Titanium complexes of chiral amino alcohol derived Schiff bases as efficient catalysts in asymmetric oxidation of prochiral sulfides with hydrogen peroxide as an oxidant

Article abstract of DOI:10.1016/j.molcata.2012.04.014

An efficient asymmetric oxidation of prochiral sulfides catalyzed by a series of simple in situ generated complexes based on chiral amino alcohol derived Schiff bases with Ti(Oi-Pr)₄ was carried out in presence of cheap and environmentally benign oxidant H₂O₂ at 0 °C. Prochiral sulfides were converted to respective chiral sulfoxides efficiently (conversion, 93%; up to ee, 98%) with this system in 10 h at 0 °C. The present study demonstrated a significant role of steric influence of the substituent attached on both aryl and alkyl moiety on the enantioselectivity. Kinetic studies of the catalytic reaction showed first order dependence on substrate and catalyst whereas it is zero for the oxidant. Kinetic studies in combination with UV-vis. spectral studies were used to propose a catalytic cycle for the sulfoxidation reaction.

Full text of DOI:10.1016/j.molcata.2012.04.014

Journal of Molecular Catalysis A: Chemical 361–362 (2012) 36–44
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Titanium complexes of chiral amino alcohol derived Schiff bases as efficient
catalysts in asymmetric oxidation of prochiral sulfides with hydrogen peroxide
as an oxidant
Prasanta Kumar Bera, Debashis Ghosh, Sayed Hasan Razi Abdi∗, Noor-ul Hasan Khan,
Rukhsana Ilays Kureshy, Hari Chandra Bajaj
Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute (CSMCRI), Council of Scientific & Industrial Research (CSIR), G.B. Marg, Bhavnagar
364 021, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient asymmetric oxidation of prochiral sulfides catalyzed by a series of simple in situ generated
complexes based on chiral amino alcohol derived Schiff bases with Ti(Oi-Pr)4 was carried out in presence
of cheap and environmentally benign oxidant H2O2 at 0 ◦C. Prochiral sulfides were converted to respective
chiral sulfoxides efficiently (conversion, 93%; up to ee, 98%) with this system in 10 h at 0 ◦C. The present
study demonstrated a significant role of steric influence of the substituent attached on both aryl and alkyl
moiety on the enantioselectivity. Kinetic studies of the catalytic reaction showed first order dependence
on substrate and catalyst whereas it is zero for the oxidant. Kinetic studies in combination with UV–vis.
spectral studies were used to propose a catalytic cycle for the sulfoxidation reaction.
Received 28 December 2011
Received in revised form 26 April 2012
Accepted 27 April 2012
Available online 8 May 2012
Keywords:
Asymmetric oxidation
Sulfoxidation
© 2012 Elsevier B.V. All rights reserved.
Schiff base
Chiral titanium complex
Hydrogen peroxide
1. Introduction
however, high chemo- and enantioselectivity remained elusive in
most cases particularly when titanium was used with H2O2 as an
Development of catalytic system for the synthesis of opti-
cally pure sulfoxides is important in organic synthesis as this
class of compounds are used as chiral auxiliary in asymmetric
synthesis [1–4], and as synthon in bioactive compounds, which
are increasingly finding application in pharmaceutical industry
[5–8]. Asymmetric sulfoxidation of prochiral sulfides is exten-
sively studied over the last two decades since its discovery by
Kagan (using Sharpless catalytic system) [9] and Modena [10].
To introduce enantioselectivity, stoichiometric amount of chiral
oxidants were either used directly [11–16] or in the presence of
Ti(Oi-Pr)4 [17–19] with limited success. Several transition met-
als such as vanadium [20–39] manganese [40–46] iron [47–50]
and titanium [51–68] and few other metals, e.g., Zr, Pt, W, Mo,
Ru and Al [69–74] with various types of chiral bidentate [52]
tridentate [59] and tetradentate ligands [64] have also been
explored to catalyze asymmetric sulfoxidation of prochiral sul-
fides with various oxidants like TBHP/CHP [59,64], PhIO [45,46],
oxaridine [16], (S)-norcamphor derived hydroperoxide [18] and
H2O2 [29,31,35,47–50,59,62]. Besides these transition metals, few
enzyme catalyzed [75–80] sulfoxidation have also been reported,
oxidant. Since, H2O2 is atom-efficient and environmentally benign
it is most preferred oxidant for all practical purpose. Toward this
aim reports from Bolm group [47–59] are significant in terms of
good product yield, selectivity and enantiomeric excess using H2O2
as an oxidant and nontoxic iron complexes of amino alcohol derived
Schiff bases as catalysts. This system however is less active and
enantioselective in the absence of an additive particularly Li/Na
salt of 4-methoxy benzoic acid. Alternatively, titanium complexes
of amino alcohol derived Schiff bases can also be used as non-
toxic catalyst with H2O2 as an oxidant but they were found to be
less enantioselective in asymmetric sulfide oxidation reaction [65].
Here we report titanium complexes of a series of amino alcohol
derived Schiff bases as effective catalyst in asymmetric oxidation
of various prochiral sulfides giving sulfoxides in moderate to excel-
lent ee (30–98%) and selectivity (90–99%) with good conversion
(58–93%), importantly, in the absence of an additive.
2. Experimental
2.1. General
All the sulfides were purchased from TCI and Acros.
∗
(1S,2R)-(+)-2-amino-1,2-diphenylethanol
amino-1,2-diphenylethanol, cumene hydroperoxide (CHP),
and
(1R,2S)-(−)-2-
Corresponding author. Tel.: +91 0278 2567760; fax: +91 0278 2566970.
