Dirhodium(II)-Catalyzed Sulfide Oxygenations: Catalyst Removal by Coprecipitation with Sulfoxides

Article abstract of DOI:10.1021/acs.joc.5b02400

The dirhodium(II) carboxylate complex Rh₂(esp)₂ (esp = α,α,α′,α′-tetramethyl-1,3-benzenedipropanoate) was shown to catalyze the sulfoxidation of organic sulfides using tert-butyl hydroperoxide as the oxidant. Due to the unique structure of Rh₂(esp)₂ and its stable Rh₂(II,II) catalyst resting state, the rhodium catalyst is able to precipitate as a Rh₂(esp)₂-sulfoxide complex following the reaction which makes separation of the catalyst from the products very convenient. The precipitated Rh₂(esp)₂-sulfoxide complexes could be reused to catalyze sulfide oxygenation reactions without considerable loss of activity. Mechanistic studies suggest that the axial ligands fine-tune the redox potential of the dirhodium(II,II) compounds and determine the predominant catalyst species in the oxidation reaction.

Full text of DOI:10.1021/acs.joc.5b02400

Article
pubs.acs.org/joc
Dirhodium(II)-Catalyzed Sulfide Oxygenations: Catalyst Removal by
Coprecipitation with Sulfoxides
Lili Zhao, Hongyang Zhang, and Yuanhua Wang*
College of Chemistry, Sichuan University, Chengdu, 610064, P. R. China
*
S Supporting Information
ABSTRACT: The dirhodium(II) carboxylate complex Rh (esp) (esp =
2
2
α,α,α′,α′-tetramethyl-1,3-benzenedipropanoate) was shown to catalyze the
sulfoxidation of organic sulfides using tert-butyl hydroperoxide as the
oxidant. Due to the unique structure of Rh (esp) and its stable Rh (II,II)
2
2
2
catalyst resting state, the rhodium catalyst is able to precipitate as a
Rh (esp) −sulfoxide complex following the reaction which makes separa-
2
2
tion of the catalyst from the products very convenient. The precipitated
Rh (esp) −sulfoxide complexes could be reused to catalyze sulfide
2
2
oxygenation reactions without considerable loss of activity. Mechanistic
studies suggest that the axial ligands fine-tune the redox potential of
the dirhodium(II,II) compounds and determine the predominant catalyst
species in the oxidation reaction.
INTRODUCTION
promising result to perform this reaction on an industrial scale.
■
By separating the catalyst from the products, there is also a
possibility to reuse the dirhodium(II) catalyst in subsequent
Homogeneous transition metal catalysts are extensively used in
organic chemistry because of their ability to catalyze important
synthetic transformations. However, to use these types of
catalysts in industry, thermal stability, and recyclability of
catalysts are needed because many industrial scale processes
8
1
reactions. These results are ascribed to the unique structure of
the dirhodium(II) complex which consists of a binuclear core
surrounded by four equatorial μ -ligands and two axial ligands
2
9
2
(Figure 2).
occur at high temperatures and on a large scale. Therefore,
metal catalyst recovery methods that are environmentally
friendly and efficient are required. We previously found that
3
RESULTS AND DISCUSSION
■
To optimize the reaction conditions for the oxidation of sulfides,
methyl phenyl sulfide 1a was chosen as a sample substrate. With
high sulfoxide/sulfone selectivities, the efficient conversion of
a into methyl phenyl sulfoxide 2a was achieved with 1 mol %
Rh (esp)2 and 5 equiv of T-HYDRO as the oxidant in
dirhodium(II) catalyst Rh (esp) was able to catalyze allylic
2
2
oxidation reactions using aqueous TBHP (70% tert-butyl
hydroperoxide in water, T-HYDRO) with a remarkably low
4,5
1
catalyst loading. Because this catalyst is highly resistant to
oxidative degradation, it was able to be recycled and reused in
consecutive runs even though the reaction occurred under
strongly oxidizing conditions. While Doyle’s dirhodium(II)
catalyst Rh (cap) have been shown to undergo one-electron
2
dichloromethane (DCM) under ambient conditions (Table 1,
7f−k
entry 1).
In the absence of T-HYDRO, no conversion was
observed (Table 1, entry 4). In the absence of catalyst, only 32%
of 2a was obtained after 6 h (Table 1, entry 3). Dioxygen did
not play a role in the reaction (Table 1, entry 5). When the
amount of T-HYDRO was reduced to 3 or 1 equiv, a signifi-
cantly lower conversion was observed (Table 1, entries 6 and 7).
