A ruthenium-grafted triazine functionalized mesoporous polymer: A highly efficient and multifunctional catalyst for transfer hydrogenation and the Suzuki-Miyaura cross-coupling reactions

Article abstract of DOI:10.1039/c3dt53158a

A new ruthenium-grafted mesoporous organic polymer Ru-MPTAT-1 has been synthesized via simple and facile in situ 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 a template, followed by grafting of Ru(ii) onto its surface. Ru-MPTAT-1 has been characterized by elemental analysis, powder XRD, HRTEM, FT-IR, UV-vis DRS, TG-DTA, FESEM and XPS characterization tools. The Ru-MPTAT-1 material showed very good catalytic activity in the Suzuki-Miyaura cross-coupling reaction for aryl halides and transfer hydrogenation reaction for a series of carbonyl compounds. The catalyst is easily recoverable from the reaction mixture and can be reused several times without appreciable loss of catalytic activity in the above reactions. Highly dispersed and strongly bound Ru(ii) sites at the mesoporous polymer surface could be responsible for the observed high activity of the Ru-MPTAT-1 catalyst in these reactions.

Full text of DOI:10.1039/c3dt53158a

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A ruthenium-grafted triazine functionalized
mesoporous polymer: a highly efficient and
multifunctional catalyst for transfer hydrogenation
and the Suzuki–Miyaura cross-coupling reactions†
Cite this: Dalton Trans., 2014, 43,
7057
Noor Salam,a Sudipta K. Kundu,b Anupam Singha Roy,a Paramita Mondal,a
Kajari Ghosh,a Asim Bhaumik*b and S. M. Islam*a
A new ruthenium-grafted mesoporous organic polymer Ru-MPTAT-1 has been synthesized via simple
and facile in situ 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 a template, followed by grafting of Ru(II)
onto its surface. Ru-MPTAT-1 has been characterized by elemental analysis, powder XRD, HRTEM, FT-IR,
UV-vis DRS, TG-DTA, FESEM and XPS characterization tools. The Ru-MPTAT-1 material showed very good
catalytic activity in the Suzuki–Miyaura cross-coupling reaction for aryl halides and transfer hydrogenation
reaction for a series of carbonyl compounds. The catalyst is easily recoverable from the reaction mixture
and can be reused several times without appreciable loss of catalytic activity in the above reactions.
Highly dispersed and strongly bound Ru(II) sites at the mesoporous polymer surface could be responsible
for the observed high activity of the Ru-MPTAT-1 catalyst in these reactions.
Received 8th November 2013,
Accepted 22nd February 2014
DOI: 10.1039/c3dt53158a
www.rsc.org/dalton
stereoselectivity, and excellent compatibility with many func-
tional groups. The efficiency of the reactions has been success-
Introduction
The selective reduction of carbonyl compounds to the corres- fully demonstrated in the synthesis of natural products,
ponding alcohols is a very important and demanding trans- organic building blocks, pharmaceuticals, and agricultural
formation in organic synthesis. Most of the methods reported derivatives. Homogeneous metal catalysts are often used as
in the literature for catalytic reduction require the use of a stoi- effective catalysts for these reactions.6–12 However, there are
chiometric amount of sensitive reagents or the use of danger- serious difficulties in the recovery and reuse of expensive
ous hydrogen gas under high pressure conditions.1–3 Transfer metal-catalysts and most of the reactions do not proceed
hydrogenation has become the centre of attraction because the without the use of expensive ligands. Heterogeneous catalysts,
hydrogen donors, such as 2-propanol, etc., are readily avail- which can be prepared by heterogenisation of homogeneous
able, inexpensive, easy to handle, and do not require elaborate counterparts (metal complexes, metal salts, metals or metal
experimental setups, such as high pressure reactors. Similarly, oxide nanoparticles) into insoluble solid porous matrices, have
the Ru-catalyzed olefination of aryl halides (Heck–Mizoroki gained increasing attention due to their easy separation tech-
reaction)4 and closely relevant Pd-catalyzed cross-couplings nique, recovery and reusability of the catalyst.13 Numerous
(e.g. Sonogashira, Suzuki–Miyaura reactions, etc.)4,5 have been heterogeneous catalysts (noble metals such as Ru, Pt, Pd, and
developed as efficient tools for the carbon–carbon and Au) have been reported as effective and reusable catalysts for
carbon–heteroatom bond formation reactions in synthetic these reactions.14–18
organic chemistry. The catalytic pathway is very simple, attrac-
In this context functionalized mesoporous materials have
