Highly atom-economic, catalyst- and solvent-free oxidation of sulfides into sulfones using 30% aqueous H2O2

Article abstract of DOI:10.1039/c2gc36073j

Highly atom-efficient oxidation of sulfides into sulfones under solvent- and catalyst-free reaction conditions using a 30% aqueous solution of H ₂O₂ at 75 °C is reported. A structurally diverse set of phenyl alkyl-, phenyl benzyl-, benzyl alkyl-, dialkyl-, heteroaryl alkyl- and cyclic sulfides were transformed into sulfones regardless of the aggregate state and electronic nature of the substituents. In spite of the heterogeneous reaction mixtures throughout the work, no difficulties with stirring and reaction progress were noted. In numerous cases, only 10 mol% excess of H ₂O₂ was used, thus contributing considerably to the high atom economy of the process. Some solid substrates required a variable excess of hydrogen peroxide; however, the reactions were performed strictly without organic solvents. The transformation was demonstrated to be amenable for scale-up with both liquid and solid sulfides. In addition, isolation and purification of the crude products can be simply done with only filtration and crystallization.

Full text of DOI:10.1039/c2gc36073j

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Highly atom-economic, catalyst- and solvent-free oxidation of sulfides into sulfones
using 30% aqueous H2O2
Marjan Jereb*
Faculty of Chemistry and Chemical Technology, Aškerčeva 5, 1000 Ljubljana, Slovenia.
E-mail: marjan.jereb@fkkt.uni-lj.si; Fax: ++386-1-241-9220; Tel: ++386-1-241-9248
† Electronic supplementary information (ESI) available.
Highly atom-efficient oxidation of sulfides into sulfones under solvent- and catalyst-free
reaction conditions using a 30% aqueous solution of H2O2 at 75 °C is reported. A
structurally diverse set of phenyl alkyl-, phenyl benzyl-, benzyl alkyl-, dialkyl-,
heteroaryl alkyl- and cyclic sulfides were transformed into sulfones regardless of the
agreggate state and electronic nature of the substituents. In spite of the heterogeneous
reaction mixtures throughout the work, no difficulties with stirring and reaction progress
were noted. In numerous cases, only 10 mol% excess of H2O2 was used, thus
contributing considerably to the high atom economy of the process. Some solid
substrates required a variable excess of hydrogen peroxide; however, the reactions were
performed strictly without organic solvents. The transformation was demonstrated to be
amenable for scale-up with both liquid and solid sulfides. In addition, isolation and
purification of the crude products can be simply done with only filtration and
crystallization.
Key words: solvent free, uncatalyzed, sulfides, sulfones, atom economy, hydrogen
peroxide
Introduction
Modern chemistry has been inseparably linked with green chemistry.1 It has been still
gaining importance in all aspects of chemistry. Sustainable development in all stages
from the development to the final production cycle has become a practical need in an
over-polluted world.2 Solvent-free reaction conditions represent one of the most notable
contributions to the protection of the environment and people's health; further benefits
include enhanced cost efficiency, waste reduction and operational simplicity.3 High
atom economy is one of the most essential contributions to the overall process
efficiency.4 Uncatalyzed reactions of neat reactants are of fundamental importance in
chemistry and one of the most desired transformations. In spite of the increasing number
of studies with neat reactants, exploitation of the intrinsic reactivity of neat reactants
remains undervalued and should receive greater attention.3c
A 30% aqueous solution of H2O2 is a non-toxic, environmentally benign, cheap and
effective oxidizer, and its only residue is water. Besides oxygen, it is one of the most
'green' oxidants and its oxidative power and positive properties have been highlighted in
several publications.5
Sulfones represent an important class of compounds due to their properties and
reactivity. The strong inductive effect of the sulfone moiety and the broad possibilities
of the subsequent transformations make them very attractive in the field of asymmetric
organocatalysis.6 Numerous sulfones and their derivatives exhibit different biological
activities and have been a subject of an extensive investigation.7 Phenyl and heteroaryl
substituted sulfones are important reactants in the Julia-type olefination reaction;8 in
addition, the Grignard reagents of sulfones are useful intermediates for the synthesis of