E-mail address: shrabdi@csmcri.org (S.H.R. Abdi).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.04.014

P.K. Bera et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 36–44
37
3,5-dichlorosalicylaldehyde,
5-chlorosalicylaldehyde,
2-
11H), 6.93 (dd, J = 7.6 Hz, 1.6 Hz 1H), 6.74 (t, J = 7.6 Hz, 1H), 5.04 (d,
J = 6.8 Hz, 1H), 4.48 (d, J = 7.0 Hz, 1H), 2.08 (br, s, 1H), 1.44 (s, 9H).
Ligand 1g: Yellow solid; yield: 94%; mp = 65–68 ◦C; 1H NMR
(200 MHz, CDCl3): ı 14.13 (s, 1 H,), 8.24 (s, 1H), 7.34–7.17 (m, 12H),
5.08 (d, J = 6.2 Hz, 1H), 4.59 (d, J = 6.2 Hz, 1H), 3.99 (s, 1H), 2.64 (s,
6H), 1.43 (s, 9H).
hydroxy-4-methoxybenzaldehyde, 2-hydroxy-1-naphthaldehyde,
2-t-butylphenol, 4-t-butylphenol, titanium(IV)isopropoxide were
purchased from Sigma–Aldrich (USA). Urea hydrogen peroxide
(UHP) and t-butyl hydrogen peroxide (TBHP) were purchased
from Merck, 30% aqueous H2O2 was from Rankem (India), salicy-
laldehyde and required solvents were from Spectrochem (India).
All reagents were used as received but the solvents were dried
and stored over activated molecular sieve under nitrogen atmo-
sphere. 1H NMR spectra of ligands, substrates and products were
obtained from Bruker-Avance-DPX-200 (200 MHz) spectrometer
using TMS as internal standard. Electronic spectra were recorded
Ligand 1h: Yellow solid; yield: 80%; mp = 216–219 ◦C; 1H NMR
(200 MHz, CDCl3): ı 14.11 (s, 1H), 8.23 (s, 1H), 7.74–7.58 (m, 17H),
7.29–7.05 (m, 15H), 6.61 (s, 1H), 5.12 (d, J = 6.6 Hz, 1H), 5.07 (s, 2H),
4.75 (d, J = 5.8 Hz, 1H), 1.12 (s, 9H).
Ligand 1i: Yellow solid; yield: 95%; mp = 142–145 ◦C; 1H NMR
(200 MHz, CDCl3): ı 13.67 (s, 1H), 7.94 (s, 1H), 7.36–7.31 (m, 5H),
7.30–7.22 (m, 5H), 6.94 (d, J = 8.4 Hz, 1H), 6.40 (d, J = 2.2 Hz, 1H),
6.33 (dd, J = 8.6 Hz, J = 2.4 Hz, 1H), 5.00 (d, J = 6.8 Hz, 1H), 4.64 (d,
J = 6.8 Hz, 1H), 3.76 (s, 3H), 2.29 (br, s, 1H).
in dichloromethane on
a Varian Cary 500 Scan UV–vis–NIR
spectrophotometer. The enantiomeric excess of sulfoxides were
determined by chiral Shimadzu-HPLC with SPD-M10A-VP and
SPD-M20A UV detector and PDR-Chiral Lnc. advanced Laser
Polarimeter (PDR-CLALP), using Daicel Chiralcel OD, OB and AD-H
chiral columns with 2-propanol/hexane mixture as eluent of the
purified product. Absolute configurations of chiral sulfoxides were
determined by comparing the sign of optical rotation (obtained
from PDR-CLALP) and elution order with the literature. The con-
version of the sulfide and sulfoxide-selectivity were determined
by integrating the methyl proton signal of sulfide, sulfoxide and
sulfone in 1H NMR spectra of the crude reaction mixture, but for
phenyl ethyl sulfoxide and phenyl benzyl sulfoxide products were
isolated.
Ligand 2: Yellow solid; yield: 94%; mp = 201–204 ◦C; 1H NMR
(200 MHz, CDCl3): ı 13.99 (s, 1H), 8.67 (s, 1H), 7.72–7.60 (m, 3H),
7.37–7.23 (m, 12H), 6.99 (d, J = 9.2 Hz, 1H), 5.08 (d, J = 6.6 Hz, 1H),
4.69 (d, J = 6.8 Hz, 1H), 2.33 (br, s, 1H).
Ligand 3: White solid; yield: 76%; mp = 100–103 ◦C; 1H NMR
(200 MHz, CDCl3): ı 7.35–7.17 (m, 11H), 6.65 (d, J = 2.4 Hz, 1H), 4.92
(d, J = 6.0 Hz, 1H), 3.89 (d, J = 6.0 Hz, 1H), 3.79 (d, J = 13.4 Hz, 1H), 3.56
(d, J = 13.4 Hz, 1H), 1.41 (s, 9H), 1.21 (s, 9H).
2.1.4. General procedure for asymmetric sulfoxidation
To a stirring solution of ligand 1a (16 ␮mol) in dry CH2Cl2
(2 ml), Ti(Oi-Pr)4 (3.7 ␮l 12.5 ␮mol) was added and stirred for 1 h at
room temperature under N2 atmosphere. Then appropriate sulfide
(0.5 mmol) was added to the above starring solution. The result-
ing solution was cooled to 0 ◦C, then aqueous hydrogen peroxide
(30%; 85 ␮l, 0.75 mmol) was added at once and the reaction mixture
was allowed to stir. After 10 h the organic layer was washed three
times with water (3× 1 ml), the product was purified by column
chromatography with silica gel and hexane/ethyl acetate solvent
mixture as an eluent. Enantiomeric excess were determined by
HPLC analysis of the purified sample using Daicel Chiralcel OD,
AD-H and OB column at ␭ = 220 nm.