tive and versatile for a wide range of organic transformations gained increasing interest and successfully utilized as drug-
because they usually offer high product yields, good regio- and delivery vehicles,19 and in adsorption,20 metal ion sensing,21
catalysis,22 selective separation23 and gas storage24 due to their
exceptionally high surface area, thermal and mechanical stabi-
aDepartment of Chemistry, University of Kalyani, Kalyani, Nadia, 741235 W.B.,
lity. To date, mesoporous materials composed of inorganic
oxides25 and inorganic–organic hybrid silicas26 have been
India. E-mail: manir65@rediffmail.com
bDepartment of Material Science, Indian Association for the Cultivation of Science,
studied extensively, but only limited research work has been
carried out on purely organic mesoporous materials, which
Kolkata 700032, India. E-mail: msab@iacs.res.in
†Electronic supplementary information (ESI) available. See DOI:
could bear strong donor sites for binding active metal ions at
10.1039/c3dt53158a
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their surface.27–29 Thus, to design an efficient heterogeneous Preparation of the catalyst
catalyst, which could support a reactive ruthenium complex via
Synthesis of mesoporous poly-2,4,6-triallyloxy-1,3,5-triazine
(MPTAT-1). MPTAT-1 has been synthesized through the
aqueous-phase polymerization of 2,4,6-triallyloxy-1,3,5-triazine
(TAT) under hydrothermal conditions using ammonium per-
sulfate (APS) as an initiator. In a typical synthesis, 2.35 g
(0.00814 mol) of SDS was dissolved in 40 ml of water with con-
stant stirring followed by the addition of 2 g (0.008 mol) of
2,4,6-triallyloxy-1,3,5-triazine. After the addition of 0.94 g
(0.00814 mol) of TMEDA 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 interaction of the organic mono-
oxygen and nitrogen donor atoms on the mesoporous frame-
work, is of great significance. This is particularly important
due to their multi-electron transfer properties, their ability to
have a wide range of oxidation states and thus potential cata-
lytic efficiency.30–32
Thus, we designed
a new functionalized mesoporous
material based on an organic polymer framework by grafting it
with an active metal centre to explore its potential application
in C–C coupling reactions and transfer hydrogenation reaction.
Very recently, a new functionalized mesoporous polymer
(MPTAT-1) has been synthesized using sodium dodecyl sulfate
as a template, which could bind to active Pd(II) sites strongly at
its surface.33 Inspired by the above observations, in the
present work we have designed a mesoporous organic polymer
catalyst through the polymerization of the 2,4,6-triallyloxy-
1,3,5-triazine monomer unit, followed by grafting ruthenium
onto its surface, and explored its potential in transfer hydro-
genation and the Suzuki–Miyaura C–C cross-coupling reactions
maintaining eco-friendly reaction conditions.
−
mers with the negative charge of C12H23SO3 of the SDS mole-
cules. 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 to the solution with vigorous stirring. A
white precipitate appeared immediately after the addition of
APS. The resulting 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 resulting precipitate was filtered, and washed
thoroughly with de-ionized water, yielding the mesoporous
polymer MPTAT-1. The surfactant was removed from the as-
synthesized mesoporous polymer 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 tem-
plate extraction procedure was repeated three times to obtain
template-free MPTAT-1.
Synthesis of ruthenium grafted mesoporous poly-2,4,6-tri-
allyloxy-1,3,5-triazine (Ru-MPTAT-1). The synthetic procedure
for grafting ruthenium onto the mesoporous poly-2,4,6-trially-
loxy-1,3,5-triazine (MPTAT-1) has been illustrated in Scheme 1.
For Ru loading, 1.0 g of the template-free mesoporous polymer
was dispersed in 20 ml of DMF taken in a round bottom flask.
To this 0.5 g ruthenium chloride was added and the suspen-
sion was refluxed for about 12 h under vigorous stirring. At the
end, the resulting solid was filtered, washed thoroughly with
methanol and finally dried under vacuum for 24 h to obtain
Ru-MPTAT-1.
Experimental section
Materials and reagents
2,4,6-Triallyloxy-1,3,5-triazine (TAT, monomer), RuCl3 and
other reactants for the catalytic reactions were obtained from
Sigma Aldrich. N,N,N′N′-Tetramethylethylenediamine (TMEDA,
promoter), ammonium persulfate (APS, radical initiator), and
sodium dodecyl sulfate (SDS, template) were purchased from
Loba Chemie, India. All other chemicals used for this investi-
gation purpose were of analytical grade produced by Merck,
India unless otherwise mentioned.
