Enantioselective hydrogenation of olefins by chiral iridium phosphorothioite complex covalently anchored on mesoporous silica

Article abstract of DOI:10.1016/j.jcat.2007.12.002

Chiral monodentate phosphorous-based ligands have proven effective for the enantioselective hydrogenation of olefins. Binol-derived monodentate phosphorothioite (PS) ligand was synthesized from binol and thiopropyltriethoxysilane, and its iridium complex was covalently anchored to mesoporous silica supports like SBA-15, MCM-41, and MCM-48. These catalysts were characterized by different physicochemical techniques and assessed for their catalytic performances in the heterogeneous asymmetric hydrogenation of itaconic acid and its derivatives. It was found that the catalytic activities and enantioselectivities of the heterogenized iridium complex (IrPSSBA-15) in the hydrogenation reactions were comparable to its homogeneous analogue. Binol-derived monodentate phosphorothioite ligand in heterogeneously anchored form (iridium complex) is a more effective catalyst than the reported monodentate phosphorous ligand systems in the hydrogenation reactions, possibly due to the changes in electronic properties around the iridium metal center. The effects of substrate-to-catalyst molar ratio, solvents, and temperature on substrate conversions and enantioselectivities of the products were investigated in hydrogenation reactions.

Full text of DOI:10.1016/j.jcat.2007.12.002

Journal of Catalysis 254 (2008) 91–100
www.elsevier.com/locate/jcat
Enantioselective hydrogenation of olefins by chiral iridium phosphorothioite
complex covalently anchored on mesoporous silica
a
b
c
a,∗
Suman Sahoo , Pradeep Kumar , F. Lefebvre , S.B. Halligudi
a
Inorganic Chemistry and Catalysis Division, National Chemical Laboratory, Pune 411008, India
b
Organic Chemistry Technology, National Chemical Laboratory, Pune 411 008, India
c
Laboratoire de Chemie Organometallique de Surface, CNRS-CPE, Villeurbanne Cedex, France
Received 11 October 2007; revised 12 November 2007; accepted 2 December 2007
Available online 4 January 2008
Abstract
Chiral monodentate phosphorous-based ligands have proven effective for the enantioselective hydrogenation of olefins. Binol-derived monoden-
tate phosphorothioite (PS) ligand was synthesized from binol and thiopropyltriethoxysilane, and its iridium complex was covalently anchored to
mesoporous silica supports like SBA-15, MCM-41, and MCM-48. These catalysts were characterized by different physicochemical techniques
and assessed for their catalytic performances in the heterogeneous asymmetric hydrogenation of itaconic acid and its derivatives. It was found that
the catalytic activities and enantioselectivities of the heterogenized iridium complex (IrPSSBA-15) in the hydrogenation reactions were compa-
rable to its homogeneous analogue. Binol-derived monodentate phosphorothioite ligand in heterogeneously anchored form (iridium complex) is
a more effective catalyst than the reported monodentate phosphorous ligand systems in the hydrogenation reactions, possibly due to the changes
in electronic properties around the iridium metal center. The effects of substrate-to-catalyst molar ratio, solvents, and temperature on substrate
conversions and enantioselectivities of the products were investigated in hydrogenation reactions.
©
2007 Elsevier Inc. All rights reserved.
Keywords: Binol; Monodentate ligand; Phosphorothioite ligand; Mesoporous silica SBA-15; Immobilization; Itaconic acid; Iridium complex; Enantioselective
hydrogenation
1
. Introduction
oselective solid-phase catalysis for asymmetric synthesis [6].
The heterogenization of a homogeneous catalyst would provide
many advantages, including easy separation, efficient recycling,
minimization of metal traces in the product, and process con-
trol, which would finally reduce the overall process cost. It has
been reported that heterogeneous catalysts are even more selec-
tive than their homogeneous analogues in some reactions [7,8].
Moreover, the potential of heterogeneous chiral catalysts has
been reported in recent reviews [9–14]. Simons et al. have re-
ported the successful immobilization of rhodium complex of
the monodentate ligand on silica support [15,16] and used in the
asymmetric hydrogenation reactions. The covalent anchoring
of ligands suffers from the lengthy process involved in func-
tionalization of ligand and effective covalent anchoring onto the
support [17,18]. Although inorganic material-immobilized cat-
alysts have some advantages, they have attracted little attention
[19–21] compared with immobilized catalysts prepared from
The pioneering work [1–4] at the start of this century brought
about a renaissance in the use of monodentate phosphorus lig-
ands in the asymmetric hydrogenation reactions. The devel-
opment of binol-derived monodentate phosphorus ligands is
a research topic of increasing interest because of their easy
preparation methods, higher stabilities, and excellent activities
and enantioselectivities in asymmetric catalysis [5]. Because
of the high cost of chiral ligands and noble metals used in
catalyst preparation, catalyst recovery becomes an important
issue for the application of enantioselective catalyst in large-
scale processes. In recent years, enormous progress has been
made in interdisciplinary research on the development of stere-
*
Corresponding author. Fax: +91 20 25902633.
