The highly selective metal-free oxidation of sulfides, tellurides and phosphines using sodium bromate in the presence of recyclable ionic liquid [bmim]HSO4, at 80°C

Article abstract of DOI:10.3184/174751916X14656595396325

The metal-free oxidation of sulfides to sulfones using sodium bromate (NaBrO₃) in [bmim]HSO₄:H₂O (3:1, v/v) at 80°C is reported. Phenylalkyl, phenylbenzyl, diaryl and heteroaryl sulfides were transformed to the corresponding sulfones. Aryl tellurides and phosphines were oxidised to the corresponding telluroxides and phosphine oxides. All the reactions proceeded smoothly and gave high yields in 20-55 min. The ionic liquid [bmim]HSO₄ was easily recovered and recycled.

Full text of DOI:10.3184/174751916X14656595396325

442 RESEARCH PAPER
VOL. 40 JULY, 442–444
JOURNAL OF CHEMICAL RESEARCH 2016
The highly selective metal-free oxidation of sulfides, tellurides and
phosphines using sodium bromate in the presence of recyclable ionic liquid
[bmim]HSO4, at 80 °C
M. Rajeswari, Anshika Lumb and Jitender M. Khurana*
Department of Chemistry, University of Delhi, New Delhi-110 007, India
The metal-free oxidation of sulfides to sulfones using sodium bromate (NaBrO ) in [bmim]HSO4:H O (3:1, v/v) at 80 °C is reported.
Phenylalkyl, phenylbenzyl, diaryl and heteroaryl sulfides were transformed to the 3corresponding sulfo2nes. Aryl tellurides and phosphines
were oxidised to the corresponding telluroxides and phosphine oxides. All the reactions proceeded smoothly and gave high yields in
20–55 min. The ionic liquid [bmim]HSO4 was easily recovered and recycled.
Keywords: oxidation, sodium bromate, [bmim]HSO4, sulfones, telluroxides and phosphine oxides
Sulfones are used extensively for the preparation of a variety
of fine chemicals or speciality chemicals. Sulfones are useful
reagents in organic synthesis, particularly in asymmetric
organocatalysis1 and are also important building blocks for the
synthesis of various biologically active molecules(Fig. 1).2
Therefore, the synthesis of functionalised sulfones is of
considerable interest. A variety of oxidising agents have
been used for the synthesis of sulfones. Transition metal-
substrate. Reactions were studied by varying the molar ratio
of substrate to oxidant (NaBrO3) and also using water as a
co-solvent. Initially, a reaction of diphenyl sulfide (1a) was
examined with sodium bromate:1a using 1:1 molar ratio in the
presence of ionic liquid [bmim]HSO4 at room temperature.
There was no reaction even after 3 h, as revealed by TLC
(petroleum ether:ethyl acetate, 85:15, v/v). The starting material
was isolated (Table 1, entry 1). The same reaction was then
attempted at 80 °C under otherwise identical conditions. The
reaction was incomplete after 2 h but yielded 43% of 2a after
separation by column chromatography (Table 1, entry 2).
The same reaction, was carried out using a 1:2 molar ratio
of substrate (1a):NaBrO3 in presence of [bmim]HSO4 at 80
°C, yielded 63% of 2a in 90 min (Table 1, entry 3). The same
reaction was then carried out using 1:3 molar ratio of 1a:NaBrO3
in the presence of [bmim]HSO4 at 80 °C and yielded 71% of 2a
in 90 min (Table 1, entry 4). The best yield of 2a corresponding
to 89% was obtained when the reaction was attempted using 1:3