Instrumentation
Powder X-ray diffraction (XRD) patterns of different samples
were analyzed with a Bruker D8 Advance X-ray diffractometer
using Ni-filtered Cu Kα (λ = 0.15406 nm) radiation. Trans-
mission electron microscopy (TEM) images of the mesoporous
polymer were obtained using a JEOL JEM 2010 transmission
electron microscope operated at 200 kV. Thermogravimetric
analysis (TGA) was carried out using a Mettler Toledo TGA/
DTA 851e. Scanning electron microscopy measurements were
General experimental procedure for transfer hydrogenation of
carbonyl compounds
performed with a JEOL JEM 6700F field-emission scanning In a typical reaction in an oven dried 100 ml RB flask,
electron microscope (FESEM). Carbon, hydrogen and nitrogen Ru-MPTAT-1 catalyst (0.05 g), isopropyl alcohol (10 ml),
contents of both the mesoporous MPTAT-1 and Ru-grafted the substrate (1 mmol) and NaOH (0.4 mmol) were stirred
polymer were analyzed using a Perkin Elmer 2400 Series II under
a nitrogen atmosphere at 60 °C. The reaction
CHN analyzer. UV-Vis spectra of the samples were recorded mixtures were collected at different time intervals and identi-
using a Shimadzu UV-2401PC double beam spectrophotometer fied by GC-MS and quantified by GC analysis. After the com-
having an integrating sphere attachment for solid samples. pletion of the reaction, the catalyst was filtered off and
The reaction products were quantified (GC data) using a washed with water followed by acetone and dried in an
Varian 3400 gas chromatograph equipped with a 30 m oven. The filtrate was extracted three times with ethyl acetate
CP-SIL8CB capillary column and a flame ionization detector (3 × 20 ml) and the combined organic layers were dried
and identified using a Trace DSQ II GC-MS equipped with a with anhydrous Na2SO4 under high vacuum. The filtrate
60 m TR-50MS capillary column.
was concentrated and the resulting residue was purified by
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its mesoporous structure. In Scheme 1, we have described the
polymerisation of the TAT monomer in the presence of
ammonium persulfate, which is a good radical initiator for
aqueous phase polymerization of olefins under mild acidic
conditions.34 Since one monomer contains three olefinic
double bonds in each of its branches, this polymerization
gives random cross-linking, leading to a highly stable and
robust cross-linked porous organic framework. Immobilization
of ruthenium and its strong binding on the MPTAT-1 surface
has been facilitated due to the presence of strong donor
N- and O-sites in the triazine moiety.27c
Characterization of Ru-MPTAT-1 material
In Fig. 1 we have shown the proposed polymeric network in
the as-synthesized MPTAT-1, template-free MPTAT-1 and Ru-
MPTAT-1. The as-synthesized mesoporous polymer synthesized
through the radical polymerization of 2,4,6-triallyloxy-1,3,5-tri-
azine in the presence of the SDS template upon acid extraction
yielded template free MPTAT-1. This template free mesoporous
polymer upon impregnation with RuCl3 in air resulted in Ru-
MPTAT-1. AAS analysis revealed that the ruthenium content in
the Ru(II)-grafted mesoporous polymer was 4.56 wt%.
Scheme 1 Organic–organic in situ radical polymerization of 2,4,6-trial-
lyloxy-1,3,5 triazine by APS by using a SDS template and grafting of Ru(II)
onto its surface to obtain Ru-MPTAT-1.
X-ray diffraction
column chromatography on silica gel to obtain the desired
product.
In Fig. 2 the small angle powder X-ray diffraction pattern of the
ruthenium loaded mesoporous organic polymer Ru-MPTAT-1 is
shown. It shows one distinct diffraction peak centered at 2θ =
2.30 degrees, corresponding to the mesophase with the
General experimental procedure for the Suzuki–Miyaura
coupling reaction
In a typical reaction a mixture of aryl halide (1.0 mmol), phenyl-
boronic acid (1.5 mmol), K2CO3 (2.0 mmol), DMF (5 ml)
and Ru-MPTAT-1 catalyst (0.05 g) was stirred at 80 °C in a
closed round-bottom flask. To study the progress of the reac-
tion the reaction mixtures were collected at different time
intervals and quantified by GC analysis. At the end of the reac-
tion, the catalyst was separated by simple filtration. The filtrate
was extracted with CH2Cl2 and passed through a pad of silica
gel. The organic phase thus collected was dried over Na2SO4,
filtered, concentrated and the residue was purified through
flash chromatography on silica gel. The product was analyzed
by 1H NMR. All prepared compounds are known and their
respective NMR spectra were compared with authentic
samples.
