Synthesis of chiral sulfoxides by enantioselective sulfide oxidation and subsequent oxidative kinetic resolution using immobilized Ti-binol complex

Article abstract of DOI:10.1016/j.jcat.2008.12.007

Chiral Ti-binol complex was immobilized onto ionic liquid modified SBA-15 and characterized by different physicochemical techniques. The catalyst was found to be highly enantioselective in the heterogeneous asymmetric oxidation of prochiral sulfides to sulfoxides and subsequent oxidative kinetic resolution of the sulfoxides using aqueous tert-butylhydroperoxide as the oxidant. A positive non-linear effect was observed in the oxidation-kinetic resolution of thioanisole using this supported catalyst. The supported catalyst was reused in multiple catalytic runs without any loss of enantioselectivity.

Full text of DOI:10.1016/j.jcat.2008.12.007

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Journal of Catalysis 262 (2009) 111–118
Contents lists available at ScienceDirect
Journal of Catalysis
www.elsevier.com/locate/jcat
Synthesis of chiral sulfoxides by enantioselective sulfide oxidation and subsequent
oxidative kinetic resolution using immobilized Ti–binol complex
Suman Sahoo a, Pradeep Kumar b, F. Lefebvre c, S.B. Halligudi a,d,∗
a
Inorganic Chemistry and Catalysis Division, National Chemical Laboratory, Pune 411008, India
Organic Chemistry Technology, National Chemical Laboratory, Pune 411 008, India
Laboratoire de Chemie Organometallique de Surface, CNRS-CPE, Villeurbanne Cedex, France
Catalysis Centre for Molecular Engineering Korea Research Institute of Chemical Technology (KRICT), Jang-dong 100, Yuseong-gu, 305-600 Daejeon, Korea
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Chiral Ti–binol complex was immobilized onto ionic liquid modified SBA-15 and characterized by
different physicochemical techniques. The catalyst was found to be highly enantioselective in the
heterogeneous asymmetric oxidation of prochiral sulfides to sulfoxides and subsequent oxidative kinetic
resolution of the sulfoxides using aqueous tert-butylhydroperoxide as the oxidant. A positive non-linear
effect was observed in the oxidation-kinetic resolution of thioanisole using this supported catalyst. The
supported catalyst was reused in multiple catalytic runs without any loss of enantioselectivity.
© 2008 Elsevier Inc. All rights reserved.
Received 24 September 2008
Revised 8 December 2008
Accepted 11 December 2008
Available online 24 December 2008
Keywords:
Sulfides
Ti–binol
Mesoporous silica
Supported ionic liquid
Non-linear effect
Kinetic resolution
Sulfoxides
1. Introduction
sulfoxidation, Brunel et al. and Furia et al. reported independently
a modified Sharpless procedure for the asymmetric oxidation of
sulfides in the presence of stoichiometric diethyl tartrate (DET)
and titanium attaining very high ee values [12–14]. Although the
enantioselective oxidation of sulfides catalyzed by chiral complexes
of transition metals, such as titanium [15–17], vanadium [18,19],
iron [20], manganese [21], Osmium [22] has been extensively stud-
ied, Kagan method and its improvements are mainly used in the
large scale preparations of sulfoxides.
Many chiral sulfoxides exhibit interesting biological activities
and show promise as therapeutic agents [1–4]. For example,
omeprazole and some of its derivatives are used as proton-pump
inhibitors to treat acid-related diseases [2]. OPC-29030 is used as a
platelet adhesion inhibitor [3]. In addition, optically active sulfox-
ides are valuable chiral auxiliaries and intermediates in contempo-
rary organic synthesis [4–6]. So, the development of methodologies
to synthesize enantiopure sulfoxides has become an important
pursuit in chemical research.