Other oxidants such as cumene hydroperoxide and 30%
aqueous hydrogen peroxide were also tested. However, the
activity of the reaction decreased when cumene hydroperoxide
was used and no conversion occurred when using 30% aqueous
hydrogen peroxide (Table 1, entries 8 and 9). The catalytic
reaction was found to be solvent dependent. Polar solvents such
as acetonitrile and actetone were not suitable because they
coordinate to dirhodium catalysts and block the axial reaction
2
4
6
oxidative mechanisms, mechanistic studies for Rh (esp)2
2
suggested a stable dirhodium(II,II) species as the catalyst
5
resting state rather than an unstable dirhodium(II,III) species.
The dirhodium(II,II) species is less prone to decomposition
which is advantageous in catalysis (Figure 1).
This paper presents a new application for the Rh (esp) -based
2
2
catalytic oxidation system in the highly selective oxidation of
organic sulfides to their corresponding sulfoxides. This organic
transformation is of synthetic interest because sulfoxides are
common functionalities in the syntheses of pharmaceuticals,
7
agrochemicals, and other fine chemicals. In this work, condi-
tions were optimized for this catalytic reaction with Rh (esp)2
2
9
site. Reactions performed in nonpolar solvents such as heptane
and a catalyst removal method was explored. The catalyst
precipitated from the reaction mixture as Rh (esp) −sulfoxide
2
2
complexes, and the catalyst was then removed by filtration.
This convenient and efficient catalyst separation method is a
Received: October 16, 2015
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.joc.5b02400
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Article
Figure 1. One-electron oxidative procedure of dirhodium(II) catalysts.
loading of 0.1 mol % Rh (esp) could give 52% 2a after 6 h
2
2
(Table 1, entry 13). Other dirhodium(II) catalysts were also
tested for this reaction. However, they were not as effective as
Rh (esp) (Table 1, entries 14−16). Interestingly, while Rh (OAc)
2
2
2
4
has a nearly identical potential for one-electron oxidation com-
1
0
pared to Rh (esp) , using Rh (OAc) only gave a 35% yield.
2
2
2
4
11
Rh (OAc) (IMes) was also tested but only gave 71% yield.
2
4
Low yields were obtained when carrying out the reaction with
12
structurally similar chelating dirhodium(II) catalysts.
Figure 2. Structure of dirhodium(II) carboxylate catalysts.
After the optimized reaction conditions were determined, the
substrate scope of the reaction was tested. A variety of organic
sulfides 1b−o were oxidized to their corresponding sulfoxides
a
Table 1. Optimization of Reaction Conditions
2
b−o with excellent selectivities and yields (Table 2). Different
substituted-phenyl groups in the methyl aryl sulfides 1b−g are
well-tolerated (Table 2, entries 1−6). However, sulfides bearing
electron-withdrawing substituents required longer reaction
times for high conversion (Table 2, entries 4−6). The ethyl sub-
stitution at sulfur did not slow down the reaction rate (Table 2,
entry 7). This method proved tolerant of the tested functional
groups such as alcohols and alkenes. For instance, sulfides
with oxidation-susceptible functionalities (1i−k) gave sulfoxides
deviation from standard reaction
conditions
sulfoxide selectivity 2a yield
b c
entry
1
(%)
(%)
none
>99
>99
>99
−
96
74
32
NR
92
87
33
−
d
2
without solvent
3
4
5
6
7
8
9
without Rh (esp)
2
2
2
i−k without affecting the sensitive functional groups (Table 2,
without T-HYDRO
entries 8−10). The less reactive diphenyl sulfide 1l and
thianthrene 1m were both oxidized in good yields (Table 2,
entries 11 and 12). Alkyl sulfides 1n−o were also oxidized to
their corresponding sulfoxides 2n−o in good yields and with
excellent chemoselectivity (Table 2, entries 13 and 14).
N instead of air
>99
2
3 equiv of T-HYDRO
98
1 equiv of T-HYDRO
>99
−
30% H O instead of T-HYDRO
2
2
cumene instead of T-HYDRO
acetonitrile instead of DCM
n-heptane instead of DCM
acetone instead of DCM
0.1 mol % Rh (esp)
99
84
64
80
74
52
35
71
39
During the reaction, many color changes occur. These color
changes were studied to give us mechanistic information about
10
11
12
13
14
15
16
95
>99
97
the oxidation reaction. UV/vis spectroscopy was used to monitor
9
the oxidation reaction of 1a (Figure 3). When Rh (esp) was
>99
>99
>99
>99
2
2
2
2
dissolved in DCM, a transparent green solution showed the
Rh (OAc) instead of Rh (esp)
2
2
4
2
4
characteristic band Rh π* to Rh σ* of Rh (esp) at 661 nm.
