Time-dependent stereoselective Heck reaction using mesoporous Pd/TiO 2 nanoparticles catalyst under sunlight

Article abstract of DOI:10.1039/c3cy00602f

Pd(0)-doped mesoporous TiO₂ nanoparticles (Pd/TiO₂) have been synthesized using an EISA method. The catalytic activity of Pd/TiO₂ nanoparticles (NPs) is significantly enhanced under solar light irradiation for the Heck reaction. The Heck coupling product gives a considerable amount of Z-isomer, due to photochemical isomerization of the initially formed E-isomer, providing a facility for the time-dependent selectivity of E/Z-isomers under sunlight at ambient conditions. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3cy00602f

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Time-dependent stereoselective Heck reaction
using mesoporous Pd/TiO2 nanoparticles catalyst
under sunlight†
Cite this: Catal. Sci. Technol., 2014,
4, 510
*
Saurabh S. Soni and Deepali A. Kotadia
Received 17th August 2013,
Accepted 29th October 2013
Pd(0)-doped mesoporous TiO2 nanoparticles (Pd/TiO2) have been synthesized using an EISA method.
The catalytic activity of Pd/TiO2 nanoparticles (NPs) is significantly enhanced under solar light irradiation
for the Heck reaction. The Heck coupling product gives a considerable amount of Z-isomer, due to
photochemical isomerization of the initially formed E-isomer, providing a facility for the time-dependent
selectivity of E/Z-isomers under sunlight at ambient conditions.
DOI: 10.1039/c3cy00602f
www.rsc.org/catalysis
cheaper, abundant and greener energy source. Very early on,
the use of solar energy for organic chemical reactions was
1. Introduction
The palladium (Pd) catalyzed Heck reaction is one of the
most important C–C coupling reactions with a remarkably
wide scope in organic synthesis.1–3 The C–C bond formation
between aryl halide and olefins provides the simplest and
efficient route for the regioselective attachment of side
chains to aromatic rings using a variety of homogeneous and
heterogeneous catalysts. A wide variety of Pd catalysts have
been reported to promote the vinylation of aryl halides, of
which many follow the “classical” Pd(0)/Pd(II) mechanism,
i.e. that includes oxidative addition, migratory insertion,
β-hydride elimination and reductive elimination.4–7 Of the
homogeneous and heterogeneous catalysts, due to beneficial
advantages like recyclability, easy separation, and use of a
small amount in the reaction, heterogeneous catalysts have
received more and more attention.8–10 In this context, many
reports are available on the Heck reaction in which Pd was
supported on various metal oxides, silica, carbon, zeolite,
graphene oxide etc.11–13 and out of these, Pd-incorporated
mesoporous materials enhance the catalytic activity and
recyclability in the reactions.11
reported and was limited to photochemical and photothermal
reactions.15–17 Besides these, in recent years there have been
many reports that summarize the sunlight-mediated reactions
carried out in organic synthesis using photoactive catalyst
materials. However, the focus of research has been mostly
directed towards the chemistry of reaction in which molecules
undergo the desired bond modifications by absorbing light.18–21
The effectiveness of a catalyst towards selectivity and
efficiency has been increased by carrying out the Heck
reactions under irradiation of UV–visible or sunlight.22–25
Fredricks et al. reported the acceleration of the Heck reaction
using UV–vis irradiation over Pd(OAc)2 and heterogeneous
catalyst Pd/Al2O3.22,23 Very recently, Waghmode et al. showed
stereoselectivity towards the trans-isomer in the Heck reac-
tion using a PdCl2/TiO2 catalyst after 5 h of UV–vis radia-
tion.25 Chaudhary and Bedekar24 reported the Mizoroki–Heck
reaction over Pd(OAc)2 and phosphine free naphthol-based
ligand under sunlight and obtained ~65% Z-isomer after 9 days
of irradiation. Except for these reports the stereoselectivity
of Heck coupling products has been strictly restricted to the
trans- or E-isomer as the major or only product. It has now
become a challenge to form a particular isomer of interest
and control the cis- and trans-isomer ratio. To overcome this
challenge their interconversions has been carried out either
by using thermal–chemical interconversion or by photo-