2.1.1. Synthesis of ligands [39]
Chiral ligands were synthesized by the condensation reaction
of readily available salicylaldehyde derivatives (or synthesized
according to Ref. [86]) with chiral 2-amino-1,2-diphenylethanol by
the modified procedure.
2.1.2. General methods for the synthesis of ligands
To a solution of salicylaldehyde derivative (1 mmol) in dry
methanol (3 ml) and anhydrous sodium sulfate (1 g), was added
a solution of 2-amino-1,2-diphenylethanol (1 mmol, 213.12 mg in
dry methanol (2 ml)) and were stirred at room temperature. After
the completion of reaction (15 h) solvent was evaporated and the
precipitate was washed with water to remove sodium sulfate and
finally dried in vacuum.
2.1.5. Characterization data of the sulfoxides and
chromatography (HPLC) condition
Phenyl methyl sulfoxide (5a) [18]: Colorless oil; 1H NMR
(200 MHz, CDCl3): ı 2.74 (s, 3H), 7.50–7.56 (m, 3H), 7.62–7.67
(m, 2H); HPLC condition: column: Daicel Chiralcel OD; eluent:
80:20 (hexane/IPA); flow rate: 0.5 ml/min; tr (R) = 14.5 min, tr
(S) = 16.7 min.
2.1.3. Characterization data of ligands
Ligand 1a [84]: Yellow solid; yield: 98%; mp = 60–63 ◦C; 1H NMR
(200 MHz, CDCl3): ı 13.41 (s, 1H), 8.15 (s, 1H), 7.38–7.26 (m, 11H),
6.92 (d, J = 2.4 Hz, 1H), 5.08 (d, J = 6.8 Hz, 1H), 4.51 (d, J = 6.6 Hz, 1H),
2.07 (br, s, 1H), 1.45 (s, 9H), 1.25 (s, 9H).
4-Methylphenyl methyl sulfoxide (5b) [18]: White solid; 1H
NMR (200 MHz, CDCl3): ı 2.42 (s, 3H), 2.71 (s, 3H), 7.33 (d, J = 7.9 Hz,
2H), 7.54 (d, J = 7.9 Hz, 2H); HPLC condition: column: Daicel Chi-
ralcel OD; eluent: 95:05 (hexane/IPA); flow rate: 0.5 ml/min; tr
(R) = 35.9 min, tr (S) = 39.2 min.
Ligand 1b [84]: Yellow solid; yield: 95%; mp = 80–83 ◦C; 1H NMR
(200 MHz, CDCl3): ı 13.14 (s, 1H), 8.08 (s, 1H), 7.36–7.26 (m, 10H),
7.09 (d, J = 7.2 Hz, 1H), 6.96 (d, J = 8.0 Hz, 1H), 6.80 (t, J = 7.4 Hz, 1H),
5.07 (d, J = 6.8 Hz, 1H), 4.56 (d, J = 6.8 Hz, 1H), 1.90 (br, s, 1H).
Ligand 1c [84]: Yellow solid; yield: 93%; mp = 61–63 ◦C; 1H NMR
(200 MHz, CDCl3): ı 14.17 (s, 1H), 7.92 (s, 1H), 7.38–7.27 (m, 11H),
6.96 (d, J = 2.2 Hz, 1H), 5.02 (d, J = 7.0 Hz, 1H), 4.53 (d, J = 7.2 Hz, 1H),
2.03 (br, s, 1H).
4-Methoxyphenyl methyl sulfoxide (5c) [18]: Yellow oil; 1H
NMR (200 MHz, CDCl3): ı 2.70 (s, 3H), 3.86 (s, 3H), 7.03 (d, J = 8.8 Hz,
2H), 7.6 (d, J = 8.8 Hz, 2H); HPLC condition: column: Daicel Chi-
ralcel OD; eluent: 90:10 (hexane/IPA); flow rate: 0.7 ml/min; tr
(R) = 24.5 min, tr (S) = 26.5 min.
4-Nitrophenyl methyl sulfoxide (5d) [47]: White solid; 1H NMR
(200 MHz, CDCl3): ı 2.80 (s, 3H), 7.85 (d, J = 8.6 Hz, 2H), 8.40 (d,
J = 8.6 Hz, 2H); HPLC condition: column: Daicel Chiralcel OJ; elu-
ent: 65:35 (hexane/IPA); flow rate: 0.5 ml/min; tr (R) = 22.2 min, tr
(S) = 25.8 min.
Ligand 1d: Yellow solid; yield: 93%; mp = 112–115 ◦C; 1H NMR
(200 MHz, CDCl3): ı 13.13 (s, 1H), 7.96 (s, 1H), 7.40–7.33 (m, 5H),
7.26–7.18 (m, 6H), 7.03 (d, J = 2.4 Hz, 1H), 6.86 (d, J = 8.8 Hz, 1H), 5.02
(d, J = 7.2 Hz, 1H), 4.50 (d, J = 7.0 Hz, 1H), 1.96 (br, s, 1H).
Ligand 1e [82]: Yellow solid; yield: 97%; mp = 48–52 ◦C; 1H NMR
(200 MHz, CDCl3): ı 13.6 (s, 1H), 8.12 (s, 1H), 7.38–7.26 (m, 11H),
7.07 (d, J = 2.4 Hz, 1H), 6.88 (d, J = 8.6 Hz, 1H), 5.05 (d, J = 7.0 Hz, 1H),
4.53 (d, J = 7.0 Hz, 1H), 2.11 (br, s, 1H), 1.24 (s, 9H).
