Mild, Selective, and Efficient Oxidation of Sulfides to Sulfoxides Catalyzed by Mn(III)-Salen Complexes

Article abstract of DOI:10.1080/10426507.2014.952005

A detailed optimization of the catalytic environment for the oxidation of sulfides promoted by Mn(salen)/H₂O₂ systems is proposed. The experimental setup is demonstrated to be active towards different substrates, providing TOFs up to 9300 h^-1, which are among the highest reported for homogeneous catalysts employed in sulfoxidation reactions.

Full text of DOI:10.1080/10426507.2014.952005

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Phosphorus, Sulfur, and Silicon and the Related
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Mild, selective and efficient oxidation of sulfides to
sulfoxides catalyzed by Mn(III)-salen complexes
Alessandro Bismutoa, Maria E. Cucciolitoab, Francesco Ruffoab, Aldo Vitaglianoab
Massimiliano Curcioc
&
a Dipartimento di Scienze Chimiche, Università di Napoli “Federico II”, Complesso
Universitario Monte S. Angelo, Via Cintia, I-80126 Napoli, Italy.
b Consorzio Interuniversitario di Reattività Chimica e Catalisi, via Celso Ulpiani 27, 70126
Bari (Italy)
c Institute for Polymers, Composites and Biomaterials (IPCB), National Research Council of
Italy (CNR), P.le Fermi 1, I-80055 Portici (Napoli), Italy.
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Accepted author version posted online: 20 Aug 2014.
To cite this article: Alessandro Bismuto, Maria E. Cucciolito, Francesco Ruffo, Aldo Vitagliano & Massimiliano Curcio (2014):
Mild, selective and efficient oxidation of sulfides to sulfoxides catalyzed by Mn(III)-salen complexes, Phosphorus, Sulfur, and
Silicon and the Related Elements, DOI: 10.1080/10426507.2014.952005
To link to this article: http://dx.doi.org/10.1080/10426507.2014.952005
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MILD, SELECTIVE AND EFFICIENT OXIDATION OF SULFIDES TO
SULFOXIDES CATALYZED BY Mn(III)-SALEN COMPLEXES
Alessandro Bismuto,a,† Maria E. Cucciolito,a,b Francesco Ruffo,a,b,1* Aldo
Vitaglianoa,b and Massimiliano Curcioc,†
a
Dipartimento di Scienze Chimiche, Università di Napoli “Federico II”,
Complesso Universitario Monte S. Angelo, Via Cintia, I-80126 Napoli, Italy.
b Consorzio Interuniversitario di Reattività Chimica e Catalisi, via Celso Ulpiani
27, 70126 Bari (Italy)
c Institute for Polymers, Composites and Biomaterials (IPCB), National Research
Council of Italy (CNR), P.le Fermi 1, I-80055 Portici (Napoli), Italy.
†These authors contributed equally.
*Address correspondence to Francesco Ruffo. E-mail: ruffo@unina.it.
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GRAPHICAL ABSTRACT
N
O
N
O
Mn
X
O
S
H2O2
S
R
R
R1
R2
R1
R2
R
R
catalysts (R= H or But)
Abstract A detailed optimization of the catalytic environment for the
oxidation of sulfides promoted by Mn(salen)/H2O2 systems is proposed. The
experimental setup is demonstrated to be active towards different
substrates, providing TOFs up to 9300 h1, which are among the highest
reported for homogeneous catalysts employed in sulfoxidation reactions.
Keywords Manganese; salen; epoxidation; sulfides; sulfoxides
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INTRODUCTION
Catalytic oxidation of sulfides provides a forthright synthetic strategy for preparing
sulfoxides,17 which are relevant intermediates of a wide range of chemically and biologically
active molecules.813
Over the last two decades, Mn(salen) complexes1416 have demonstrated their efficiency by
covering a broad portion in the field of oxidation reactions, coupled together with a large number
of oxidants.17, 18 Hydrogen peroxide is among the most convenient ones, for it reacts without
providing any toxic by-product and it can be smoothly employed, even on an industrial
scale.1921
Despite literature displays several studies regarding enantioselection enhancements in
Mn(salen)/H2O2 promoted sulfide oxidation,22,23 a thorough optimization of the reaction
variables in order to achieve the highest activity had yet to be carried out. Herein we report the
results related to the optimization of the oxidation conditions for a wide scope of sulfides,
obtained via a thorough screening of solvents, additives and complexes. As described below, this
study has led to achieve unprecedented TOFs for homogenously catalyzed sulfoxidation
reactions.