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variously functionalized sulfones.9 1,2-Diarylethenyl sulfones serve as masked
diarylethynes, intermediates for carbon-rich materials and molecular wires.10 Good
hydrolytic stability in basic and acidic media, resistance to oxidation and corrosion,
excellent thermal stability and good electrical properties make polymeric sulfones
attractive functional materials.11
Oxidation of sulfides into sulfones has been extensively studied, and H2O2 was
13
frequently utilized in combination with different catalysts i.e. MoO3,12 CH3ReO3,
dioxo-molybdenum(VI) complex,14 NH4Cl,15 polyoxometalate-cored dendrimers,16
silica-vanadia catalyst,17 silica sulfuric acid,18 heterogeneous TiO2,19 tetra-
(tetraalkylammonium)octamolybdate,20 immobilized molybdenum heteropolyacid,21
TaC and NbC,22 sodium tungstate/PTC/phenylphosphonic acid,23 MoO2Cl2,24
Cp'Mo(CO)3Cl,25
tantalum(V)
and
niobium(V),26
cyanuric
chloride,27
[SeO4{WO(O2)}2]2–,28 composite oxide catalyst LiNbMoO6,29 carbon-based solid acid,30
1,3,4-triazo-2,4,6-triphosphorine-2,2,4,4,6,6-tetrachloride,31 H3BO3, and others. Some
32
of the other utilized oxidizers were H5IO6,33,34,35 ozone,36 an aqueous NaOCl,37
HOF·CH3CN,38 oxygen/2-methylpropanal,39 NaIO4,40 Oxone,41 and others.
As a rule, all oxidations were performed in different organic solvents. Oxidation also
worked well in the supercritical carbon dioxide as an environmentally benign reaction
medium.42,43 The uncatalyzed oxidation of sulfides in acetonitrile could be promoted by
ultrasound,44 and it could be performed in T-shape micromixer as well.45 The oxidation
of sulfides into sulfoxide could be accomplished with 30% aqueous H2O2 under solvent-
free reaction conditions;46 in addition, sulfoxidation in the presence of a chiral
aluminium(salalen) catalyst was highly enantioselective.47
Hydrogen peroxide exhibited surprisingly low reactivity in the sulfoxidation of
sulfides,48 while similar observations were made in sulfoxidation with ozone;49 further
oxidation to sulfones is even more demanding. We were interested if 30% aqueous
hydrogen peroxide alone is able to oxidize sulfides into sulfones under solvent- and
catalyst-free conditions. We report on uncatalyzed sulfonation of neat sulfides with 30%
aqueous H2O2.
Results and discussion
Initially, the reactivity of thioanisole 1a with 2.2 equivalents of 30% aqueous H2O2 was
examined at 75 °C. Full conversion to the phenyl methyl sulfone 2a was observed over
two hours, and 2a was isolated in a good yield (Table 1, entry 1). Thianisole and H2O2
form two separable phases at room temperature, but reaction took place surprisingly
well at the elevated temperature. Further, more challenging sulfides 1b-1e with
increasing hydrophobicity were tested. Sulfones 2b-2e were isolated in good yields;
however, the reaction times were growing with the hydrophobicity (entries 2–5). In the
case of 1e, the best reaction progress was noted when consecutive portions of 1
equivalent of H2O2 were added over several hours. Methoxy substituted thioanisoles 1f-
1h were effectively oxidized into the corresponding sulfones 2f-2h in high yields
(entries 6–8), and 4-methoxy derivative 1f was the most reactive. 4-Methylphenyl ethyl
sulfide 1i furnished sulfone 2i in a 65% yield (entry 9). 4-Fluoro- and 2-fluorophenyl
substituted sulfides 1j and 1k were transformed into their sulfones 2j and 2k with 2.2
equivalents of H2O2 in three hours at 75 °C. 2,4-Difluorothioanisole 1l was much less
reactive and required 8 hours for complete conversion into 2l (entry 12).
Chlorothioanisoles 1m and 1n required 3 equivalents of H2O2 for fast and efficient
conversion into 2m and 2n (entries 13 and 14), much less reactive was 1o giving 2o
(entry 15). 4-Acetylthioanisole 1p exhibited surprisingly high reactivity in this
oxidation, while 4-nitrothianisole 1q required 5 equivalents of H2O2 and 8 hours for full