Fluorophenyl methyl sulfoxide (5e) [29]: Colorless oil; 1H NMR
(200 MHz, CDCl3): ı 2.46 (s, 3H), 6.99 (m, 2H), 7.25 (m, 2H);
HPLC condition: column: Daicel Chiralcel OD; eluent: 92:08 (hex-
ane/IPA); flow rate: 0.4 ml/min; tr (R) = 32.3 min, tr (S) = 34.7 min.
4-Bromophenyl methyl sulfoxide (5f) [18]: White solid; 1H NMR
(200 MHz, CDCl3): ı 2.72 (s, 3H), 7.53 (d, J = 8.6 Hz, 2H), 7.6 (d,
Ligand 1f [82]: Yellow solid; yield: 96%; mp = 166–169 ◦C; 1H
NMR (200 MHz, CDCl3): ı 13.60 (s, 1H), 8.07 (s, 1H), 7.39–7.25 (m,

38
P.K. Bera et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 36–44
J = 8.6 Hz, 2H); HPLC condition: column: Daicel Chiralcel OB; elu-
ent: 80:20 (hexane/IPA); flow rate: 0.8 ml/min; tr (S) = 15.2 min, tr
(R) = 20.0 min.
2-Bromophenyl methyl sulfoxide (5g) [48]: Yellow oil; 1H NMR
(200 MHz, CDCl3): ı 2.83 (s, 3H), 7.35 (m, 1H), 7.60 (m, 2H), 7.95 (m,
1H); HPLC condition: column: Daicel Chiralcel AD-H; eluent: 90:10
(hexane/IPA); flow rate: 0.5 ml/min; tr1 = 16.0 min, tr2 = 18.9 min.
4-Chlorophenyl methyl sulfoxide (5h) [18]: Colorless oil; 1H
NMR (200 MHz, CDCl3): ı 2.73 (s, 3H), 7.51 (d, J = 8.6 Hz, 2H), 7.60
(d, J = 8.6 Hz, 2H); HPLC condition: column: Daicel Chiralcel OB; elu-
ent: 80:20 (hexane/IPA); flow rate: 0.7 ml/min; tr (R) = 11.4 min, tr
(S) = 16.9 min.
2-Chlorophenyl methyl sulfoxide (5i): Colorless oil; 1H NMR
(200 MHz, CDCl3): ı 2.83 (s, 3H), 7.5 (m, 3H), 7.96 (m, 1H); HPLC con-
dition: column: Daicel Chiralcel AD-H; eluent: 90:10 (hexane/IPA);
flow rate: 0.5 ml/min; tr (S) = 15.5 min, tr (R) = 18.5 min.
Benzyl phenyl sulfoxide (5j) [18]: White solid; 1H NMR
(200 MHz, CDCl3): ı 4.00 (d, J = 12.6 Hz, 1H), 4.11 (d, J = 12.6 Hz,
1H), 6.98 (m, 2H), 7.26 (m, 4H), 7.42 (m, 5H); HPLC condition: col-
umn: Daicel Chiralcel OD; eluent: 90:10 (hexane/IPA); flow rate:
0.5 ml/min; tr (R) = 24.0 min, tr (S) = 28.8 min.
Fig. 1. Temperature dependence of asymmetric oxidation of C6H5SCH3 catalyzed
by Ti–(1R,2S)-1a complex with aqueous H2O2 (30%, 1.1 equiv.).
Ethyl phenyl sulfoxide (5k) [18]: Colorless oil; 1H NMR
(200 MHz, CDCl3): ı 1.20 (t, J = 7.4 Hz, 3H), 2.68–3.00 (m, 2H),
7.50–7.55 (m, 3H), 7.60–7.64 (m, 2H); HPLC condition: column: Dai-
cel Chiralcel OD; eluent: 90:10 (hexane/IPA); flow rate: 0.5 ml/min;
tr (R) = 25.9 min, tr (S) = 30.9 min.
an increase in ee (72%) of the product till 0 ◦C. On further decrease
in the reaction temperature (up to −20 ◦C) there was a reduction
in conversion but the selectivity and enantioselectivity remained
unchanged (Fig. 1). Therefore, subsequent catalyst evaluation runs
were conducted at 0 ◦C (taken as optimum) to optimize other reac-
tion parameters.
2.1.6. Reaction procedure to study the kinetic
The in situ generated titanium complex of (1S,2R)-1a
(0.00375–0.0075 M) in CH2Cl2 was stirred with 2-phenyl methyl
sulfide (0.15–0.3 M) for 10 min before cooled to 0 ◦C, then 30% aque-
ous H2O2 (0.75–1.75 M) was added and allowed to stirred. 20 ␮l
aliquots were drawn from the reaction mixture, quenched with
sodium sulfite solution, dried with anhydrous sodium sulfate and
injected in GC to analyzed the conversion.
3.2. Solvent and catalyst screening
First we varied solvents for the above reaction keeping other
reaction parameters constant (Fig. 2). Only representative solvents
among various commercially available solvents were used here
while excluding alcoholic solvents, DMSO and DMF for their either
known reactivity in the oxidative reaction condition or difficulty in
the post-catalytic processing of the reaction mixture. Clearly no
other solvent studied here could match the performance of the
solvent CH2Cl2.