chemical means.26–29 For the selective formation of the Z-isomer,
special conditions for the Heck reaction have also been
reported by some groups, which involved the design of ligands
or other parameters or the use of bioactive materials.30–32
Therefore, it is desirable to have a new catalyst structure which
could be designed in such a way that the vast potential of
sunlight can be exploited instead of energy-consuming heat as
the source to drive the reactions, with good efficiency and
As per one of green chemistry's principles stating that
“the energy requirements of chemical processes should be
recognized for their environmental and economical impacts
and should be minimized”,14 it would be a significant break-
through if we could enhance the catalytic activity of Pd at
ambient temperature and pressure using sunlight as the
Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar, 388
120Gujarat, India. E-mail: soni_b21@yahoo.co.in; Tel: +91 2692 226856 ext. 216
† Electronic supplementary information (ESI) available: SEM, TEM, EDS and
FTIR data of Pd/TiO2 catalyst is shown. The characterization of coupling
products by GC–MS and 1H and 13C NMR spectra is shown. See DOI: 10.1039/
c3cy00602f
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stereoselectivity. With this goal in mind, we have incorpo-
rated Pd into mesoporous TiO2 nanoparticles to catalyze
various Heck reactions under sunlight. The idea behind
selecting the titania support is the peculiar properties of its
anatase phase which exhibits strong photocatalytic activity
under sunlight and its potential use for environmental and
biological remediation purposes.33–35
2. Results and discussion
The ordered mesoporous Pd/TiO2 nanoparticles (NPs) were
synthesized based on our earlier report using an Evaporation
Induced Self Assembly (EISA) route.36 In a departure from
this procedure, Pd(OAc)2 was introduced in an initial sol for
uniform distribution of Pd in the titania matrix. As the P123
[(PEO)20–(PPO)70–(PEO)20] block copolymer contains polyeth-
ylene oxide (PEO) units, which have the capability to reduce
metal salts37–40 to nanoparticles, Pd2+ ions were reduced to
Pd0 (Pd black) in situ. During the solvent evaporation, ~50%
RH was maintained to obtain mesophase formation and at
the end of two days a sticky material was obtained. In order
to obtain photoactive mesoporous crystalline anatase TiO2
the obtained material was calcined at 450 °C for 30 min at a
heating rate of 2 °C min−1. After calcination the resultant
material obtained was a free-flowing powder mesoporous in
nature, which was subjected for structural and morphological
characterization. The powder diffraction pattern (Fig. 1) of
the Pd/TiO2 NPs is analyzed, and from the Bragg reflections
the anatase phase of titania was clearly identified, but no
specific peaks were obtained for the Pd NPs. This might be
due either to a low % Pd in the titania matrix or to merging
of the characteristic peaks of Pd(0) nanoparticles which
appear at ~39° (1 1 1) and ~47° (2 0 0) with the anatase titania
peaks.40 The average particle size calculated using the
Scherrer equation is 13.8 nm.
Fig.
2
TEM micrographs and DLS study (inset) or particle size
distribution of Pd-doped titania mesoporous materials.
were of palladium NPs distributed uniformly on the titania
surface. The particle size obtained from DLS measurements
(inset of Fig. 2) is in the range of 12–15 nm. A similar kind of
observation can be predicted from SEM measurement (Fig. 3)
which shows the particle size in the range of 12–15 nm with
a regular particle shape. The obtained particle sizes from
TEM and DLS measurements are compatible with that
acquired from XRD analysis. Energy dispersive spectroscopy
(EDS) analysis (ESI,† S3) reveals the presence of 1.23 wt% of
Pd along with Ti (47.71 wt%) and O (51.06 wt%), which
proves the presence of Pd in the composites as well as
in aggregates.
In order to obtain information regarding the surface area
and porosity of the Pd/TiO2 NPs, nitrogen adsorption–desorption
isotherm measurements were carried out (Fig. 4). The result
shows a type-IV isotherm with a large hysteresis indicating a
3D intersection network of porous structure and that capillary
condensation of N2 is occurring during the adsorption process.