E-mail address: sb.halligudi@ncl.res.in (S.B. Halligudi).
0021-9517/$ – see front matter © 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2007.12.002

92
S. Sahoo et al. / Journal of Catalysis 254 (2008) 91–100
organic polymeric supports. The immobilized chiral transition
metal catalysts prepared from inorganic materials can prevent
intermolecular aggregations of the active species because of
their rigid structures. These catalysts often exhibit superior ther-
mal and mechanical stabilities, and they do not swell or dissolve
in organic solvents.
Recently, an iridium complex of monodentate phospho-
ramidite ligand has been reported for asymmetric hydrogena-
tion reactions in homogeneous conditions [22]. Hydrogenation
products like chiral 2-substituted succinic acids have attracted
much recent interest for their utility as chiral building blocks
[
23,24]. In this paper, we report synthesis of a binol-derived
monodentate triethoxysilyl phosphorothioite PS ligand (here-
after PS) and its iridium complex covalently anchored onto
high-surface area mesoporous silica supports, along with their
applications in the asymmetric hydrogenation of itaconic acid
and its derivatives. The effects of reaction parameters such as
substrate-to-catalyst molar ratio, temperature, hydrogen pres-
sure, catalyst concentration, and solvents on the optimum sub-
strate conversions and product enantioselectivities in the hydro-
genation of olefinic substrates are explored.
Scheme 1. Synthesis of the monodentate triethoxysilylphosphorothioite ligand.
2H), 3.40–3.52 (m, 6H), 7.08 (1H), 7.27–7.40 (m, 7H), 7.84–
7.95(m, 4H) 31P NMR (CDCl3): δ 115. FTIR (cm ): 3059,
−1
2942, 2839, 1460, 740, 809, 459.
2
(
.2.2. Synthesis of homogeneous iridium complex of PS ligand
IrPS)
The homogeneous iridium phosphorothioite complex was
synthesized as reported previously [22]. [Ir(COD)Cl]2 (65 mg,
.096 mmol) was placed in a 10-ml Schlenk flask, and the entire
2
. Experimental
2
.1. Chemicals
0
apparatus was evacuated and backfilled with N2 three times to
establish an inert atmosphere. Dry, degassed dichloromethane
[
Ir(COD)Cl]2, S-binol, 3-thiopropyltriethoxysilane, TEOS,
pluronic-123, dimethylitaconate, itaconic acid, and triethyl-
amine were purchased from Aldrich. Diethylitaconate was
prepared from itaconic acid by esterification with ethanol.
Phosphorous trichloride and cetyltrimethylammonium bromide
(
1 ml) and PS ligand (100 mg, 0.192 mmol) were added, and the
reaction mixture was stirred at room temperature for 10 min.
The solvent was removed under vacuum to give a homoge-
1
1
neous IrPS complex. H NMR (300 MHz, CDCl3): H NMR
CDCl3): δ 0.65 (m, 2H), 1.26–1.35 (t, 12H), 1.63 (m, 2H),
(
CTAB) were procured from Loba Chemie, India and were used
(
as received without further purification. Dichloromethane, ethyl
acetate, toluene, methanol, tetrahydrofuran, and diethyl ether
were purchased from Ranbaxy, India and distilled before their
use following the standard procedure. For the quantitative esti-
mation of enantiomers, the racemic products were obtained by
Pd-charcoal-catalyzed reduction.
1
3
7
.95–1.99 (m, 2H), 2.74–2.83 (m, 4H), 3.14–3.24 (m, 4H),
.41–3.53 (m, 6H), 5.13–5.24 (m, 2H), 5.29–5.40 (m, 2H),
31
.08 (1H), 7.27–7.40 (m, 7H), 7.84–7.95 (m, 4H) P NMR
−1
(CDCl3): δ 83.1. FTIR (cm ): 3059, 2942, 2839, 1460, 740,
8
09, 459.
