Triazine functionalized ordered mesoporous polymer: A novel solid support for Pd-mediated C-C cross-coupling reactions in water

Article abstract of DOI:10.1039/c1gc15045f

A new functionalized mesoporous polymer (MPTAT-1) has been synthesized via organic-organic radical polymerization of 2,4,6-triallyloxy-1,3,5-triazine (TAT) in aqueous medium in the presence of an anionic surfactant (sodium dodecyl sulfate) as template. Powder XRD and TEM image analysis suggests the presence of ordered 2D-hexagonal arrangement of pores in the material. N₂ sorption analysis reveals a moderately good surface area 135 m² g^-1 for this mesoporous polymer. The template free MPTAT-1 acts as an excellent support for immobilizing Pd(ii) at its surface and the resulting material showed very good catalytic activity in several C-C cross-coupling reactions like Mizoroki-Heck, Sonogashira and Suzuki-Miyaura in an environmentally benign reaction medium, water. The catalyst exhibits very high catalytic activity for the coupling of various aryl halides including aryl chlorides with alkenes or alkynes and the sodium salt of (trihydroxy) phenylborate. Due to strong binding with the functional groups of the polymer, the anchored Pd(ii) could not leach out from the surface of the mesoporous catalyst during the reaction and it has been reused several times without appreciable loss in catalytic activity.

Full text of DOI:10.1039/c1gc15045f

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PAPER
Triazine functionalized ordered mesoporous polymer: a novel solid support
for Pd-mediated C–C cross-coupling reactions in water†
a
a
b
a
Arindam Modak, John Mondal, Manickam Sasidharan and Asim Bhaumik*
Received 12th January 2011, Accepted 1st March 2011
DOI: 10.1039/c1gc15045f
A new functionalized mesoporous polymer (MPTAT-1) has been synthesized via organic–organic
radical polymerization of 2,4,6-triallyloxy-1,3,5-triazine (TAT) in aqueous medium in the presence
of an anionic surfactant (sodium dodecyl sulfate) as template. Powder XRD and TEM image
analysis suggests the presence of ordered 2D-hexagonal arrangement of pores in the material. N
2
2
-1
sorption analysis reveals a moderately good surface area 135 m g for this mesoporous polymer.
The template free MPTAT-1 acts as an excellent support for immobilizing Pd(II) at its surface and
the resulting material showed very good catalytic activity in several C–C cross-coupling reactions
like Mizoroki–Heck, Sonogashira and Suzuki–Miyaura in an environmentally benign reaction
medium, water. The catalyst exhibits very high catalytic activity for the coupling of various aryl
halides including aryl chlorides with alkenes or alkynes and the sodium salt of
(
trihydroxy)phenylborate. Due to strong binding with the functional groups of the polymer, the
anchored Pd(II) could not leach out from the surface of the mesoporous catalyst during the
reaction and it has been reused several times without appreciable loss in catalytic activity.
Introduction
surfactant templating route can carry suitable functional groups
at their surface, which can effectively interact with relevant
metal ion/molecules and therefore are potential candidates for
1
Since the first discovery of ordered mesoporous silicas these
materials have attracted widespread attention in both academics
and industry due to their huge potential applications. Today
13
heterogeneous catalysts.
2
In this context it is pertinent to mention that palladium
catalysed C–C cross-coupling reactions like Suzuki, Heck,
Sonogashira, are extremely important from the synthetic organic
mesoporous materials have been employed successfully in gas
3
4
5
6
7
adsorption and storage, ion-exchange, catalysis, sensing,
and so on. In general, these materials can be synthesised by
means of a soft templating route using either a cationic or
14
chemistry point-of-view. One of the most challenging tasks in
organic chemistry today is to design efficient and eco-friendly
catalysts for C–C bond formation reactions. Homogenous Pd-
catalysts used in these reactions have several draw backs due to
tedious and time consuming work-up procedures associated with
inefficient recycling. A heterogeneous Pd-catalyst can solve these
problems very efficiently and thus supported Pd-catalysts can
1,8
anionic surfactant as the structure directing agent or by a
hard templating approach, such as those employing colloidal
9
particles. Removal of surfactant molecules through solvent
extraction produces functionalized mesopores having tuneable
pore dimensions, which can act as a scaffold for the synthesis of
nanomaterials due to the presence of active functional groups at
the mesopore surface. To date, mesoporous materials composed
of inorganic oxides and inorganic–organic hybrid silica
materials have been studied extensively, but very limited research
work has been carried out on purely organic mesoporous
15
offer high activity and selectivity in the C–C coupling reactions.
Thus it is highly desirable to develop an efficient supported Pd-
catalyst based on a functionalized mesoporous material. Func-
tionalized mesoporous silica, metal–organic frameworks, resins,
gels, metal oxides, clays etc., containing suitable binding sites
could serve as the desired heterogeneous matrix for the binding
of active metal centres and thus subsequent used in catalytic
10
11
1
2
materials prepared through a surfactant templating pathway.
