Selective sulfoxidation with hydrogen peroxide catalysed by a titanium catalyst

Article abstract of DOI:10.1039/c4cy00965g

A moisture-tolerant cyclopentadienyl-silsesquioxane titanium complex efficiently catalyses the selective oxidation of various types of sulfides to either sulfoxides (TOFs up to 32 530 h^-1) or sulfones with H₂O₂ (30% in water) under mild conditions.

Full text of DOI:10.1039/c4cy00965g

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Selective sulfoxidation with hydrogen peroxide
catalysed by a titanium catalyst†
Cite this: DOI: 10.1039/c4cy00965g
a
b
b
b
Lorena Postigo, Maria Ventura, Tomás Cuenca, Gerardo Jiménez*
and Beatriz Royo*a
Received 25th July 2014,
Accepted 20th August 2014
A moisture-tolerant cyclopentadienyl–silsesquioxane titanium complex efficiently catalyses the selective
DOI: 10.1039/c4cy00965g
−
1
oxidation of various types of sulfides to either sulfoxides (TOFs up to 32 530 h ) or sulfones with H
(30% in water) under mild conditions.
2 2
O
www.rsc.org/catalysis
5
2
titanium complex [Ti{η -C H SiMe OPh Si O -κ O }Cl] (1)
5
4
2
7
7
11
2
Introduction
(
Fig. 1). To the best of our knowledge, this is the first report
The development of highly efficient catalytic systems for
the oxidation of sulfides is of much current interest because
the final products, sulfoxides and sulfones are important
intermediates for the synthesis of chemically and biologically
on the oxidation of sulfides by titanium silsesquioxane com-
plexes using aqueous H O as the primary oxidant. So far,
2
2
the use of titanium silsesquioxanes in oxidation reactions
10–14
has been limited to catalytic epoxidation of olefins.
1
significant molecules and also in the context of desulfurisation
2
of fuels.
The use of an environmentally benign oxidant such as Results and discussion
H O represents an important goal in oxidation reactions
2
2
We first investigated the oxidation of methylphenylsulfide
(thioanisole) as a model substrate to explore the potential of
since it is non-toxic, cheap and forms water as the only
by-product. Furthermore, the selective synthesis of sulfoxides
3
1
as a sulfoxidation catalyst and optimise the reaction condi-
tions. Such oxidation was initially performed in methanol
using a 0.5 mol% loading of 1 and 1 equiv. of H (30% in
over the fully oxidised sulfones represents another important
challenge.
2
O
2
In recent years, some homogeneous H O -based catalytic
2
2
water) at ambient temperature. We were pleased to find that
thioanisole was quantitatively and chemoselectively oxidised
to methylphenylsulfoxide in 5 min (Table 1, entry 1). The
reaction mixture was examined after 24 h, confirming that no
overoxidation process to its corresponding sulfone occurred.
In addition, blank experiments confirmed that no reaction
took place in the absence of the catalyst.
systems mediated by transition metals for the oxidation of
4
,5
sulfides have been reported in the literature.
However,
most of them present disadvantages, such as the use of excess
oxidant, low chemoselectivity, and limited applicability. Notable
among these are the efficient catalytic systems based on
6
7
8
Ti, V, and Fe using equimolecular amounts of H O .
2
2
Recently, we disclosed the excellent performance of robust
The catalyst loading could be lowered to 0.01 mol%
(Table 1, entry 4) without appreciable deterioration of the
effectiveness of the catalyst, achieving turnover frequency
cyclopentadienyl–silsesquioxane titanium complexes in olefin
9
epoxidation using H O as the oxidant. For the first time,
2
2
we demonstrated the applicability of H O as the oxidant in
2
2
−
1
(
TOF) values up to 32 530 h . These TOF numbers are among
reactions catalysed by titanium silsesquioxanes. Encouraged
by these exciting results, we decided to explore further their
catalytic activity in oxidation processes. Herein, we report an
efficient method for the selective oxidation of sulfides to
either sulfoxides or sulfones under green conditions using
H O as the oxidant and methanol as solvent with the
6
the highest reported for titanium-catalysed reactions.
2
2
a
Instituto de Tecnología Química e Biológica da Universidade Nova de Lisboa. Av.
da República, EAN, 2780-157 Oeiras, Portugal. E-mail: broyo@itqb.unl.pt
b
Departamento de Química Orgánica y Química Inorgánica, Universidad de Alcalá,
Campus Universitario, 28871 Alcalá de Henares, Spain
†
Electronic supplementary information (ESI) available: NMR and MS-ESI
spectra of the compounds. See DOI: 10.1039/c4cy00965g
Fig. 1 Titanium cyclopentadienyl–silsesquioxane complex (1).