2
.2.3. Synthesis of siliceous support
The pure siliceous supports, such as MCM-41, MCM-48,
2
2
.2. Catalyst preparation
and SBA-15, were prepared as described previously [25–27].
.2.1. Synthesis of triethoxysilyl phosphorothioite ligand (PS)
Scheme 1 depicts the synthesis of the monodentate PS lig-
2.2.4. Synthesis of ligand functionalized SBA-15
and. S-binol (0.5 g) in 3 ml of phosphorous chloride (PCl3)
was heated under reflux for 8 h. Then excess PCl3 was re-
moved by evaporation under vacuum. The resultant solid was
subjected to azeotropic distillation with toluene and dried un-
der vacuum. The resulting residue was dissolved in toluene
3 g of SBA-15 and 0.5 g of the PS ligand were mixed in
50 ml of chloroform and refluxed for 18 h (Scheme 2). The
resulting material was filtered and washed with chloroform for
◦
several times and dried under vacuum at 50 C to give ligand-
functionalized SBA-15 (PSSBA-15). Ligand functionalization
of MCM-41 and MCM-48 was carried out similarly to obtain
PSMCM-41 and PSMCM-48, respectively.
(
10 ml) and added to a solution of 0.38 g (1.8 mmol) of thio-
propyltriethoxysilane and 0.6 ml triethylamine in 5 ml of dry
◦
tetrahydrofuran at 0 C. The resulting mixture was diluted with
diethyl ether (12 ml), filtered over a plug of silica, and washed
with 50 ml diethyl ether. The solvent was then removed under
vacuum. Column chromatography on silica gave pure PS lig-
2.2.5. Covalent anchoring of iridium complex onto
ligand-modified SBA-15
[Ir(COD)Cl] (0.05 g) dissolved in 50 ml of dichloromethane
2
1
and. [α]D = +57.53 (C = 2.01, CHCl3) H NMR (CDCl3): δ
was slowly added to 3 g of the PSSBA-15, and the mix-
ture was stirred at room temperature overnight. The resulting
0
.64 (m, 2H), 1.25–1.33 (t, 12H), 1.65 (m, 2H), 1.93–1.97 (m,

S. Sahoo et al. / Journal of Catalysis 254 (2008) 91–100
93
substrate and product enantioselectivities were estimated from
gas chromatographic analysis following a standard method. Af-
ter completion of the reaction, the reaction mixture was filtered
to remove the catalyst, and the solvent was removed under
reduced pressure. The resultant mixture was dried and puri-
fied by column chromatography on silica gel as the stationary
phase (petroleum ether/ethyl acetate, 90/10). The optical rota-
tion of products was measured using a Jasco P-1020 polarime-
ter; for example, dimethylmethylsuccinate: colorless liquid,
◦
enantiomeric excess was found to be 94% (GC condition: 90 C
isothermal, 60 min, minor isomer: 36.7, major isomer: 40.0)
1
H NMR (CDCl3, 200 MHz): δ = 3.61 (s,1H), 3.63 (s, 1H),
Scheme 2. Covalent anchoring of the ligand onto the mesoporous silica SBA-15
support and complexation with iridium.
2
.66–2.74 (2H, m), 2.36–2.39 (1H, m), 1.13–1.17 (3H, d),
+
GCMS m/z (relative intensity): 161 (0.05) [M ], 129 (20.30),
1
00 (10.93), 87 (8.68), 69 (10.50), 59 (100), 41 (31.91), ab-
solid was filtered, washed repeatedly with dichloromethane,
and then dried under vacuum at 40 C to get an iridium com-
plex covalently anchored onto SBA-15 (hereafter IrPSSBA-15).
Similarly, IrPSMCM-41 and IrPSMCM-48 were prepared by
following the above procedure.
◦
solute configuration—R, optical rotation, [α]D = +4.5 , C = 3
◦
◦
in chloroform (literature value: [α]D = +4.9 , C = 2.9 in chlo-
roform).
3
. Results and discussion
2
.3. Catalyst characterization
Our main objective in this study was to achieve ready cova-
lent anchoring of monodentate binol-derived ligands to meso-
porous silica support through the phosphorous heteroatom in
minimum number of steps. For the covalent anchoring of the
phosphoramidite ligand through the nitrogen part of the ligand,
the nitrogen precursor must be monoalkylated, because a sec-
ondary amine is needed as reported in literature. The monoalky-
lation of aminopropyltriethoxysilane was difficult, because it
gave both monoalkylated and dialkylated product, making the
procedure tedious. The precursor for covalent anchoring of
the monodentate phosphite ligand is not readily available; we
attempted it using readily available thiopropyltriethoxysilane.