Purely organic mesoporous materials synthesized through a
16
reactions. However, only very limited number of materials can
be used as a catalyst in the presence of water as the reaction
a
17
Department of Materials Science, Indian Association for the Cultivation
medium, which is most environmentally benign solvent. Most
of Science, Jadavpur, Kolkata, 700 032, India. E-mail: msab@iacs.res.in
of these C–C coupling reactions proceed in the presence of
environmentally hazardous solvent like DMF, DMSO, THF
or acetonitrile etc. So, designing a new catalyst and grafting
it with active metal centres to explore its potential application in
b
Department of Chemistry, Faculty of Science and Engineering, Saga
University, 1 Honjo-machi, Saga, 840-8502, Japan
†
Electronic supplementary information (ESI) available: Spectroscopic
data. See DOI: 10.1039/c1gc15045f
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C–C coupling reactions in water is very demanding. Herein, we
first report the synthesis of a new ordered mesoporous polymer
containing 2,4,6-triallyloxy-1,3,5-triazine moiety, grafted with
Pd(II) at its surface and its use in Heck, Sonogashira and
Suzuki C–C cross-coupling reactions maintaining eco-friendly
conditions.
molecules. The pH of the gel was maintained at ca. 7.0. Finally,
5.2 g (0.016 mol) of APS dissolved in 10 ml of distilled water
was added quickly into the solution with vigorous stirring. A
white precipitate appeared immediately after the addition of
APS. The resultant slurry was stirred for another 3 h at room
temperature and then autoclaved at 348 K for 3 days without
stirring. The final pH of the synthesis gel was ca. 4.0–5.0. After
aging, the resultant precipitate was filtered, washed thoroughly
with de-ionized water, yielding mesoporous polymer MPTAT-1.
The surfactant was removed from the synthesised material by
extraction of 1.0 g of the MPTAT-1 material with 0.5 g acetic
acid taken in 50 ml water under 4 h stirring at room temperature.
This template extraction procedure was repeated three times to
obtain template-free MPTAT-1.
Experimental section
Materials
2
,4,6-Triallyloxy-1,3,5-triazine (TAT, monomer), palladium ac-
etate, styrene, phenyl acetylene and other reactants for the cat-
alytic reactions were obtained from Sigma Aldrich. N,N,N¢,N¢-
tetramethylethylenediamine (TMED, promoter), ammonium
persulfate (APS, radical initiator), sodium dodecyl sulfate
Synthesis of palladium grafted mesoporous
poly-2,4,6-triallyloxy-1,3,5-triazine (Pd-MPTAT-1)
(
SDS, template) were purchased from Loba Chemie. All other
chemicals used for this investigation purposes were of analytical
grade produced by E-Merck, unless mentioned otherwise.
Synthetic procedure for grafting of palladium onto the meso-
porous poly-2,4,6-triallyloxy-1,3,5-triazine (MPTAT-1) has
been illustrated in Scheme 1. For Pd loading, 1.0 g of the
template-free mesoporous polymer was taken in a round bottom
flask and it was dispersed in 25 ml of glacial acetic acid. To
this 0.5 g palladium acetate was added and the suspension
was refluxed for about 12 h under a nitrogen atmosphere with
vigorous stirring until the colour changed to black. At the end,
the resulting dark solid was filtered and washed thoroughly with
THF, methanol and acetone, and finally dried under vacuum for
Instrumentation
Carbon, hydrogen and nitrogen contents of both the meso-
porous polymer and Pd-grafted polymer were analyzed using
a Perkin Elmer 2400 Series II CHN analyzer. X-Ray diffrac-
tion patterns of the powder samples were obtained with a
Bruker AXS D8 Advanced SWAX diffractometer using Cu-Ka
(
0.15406 nm) radiation. N
2
adsorption/desorption isotherms of
the samples were recorded using Bel Japan Inc. Belsorp-HP at
2
4 h to obtain Pd-MPTAT-1.
7
3
7 K. Prior to the measurement, the samples were degassed at
93 K for 12 h under high vacuum conditions. Transmission
electron microscopy (TEM) images of the mesoporous polymer
were obtained using a JEOL JEM 2010 transmission electron
microscope operating at 200 kV. The samples were prepared
by dropping a colloidal solution onto the carbon-coated cop-
per grids. Scanning electron microscopic measurements were
performed with a JEOL JEM 6700F field-emission scanning
electron microscope (FESEM). Pd content was determined by
atomic absorption spectroscopic (AAS) analysis by using a
Shimadzu AA-6300 AAS spectrometer fitted with a double
beam monochromator. UV-visible diffuse reflectance spectra
were recorded on a Shimadzu UV 2401PC with an integrating
sphere attachment. BaSO was used as the background standard.
4
FT–IR spectra of these samples were recorded using a Nicolet
MAGNA-FT IR 750 Spectrometer Series II.
Synthesis of mesoporous poly-2,4,6 -triallyloxy-1,3,5-triazine
(
MPTAT-1)
MPTAT-1 has been synthesised through the aqueous-phase
polymerisation of 2,4,6-triallyloxy-1,3,5-triazine (TAT) under
hydrothermal condition by using ammonium persulfate (APS)
as an initiator. In a typical synthesis, 2.35 g (0.00814 mol) of
SDS was dissolved in 40 ml of water with constant stirring
followed by the addition of 2 g (0.008 mol) of 2,4,6-triallyloxy-
Scheme 1 Organic–organic in situ radical polymerization of 2,4,6-
triallyloxy-1,3,5-triazine by APS using a SDS template and grafting
on Pd(II) in acetic acid medium.