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RESULTS AND DISCUSSION
The oxidation of thioanisole (S-1) to methylphenyl sulfoxide (SO-1) using complex rac-
[MnIII(salen)]Cl (I-Cl) as catalyst was taken as reference reaction (Figure 1).
Preliminary tests were carried out in methanol in the conditions reported in Table 1.
Hydrogen peroxide is not active in the absence of any metal catalyst or in the presence of simple
Mn(OAc)2 (entries 1 and 2). The performance of I-Cl was already significant under these not
optimized conditions, as a 0.025% loading led to a good conversion of the substrate within 40
min (entry 8). The resulting TOF (3600 h-1) is already among the highest reported so far for
homogeneous catalysts.2426
The convenient conditions of entry 6 were selected for the screening of solvents shown in
Table 2. This catalyst loading was chosen because the corresponding conversion is useful to
allow a clear distinction of the effects within the reaction media.
The negligible conversion in DCM (entry 2) can be attributed to the non-miscibility of the
solvent with the oxidizing agent. The highest conversions were observed in protic polar solvents
(entries 1 and 5). This is plausibly due to their capability in stabilizing the polar TS, which leads
to a reduced activation energy.
Hence, methanol was considered as the solvent of choice for the successive optimizations.
It is also known that the presence of an additional ligand plays a fundamental role in
oxidation catalysis by influencing the redox potential of the couple Mn(V)/Mn(III).27 Therefore,
a thorough screening was carried out by using several aliphatic or aromatic nitrogen heterocycles
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(table 3). These ligands, which are easily available and inexpensive, were selected in order to
have a homogeneous set with finely modulated basic features.
By using the catalyst loading of 0.1 mol% (entries 1-14), imidazole and N-methylimidazole
were found to be the most effective, promoting very similar conversions (entries 5 and 6).
The conditions were further refined (entries 15-22) by optimizing both the imidazole :
substrate ratio (A/S1) and the catalyst loading. The effect of pH (entries 21 and 22) was also
taken into account. The experimental setup of entry 20 promoted a 62% conversion with a very
convenient catalyst loading (0.010%). The corresponding TOF is 9300 h1.
These conditions were then used for a screening of ligands (28-32), which do present steric
hindrance modulated by the presence of t-butyl groups in positions 3 and 5 of the aromatic rings
(Table 4).
This screening, although it is not immediately to rationalize, allows the identification of I-
Cl as the most active catalyst within the set. Finally, to assess the effects of the ion pair and
modulate the activity of the catalyst as a function of different counterions, other complexes of
type I were synthesized and used in catalysis (Table 5).
With the exception of the low performing bromide, the data indicate that a good
coordinating ligand enhances the activity of the catalyst. This result can be explained by
admitting that the reaction product may coordinate the metal center, thus inhibiting its activity. In
order to validate this hypothesis, some kinetic studies were addressed to disclose the effects of
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the catalyst concentration on the activity. Figure 2 shows that the catalyst undergoes a faster
deactivation at low concentration (▲). This evidence can be interpreted by assuming that the
sulfoxide has a sufficient coordinating behaviour to compete with the other reactants.
To support this hypothesis, the effect of added DMSO (as DMSO-d6) was investigated
(Figure 3). As postulated, the conversion decreased from complete to ~30% within 20 minutes,
confirming that the metal centre is deactivated by the sulfoxide coordination.
Considering the pronounced catalyst deactivation in diluted solutions, the substrates
screening was carried out with a catalyst concentration of 0.125 mM. The reactions were
quenched after 20 min in order to compare the activities to the complete conversion of
thioanisole reported in figure 2.
These results show that the catalytic system is active towards a wide scope of substrates
with different stereoelectronic properties, thus validating the suitability of the presented system
for the synthesis of a large variety of sulfoxides. The trend within p-substituted aryl methyl
sulfides is in general agreement with the electronic properties of the substituent, i.e. the
conversion increases with its donor ability. A remarkable exception is given by S-6, whose
amino function plausibly coordinates to the metal center. The comparison among S-1, S-8 and S-
12 discloses the effects due to an increase of the steric hindrance.
In conclusion, we have achieved a detailed optimization of the catalytic environment for
the Mn(salen) promoted sulfides oxidation. The experimental setup proved to be active towards
different substrates, providing TOFs among the highest reported for homogeneous catalysts
employed in this reaction.