Results and discussion
Formation of mesophase
In the presence of the anionic structure directing agent SDS,
the organic–organic supramolecular templating interaction
takes place in aqueous medium. Under the acidic pH con-
ditions the positively charged protonated N-atoms of the tri-
azine moiety preferentially interact with the negatively charged
polar head groups of the dodecyl sulfonate moiety i.e.
−
C12H23SO3 and condensed around the micelle to form a con-
tinuous mesophase. The resulting array of organic mesophases
Fig. 1 Schematic synthetic pathway for MPTAT-1 (as-synthesized),
upon removal of the surfactant via acid extraction has retained MPTAT-1 (template free) and Ru-MPTAT-1.
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area of Ru-MPTAT-1 suggests that a considerable amount of
Ru-centers have been anchored to the polymer. Pore size distri-
bution (PSD) of this sample employing non-local density func-
tional theory (NLDFT) is shown in the inset of Fig. 3. The pore
size distribution peaks are centered at 1.5 nm, 2.8 nm and
4.73 nm suggesting that the material contains a wide range of
pores in the supermicropore to the mesopore region. The pore
volume of the material is 0.0364 cc g−1
.
Electron microscopy analysis (SEM and TEM)
A field emission scanning electron microscopy (FESEM) image
of Ru-MPTAT-1 is shown in Fig. 4. This FESEM image indicates
uniform spherical morphology of the Ru-MPTAT-1 material
with the average dimension of ca. 64 nm. Here the spherical
particles get aggregated among themselves to form a large
assembly of particles.
HRTEM images of Ru-MPTAT-1 are shown in Fig. 5. These
images showed the presence of ruthenium species (dark spots)
and the hollow sphere nanostructure.35,36 This Ru-grafted
mesoporous organic polymer consists of almost all spherical
shaped nanoparticles together with a smooth shell wall. These
hollow spheres are uniformly distributed throughout the speci-
men grid and ruthenium species are anchored to a considerable
extent on the surface of these hollow spheres. The size of these
nanospheres showed a wide size distribution of ca. 50–180 nm.
Fig. 2 Small angle powder XRD patterns of Ru-MPTAT-1 material.
interplanar distance of 3.85 nm. The XRD pattern of the tem-
plate-free MPTAT-1 shows the 100 diffraction peak at 2θ = 2.08
degrees, whereas after Ru-grafting the 2θ value increases con-
siderably, suggesting that the contraction of pore walls occurred
during the anchoring of ruthenium to the mesopore surface.33
N2 adsorption–desorption
Bonding and the framework structure
In Fig. 3 the nitrogen adsorption–desorption isotherm of Ru-
MPTAT-1 material is shown. This isotherm is closely related to
the type IV isotherm, which is the characteristic of mesoporous
materials. However, no sharp capillary condensation was
observed. The BET surface areas for the template-free MPTAT-1
and Ru-MPTAT-1 material are 135 m2 g−1 (ref. 33) and 30 m2
g−1, respectively. The considerable decrease in the BET surface
FTIR spectra of both mesoporous polymers MPTAT-1 and Ru-
MPTAT-1 are shown in Fig. 6. Both the samples showed two
distinct C–H vibrations in the region 2932–2950 cm−1, corres-
ponding to non-identical C–H bonds in the triazine frame-
work. Both samples exhibited a very strong band at 1574 cm−1
due to ring >CvN stretching vibration together with two
additional sharp peaks at 1142 cm−1 (δ∼C–N) and 1420 cm−1
(δ∼CH2). Furthermore, the allylic double bond (–CvC–)
stretching of the 2,4,6-triallyloxy-1,3,5-triazine moiety at
Fig. 3 N2 adsorption–desorption isotherm of Ru-MPTAT-1. Adsorption
points are marked by filled circles and those for desorption are marked
by open circles. NLDFT pore size distribution is shown in the inset.
Fig. 4 FESEM image of Ru-MPTAT-1 materials.
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involving two major steps corresponding to the collapse of the
mesostructure and the polymeric framework.