1
,3,5-triazine. After the addition of 0.94 g (0.00814 mol) of
Catalytic reactions
TMED to this mixture, concentrated HCl (12 N) was added
dropwise to obtain a clear solution. This helps the protonation
of the N-atoms of the TAT molecules, which facilitates the ionic
The catalytic activity of this Pd grafted MPTAT-1 poly-
mer has been tested for several C–C cross-coupling reac-
tions like Mizoroki–Heck, Sonogashira, Suzuki–Miyaura in
-
interaction with the negative charge of C12
H
23SO
3
of the SDS
1
318 | Green Chem., 2011, 13, 1317–1331
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environmentally benign conditions. For the Mizoroki–Heck
coupling reaction, 1 mmol of arylhalide, 2 mmol of potassium
from pet-ether/ethyl acetate to obtain compounds having
sufficient purity which were further characterized by NMR spec-
troscopy. The products were isolated, purified and then identified
carbonate (K
2
CO
3
) were mixed with 10 mL water along with
1
13
5
ml ethanol and placed in a batch reactor. To this 0.02 g of
by H NMR, C NMR, FTIR, CHN analysis etc. methods.
Melting points of those compounds are in good agreement
with those reported in literature. NMR spectra were measured
on a Bruker Avance DPX 300 NMR (300 MHz) spectrometer
using TMS as the internal standard. The detail experimental
procedures of the individual coupling reactions are given below.
Pd-MPTAT-1 polymer catalyst was added, and the mixture was
◦
heated to 110 C in an oil bath with continuous stirring. Finally
1
.5 mmol of styrene/acrylic acid was added to start the coupling
reaction. For less reactive aryl chlorides 2 mmol of a phase
31
transfer catalyst (TBAB) was added in addition to our Pd-
MPTAT-1 catalyst. The progress of the reaction was monitored
by TLC and it has been found that 10–12 h is required for full
conversion of aryl iodides and bromides, but a much longer
reaction time such as 20–24 h is required for less reactive aryl
chloridecoupling. For isolation of the products atthe end of each
catalytic reaction, the heterogeneous catalyst was first separated
out by filtration, then the filtrate was evaporate by a rotary
evaporator to remove ethanol and finally the concentrated part
was extracted with ethyl acetate (3 ¥ 20 ml) and thoroughly
washed with water and brine solution. The organic layer was
Results and discussion
Formation of mesophase
In the presence of the anionic structure directing agent SDS,
organic-organic supramolecular assembly takes place in aque-
ous medium. At the acidic pH conditions the positively charged
protonated N-atoms of the triazine moiety preferentially interact
-
with the negatively charged polar head groups i.e. C12
H
23SO
3
and condensed around the micelle to form a continuous
mesophase. The resulting array of organic mesophases upon re-
moval of the surfactant by acid extraction retains its mesoporous
structure. In Scheme 1 we have described the polymerisation of
TAT monomer in the presence of ammonium persulfate, which
is a good radical initiator for aqueous phase polymerisation
collected and dried over Na
2
SO
4
, and finally the remaining
◦
solvent was removed under vacuum at 25 C. Crude product
was isolated and repeatedly re-crystallized from ethyl acetate–
hexane mixture to obtain compounds having sufficient purity
for further characterization purposes.
Use of Pd-MPTAT as catalyst for Sonogashira cross-coupling
reaction in triethanol amine has been carried out without
addition of any external base or solvent. Thus in a typical
synthesis, 1.0 mmol of arylhalide, 0.05 mmol of CuI were
mixed with 12 mL of triethanol amine and placed in a batch
reactor. To this solution 0.02 g of Pd-MPTAT-1 was added. The
20
of olefins under mildly acidic condition. Since one monomer
contains three olefinic double bonds in each of its branches, its
polymerisation gives random cross-linking, leading to a highly
stable and robust organic framework.
Immobilization of palladium and its strong binding at the
MPTAT-1 surface has been facilitated due to the presence of
◦
mixture was then heated to 80 C in an oil bath under nitrogen
11d
strong donor sites in the triazine moiety.
atmosphere with continuous stirring. The coupling reaction
started immediately after addition of 1.5 mmol phenylacetylene
to the mixture. The progress of the coupling reaction was
monitored by TLC and it took 8–12 h for complete conversion
depending on the substrate. For isolation of the products at the
end of the catalytic reaction, the catalyst was first separated
out by filtration and thoroughly washed with water and hexane.
The filtrate was then extracted with hexane three times (3 ¥
Mesophase
In Fig. 1 small angle powder diffraction patterns of both (a) the
as synthesized MPTAT-1 and (b) the template-free MPTAT-1
are shown. For as synthesized sample, in addition to a large
diffraction peak at 2q ca. 2.0, two additional weak intensity
21,22
peaks suggesting the presence of ordered mesophase.