Rh (OAc) (IMes) instead of Rh (esp)
2
2
2
2
2
2
4
2
Figure 3 shows that the band Rh π* to Rh σ* shifts 107 nm after
Rh (MPDP) instead of Rh (esp)
2
2
2
2
2
2
a
substrate 1a was added. Previous reports have shown that the
energy of Rh π* to Rh σ* HOMO to LUMO transition is
Reaction conditions: 1a (0.9 mmol), Rh (esp) (0.009 mmol), T-HYDRO
2
2
2
2
(
4.5 mmol), DCM (1.8 mL), rt, 6 h, air atmosphere, unless otherwise
11,13
b
1
strongly affected by axially coordinated ligands.
This large
noted. Determined by H NMR; selectivity = 2a(mol) /(2a(mol) +
3
time: 8 h.
c
d
shift indicates that 1a strongly binds to rhodium. Upon adding
one portion of T-HYDRO, the UV/vis spectrum remained
relatively unchanged and there was no band that corresponded
a(mol)) × 100%. Yield after column chromatography. Reaction
also gave less yield when compared to reactions run in DCM
to typical one-electron oxidized Rh (II,III) species (about 800 nm).
2
(Table 1, entries 10−12). It is important to note that a catalyst
This suggests that Rh (II,II) is the catalyst resting state.
2
B
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Article
a
Table 2. Oxidation of Various Sulfides with T-HYDRO Catalyzed by Rh (esp)
2
2
a
Reaction conditions: sulfides 1 (0.9 mmol), Rh (esp) (0.009 mmol), T-HYDRO (4.5 mmol), DCM (1.8 mL), rt, air atmosphere, unless
2
2
b
1
c
d
otherwise noted. Determined by H NMR; selectivity = 2(mol)/(2(mol) + 3(mol)) × 100%. Yield after column chromatography. 1.5 eq. of
T-HYDRO. 3 equiv of T-HYDRO.
e
Over the course of the reaction, the color gradually became
red which implies that the product sulfoxide 2a becomes
coordinated to the dirhodium(II) compound as the concen-
tration of substrate 1a decreases. At the end of the reaction, the
band Rh π* to Rh σ* was shifted to 506 nm.
that one of the two axial ligands could be displaced and leave one
9c,d,13
vacant axial coordination site on the dirhodium(II) complexes.
In other Rh (esp) -catalyzed reactions such as intermolecular
2
2
amination, DCM is not a suitable solvent, as it is oxidized
(likely through C−H abstraction) under the strong oxidation
2
2
4
,14
The Cl− can then axially coordinate to one-
After the reaction was completion and workup, the Rh (esp) −
conditions.
2
2
+
sulfoxide complexes directly precipitated and were recovered by
electron oxidized Rh (esp)2 to result in the Rh (esp) Cl
2
2
2
simple filtration. The structure of the precipitated Rh (esp) −
adduct which is inactive for catalysis (Scheme 1).
2
2
bis-sulfoxide complex 2c was confirmed by X-ray crystallography (see
Interestingly, even though we also observed the formation
of Rh (esp) Cl in DCM upon addition of T-HYDRO, the
Supporting Information (SI)). Actually, it was reported in the solution
2
2
C
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our oxidation conditions (Figure 4, I). Unlike other oxidation
cases, we observed the reaction mixture change from a purple
red color to green over the course of the 2 h reaction and the
Rh π* to Rh σ* band shifted back to around 620 nm. X-ray
2
2
crystal analysis helped discern these changes (see SI
Rh (esp) −bis-2d: its formation and isolation are the same as
2
2
Rh (esp) −bis-2c). It is important to note that Rh (esp)
2
2
2
2
coordinates to sulfoxide 2d through the oxygen atom and not
the sulfur atom. It is also interesting that the green color slowly
turned to red after 10 min and a band at 847 nm could be
observed in the UV/vis spectra; this is typical for a one-electron
4
oxidized Rh (II,III) species. This band was similar to the band
2
observed in the absence of substrate (Figure 4, III). This suggests
that the Rh (esp) Cl adduct was formed. Sulfoxide 2d coordinates
2
2
to the metal center through the weak electron donor oxygen
atom which is not able to prevent DCM oxidation.
Figure 3. (A) Rh (esp) in CH Cl (λ = 661 nm). (B) Rh (esp)2
Another factor besides ligand coordination ability (i.e., redox
potential) could also explain why DCM oxidation does not
occur in this catalytic reaction. It has been reported that axial
ligands coordinating to a dirhodium(II) center can change the
2
2
2
2
max
2
and 1a in CH Cl (λ = 554 nm). (C) Rh (esp) , 1a and T-HYDRO
2
2
max
2
2
in CH Cl (1 h, λ = 554 nm). (D) Rh (esp) , 1a and T-HYDRO in
2
2
max
2
2
CH Cl (3 h, λ = 514 nm). (E) Rh (esp) , 1a and T-HYDRO in
2
2
max
2
2
CH Cl (6 h, λ = 506 nm).