ratio of substrate (1a):NaBrO3 in [bmim]HSO:H2O (3:1, v/v) at
80°C. The reaction was complete in 30 min as revealed by TLC
using petroleum ether:ethyl acetate (85:15, v/v) (Table 1, entry
5). The addition of water as co-solvent appeared to enhance
the rate of reaction. While reactions carried out at a higher
temperature (entry 6) did not affect the yield, reactions carried
out using a higher amount of water as solvent, reduced the
yields (entries 7–8). It can be concluded that 1:3 molar ratio
of substrate (1a):NaBrO3 in [bmim]HSO4:H2O (3:1 mL, v/v) at
80 °C is the optimum condition for the oxidation of sulfides
to sulfones. The generality of the procedure was examined for
a variety of sulfides containing aromatic and aliphatic groups
under the optimum reaction conditions. All the reactions
based catalysts such as Si-V10-2(silica-vanadia),3 MeO3Re,4
Mn(III)Schiff-base,5 Ti(OiPr)4,
[(n-C4H9)4N]4(α-Mo8O26)7
6
and SiO2-W2-imidazole8 have been used for the preparation of
sulfones in organic solvents. Other oxidising agents include
polyvalent iodine,9 oxone,10 ozone,11 aqueous NaOCl,12 oxygen,13
NaIO414 and IBX.15 However, some of these are expensive,
require excessive use of oxidants and are difficult to prepare.
Sodium bromate is an inexpensive important oxy-halo
oxidant which has proved to be a versatile reagent in effecting
a variety of oxidation reactions,16 and for the oxidative cleavage
of alkyl and trimethylsilyl ethers and ethylene acetals to the
corresponding carbonyl compounds.17,18 Imidazolium cation
based ionic liquids have received considerable attention in
recent years19 because of their unique physical and chemical
properties such as thermal stability, recyclability and ability to
dissolve a large range of organic and inorganic compounds.20
In continuation of our interest in exploring oxidation
reactions in ionic liquids,21 we decided to explore the oxidation
of sulfides, selenides, tellurides and phosphines with sodium
bromate in the Brønsted acidic ionic liquid [bmim]HSO4 under
appropriate conditions.
Result and discussion
In this paper, we report the successful oxidation of sulfides
to the corresponding sulfones, tellurides to telluroxides and
phosphines to phosphine oxides using a 1:3 molar ratio of
substrate:sodium bromate in the presence of the ionic liquid
[bmim]HSO4:H2O (3:1, v/v) at 80°C.
The optimum reaction conditions were identified by
examining reactions using diphenyl sulfide (1a) as a model
Table 1 Optimisation for the oxidation of diphenylsulfide (1a) with sodium
bromate (NaBrO3) in [bmim]HSO4 under different conditions
Substrate:
Oxidant
Temperature/ Time/ Yield/
Entry
IL:H2O
°C
r.t.
min
180
120
90
%
a
1
2
3
4
5
6
7
8
1:1
1:1
1:2
1:3
1:3
1:3
1:3
1:3
[bmim]HSO4:H2O (4:0)
[bmim]HSO4:H2O (4:0)
[bmim]HSO4:H2O (4:0)
[bmim]HSO4:H2O (4:0)
[bmim]HSO4:H2O (3:1)
[bmim]HSO4:H2O (3:1)
[bmim]HSO4:H2O (1:1)
[bmim]HSO4:H2O (1:3)
–
80
80
43
63
71
89
90
82
75
O
N
O
O
O
S
S
S
S
80
90
S
N
N
80
30
N
100
80
30
50
Used as fungicide
Analogue of thrombocytopenia inhibitor
80
90
Fig. 1 Sufones are important building blocks in the synthesis
aNo reaction.
of various biologically active molecules.
* Correspondent. E-mail: jmkhurana@chemistry.du.ac.in

