Asymmetric oxidation of sulfides catalyzed by (R)-6,6'-dibromo-BINOL derived titanium complex

Article abstract of DOI:10.1080/00397911.2020.1786121

An efficient asymmetric oxidation of sulfides was achieved using (R)-6,6'-dibromo-BINOL as chiral ligand in combination with Ti(OⁱPr)₄ using 70% aqueous tertiary butyl hydroperoxide as oxidant. The resulting sulfoxides had high enantiopurities and good yields. A range of aryl alkyl and aryl benzyl sulfides were oxidized to the corresponding sulfoxides with 78–95% ee in 72–80% yields.

Full text of DOI:10.1080/00397911.2020.1786121

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
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ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Asymmetric oxidation of sulfides catalyzed by
(R)-6,6'-dibromo-BINOL derived titanium complex
Narotham V. Maddireddy, Himanshu M. Godbole, Girij P. Singh, Suvarna G.
Kini & Gautham G. Shenoy
To cite this article: Narotham V. Maddireddy, Himanshu M. Godbole, Girij P. Singh,
Suvarna G. Kini & Gautham G. Shenoy (2020): Asymmetric oxidation of sulfides catalyzed
by (R)-6,6'-dibromo-BINOL derived titanium complex, Synthetic Communications, DOI:
10.1080/00397911.2020.1786121
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SYNTHETIC COMMUNICATIONSV
https://doi.org/10.1080/00397911.2020.1786121
Asymmetric oxidation of sulfides catalyzed by (R)-6,6’-
dibromo-BINOL derived titanium complex
Narotham V. Maddireddya, Himanshu M. Godbolea, Girij P. Singha,
Suvarna G. Kinib, and Gautham G. Shenoyb
b
aLupin Limited, Pune, India; Manipal Academy of Higher Education, Manipal, India
ABSTRACT
ARTICLE HISTORY
Received 4 February 2020
An efficient asymmetric oxidation of sulfides was achieved using (R)-
6,6’-dibromo-BINOL as chiral ligand in combination with Ti(OiPr)4
using 70% aqueous tertiary butyl hydroperoxide as oxidant. The
resulting sulfoxides had high enantiopurities and good yields. A
range of aryl alkyl and aryl benzyl sulfides were oxidized to the cor-
responding sulfoxides with 78–95% ee in 72–80% yields.
KEYWORDS
(R)-6,6’-dibromo-BINOL;
chiral sulfoxides; kinetic
resolution; asymmet-
ric oxidation
GRAPHICAL ABSTRACT
Introduction
Chiral sulfoxides are utilized as chiral auxiliaries in various asymmetric reactions.[1]
Some of them like esomeprazole, armodafinil, aprikalim, oxisurane and ustiloxine[2]
exhibit biological activities and thus find use as therapeutic agents. Generally, enantio-
pure sulfoxides are obtained either by nucleophilic displacement of diastereomeric sulfi-
nates or resolution of racemic mixtures or by asymmetric oxidation of sulfides.[3] Over
the past two decades, the catalytic asymmetric oxidation of sulfides has been investi-
gated extensively using transition metal complexes of various chiral ligands. Several
catalyst systems for the asymmetric oxidation of sulfides have been reported including
Ti(IV)-BINOL,[4] C2-symmetric diols,[5] b-amino alcohols,[6] and organic catalysts.[7]
Since the pioneering work on the asymmetric oxidation of sulfides involving the use of
[8]
Ti(OiPr)4/chiral tartrate systems reported by Kagan et al. and Modena et al.
the
enantioselective oxidation of sulfides catalyzed by chiral complexes of transition metals
such as titanium,[9] vanadium,[10] manganese,[11] iron,[12] with various types of chiral
bidentate,[13] tridentate,[14] and tetradentate ligands have also been explored to catalyze
asymmetric sulfoxidation of prochiral sulfides.
CONTACT Narotham V. Maddireddy
narothammadireddy@lupin.com
Lupin Limited, Pune, Maharashtra, India
Supplemental data for this article can be accessed on the publisher’s website.
ß 2020 Taylor & Francis Group, LLC