13
2
2
max
redox potential of the complex. Therefore, cyclic voltammetry
(
CV) experiments were performed to measure the redox
Scheme 1. Formation of Rh (esp) Cl
2
2
potentials over the course of the reaction (Figure 5). Interestingly,
−
4
−1
Figure 5. I (a) Rh (esp) (5.0 × 10 mol L in CH Cl solution)
2
2
2
2
−
4
−1
(
E1/2 = 781 mV). (b) Rh (esp) (5.0 × 10 mol L in CH Cl
solution) + 1a (20 equiv) (E = 642 mV). (c) Rh (esp) (5.0 × 10
mol L in CH Cl solution) + 2c (20 equiv) (E = 623 mV). II (a)
2
2
2
2
−
4
1/2
2
2
−1
2
2
1/2
−4
−1
Rh (esp) (5.0 × 10 mol L in CH Cl solution) (E = 781 mV).
2
2
2
2
1/2
−4
−1
(
b) Rh (esp) (5.0 × 10 mol L in CH Cl solution) + 1d (20
2 2 2 2
catalysis continued once sulfide was added (Figure 4, II). We
propose that Rh (esp) Cl does not exist in the presence of
−4
−1
equiv) (E1/2 = 553 mV). (c) Rh (esp) (5.0 × 10 mol L in CH Cl
2
2
2
2
2
2
solution) + 2d (20 equiv) (E1/2 = 523 mV).
sulfide because sulfides are strong ligands that occupy the
axial sites of the dirhodium(II) catalyst, avoiding generated
+
Rh (esp)2 to react with DCM (Scheme 1). Because sulfoxides
the Rh (II,II)/Rh (II,III) has a redox couple that is 139 mV more
2 2
2
15
have been shown to be ambidentate ligands, we tested our
accessible than that for Rh (esp) when 1a was added. The redox
2 2
+
hypothesis by subjecting methyl 2-methoxyphenyl sulfide 1d to
potential of Rh (esp) -2d (E = 523 mV vs Fc/Fc ) was lower
2 2 1/2
Figure 4. I (A) Rh (esp) in CH Cl (λ = 666 nm). (B) Rh (esp) and 1d in CH Cl (λ = 557 nm). (C) Rh (esp) , 1d, and T-HYDRO in
2
2
2
2
max
2
2
2
2
max
2
2
CH Cl (1 h, λ = 557 nm). (D) The oxidation reaction mixture of 1d when the reaction finished (λmax = 620 nm). (E) The oxidation reaction
2
2
max
mixture of 1d stood for 10 min after the reaction was finished (λmax = 847 nm). II (A) Rh (esp) and T-HYDRO in Benzene (λ = 620 nm). (B)
2
2
max
Rh (esp) and T-HYDRO in DCM (λ = 848 nm). III (A) The oxidation reaction mixture of 1d stood for 10 min after the reaction was finished
2
2
max
(
λmax = 847 nm). (B) Rh (esp) and T-HYDRO in DCM (λ = 848 nm).
2 2 max
D
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than that of the redox potential of Rh (esp) -2c (E = 623 mV
vs Fc/Fc ). This lower redox potential further explains why
oxidation reaction. Further experiments will be performed to
elucidate the mechanism in detail and to broaden the scope of
the reaction.
2
2
1/2
+
the Rh (esp) Cl adduct can form when 2d is an axial ligand. This
2
2
experiment also shows that the axial ligand at the Rh center can
fine-tune the redox potentials of the dirhodium(II,II) compounds
and affect the predominant catalyst species during the reaction.
After establishing the high stability of Rh (esp) during the
reaction, we investigated the recyclability and reusability of
the catalyst using 1a as a model substrate. At the end of the
EXPERIMENTAL SECTION
■
General. All reagents were purchased unless otherwise noted.
Reactions were run under atmospheric conditions at room temperature.
CH Cl was distilled from CaH . Ethyl acetate, n-hexane, and diethyl
2
2
2
2
2
ether were purchased from commercial sources and used without
reaction and workup, the Rh (esp) catalyst in Rh (esp) −bis-
2
2
2
2
further purification. Rh (esp) was prepared according to the
2
2
1
7,18
2a complex form was recovered by filtration, washed with cold
literature.
Rh (MPDP) and Rh (OAc) (IMes) preparations have
2 2 2 4
19,20
ethyl acetate, and dried. It was then directly reused in a sub-
sequent experiment with fresh 1a and oxidant under similar
reaction conditions. Four recycling experiments were carried
out, and the results are summarized in Table 3. The catalyst
been previously reported.