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EXPERIMENTAL
NMR spectra were recorded on Bruker Avance 400 MHz and Varian VXR 200 MHz
spectrometers and the reported chemical shifts in parts per million (ppm) are relative to internal
1
CHCl3 (7.26 ppm for H). Solvents and reagents were purchased from Sigma-Aldrich and were
used as received. The catalysts were prepared according to literature procedures (28-32).
Catalytic experiments were carried out as follows: the sulfide, the additive and H2O2 were
added in this order to a solution of the catalyst in 1 mL of MeOH. The relative amounts are
reported in the footnotes of the Tables. After the chosen time, the reaction was quenched with a
saturated aqueous Na2SO3 solution (1 mL). The mixture was extracted with diethyl ether (4x5
mL) and the organic phase was dried over anhydrous Na2SO4. The solvent was removed under
vacuum and the sample dissolved in CDCl3 for the 1H-NMR analysis.
Kinetic experiments were run as follows: methyl phenyl sulphide (S-1), N-
methylimidazole, DMSO-d6 and H2O2 were added in this order to a solution of I-Cl in CD3OD.
The relative amounts are reported in the footnotes of Figures 2 and 3. At fixed intervals of time,
0.100 mL of the reacting mixture were taken and added to a CDCl3 solution (0.40 mL). The
resulting solution was immediately analysed through 1H NMR spectroscopy.
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Gaillon, L.; Bedioui, F. Chem. Commun. 2001, 1458-1459.
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CF3SO3Ag solution.
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O
O
S
N
O
N
O
Mn
Cl
S
S
I-Cl, H2O2
tBu
tBu
O
tBu
tBu
SO2-1
SO-1
S-1
I-Cl
Figure 1. Oxidation of thioanisole (S-1) to methyl phenyl sulfoxide (SO-1). Sulfone SO2-1 is the
by-product.
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Figure 2. Kinetic trends for I-Cl at different substrate:catalyst ratios. ♦ S-1:0.25 mmol; H2O2:
0.35 mmol; A5: 0.25 mmol; I-Cl: 1.25×104 mmol; ▲ S-1: 0.25 mmol; H2O2: 0.35 mmol; A5:
0.25 mmol; cat: 2.5×105 mmol; 1 mL CD3OD.
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Figure 3. Deactivation study: ▲ S-1: 0.25 mmol; H2O2: 0.35 mmol; A5: 0.25 mmol; DMSO-d6:
0.125 mmol; I-Cl: 1.25×104 mmol. 1 mL CD3OD.
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Table 1 Oxidation of thioanisole with complex I-Cl.a
H2O2/S-
SO-1 : SO2-
Entry
Catalyst (mol%)
Conversionb
1
1
1
2
3
4
5
6
7
8
9
-
1.4
1.4
1.4
1.1
2.0
1.4
1.4
1.4
1.4
<5%
6%
-
Mn(OAc)2 (0.100)
I-Cl (0.200)
I-Cl (0.100)
I-Cl (0.100)
I-Cl (0.100)
I-Cl (0.050)
I-Cl (0.025)
I-Cl (0.010)
>99:1
>99:1
>99:1
99:1
87%
76%
91%
87%
66%
60%
12%
>99:1
>99:1
>99:1
>99:1
a Reaction conditions: 0.25 mmol S-1, RT, 40 min, 1 mL MeOH.
b Evaluated by 1H NMR spectroscopy.
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Table 2 Solvents screening.a
Entr
Solvent (1 mL)
y
Conversionb
SO-1 : SO2-1
1
2
3
4
5
6
7
8
9
MeOH
DCM
87%
<1%
4%
>99:1
-
MeCN
>99:1
-
THF
<1%
64%
34%
23%
14%
21%
EtOH
>99:1
>99:1
>99:1
>99:1
>99:1
MeOH:hexane 1:1 v/v
MeOH:hexane 1:2 v/v
MeOH:hexane 1:9 v/v
MeOH:H2O 1:1 v/v
a Reaction conditions: S-1: 0.25 mmol; H2O2: 0.35 mmol; I-Cl:2.5×104 mmol, RT, 40 min.
b Evaluated by 1H NMR spectroscopy.