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N. V. MADDIREDDY ET AL.
Scheme 1. Synthesis of (R)-6,6’-dibromo-BINOL.
1,10-Binaphthyl-2,20-diol (BINOL) and its derivatives have generated particular inter-
est because their versatile backbone can be modified, thereby affecting the reaction
environment by influencing properties of the metal center. Uemura and coworkers
reported the asymmetric oxidation of sulfides by using a Ti(IV) complex generated
from Ti(OiPr)4 and chiral binaphthol[4]. Moderate yields and enantiomeric excesses
were obtained at low temperatures; whereas high enantiomeric excesses and very low
yields were reported at 25 ꢀC. In these experiments CCl4 was used as solvent.
Enantiomers of 6,60-dibromo-BINOL were effectively used as chiral ligands in various
asymmetric reactions namely Mannich-Type reaction,[15] Strecker-type reaction,[16]
Diels-alder reaction,[17] carbonyl-ene cyclization,[18] Friedel-crafts reaction.[19] However,
there have been no successful reports for the application of 6,60-dibromo-BINOL as
chiral ligand for the asymmetric oxidation of sulfides. Xian and coworkers reported[20]
a single reaction for the asymmetric oxidation of thioanisole using (S)-6,60-dibromo-
BINOL as chiral ligand. In this, lower enantioselectivities (enantiomeric excess is up to
43%) were observed when the reaction was performed at 0 ꢀC. However, when the reac-
tion was stirred for prolonged time at 25 ꢀC, enantioselectivity was improved but chem-
ical yield was very low (35%). This decrease in yield is attributed due to the formation
of more sulfone. Here, decane solution of tertiary butyl hydroperoxide was used as oxi-
dizing agent. In the presence of this oxidizing agent the rate of kinetic resolution is so
fast that the formation of sulfone increases rapidly, and the yield decreases drastically
with time. Therefore, finding suitable reaction conditions by using 6,60-dibromo-BINOL
as chiral ligand, which can minimize the sulfone formation and also afford good
enantiomeric excess still remains an attractive goal. In this context, we wish to report
efficient oxidation of sulfides to yield the sulfoxides with a high enantiomeric excess
and good yields. By using 70% aq. tertiary butyl hydroperoxide instead of tertiary butyl
hydroperoxide (in decane) as oxidizing agent we found that the oxidizing agent has a
profound effect on the rate of kinetic resolution. With aq.solution of tertiary butyl
hydroperoxide, kinetic resolution occurred in a controlled way, thus it controls the for-
mation of sulfone.
Results and discussion
Synthesis of (R)-6,6’-dibromo-BINOL
(R)-6,60-dibromo-BINOL was synthesized from the commercially available (R)-BINOL
in single step by following the reaction (as shown in Scheme 1), reported in the
literature.[21]