The use of this new ligand in covalent anchoring to the meso-
porous support is reported here for the first time. The mon-
odentate triethoxysilyl phosphorothioite ligand (PS) was easily
synthesized from commercially available starting materials in
Iridium contents in the catalyst materials were estimated
by inductively coupled plasma atomic emission spectrometry
ICP-AES). The elemental analysis (C and N) was carried out
with Carlo Erba Instruments EA1108 elemental analyzer. The
specific surface areas of the catalysts were measured by N2
(
physisorption at liquid nitrogen temperature using a Quan-
tachrome Nova-1200 surface area analyzer. Samples were de-
◦
gassed in N flow for 12 h at 100 C before N physisorption
2
2
measurements. Low-angle powder XRD patterns of the samples
were collected on a Philips X’Pert Pro 3040/60 diffractometer
using CoKα radiation (λ = 0.17890 nm), an iron filter, and an
X’celerator as a detector. Samples were prepared by placing the
droplets of a suspension of solid in isopropanol on a polymer
microgrid supported on a copper grid for TEM measurements.
31
1
31
A
P MAS NMR spectrum of the catalyst was recorded using a
few steps. The H and P NMR of the PS ligand confirmed
the presence of the phosphorous-anchoring moiety in the lig-
and. The optical rotation data confirmed the retention of chi-
rality of the ligand after functionalization with the thiopropy-
ltriethoxysilane group. The homogeneous complex (IrPS) was
Bruker DSX-300 spectrometer at 121.5 MHz with high-power
decoupling using a Bruker 4-mm probe head. The spinning rate
was 10 kHz, and the delay between the two pulses was varied
31
between 1 and 30 s to ensure complete relaxation of the P nu-
clei. The chemical shifts were measured taking 85% H3PO4 as
the reference. A Shimadzu FT-IR-8201PC unit, in DRS mode,
synthesized from the iridium precursor, [Ir(COD)Cl] , with an
2
iridium-to-ligand ratio of 1, as reported in case of the iridium
−
1
with a measurement range of 450–4000 cm , was used to ob-
tain the FT-IR spectra of solid samples.
monodentate complex [22]. Mesoporous silica supports, such
as SBA-15, MCM-41, and MCM-48, were functionalized with
the monodentate triethoxysilyl phosphorothioite ligand, after
which complexation with iridium was carried out to obtain co-
valently anchored catalysts. These catalysts were characterized
for their physicochemical properties by different techniques to
establish their integrity and stability for use as heterogeneous
catalysts in asymmetric hydrogenation reactions. The effica-
cies of these catalysts were assessed in the enantioselective
hydrogenation of itaconic acid and its derivatives under differ-
ent reaction conditions. The reaction conditions were optimized
to get higher substrate conversions and enantioselectivities (ee)
for products.
2
.4. Catalyst testing
In a typical reaction, olefin (10 mmol) and IrPSSBA-15
0.20 g, 0.006 mmol of iridium) in 50 ml of dichloromethane
(
were placed in a 300-ml Parr autoclave, which was purged
with hydrogen five times and then pressurized with 20 bar H2
◦
and stirred at 40 C. Samples were withdrawn at regular in-
tervals and analyzed in a Shimadzu 14B gas chromatograph
equipped with a flame ionization detector using HP-chiral capil-
lary (30 m×0.320 mm×0.25 µm) column. The conversions of

94
S. Sahoo et al. / Journal of Catalysis 254 (2008) 91–100
Table 1
Physicochemical properties of the materials
No.
Catalyst
Surface area BET
Pore volume
(cm g
Average pore
diameter (Å)
2
−1
3
−1
)
(m g
)
1
2
3
4
5
6
7
8
9
MCM-41
MCM-48
SBA-15
PSMCM-41
PSMCM-48
PSSBA-15
IrPSMCM-41
IrPSMCM-48
IrPSSBA-15
1093
983
730
685
556
617
441
430
572
0.88
0.75
1.15
0.57
0.51
0.54
0.28
0.23
0.30
32.6
27
79
24
20
68
19
17
20
onal lattice (Figs. 1a and 1b) and indicate a significant degree
of long-range ordering in the structure. The XRD patterns of
MCM-48 (Fig. 1c) showed an intense peak corresponding to
the (211) reflection along with a shoulder peak at the (220)
reflection, both typical of cubic cells. The phosphorothioite
ligand-modified sample (PSSBA-15) showed decreased inten-
sities of all peaks, with a marginal shift toward lower 2θ values,
indicating silylation inside the mesopores of SBA-15. The peak
intensities at the (100), (110), and (200) reflections of IrPSSBA-
1
5 were further decreased, indicating immobilization of the
iridium complex inside the mesoporous channels of SBA-15.
