National Chemical Laboratory, Pune-411008, India.
Corresponding author: Suresh B. Waghmode (e-mail: suresh@chem.unipune.ac.in).
Can. J. Chem. 91: 348–351 (2013) dx.doi.org/10.1139/cjc-2012-0377
Published at www.nrcresearchpress.com/cjc on 5 December 2012.

Waghmode et al.
349
Scheme 1. Photochemical Heck reaction between iodobenzene and
Table 2. Reaction of iodobenzene with ethyl acrylate using different
ethyl acrylate.
palladium catalysts.
Selectivity (%)
EtO2C
CO2Et
hυ, PdCl2, TEA
temp. 45 3 ºC
CO2Et
Sr. No.
Catalyst
Conversion (%)
Trans
Cis
+
I
+
1
PdCl2
Pd(OAc)2
99
53
55
84
92b
3
88
91
88
87
94
100
92
94
12
9
12
13
6
—
8
5
cis
trans
2
3
4
5
6
7
8
Pd(NH3)4Cl2.H2O
Pd(NH3)4(OAc)2
Na2PdCl6. 4 H2O
10% Pd/C
[(C6H5)3P]4Pd
PdCl2 and 5% PPh3
Table 1. Reaction of iodobenzene with ethyl acrylate using different
solvents.
Selectivity (%)
43
72
Entry
Solvents
Conversion (%)
Trans
Cis
1
DMF
DMF
DMAc
NMP
DMSO
ACN
99
12
99
87
27
11
88
100
87
94
71
12
—
13
6
29
26
—
Note: Reaction conditions: iodobenzene (1 mmol), ethyl acrylate (3 mmol),
triethyl amine (3 mmol), 5 mL DMF, and 0.5 mol% catalyst were stirred for
3 h under a 400 W mercury vapor lamp at 45 3 °C.
bTime = 5 h.
2
3
4
5
6
7
74
—
conversion with 78% and 53% selectivity for the trans isomer,
respectively (Table 3; entries 9, 10). The effect of substitution on
various positions in an aromatic ring was studied and activity
order was as follows, ortho- > meta- > para-iodomethylbenzoate.
We found that bromo- and chloro-substituted aryl iodides gave
selectively iodo-coupled products (Table 3; entries 16, 17). Di-
substituted products were observed for the reaction between 1,3
di-iodobenzene and ethyl acrylate with 100% conversion showing
74% selectivity for di-substituted product (Table 3; entry 18).
Gray colored Pd0 nanoparticles were observed during the pho-
tochemical C–C coupling reaction. The synthesis of Pd nanopar-
ticles was confirmed by UV–vis adsorption spectra and TEM
analysis (see the Supplementary data). The UV spectrum of PdCl2
in DMF showed absorbance at 422 nm, owing to d–d transitions,22
whereas Pd0 did not show surface plasmon bands in the UV–vis
range.23 TEM analysis showed cubic crystalline structure of palla-
dium particles with a size of 120 10 nm (see the Supplementary
data). X-ray diffraction pattern of the Pd nanoparticles was re-
corded, and were found to match earlier reports (JCPDS No. 46-
1043).24 The observed binding-energy values of Pd 3d5/2 and 3d3/2
core levels were 335.3 and 340.7 eV, respectively25. X-ray photo-
emission spectra confirmed the formation of Pd0 particles. Some
Pd2+ was also observed at 336.5 eV, attributed to unreacted PdCl2.
After the completion of the reaction, Pd nanoparticles were
recovered by centrifugation and washed several times with DMF.
These nanoparticles were not active under photochemical reac-
tion conditions, which suggest that the oxidative insertion of me-
tallic palladium into iodobenzene to form an intermediate species
was not possible under the photochemical reaction conditions,
whereas PdCl2 easily formed the intermediate species. However,
the recovered palladium nanoparticles were thermally recycled at
130 °C, and the catalytic activity was found to be same, even for
the sixth recycle (Supplementary data). Further, to study the recy-
clability of PdCl2 towards the photochemical Heck reaction, we
carried out the reaction using iodobenzene and ethyl acrylate.