The
2
0 ml) and finally washed with brine. The organic layers thus
template-free sample shows one strong peak at 2q = 2.08,
which indicates little contraction of pore wall upon removal
of template from the as synthesized polymer. A transmission
collected were combined and dried over anhydrous MgSO
4
.
Solid compounds were obtained on evaporation of the organic
◦
solvent under vacuum at 25 C. The crude products were
repeatedly re-crystallized from hot ethanol to obtain purified
products.
We carried out the Suzuki–Miyaura cross-coupling reaction
18
using sodium salt of phenyl trihydroxyborate as the coupling
agent, instead of using conventional phenyl boronic acid, with
aryl halides in water under base-free conditions. In a typical
synthesis, 1 mmol aryl halide, 1.2 mmol phenyl trihydroxyborate
were taken in a round bottom flask. The flask was charged with
5
: 1 (by volume) mixture of water–DMF and 0.02 g catalyst.
◦
The mixture was refluxed at 90 C with continuous stirring.
Progress of the reaction was monitored by using TLC technique.
It took 8–10 h for completion of reaction. At the end of the
reaction, the catalyst was first separate out by filtration, and this
is followed by washing with water and diethylether. The filtrate
was extracted thrice with 1 : 1 mixture of diethyl ether and hexane
Fig. 1 XRD patterns of (a) the synthesized and (b) acid extracted
(
3 ¥ 20 ml). The crude product was repeatedly re-crystallized
MPTAT-1.
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Fig. 2 Transmission electron micrograph of MPTAT-1.
electron microscopic image of MPTAT-1 is shown in Fig. 2.
In this image low electron density spots (pores) were seen
throughout the specimen and these are arranged in a honeycomb
like 2D-hexagonal array. The average pore diameter as estimated
from the HR TEM image was ca. 3.8 nm, which agrees
reasonably well with other mesoporous materials synthesized
23
through anionic SDS templating route. The pore dimension
of this material was further confirmed from the FFT pattern as
shown in the inset of the Fig. 2. Bright diffraction spots and their
hexagonal arrangement were quite clear. This HR TEM image
also reveals a wall thickness of ca. 1.1 nm. Well defined spherical
external morphologies of this material were observed from the
corresponding FE SEM image as shown in Fig. 3. The particles
are spherical in nature and have a diameter of ca. 50 nm. These
nanospheres adhere among themselves at the surface to form
large aggregated particles, which could be due to the random
polymerisation of the 2,4,6-triallyloxy-1,3,5-triazine monomer.
Porosity and surface area
Fig. 3 Scanning electron micrographs of MPTAT-1 (a) and Pd-
MPTAT-1 (b).
In Fig. 4 nitrogen adsorption/desorption isotherms of the
template-free MPTAT-1 are shown. This isotherm is closely
related to a type IV isotherm, characteristic of mesoporous
24
chemical properties like UV-visible absorbance, SEM-EDX,
TEM, FTIR, CHN analysis, AAS etc. Elemental analysis reveals
the presence of C, H, and N contents in this mesoporous polymer
before and after loading of Pd. It was found that the MPTAT-1
before loading possesses C = 54.89%, H = 7.93%, N = 10.39%,
whereas in Pd-MPTAT-1 it was C = 52.33%, H = 6.55%, N =
10.3%. Retention of C, H, N contents confirm the stability of
the triazine framework under very drastic conditions such as
glacial acetic acid during metal loading. The metal (Pd) content
of polymer anchored Pd(II) complex as determined by AAS
suggested 5.16 wt%, and for 0.02 g of Pd-MPTAT-1 catalyst, the
loading of Pd was supposed to be 0.8 mol% with respect to 1 mol
reactant, which is in good agreement with the amount of Pd-
catalyst used (0.25–3 mol% Pd) in the reports on Pd-mediated
materials. However, in contrast to the sharp capillary con-
densation usually observed for mesoporous solids, here a
gradual and very broad uptake of N
2
occurred over the entire
high P/P scale. From the nitrogen sorption isotherm BET
0
2
-1
surface area of the sample was found to be 135 m g . In
this context it is pertinent to mention that a purely organic
mesoporous polymer usually has a small surface area, whereas
inorganic mesoporous materials and organic-inorganic hybrid
25
11,26
mesoporous materials have a high surface area.
Pore size
distribution (PSD) of the sample MPTAT-1, is shown in the
inset of Fig. 4 employing NLDFT (non-local density functional
theory) method. From this PSD, a broad distribution having
maximum pore width of 3.7 nm is observed.
27,28
cross-coupling reactions.
FT IR spectra of both mesoporous
Characterization of Pd-MPTAT-1 catalyst
polymers MPTAT-1 and Pd-MPTAT-1 are shown in Fig. 5. Both
the samples showed two distinct C–H vibrations in the region
Due to the insolubility of Pd-MPTAT-1 in all common organic
solvents, its structural investigation was limited to only phsico-
-
1
2932–2950 cm , corresponding to non-identical C–H bonds in
1
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Fig. 4
2
N adsorption/desorption isotherms of MPTAT-1 at 77 K. Ad-
sorption points are marked by filled circles and desorption points by
empty circles. Pore size distribution is shown in the inset.