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Scheme 2. Asymmetric oxidation of methyl phenyl sulfide.
(R)-BINOL was reacted with Br2 at ꢁ70ꢀ to ꢁ65 ꢀC in dichloromethane to get (R)-
6,60-dibromo-BINOL (R)-2.
In order to find the optimal conditions for asymmetric sulfoxidation using (R)-6,60-
dibromo-BINOL as chiral ligand, methyl phenyl sulfide was chosen as a probe substrate
(Scheme 2). Different metal complexes like Ti(OiPr)4, VO(acac)2, Cu(acac)2, Fe (acac)3
were explored in an attempt to find the suitable metal complex for this system. The oxi-
dation of sulfide using VO(acac)2, Cu(acac)2 and Fe(acac)3 afforded sulfoxide with very
low enantiomeric excess (Table 1, entries 2–4). The use of Ti(OiPr)4 afforded the sulfox-
ide with good enantiomeric excess (Table 1, entry 1 and 5). Hence, Ti(OiPr)4 was
selected as the suitable metal complex.
The effect of varied mole ratios of (R)-6,60-dibromo-BINOL and Ti(OiPr)4 on the asym-
metric oxidation of methyl phenyl sulfide (Table 2) was studied. Mole ratios of (R)-6,60-
dibromo-BINOL were differed from 5 to 20mole % and mole ratios of Ti(OiPr)4 were dif-
fered from 2.5 to 10mole %. It was observed that with an increase in mole ratios, there was
no improvement in enantiomeric excess of sulfoxide. However, with the increase in mole
ratios, sulfone formation was more and due to this sulfoxide was obtained with low yield.
Based on the experimental results, 10mole % of (R)-6,60- dibromo-BINOL and 5 mole % of
Ti(OiPr)4 was selected as the optimum mole ratios for the reaction.
The impact of time and temperature on the outcome of oxidation was also studied.
The reaction proceeds by two successive steps: initially sulfide is enantioselectively oxi-
dized into sulfoxide and then oxidative kinetic resolution takes place. During kinetic
resolution, predominantly, the less favored sulfoxide enantiomer gets over oxidized to
form sulfone, which in turn enhances the enantiomeric excess of phenyl
methyl sulfoxide.
When the reaction was carried out at 0 ꢀC, sulfoxide was obtained with low enantio-
meric excess and high yield (Table 3, entries 1 and 2). Reason for low enantiomeric
excess is, at 0 ꢀC the rate of kinetic resolution is slow and ineffective. When the reaction
was carried out at 25 ꢀC, sulfoxide was obtained with high enantiomeric excess and low
yield (Table 3, entry 5). Reason for low yield is, at 25 ꢀC kinetic resolution starts taking
place even before the complete conversion of sulfide. Due to this, formation of sulfone
is more. To get a compromise between enantiomeric excess and yield, initially the reac-
tion was stirred at 0 ꢀC till the maximum conversion of sulfide took place and then
increased the temperature to 25 ꢀC to allow the kinetic resolution to take place. With
this telescopic method, sulfoxide was obtained with high enantiomeric excess and good
yield (Table 3, entry 6).
The influence of different solvents on the oxidation of methyl phenyl sulfide
(Table 4) was also explored. The oxidation of sulfide using either CH2Cl2 or hexane
(Table 4, entries 3 and 4) afforded sulfoxide with low enantiomeric excess and low
yield. The use of CCl4 (Table 4, entry 2) afforded sulfoxide with good enantiomeric

