A simple metal-free catalytic sulfoxidation under visible light and air

Article abstract of DOI:10.1039/c2gc36683e

A metal-free aerobic selective sulfoxidation photosensitized by Rose Bengal or solid-supported Rose Bengal has been developed. The reaction utilizes visible light as the driving force and molecular oxygen as the oxidant. Among the advantages of the develo

Full text of DOI:10.1039/c2gc36683e

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A highly efficient metal-free, visible-light promoted aerobic sulfoxidation with remarkably simple
workup protocols.

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ARTICLE TYPE
A Simple Metal-free Catalytic Sulfoxidation under Visible Light and Air
Xiangyong Gu, Xiang Li, Yahong Chai, Qi Yang, Pixu Li* and Yingming Yao*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
5
A metalꢀfree aerobic selective sulfoxidation photosensitized by Rose Bengal or solidꢀsupported Rose
Bengal has been developed. The reaction utilizes visible light as the driving force and molecular oxygen
as the oxidant. Highlighted by its high efficiency and selectivity, extremely simple operation and workup
procedure, and minimal waste generation, the process is an excellent example of perfect green chemistry.
considered a “nearꢀperfect” reaction as it proceeds in a highly
atomꢀeconomical and almost wasteꢀfree manner.
Introduction
10 “Green chemistry” is one of the central topics both in academia
and industry since early 1990’s. The fundamental “Twelve
Principles” have become the guidelines for the development of
“green” chemical reactions or processes.1 A “perfect chemical
reaction”, one that gives only the desired products with 100%
15 selectivity and 100% yield without unwanted wastes2, is the
ultimate goal for chemists. In addition to that, it is believed an
50 Results and discussion
The sulfoxidation of sulfides is important in many fields, such as
pharmaceuticals,6 fossil fuels desulfurization,7 industrial
wastewater treatment,8 and chemical warfare agent disposal.9 The
main side reaction of sulfoxidation is the formation of sulfone
ideal chemical reaction or process should be highly atomꢀ 55 through overꢀoxidation. Many methods have been developed to
economical, environmentally benign, and energyꢀsaving.
Although most organic molecules do not absorb visible light,
20 Nature has shown us through photosynthesis that visible light is
an “unlimited” energy source for chemical synthesis with the aid
achieve selective sulfoxidation.10 Among all the oxidants,
hydrogen peroxide has been studied extensively as a “green
oxidant” for sulfoxidation.10c Nonetheless, a catalytic selective
sulfoxidation using molecular oxygen as the terminal oxidant
of chromophores. Therefore, a reaction driven by visible light at 60 would be more advantageous over existing methods. Although
ambient temperature is considered to be sustainable. The recent
seminal work on photoredox catalysis by the MacMillan group3
25 and the Yoon group4 have attracted significant attention to the
applications of visible light photochemistry in organic synthesis.
the mechanism of sulfoxidation by molecular oxygen via
photochemical irradiation has been investigated in depth in
physical chemistry,11 the applications to organic synthesis have
only been reported recently.12 Our interest in practical synthetic
Several new exciting visible light photochemical processes 65 methods of sulfurꢀcontaining compounds under visible light
sensitized by polybipyridyl metal complexes or organic dye
photoredox catalysts were reported since then.5
photoredox conditions using molecular oxygen as the terminal
oxidants,13 led us to investigate the selective sulfoxidation of
thioanisole. Initial screening experiments examined the nature
and effect of the photosensitizer and solvent. The results are
30
Oxidation is a fundamental organic transformation and has been
studied extensively. However, oxidation is also one of the most 70 shown in Table 1. In the first round of catalyst screening, a
problematic reactions. Many classical oxidants are toxic and
some heavy metal containing oxidants often generate large
35 amounts of toxic waste. Moreover, many efficient but expensive
oxidants have to be used stoichiometrically. Therefore, a catalytic,
solution of thioanisole in acetonitrile was added 2 mol% of
various catalysts (entries 1ꢀ9). The reactions were carried out in
open vessels under 14 W compact fluorescent lights for 24 hr.
With the assistance of a catalytic amount of hydrochloric acid,
environmentally friendly, and economical oxidation process is 75 Rose Bengal appeared to be most efficient and selective catalyst
always in demand. Among all the oxidants, molecular oxygen is
an ideal reagent as it is “virtually unlimited” and free just as
40 visible light. Numerous sophisticated oxidations occur in
biological systems using molecular oxygen as the oxidant.