But the mesoporous structure of the support remained intact
under the conditions used for immobilization. Similar results
were observed for MCM-41- and MCM-48-supported catalysts
(Figs. 1b and 1c). These results indicate an ordered meso-
porosity of the supports even after the incorporation of organic
functional groups and iridium complexes.
3
.1.2. N2 sorption study
The specific surface area, pore volume, and pore diame-
ters estimated from N2 sorption studies of ligand-functionalized
and iridium complex immobilized materials are presented in
Table 1. BET surface areas and BJH pore size distributions
◦
were calculated using N2 adsorption at −196 C. Ligand func-
tionalization and iridium complex immobilization affected the
surface area and pore distribution of the modified samples.
The samples displayed a type IV isotherm (as defined by IU-
PAC) with H1 hysteresis and a sharp increase in pore volume
3
adsorbed above a P/P0 of 0.7 cm /g (Fig. 2), which is a
characteristic of highly ordered mesoporous materials. The tex-
tural properties of SBA-15 were substantially maintained over
ligand functionalization and on subsequent complexation of
Fig. 1. Low angle powder XRD patterns of the materials (a) SBA-15,
PSSBA-15, IrPSSBA-15, (b) MCM-41, PSMCM-41, IrPSMCM-41,
[
Ir(COD)Cl]2. The parent SBA-15 sample exhibited a maxi-
2
(c) MCM-48, PSMCM-48, IrPSMCM-48.
mum pore diameter (79 Å) and surface area (730 m /g), as
shown in Table 1. Ligand functionalization of mesoporous sil-
ica resulted in a shift of the pore maximum to smaller diameters
3
3
.1. Catalyst characterization
2
and a decrease in surface area (617 m /g). The complexation
led to a further decrease in surface area and pore volume. The
same trend was observed for the other two supports.
.1.1. Low-angle XRD
The powder XRD patterns of ligand-functionalized and
complex immobilized supports and corresponding parent sup-
ports are shown in Fig. 1. The parent MCM-41 and SBA-15
supports exhibited three XRD peaks assigned to reflections at
3.1.3. Microscopic analysis
SEM images of IrPSSBA-15 (Fig. 3) show that the rope-like
micromorphology of SBA-15 remained intact even after func-
tionalization with ligand and iridium complex. TEM measure-
(
100), (110), and (200), which are characteristic of 2D hexag-

S. Sahoo et al. / Journal of Catalysis 254 (2008) 91–100
95
ments were carried out to study the morphology of the SBA-15
and IrPSSBA-15 catalysts (Fig. 4). TEM images of these cat-
alysts showed retention of the periodic structure of the parent
SBA-15 precursor, confirming that the hexagonally arranged
mesopores of SBA-15 were retained after modification with lig-
and and iridium complex.
3.1.4. Nuclear magnetic resonance
MAS NMR is a good technique for investigating ligand
functionalization and complex anchored onto the mesoporous
support. Because the PSSBA-15 and IrPSSBA-15 samples con-
tained small amounts of phosphorus, we used cross-polarization
1
31
31
( H– P) MAS NMR to enhance the sensitivity of the
P
1
31
signal. The H– P coupled CP-MAS NMR spectra of the lig-
and functionalized SBA-15 (PSSBA-15) and iridium complex-
anchored SBA-15 (IrPSSBA-15) are depicted in Figs. 5a
and 5b, respectively. The 31P peak corresponding to PSSBA-
1
5 exhibits a marginal shift at δ 113.6 ppm, compared with that
of neat ligand at δ 115 ppm, possibly due to the different ligand
environments. IrPSSBA-15 exhibits a 31P peak at δ 81.6 ppm,
clearly indicating that the iridium complex has covalently an-
chored to the modified SBA-15 support, and the shift clearly
confirms the presence of the iridium complex in a new environ-
ment [22].