The specific surface area for Pd/TiO2 was calculated according
TEM analysis (Fig. 2) confirmed the generation of uniform
spherical nanoparticles with an average diameter of ~15 nm
and, as observed, the black dots with diameters of 6–7 nm
Fig.
1 X-Ray diffraction analysis of Pd-doped titania mesoporous
materials.
Fig. 3 SEM micrographs of Pd-doped titania mesoporous materials.
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to the BET equation and was 107.27 m2 g−1 and the total
pore volume was 0.0312 cm3 g−1 which corresponds to an
average pore diameter (4V/A by BET) of 2.8 nm. The pore size
distribution derived from the isotherm data using the
Barrett–Joyner–Halenda (BJH) model was quite broad, with
a maximum at 3.15 nm. The above measurements show that
Pd/TiO2 NPs are porous and have a large surface area;
therefore, they can be a good candidate as a catalyst for numer-
ous reactions.
Scheme 1 Heck reaction compared under sunlight with open and
black-coated flask.
The photocatalytic activity of the mesoporous Pd/TiO2
catalyst was evaluated under sunlight for the Heck reaction
of iodobenzene with styrene using Et3N as base and DMF
(dimethylformamide) as solvent. The reaction was exposed to
sunlight with a maximum light power ~30 mW cm−2 for
6–7 days and the product formation was checked on each day
with the help of GC–MS. In order to confirm whether the
reaction is photochemical or thermal, the reaction was
carried without irradiation of sunlight (by covering the flask
with black paper) keeping the other reaction parameters con-
stant (Scheme 1). All the reactions were performed three
times maintaining the same reaction conditions (sunlight
intensity may vary a little in the range of 30 mW cm−2
5 mW cm−2) to check the reproducibility of the reaction, and
the results for coupling yield obtained were approximately
the same obtained in the first run.
Table
1 Test experiments for optimization of the Heck coupling
reactions of iodobenzene and styrenea
Entry
Catalyst
%Yield under lightb
%Yield under darkb
1
2
3
4
Pd/TiO2
TiO2
Pd(OAc)2
Pd/TiO2
91
25
43
42c
33
16
28
<10d
a
Reaction conditions: aryl halides (1.28 mmol), alkenes (1.66 mmol),
triethylamine (2.56 mmol), dodecane (40 mg, as internal standard),
Pd/TiO2 (0.5 wt% of Pd) and DMF (5 ml), for 3 days under sunlight.
b
% yields are the isolated yield and based on average results
c
(reaction carried out thrice for reproducibility check). Flask coated
with black paper. d 7 days of reaction.
The result clearly shows that the present reaction is a
photo-induced reaction as in the presence of sunlight the
coupling product was found in 91% yield whereas in the
absence of sunlight the reaction progress was very slow,
giving 33% yield (Table 1, entry 1). The effect of TiO2 and
Pd(OAc)2 as an individual catalyst for the same reaction
under sunlight was also carried out and the result shows
formation of the coupling product in low yield (Table 1, entry
2 and 3). This proves a synergistic effect of Pd and TiO2
(Pd/TiO2) NPs for the enhancement of coupling yields in
heterogeneous fashion under sunlight which is also supported
by the Pd leaching test. When the day temperature is at its
highest in the afternoon, the Pd/TiO2 catalyst was separated
from the reaction mixture and the reaction was allowed to
proceed under similar conditions. We found that no further
reaction was observed as no coupling product formation was
observed (GC–MS), showing no evidence of Pd leaching into
the products which was also confirmed by ICP-OES test (which
showed 0.667 ppm of Pd in the reaction mass), providing
evidence of TiO2 acting as a good scaffold for Pd catalyst
precursors. The ultimate conclusion that the reaction proceeds
in a heterogeneous fashion is also supported by a plausible
mechanism. The mechanism can be explained as follows: upon
sunlight irradiation, electrons from titania are excited from the
valence band into the unoccupied conduction band and posi-
tive holes in the valence band. These electrons from the con-
duction band can easily transfer to the Pd surface generating
electron heterogeneity at the Pd/TiO2 surface. Thus, it is
reasonable to expect that under sunlight the Pd sites with the
energetic electrons at the Pd/TiO2 surface will exhibit superior
catalytic activity to that of palladium salt or TiO2 alone. The
most interesting thing to note here is the stereoselectivity of
the reaction which shows formation of Z-stilbene in a major
amount compared to E-stilbene which is contrary to the usual
selectivity of the reaction. The formation of Z-stilbene might be
due to photoisomerization which is studied by optimization of
the same model reaction again over a period of time in
the presence of sunlight. The catalytic photoisomerization of
cis-/trans-stilbene and derivatives has previously been very well
studied which makes our interpretation even stronger.24,26–29 It
is very well reported that the photochemical isomerization of
arylethenes (stilbene derivatives) usually takes place in the
Fig. 4 N2 adsorption–desorption isotherms of Pd/TiO2 and (b) the
corresponding pore size distribution derived from the adsorption
isotherm.