One of the major approaches towards enantiomeric pure sul-
foxides is the stereospecific substitution of optically active sulfi-
nates with Grignard reagents (Andersen’s method) [7]. However,
it suffers from important drawbacks such as the tedious proce-
dure and, the limited substrate scope [8]. Other attempts involve
stoichiometric chiral oxidants, such as oxaziridines [9], hydroperox-
ides [10] but this approach is less attractive due to high cost and
restricted availability. The only practical synthetic route towards
chiral sulfoxides that has already found acceptance by industry
during the last two decades is the catalytic enantioselective oxida-
tion of prochiral sulfides [11]. Based on transition metal-promoted
Kinetic resolution of sulfoxides is the reaction of two enan-
tiomeric sulfoxides at different rates [23]. There are numerous
examples of the oxidative kinetic resolution of sulfoxides to sul-
fones, most of which were discovered during the investigation of
asymmetric sulfide oxidation [23–25]. Komatsu and co-workers re-
ported kinetic resolution when carrying out sulfur oxidations us-
ing a titanium–binaphthol catalyst with arylmethylsulfoxides of up
to 99% ee being obtained in 26% yield [24]. Recently Jia and co-
workers optimized this and reported that carrying out the kinetic
◦
resolution step at 25 C gave the best results, obtaining phenyl-
methylsulfoxide in 29% yield and 95% ee [25]. Synthesis of chiral
sulfoxides and their applications has been reviewed several times
in the literature [26–30].
Many times the successes in organic transformations by soluble
asymmetric catalysts affording high purity optical compounds are
not reflected in the industrial front due to severe limitations in-
cluding lower turn over numbers, difficulties in recovery, recycling
Corresponding author. Fax: +82 42 860 7676.
E-mail address: sb_halligudi123@yahoo.co.in (S.B. Halligudi).
*
0021-9517/$ – see front matter © 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2008.12.007

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S. Sahoo et al. / Journal of Catalysis 262 (2009) 111–118
and handling of the soluble catalysts. Therefore, the asymmetric
heterogeneous catalysts are preferable because of their ease of
handling and separation properties from a technical and commer-
cial point of view, provided the catalyst performance is satisfactory.
Mesoporous silica had attracted much attention in many fields of
science and engineering such as adsorption, separation, and catal-
ysis due to their unique pore structures [31,32]. In particular, their
remarkable textural properties such as high surface area and large
pore volume, good hydrothermal stability with varying pore size
make them well suitable for application as catalyst supports es-
pecially in asymmetric catalysis [33–37]. A number of research
papers have been published in the field of heterogeneous sulfide
oxidation [38–42]. But a very few reports are there on the immobi-
lization of chiral titanium complex on to mesoporous silica [43,44].
All these heterogeneous catalysts (Ti-MCM-41) provided very low
enantiomeric excess (13%) of the sulfoxides.
Recently we have reported the successful immobilization of chi-
ral Mn(III) salen complex by supported ionic liquid phase (SILP)
strategy [45]. This involves immobilization of a thin film of ionic
liquid containing organometallic complex onto a high surface area
support. This allows fixing molecular catalysts in a widely tai-
lorable environment without the drawbacks of complex grafting
chemistry [46–49]. This supported ionic liquid phase (SILP) catalyst
allows the application of fixed-bed reactors and makes the sepa-
ration and catalyst recycling easy. In this paper, we have further
utilized this SILP strategy for the immobilization of chiral Ti–binol
complex onto the ionic liquid modified mesoporous silica (SBA-15)
support. This catalyst system was applied in the sulfide oxida-
tion and subsequent kinetic resolution of sulfoxides and found to
be comparable to the homogeneous system in activity and enan-
tioselectivity. Also the non-linear effect was addressed using this
catalyst system.
Ti contents in the resulting solids were estimated by ICP-AES.
The elemental analysis (C and N) was carried out with a Carlo
Erba Instruments EA1108 elemental analyzer. Nitrogen adsorption
◦
and desorption isotherms were measured at −196 C on a Quan-
tachrome Autosorb 1 sorption analyzer. The samples were out
◦
gassed for 3 h at 250 C under vacuum in the degas port of the
adsorption analyzer. The specific surface area was calculated us-
ing the BET model. The pore size distributions were obtained from
the adsorption branch of the nitrogen isotherms by Barrett–Joyner–
Halenda method. The XRD patterns of the samples were collected
on a Philips X’Pert Pro 3040/60 diffractometer using CuKα ra-
diation (λ = 1.5418 Å), iron as filter and X’celerator as detector.