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N. V. MADDIREDDY ET AL.
Table 1. Asymmetric oxidation of methyl phenyl sulfide with different metal complexes under reac-
tion conditionsa.
Metal complex
Entry
Metal complex
(R)-2 (mol%)
(mol %)
Yieldb (%)
eec (%) (config.)d
1
2
3
4
Ti(OiPr)4
Vo(acac)2
Cu(acac)2
Fe(acac)3
10
10
10
10
5
5
5
5
72
73
75
78
85 (R)
25 (R)
16 (R)
21 (R)
aReaction conditions: Methyl phenyl sulfide (1 mmol), (R)-6,6’-dibromo-BINOL (10 mol%), metal complex, 70% aq. tertiary
butyl hydroperoxide (2 equiv.), water (0.5 equiv.), toluene, at room temperature. Monitored the reaction using TLC
until all the sulfide was consumed.
bIsolated yield after column chromatography of the crude product.
cDetermined by HPLC analysis on Chiralpak IC column.
dAbsolute configuration was determined by the comparison of the sign of the specific rotation with that of reported in
the literature.[9a]
Table 2. Asymmetric oxidation of methyl phenyl sulfide with different mole ratios of (R)-2 and
Ti(OiPr)4 under reaction conditionsa.
Ti(OiPr)4
(mol%)
Entry
(R)-2-Ti(OiPr)4 (mol/mol)
(R)-2 (mol%)
Yieldb (%)
eec (%) (config.)d
1
2
3
4
1:0.5
1:0.5
1:0.5
1:0.5
20
15
10
5
10
7.5
5
68
74
80
81
87 (R)
85 (R)
95 (R)
82 (R)
2.5
aReaction conditions: Methyl phenyl sulfide (1 mmol), (R)-6,6’-dibromo-BINOL, Ti(OiPr)4, 70% aq. tertiary butyl hydroper-
oxide (2 equiv.), water (0.5 equiv.), toluene. Monitored the reaction using TLC until all the sulfide was consumed.
bIsolated yield after column chromatography of the crude product.
cDetermined by HPLC analysis on Chiralpak IC column.
dAbsolute configuration was determined by the comparison of the sign of the specific rotation with that of reported in
the literature.[9a]
excess and good yield. The use of toluene as solvent afforded the sulfoxide with signifi-
cance enhancement in enantioselectivity (Table 4, entry 1). Moreover, toluene as solvent
offers significant safety benefits over CCl4.
The impact of different oxidants on the oxidation of methyl phenyl sulfide
was also examined (Table 5). Among these oxidants, with cumene hydroperoxide
and 30% aq. H2O2 (Table 5, entries 2 and 3), sulfoxide was obtained with moderate
yield and very low enantiomeric excess. With 70% aq. tertiary butyl hydroperoxide
(Table 5, entry 1), sulfoxide was obtained with high enantiomeric excess and good
yield. With tertiary butyl hydroperoxide (in decane), the reaction proceeded so
quickly that within 3 h at 25 ꢀC, yield of sulfoxide was decreased to as low as 30%
(Table 5, entry 4) and after 14 h at 25 ꢀC, no product could be isolated (Table 5,
entry 5). It was observed that compared to tertiary butyl hydroperoxide (in decane),
with 70% aq. tertiary butyl hydroperoxide the reaction proceeded in a controlled
way so that the formation of sulfone is considerably reduced. Based on these
results, 70% aq. tertiary butyl hydroperoxide was selected as the suitable oxidiz-
ing agent.

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Table 3. Asymmetric oxidation of methyl phenyl sulfide with varied temperature and time under
reaction conditionsa.
Entry
Temperature (ꢀC)
Time
Yieldb (%)
eec (%) (config.)d
1.
2.
3.
4.
5.
6.
0
0
25
25
25
0
9 h
14 h
4 h
9 h
14 h
5 h
88
85
81
72
62
80
30 (R)
42 (R)
69 (R)
81 (R)
92 (R)
95 (R)
25
9 h
aReaction conditions: Methyl phenyl sulfide (1 mmol), (R)-6,6’-dibromo-BINOL (10 mole%), Ti(OiPr)4(5 mole%), 70% aq.
tertiary butyl hydroperoxide (2 equiv.), water (0.5 equiv.), toluene. Monitored the reaction using TLC until all the sul-
fide was consumed.
bIsolated yield after column chromatography of the crude product.
cDetermined by HPLC analysis on Chiralpak IC column.
dAbsolute configuration was determined by the comparison of the sign of the specific rotation with that of reported in
the literature.[9a]
Table 4. Asymmetric oxidation of methyl phenyl sulfide catalyzed by (R)-2-Ti(OiPr)4 in differ-
ent solventsa.
Entry
Solvent
Toluene
CCl4
CH2Cl2
Hexane
Yieldb (%)
eec (%) (config.)d
1.
2.
3.
4.
80
72
63
67
95 (R)
74 (R)
48 (R)
52 (R)
aReaction conditions: Methyl phenyl sulfide (1 mmol), (R)-6,6’-dibromo-BINOL(10 mol%), Ti(OiPr)4 (5 mol%), 70% aq. ter-
tiary butyl hydroperoxide (2 equiv.), water (0.5 equiv.), solvent. Monitored the reaction using TLC until all the sulfide
was consumed.
bIsolated yield after column chromatography of the crude product.
cDetermined by HPLC analysis on Chiralpak IC column.
dAbsolute configuration was determined by the comparison of the sign of the specific rotation with that reported in
the literature.[9a]
In this invention, after charging Ti(OiPr)4 and (R)-6,60-dibromo-BINOL, 0.5 mole equiv.
water is added to the catalytic system. Hypothesis on the effect of water is that the addition
of water into [Ti(OiPr)2(BINOLato)] hydrolyzes a Ti-O-iPr bond to form a m-oxo bridge
between two dimers (Ti-O-Ti), which acts as active catalyst for the asymmetric oxidation.
After establishing the optimum reaction conditions for asymmetric oxidation of phe-
nyl methyl sulfide, these reaction conditions i.e., 5 mol % Ti(OiPr)4, 10 mol % (R)-6,60-
dibromo-BINOL, 0.5 mole. equiv. water, 2 mole equiv. 70% aq. tertiary butyl hydroper-
oxide, toluene as solvent, were then applied in the asymmetric oxidation of a number of
aryl alkyl and aryl benzyl sulfides. The results are summarized in Table 6.