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excited state through rotation around the CC double bond.
For instance, highly branched stilbene dendrimers with molec-
ular weights of over 6000 underwent E–Z isomerization in less
than 10 ns, the lifetime of the excited singlet state.41,42 Analysis
of first two days' results shows formation of both the isomers
in moderate yield with the E-isomer in excess (Fig. 5). With
increasing the time of sunlight exposure by three days, forma-
tion of the Z-isomer eventually increased which is, as said
earlier, due to photochemical isomerization of stilbene-like
compounds when exposed to light. After that a sudden fall in
percentage yield of the E-isomer and increase in the Z-isomer
was obtained, which decreases a little on day 4. A reverse trend
again was observed on day 5 as the ratio of the E-isomer
Fig.
6
Possible mechanism for E/Z photoisomerization in the
presence of Pd/TiO2.
Table 2 The Heck coupling reactions of aryl halides with alkenesa
increases and Z-isomer decreases.
A maximum of 72%
coupling yield was obtained at day 3 for the Z-isomer and 63%
yield of the E-isomer on day 5. The possible mechanism for the
photoisomerization can be explained by the double-bond-shift
of stilbene over Pd/TiO2 (Fig. 6) which follows the same mecha-
nistic pathway earlier reported by Hashimoto et al. for the
photoisomerization of 1-butene over Pd-loaded TiO2.43
No.
R1
R2
X
Yieldb (E/Z)c
1
2
3
4
5
6
7
8
9
–H
–C6H5
–C6H5
–C6H5
–C6H5
I
I
I
Br
Br
Br
I
I
I
94 (22 : 78)
90 (25 : 75)
92 (18 : 82)
80 (20 : 80)
77 (24 : 76)
76 (21 : 79)
90 (30 : 70)
89 (21 : 79)
85 (27 : 73)
As the reaction occurs on the surface of the Pd/TiO2,
stilbene produced will be adsorbed on the surface and lead
to the formation of stilbene carbenium ion by abstraction of
hydride. The photogenerated electrons from the titania
surface transform to Pd particles under the irradiation of
sunlight followed by addition of hydride to the stilbene
carbenium ion. These photogenerated electrons and holes
finally recombine after addition of hydride to the stilbene
carbenium ion, resulting in the formation of the stilbene
isomer.33 The formation of E- and Z-isomers using Pd/TiO2 is
very selective and proceeds in a controlled manner under
sunlight. Depending upon the requirement, one can have
time-dependent selection of the desired isomeric product.
With these results in hand, we carried out the Heck coupling
reaction of various substrates by photocatalysis (Table 2). A
variety of aryl halides reacts with various alkenes such as
acrylates and styrene, to give the required products with good
to excellent yields. Interestingly, the ability of Pd/TiO2 to pro-
mote coupling reactions of different alkenes with a variety of
–OCH3
–COOH
–H
–COOH
–H
–H
–COOH
–OCH3
–C6H5
–CO2C4H9-t
–CO2C4H9-t
–CO2C4H9-t
–CO2C4H9-t
a
Reaction conditions
–
aryl halides (1.28 mmol), alkenes
(1.66 mmol), triethylamine (2.56 mmol), dodecane (40 mg, as internal
standard), Pd/TiO2 (0.5 wt% of Pd) and DMF (5 ml) under sunlight for
3 days. b Isolated yields. c Isomer ratio determined by NMR.