The 31P MAS NMR spectrum of the catalyst was recorded using a
Bruker DSX-300 spectrometer at 121.5 MHz with high-power de-
coupling using a Bruker 4-mm probe head. The spinning rate was
10 kHz, and the delay between the two pulses was varied between
1 and 30 s to ensure complete relaxation of the 31P nuclei. The
chemical shifts were given relative to external 85% H3PO4. NMR
spectra were recorded on DRX-300 (200 MHz for 1H and 50.3 MHz
for 13C) spectrophotometer using CDCl3 as a solvent and Me4Si as
the internal standard. Shimadzu FT-IR-8201PC unit, in DRS mode,
with a measurement range of 450–4000 cm−1, was used to obtain
the FT-IR spectra of solid samples.
2.2. Catalyst preparation
2.2.1. 1-(3-Trimethoxysilylpropyl)-3-methylimidazoliumchloride (1)
A mixture of 1-methylimidazole (3.36 g, 40 mmol) 3-chloropro-
pyltrimethoxysilane (9.63 g, 40 mmol) was heated under argon
◦
atmosphere at 95 C for 24 h (Scheme 1). After cooling viscous oil
was obtained. 1H NMR (CDCl3): δ = 0.71–0.78 (m, 2H), 1.25–1.33
(t, 9H), 1.85–1.91 (m, 2H), 3.40–3.52 (m, 6H), 4.11 (s, 3H), 8.26 (d,
1H), 8.34 (d, 1H), 8.53 (s, 1H); 13C NMR (CDCl3): δ = 7.0 (SiCH2),
11.2 (CH3), 25.1 (CH2), 36.4 (NCH3), 51.4 (CH3O), 52.2 (CH2N),
122.8, 124.3, 136.1.
2. Experimental
2.1. General
2.2.2. 1-(3-Trimethoxysilylpropyl)-3-methylimidazoliumhexafluoro-
phosphate (2)
All the solvents procured from Merck (AR grade), India were
distilled and dried prior to their use. Tetraethyl orthosilicate
(TEOS), all the sulfides, Ti(OiPr)4, (S)-binol, racemic binol, and
tert-butylhydroperoxide were procured from sigma Aldrich. All the
racemic sulfoxides were synthesized from sulfides by oxidation
with tert-butylhydrogenperoxide using Ti(OiPr)4. The ionic liquid
[BMIM]PF6 and the mesoporous silica were synthesized as de-
scribed in the literature [50,51].
To a solution of the above salt (3.2 g, 10 mmol), in ace-
tone (15 mL), potassium hexafluorophosphate (1.9, 10.5 mmol) was
added in one portion. The mixture was stirred at room tempera-
ture for 3 days. After this time the mixture was filtered and the
solvent evaporated under reduced pressure to give 2 (Scheme 1).
The residue was taken up with chloroform.
Scheme 1. Synthesis of ionic liquid modified SBA-15.

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S. Sahoo et al. / Journal of Catalysis 262 (2009) 111–118
113
◦
20 C; hexane/iPrOH, 9:1; flow rate, 0.5 mL/min (230 psi)) tR1
=
22.09 min (minor isomer), tR2 = 24.6 min (major isomer), >99.9%
ee; 1H NMR (300 MHz, CDCl3): d = 2.42 (s, 3H), 2.71 (s, 3H), 7.32–
7.35 (d, 2H, J = 7.8 Hz), 7.53–7.56 ppm (d, J = 7.8 Hz, 2H); EIMS:
+
Scheme 2. Immobilizaton of Ti-(S)-binol onto ionic liquid modified SBA-15.
m/z (%): 154 (83) [M] , 139 (100).
(S)-p-bromophenylmethylsulfoxide: [α]D = −150.1 (c = 0.43,
1H NMR (CDCl3), δ = 0.61–0.70 (m, 2H), 1.27–1.35 (t, 9H), 1.88–
1.94 (m, 2H), 3.89 (s, 3H), 4.23 (t, 2H), 7.55 (d, 1H), 7.58 (d, 1H),
8.90 (s, 1H), 13C NMR (CDCl3), δ = 6.9 (SiCH2), 10.2 (CH3), 24.9
(CH2), 36.0 (NCH3), 51.3 (CH2O), 51.5 (CH2N), 122.4, 124.0, 136.3.