After completion of the reaction first cycle, in the first addition,
all of the reactants except PdCl2 were added to the earlier reaction
mixture, and the reaction was continued for 3 h. This gave an
excellent conversion up to the fifth addition of reagents (see the
Supplementary data).
Xylene
NR
Note: Reaction conditions = iodobenzene (1 mmol), ethyl acrylate (3 mmol),
triethyl amine (3 mmol), 5 mL solvent, and 0.5 mol% PdCl2 were stirred for 3 h under
a 400 W mercury vapour lamp at 45 3 °C; NR, no reaction.
I > Br > Cl. Our efforts to characterize the intermediate species by
spectroscopic methods (UV spectrometer), did not give any infor-
mation, and we speculate that the half-life of the intermediate
species was too small for detection. Nonpolar solvents do not
show activity (entry 6). The order of activity was DMF ϳ DMAc >
NMP > DMSO > ACN > xylene.
The catalytic activity of different palladium sources for the C–C
coupling under photochemical conditions was studied and the
results are summarized in Table 2. It was observed that palladium
with oxidation states 0, II, and IV were active, whereas Pd/C did
not show any activity. Among the Pd salts, PdCl2 showed excellent
catalytic activity as well as selectivity towards the trans-ethyl cin-
namate (Table 2 entry 1). The results of this study indicate that
chloride-based Pd compounds are catalytically more active com-
pared with other Pd compounds (Table 2). The solubility of the
base affects the yield of the reaction; photochemical Heck reac-
tion with organic bases, such as TEA and TBA, showed excellent
yields compared with inorganic bases such as Na2CO3 and K2CO3.
This could be attributed to the homogeneity of these bases in the
reaction mixture. The study of different solvents indicated that
TEA was the best among the studied bases. The effect of PdCl2
concentration and UV–vis duration for the iodobenzene conver-
sion was studied. Complete conversion was obtained within 3 h
for iodobenzene; continuing the reaction increased cis-ethyl cin-
namate with increase in irradiation time. After 25 h of irradiation
the selectivity for trans- and cis-ethyl cinnamate at a ratio of 5:4 is
obtained. This observation suggests that the trans product was
isomerized to cis under UV–vis.21
The applicability of this methodology was studied for various
substituted aryl iodides and olefins, and the results are summa-
rized in Table 3. Reaction of iodobenzene with ethyl, methyl, or
butyl acrylate gave satisfactory conversion and selectivity (Table 3;
entries 1–3). Styrene and acryl amide (entries 4, 5) gave 46% and
10% conversion with 32% and 47% selectivity for the trans product,
respectively. Ortho-iodophenol gave 55% conversion with 100% se-
lectivity for the trans product (entry 6). Ortho- and para-anisole
gave 50% and 85% conversion with 51% and 78% selectivity for the
corresponding trans products, respectively (entries 7, 8). Gener-
ally, ortho-substituted compounds required more reaction time
compared with meta- and para-substituted aryl iodides. Amino-
and nitro-substituted substrates were not active under photo-
chemical reaction conditions. Owing to the optical absorption of
UV–vis by these functional groups, the rate of C–C bond formation
was retarded. Ortho- and para-iodoacetanilide gave 99% and 80%
Conclusions
In conclusion, we have developed a simple PdCl2-catalyzed pho-
tochemical C–C coupling reaction for various substituted aryl io-
dides and olefins that favors a good yield for the corresponding
coupled products at ambient reaction conditions. The Heck reac-
tion gave minor amounts of cis product, owing to photochemical
isomerization of the initially formed trans product. Polar aprotic
solvents gave excellent yields of the desired product, compared
with nonpolar solvents. Chloride-based Pd sources were more cat-
Published by NRC Research Press

350
Can. J. Chem. Vol. 91, 2013
Table 3. The photochemical Heck coupling of aryl halides and olefins using PdCl2.