activated and non-activated aryl iodides is satisfactory. Aryl
iodides possessing either an electron-withdrawing (–COOH)
or electron-releasing (–OCH3) substituent on the benzene
ring efficiently cross-coupled with styrene and acrylates,
regardless of the substituent position to afford the products
in good to excellent yields. The reactivity of iodides is higher
than bromo-derivatives because of their role of photo-
chemical cleavage of the Ar–I bond for the SRN1-type substitu-
tion reaction assisted by sunlight. Apparently Pd/TiO2 results
in a much higher catalytic activity which can be observed
from the high turnover frequency (TOF = 2.89 h−1) of the cat-
alyst (ESI,† Table S1). The stilbene or cinnamate derivatives
formed were found to have a considerable amount of
the Z-isomer in most of the cases. This method thus proves
to be a versatile one for the synthesis of olefins with
Z-stereochemistry.
As the catalyst is heterogeneous in nature, it can easily be
recovered after work up. The recovered Pd/TiO2 was washed
with diethyl ether and dried under vacuum at 80 °C and
further used for the next run. The time profile for the model
reaction using recovered catalyst does not vary much in days.
Pd/TiO2 was effectively used for 3 runs showing the same
trend (i.e. formation of the Z-isomer) with a little loss in
catalytic activity. The recovered Pd/TiO2 was characterized by
Fig. 5 Formation of E/Z isomers with the exposure of sunlight.
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FTIR and the Pd content was measured using EDS analysis
which shows 1.19 wt% of Pd, signifying that no major loss of
Pd occurs during the reaction.
FTIR spectrophotometer (Canada). All the coupling reactions
were performed in a container to maintain a constant tem-
perature and kept above a mirror to utilize the reflected
sunrays. Reactions were monitored by thin layer chromatog-
raphy using 0.25 mm E. Merck silica gel-coated glass plates
(60F-254) with UV light to visualize the course of reaction
and GC–MS (GC–Mass Spectrometer, Perkin Elmer, Auto
3. Conclusions
In the study, palladium metal-doped titanium dioxide (Pd/TiO2)
has been successfully prepared in a stable form and tested as
a heterogeneous photocatalyst for the Heck reaction. The
synthesized Pd/TiO2 was characterized by XRD, BET,
SEM-EDS, TEM, and DLS techniques. From these various
analyses the particle size obtained for the Pd/TiO2 meso-
porous nanomaterials is in the range of 12–15 nm. Meso-
porous Pd/TiO2 exhibited high activity and stereoselectivity
for the Heck reaction, showing photoisomerization under
sunlight owing to the synergistic effect of Pd and TiO2 both.
The synthetic reaction conditions here are environmentally
benign, providing a facility for the time-dependent selectivity
of E/Z-isomer under sunlight at ambient conditions with
good recyclability.
1
system XL, GC+). The coupling products were analyzed by H
and 13C NMR (400 MHz Bruker Scientific, Switzerland)
spectroscopic techniques.
4.2 Synthesis of Pd/TiO2 catalyst
Ordered mesoporous Pd/TiO2 materials were synthesized
through a simple Evaporation Induced Self Assembly process
in the presence of triblock polymer P123 as structure
directing agent. To homogeneously distribute Pd nanoparticles
into the titania framework, we utilized a multicomponent
assembly approach where the surfactant, titania and Pd were
assembled in a one-step process. In particular, HCl (37%)
(6.4 g, 63 mmol) was added dropwise to Ti(iPrO)4 (10.24 g,
35 mmol) in an ice-bath at 0 °C under stirring and after
completion of addition the reaction mass was allowed to stir
further for 3 h at room temperature. On the other hand, the
calculated amount of Pd(OAc)2 (30 mg, 0.133 mmol) was added
to P123 ethanolic solution (2 g P123 in 25 g ethanol) and stirred
vigorously. The dispersion formed was added to the Ti/HCl
mixture and further stirred for 3 h for complete homogeniza-
tion before it was then transferred into a Petri dish for ageing.