31P NMR (CDCl3), δ = 143.6.
acetone), HPLC (Chiralcel OJ-H column: UV detection at λ =
◦
254 nm; 20 C; hexane/iPrOH, 9.5:0.5; flow rate, 0.5 mL/min
(250 psi)) tR1 = 48.2 min (major isomer), tR2 = 46.1 min (minor
isomer), >99.9% ee; 1H NMR (300 MHz, CDCl3): δ = 2.74 (s, 3H),
7.53–7.55 (d, J = 8.4 Hz, 2H), 7.67–7.70 ppm (d, J = 8.4 Hz, 2H);
+
+
EIMS: m/z (%): 220 (66) [M + 1] , 218 (65) [M − 1] , 205 (100),
2.2.3. Synthesis of ionic liquid modified SBA-15 (ILSBA-15)
203 (98).
The ionic liquid 2 (2.81 g, 6.5 mmol) was dissolved in chlo-
roform (50 mL) and treated with mesoporous silica (dried under
(S)-p-fluorophenylmethylsulfoxide: [α]D = −128.6 (c = 1.5,
acetone), 98.6% ee; 1H NMR (300 MHz, CDCl3): δ = 2.72 (s, 3H),
7.20–7.26 (m, 2H), 7.63–7.68 ppm (m, 2H); EIMS: m/z (%): 158
◦
vacuum and heated at 180 C overnight, 4.00 g). The mixture was
◦
+
heated under reflux (65 C) for 26 h. After cooling to room temper-
(60) [M] , 143 (100), 115 (1), 95 (36), 75 (39).
ature, the solid was isolated by filtration and washed with chloro-
form (50 mL) and diethyl ether (50 mL). The solid was dried under
reduced pressure to give a powder.
(S)-m-bromophenylmethylsulfoxide: [α]D = −110.4 (c = 1.33,
acetone), HPLC (Chiralcel OD-H; flow rate 0.8 mL/min; hexane/i-
PrOH, 8/2): tR1 = 14.3 min (minor isomer), tR2 = 11.0 min (Major
isomer) >99.9% ee; 1H NMR (300 MHz, CDCl3): δ = 2.74 (s, 3H),
7.40–7.43 (m, 1H), 7.53–7.54 (m, 1H), 7.62 (m, 1H), 7.80–7.81 ppm
2.2.4. Synthesis of homogeneous Ti–binol complex
+
+
In a round bottom flask (S)-binol (0.2 g, 2 mmol) in 20 mL of
CCl4 was taken. To this ligand solution, Ti(OiPr)4 (0.1 g, 1 mmol)
was added drop wise and stirred for 5 min. Then water (0.18 g,
10 mmol) was added and stirred for another 1 h. Then the sol-
vent was evaporated under reduced pressure to provide an orange
colored Ti–binol complex (Scheme 2). This complex was charac-
(m, 1H); EIMS: m/z (%): 220 (81) [M + 1] , 218 (81) [M − 1] , 205
(93), 203 (96).
(S)-p-nitrophenylmethylsulfoxide: [α]D = −126.5 (c = 1.2, ace-
tone), 89.1% ee; 1H NMR (300 MHz, CDCl3): d = 2.79 (s, 3H), 7.82–
7.85 (m, 2H), 8.38–8.40 ppm (m, 2H); EIMS: m/z (%): 185 (100)
+
[M] , 170 (29), 140 (11).
terized by FTIR spectroscopy. FTIR (KBr pellet): 3522, 3056, 1587,
(S)-phenylethylsulfoxide: [α]D = −97.1 (c = 1.3, acetone), HPLC
−1
1463, 1338, 1240, 1079, 975, 819, 791 cm
.
(Chiralcel OD-H; flow rate 0.8 mL/min, hexane/i-PrOH, 8/2): tR1
=
17.8 min (Minor isomer), tR2 = 9.5 min (major isomer), 75.5% ee;
1H NMR (300 MHz, CDCl3): d = 1.19–1.24 (t, J = 7.5 Hz, 3H), 2.75–
2.96 (m, 2H), 7.51–7.65 ppm (m, 5H); EIMS: m/z (%): 154 (20)
2.2.5. Supported chiral Ti–binol materials (TiILSBA-15)
To a stirred solution of (S)-binol (0.2 g, 2 mmol) in 10 mL of
CCl4, Ti(OiPr)4 (0.1 g, 1 mmol) and water (0.18 g, 10 mmol) were
added one after the other drop wise. After that [BMIM] PF6 (0.5 g)
in 10 mL of CH2Cl2 was added (Scheme 2). To this solution the
ionic liquid modified mesoporous silica (1.5 g) was added. The
mixture was stirred for 1 h, and then evaporated under reduced
pressure for 3 h to give a powder of TiILSBA-15.