3. Results and discussion
3.1. Temperature effect on in situ generated complex catalyzed
reaction
The chiral Schiff base ligands 1a–1i and 2 used in the present
study were conveniently synthesized [39] by the condensation of
(1S,2R)/(1R,2S)-2-amino-1,2-diphenylethanol with salicylaldehyde
derivatives having varying degree of steric and electronic features.
The titanium complexes of these ligands (0.016 mmol) and hydro-
genated form of (1R,2S)-1a, i.e., 3 were generated in situ by the
addition of Ti(Oi-Pr)4 (0.013 mmol) as a metal source (Scheme 1).
The in situ generated complexes were used to catalyze asymmetric
oxidation of phenyl methyl sulfide 4a as a representative substrate
with H2O2 (30% aqueous) as an oxidant in CH2Cl2.
To begin with, in situ generated Ti complex of (1R,2S)-1a
(2.5 mol%) was used as catalyst in the oxidation of 4a at room
temperature (27 2 ◦C) where 74% conversion with 85% selec-
tivity for the product (R)-phenyl methyl sulfoxide (ee, 53%) was
obtained in 10 h. On decreasing the reaction temperature there was
Fig. 2. Effects of solvents on asymmetric oxidation of C6H5SCH3 catalyzed by
Ti–(1R,2S)-1a complex with aqueous H2O2 (30%, 1.1 equiv.) at 0 ◦C.
Scheme 1. Synthesis of chiral ligand and expected structure of the in situ generated catalytically active complex.

P.K. Bera et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 36–44
39
Table 1
Catalytic asymmetric oxidation of methyl phenyl sulfide using various chiral ligands.a
O
O
S
S
S
Ti(Oi-Pr)4 / ligand
H2O2 (30%)
Solvent
O
4a
5a
6a
Ph
N
Ph
Ph
Ph
Ph
Ph
NH
OH
OH
N
OH
R3
OH
OH
t-Bu
OH
R2
R1
t-Bu
3
Ligand :
Ligand : 2
Ligands : 1a-i
1e: R1 = t-Bu, R2 = H, R3 = H
1f: R1 = H, R2 = H, R3 = t-Bu
1a: R1 = t-Bu, R2 = H, R3 = t-Bu
1b: R1 = R2 = R3 = H
1
2
3
1c: R1 = Cl, R2 = H, R3 = Cl
1g
: R = t-Bu, R = H, R = CH2NMe2
1h: R1 = t-Bu, R2 = H, R3 = CH2PPh3Cl
1i: R1 = H, R2 = OMe,R 3 = H
1
2
3
1d
: R = H, R = H, R = Cl
Entry
Ligands
Conversionb (%)
Selectivityb (%)
eec (%)
Config.d
1
2
3
4
5
6
7
8
9
(1S,2R)-1a
(1R,2S)-1a
(1S,2R)-1b
(1S,2R)-1c
(1S,2R)-1d
(1S,2R)-1e
(1S,2R)-1f
(1S,2R)-1g
(1S,2R)-1h
(1S,2R)-1i
(1S,2R)-2
81
76
48
55
53
64
49
57
54
50
51
66
96
95
95
90
91
97
92
90
94
91
93
82
84
72
25
13
12
79
32
25
07
31
15
05
R (+)
S (−)
R (+)
R (+)
R (+)
R (+)
R (+)
R (+)
R (+)
R (+)
R (+)
R (+)
10
11
12
(1R,2S)-3
a
Reaction condition: C6H5SCH3 (0.5 mmol), Ti(Oi-Pr)4 (0.013 mmol), Ligand (0.016 mmol), H2O2 (30%; 0.55 mmol) at 0 ◦C for 10 h in CH2Cl2 (2 ml).
Determined by 1H NMR measurement of sulfide, sulfoxide and sulfone relative concentration.
Determined by HPLC with a Daicel Chiralcel OD column, flow rate = 0.5 mL/min. hexane/i-PrOH = 8/2 (v/v).
b
c
d
The absolute configuration were assigned by comparing HPLC elution orders and sign of optical rotations with literature data.
Conventionally by switching over to the opposite configuration
10 h and the results are given in Table 1. All the ligands listed here
were able to generate the catalyst with moderate to high activity
and chemo-selectivity but highest enantioselectivity was observed
for the ligand with t-butyl group at 3 and 5 positions of salicylalde-
hyde moiety. Both 3 and 5 positions are extremely sensitive toward
of the catalyst, it is expected that the activity and chemo-
/enantioselectivity of the catalyst would remain the same except
for the opposite configuration of the product. However, to our
advantage when we took in situ generated titanium complex with
(1S,2R)-1a under the identical reaction conditions, we got the
product with opposite configuration, but with relatively higher
conversion and enantioselectivity (ee, 84%; Table 1, entry 2)
In order to understand the product profile during the catalytic
run over a period of time, in situ generated titanium complex with
(1S,2R)-1a was used to catalyze oxidation of phenyl methyl sul-
fide under the above optimized reaction condition over a period of
12 h. An hourly withdrawal of aliquot from the reaction was mon-
itored on HPLC and 1H NMR. The data as given in Fig. 3 indicate
that the conversion increases steadily up to 10 h and attained a
maximum value of 84% while chemo-selectivity (95–96%) remains
constant throughout the course of the reaction. As far as the enan-
tioselectivity is concern, only marginal increase (ee, from 74 to 84%)
was observed. This reaction profile, particularly with no change in
chemo-selectivity, suggests that under our reaction condition the
oxidative kinetic resolution of the product is either not occurring
or negligible as reported elsewhere [29].