JOURNAL OF CHEMICAL RESEARCH 2016 443
O
S
S:NaBrO3 (1:3)
R S R'
R
R'
[bmim]HSO4:H2O (3:1),
80 °C
O
R = Alkyl, Aryl
R' = Alkyl, Aryl
R = Alkyl, Aryl
R'= Alkyl, Aryl
O
O
S
S
S:NaBrO3 (1:3)
[bmim]HSO4:H2O (3:1),
80 °C
Cyclic aryl sulfones
Cyclic aryl sulfides
2a–l
1a–l
Scheme 1 Oxidation of sulfides to sulfones.
Table 2 Reaction of sulfides with sodium bromate (1:3 ratio) in [bmim]HSO4:H2O (3:1, v/v) at 80 °C
Reactant
S. No
Product (2)b
Time/min
Yield/%a
R
R’
1
2
3
4
5
6
7
8
9
10
11
12
C6H5
C6H5
Diphenyl sulfone
Di(4-chlorophenyl) sulfone
Dibenzyl sulfone
4-Chlorophenyl phenyl sulfone
4-Bromophenyl phenyl sulfone
4-Methylphenyl phenyl sulfone
Benzyl phenyl sulfone
Phenyl n-propyl sulfone
n-Dodecyl phenyl sulfone
3-Bromothiophene-1,1-dioxide
2H-1,4-Benzothiazine-3(4H)-one, 1,1-dioxide
9H-Thioxanthene-9-one-10,10-dioxide
30
30
35
20
30
25
30
20
35
30
55
45
89
92
82
84
88
90
87
91
89
83
85
92
4-ClC6H4
C6H5CH2
4-ClC6H4
4-BrC6H4
4-(H3C)C6H4
C6H5CH2
C6H5
n-C5H25
3-Bromothiophene
2H-Benzo[e][1,2]thiazin-3(4H)-one
9H-Thioxanthene
4-ClC6H4
C6H5CH2
C6H5
C6H5
C6H5
C6H5
n-C3H7
C6H5
a% of yield.
bLiterature melting points and reported melting points are given in the ESI file.
Table 3 Oxidation of tellurides and phosphines with sodium bromate (1:3
proceeded smoothly and gave the corresponding sulfones in
high yields. The results are summarised in Scheme 1, (Table 2,
entries 1–12). Both cyclic and acyclic sulfides gave sulfones in
high yield.
molar ratio) in [bmim]HSO4:H2O (3:1, v/v) at 80 °C
O
NaBrO3, 80 oC
Ar Te Ar'
Ar Te Ar
'
(1)
(2)
[bmim]HSO4:H2O
4a–d
3a–d
The oxidation of diphenyl selenide was attempted under a
variety of conditions by changing the molar ratio of substrate to
NaBrO3, the ratio of the ionic liquid:H2O and the temperature.
However, all the reactions resulted into mixtures and did not
give either diphenyl selenoxide or diphenyl selenone in high
yields. Interestingly, we observed that diaryl tellurides gave the
corresponding telluroxides, using 1:3 ratio of substrate:NaBrO3
and 3:1 ratio of [bmim]HSO4:H2O at 80 °C (Eqn 1). Triaryl
and trialkyl phosphines also underwent oxidation under
these conditions to give the corresponding phosphine oxides
(Eqn 2). All the products were characterised by IR, NMR and
by comparison of their melting points with the literature data.
These results are listed in Table 3 and in the ESI.
The recyclability of [bmim]HSO4 was investigated in the
reaction of diphenyl sulfide with sodium bromate under the
same experimental conditions. The aqueous medium along
with [bmim]HSO4 obtained after separation of the product was
reused again. Comparable yields of the product were obtained
even after reusing the recovered medium for three times as
shown in Fig. 2.
NaBrO3, 80 oC
R3P
R3P
O
[bmim]HSO4:H2O
5a–c
6a–c
R = Aryl, Alkyl
S. No Reactant
Product (4, 6)b
(C6H5)2Te=O
[3-(CH3O)C6H4]2Te=O
[4-(CH3O)C6H4]2Te=O
[4-(H3C)C6H4]2Te=O
(C6H5)3P=O
Time/min Yield/%a
13
14
15
16
17
18
19
(C6H5)2Te
30
55
35
30
25
25
30
85
79
89
92
95
82
85
[3-(CH3O)C6H4]2Te
[4-(CH3O)C6H4]2Te
[4-(H3C)C6H4]2Te
(C6H5)3P
(C6H11)3P
(n-C4H9)3P
(C6H11)3P=O
(n-C4H9)3P=O
a% of yield.
bLiterature melting points and reported melting points are given in the ESI file.
PerkinElmer FTIR Spectrum-2000. 1H NMR spectra were recorded on
a Hitachi FT-NMR model R-600 (60 MHz) with CDCl3 or DMSO as
solvent and TMS as internal standard. All phosphine compounds were
purchased from Spectrochem and Sigma Aldrich and used as such. The
ionic liquid was prepared by reported procedure.22 All the sulfides23
and tellurides24 were prepared by the reported literature.
Experimental
The products were identified by m.p. and mixed m.p. (wherever
applicable) on Buchi melting point apparatus M-560, IR and NMR
spectra (see ESI). IR spectra were recorded in chloroform on

444 JOURNAL OF CHEMICAL RESEARCH 2016
M.R. and A.L. thank C.S.I.R., New Delhi for the grant of JRF
and SRF. A.L. also thanks to University of Delhi, India for the
grant of UTA.
Electronic Supplementary Information
Melting points and spectral data are available through:
stl.publisher.ingentaconnect.com/content/stl/jcr/supp-data.
Received 2 March 2016; accepted 30 April 2016
Paper 1603960 doi: 10.3184/174751916X14656595396325
Published online: 20 June 2016
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Oxidation of sulfides, tellurides and phosphines; general procedure
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Conclusion
In conclusion, we have developed a metal-free oxidation of
sulfides to sulfones using sodium bromate (NaBrO3) in [bmim]
HSO4:H2O (3:1, v/v) at 80°C. Structurally diverse phenyl alkyl,
phenyl benzyl, diaryl, and heteroaryl sulfides were transformed
to corresponding sulfones. Tellurides and phosphines are
oxidised to corresponding telluroxides and phosphine oxides.
All the reactions proceeded smoothly and gave high yields in
20–55 min.
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