Ethanol was subsequently evaporated at room temperature at a
relative humidity of 60% which was maintained for 24 h. This
was subsequently dried and kept in an oven at 60 °C and for
an additional 24 h for ageing. The as-made mesostructured
hybrids were calcined at 450 °C in air for 30 min at a heating
rate of 2 °C min−1 to remove surfactant and to obtain highly
ordered mesostructured Pd/TiO2.
4. Experimental section
4.1 Materials and method
The block co-polymer surfactant (P123, PEO–PPO–PEO
MW-5750 g mol−1), Ti(OC(CH3)3)4, Pd(OAc)2 and the starting
substrates for the Heck coupling were purchased from
Sigma-Aldrich, India. HCl and ethanol used were of
analytical grade laboratory reagent. The synthesized
Pd-doped nanoparticles were characterized for their structure
and texture by XRD, BET, TEM, SEM, EDS and DLS. The XRD
measurements were performed using a Rigaku D/max 2500
X-ray Diffractometer. Diffraction patterns were recorded with
Cu Kα radiation (40 kV, 30 mA) over a 2θ range of 3–80 °C at
a scan rate of 2° min−1. The BET surface area, average pore
volume, and average pore diameter were measured by
physisorption of N2. The isotherms were elaborated
according to the BET (Brunauer, Emmett and Teller) method
for surface area calculation, and pore size distribution curves
derived from the desorption branches for the porous
materials with BJH (Barrett–Joyner–Halenda) methods using
a Micromeritics Gemini 2375 Surface area analyzer. The TEM
measurements were conducted by using a Philips Tecnai 20
(Holland) transmission electron microscope at an accelera-
tion voltage of 200 kV with a W emitter and LaB6 electron
source. A single drop of the sample was deposited on a
carbon-coated copper grid and dried at room temperature
under atmospheric pressure. The SEM observations were
carried out using a JSM-7600F Field Emission Gun-Scanning
Electron Microscope (FEG-SEM) at an acceleration voltage of
0.1 to 30 kV. The DLS measurements were carried out using a
Malvern instrument, Zetasizer ZS-90 (for nanosizer) Particle
size analyzer. The ICP analysis was carried out using an
Atomic Emission Spectrometer Perkin Elmer Optima 3300 RL
with SCD detector; detection limit: up to ppb level. The IR
measurements of Pd/TiO2 were carried out using an ABB
4.3 General procedure for the Heck reactions
Aryl halides (1.28 mmol), alkenes (1.66 mmol), triethylamine
(2.56 mmol), dodecane (40 mg, as internal standard), Pd/TiO2
(0.5 wt% of Pd) and DMF (5 ml) were taken in an Erlenmeyer
flask. The mixture was kept under sunlight for 3–7 days. After
completion of the reaction (confirmed by TLC analysis and
GC–MS analysis), the mixture was filtered through a short
pad of Celite to remove the catalyst. To the filtrate, diethyl
ether and water were added and the layers were separated.
The aqueous layer was extracted twice with diethyl ether and
the combined organic layers were washed with brine, dried
over Na2SO4, filtered and concentrated in vacuo to get the
desired product. All the coupling products have been
previously reported and their identities were confirmed by
comparison of their 1H, 13C NMR and GC–MS spectral data
with the values of authentic samples/earlier reports. (Note:
Complete care has been taken to maintain the light intensity
(maximum to minimum) between 25 and 30 mW cm−2 for
the reaction carried out during inter–intra days.) The filtered
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off solid catalyst was washed with acetone and dried under
vacuum after each cycle, and then reused for the next reaction.
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Acknowledgements
Authors are thankful to SICART (Vallabh Vidyanagar) for
TEM and GC–MS analysis, SAIF (IIT Bombay) for SEM and
EDS analysis, Department of Physics for XRD analysis and
Department of Material Science for BET analysis. One of the
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providing financial assistance in terms of CSIR-SRF.
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