The in situ generated complexes from ligands 1b–1i, 2 and 3
were then evaluated for their catalytic performance in the oxidation
of phenyl methyl sulfide under the above optimized condition over
Fig. 3. The time profile diagram of oxidation of C6H5SCH3 catalyzed by Ti–(1R,2S)-1a
complex with aqueous H2O2 (30%, 1.1 equiv.) at 0 ◦C.

40
P.K. Bera et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 36–44
Table 2
Optimization of metal mol (%) and metal to ligand ratio.a
O
S
O
S
S
Ti(Oi-Pr)4 / (1S,2R)-1a
O
+
H2O2 (30%)
DCM, 0 o
C
6a
4a
5a
Entry
Ti(Oi-Pr)4 (mol %)
Ti: Ligand (mol ratio)
Conversionb (%)
Selectivityb (%)
eec (%)
1
2
3
4
5
6
7
8
9
1
1.5
2
2.5
5
2.5
2.5
2.5
2.5
2.5
2.5
1:1.25
1:1.25
1:1.25
1:1.25
1:1.25
1:0.67
1:1
1:1.5
1:2
1:2.5
1:0
66
71
72
79
81
70
77
71
69
64
16
94
96
95
96
94
95
95
96
97
95
95
61 (R)
75 (R)
68 (R)
84 (R)
77 (R)
72 (R)
76 (R)
75 (R)
75 (R)
74 (R)
00
10
11d
a
Reaction condition: C6H5SCH3 (0.5 mmol), Ti(Oi-Pr)4 (0.013 mmol), (1S,2R)-1a, H2O2 (30%; 0.55 mmol) at 0 ◦C for 10 h in CH2Cl2 (2 ml).
Determined by 1H NMR measurement of sulfide, sulfoxide and sulfone relative concentration.
Determined by HPLC with a Daicel Chiralcel OD column, flow rate = 0.5 mL/min. hexane/i-PrOH = 8/2 (v/v).
Reaction was carried for 24 h.
b
c
d
the steric and electronic features of the substituent as far as the
conversion in 10 h, but a decrease in the product selectivity by
giving over oxidized product sulfone in excess, however there was
not much change in the ee of the desired product.
enantioselectivity is concern. For instance there was only marginal
decrease in the ee of the product (79%) when ligand 1e with posi-
tions 3 and 5 were occupied by t-butyl group and H, respectively
(entry 6). However, reversing the positions of t-butyl and H as in
1f (entry 7) caused drastic decrease in ee of the product (32%). At
the same time a polar (CH2–N(CH3)2 or charged ([CH2–P(Ph)3]+Cl−)
substituents at 5 position (entries 8 and 9) brought down the enan-
tioselectivity drastically despite its position 3 was occupied by a
t-butyl group. The ligands bearing marginal electron withdrawing
capacity, e.g., Cl in ((1S,2R)-1c and (1S,2R)-1d) and electron donat-
ing group like OMe in ((1S,2R)-1i) gave poor enantioselectivity
(entries 4, 5 and 10). Ligands (1S,2R)-2 and (1R,2S)-3 (hydrogenated
form of 1a) too fared badly in terms of enantioselectivity (entries
11, 12).
In titanium catalyzed reactions, bridged ␮-oxo species is gen-
erated by the addition of stoichiometric amount of water and it
is reported that thus generated di-␮-oxo species is often more
catalytically active and enantioselective [54]. However, when we
added stoichiometric amount of water to the in situ generated Ti
complex of 1a, there was an overall drop in the conversion, selec-
tivity and enantioselectivity (entry 5) for the oxidation of phenyl
methyl sulfide.
We also examined the applicability of other oxidants, e.g., CHP
(cumene hydroperoxide), TBHP (t-butyl hydroperoxide), UHP (urea
hydrogen peroxide) under the above optimized reaction condi-
tions in the enantioselective oxidation of phenyl methyl sulfide
(Fig. 4). Among these oxidants CHP and TBHP gave fairly good con-
version and product selectivity but the ee obtained was only 13
and 4%, respectively. Whereas UHP, (as compared to H2O2) effec-
tively oxidized the sulfide affording higher conversion (95%) but
with somewhat less selectivity (87%) and ee (75%).
3.3. Fine tuning of catalytic activity
As the ligand (1S,2R)-1a gave overall better results, it was
selected for further refinement of the reaction parameters, i.e.,
loading of the active catalyst keeping metal to ligand ratio 1:1.25.
Accordingly, for the fixed amount of the substrate (0.5 mmol), the
mol% of the Ti(Oi-Pr)4 was varied over 1–5 mol% (Table 2, entries
1–5) where 2.5 mol% of Ti(Oi-Pr)4 with a metal to ligand (1S,2R)-1a
ratio 1:1.25 was found to be optimum.
3.4. Scope of catalysis
Based on the optimization study, 2.5 mol% of Ti(Oi-Pr)4, 1:1.25
Ti to ligand ratio (ligand (1S,2R)-1a) and 1.5 equiv. of 30% aqueous
H2O2 was found to be optimum for the oxidation of phenyl
methyl sulfide at 0 ◦C for 10 h. These sets of parameters were then
We next varied metal to ligand ratio keeping the mol% of Ti(Oi-
Pr)4 as 2.5% and other reaction parameters fixed (Table 2, entries
6–11). Only marginal variation in the conversion and enantioselec-
tivity was observed on deviating from the above optimized metal
to ligand ratio of 1:1.25. Nevertheless, higher and equal stoichio-
metric content of metal as against ligand there was slight reduction
in conversion and ee of the product (entries 6 and 7) but increasing
ligand ratio caused steady decrease in the conversion while prod-
uct selectivity and ee remain steady (entries 8 and 9). It is to be
noted that in the absence of a ligand the oxidation reaction failed
to proceed beyond 16% conversion even for extended reaction time
(24 h) to give the racemic product with 95% selectivity for sulfoxide.