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Table 1 Selective sulfoxidation of thioanisole to methylphenylsulfoxide
sulfides (Table 2, entries 7–9) were selectively oxidised in excel-
lent yields (85–98%) within short reaction times (5–60 min). It
is worth emphasising the high tolerance displayed by 1 toward
a
catalysed by 1
sensitive functional groups such as CC, OH, OMe, N O,
2
COOEt and COOH. However, when using this protocol for
oxidation of thiophene derivatives (benzothiophene and
dibenzothiophene), the corresponding sulfones were formed
along with the sulfoxides almost simultaneously. The low selec-
tivity observed with diaryl sulfides is a general behaviour in
b
c
Entry
Cat. (mol%)
% Yield
TOF
1
2
3
4
0.5
0.25
0.1
>99
>99
58
2385
4771
6987
32 530
1
5
electrophilic oxidation.
0.01
27
Based on these results, we decided to pursue the oxidation
of sulfides to sulfones by increasing the substrate : oxidant
molar ratio. Thus, when the oxidation of thioanisole was
performed in methanol using 0.5 mol% loading of 1 and
a
All reactions were carried out with 0.54 mmol of thioanisole and
equiv. of H (30% aqueous solution) in MeOH at ambient
1
2 2
O
b
1
temperature in 5 min. Yield determined by H NMR spectroscopy
using mesitylene (0.26 mmol) as the internal standard. TOFs
c
(turnover frequencies) calculated at 5 min of reaction.
2
2 2
equiv. of H O (30% in water) at room temperature, a mix-
ture of the sulfoxide and sulfone was observed. Nevertheless,
we were delighted to find that a slight increment of the
oxidant amount up to 2.3 equiv. and a reaction temperature
of 50 °C led to quantitative oxidation of thioanisole to
methylphenylsulfone in a reasonable short reaction time
We investigated the reuse and the selectivity of the catalyst
by adding to the reaction mixture new charges of substrate
and oxidant in a ratio of 1 : 1. Interestingly, the catalyst was
reused for up to 14 cycles without any loss in its activity and
selectivity. The total turnover attained was 2800. No sulfone
was present in the reaction mixture after being stirred for
ca. 6 h, indicating the remarkable chemoselectivity and stability
of our catalyst under these reaction conditions.
To explore the scope of 1 as a sulfoxidation catalyst, we
studied the oxidation of a wide variety of substrates with
aqueous hydrogen peroxide under the optimised reaction
conditions found for thioanisole (0.25 mol% of 1, 1 equiv. of
(
>99% yield in 3 h, Table 3, entry 1). Remarkably, a quasi-
equivalent amount of H O was sufficient to achieve the 99%
2
2
yield of sulfone, indicating the high efficiency of this oxida-
tion. The effect of the catalyst amount on the reaction rate
was evaluated in methanol; decreasing the amount of the
catalyst to 0.25 and 0.1 mol% significantly slowed down the
oxidation process (Table 3, entries 2 and 3, respectively). The
oxidation in different solvents under the optimum reaction
conditions was also investigated. As shown in Table 3, methanol
and acetonitrile yielded quantitative methylphenylsulfone
in 3 h, while lower yields were obtained in dichloromethane
and acetone (Table 3, entries 1, 4, 5, and 6, respectively).
Further, we explored the scope of the oxidation of sulfides
to sulfones with a wide variety of substrates under the
optimised reaction conditions using 0.5 mol% of catalyst 1
2 2
H O , in methanol, and at room temperature). As summarised
in Table 2, various aryl-alkyl (Table 2, entries 1–6) and dialkyl
a
Table 2 Selective oxidation of sulfides to sulfoxides catalysed by 1
b
Entry
1
Substrate
t (min)
% Yield
5
98
and 2.3 equiv. of H
(
(
O
in methanol at 50 °C. Various alkyl-aryl
Table 4, entries 1–7), dialkyl (Table 4, entries 8–10), and diaryl
Table 4, entries 11–13) sulfides were oxidised quantitatively
2
2
2
5
87
to their corresponding sulfones with high efficiency within
3
4
5
88
98
Table 3 Optimisation of reaction conditions for oxidation of thioanisole
a
to methylphenylsulfone catalysed by 1
60
5
6
30
30
88
97
b
Entry
Cat. (mol%)
% Yield
Solvent
1
2
3
4
0.5
0.25
0.1
0.5
0.5
0.5
>99
76
53
>99
56
60
MeOH
MeOH
MeOH
NCMe
7
8
5
98
85
15
c
5
2 2
CH Cl
9
15
87
c
6
Acetone
a
All reactions were carried out with 0.54 mmol of thioanisole and
a
b
All reactions were carried out with 0.54 mmol of the substrate and
equiv. of H (30% aqueous solution) in MeOH at ambient
2.3 equiv. of H O (30% aqueous solution) in MeOH in 3 h at 50 °C. Yield
determined by H NMR spectroscopy using mesitylene (0.26 mmol)
as the internal standard. Reaction time: 5 h.