XPS analysis
In Fig. 8, XPS data of Ru-MPTAT-1 material are shown. The
XPS spectrum of Ru-MPTAT-1 shows two major peaks with
binding energies 286.4 eV and 532.2 eV. A weak peak at ca. 400
eV is attributed to the N 1s of triazine nitrogen atoms. The
binding energies of 284.1 and 280.0 eV are usually observed
for the Ru 3d3/2 and Ru 3d5/2 photoelectron lines for metallic
ruthenium.37 The observed main peak at 286.4 eV could be
assigned due to the combination of C 1s of the carbon atoms
of the organic polymeric network and Ru 3d3/2 in Ru-MPTAT-1.
On the other hand in the Ru 3d region (Fig. 8A), a weak peak
at 277.8 eV could be assigned due to Ru 3d5/2 of Ru(II) species
present at the surface of the polymeric network.38 Thus, this
X-ray photoelectron spectroscopy result suggested that the +2
oxidation state of ruthenium is present in our Ru-grafted
mesoporous polymer Ru-MPTAT-1.
UV-vis spectroscopy study
The UV-vis absorbance spectrum of MPTAT-1 exhibits no
strong characteristic absorbance, except for two very weak
peaks at 207 and 266 nm due to very weak π→π* transitions.
But a distinguishable change in absorbance is observed in the
case of Ru-MPTAT-1 material (Fig. 9). This material displays
two intense absorptions in the region 250–650 nm which can
be assigned to the metal to ligand charge transfer along with
intra-ligand transitions. The high intensity bands with peaks
at around 340 and 256 nm have been designated as n→π* and
π→π* transitions, respectively. This phenomenon could be due
to the relative destabilization of the ruthenium t2g orbital poss-
ibly due to the electron donation from the two anionic chlor-
ide ligands to the ruthenium center.39,40
Fig. 5 HRTEM images of Ru-MPTAT-1.
Catalytic activity
Since mesoporous materials supported with active Pd metal
sites exhibit high catalytic activity in a wide range of important
reactions,33 we have explored the catalytic activity of Ru-
MPTAT-1 in the C–C cross-coupling reactions of different aryl
halides and transfer hydrogenation of ketones in the presence
of NaOH as a base.
1840 cm−1 has disappeared and the intensity of another at
1719 cm−1 has diminished considerably, suggesting near com-
pletion of polymerization reaction. All the framework
vibrations are assigned and given in tabular form at the
bottom of the FTIR spectra and this result suggested the pres-
ence of the 2,4,6-triallyloxy-1,3,5-triazine moiety in MPTAT-1
and Ru-MPTAT-1 materials.
Catalytic transfer hydrogenation of ketones
Ruthenium mediated transfer hydrogenation reactions are
found to be effective catalytic systems in which hydrogen is
Thermal analysis
The organic content and the framework stability of the Ru- transferred from one organic molecule to another under liquid
MPTAT-1 sample are estimated quantitatively from the thermo- phase reaction conditions and this made us to carry out this
gravimetric (TG) and differential thermal analyses (DTA). TGA– reaction over Ru-MPTAT-1. The catalytic activity of Ru-MPTAT-1
DTA plots of Ru-MPTAT-1 are shown in Fig. 7. The TGA of this in the transfer hydrogenation of various aliphatic and aromatic
material showed the first weight loss below 100 °C due to de- ketones in the presence of isopropanol and a base as a promo-
sorption of physisorbed water. This was followed by a gradual ter has been explored (Scheme 2). The reaction conditions for
decrease in the weight after 250 °C. Thus this thermal analysis the transfer hydrogenation of acetophenone have been opti-
data suggested that the Ru-MPTAT-1 sample is stable up to mized. In order to optimize the reaction conditions, different
250 °C. Above 250 °C the Ru-MPTAT-1 structure collapses solvents and bases were studied.
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Fig. 6 FT IR spectra of MPTAT-1 (a) and Ru-MPTAT-1 (b). Vibration bands are tabulated at the bottom of the spectra.
Fig. 7 TGA (a) and DTA (b) profiles of Ru-MPTAT-1 material.
We have carried out the transfer hydrogenation over Ru-
MPTAT-1 in the presence of various solvents and NaOH as a
base, and the results are summarized in Table 1. Methanol,
ethanol, and isopropanol were taken for investigations and iso-
propanol is found to be a suitable reducing agent for the
maximum conversion of acetophenone. As is seen in Table 1,
the use of methanol resulted in poor conversion of the sub-
strate (entry 1) when compared to ethanol and isopropanol.