+
[M] , 126 (54), 97 (15), 78 (100), 51 (32).
(S)-4-methoxyphenylmethylsulfoxide: [α]D = −76 (c = 1.1, ace-
tone), HPLC (Chiralcel OJ-H; flow rate 0.5 mL/min; hexane/i-PrOH,
9/1): tr (R) = 38.8 min, tr (S) = 41.08 min, 1H NMR (400 MHz):
d = 7.61–7.58 (d, J = 8.9 Hz, 2H), 7.04–7.02 (d, J = 8.9 Hz, 2H),
3.86 (s, 3H), 2.71 (s, 3H);
(S)-4-chlorophenylmethylsulfoxide (6d): [α]D = −120 (c = 1.6,
acetone), HPLC (Chiralcel OB; flow rate 0.8 mL/min; hexane/i-
PrOH, 8/2): tR1 = 15.2 min (Minor isomer), tR2 = 10.5 min (Major
isomer) 1H NMR (400 MHz): d = 7.59 (d, J = 8.6 Hz, 2H), 7.51 (d,
J = 8.6 Hz, 2H), 2.73 (s, 3H).
2.3. Catalytic experiments
2.3.1. Typical experimental procedure for heterogeneous asymmetric
sulfoxidation and catalyst recycling
Solvent (10 mL) and thioanisole (0.5 mmol) were added to the
solid-state catalyst TiILSBA-15 (0.4 g) obtained above. The mix-
ture was stirred for 15 min before TBHP (70% in water, 1 g,
7.5 mmol) was added dropwise at room temperature, and the het-
erogeneous mixture was stirred at room temperature for 20 h.
After the isolation of the solids by filtration, the insoluble cata-
lyst was recharged with CCl4 (10 mL), substrates (0.5 mmol), and
oxidant (0.75 mmol) for the next run. The filtrate was concen-
trated and the residue was submitted to column chromatography
on silica gel using pet ether/ethyl acetate (1:1) as eluent to give
(S)-methylphenylsulfoxide as colorless oil in 38% yield; The optical
rotation of products was measured by Jasco P-1020 polarimeter.
[α]D = −134.5 (c = 1.1, acetone); [α]D = +135 (c = 1, acetone),
(R)-enantiomer, 99.2% ee; 1H NMR (300 MHz, CDCl3): δ = 2.74 (s,
3 H), 7.52–7.55 (m, 3H), 7.64–7.68 ppm (m, 2H); EIMS: m/z (%):
3. Results and discussion
The Ti–binol complex immobilized onto the ionic liquid modi-
fied SBA-15 support was characterized by different physicochemi-
cal techniques for the confirmation of its presence and the struc-
tural integrity of the mesoporous silica support after immobiliza-
tion. The catalyst was assessed in the asymmetric oxidation of
sulfides.
3.1. Catalyst characterization
3.1.1. Low angle XRD
The well-defined XRD patterns were obtained for all the sam-
ples which are similar to those recorded for SBA-15 materials as
described by Zhao et al. [47]. XRD patterns of calcined SBA-15
and TiILSBA-15 materials (shown in Figs. 1(a) and 1(b) respec-
tively) consists of three well-resolved peaks in the 2θ range of 0.8
to 1.8 correspond to the (100), (110), (200) reflections which are
associated with p6mm hexagonal symmetry in the materials. The
peak intensities at (100), (110) and (200) reflections of TiILSBA-15
were decreased, indicating the immobilization of Ti–binol complex
+
140 (100) [M] , 125 (98), 97 (58), 77 (45), 51 (63). The ee value
was determined by performing HPLC (CLASS-VP) using a Chiralcel
◦
OD-H column: UV detection at λ = 254 nm; 20 C; hexane/iPrOH,
9:1; flow rate, 0.5 mL/min (250 psi); tR1 = 24.5 min (minor iso-
mer), tR2 = 28.3 min (major isomer).