2 2
1
1
2
O
2
b
c
temperature using 0.25 mol% of the catalyst. Isolated yields.
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a
Table 4 Selective oxidation of sulfides to sulfones catalysed by 1
characteristic pattern arising from the natural abundance of
b
titanium isotopes and it was assigned to the aquo peroxo-
Entry
1
Substrate
t (h)
% Yield
+
titanium species [M(O–O)(H
2
O)] , which might be the catalyti-
3
>99 (98)
>99 (70)
cally active species.† The MS-ESI experiments were performed
in acetonitrile instead of methanol (the solvent used in the
catalytic assays) due to the poor solubility of the titanium
species in methanol. Recently, Licini and Zonta have reported
an elegant mechanistic study of the oxidation of sulfides with
2
3
4
3
3
5
>99 (85)
>99 (84)
2 2
H O in methanol using a titanium(IV) aminotriphenolate
1
6
complex. Based on kinetic experiments and theoretical
calculations, they proved the fundamental role of methanol
2 2
in the activation of H O in sulfoxidation. In order to clarify
5
3
>99 (85)
if MeOH could coordinate to the titanium centre of 1, we
decided to study the stability of 1 in methanol by preparing a
solution of 1 in deuterated chloroform, adding some drops of
methanol, and monitoring the sample by NMR spectroscopy.
We observed that complex 1 was maintained intact after 24 h
in methanol at room temperature. Complex 1 also exhibited
remarkable stability towards water; wet methanol solutions of
6
7
0.5
3
>99 (95)
>99 (72)
8
9
3
3
3
>99 (98)
>99 (97)
>99 (91)
1
could be kept for 48 h without exhibiting any decomposi-
tion. These observations corroborate that methanol does not
coordinate to complex 1. The use of methanol does not seem
to accelerate the oxidation reaction, since similar activity of
catalyst 1 was found when the reactions were performed in
acetonitrile (Table 3, entries 1 and 4, respectively).
1
0
1
c
1
5
5
>99 (60)
>99 (56)
c
1
1
2
3
c
5
>99 (70)
Conclusions
In summary, the titanium complex 1 acts as a highly efficient
catalyst for the chemoselective oxidation of sulfides either to
sulfoxides or to sulfones with near-stoichiometric amount of
a
All reactions were carried out with 0.54 mmol of the substrate and
.3 equiv. of H (30% aqueous solution) with 0.5 mol% of catalyst
in MeOH at 50 °C. Yield determined by H NMR spectroscopy
2
1
2 2
O
b
1
using mesitylene (0.26 mmol) as the internal standard. The numbers
in parentheses are isolated yields. Yield determined by GC analysis,
reactions done with 2 mol% of catalyst 1. The numbers in parentheses
are isolated yields.
aqueous H
titanium silsesquioxane complex catalysing the oxidation of
sulfides under environmentally friendly (H O /methanol) and mild
2 2
O . This method represents the first example of a
c
2 2
conditions. It is worth noting that catalyst 1 showed remarkable
stability towards water, being reused for up to 14 cycles without
appreciable loss in its activity. We are currently investigating
the potential of this type of catalyst in other oxidation processes
with hydrogen peroxide.
relatively short periods of time. Results are summarised
in Table 4. Significantly, challenging compounds such as
benzothiophene (BTP) and dibenzothiophene (DBT) were also
oxidised to their corresponding sulfones in quantitative
yields (Table 4, entries 12 and 13), although a higher amount
of catalyst was required for these substrates (2 mol% of 1).
The generally accepted mechanism for titanium-based oxi-
dation catalysts involves the formation of a peroxo-titanium
Experimental section
Materials and methods
5
The compound Ti(η -C
5
H
4
SiMe
2
Cl)Cl
3
was synthesised
2
17
active species, in which the peroxo group is η coordinated to
according to the method described in the literature. All
other reagents were used as received from commercial sup-
pliers without further purification. All manipulations were
carried out under a nitrogen atmosphere using standard
Schlenk techniques and solvents were purified from appro-
priate drying agents. Deuterated solvents were degassed and
6
the metal centre. In order to gain insight into the species
formed during the course of the reaction, catalyst 1 was
treated with a 10-fold excess of H O (30% in water) in aceto-
2
2
nitrile at ambient temperature, and the reaction was followed
by MS-ESI. The MS-ESI spectrum of 1 displays an intense
peak at m/z 1096.8 corresponding to the titanium ion
1
13
29
stored over molecular sieves. H, C, and Si NMR spectra
were recorded on a Bruker Avance III 400 MHz. Electrospray
mass spectra (MS-ESI) were recorded on a Micromass Quattro
LC instrument; nitrogen was employed as drying and
nebulising gas.