Having optimized the catalyst loading and metal to ligand ratio
for the oxidation of phenyl methyl sulfide, we next examined
the effect of stoichiometry of H2O2 (with respect to substrate)
and role of water as an additive on the conversion selectivity
and ee of the product (Table 3). It is evident from the entries 1–4
that on increasing the amount of H2O2 there was an increase the
Table 3
The effect of oxidant and additives on asymmetric oxidation of methyl phenyl
sulfide.a
Entry
H2O2 (equiv.)
Conversionb (%)
Selectivityb (%)
eec (%)
1
2
3
1.1
1.5
2
2.5
1.1
79
92
>99
>99
68
96
93
80
66
91
84 (R)
84 (R)
83 (R)
85 (R)
59 (R)
4
5d
a
Reaction condition: C6H5SCH3 (0.5 mmol), Ti(Oi-Pr)4 (0.013 mmol) in CH2Cl2
(2 ml), at 0 ◦C for 10 h, otherwise mentioned.
b
Determined by 1H NMR measurement of sulfide, sulfoxide and sulfone relative
concentration.
c
Determined by HPLC with a Daicel Chiralcel OD column, flow rate = 0.5 mL/min.
hexane/i-PrOH = 8/2 (v/v).
d
2.5 mol % water was added as an additive after the complex formation.

P.K. Bera et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 36–44
41
O
Ti(Oi-Pr)4 / Ligand
S
S
Ar
R
1.5 equiv. H2O2 (30%)
DCM, 0 oC
Ar
R
4
5
Ar = 4-OMe-C6H4, R = Me Ar = 2-Cl-C6H4, = R = Me
Ar = 4-F-C6H4, = R = Me
Ar = 2-Br-C6H4, = R = Me
Ar = 4-Cl-C6H4, = R = Me Ar = Ph, R = Me
Ar = 4-Br-C6H4, = R = Me Ar = Ph, R = Et
Ar = 4-NO2-C6H4, = R = M Ar = Ph, R = Bn
Scheme 2. Titanium catalyzed asymmetric oxidation of sulfides.
Fig. 4. Screening of oxidants for oxidation of C6H5SCH3 catalyzed by Ti–(1S,2R)-1a
complex with aqueous H2O2 (30%, 1.1 equiv.) at 0 ◦C.
examined for their suitability in the oxidation of various other
substrates (Table 4, Scheme 2).
The data suggest that more than electronic, steric factor has
got pronounced effect on the enantioselectivity of the product. For
example, the ee obtained with 2-Br- and 2-Cl-phenyl methyl sulfide
(entries 7 and 9) were 39 and 30%, respectively, while the products
from 4-Br- and 4-Cl-phenyl methyl sulfide (entries 6 and 8) oxida-
tion the ee obtained were 82 and 86%, respectively. Similar trend in
the ee of the product was obtained when methyl group of phenyl
methyl sulfide (ee, 84%) was replaced with benzyl (ee, 65%) or ethyl
group (ee, 70%). Distinctly, highest ee (98%) was obtained with
highly electron deficient 4-NO2-phenyl methyl sulfide (entry 4),
but with conversion 58% and selectivity 84%. By comparison, a rel-
atively less electron deficient 4-F-phenyl methyl sulfide provided
ee 80% but with 85% conversion and 91% selectivity. But this consis-
tency was not extended to electron rich 4-OMe-phenyl methyl sul-
fide and 4-Me-phenyl sulfide, both of which gave relatively higher
ee 82% (93% conversion, 93% selectivity) and 85% (87% conversion,
94% selectivity), respectively, with excellent conversion and selec-
tivity. Hence it can be concluded that the electronic effect of the
substituent on the aryl moiety do not have any regular effect in
enantioselectivity.
Scheme 3. Kinetics of sulfoxidation reaction with 2-chlorophenyl methyl sulfide as
substrate.
3.5. Kinetics of the reaction
It is difficult to evaluate the precise order of sulfoxidation
(Scheme 3) reaction because of the parallel over oxidation of
sulfoxide to form sulfone toward the end of the reaction [59]. Nev-
ertheless we studied the kinetics of sulfoxidation of 2-chlorophenyl
methyl sulfide (4i) as a function of catalyst, substrate and oxidant
concentration. The choice of 2-chlorophenyl methyl sulfide (4i) as
model substrate was due to the fact that it is not over-oxidized
under the employed reaction conditions until 40% of the substrate
get oxidized to sulfoxide, thus reduces the complexity that may
arise due to the further oxidation reaction.