3.1.5. FTIR
The FTIR spectra of PSSBA-15 and IrPSSBA-15 are given
in Figs. 6a and 6b. SBA-15 showed characteristic FT-IR
−
1
peaks at 2900–3800, 1080, and 450 cm due to O–H of the
silanols, adsorbed water molecules, and Si–O–Si stretching vi-
brations, respectively. PSSBA-15 showed an additional peak at
1460 cm due to the C=C stretching of the aromatic ring and
Fig. 2. Nitrogen adsorption desorption isotherms of SBA-15, PSSBA-15,
IrPSSBA-15.
−
1
Fig. 3. SEM photographs of (a) SBA-15 and (b) IrPSSBA-15.
Fig. 4. TEM photographs of (a) SBA-15 and (b) IrPSSBA-15.

96
S. Sahoo et al. / Journal of Catalysis 254 (2008) 91–100
Fig. 6. FTIR spectra of PSSBA-15 and IrPSSBA-15, (a) wave number range
− −1
1
(
500–900 cm ), (b) wave number range (500–4000 cm ).
31
Fig. 5. P MAS NMR spectra of (a) PSSBA-15 and (b) IrPSSBA-15.
−
975 and 2845 cm 1 due to C–H and C–C stretching modes of
gas–solid–liquid system compared with a homogeneous liquid
system [29]. The results presented in Table 2 clearly demon-
strate that dimethylitaconate was hydrogenated with very high
conversion (up to 99%) and high product enantioselectivity
2
the propyl spacer, respectively. The bands observed in the range
30–800 cm are due to the O–P–O stretching (Fig. 6a) and
−1
7
−
1
P–S stretching band (460 cm ) [28] merging with Si–O–Si
stretching vibrations. Similar results were observed for the
MCM-41 and MCM-48 supports. FT-IR results support the suc-
cessful incorporation of the metal complex onto the surface of
mesoporous silica.
(
up to 94% ee), which are comparable with the homogeneous
analogue (entry 2, Table 2). It has been reported that higher
catalyst loadings (substrate: catalyst molar ratio = 50:1) are
usually required to get higher enantioselectivity [30–32]. But,
surprisingly, we have found excellent activity and enantioselec-
tivity (ee) even with a substrate-to-catalyst molar ratio of 1660,
which is nearly 30-fold higher than the reported iridium phos-
phoramidite complex [22] and the other rhodium monodentate
phosphorous complexes. This can be attributed to an increase in
electron density around the metal center [33,34]. Phosphorus-
sulfur π-bond formation is less favored geometrically than
phosphorus-oxygen π-bond formation [35]; thus, an increase
in electron density may occur around the iridium metal cen-
ter compared with the reported phosphite and phosphoramidite
ligands, which favor the oxidative addition of hydrogen in the
catalytic hydrogenation reaction cycle [36]. Thus, our catalyst
reported here provides high enantioselectivity in asymmetric
hydrogenation reactions under milder reaction conditions.
Increasing the substrate-to-catalyst molar ratio from 1660
to 3000 caused no significant decrease in the conversion of
dimethylitaconate, but did produce a decrease in the enantiose-
3
.2. Catalytic results
The efficiency of the three immobilized catalysts prepared
was assessed in the enantioselective hydrogenation of itaconic
acid and its derivatives. These results are summarized in Ta-
ble 2. Dimethylitaconate was chosen as the test substrate, and
the heterogeneous enantioselective liquid-phase hydrogenation
was carried out with a substrate to catalyst molar ratio of 1660:1
in dichloromethane at 40 C and 20 bar H . A homogeneous
liquid phase reaction was also performed with neat iridium
complex (IrPS) under similar conditions for comparative pur-
pose. It was found that the activity (in terms of turnover num-
ber [TON], defined as mole of substrate converted per mole of
iridium) of neat complex was twice as high as in the hetero-
geneous system. This could be due to a slower interaction be-
tween the substrate and the catalyst in heterogeneous triphasic
◦
2

S. Sahoo et al. / Journal of Catalysis 254 (2008) 91–100
97
Table 2
Table 3
a
Asymmetric hydrogenation of Itaconic acid and its derivatives
Effect of solvent in the enantioselective hydrogenation of dimethylitaconate
with IrPSSBA-15
a
b
c
Entry
Solvent
Conversion
mol%)
TON
ee (%)
(
1
2
3
4
5
6
Dichloromethane
Chloroform
Acetone
Ethyl acetate
Methanol
99
99
95
99
94
25
1650
1650
1583
1650
1533
416
94
94
93
94
89
–
b
c
Entry
1
R
R
Conversion
mol%)
TON
ee
(
(%)
Me
Me
H
Me
Me
H
99
99
98
90
85
82
98
97
1650
1650
1633
1500
1416
1366
3005
1981
94
96
93
91
94
94
57
73
d
2
3
Toluene
e
a
4
Et
Et
Reaction conditions: 10 mmol of the substrate, substrate to catalyst molar
f
◦
5
Me
Me
Me
Me
Me
Me
Me
Me
ratio = 1660:1, solvent 50 ml, temperature 40 C, 20 bar H2, time 20 h.
g
b
6
7
TON (turn over number) mole substrate converted per mole of Ir.