Fig. 6 UV-Visible absorbance of MPTAT-1 (a) and Pd-MPTAT (b).
29,30
respectively
(Fig. 6b). The presence of a high coordinating
Fig. 5 FTIR spectra of MPTAT-1 (a) and Pd-MPTAT-1 (b).
ligand like triazine around Pd(II) actually shift those absorbance
band towards the higher frequency region resulting in a slight
deviation from theoretical values. Another strong absorbance
at 645 nm i.e. in the visible region may be due to a change in
p→p* and n→p* transitions after Pd grafting in MPTAT-1.
Based on those results such as elemental analysis, FTIR, UV-
visible absorbance, it can be said that triazine moiety has been
incorporated into the polymeric framework of MPTAT-1.
the triazine framework. Further mesoporous polymer sample (a)
-
1
exhibits very strong and broad absorption band at 3435 cm ,
which is mainly due to N–H stretching. After Pd loading, in
-
1
sample (b), this band at 3435 cm diminished in intensity
and slightly shifted towards the lower frequency region. Both
-
1
samples (a) and (b) exhibited very strong band at 1563 cm due
to ring >C N stretching vibration and two additional sharp
peaks at 1142 cm- (n ~ C–N) and 1420 cm (n ~ CH
1
-1
). Further,
the allylic double bond (-C C-) stretching of 2,4,6-triallyloxy-
2
Catalysis
-
1
1
,3,5-triazine moiety at 1840 cm , has disappeared and the
Pd-MPTAT-1 acts as a very efficient catalyst in C–C coupling
reactions of different aryl halides. The reaction shows good
diversity in the presence of both electron donating and electron
withdrawing groups. In the following three sections we have
described C–C coupling reactions over Pd-MPTAT-1.
-
1
intensity of another at 1714 cm has diminished considerably,
suggesting near completion of polymerization reaction. Thus
these framework vibrations suggested the presence of 2,4,6-
triallyloxy-1,3,5-triazine moiety in MPTAT-1 and Pd-MPTAT
materials. Further, UV absorbance spectra (Fig. 6a) of MPTAT-
1
exhibits no strong characteristic absorbance, except two very
a) Pd-MPTAT-1 catalysed Mizoroki–Heck cross-coupling
in water–ethanol mixture. Pd-MPTAT-1 catalyst exhibits very
high catalytic activity and trans-selectivity in Mizoroki–Heck
cross-coupling in water. Since from its discovery in early 1970s,
Heck–Mizoroki cross-coupling has been widely used as a tool
in organic synthesis because of the direct incorporation of the
weak peaks at 207 and 266 nm due to very weak p→p* tran-
sitions. But a distinguishable change in absorbance is observed
in case of Pd-MPTAT-1 material. The UV-visible absorbance
spectrum of this Pd-grafted polymer shows different strong
peaks at 212, 251, 290, 362 and 645 nm. The low-spin Pd(II)
complex may exhibit three spin-allowed d–d transitions from
the lower lying d
bands are observed at 212 nm, 251 nm and 290 nm, which may be
designated as A1g→ Eg, A1g→ B1g and A1g→ A2g transitions
15
olefin moiety in the aromatic framework. This method has
been conveniently applied to total synthesis in organic chemistry
because of the tolerance of several functional groups, without
2
2
2
z
orbital to the higher empty d
x
-y orbital. The
1
1
1
1
1
1
31
the need to protect them. Several supported Pd-nanoparticles
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3
2
and Pd-immobilized mesoporous silica are found to serve
for that purpose, but a very few of them can be applied in
water, which is most desirable eco-friendly solvent. Our Pd-
containing mesoporous catalyst can be very efficiently utilized in
water–ethanol mixed solvent system. Results are summarized in
Table 1, which shows that Pd-MPTAT-1 is active for a variety of
functional groups, yielding only E-selective coupling products. It
has been observed that the reaction goes well not only with aryl
iodides and bromides but also with aryl chlorides. However, the
yield of the coupling product for aryl chlorides are somewhat
lower and requires a prolonged time compare to their bromo
and iodo analogues. Coupling of aryl chlorides in water has
been carried out in presence of a phase transfer agent such as
Ever since its discovery, it has had tremendous application in
heterocyclic chemistry, the synthesis of natural products and
34
pharmaceuticals for incorporating an alkyne moiety directly in
the compound. The original Sonogashira reaction often required
degassed organic solvent, inert atmosphere, strong base, pro-
longed reaction time and phosphine containing catalyst. Here
we have shown that new catalyst Pd-MPTAT-1 is efficient in
this reaction under eco-friendly condition by using triethanol-
amine as solvent as well as base. Thus, the use of degassed
19
organic solvent and the environmentally toxic phosphine
ligand-containing catalyst is eliminated. After judicial screening
with base, solvent, temperature etc., we have observed that
instead of adding additional base to the medium, triethanol
amine alone can serve the dual character i.e. both as a base as
33
NBu
4
Br (tetrabutyl ammonium bromide). The reaction was
conveniently carried out at 110 C for bromo- and iodobenzenes
but for activation of chlorobenzene, a higher temperature at
about 150 C is required. Lowering in temperature often results
in incomplete conversion of the starting materials. Several
organic and inorganic bases were screened and it was observed
◦
◦
well as solvent and the optimised temperature should be 90 C
for a successful completion of the coupling reaction with our
present catalytic system. In the absence of 0.05 mmol equivalent
CuI, the Sonogashira reaction goes well but the conversion
was poor (~25%). Using CuI as the co-catalyst dramatically
improves conversion for iodo and bromo compounds. However,
moderate conversion was found for chlorobenzene derivatives.