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conversion into 2q (entries 16 and 17). Both sulfides are solid and reaction took place
well anyway. The trifluoroethyl group reduced reactivity in comparison with the ethyl
group (entries 18 and 9), and sulfone 2r was obtained in a moderate yield.
Phenyl cyclopropylmethyl sulfide 1s smoothly yielded 2s with 10 mol% excess of H2O2
only (entry 19). The branching of the alkyl substituent on the sulfur atom in 1t and 1u
did not hamper the oxidation into 2t and 2u (entries 20 and 21). Phenyl 2-bromoethyl
sulfide 1v was converted into sulfone 2v in good yield (entry 22). Oxidation of ethyl 2-
(phenylthio)acetate 1w proceeded with lower selectivity, and 2w was obtained in low
yield (entry 23). Oxidation of phenylthioacetonitrile 1x was surprisingly fast in
comparison with 1w, and 2x was isolated in good yield (entry 24). Prolonged reaction
time led to the less selective reaction and a lower yield of 2x.
Sulfides with unsaturated alkyl substituents were also examined in solvent-free
oxidation. Allyl phenyl sulfide 1y was smoothly converted into 2y (entry 25). The more
sensitive phenyl vinyl sulfide 1z was oxidized into 2z in a short reaction time; however,
the yield was somewhat lower (entry 26). Further, aryl and benzyl propargyl sulfides
1aa and 1bb could be selectively oxidized into 2aa and 2bb in good yields (entries 27
and 28), and prolonged reaction time deteriorated the reaction outcome. Benzyl ethyl
sulfide 1cc and benzyl butyl sulfide 1dd were efficiently oxidized into sulfones 2cc and
2dd proving benzyl sulfides to be suitable substrates for this oxidation.
Heterocyclic sulfones exhibit different biological activities and are important in
medicinal chemistry. 2-Methylthiopyridine 1ee and 1-methyl-2-methylthioimidazole 1ff
were converted into sulfone derivatives 2ee and 2ff; however, a little higher excess of
H2O2 was needed (entries 31 and 32). The reactivity of both substrates was relatively
low.
Further, the reactivity of alkyl substituted sulfides was examined. Dimethyl sulfide 1gg
and 2.2 equivalents of 30% aqueous H2O2 were stirred in a tightly closed conical vial.
Two phases turned into one after 30 minutes of stirring, and full conversion into sulfone
2gg took place in 16 hours (entry 33). Ethyl hexyl sulfide 1hh as highly nonpolar
compound was surprisigly reactive with aqueous H2O2 giving sulfone 2hh in high yield
(entry 34). Cyclohexyl ethyl sulfide 1ii and cyclohexyl butyl sulfide 1jj were efficiently
converted into sulfones 2ii and 2jj in high yields (entries 35 and 36). 2-Hydroxyethyl
butyl sulfide 1kk was selectively oxidized into sulfone 2kk, and no oxidation of
hydroxy group took place (entry 37).
The oxidation method was finally tested on several solid and substantially hydrophobic
starting sulfides. Reactions were performed at 75 °C, and some of the reagents melted
below the reaction temperature, while others remained solid. No difficulties due to the
heterogeneous reaction mixtures were noted. Solid substrates could be considered to be
demanding substrates in such a reaction, and this is particularly true for the substrates
with melting points above 75 °C. A variable excess of H2O2 was needed in these
reactions; however, reactions were conducted without organic solvent. Phenyl benzyl
sulfide 1ll was oxidized into sulfone 2ll with a three-fold excess of aqueous H2O2,
which was added consequtively in four portions (entry 38). 4-Methoxybenzyl phenyl
sulfide 1mm was efficiently oxidized into sulfone 2mm by using a three-fold excess of
H2O2 in 5 hours at 75 °C (entry 39). 4-Methoxybenzyl 4-methylphenyl sulfide 1nn
similarly furnished its sulfone 2nn in
a
good yield (entry 40). 4-
Methylthiobenzophenone 1oo and 4-methyl-4'-methylthiobenzophenone 1pp yielded
the corresponding sulfones 2oo and 2pp in high yields using a three-fold excess of H2O2
in 8 and 9 hours, respectively (entries 41 and 42).
Finally, cyclic sulfides were tested in sulfonation reaction. Oxidation of
benzo[b]thiophene 1qq produced sulfone 2qq in moderate yield (entry 43).