(entry 4). Almost no overꢀoxidation was observed. Compared to
the metal complex photoredox catalysts, Rose Bengal, an organic
dye, is readily accessible and inexpensive. The reactivity of Rose
Bengal in different solvents was then examined (entries 10ꢀ17),
However, molecular oxygen is not a reactive oxidant and in many 80 with ethanol being the optimal solvent for this oxidation. The
cases the atomꢀeconomy is low (50% efficiency). Herein, we
report our findings utilizing visible light and molecular oxygen
45 for a selective sulfoxidation process. We believe this process
meets the requirements of green chemistry and could be
reaction was completed within 24 hr of light irradiation and the
selectivity was excellent. Furthermore, the reaction time in EtOH
could be shortened to 12 hr and selectivity was increased to >
99:1 in the presence of 0.2 equivalents of HCl. To ascertain that
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selectivity (entry 12).15 Compared to arylalkylthioethers, the rate
reaction was carried out in the dark and no oxidation occurred 40 of dialkylthioether sulfoxidation was generally faster (entry 13ꢀ
visible light was necessary for the reaction to proceed, a control
(entry 18). Similarly, a control reaction in the absence of Rose
Bengal did not proceed under identical conditions (entry 19).
15). LꢀMethionine derivative was oxidized under the standard
conditions as well, implying that the oxidation may be used in the
sulfoxidation of biologically interesting methionine containing
proteins or peptides.16 1ꢀ(Methylsulfinyl)ꢀ4ꢀnitrobenzene was not
45 oxidized because nitrobenzene is known to be a radical trap,
which suggests that the oxidation may go through a radical
pathway (entry 17).
5
Table 1. Screening of catalysts and solvent
Table 2. Homogeneous catalysis: Visible light promoted aerobic
oxidation of various sulfidesa
Entry
Catalyst
Solvent
GC yield
Selectivityb
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
DMF
DMSO
MeOH
CH2Cl2
EtOH
EtOH
NMP
1
2
Ru(bpy)3Cl2ꢁ6H2O
Ir(ppy)3
50 %
N.R.
16 %
80 %
trace
trace
trace
7.5 %
40 %
19 %
7 %
75 %.
N.R.
>99:1
ꢀ
>99:1
98:2
ꢀ
ꢀ
ꢀ
ꢀ
>99:1
>99:1
>99:1
3:1
ꢀ
98:2
>99:1
ꢀ
Reaction
time
Entry
Product
Yieldb Selectivityc
3
Rose Bengal
Rose Bengal
Rodanmin B
Flourensin
Alizarin Red
Erosin B
4c
1
12 h
> 99%
>99:1
5
6
7
O
S
8
2
3
12 h
12 h
> 99%
> 99%
>99:1
>99:1
9c
Erosin B
10
11
12
13
14
15c
16
17
18cd
19ce
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
Rose Bengal
ꢀ
>99 %
>99 % (12 h)
N.R.
4
48 h
> 99%
99:1
MeNO2
EtOH
EtOH
3 %
N.R.
N.R.
>99:1
ꢀ
5
6
7
8
9
12 h
48 h
12 h
12 h
18 h
> 99%
67%de
93%
>99:1
>99:1
98:2
a: Reaction conditions: Thioanisole (1 mmol), catalysts (2 mol%), solvent
(2 ml), opened to the air at room temperature, irradiating with 14 w
fluorescent lamp for 24 hr. b: Based on GC area. c: 0.2 eq conc. HCl was
10 added. d: reaction was carried out in dark. e: no catalyst was added
73%e
99%
99:1
The sulfoxidation of thioanisole was very clean and the only nonꢀ
volatile material besides the sulfoxide product was the catalyst.
Therefore, an extremely simple workup/purification procedure
15 was developed by passing the crude reaction mixture through a
short plug of basic alumina to remove the acidic Rose Bengal.
Subsequent evaporation of the EtOH from the filtrate afforded a
pure product without additional purification.
>99:1
10
12 h
99%
>99:1
11
12
24 h
12 h
99
>99:1
>99:1
20 This reaction and the workup process meet many requirements of
green chemisty: a) the reaction is driven by visible light and
occurs at ambient temperature; b) the reaction uses molecular
oxygen from air as the oxidant; c) high atomꢀeconomy is
achieved as no other reductant is involved; d) nonꢀmetal, cheap
25 organic dye with relatively low loading is used as the catalyst; e)
very low Eꢀfactor14 was achieved because minimal waste is
generated; f) the reaction uses environmentally benign ethanol as
the solvent that is easily recycled during workup.
