An efficient solvent free catalytic oxidation of sulfides to sulfoxides with hydrogen peroxide catalyzed by a binaphthyl-bridged Schiff base titanium complex

Article abstract of DOI:10.1016/j.tetlet.2006.07.133

A titanium binaphthyl-bridged Schiff base complex proved to be an efficient catalyst for the hydrogen peroxide oxidation of aromatic and aliphatic sulfides to the corresponding sulfoxides in satisfactory yields under solvent-free conditions.

Full text of DOI:10.1016/j.tetlet.2006.07.133

Tetrahedron Letters 47 (2006) 7233–7235
An efficient solvent free catalytic oxidation of sulfides
to sulfoxides with hydrogen peroxide catalyzed by a
binaphthyl-bridged Schiff base titanium complex
Margherita De Rosa, Marina Lamberti, Claudio Pellecchia, Arrigo Scettri,
Rosaria Villano and Annunziata Soriente*
`
Dipartimento di Chimica, Universita degli Studi di Salerno, via S. Allende, I-84081 Baronissi, Salerno, Italy
Received 23 June 2006; revised 24 July 2006; accepted 25 July 2006
Available online 22 August 2006
Abstract—A titanium binaphthyl-bridged Schiff base complex proved to be an efficient catalyst for the hydrogen peroxide oxidation
of aromatic and aliphatic sulfides to the corresponding sulfoxides in satisfactory yields under solvent-free conditions.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
and halogen-free conditions,9 an iron tetrakis-(pentaflu-
orophenyl)-porphyrin complex,10 HPA having Keggin
structure,11 a silica-based tungstate interphase cata-
lyst.12 A variety of Mt-(salen) complexes13–15 are able
to catalyze the asymmetric oxidation of sulfides to
sufoxides with hydrogen peroxide, as the terminal
oxidant, but a drawback of these convenient methods
is often the use of chlorohydrocarbon solvents or anhy-
drous H2O2 in ethanol6 and the over-oxidation to sulf-
ones as side products.
The oxidation of sulfides is the major method for the
synthesis of sulfoxides which are versatile intermediates
in organic synthesis (usable for C–C bond formation1
and functional group transformations2) and are useful
for the preparation of biologically and medically impor-
tant compounds.3
Although many reagents are available for the oxidation
of sulfides to sulfoxides,4 the catalytic processes with
environmentally benign oxidants have become increas-
ingly important.
Recently, we became interested in the use of titanium
(salen)-catalysts in oxidation reactions and we reported
the preparation and the characterization of a new octa-
hedral titanium complex bearing a bulky binaphthyl-
bridged Schiff base ligand (rac-1, Fig. 1) and we showed
that the C1-symmetric cis-b isomer is preferentially
formed.16
Apart from molecular oxygen, recently, hydrogen per-
oxide has been a very attractive ‘green oxidant’ offering
the advantages that it is cheap, environmentally benign,
readily available and produces only water as a by-prod-
uct.5 Nevertheless, hydrogen peroxide alone has to be
used carefully due to the possibility of an over-oxidation
reaction.6
Cl
Many catalysts have been specifically developed for
hydrogen peroxide: tungsten catalyst systems,7 a di-
nuclear iron complex,8 a tungsten catalyst coupled with
an acidic phase-transfer catalyst under organic solvent
tBu
N
N
O
Ti
O
Cl
tBu
Keywords: Salen ligands; Ti(IV) complexes; Sulfoxidation; Hydrogen
peroxide; Sulfoxide; Oxidation.
cis−β-1
Figure 1. The titanium binaphthyl-bridged Schiff base complex 1.
*
Corresponding author. Tel.: +39 89965372; fax: +39 89965296;
e-mail: titti@unisa.it
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.07.133