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N. V. MADDIREDDY ET AL.
Table 5. Asymmetric oxidation of methyl phenyl sulfide catalyzed by (R)-2-Ti(OiPr)4 using differ-
ent oxidants.a
Entry
Oxidant
Yieldb (%)
eec (%) (config.)d
1.
2.
3.
4.
70% aq. TBHP (2 equiv.)
CHP (2 equiv.)
30% aq. H2O2 (2 equiv.)
TBHP (in decane)
(2 equiv.)
80
60
65
30
95 (R)
20 (R)
32 (R)
85 (R)
5.
TBHP (in decane)
(2 equiv.)
0
–
aReaction conditions: Methyl phenyl sulfide (1 mmol), (R)-6,6’-dibromo-BINOL(10 mol%), Ti(OiPr)4 (5 mol%), Oxidant,
water (0.5 equiv.), toluene. Monitored the reaction using TLC until all the sulfide was consumed.
bIsolated yield after column chromatography of the crude product.
cDetermined by HPLC analysis on Chiralpak IC column.
dAbsolute configuration was determined by the comparison of the sign of the specific rotation with that reported in
the literature.[9a]
Table 6. Asymmetric oxidation of Sulfides with (R)-2-Ti(OiPr)4 under reaction conditionsa.
eec (%)
Entry
R
R’
Yieldb (%)
(config.)d
1.
2.
3.
4.
5.
6.
Phenyl
Phenyl
p-Tolyl
4-Methoxy phenyl
4-Bromo phenyl
Phenyl
Methyl
Ethyl
Methyl
Methyl
Methyl
Benzyl
80
78
80
76
72
76
95 (R)
80 (R)
88 (R)
90 (R)
82 (R)
78 (R)
aReaction conditions: Sulfide (1 mmol),(R)-6,6’-diphenyl-BINOL (10 mol%), Ti(OiPr)4 (5 mol%), 70% aq. tertiary butyl hydro-
peroxide (2 equiv.), water (0.5 equiv.), toluene. Monitored the reaction using TLC until all sulfide was consumed.
bIsolated yield after column chromatography of the crude reaction mixture.
cFor entries 1–6, except for entry 3, chiral purity was determined by HPLC analysis on Chiralpak IC column, for entry 3
by using Chiralpak IA column.
dAbsolute configuration was determined by the comparison of the sign of the specific rotation with that reported in
the literature.[9a,22]
As shown in Table 6, we have extended the applicability of the optimized reaction condi-
tions to different alkyl aryl and aryl benzyl sulfides. Optically active sulfoxides were obtained
with high enantiomeric excesses (up to 89%) with good chemical yields (up to 80%).
Conclusion
In summary, an efficient enantioselective sulfide oxidation is effected using (R)-6,60-
dibromo-BINOL as chiral ligand. An important feature of this invention is control