Interestingly, the reaction performed using isopropanol
(iPrOH) as the hydride donor gave the overall highest yield.
The effect of the bases on the catalytic performance of this
system has been investigated in detail. In transfer hydrogen-
ation, the base facilitates the formation of ruthenium alkoxide
Fig. 8 XPS spectra of the Ru 3d region (A) and full scale (B) pattern of
Ru-MPTAT-1.
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81–93% of conversion (entries 3 and 4). In the absence of a
base, no conversion of ketones into alcohol was observed
(entry 8). Similarly, the use of triethylamine as a base also
leads to trace conversion (entry 6). As a result NaOH was
selected as the optimum base. We observed the best yield in
the transfer hydrogenation reaction of acetophenone using
NaOH as a base and iPrOH as a solvent at 60 °C for 5 h.
The transfer hydrogenation reaction of a series of carbonyl
compounds has been carried out over Ru-MPTAT-1 using
i-PrOH and NaOH at 60 °C and the results are summarized in
Table 2. All the reactions are carried out under identical con-
ditions to allow a reasonably good comparison of results. The
products are identified by GC and GC-MS after doing the
required workup. The complex catalyzes the transfer hydrogen-
ation of ketones to their corresponding alcohols with good to
excellent conversions in all the cases. Acetophenone with
different substituents of varying electronic properties is efficien-
tly reduced to the corresponding secondary alcohols in excellent
conversions. The presence of electron-withdrawing or electron-
donating substituents on the aromatic ring has a significant
effect on the conversion. In general, an electron-withdrawing
substituent (4′-bromo, 4′-chloro, 4′-nitro and 4′-cyano) on the
aryl ring gave higher conversions (entries 2–4) when compared
to that of acetophenone whereas an electron-donating substitu-
ent (4′-methyl, 4′-methoxy and 4′-hydroxy) on the ring gave
lower conversions (entries 5–7) than that of acetophenone. On
the other hand, benzophenone is quantitatively reduced to 1,1-
diphenylmethanol and shows the conversion of 82% (entry 8).
Furthermore, the ruthenium complex has been demonstrated to
be a very active catalyst toward a variety of alkyl ketones. 2-Buta-
none and 2-pentanone underwent transfer hydrogenation to
afford the corresponding alcohols in 65% and 69% respectively
(entries 9 and 10). Similarly, the Ru-MPTAT-1 shows good cata-
lytic activity for the transfer hydrogenation of cyclic ketones. Par-
ticularly, 1-cyclopentanone has been reduced to 1-cyclopentanol
almost quantitatively over Ru-MPTAT-1 (entry 11).
Fig. 9 UV-Visible absorbance of Ru-MPTAT-1.
Scheme 2 Ru-MPTAT-1 catalyzed transfer hydrogenation of carbonyl
compounds.
Table 1 Optimization of reaction conditionsa
Entry
Alcohol
Base
Yieldb (%)
1
2
3
4
5
6
7
8
MeOH
EtOH
NaOH
NaOH
NaOH
KOH
LiOH
Et3N
30
48
93
81
43
Trace
38
Suzuki–Miyaura cross-coupling reaction
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
The Suzuki–Miyaura cross-coupling reaction represents a very
powerful method for the C–C bond formation reaction in the
organic synthesis. Construction of biaryl compounds via the
Suzuki–Miyaura reaction is an interesting area in organic syn-
thesis. The importance of biaryl units as molecular com-
ponents in pharmaceuticals, herbicides and natural products,
as well as in engineering materials such as conducting poly-
mers, molecular wires and liquid crystals, has attracted enor-
mous interest. We started our investigation for the Suzuki–
K2CO3
—
—
a Reaction conditions: acetophenone (1 mmol), Ru-MPTAT-1 (0.05 g),
base (0.4 mmol), solvent (8 ml), temperature (60 °C), time 5 h, N2
atom. b Yield determined by GC and GC-MS analyses.
by abstracting proton from the alcohol and subsequently alk- Miyaura reaction (Scheme 3) with the coupling of aryl halides
oxide undergoes β-elimination to give ruthenium hydride, and phenylboronic acid as a model reaction in the presence of
which is an active species in this reaction. This mechanism the ruthenium catalyst. In order to optimize the reaction con-
has been proposed by several researchers for the ruthenium ditions, the performance of a ruthenium-catalyzed Suzuki–
catalyzed transfer hydrogenation reaction by metal hydride Miyaura reaction is known to be governed by a number of
intermediates.41 The substrate was allowed to react with a cata- factors such as the bases, solvents, reaction temperatures, etc.