Using these optimized reaction conditions the efficiency of
our catalyst has been investigated for Sonogashira coupling
with various aryl halides and phenyl acetylene, the results are
summarized in Table 2. From the experimental findings we infer
that our present catalyst has very good tolerance for a wide
variety of substrates. A controlled experiment using aryl halide,
phenyl acetylene, CuI and triethanolamine has been performed
in the absence of Pd-MPTAT-1 catalyst and the result shows
practically no conversion of the reactant. All those experimental
findings again suggested the catalytic role played by Pd-grafted
mesoporous polymer Pd-MPTAT-1 in the coupling reaction. In
Fig. 8 we have shown the possible mechanism of Sonogashira
coupling schematically. Coupling between aryl halides and
◦
that K
system in water–ethanol mixture. Other inorganic and organic
bases were not as effective as K CO . With these optimised
2
CO
3
was the most efficient base for our present catalytic
2
3
reaction conditions the coupling of various substituted and
non-substituted aryl halides with styrene and acrylic acid were
examined to explore the scope and generality of this catalyst.
A controlled experiment was performed in a batch using
iodobenzene, styrene, K
2
CO in water–ethanol mixture without
3
addition of our catalyst. No conversion of iodobenzene again
strengthens the novelty of our catalyst. A detailed catalytic cycle
has been shown schematically in Fig. 7, where the Heck coupling
product is supposed to be formed through the intermediates
A, B, C, D, E and F species at the surface of the Pd-grafted
mesoporous polymer matrix.
Fig. 7 Catalytic cycle of Mizoroki–Heck cross-coupling reaction over
Pd-MPTAT-1.
b) A unique approach for Sonogashira cross-coupling using
Pd-MPTAT-1 catalyst. Another powerful method for C(sp)-
2
C(sp ) cross-coupling in organic chemistry is the Sonogashira
15
coupling, where an aryl halide is used to couple with a
terminal alkyne in the presence of a base and Pd-catalyst.
Fig. 8 Catalytic cycle of the Sonogashira cross-coupling reaction over
Pd-MPTAT-1.
1
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a
Table 1 Heck coupling of aryl halides over Pd-MPTAT-1 catalyst
b
Entry
1
Aryl halide
Product
Time (h)
8
Yield (%)
Reference†
95
85
95
S1
S1
S2
2
3
10
7
4
10
85
S2
5
6
10
8
85
90
S3
S3
7
8
9
12
12
14
75
75
70
S4
S5
S5
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Table 1 (Contd.)
b
Entry
Aryl halide
Product
Time (h)
9
Yield (%)
Reference†
S7
1
0
84
1
1
1
1
2
3
6
8
6
95
84
90
S8
S8
S9
1
1
4
5
24
22
55
60
S1
S2
1
1
6
7
24
20
55
50
S7
S9
1
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Table 1 (Contd.)
b
Entry
Aryl halide
Product
Time (h)
14
Yield (%)
Reference†
S10
1
8
80
a
◦
Reactions were carried out using 1 mmol aryl halide, 1.5 mmol alkene, 2 mmol base, 10 ml water, 5 ml ethanol at 110 C with 0.02 g Pd-MPTAT-1
b
1
catalyst. Yields refer to those purified products based on H NMR and GC analysis.
terminal alkyne has taken place through the formation of several
intermediates A, B, C and through the formation of an organo-
copper intermediate.
we found the coupling reaction proceeded well when performed
in water–DMF (5 : 1) as the solvent in the presence of phenyl
◦
trihydroxyborate salt at 90 C. The reaction temperature has also
a significant effect for the rate of coupling. At lower temperatures
◦
c) An alternative route for base-free Suzuki-Miyaura cross-
coupling using Pd-MPTAT-1 catalyst with the sodium salt
of phenyl trihydroxyborate. Suzuki–Miyaura cross-coupling
(40–70 C), low conversion was observed and increasing the
temperature dramatically increases not only the reaction rate
but also the yield of the final product. Thus the optimum
temperature was found to be 90 C with our present catalytic
15
◦
is an efficient method for C–C bond formation in organic
chemistry for preparing unsymmetrical bi-aryls from aryl halide
and phenyl boronic acid with tolerance over a wide range of
substrates. Pd nanoparticles immobilized on solid supports are
the best choice as heterogeneous catalysts for this cross-coupling
system. All results of the Suzuki–Miyaura coupling products
have been summarized in Table 3. A controlled experiment
was performed using bromobenzene as the substrate with the
sodium salt of phenyl trihydroxyborate as organo-boron species
in the absence of the catalyst (Pd-MPTAT-1). Practically no
conversion of the reactant was observed, supporting the catalytic
role of Pd-MPTAT-1. The result shows that the catalyst has
tolerance for a wide range of substrates with different functional
groups from electron donating to electron withdrawing and
also to derivatives of chlorobenzene. Reactions with a chloro-
functionalized phenyl ring were not that efficient as compared
with their bromo- or iodoanalogues, and require prolonged
heating. Chlorobenzenes containing an electron withdrawing
group require much less time for a better conversion. This
has been a successful attempt by us to convert much cheaper
chlorobenzene derivatives (compared to their -Br, -I analogues)
to their corresponding biphenyl derivatives by using Suzuki–
Miyaura cross-coupling in an environment friendly medium,
water. A schematic diagram regarding the catalytic cycle of
the Suzuki–Miyaura cross-coupling has been shown in Fig. 9.