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Thiochroman-4-one 1rr as more polar and liquid substrate led to its sulfone 2rr in 75%
in short reaction time (entry 44). Dibenzothiophene 1ss remained solid throughout the
reaction, and oxidation to sulfone 2ss surprisingly proceeded succesfully in a
heterogeneous mixture with a nine-fold excess of H2O2 added in several portions (entry
45). Oxidation of 1ss and its derivatives is one of the most important tasks in oil
refining industries for the removal of sulfur. High environmental standards require
maximum sulfur content in ppm level,50 and this process has been intesively studied.51,
52, 53
Thioxanthen-9-one 1tt was anticipated to be a 'difficult' substrate under our
reaction conditions, due to its high melting point and low solubility. It was oxidized into
sulfone 2tt in a good yield with a nine-fold excess of H2O2 added in consecutive
portions (entry 46). 1,3-Dithiane 1uu dissolved immediately in aqueous H2O2 and
yielded disulfone 2uu in 3 hours at 75 °C (entry 47). The crude product was washed
with water and crystallized from acetone giving 59% of 2uu as white crystalls.
The reaction was demonstrated to be amenable for scale-up with liquid and solid
substrates. After 3 hours of stirring of neat thionisole 1a (20 mmol) and 5 mmol of 30%
H2O2 at 75 °C, 20 mmol of H2O2 was added. Two phases at the beggining of the
reaction slowly turned into one. After 17 h of stirring 19 mmol of H2O2, and after 47 h
additional 10 mmol of H2O2 were added. Full conversion into sulfone 2a was reached in
65 h, and the product already began to partly crystallize during the reaction. Upon
1
cooling 2a was filtered off as white crystalls. The product gave a clean H NMR
spectrum without further purification. The yield (91%) is significantly higher than on a
0.3 mmol scale (68%).
As a highly volatile substrate, dimethyl sulfide 1gg was tested. The oxidation was
performed in a tightly closed Ace pressure tube; 30% aqueous H2O2 (66 mmol) and 1gg
(30 mmol) formed two separated phases, which turned into one after 30 minutes of
heating. The mixture was stirred for 33 h at 75 °C. The tube was cooled and 3 mmol of
H2O2 were added, and the mixture was further stirred for 25 h (full conversion in 58 h).
Upon cooling, 2gg was filtered off (1.63 g). The mother liquor was concentrated in the
air furnishing additional 0.84 g, in total (88%) of dimethyl sulfone 2gg. The yield is in
comparison with 1 mmol scale (79%) considerably higher.
Scaling-up on solid substrates was tested on benzyl phenyl sulfide 1ll and on 4-
methxoxybenzyl 4-methylphenyl sulfide 1nn in a similar way as exemplified on the
case of 1ll. 2 mmol of 1ll and 4.4 mmol of H2O2 were stirred at 75 °C and after 3, 8 and
18 h 2 mmol of H2O2 were added each time. 1nn melted soon after starting of heating,
and two phases were present for approx. 2 h, and then a white solid crystallized. Full
conversion in the heterogeneous reaction mixture was achieved in 27 h. The crude
product was washed with water, filtered off and purified by crystallization from
methanol giving 2nn (0.43 g, 78%) as white solid.
Conclusions
In this paper, we describe an environmentally friendly protocol for the oxidation of
variously substituted sulfides into sulfones at 75 °C using 30% aqueous H2O2 under
solvent- and catalyst-free reaction conditions. In numerous cases, only 10% excess of
H2O2 was utilized, thus contributing to the high atom economy of the oxidation. The
transformation worked well with the both liquid and solid sulfides in spite of a
heterogenous reaction mixture, and no difficulties with stirring were noted. Highly
nonpolar, hydrophobic sulfides were also surprisingly efficiently oxidized into sulfones,
although a variable excess of aqueous H2O2 was needed. Oxidation worked well with
sulfides bearing electron-withdrawing- as well as electron-donating groups. Unsaturated
i.e. allyl-, vinyl- and propargyl sulfides were efficiently oxidized into their sulfones.