91%ef
13
14
6 h
6 h
> 99%
> 99%
>99:1
>99:1
15
16
6 h
8 h
98%
>99:1
> 99%
>99:1g
30 To test the generality of this reaction, a series of thioethers were
subjected to the optimized reaction conditions (Table 2). The
results demonstrate that dialkylthioethers and alkylarylthioethers
almost uniformly afforded excellent yields and selectivity.
Diphenylthioether gave a slow reaction possibly due to the low
35 electron density on sulfur atom (entry 6). Phenylallylthioether
only gave the desired sulfoxide in 73% yield along with an
unknown byproduct (entry 8). Interestingly, the dithio derivative
selectively gave monoꢀsulfoxidation in excellent yield and
17
24 h
N.R.
-
50 a: reaction conditions: thioether (1 mmol), Rose Bengal (2 mol%), 0.05 M
HCl in EtOH (4 ml, 20 mol%), opened to the air at room temperature,
using 14w fluorescent lamp as visible light source. b: isolated yield. c:
selectivities were determined based on GC peak areas. d: GC conversion
of sulfide is 67% after 48 h. e: isolated by silica gel chromatography. f:
55 d.r.>99:1. g: selectivity was determined by HPLC peak areas.
To further demonstrate that the reaction is practical and scalable,
a 50 mmol scale reaction was carried out in an Erlenmeyer flask
2
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under sunshine on the roof of the chemistry building (Figure 1).
The reaction showed identical results as that carried out in the
laboratory hood.
5
6
7
8
9
48 h
48 h
24 h
24 h
72 h
>99%
53%de
78%
98:2
99:1
94:6
99:1
98:2
77%e
96%
5
Figure 1. Reaction under sunshine
The recycling of catalyst is a very important factor of green
chemistry. To verify the catalyst could be reused, a thioanisole
sulfoxidation reaction mixture was exposed to light and air for 12
10
36 h
99%
98:2
10 hours after the reaction was completed, followed by the addition
of a second equivalent of thioanisole. To our delight, the second
sulfoxidation started without any problem and was completed
within the same reaction timeframe, indicating the organic dye
catalyst was not decomposed. This observation stimulated us to
15 seek a “greener” protocol to eliminate the catalyst waste and the
waste generated during workup. The idea was to immobilize the
catalyst so it could be easily recycled by a simple filtration.
Attaching Rose Bengal on a resin with an ester linker was
reported in the 1970’s.17 Different types of heterogeneous
11
12
72 h
24 h
>99%
87%ef
>99:1
96:4
13
14
12 h
12 h
98%
>99:1
>99:1
>99%
15
12 h
99%
>99:1
a: reaction conditions: thioether (1 mmol), polymer supported Rose
Bengal (10 mol%), 0.05 M HCl in EtOH (4 ml, 20 mol%), opened to the
air at room temperature, using 14 w fluorescent lamp as visible light
source. b: isolated yield. c: selectivities were determined based on GC
40 peak areas. d: GC conversion of sulfide is 53% after 48 h. e: isolated by
silica gel chromatography. f: d.r.>99:1.
20 catalysts for sulfide oxidation were discovered in the last
18
decades.12f,
It was decided to immobilize Rose Bengal onto
Wang resin with a stronger amide linker (Figure 2).
To confirm the idea of catalyst recycling, the solidꢀsupported
catalyst was reused in the sulfoxidation of thioanisole for 10
45 times. As shown in Chart 1, the oxidation results remained
unchanged.
GC yield (%)
Selectivity (%)
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
25
Figure 2. Preparation of solidꢀsupported Rose Bengal.
The solidꢀsupported catalyst was subjected to the sulfoxidation
reaction. As shown in Table 3, similar results were obtained
except the rates of several heterogeneous reactions were
30 somewhat slower. Nonetheless, the workup procedure was
improved significantly with the use of a solidꢀsupport catalyst.
Filtration
to
recover
the
catalyst,
followed
by
evaporation/recovery of solvent, afforded the pure product.