For the H, C NMR spectra ( H NMR 300, 400 MHz, 13C NMR
1
13
1
75, 100 MHz), CDCl was used as the solvent. Chemical shifts are
3
reported in δ (ppm) relative to TMS (δ = 0.00 ppm). Electrochemical
measurements were performed in anhydrous CH Cl with tetrabuty-
2
2
a
lammonium perchlorate as the supporting electrolyte at a scan rate of
Table 3. Catalyst Recycling Experiments
−1
5
0 mV s . The electrochemical measurements system was constructed
using a Pt disk working electrode, a Pt wire counter electrode, and a
cycle
Rh (esp) recovery yield (%)
product yield (%)
2
2
−1
Ag/AgNO (0.1 mol L in acetonitrile) reference electrode. For cyclic
3
1
2
3
4
94
87
88
87
91
90
99
98
voltammetry, data were referenced to the ferrocene/ferrocenium redox
+
couple (Fc/Fc ). Melting points were uncorrected. The high-resolution
mass spectra (HRMS) were performed on a mass spectrometer with a
TOF (for ESI).
a
Reaction condition: Rh (esp) (1 mmol %), 1a, T-HYDRO, CH Cl ,
Typical Sulfide Oxidation Procedure. The oxidation reaction
was carried out in a 10 mL tube containing a magnetic stirrer. CH Cl
2
2
2
2
2
rt, 6 h, air.
2
(1.8 mL) and methyl phenyl sulfide 1a (112.0 mg, 0.9 mmol) were
added into the reactor charged with Rh (esp) (6.8 mg, 0.009 mmol)
2
2
recovery rate and product yield remained relatively constant,
which confirms the robustness of this oxidation protocol.
Another recycling experiment was performed by using a
under stirring and then sealed by a rubber plug with a needle.
T-HYDRO (0.64 mL, 4.5 mmol) was then added via syringe. After the
reaction was complete, the reaction mixture was quenched by the
addition of 10 mL of saturated sodium thiosulfate solution and extracted
with ethyl acetate (3 × 5 mL). The combined organic layers were dried
by anhydrous Na SO , filtered, and concentrated under reduced
batch of Rh (esp) catalyst to successively catalyze six different
2
2
sulfide oxidation reactions (Scheme 2). After each reaction,
the Rh (esp) −sulfoxide complexes were collected and used
2
2
2
4
directly to catalyze the next reaction. It is noteworthy that there
was no obvious decrease in sulfoxide yield when using the
recovered catalyst compared to a new batch of catalyst. At
the end of the recycling experiment, the Rh (esp) −sulfoxide
pressure to give a crude product. The crude product was analyzed by
1
H NMR to obtain the selectivity of the oxidation reaction. The crude
product was purified by silica gel flash chromatography (n-hexane/
7f
EtOAc = 5/1, then 1/3) to give methyl phenyl sulfoxide (2a) (119 mg,
2
2
1
9
2
6%) as a yellow oil. H NMR (400 MHz, CDCl ) δ 7.65−7.62 (m,
3
complex was passed through a column to obtain pure Rh (esp)
2
2
13 1
H), 7.55−7.46 (m, 3H), 2.71 (s, 3H). C{ H} NMR (100 MHz,
CDCl ) δ 145.6, 131.0, 129.3, 123.4, 43.9. HRMS (ESI-TOF): m/z
calculated for C H OS ([M + H] ) 141.0369, found 141.0346.
Methyl 4-methylphenyl sulfoxide (2b): Typical procedure was
catalyst.
3
On the basis of the above results and the literature available
+
7
8
involving sulfoxidation, a possible catalytic cycle has been
7f
16
proposed that involves a single electron transfer (SET). First,
a one-electron transfer from sulfenyl sulfur to tert-butylperoxy
radical (t-BuOO•) generated by Rh (esp) occurs to give the
followed and flash chromatography (silica gel, n-hexane/EtOAc = 5/1,
1
then 1/3) afforded 133 mg (97%); yellow oil. H NMR (300 MHz,
CDCl ) δ 7.50 (d, J = 8.2 Hz, 2H), 7.29 (d, J = 8.2 Hz, 2H), 2.67 (s,
2
2
3
16
13
1
corresponding sulfenium radical (Scheme 3). The resultant
sulfenium radical then undergoes fast chemical transformation
with tert-butylperoxy radical to afford the corresponding sulfoxide.
The one-electron oxidation to dirhodium(II,III) is the rate-
limiting step which means that the catalyst resting state is the
stable dirhodium(II,II) complex. This helps avoid oxidative
degradation and catalyst deactivation which allows for the
Rh (esp) catalyst to be recovered and reused again. The details
3H), 2.38 (s, 3H). C{ H} NMR (75 MHz, CDCl
29.9, 123.8, 43.9, 21.2.