The coupling product between the aryl halide and the phenyl
trihydroxyborate salt proceeds through the formation of A, B,
and C intermediates, at the surface of the mesoporous polymer
matrix (Fig. 9). The first step is the oxidative addition of aryl
halide with Pd(0) to form B. In the second step nucleophillic
addition of phenyl trihydroxyborate has taken place with B
35
reaction. The Suzuki cross-coupling reaction over Pd-MPTAT-
was carried out in a mixed solvent system i.e. DMF and water
1 : 5 by volume) using aryl iodides, bromides and chlorides
as substrates and the sodium salt of phenyl trihydroxyborate
1
(
18
as an alternative to organoboron species. Using borate salt
instead of phenyl-boronic acid means phenyl boronic acid can
no longer take part in the transmetalation step unless it is
activated to a borate salt by employing some suitable base.
Several drawbacks for a conventional Suzuki coupling using
boronic acid and the external base is thus solved by using the
key intermediate directly into the reaction mixture. Dissolving
phenyl boronic acid in toluene and adding concentrated NaOH
solution to this mixture precipitates out the sodium salt of phenyl
trihydroxyborate as a free flowing white powder. Addition of
NaOH continues until the complete precipitation results. The
precipitate was filtered and used for Suzuki-Miyaura coupling
reactions without any further purification. Using this base-free
protocol we have carried out C–C coupling reactions in water–
DMF mixture (5 : 1), where DMF is used mainly to solubilise
some typically insoluble organic halides. We have repeated the
experiments to find out the best possible outcome regarding the
reaction conditions by using our catalyst (Pd-MPTAT-1). So far
36
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a
Table 2 Sonogashira cross-coupling reaction of aryl-halides over Pd-MPTAT-1 catalyst
b
Entry
1
Aryl halide
Product
Time (h)
10
Yield (%)
Reference†
S11
85
80
88
2
3
14
14
S11
S12
4
5
10
12
90
80
S12
S13
6
7
8
14
16
26
75
70
60
S13
S14
S12
a
◦
Reactions were carried out using 1 mmol aryl halide, 1.5 mmol alkyne, 0.05 mmol CuI, 12 ml triethanol amine under 90 C refluxing condition with
b
1
0
.02 g Pd-MPTAT-1 catalyst. Yields refer to those purified products based on H NMR and GC analysis (supporting information†).
1
326 | Green Chem., 2011, 13, 1317–1331
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a
Table 3 Base free Suzuki-Miyaura cross-coupling reaction of aryl-halides over Pd-MPTAT-1 catalyst
b
Entry
1
Aryl halide
Product
Time (h)
8
Yield (%)
Reference†
95
85
95
S15
S15
S16
2
3
10
6
4
5
6
8
88
80
85
S16
S17
S17
10
8
7
8
12
13
80
80
S18
S19
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Table 3 (Contd.)
b
Entry
9
Aryl halide
Product
Time (h)
14
Yield (%)
Reference†
S19
78
85
85
75
78
70
1
1
1
1
1
0
1
2
3
4
10
S20
S20
S21
S22
S23
8
14
14
12
1
5
12
72
S23
1
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Table 3 (Contd.)
b
Entry
Aryl halide
Product
Time (h)
22
Yield (%)
Reference†
1
6
65
S15
1
7
20
68
S16
1
1
8
9
24
26
60
68
S17
S20
a
◦
Reactions were carried out using 1 mmol Aryl halide, 1.2 mmol sodium salt of phenyltrihydroxyborate, (1 : 5) by volume DMF : water under 90 C
b
1
refluxing condition with 0.02 g Pd-MPTAT-1 catalyst. Yields refer to those purified products based on H NMR and GC analysis (supporting
information†).
resulting in the formation of C within the mesoporous polymer.
Finally the reductive elimination from C regenerates A.
sort of metal leaching during the catalytic reactions, rendering
highly activity and selectivity of the catalyst. High catalytic
activity of the Pd-catalysts in the C–C coupling reactions has
been explained through the reactive centres involving N–Pd–
Reusability of catalyst
37,38
O binding sites.