Table 3: Heterogeneous catalysis: Visible light promoted aerobic
35 oxidation of various sulfidesa
1
2
3
4
5
6
7
8
9
10
Recycle Number
Reaction
time
Entry
Product
yieldb
Selectivityc
Chart 1. Solidꢀsupport catalyst recycling
1
18 h
>99%
>99:1
Photosensitized sulfoxidation is generally accepted to occur via
50 two main mechanisms, singlet oxygen oxidation through energy
transfer and radical pathway through electronꢀtransfer.11a, 11d, 11f
Diphenyl sulfide is known to be unreactive toward singlet oxygen.
But under our optimized conditions, diphenyl sulfide was
oxidized to diphenyl sulfoxide, albeit at a slower rate (Table 2,
55 entry 6 and Table 3, entry 6). 1ꢀ(Methylsulfinyl)ꢀ4ꢀnitrobenzene
containing a radical trap in the molecule did not participate in the
reaction (Table 2, entry 17). These two results suggested that the
O
S
2
3
24 h
24 h
>99%
> 99%
>99:1
99:1
4
24 h
98%
97:3
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sulfoxidation might go through radical mechanism. It was
6. S. Caron, R. W. Dugger, S. G. Ruggeri, J. A. Ragan and D. H. B.
Ripin, Chem. Rev., 2006, 106, 2943.
consistant with that the oxidation of thioanisole was suppressed
by benzoquinone, a radical quencher. Only 12% of PhS(O)Me
was formed after 24 hours with 1 equivilent of benzoquinone
under otherwise identical conditions. Moreover, trace amount of
diphenyl sulfone was observed when diphenyl sulfoxide was
added to the reaction of thioanisole sulfoxidation. It indicated that
the formation of persulfoxide, an intermediate in singlet oxygen
pathway, was neglectable.19 All of the above evidence suggested
10 that the sulfoxidation went through electronꢀtransfer pathway and
superoxide anion might involved in the reaction. However,
singlet oxygen oxidation through energy transfer mechanism
could not be ruled out because it might operate with other
thioether substrates, such as dialkyl sulfides.
60
7. (a)S. Otsuki, T. Nonaka, N. Takashima, W. H. Qian, A. Ishihara, T.
Imai and T. Kabe, Energy & Fuels, 2000, 14, 1232; (b)F. M. Collins,
A. R. Lucy and C. Sharp, J. Mol. Catal. A-Chem., 1997, 117, 397.
8. M. Henze, P. Harremoes, J. l. C. Jansen and E. Arvin, Wastewater
Treatment: Biological and Chemical Processes, SpringerꢀVerlag,
Berlin Heidelberg 2002.
5
65
9. Y. C. Yang, J. A. Baker and J. R. Ward, Chem. Rev., 1992, 92, 1729.
10. (a)E. Wojaczynska and J. Wojaczynski, Chem. Rev., 2010, 110, 4303;
(b)P. Kowalski, K. Mitka, K. Ossowska and Z. Kolarska,
Tetrahedron, 2005, 61, 1933; (c)K. Kaczorowska, Z. Kolarska, K.
Mitka and P. Kowalski, Tetrahedron, 2005, 61, 8315; (d)M.
Madesclaire, Tetrahedron, 1986, 42, 5459; (e)K. A. Stingl and S. B.
Tsogoeva, Tetrahedron-Asymmetry, 2010, 21, 1055.
70
15 Conclusions
11. (a)J. J. Liang, C. L. Gu, M. L. Kacher and C. S. Foote, J. Am. Chem.
Soc., 1983, 105, 4717; (b)K. Nahm and C. S. Foote, J. Am. Chem.
Soc., 1989, 111, 1909; (c)K. Nahm, Y. Li, J. D. Evanseck, K. N.
Houk and C. S. Foote, J. Am. Chem. Soc., 1993, 115, 4879; (d)E.
Baciocchi, T. Del Giacco, F. Elisei, M. F. Gerini, M. Guerra, A. Lapi
and P. Liberali, J. Am. Chem. Soc., 2003, 125, 16444; (e)M. Ganesan,
V. K. Sivasubramanian, S. Rajagopal and R. Ramaraj, Tetrahedron,
2004, 60, 1921; (f)S. M. Bonesi, I. Manet, M. Freccero, M. Fagnoni
and A. Albini, Chem. Eur. J., 2006, 12, 4844; (g)A. Greer, Acc.
Chem. Res., 2006, 39, 797; (h)K. L. Stensaas, B. V. McCarty, N. M.