-Methoxyphenyl methyl sulfoxide (2c): Typical procedure was
3
) δ 142.3, 141.3,
1
7f
4
followed and flash chromatography (silica gel, n-hexane/EtOAc = 5/1,
1
then EtOAc) afforded 145 mg (96%); yellow oil. H NMR (300 MHz,
CDCl ) δ 7.53 (d, J = 8.7 Hz, 2H), 6.96 (d, J = 8.7 Hz, 2H), 3.78 (s,
3
13
1
3
1
H), 2.63 (s, 3H). C{ H} NMR (75 MHz, CDCl ) δ 161.7, 136.8,
3
25.4, 114.5, 55.3, 43.6. HRMS (ESI-TOF): m/z calculated for
+
2
2
C H O S ([M + H] ) 171.0474, found 171.0442.
8
10
2
7k
of how Rh (esp) controls the oxidation state of the catalyst
2-Methoxyphenyl methyl sulfoxide (2d): Typical procedure was
2
2
resting state during the reaction and how the chelating ligand
affects the reaction remain to be investigated.
followed and flash chromatography (silica gel, n-hexane/EtOAc = 5/1,
then EtOAc) afforded 145 mg (96%); yellow oil. H NMR (300 MHz,
1
CDCl ) δ 7.79 (dd, J = 7.7, 1.6 Hz, 1H), 7.46−7.38 (m, 1H), 7.18−
3
CONCLUSION
A simple, efficient, and environmentally friendly method for the
oxidation of sulfides to sulfoxides using Rh (esp) as catalyst
7.13 (m, 1H), 6.89 (d, J = 8.2 Hz, 1H), 3.85 (s, 3H), 2.74 (s, 3H).
■
13
1
C{ H} NMR (75 MHz, CDCl ) δ 154.6, 132.9, 131.8, 124.4, 121.5,
3
1
10.5, 55.5, 41.1. HRMS (ESI-TOF): m/z calculated for C H O S
8
10
2
2
2
+
([M + H] )171.0474, found 171.0451.
and safe aqueous T-HYDRO as oxidant has been developed.
Because of the unique structure of the dirhodium(II) compound,
catalyst/product separation is very simple. It was demonstrated
that Rh (esp) could be reused without considerable loss of
7f
4
-Chlorophenyl methyl sulfoxide (2e): Typical procedure was
followed and flash chromatography (silica gel, n-hexane/EtOAc = 5/1,
1
then 1/3) afforded 151 mg (98%); yellow oil. H NMR (300 MHz,
2
2
CDCl ) δ 7.58 (d, J = 8.6 Hz, 2H), 7.49 (d, J = 8.6 Hz, 2H), 2.70 (s,
3
1
3
1
activity due to a stable Rh (II,II) catalyst resting state during the
3H). C{ H} NMR (75 MHz, CDCl ) δ 144.2, 137.2, 129.6, 124.9, 44.0.
2
3
E
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Scheme 2. Catalyst Recycling Experiments
7
i
4
-Nitrophenyl methyl sulfoxide (2g): Typical procedure was
Scheme 3. Possible Mechanism
followed and flash chromatography (silica gel, n-hexane/EtOAc = 5/1,
then EtOAc) afforded 144 mg (88%); white solid, mp 153−155 °C.
1
H NMR (400 MHz, CDCl ) δ 8.36 (d, J = 8.9 Hz, 2H), 7.86 (d, J =
3
.9 Hz, 2H), 2.78 (s, 3H). 13C{ H} NMR (100 MHz, CDCl ) δ 153.3,
1
8
1
3
49.5, 124.7, 124.5, 43.9. HRMS (ESI-TOF): m/z calculated for
+
C H NO S ([M + H] ) 186.0219, found 186.0236.
7
7
3
7f
Phenyl ethyl sulfoxide (2h): Typical procedure was followed and
flash chromatography (silica gel, n-hexane/EtOAc = 5/1, then 1/3)
1
afforded 135 mg (99%); yellow oil. H NMR (300 MHz, CDCl ) δ
3
7
1
1
.62−7.54 (m, 2H), 7.53−7.43 (m, 3H), 2.88 (m, 1H), 2.80−2.67 (m,
H), 1.17 (t, J = 7.4 Hz, 3H). 13C{ H} NMR (75 MHz, CDCl ) δ
1
3
43.1, 130.8, 129.0, 124.0, 50.1, 5.8.
7g
+
2-(Phenylsulfinyl)ethanol (2i): Typical procedure was followed
HRMS (ESI-TOF): m/z calculated for C H ClOS ([M + H] )-
7
7
and flash chromatography (silica gel, n-hexane/EtOAc = 5/1, then
EtOAc) afforded 122 mg (81%); yellow oil. H NMR (300 MHz,
1
74.9979, found 174.9945.