% ee was calculated by GC analysis using HP-Chiral column.
h
c
i
8
a
Reaction conditions: 10 mmol of the substrate, substrate to catalyst molar
3.2.1. Effect of solvent
◦
ratio = 1660:1, dichloromethane 50 ml, temperature 40 C, 20 bar H , time
2
To investigate the effect of solvent on the substrate conver-
sion and enantioselectivity, we carried out the hydrogenation of
dimethylitaconate with the IrPSSBA-15 catalyst using different
solvents. The results, along with the reaction conditions, are
presented in Table 3. Dichloromethane, chloroform, and ethyl
acetate were found to be the most effective solvent systems. The
enantioselectivities varied within 1 to 2% and were excellent
(up to 94%) in all the solvents studied. In protic polar solvent
(methanol), a slight decrease in the product enantioselectivity
was obtained, which is in consistent with previous reports [39].
The lack of activity in toluene might be due to the tendency of
2
0 h;
b
TON (turn over number) mole substrate converted per mole of Ir.
c
%
ee was calculated by GC analysis using HP-Chiral column.
d
The reaction was carried out under homogeneous condition, which was
completed within 12 h.
e
The product was analyzed by converting the acid to the corresponding
methyl ester.
f
Using IrPSMCM-41 as catalyst.
Using IrPSMCM-48 as catalyst.
The substrate to catalyst molar ratio taken was 3000:1.
The substrate to catalyst molar ratio taken was 2000:1.
g
h
i
6
iridium to form stable η -arene complexes with aromatic com-
lectivity of dimethylmethylsuccinate (Table 2, entries 7 and 8).
Increasing the iridium-to-ligand molar ratio from 1:1 to 1:2
produced no significant changes in catalyst activity and selec-
tivity in the hydrogenation of dimethylitaconate. Consequently,
iridium complex with a single monodentate phosphorothioite
ligand covalently anchored to mesoporous silica supports are
the most active catalysts for enantioselective hydrogenation
reactions—a significant and important finding of our studies.
All three siliceous mesoporous supports resulted in the same
product enantioselectivity in the hydrogenation of dimethyli-
taconate, but with different TONs. IrPSSBA-15 proved to a
more efficient catalyst (99% conversion and 1650 TON) com-
pared with IrPSMCM-41 (conversion, 85%; TON, 1416) and
IrPSMCM-48 (conversion, 82%; TON, 1366) (Table 2, entries
pounds, as has been reported in rhodium complexes [40,41].
3
.2.2. Effect of reaction time
The effect of reaction time over substrate conversion and
enantioselectivity in the hydrogenation of dimethylitaconate is
illustrated in Fig. 7. Fig. 7a shows that substrate conversion in-
creased as a function of time for all of the catalysts, reaching
optimum conversion in 20 h for IrPSSBA-15 (>99%), followed
by the others. However, the ee values reached maximum from
the very beginning of the reactions shown in Fig. 7b. No sig-
nificant effect of reaction time on the product ee was seen;
a maximum ee around 90% was obtained after 1 h of reaction
with all three catalysts.
1
, 5, and 6) in the foregoing reaction. IrPSSBA-15 was an ef-
3
.2.3. Effect of temperature
ficient catalyst because the SBA-15 support afforded less dif-
fusional resistance for the substrate molecules to interact with
active sites of the complex in its mesoporous channels, having a
larger pore diameter compared with the MCM-41 and MCM-48
supports [37,38].
Enantioselective hydrogenation of itaconic acid and diethyl-
itaconate was carried out under similar conditions. Diethyl-
itaconate was hydrogenated to the corresponding succinic acid
derivative with 90% conversion and 91% ee (Table 2, entry 4),
and, similarly, itaconic acid also was hydrogenated to the cor-
responding chiral product with higher conversion (98%) and
optimum ee (93%) (Table 2, entry 3), indicating no significant
influence of substitution over substrate molecules.