+
+
[
C
49
H
45
O
12Si
8
Ti] , [M-Cl] . Immediately after addition of
H O , a new intense peak at m/z 1146.9 appeared in the mass
2 2
spectrum while the peak at 1096.8 completely disappeared
from the spectrum. The peak at m/z 1146.9 displayed the
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5
2
Synthesis of Ti(η -C H SiMe OPh Si O -κ O )Cl (1). A
5
4
2
7
7
11
2
Notes and references
solution of NEt (0.40 mL, 2.88 mmol) in CH Cl (25 mL) was
3
2
2
5
added to a mixture of Ti(η -C
5
H
4
SiMe
2
Cl)Cl
3
(0.3 g, 0.96 mmol)
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and Ph Si O (OH) (0.89 g, 0.96 mmol) and finely mashed.
7
7
9
3
The reaction mixture was heated at 70 °C and stirred for
days. The resultant suspension was cooled down to room
3
temperature and the ammonium salt was removed by
filtration. The filtrate was concentrated under vacuum and
the residue was extracted into toluene (5 × 15 mL). The
toluene solution was concentrated (35 mL) and cooled at
−
20 °C to give 1 as a pale yellow solid. Yield: 1.03 g (95%).
1
H NMR (CDCl
3 2
, 400 MHz): δ 0.43 (s, 6H; SiMe O), 6.89, 6.97
(
m, 2 × 2H; C H ), 7.33, 7.41, 7.66 (m, 14H, 7H, 14H; Ph).
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S. Hussain, D. Talukdar, S. K. Bharadwaj and M. K. Chaudhuri,
Tetrahedron Lett., 2012, 53, 6512–6515; (b) K. Kamata,
T. Hirano, R. Ishimoto and N. Mizuno, Dalton Trans.,
5
4
1
3
C NMR (CDCl ): δ 0.9 (SiMe O), 123.8, 124.1, 125.8, 125.9,
3
2
1
1
(
−
26.6, 126.9, 127.7, 127.8, 127.9, 128.0, 128.2, 128.5, 128.9,
29.1, 130.2, 130.4, 130.43, 130.5, 130.6, 130.7, 131.6, 131.9
2
9
C H and C H ). Si NMR (CDCl ): δ −6.7 (SiMe O), −70.3,
5
4
6
5
3
2
69.4, −67.3, −66.5, −65.8. Anal. calc for C49
1133.92): C, 51.86; H, 3.97. Found: C, 51.83; H, 4.01. MS
H
8
45ClO12Si Ti
(
(
+
45 8
ESI-TOF): m/z [M-Cl] calc for C49H O12Si Ti, 1097.0; found:
+
1
096.8 [M-Cl] .
2
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mesitylene (0.26 mmol, 37 μl) in MeOH (1.25 mL), 1 equiv. of
H O (30% aqueous solution, 60 μl) was added. The course of
2
2
1
the reaction was monitored by H NMR or analyzed by GC.
In the latter case, aliquots of 0.1 mL were taken at regular
intervals, treated with MnO in 0.4 mL of CH Cl , and filtered
2
2
2
over Celite previously to be analysed. The products were char-
1
13
acterized by H and C NMR spectroscopy.
Catalytic oxidation of sulfides to sulfones
A procedure similar to that described for sulfoxides was
adopted by using 2.3 equiv. of H O (30% aqueous solution,
2
2
5 Selected references for sulfide oxidation to sulfones: (a)
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1
40 μl) and heating to 50 °C.
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4
573–4576; (b) K. Bahrami, M. M. Khodaei, V. Shakibaian,
mentioned reactions were performed without mesitylene and
kept until completion of the reaction. The final mixture was
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2 2 2
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1
2
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1
8
with the reported data.
Acknowledgements
We gratefully acknowledge financial support from FCT of
Portugal, POCI 2010 and FEDER through projects PTDC/
QEQ-QIN/0565/2012, CRUP and the Ministerio de Educación y
Ciencia for Bilateral Action Program. We thank FCT for
REDE/1517/RMN/2005. M.V. thanks the University of Alcalá
for a fellowship. C. M. Almeida is acknowledged for providing
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1
1
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