1
7
f
4
-Bromophenyl methyl sulfoxide (2f): Typical procedure was
CDCl ) δ 7.65−7.56 (m, 2H), 7.54−7.42 (m, 3H), 4.32 (s, 1H),
followed and flash chromatography (silica gel, n-hexane/EtOAc = 5/1,
3
1
then EtOAc) afforded 168 mg (87%); yellow oil. H NMR (400 MHz,
4.15−4.09 (m, 1H), 3.97−3.85 (m, 1H), 3.11−3.03 (m, 1H), 2.94−
2.87 (m, 1H). 13C{ H} NMR (75 MHz, CDCl
1
) δ 143.0, 131.1, 129.3,
CDCl ) δ 7.63 (d, J = 8.4 Hz, 2H), 7.50 (d, J = 8.4 Hz, 2H), 2.69
3
3
13
1
(
1
s, 3H). C{ H} NMR (100 MHz, CDCl ) δ 144.9, 132.6, 125.5,
25.2, 44.0.
123.9, 59.6, 56.0. HRMS (ESI-TOF): m/z calculated for C
([M + H] ) 171.0474, found 171.0421.
8
H
O
10
2
S
3
+
F
DOI: 10.1021/acs.joc.5b02400
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
7
g
1
Allyl phenyl sulfoxide (2j): Typical procedure was followed and
Rh (esp) −bis-2j. H NMR (400 MHz, CDCl ) δ 7.71−769 (m,
2
2
3
flash chromatography (silica gel, n-hexane/EtOAc = 5/1, then 1/3)
afforded 131 mg (89%); yellow oil. H NMR (300 MHz, CDCl ) δ
4H), 7.52−7.43 (m, 6H), 7.08 (t, J = 7.5 Hz, 2H), 6.85 (s, 2H), 6.85−
6.81 (m, 4H), 5.79−5.67 (m, 2H), 5.35−5.32 (m, 2H), 5.20−5.15 (m,
2H), 4.08−4.03 (m, 2H), 3.88−3.79 (m, 2H), 2.57 (s, 8H), 0.93 (s,
24H).
1
3
7
3
1
.65−7.44 (m, 5H), 5.71−5.57 (m, 1H), 5.36−5.14 (m, 2H), 3.60−
.46 (m, 2H). 13C{ H} NMR (75 MHz, CDCl ) δ 142.8, 131.0, 128.9,
1
3
25.1, 124.2, 123.7, 60.7.
7
h
Benzyl phenyl sulfoxide (2k): Typical procedure was followed
and flash chromatography (silica gel, n-hexane/EtOAc = 5/1) afforded
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.joc.5b02400.
■
*
S
1
1
60 mg (84%); white solid, mp 127−128 °C. H NMR (300 MHz,
CDCl ) δ 7.50−7.32 (m, 5H), 7.32−7.19 (m, 3H), 7.01−6.93 (m,
3
13
1
2
H), 4.08 (d, J = 12.6 Hz, 1H), 3.99 (d, J = 12.6 Hz, 1H). C{ H}
NMR (75 MHz, CDCl ) δ 142.7, 131.1, 130.3, 129.1, 128.8, 128.4,
Pictures of solution color of the sulfide oxidation
3
1
28.2, 124.4, 63.5.
Diphenyl sulfoxide (2l): Typical procedure was followed and flash
chromatography (silica gel, n-hexane/EtOAc = 5/1, then 1/1) afforded
reaction, complete recycling experiments, and copies of
7
f
1
13
H and C spectra for all products (PDF)
Crystallographic data for Rh (esp) −bis-2c (CIF)
1
2
2
1
74 mg (97%); yellow oil. H NMR (400 MHz, CDCl ) δ 7.70−7.64
3
13 1
Crystallographic data for Rh
(esp)
2
−bis-2d (CIF)
2
(
m, 4H), 7.52−7.43 (m, 6H). C{ H} NMR (100 MHz, CDCl )
3
δ 145.6, 131.1, 129.4, 124.8.
7
j
Thianthrene 5-oxide (2m): Typical procedure was followed and
AUTHOR INFORMATION
■
flash chromatography (silica gel, n-hexane/EtOAc = 5/1) afforded
Corresponding Author
E-mail: yhwang@scu.edu.cn.
Notes
1
2
03 mg (99%); white solid, mp 146−148 °C. H NMR (300 MHz,
*
CDCl ) δ 7.94−7.91 (m, 2H), 7.63−7..61 (m, 2H), 7.57−7.52 (m,
H), 7.45−7.39 (m, 2H). C{ H} NMR (75 MHz, CDCl ) δ 141.3,
3
13
1
2
1
3
The authors declare no competing financial interest.