We used the various catalysts to study the effect of temper-
◦
ature (range, 30–50 C) in the hydrogenation of dimethylita-
◦
conate. The results indicate that a critical temperature of 40 C
is required to acquire the activation energy for hydrogenation
for the three immobilized catalyst systems. At this temperature,
the maximum conversion of dimethylitaconate (up to 94%) was
obtained by all of the catalyst systems under the reaction con-
◦
ditions studied. Raising the temperature above 40 C produced
no further change in substrate conversion for all the catalysts,
as shown in Fig. 8a. Temperature did not affect the product ee;
however, a slight decrease in enantioselectivity was observed
◦
with a temperature increase from 40 to 50 C (Fig. 8b) under
the reaction conditions specified here.

98
S. Sahoo et al. / Journal of Catalysis 254 (2008) 91–100
Fig. 7. Effect of reaction time (a) conversion of dimethylitaconate and (b) enan-
tiomeric excess of dimethylmethylsuccinate. Reaction conditions: 10 mmol
Fig. 8. Effect of reaction temperature (a) conversion of dimethylitaconate
and (b) enantiomeric excess of dimethylmethylsuccinate. Reaction condi-
tions: 10 mmol of the substrate, substrate to catalyst molar ratio = 1660:1,
of the substrate, substrate to catalyst molar ratio = 1660:1, dichloromethane
◦
5
0 ml, temperature 40 C, 20 bar H .
2
dichloromethane 50 ml, 20 bar H , time 20 h.
2
3
.2.4. Effect of hydrogen pressure
The hydrogen pressure inside the reaction vessel had a pro-
active species of the catalyst into the reaction mixture; the cat-
alyst truly acted as if it were heterogeneous.
nounced effect on substrate conversion in the hydrogenation of
dimethylitaconate, as shown in Fig. 9a. But the enantioselectiv-
ity of the heterogeneous catalyst did not significantly depend on
a hydrogen pressure >10 bar, as shown in Fig. 9b. As shown in
Fig. 9a, the conversion of dimethylitaconate increased markedly
with increasing hydrogen pressure. The maximum substrate
conversion and ee were achieved at 20 bar hydrogen pressure,
beyond which no further enhancement in the conversion or ee
was achieved.
We evaluated the recyclability of the IrPSSBA-15 catalyst in
the hydrogenation of dimethylitaconate in five runs; the results
are presented in Table 4. After each run, the catalyst (orange-
colored) was washed repeatedly with dichloromethane, dried
◦
under vacuum at 70 C for 2 h, and then used in the hydrogena-
tion reaction with a fresh reaction mixture. Dimethylitaconate
conversion was practically the same (99%) in all five cycles,
with a marginal decrease at the fourth and fifth cycles with no
change in the enantioselectivity of the product (94%). Thus, by
recycling the immobilized catalyst five times, the substrate-to-
catalyst ratio (s/c) of the whole reaction was increased to 8300,
with the activity remaining the same with an enantioselectiv-
ity >94%. In comparison, a homogeneous reaction at s/c 8300
gave lower enantioselectivity (49%). This finding demonstrates
the successful immobilization of the iridium complex onto the
support.
3
.2.5. Catalyst stability
To investigate whether any active species of the catalyst were
leaching into the reaction medium, we carried out hydrogena-
tion of dimethylitaconate to give dimethylmethylsuccinate un-
der selected conditions with the IrPSSBA-15 catalyst. The reac-
tion was stopped after 2 h, after which the autoclave was cooled
to room temperature and conversion of dimethylitaconate was
estimated. Then the catalyst was separated by filtration, the fil-
trate was added to the vessel, and the reaction was continued
with fresh hydrogen pressure (20 bar) for another 20 h. No in-
crease in substrate conversion was observed; it remained the
same as was estimated in the presence of the catalyst. This
observation confirms the complete absence of leaching of any
4. Conclusion
We have demonstrated a new, effective class of heteroge-
neous catalyst through synthesis of monodentate phosphoroth-
ioite iridium complex covalently anchored to mesoporous silica

S. Sahoo et al. / Journal of Catalysis 254 (2008) 91–100
99
with a substrate-to-catalyst ratio 30 times higher than that used
in the previously reported monodentate ligand-based catalytic
systems. The catalyst was reused at least five times, and main-
tained the same activity and selectivity. Thus, we consider this
protocol to be a readily accessible pathway to highly enantios-
elective immobilized hydrogenation catalysts.
Acknowledgment
S.S. thanks CSIR, New Delhi, for the award of a research
fellowship.
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