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Solid sulfides may be considered to be demanding substrates for this reaction; this is
particulary true for sulfides with melting points above 75 °C. The latter were also
oxidized under these conditions. The feasibility of the scale-up was demonstrated on
liquid and solid substrates. Since numerous sulfones are solid, they could be isolated by
filtration and purified by crystallization. This is an important, multiple advantage of this
method, since the extraction, chromatography and organic solvents could be greatly
avoided.
Experimental
Representative procedure of the non-catalyzed oxidation of sulfides into sulfones using 30%
aqueous solution of H2O2 under SFRC (small scale)
To thioanisole 1a (0.3 mmol, 37 mg) a 30% aqueous solution of H2O2 (0.66 mmol, 75
mg) was added and the mixture was stirred at 75 °C until the complete consumption of
the starting material (TLC check). The crude reaction mixture was diluted with 5 mL of
ethyl acetate, dried over anhydrous Na2SO4, and filtered off and solvent evaporated. The
crude product could be crystallized; however due to the relatively great losses on such a
scale, filtration over short pad of silica gel gave pure phenyl methyl sulfone 2a in higher
yield (32 mg, 68%).
Representative procedure of the non-catalyzed oxidation of liquid sulfide into sulfone using 30%
aqueous solution of H2O2 under SFRC (scale-up)
To thioanisole 1a (20 mmol, 2.48 g) a 30% aqueous solution of H2O2 was added (5
mmol, 0.57 g), and the mixture was heated at 75 °C with stirring for 3 h in a round-
bottom flask equipped with a reflux condenser. The mixture was cooled and 20 mmol of
30% aqueous solution of H2O2 was added (2.28 g). Two phases at the beginning of the
reaction slowly turned into one. The mixture was further heated and stirred, and
additional H2O2 was added (19 mmol, 2.17 g after 15 h and 10 mmol, 1.14 g after 40 h).
Full conversion was reached in 65 h. The product already began to crystallize in part
already during the reaction. The mixture was cooled, and the product 2a (2.87 g, 91%)
1
as white solid was filtered off. The product gave a clean H NMR spectrum and could
be used without further purification. The yield (91%) is significantly higher than on a
0.3 mmol scale (68%).
2-Fluorophenyl ethyl sulfone (2k). Colorless oil, yield 78%, (0.3 mmol of 1k, 0.66
mmol of H2O2), r.t. = 3 h. 1H NMR: δ 1.31 (t, J = 7.5, 3H), 3.33 (q, J = 7.5, 2H), 7.23–
19
7.28 (m, 1H), 7.33–7.38 (m, 1H), 7.64–7.69 (m, 1H), 7.93–7.98 (m, 1H); F NMR: δ -
109.6 (m, 1F); 13C NMR: δ 7.1, 50.0 (d, J = 2.6 Hz), 117.1 (d, J = 21.4 Hz), 124.8 (d, J
= 3.8 Hz), 126.3 (d, J = 14.9 Hz), 130.8, 136.1 (d, J = 8.4 Hz), 159.6 (d, J = 255.4 Hz);
IR (neat) 1473, 1451, 1317, 1264, 1225, 1143, 1123, 1072, 826, 766, 738 cm-1; HRMS:
(ESI) calcd for C8H10FO2S 189.0386, found 189.0386 (M+1).
4-Methylphenyl-(2,2,2-trifluoroethyl) sulfone (2r). White crystals, yield 58%, (0.2
mmol of 1r, 0.44 mmol of H2O2 after 0 h and after 1 h), r.t. = 3.5 h, mp 89.5–91.2 °C.
1H NMR: δ 2.48 (s, 3H), 3.88 (q, J = 9.0 Hz, 2H), 7.40 (d, J = 8.1 Hz, 2H), 7.85 (d, J =
8.1 Hz, 2H); 19F NMR: δ -61.9 (t, J = 9.0 Hz, 3F), 13C NMR: δ 21.7, 58.5 (q, J = 31.2
Hz), 121.1 (q, J = 277.9 Hz), 128.5, 130.1, 135.5, 146.1; IR (neat): 1342, 1316, 1303,
1269, 1253, 1248, 1149, 1125, 1081 cm-1; HRMS: (ESI) calcd for C9H10F3O2S
239.0354, found 239.0357 (M+1). Anal Calcd for C9H9F3O2S: C, 45.38; H, 3.81.
Found: C, 45.83; H, 3.59.