Time dependence plot (Fig. 5) showed that the concentration of
the product increases linearly with time up to 1.25 h, thereafter it
started levels-off. Therefore in order to find out the dependence of
Table 4
Asymmetric oxidation of aryl alkyl sulfides by 1b/Ti(Oi-Pr)4 system under optimized reaction condition.a
O
O
O
R
Ti(Oi-Pr)4 / ligand
H2O2 (30%)
DCM, 0 oC
S
S
S
Ar
R
Ar
R
Ar
6
4
5
Entry
Sulfides (4)
Ar
Sulfoxides (5)
R
Conversionb (%)
Selectivityb (%)
eec (%)
Config.d
1
2
3
4
5
6
7
8
9
C6H5
CH3 (4a)
CH3 (4b)
CH3 (4c)
CH3 (4d)
CH3 (4e)
CH3 (4f)
CH3 (4g)
CH3 (4h)
CH3 (4i)
Bn (4j)
92
87
93
58
85
86
75
83
61
87
89
93
94
93
84
91
91
98
92
84
85
82
98
80
82
39
86
30
65
70
R (+)
R (+)
R (+)
R (+)
R (+)
R (+)
R(+)
R (+)
R (+)
R (+)
R (+)
4-CH3C6H4
4-OCH3C6H4
4-NO2C6H4
4-FC6H4
4-BrC6H4
2-BrC6H4
4-ClC6H4
2-ClC6H4
C6H5
>99
90
92
10
11
C6H5
C2H5 (4k)
a
Reaction condition: Substrate (0.5 mmol), sulfide/Ti/(1S,2R)-1a/H2O2 = 100:2.5:3.1:150, in CH2Cl2 (2 ml) at 0 ◦C for 10 h.
Determined by 1H NMR measurement of sulfide, sulfoxide and sulfone relative concentration.
Determined by HPLC with Daicel Chiralcel OD and AD-H and OB column.
b
c
d
The absolute configuration were assigned by comparing HPLC elution orders and the sign of optical rotations with literature data.

42
P.K. Bera et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 36–44
10
8
1.0
0.8
0.6
0.4
0.2
Slope ~ 1
6
4
2
0.0
0.0
0.5
1.0
0
log [Catalyst]
0
20 40 60 80 100 120 140 160 180 200
Fig. 6. Plot of log kobs versus log[catalyst] at 0 ◦C, [substrate] = 0.25 M, [oxidant]
= 1.25 M.
Time(min)
Fig. 5. Time dependence plot of sulfoxidation reaction at 0 ◦C, [substrate] = 0.25 M,
[catalyst] = 0.00625 M, [oxidant] = 1.25 M.
1.0
0.8
rate (kobs) on substrate and reagents, all reactions were studied for
only 1 h.
3.6. Dependence of the rate on the individual concentration of
substrate and reagents
0.6
Slope ~ 1
0.4
0.2
0.0
The effect of the catalyst concentration on the reaction rate was
evaluated by varying the catalyst concentration from 0.00375 M to
0.0075 M (Table 5) maintaining all the others reagent concentra-
tion constant. The plot of the log(kobs) for the formation of product
versus log[catalyst] (Fig. 6) gave a straight line with slope equal
to one passes through the origin clearly indicated the first order
dependence on catalyst.
0.0
0.5
1.0
1.5
log [Substrate]
We next varied substrate concentration over 0.15 M to 0.30 M
(Table 5), keeping all other parameter same. The linear plot of
log kobs versus log[substrate] (Fig. 7) with slop of one was obtained
indicates the first order dependence on substrate concentration.
Finally there was no change in there rate on the variation of oxidant
concentration (0.75 M to 1.75 M) indicated zero order dependent of
oxidant (Table 5).
Fig. 7. Plot of log(kobs) versus log[substrate] at 0 ◦C, [catalyst] = 0.00625 M and [oxi-
dation] = 1.25 M.
(Fig. 8) which disappeared on the addition of Ti(Oi-Pr)4 with the
emergence of a shoulder at 296 nm. These changes indicate the for-
mation of a probable complex Ti-1a. The addition of substrate to the
complex Ti-1a showed strong hyperchrorism for the substrate peak
at 253 nm possibly due to the formation of an intermediate X. This
3.7. Spectroscopic analysis of the reaction
To understand the reaction in detail, UV–vis analysis was car-
ried out by following the sequential addition of Ti(Oi-Pr)4, substrate
(phenyl methyl sulfide) and H2O2 to the ligand 1a in a manner sim-
ilar to the catalytic sulfoxidation protocol (Scheme 4). The ligand
1a showed two characteristic peaks at 263 and 332 nm in CH2Cl2
1
c
0.4
d
Table 5
0.2
0.0
PhSMe
Dependence of rate of the reaction on the catalyst, substrate and oxidant concen-
PhSOMe
tration for sulfoxidation reaction at 0 ◦C in CH2Cl2.
b
a
[Catalyst] × 103
M
[Substrate] × 102
M
[Oxidant] × 10 M kobs × 104 M min−1
300
400
500
Effect of catalyst concentration
Wavelength (nm)
3.75
6.25
7.50
25
25
25
12.5
12.5
12.5
5.3 0.03
8.2 0.02
10.1 0.02
0
Effect of substrate concentration
6.25
6.25
6.25
6.25
15
20
25
30
12.5
12.5
12.5
12.5
5.6 0.03
7.0 0.02
8.2 0.03
9.4 0.04
300
400
500
Wavelength (nm)
Effect of oxidant concentration
Fig. 8. UV–visible spectra of 26 mM solution of (1S,2R)-1a in CH2Cl2 (a) after sequen-
tial addition of 1.85 ␮l Ti(Oi-Pr)4 (b), 0.92 ␮l phenyl methyl sulfide (c) and 42.5 ␮l of
30% aqueous H2O2 (d) to the above solution. The inset shows the UV–visible spectra
of substrate and the product.
6.25
6.25
6.25
25
25
25
7.5
12.5
17.5
8.3 0.03
8.2 0.03
8.2 0.03

P.K. Bera et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 36–44
43
Scheme 4. Possible pathway of the reaction.
step is likely to be the rate determining step (slow step), since the
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fast step and is responsible for the intramolecular oxygen transfer
to the coordinated sulfide resulting in the formation of the product
sulfoxide and regeneration of the complex Ti-1a, which enters in
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