lytic amount of complex in the presence of different bases
To verify the solvent effect in Suzuki–Miyaura cross-coup-
such as NaOH, KOH, LiOH, K2CO3 and Et3N, with isopropanol ling reactions, a series of reactions were investigated by taking
(Table 1). It was found that the use of NaOH and KOH leads to this model reaction over Ru-MPTAT-1 in different solvents.
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Table 2 Catalytic transfer hydrogenation of carbonyls over Ru-MPTAT-1a
Entry
1
Substrate
Product
Conversionb (%)
93
2
94
3
4
5
97
95
89
6
7
85
84
8
82
9
65
69
90
10
11
a Reaction conditions: acetophenone (1 mmol), Ru-MPTAT-1 (0.05 g), NaOH (0.4 mmol), iPrOH (8 ml), temperature (60 °C), time 5 h, N2 atom.
b Yield determined by GC and GC-MS analyses.
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Scheme 3 Ru-MPTAT-1 catalyzed Suzuki–Miyaura cross-coupling reactions.
Quantitative yields were observed with polar solvents such as bearing either electron-donating or electron-withdrawing sub-
DMF and methanol (Table 3). In contrast, unsatisfactory yields stituents in the para positions afforded the corresponding
were obtained with nonpolar solvents such as toluene, THF or biphenyls in good to excellent yields. A heteroarylbromide,
p-xylene. The catalytic activity was also observed in water and 2-bromopyridine, was coupled efficiently with phenylboronic
the desired product was obtained in 42% yield (Table 1, entry acid to give the desired product in excellent yield (Table 4,
7). Since the starting 4-bromoanisole and the biphenyl product entry 11). The cross-coupling of sterically hindered arylhalides,
showed poor solubility in water, but high solubility in DMF, the 2-iodonaphthalene and 2-nitrobromobenzene, also proceeded
presence of DMF could accelerate the reaction rate. Conse- quite well (Table 4, entries 4, 10).
quently, DMF was chosen as the medium of choice for this
Although there are some efficient Ru-based heterogeneous
coupling. The effect of the added base on the catalytic perform- catalysts like inorganic SiO2 supported Ru(II) complexes for
ance of this system was investigated by employing various bases. efficient transfer hydrogenation of ketones42 and the Suzuki–
Comparison of inorganic bases utilized showed that carbonate Miyaura cross-coupling reactions,43 the use of a Ru-grafted
bases were more stable than the other ones. Within a short time mesoporous organic polymer in both these reactions for the
the reaction proceeded giving high yields in the presence of synthesis of value added fine chemicals is rarely found in the
Na2CO3 or K2CO3. We found that using K2CO3 as a base in DMF literature. O and N donor sites of the 2,4,6-triallyloxy-1,3,5 tri-
at 80 °C gave the coupled product in 96% yield. The other in- azine moiety of the polymeric framework could chelate the
organic bases such as NaOH, K3PO4, NaOAc and organic bases reactive Ru(II) sites strongly at the mesopore surface
like Et3N were not as effective as K2CO3, and only afforded mod- (Scheme 1), leading to high catalytic activity.
erate to low yields of coupling products (entries 8–11). As a
Recycling of the catalyst
result, K2CO3 was selected as the optimum base. In the absence
of the base, no coupling reaction was observed (entry 12). Thus,
the optimized conditions for the Suzuki cross-coupling reac-
tions are K2CO3 as a base and DMF as a solvent at 80 °C.
To examine the scope of this coupling reaction, a variety of
aryl halides were coupled with different phenylboronic acids
in DMF in the presence of Ru-MPTAT-1 using K2CO3 as a base.
The experimental results are summarized in Table 4 (1H NMR:
ESI†). A control experiment indicated that the coupling reac-
tion did not proceed in the absence of a catalyst. Aryl halides
For a heterogeneous catalyst, it is important to examine the
ease of separation, recoverability and reusability. The reusabil-
ity of the mesoporous Ru-MPTAT-1 catalyst was investigated in
transfer hydrogenation and the Suzuki–Miyaura cross coupling
reactions. After each run the filtrate was concentrated and the
resulting residue was purified. After a simple wash using ethyl
acetate and drying, the catalyst was reused for the next run
and almost consistent activity was observed for the next six
consecutive cycles (Fig. 10, reaction conditions are given in the
footnote). The strong interaction between ruthenium and the
highly functionalized polymeric surface due to the presence of
the three N-atoms in the triazine moiety and the three allylic
O-atoms facilitates strong coordination with the Ru centres.