(S)-p-toluylmethylsulfoxide: [α]D = −184.8 (c = 1.08, acetone),
HPLC (Chiralcel OD-H column: UV detection at λ = 225 nm;

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S. Sahoo et al. / Journal of Catalysis 262 (2009) 111–118
mesoporous solids. As the relative pressure increases (p/p0 > 0.6),
all isotherms exhibited a sharp step characteristic of capillary con-
densation of nitrogen within uniform mesopores, where the p/p0
position of the inflection point is correlated to the diameter of
the mesopore. This data confirmed the presence of chiral Ti–binol
complex in the mesoporous SBA-15 support, and the intactness of
the textural property of the support after functionalization.
3.1.3. Microscopic analysis
As can be seen from the SEM images of TiILSBA-15 shown in
Fig. 3(a), the micromorphology i.e., rope like structure of meso-
porous SBA-15 remained the same even after modification with
ionic liquid and Ti–binol complex.
Fig. 3(b) depicts TEM images obtained for the sample TiILSBA-
15 catalyst as definitive evidence of the retention of structure or-
dering of SBA-15 after immobilization. The presence of equidistant
parallel fringes demonstrates the nature of separation between lay-
ers and the unique well-packed arrangement of such monolayers.
The well ordered hexagonal array of mesopores was also observed.
Fig. 1. Low angle XRD patterns of (a) SBA-15 and (b) TiILSBA-15.
inside the mesoporous channels of ionic liquid modified SBA-15.
However, the mesoporous structure of the support remained intact
under the conditions used for immobilization.
3.1.4. Nuclear magnetic resonance
Regarding the successful immobilization of ionic liquid and the
chiral metal complex, the NMR spectra were obtained for the accu-
rate characterization. When looking at the proton spectra (Fig. 4),
it showed very sharp lines due to species with a high mobility,
3.1.2. N2 sorption study
The N2 adsorption–desorption isotherms of SBA-15, ILSBA-15
and TiILSBA-15 are shown in Fig. 2. Isothermal N2 adsorption mea-
surements have allowed determining BET surface area, pore size
distribution and total pore volume for the prepared materials. All
these parameters decrease after the immobilization of ionic liq-
uid and subsequent metal complex incorporation. The surface area
+
probably the BMIM species. The peaks at 6.8–7.9 corresponds
to the aromatic protons of the binol ligand and the imidazolium
moiety of the ionic liquid present and the peaks at 0.27–3.29 cor-
respond to the aliphatic side chain of the imidazolium moiety. The
13C NMR spectra (Fig. 5) shows peaks at 136 and 121, which are
due to the aromatic ring carbons and the peaks 13–49 confirm
the presence of linker propyl group, ethoxy (SiOCH2CH3). 29Si MAS
NMR spectra of the parent SBA-15 exhibited a broad peak and was
dominated by an intense peak at −110 ppm assigned to Si(OSi)4
and one shoulder peak at −102 ppm due to Si(OSi)3OH (Q3) struc-
tural units present in SBA-15. On incorporation of the imidazolium
moiety, in addition to the aforementioned three peaks, one peak
at 66 ppm appeared (Fig. 6) which is the evidence of presence
of T3 moiety only. No peak appeared at −45 and 58 ppm, in-
dicates the absence of free silane moiety physically adsorbed on
−1
observed for ILSBA-15 and TiILSBA-15, are 335 and 116 m2 g
−1
respectively compared to 730 m2 g
for parent SBA-15. The av-
erage pore volume was decreased from 1.10 to 0.69 and 0.26.
Also the average pore diameters decreased from 63 to 25 Å af-
ter immobilization. The pore size distribution was calculated from
the Kelvin equation and is presented as a BJH plot (inset picture,
Fig. 2). Although the immobilized samples showed a decrease in
surface area, pore diameter and pore volume, isotherms of all the
materials were of type IV, as defined by IUPAC and exhibited a
H1-type broad hysteresis loop, which was typical of large-pore
Fig. 2. N2 adsorption–desorption isotherms of samples: (a) SBA-15, (b) ILSBA-15 and (c) TiILSBA-15. Inset picture: Pore size distribution of these materials.