In Fig. 10 we have plotted the conversion
After the reaction was over, the Pd-MPTAT-1 catalyst was
recovered through filtration under vacuum. Recovered solid
was extensively washed with water, dichloromethane, acetone,
and THF, until GC analysis of the filtrate shows no detectable
amounts of reagents and products. The catalyst was then dried
under vacuum overnight before performing the re-usability test.
The strong interaction between Pd(II) and the highly function-
alized polymeric surface due to the presence of the three N-
atoms in the triazine moiety and the three allylic O-atoms, which
strongly coordinate with the Pd centres, thereby preventing any
levels for the Pd-MPTAT catalyst in Suzuki–Miyaura coupling
reaction between iodobenzene and the sodium salt of phenyl
trihydroxylborate in water–DMF. As seen from the plot that the
catalyst is reusable up to sixth times and only a small decrease
in the product yield from 92 to 78% occurred.
Proof of heterogeneity of the catalyst
We have checked the heterogeneity criteria of our Pd-MPTAT-1
39
catalyst by the hot filtration test with bromo benzene and the
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removal of the catalyst or the catalyst is truly heterogeneous in
nature because no evidence of leaching of the palladium from
solid support was observed as confirmed from atomic absorption
spectroscopic (AAS) measurement of the hot filtrate solution.
Further, during the course of those reactions no evidence for the
decomposition of the polymeric solid support was observed.
All these results clearly demonstrate that surface bound Pd
remained intact within the solid framework and even a trace
amount of Pd was not leached out from the solid catalyst to the
solution. Thus our novel polymeric Pd-support is an outstanding
heterogeneous catalyst for several C–C cross-coupling reactions
in the presence of water.
Solid phase poisoning test
Further the heterogeneous nature of the catalyst was checked
by employing a solid-phase poisoning test, where we have used
3
-mercaptopropyl-functionalized silica (commercially available)
as an efficient Pd scavenger. In a typical experimental procedure,
iodobenzene (1 mmol), styrene (1.5 mmol), K CO (2 mmol),
Fig. 9 Catalytic cycle of Suzuki-Miyaura cross-coupling reaction over
Pd-MPTAT-1.
2
3
water (10 ml) and ethanol (5 ml) were mixed in a round bottom
flask along with 0.02 g Pd-MPTAT-1 catalyst and 0.05 g 3-
mercaptopropyl-functionalized silica. The whole mixture was
◦
refluxed in an oil bath for about 12 h at 110 C. No change
in conversion was observed vis- a` -vis our previous experiment
(Table 1, entry 1, yield 95% in both cases), which clearly
demonstrates that our novel catalyst is heterogeneous in nature.
Since –SH functionalized mesoporous silica forms a complex
with the leached palladium, it can deactivate the catalyst
otherwise. Thus, we may conclude that there is no question of
palladium leaching from Pd-MPTAT-1 to the solution during a
Heck coupling reaction.
Conclusions
In summary, we have successfully synthesized a novel organic
mesoporous polymer through facile and simple polymerization
of 2,4,6-triallyloxy-1,3,5-triazine using APS as radical initiator
and SDS as the template under acidic pH conditions. TEM
image analysis suggests the presence of 2D hexagonal mesopores
throughout the polymer. The template free polymer when
anchored with Pd(II) shows high catalytic activity in several C–
C cross-coupling reactions like Heck, Sonogashira and Suzuki–
Miyaura under eco-friendly conditions without using phosphine
containing catalyst or environmentally hazardous solvents.
Under certain circumstances chloroarenes also responded to
coupling reactions to some extent in the presence of this catalyst.
Aryl chlorides normally remain inactive under cross-coupling
conditions and there are a very few catalysts available which
can activate chloro-compounds as well. Pd-MPTAT-1 is very
active in the presence of water, which is a green solvent and can
be recycled several times without much loss in reactivity. Thus
this C–C cross-coupling reactions presented herein could be a
versatile tool for the production of several value added organics
in a green chemical route.
Fig. 10 Recycle efficiency of Pd-MPTAT-1 in Suzuki-Miyaura
coupling reaction between iodobenzene and sodium (trihydroxyl)-
phenylborate in water–DMF.
sodium salt of phenyl trihydroxyborate (Suzuki) and the solid
40
phase poisoning test with bromo benzene and styrene (Heck).
Hot filtration test
To check whether Pd is being leached out from the solid catalyst
to the solution or weather the catalyst is truly heterogeneous
in nature, Suzuki cross-coupling reaction between iodoben-
zene and phenyl trihydroxyborate has been selected for the
investigation purposes. So, to serve for that purpose, 1 mmol
iodobenzene and 1.2 mmol phenyl trihydroxyborate were taken
in a two neck round bottom flask. Then 5 : 1 mixture of water
and DMF (by volume) was added along with 0.02 g of Pd-
34
◦
MPTAT-1 catalyst maintaining 80 C during the reaction. After
continuing the reaction for about 4 h, catalyst was separated
by filtration and the filtrate was then analyzed by a GC to
check the conversion values which was found to be 25%. The
filtrate was then continued to reflux for another 6 h without
any catalyst. After completion of 14 h, GC analysis revealed no
increase in conversion values (25%) in absence of catalyst. This
result suggested that the coupling reaction cannot proceed after
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