R'
R
R
R
R'
R'
h , PdCl2, TEA
temp. 45 3 ºC
+
I
+
cis
trans
R- H, OH, OMe, NHCOMe, CO2Me, COMe, Cl, Br, I
R=- Ph, CO2Me, CO2Et, CO2Bu, CO2NH2
Selectivity (%)
Trans Cis
Entry
R
X
R=
Conversion (%)
1
2
H
H
I
I
CO2Et
CO2Me
CO2nBu
Ph
99
99
88
89
78
32
47
100
51
64
78
53
77
84
88
—
91
12
11
3
H
I
100b
46b
10b
22
68
53
—
49
36
22
47
23
16
12
—
9
4
H
I
5
H
I
CONH2
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
6
2-OH
I
55b
50b
100b
99c
80c
100b
84c
70c
7
8
9
2-OMe
4-OMe
2-NHCOMe
4-NHCOMe
2-CO2Me
3-CO2Me
4-CO2Me
4-COMe
4-COMe
4-Br
I
I
I
I
I
I
I
Br
I
10
11
12
13
14
15
16
17
18
NR
79c
I
I
I
100b
85b
100b
63
79
74#
37
21
19
4-Cl
3-I
Note: Reaction conditions = substrate (1 mmol), olefin (3 mmol), triethylamine (3 mmol), 5 mL DMF, and 0.5 mol%
PdCl2 were stirred for 3 h under a 400 W (mercury vapour lamp) UV-visible light irradiation at 45 3 °C.
bReaction time = 5 h.
cReaction time = 7 h.
#Di-substituted trans compound.
alytically active compared with other Pd sources. Organic bases
were superior to inorganic bases. We believe that during
the reaction, PdCl2 easily formed intermediate species with
iodobenzene, solvent, base, and reagent, which accelerated the
rate of reaction. Recovered palladium nanoparticles were unable
to form intermediate species, which formed with PdCl2 upon
UV–vis. The recovered palladium nanoparticles were easily sepa-
rated and reused several times as a heterogeneous catalyst under
thermal Heck reaction conditions without significant loss of cat-
alytic activity. The PdCl2 was active up to the fifth addition of all
the reagents.
(75 MHz) NMR spectra were recorded in CDCl3 using a Varian
mercury spectrometer. XPS measurements were made with a VG
ESCALAB 3000 at room temperature, and MgK␣ X-ray radiation
was employed.26
Supplementary data
Supplementary data are available with the article through the
journal Web site at http://nrcresearchpress.com/doi/suppl/10.1139/
cjc-2012-0377.
Acknowledgements
The authors S.B.W. and S.S.A. thank the Department of Science
Technology, New Delhi, India, and the Bhabha Atomic Research
Centre, Mumbai, for financial support and fellowship, respec-
tively.
Experimental section
General procedure for the photochemical Heck reaction
To a 25 mL round bottom flask 1 mmol of aryl iodide, 3 mmol of
olefin, 3 mmol of triethyl amine (TEA) in dimethyl formamide
(DMF; 5 mL) were mixed, and to this mixture 1 mg of PdCl2 was
added. The resulting reaction mixture was irradiated under a
400 W mercury vapor lamp with continuous stirring in a closed
box, free from stray light. The reaction temperature was mea-
sured directly in a reaction vessel. The mercury vapor lamp was
kept vertically in the quartz tube, which was equipped with the
cold water circulation system to minimize the heating effect. The
progress of the reaction was monitored by GC (HP-5890) equipped
with capillary column (HP-5, 30 m × 0.32 mm × 0.25 ␮m). After
completion, the reaction mixture was diluted with water (10 mL)
and the products were extracted using ethyl acetate (3 × 10 mL).
The combined organic layer was dried over anhydrous sodium
sulfate and purified by using column chromatography over silica-
gel [hexane or hexane/ethyl acetate (9:1)]. The GC–MS spectra were
taken with a Shimadzu QP-5050 equipped with TCD and a capil-
lary column (DB-5). FTIR spectra were recorded in Nujol mull on a
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