This prevents the leaching of Ru(II) species during the catalytic
reactions, rendering high activity and selectivity to the catalyst.
As can be seen from the figure, the catalyst can be efficiently
recycled and reused for repeating cycles without an appreci-
able decrease in product yield.
Table 3 Optimization of reaction conditions for C–C coupling
reactionsa
Entry
Solvent
Base
Yieldsb (%)
1
2
3
4
5
6
7
8
DMF
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
NaOH
NaOAc
Et3N
96
12
54
25
10
12
59
80
Proof of heterogeneity of the catalyst
Toluene
Methanol
Acetonitrile
p-Xylene
THF
H2O
DMF
DMF
DMF
DMF
DMF
An important point concerning the use of a heterogeneous
catalyst is its lifetime, particularly for industrial and pharma-
ceutical applications. Heterogeneity of the catalyst was exam-
ined by the “hot filtration test” for the transfer hydrogenation
reaction.
9
27
36
13
10
11
12
Hot filtration test
—
No reaction
To examine whether ruthenium was being leached out from
the solid support to the reaction medium during the course of
the catalytic reaction, a control experiment has been carried
a Reaction conditions: 4-bromoanisole (1.0 mmol), phenylboronic
acid (1.5 mmol), Ru-MPTAT-1 (0.05 g), base (2.0 mmol), solvent (5 ml).
b Yields were determined by GC and GC-MS analyses.
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Table 4 Suzuki–Miyaura cross-coupling reaction of aryl halides and arylboronic acids using Ru-MPTAT-1a
Entry
1
ArX
Productb
Time (h)
5
Yieldc (%)
96
2
3
4
5
5
6
89
95
65
5
6
6
8
89
70
7
8
9
8
6
6
83
86
88
10
8
78
11
12
6
79
47
12
13
14
12
8
39
81
15
8
86
a Reaction conditions: aryl halides (1.0 mmol), phenylboronic acid (1.5 mmol), Ru-MPTAT-1 (0.05 g), K2CO3 (2.0 mmol), DMF (5 ml); all the
reactions were carried out in air. b Products were identified by comparison of their IR, GC-MS and 1HNMR spectral data reported in the literature.
c Yield of the product was determined by GC and GC-MS analyses using dodecane as the internal standard.
out on the transfer hydrogenation reaction over the Ru- 3 h and the filtrate was allowed to react up to the completion
MPTAT-1 mesoporous catalyst to know whether the reaction of the transfer hydrogenation reaction (6 h). It was noted that
proceeded in a heterogeneous or homogeneous fashion.44 For after filtration of the catalyst from the reactor at the reaction
the rigorous proof of heterogeneity, a test was carried out by temperature, transfer hydrogenation reactions do not proceed
filtering the catalyst from the reaction mixture at 60 °C after further. Atomic absorption spectrometric analysis of the
7066 | Dalton Trans., 2014, 43, 7057–7068
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New Delhi, India for their research fellowships. PM is thankful
to the UGC, New Delhi, India for her Kothari fellowship. AB
thanks DST, New Delhi for providing instrumental facilities
through the DST Unit on Nanoscience and DST-SERB project
grants. We acknowledge the Department of Science and Tech-
nology (DST) and the University Grant Commission (UGC)
New Delhi, India for providing support to the the Department
of Chemistry, University of Kalyani under PURSE, FIST and
SAP programs.
Fig. 10 Recycle efficiency of Ru-MPTAT-1 for the Suzuki–Miyaura
cross-coupling and transfer hydrogenation reactions. Reaction con-
ditions for the transfer hydrogenation reactions: acetophenone
(1 mmol), Ru-MPTAT-1 (0.05 g), and NaOH (0.4 mmol), iPrOH (8 ml),
temperature (60 °C), time 5 h, N2 atom; for the Suzuki–Miyaura cross-
coupling: iodobenzene (1.0 mmol), phenylboronic acid (1.5 mmol), Ru-
MPTAT-1 (0.05 g), K2CO3 (2.0 mmol), DMF (5 ml), temperature (80 °C).
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