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4-Methoxyphenyl propargyl sulfone (2aa). White crystals, yield 82%, (0.3 mmol of
1aa, 0.50 mmol of H2O2 after 0 h and 0.25 mmol after 0.5 h), r.t. = 2.2 h, mp 76.6–79.3
°C. 1H NMR: δ 2.37 (t, J = 2.7 Hz, 1H), 3.90 (s, 3H), 3.94 (d, J = 2.7 Hz, 2H), 7.04 (d, J
= 8.9 Hz, 2H), 7.91 (d, J = 8.9 Hz, 2H); 13C NMR: δ 48.6, 55.7, 71.9, 76.0, 114.3,
129.0, 131.1, 164.2; IR (neat): 3267, 2950, 1595, 1579, 1497, 1365, 1323, 1305, 1298,
1264, 1246, 1229, 1217, 1171, 1132, 1086, 1016, 881, 834, 826, 804, 756, 697 cm-1;
HRMS: (ESI) calcd for C10H11O3S 211.0429, found 211.0426 (M+1). Anal Calcd for
C10H10O3S: C, 57.13; H, 4.79. Found: C, 57.32; H, 4.49.
Cyclohexyl butyl sulfone (2jj). Colorless oil, yield 87%, (0.3 mmol of 1jj, 0.66 mmol of
H2O2), r.t. = 4 h. 1H NMR: δ 0.96 (t, J = 7.4 Hz, 3H), 1.18–1.36 (m, 3H), 1.43–1.60 (m,
4H), 1.71–1.76 (m, 1H), 1.78–1.86 (m, 2H), 1.91–1.98 (m, 2H), 2.12–2.19 (m, 2H),
2.84 (tt, J = 12.2, 3.5 Hz, 1H), 2.88–2.93 (m, 2H); 13C NMR: δ 13.6, 21.9, 23.3, 25.0,
25.1, 25.1, 49.1, 60.7; IR (neat): 2935, 2859, 1453, 1296, 1264, 1126, 1110 cm-1;
HRMS: (ESI) calcd for C10H21O2S 205.1262, found 205.1259 (M+1).
Acknowledgement
Dr. D. Žigon at the Mass Spectroscopy Centre at the ‘Jožef Stefan’ Institute in Ljubljana
for HRMS, Mrs. T. Stipanovič and Prof. B. Stanovnik for the elemental combustion
analyses and Ministry of Higher Education, Science and Technology (P1-0134) for
financial support are gratefully acknowledged.
Notes and references
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Graphical abstract
Highly atom-economic, solvent- and catalyst-free oxidation of wide variety of liquid
and solid sulfides into sulfones with 30% aqueous H2O2 is reported.

Green Chemistry
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Table 1 Solvent- and catalyst-free oxidation of
a
sulfides with 30% aqueous H2O2
Product
t (h) Yield
(%)b
1
2
3
4
5
2a
2b
2c
2d
2e
2
3.2
68
65
5.75 64
8.25 69
11
79
6
2f
1.5
95
7
2g
2.5
87
8
2h
3.75 80
9
2i
3
3
3
65
80
78
10
11
2j
2k
12
2l
8
80
79
13
14
2m
2n
1.5
1.75 93
12.5 50
15
2o
16
17
2p
2q
2
8
82
85
18
19
2r
2s
3.5
3.5
58
85
20
21
22
23
24
25
26
2t
2.5
2.5
3
78
84
72
41
2u
2v
2w
2x
2y
2z
12
2.25 68
4.5 80
0.85 65

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27
2aa
2.2
82
28
29
30
2bb
2cc
1.75 77
3
4
80
74
2dd
31
2ee
2ff
11
77
32
8
50
79
33
34
35
36
37
38
39
2gg
2hh
2ii
16
0.85 90
3
4
83
87
2jj
2kk
2ll
0.75 79
4
5
81
81
2mm
40
41
42
2nn
2oo
2pp
5
8
9
82
87
87
43
2qq
3
46
O
44
45
2rr
2ss
1.5
17
75
81
S
O2
46
47
2tt
17
3
79
59
O2
S
2uu
SO2
a Reaction conditions: 1 (0.2 or 0.3 mmol), 30% aq. H2O2
(2.2 or more equiv.) stirred at 75 °C. b Isolated yields.