29.8,129.7, 128.9, 128.3, 124.3.
7
g
Dibutyl sulfoxide (2n): Typical procedure was followed and flash
ACKNOWLEDGMENTS
We are grateful for financial support from National Science
Foundation of China (grant no. 21272162).
chromatography (silica gel, n-hexane/EtOAc = 5/1, then 1/1) afforded
■
1
1
(
6
28 mg (89%); yellow oil. H NMR (300 MHz, CDCl ) δ 2.69−2.52
3
m, 4H), 1.75−1.63 (m, 4H), 1.55−1.33 (m, 4H), 0.91 (t, J = 7.3 Hz,
H). 13C{ H} NMR (75 MHz, CDCl ) δ 52.0, 24.4, 21.9, 13.5.
3
7i
1
Methyl 3-(methylsulfinyl)propanoate (2o): Typical procedure
REFERENCES
■
was followed and flash chromatography (silica gel, n-hexane/EtOAc =
1
(1) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals; John Wiley & Sons: New York, 1988.
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5
/1, then EtOAc) afforded 93 mg (70%); yellow oil. H NMR (300
MHz, CDCl ) δ 3.67 (s, 3H), 3.07−2.96 (m, 1H), 2.90−2.74 (m, 3H),
3
13
1
2
3
.55 (s, 3H). C{ H} NMR (75 MHz, CDCl ) δ 171.4, 52.0, 48.7,
3
8.6, 26.5. HRMS (ESI-TOF): m/z calculated for C H O S
5
10
3
+
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(
[M + H] ) 151.0423, found 151.0421.
Procedure for Precipitating the Rh (esp) −Bis-sulfoxide
2
2
Complexes and Catalyst Recycling Experiment. The oxidation
reaction was performed in a round-bottomed flask containing a
magnetic stirrer at room temperature. DCM and sulfide 1 were added
into a reactor charged with Rh (esp) under stirring. The reactor was
2
2
then sealed by a rubber plug with a needle. T-HYDRO was subsequent
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2
4
of the solvent, precipitating agents were added to precipitate the
Rh (esp) −bis-sulfoxide complexes. The red catalysts complexes were
2
2
separated by filtration. The product sulfoxides were obtained after the
solvents were removed under reduced pressure. The obtained catalysts
complexes were washed with cold ethyl acetate and then dried in air
with high recovery yield (based on the Rh (esp) ), which would be
2
2
directly used as catalysts in the next cycle. Pure Rh (esp) would be
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2
2
recovered by decomplexing the Rh (esp) −bis-sulfoxides through a
2
2
silica gel column using n-hexane/ethyl acetate as elution solvents.
1
Rh (esp) −bis-2a. H NMR (300 MHz, CDCl ) δ 7.77−7.75 (m,
2
2
3
4
(
H), 7.51−7.45 (m, 6H), 7.10−7.08 (m, 2H), 6.83 (s, 2H), 6.83−6.81
m, 4H), 2.97 (s, 6H), 2.55 (s, 8H), 0.92 (s, 24H). Cocrystallized
solvent molecules were not counted.
1
Rh (esp) −bis-2c. H NMR (400 MHz, CDCl ) δ 7.72 (d, J =
2
2
3
8
2
(
.4 Hz, 4H), 7.09 (t, J = 7.5 Hz, 2H), 6.95 (d, J = 8.4 Hz, 4H), 6.87 (s,
H), 6.84−6.82 (m, 4H), 3.87 (s, 6H), 3.04 (s, 6H), 2.58 (s, 8H), 0.93
s, 24H).
1
Rh (esp) −bis-2e. H NMR (400 MHz, CDCl ) δ 7.71 (d, J =
2
2
3
8
2
.4 Hz, 4H), 7.42 (d, J = 8.5 Hz, 4H), 7.11 (t, J = 7.8 Hz, 2H), 6.85 (s,
H), 6.85−6.83 (m, 4H), 3.05 (s, 6H), 2.57 (s, 8H), 0.92 (s, 24H).
1
Rh (esp) −bis-2i. H NMR (400 MHz, CDCl3) δ 7.75−7.73 (m,
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2
2
4
6
0
H), 7.54−7.48 (m, 6H), 7.09 (t, J = 7.5 Hz, 2H), 6.82−6.79 (m, 4H),
.79 (s, 2H), 4.26−4.12 (m, 4H), 3.28−3.18 (m, 4H), 2.55 (s, 8H),
.91 (s, 24H). Cocrystallized solvent molecules were not counted.
G
DOI: 10.1021/acs.joc.5b02400
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The Journal of Organic Chemistry
Article
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DOI: 10.1021/acs.joc.5b02400
J. Org. Chem. XXXX, XXX, XXX−XXX