7234
M. De Rosa et al. / Tetrahedron Letters 47 (2006) 7233–7235
Table 1. Oxidation of methyl p-tolyl sulfide using 30% aqueous H2O2
catalyzed by 1
Table 2. Oxidation with 30% aqueous H2O2 of sulfides to sulfoxides
catalyzed by 1
Entry Catalyst Reaction time Yield of
Yield of
O
(mol %) (min)/T (°C) Sulfoxide (%) Sulfone (%)
S
H2O2 (1 eq.), 1 (0.5 mol %)
S
R1
R2
R1
R2
1a
2
3
4
5
6
1
1
—
0.5
0.5
0.5
1200/rt
15/rt
180/rt
15/rt
120/4
360/4
60
80
39
82
72
92
10
12
—
12
4
solvent free
2
3
Entry R1
R2 Time Yield of
Yield of
(min) sulfoxide (%) sulfone (%)
1
2
3
4
5
6
7a
o-(MeO)C6H4
p-(MeO)C6H4
p-ClC6H4
p-(NO2)C6H4
p-(NO2)C6H4
Me(CH2)7
Me 15
Me 15
Me 15
Me 15
Me 120
Me 15
15
95
86
77
39
74
83
68
2
8
8
a CH2Cl2 was used as the solvent in the reaction.
11
3
4
13
—
We also reported some preliminary results concerning
its catalytic activity in the epoxidation of allylic alco-
hols: the epoxyalcohols were obtained in very high regio
and chemoselective way by the MW exposure of the
mixture in solvent free-conditions.16
S
S
a Diastereoselectivity trans/cis 98/2 determined by 1H NMR analysis
(400 MHz) on the crude reaction mixture.
In the present work we wish to report an efficient, sol-
ventless protocol in which H2O2 has been used as the
oxidizing agent in the presence of the complex 1 for
the chemoselective oxidation of sulfides to their sulfox-
ides in high chemical yield and short reaction times.
(cfr. entries 4 and 5). As shown in entry 6, the catalyst
1 under the same reaction conditions is also effective
in the oxidation of a dialkyl sulfide affording the corre-
sponding sulfoxide in excellent yield and in a very short
time. At last, when the reaction was performed on the
cyclic thioacetal (entry 7), an exclusive process of mono-
sulfoxidation was found to take place with high level of
trans-diastereoselectivity.
Initially, we performed a set of preliminary experiments
on methyl p-tolyl sulfide as a model substrate using 30%
aqueous hydrogen peroxide in the presence of a catalytic
amount of 1 at room temperature under solvent free
conditions. The results are depicted in Table 1.
The reaction was initially carried out in CH2Cl2 (entry 1)
using 1% mol of catalyst and produced the sulfoxide in
60% yield while, interestingly, under solvent free condi-
tions (entry 2), methyl p-tolyl sulfoxide was obtained in
80% yield and sulfone in 12% yield after only 15 min. A
reduction of catalyst loading (entry 4) did not cause a
lowering of yield and chemoselectivity. It is noteworthy
that in a blank experiment (entry 3), no significant
oxidation was observed under similar reaction condi-
tions in the absence of 1, and only a low conversion
was observed in the presence of 2 equiv of H2O2 after
prolonged reaction times.
In conclusion, the results reported in this letter clearly
indicate that using as low as 0.5% mol of catalyst 1 it
is possible to obtain a mild and chemoselective synthesis
of sulfoxides from both aromatic and aliphatic sulfides.
The procedure is solvent-free and requires 1 equiv of
benign oxidant H2O2. Lastly, very short reaction times
are needed to obtain a high yield of sulfoxides.
2. Catalyst preparation
The catalyst 1 was prepared as reported in Ref. 16.
In order to improve the chemoselectivity, the reaction
was carried out at the lower temperature of 4 °C and
under these conditions the sulfoxide was isolated in higher
yield simply by prolonging the reaction time (entries 5
and 6) while the occurrence of the process of over-oxida-
tion to sulfone was reduced in a not negligible way.
3. General procedure for the oxidation of sulfides
to sulfoxides
A 30% hydrogen peroxide solution (1 equiv, 1 mmol)
was added to a vial containing the sulfide 2 (1 mmol)
and complex 1 (0.5%, 0.005 mmol). After stirring at
room temperature for the reported time, 10% solution
of Na2SO3 (2 ml) was added. The mixture was extracted
with AcOEt and the combined organic layers were dried
over anhydrous Na2SO4 and concentrated in vacuo. The
remaining oil was purified by chromatography on silica
gel (Etp/AcOEt 1/1 v/v).
To study the scope of this procedure, a series of various
substituted sulfides including alkyl aryl sulfides and dialk-
yl sulfides was reacted according to optimized reaction
conditions (Table 2).
The results are reported in Table 2: it can be observed
that in the series of methyl aryl sulfides (Table 2, entries
1–5) the yields of sulfoxide are high and the electronic
nature and the position of the substituents have a mini-
mal effect on the efficiency of the reaction, although the
presence of a nitro group on the aromatic ring required
longer reaction time to obtain comparable efficiency
Acknowledgements
This work was supported by the Italian Ministry of
Education (MIUR).

M. De Rosa et al. / Tetrahedron Letters 47 (2006) 7233–7235
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