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7
over the formation of over-oxidation product, sulfone. The presence of sulfone can
make the isolation of pure sulfoxide tedious and it also impacts adversely on the
overall yield of sulfoxide. With these improved reaction conditions, methyl phenyl
sulfoxide can be prepared with high enantiomeric excess (up to 95%) and good
yield (80%).
Experimental
General information
All the sulfides and all other reagents were procured from Sigma Aldrich and used dir-
ectly without additional treatment. Commercial grade solvents were used for reaction
and purification.1H NMR (500 MHz) and 13C NMR (125 MHz) were obtained as solu-
tions in deuterium substituted reagent on Bruker 500 MHz AVANCE III HD, Software-
Topspin 3.5. Chemical shifts are reported in parts per million (ppm, d). Melting points
were recorded on Buchi M-560 melting point apparatus. Specific optical rotations were
measured on Rudolph Research Analytical, Autopol V plus instrument. The ee values of
the purified products were determined by HPLC on a chiral stationary phase using
Chiralpak IA, Chiralpak IC columns with n-hexane/i-PrOH mixture as an eluent.
Absolute configurations were assigned by comparison of the sign of the specific optical
rotations with literature data.
General procedure for the catalytic oxidation of sulfides
To a solution of (R)-6,60-dibromo-BINOL (0.72g, 1.62 mmol) in toluene (20 mL) were
added Ti(OiPr)4 (0.22 mL, 0.81 mmol), water (0.14 mL, 8 mmol) and stirred for 30 min
at room temperature. To the resulting homogeneous solution was added methyl phenyl
sulfide (2.0 g, 16.0 mmol), and the mixture was stirred at room temperature for 60 min.
The solution was then cooled to 0–5 ꢀC, 70% aqueous TBHP solution (4.4 mL, 32 mmol)
was added slowly and stirred at 0–5 ꢀC for 5 h. Increased the temperature to 25–30 ꢀC
and stirred for 9 h. The reaction mixture was concentrated under reduced pressure to
obtain the crude product. This was further purified by column chromatography (ethyl
acetate, cyclohexane 1:1) to get 1.80 g, (80% yield), of (R)-methyl phenyl sulfoxide (1a).
[a]20 ¼ þ136 (c ¼ 1.7 in acetone); lit: [a]D ¼ þ130 (c ¼ 1.7 in acetone) for (R), 89%
D
1
ee;[9a] H NMR (500 MHz, CDCl3): d 2.75 (s, 3H), 7.51–7.57 (m, 3H), 7.66–7.68 (m,
2H); 13C NMR (125 MHz, CDCl3) d 43.9, 123.5, 123.5, 129.3, 129.3, 131.0, 145.6; HPLC:
tR (R) ¼ 24.1 min (major isomer), tR (S) ¼ 26.8 min (minor isomer) (Chiralpak IC col-
umn, n-Hexane/i-PrOH ¼ 80:20, flow rate ¼ 1.0 mL/min, wavelength ¼ 240 nm).
Acknowledgments
The authors are thankful to the M/S Lupin Limited and the authorities of Manipal Academy of
Higher Education for the research program. We also acknowledge the valuable guidance, support
and suggestions from Dr. P. R. Upadhyay, Dr. Milind Sindkhedkar and Dr. Vijaya Desai.

8
N. V. MADDIREDDY ET AL.
Supporting informations
Full experimental details, 1H & 13C NMR spectra of the chiral ligand [(R)-2] and sulfoxides
(1a–1f), HPLC traces of sulfoxides (1a–1f) can be found via the “Supplementary Content” section
of this article’s webpage.
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