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Table 3 Additives screening.a
O
N
H
N
H
N
N
N
N
N
NH
A4
N
N
H
A6
O
A1
A2
A3
A5
N
N
N
N
N
N
N
N
O
N
N
H
H
Cl
A7
A8
A9
A10
A11
A12
A13
I-Cl
A/S-
SO-1 : SO2-
1
Entry
Additive
Conversionb
(mol%)
1
1
-
0.100
0.100
0.100
0.100
0.100
0.100
0.100
0.100
0.100
0.100
0.100
0.100
0.100
0.100
0.050
0.050
0.050
0.050
-
87%
60%
72%
55%
100%
100%
67%
65%
22%
94%
78%
86%
53%
32%
87%
92%
100%
83%
>99:1
>99:1
>99:1
>99:1
91:9
2
0.60
0.60
0.60
0.60
0.60
0.60
0.60
0.60
0.60
0.60
0.60
0.60
0.60
0.30
0.60
1.00
1.20
A1
A2
A3
A4
A5
A6
A7
A8
A9
A10
A11
A12
A13
A4
A4
A4
A4
3
4
5
6
85:15
98:2
7
8
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
99:1
9
10
11
12
13
14
15
16
17
18
97:3
95:5
92:8
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19
20
0.010
0.010
1.00
34%
>99:1
>99:1
A4
A5
62% c
1.00
21
22
A5d
A5d
0.010
0.010
1.00
1.00
24%
15%
>99:1
>99:1
a Reaction conditions: S-1: 0.25 mmol; H2O2: 0.35 mmol, 1 mL MeOH, RT, 40 min.
b Evaluated by 1H NMR spectroscopy.
c TON = 6200, TOF = 9.3×103 h1.
2
d Performed by adding 0.100 mL of 1M HOAc/OAc (entry 21) and 1M HPO4 /PO43 (entry 22)
aqueous solution.
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Table 4 Ligands screening.a
R1 = R2 = R3 = tBu
R1, R2 = H; R3 = tBu
R1 = R2 = tBu; R3 = H
R1 = R3 = tBu; R2 = H
R1 = R2 = R3 = H
I-Cl
N
N
II-Cl
III-Cl
IV-Cl
V-Cl
Mn
R1
O Cl O
R2
R3
R3
Entry
Catalyst
Conversionb
62%
SO-1 : SO2-1
>99:1
1
2
3
4
5
I-Cl
II-Cl
III-Cl
IV-Cl
V-Cl
23%
>99:1
12%
>99:1
48%
>99:1
23%
>99:1
a Reaction conditions: S-1:0.25 mmol; H2O2: 0.35 mmol; A5: 0.25 mmol; cat: 2.5×105 mmol, 1
mL MeOH, RT, 40 min.
b Evaluated by 1H NMR spectroscopy.
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Table 5 Screening of counterions.33, a
Entry
Catalyst
I-Cl
Conversionb
62%
SO-1:SO2-1
>99:1
1
2
3
4
I-PF6
42%
>99:1
I-CF3SO3
18%
>99:1
I-Br
11%
>99:1
a
Reaction conditions: S-1: 0.25 mmol; H2O2: 0.35 mmol; A5: 0.25 mmol; cat : 2.5×105 mmol,
1 mL MeOH, RT, 40 min.
b Evaluated by 1H NMR spectroscopy.
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Table 6 Substrates screening.a
S
S
S
S
S
MeO
O2N
S
Cl
Br
S-1
S-2
S-3
S-4
S-5
S
S
S
S
Me(CH2)11
Et
MeOC
H2N
S-7
S-9
S-10
S-8
S-6
S
S
S
Ph
Bn
S-11
S-12
S-13
Entry
1
Substrate
S-1
Conversionb
100%
71%
SO:SO2
>99:1
97:3
TOF (h1)
6000
4260
3360
3900
4440
2040
3120
5520
6000
3960
6000
3600
3000
2
S-2
3
56%
96:4
S-3
4
65%
96:4
S-4
5
74%
95:5
S-5
6
34%
>99:1
97:3
S-6
7
52%
S-7
8
92%
>99:1
>99:1
97:3
S-8
9
100%
66%
S-9
10
11
12
13
S-10
S-11
S-12
S-13
100%
60%
95:5
>99:1
50%
95:5
a Reaction conditions: S: 0.25 mmol; H2O2: 0.35 mmol; A5: 0.25 mmol; I-Cl:1.25×104 mmol, 1
mL MeOH, RT, 20 min.
b Evaluated by 1H NMR spectroscopy.
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