Touchette and J. B. Brock, Tetrahedron, 2006, 62, 10683; (i)C. Wang,
Z. G. Xie, K. E. deKrafft and W. L. Lin, J. Am. Chem. Soc., 2011,
133, 13445; (j)S. M. Bonesi, M. Fagnoni and A. Albini, J. Sulfur
Chem., 2008, 29, 367; (k)S. M. Bonesi, M. Fagnoni and A. Albini,
Eur. J. Org. Chem., 2008, 2612.
In conclusion, a “nearꢀperfect” sulfoxidation with excellent yields,
selectivity, and without unwanted waste was developed. The
protocol is very practical for the simple workup procedures in
both homogeneous and heterogeneous reactions. Most
20 importantly, the use of renewable feedstocks namely, visible light
and air, and an environmentally benign solvent in a highly atomꢀ
economical and energyꢀefficient manner constitutes an ideal
example of green chemistry.
75
80
Notes and references
25 Key Laboratory of Organic Synthesis of Jiangsu Province, College of
Chemistry, Chemical Engineering, and Materials Science, Soochow
University,199 RenAi Road, Suzhou, Jiangsu, 215123, China. Tel: +86-
512-65880826; E-mail: lipixu@suda.edu.cn
Key Laboratory of Organic Synthesis of Jiangsu Province, College of
30 Chemistry, Chemical Engineering, and Materials Science, Soochow
University,199 RenAi Road, Suzhou, Jiangsu, 215123, China. Tel: +86-
512-65882806; E-mail: yaoym@suda.edu.cn
85
12. (a)E. Baciocchi, C. Chiappe, C. Fasciani, O. Lanzalunga and A. Lapi,
Org. Lett., 2010, 12, 5116; (b)P. F. Zhang, Y. Wang, H. R. Li and M.
Antonietti, Green Chemistry, 2012, 14, 1904; (c)H. J. Xu, Y. C. Lin,
X. Wan, C. Y. Yang and Y. S. Feng, Tetrahedron, 2010, 66, 8823;
(d)S. M. Bonesi, E. Carbonell, H. Garcia, M. Fagnoni and A. Albini,
Appl. Catal. B-Environ., 2008, 79, 368; (e)J. M. Zen, S. L. Liou, A. S.
Kumar and M. S. Hsia, Angew. Chem. Int. Ed., 2003, 42, 577; (f)A.
M. Khenkin and R. Neumann, Catal. Lett., 2000, 68, 109.
90
Acknowledgment. Financial support is provided by NSFC (21102097),
35 the Specialized Research Fund for the Doctoral Program of Higher
Education (20103201120006), Beijing National Laboratory for Molecular
Sciences (BNLMS), and the Priority Academic Program Development of
Jiangsu Higher Education Institutions (PAPD). We thank Professor
Xingwang Wang and Mr. Zhaomin Liu for helpful discussions and
40 suggestions.
95
13. Y. Cheng, J. Yang, Y. Qu and P. Li, Org. Lett., 2012, 14, 98.
14. R. A. Sheldon, Chem. Soc. Rev., 2012, 41, 1437.
† Electronic Supplementary Information (ESI) available: Experimental
procedures, NMR and Mass spectra data of sulfoxides.
15. Z. M. Liu, H. Zhao, M. Q. Li, Y. B. Lan, Q. B. Yao, J. C. Tao and X.
W. Wang, Adv. Synth. Catal., 2012, 354, 1012.
100
105
110
45 1. P. T. Anastas and J. C. Warner, Green Chemistry, Theory and
Practice, Oxford University Press, Oxford, 1998.
16. (a)R. L. Levine, J. Moskovitz and E. R. Stadtman, IUBMB Life, 2000,
50, 301; (b)G. Jori, G. Galiazzo and E. Scoffone, Int. J. Protein Res.,
1969, 1, 289.
2. R. Noyori, Adv. Synth. Catal., 2001, 343, 1.
3. D. A. Nicewicz and D. W. C. MacMillan, Science, 2008, 322, 77.
4. M. A. Ischay, M. E. Anzovino, J. Du and T. P. Yoon, J. Am. Chem.
17. (a)A. P. Schaap, A. L. Thayer, E. C. Blossey and D. C. Neckers, J.
Am. Chem. Soc., 1975, 97, 3741; (b)E. C. Blossey and D. C. Neckers,
J. Am. Chem. Soc., 1973, 95, 5820.
50
Soc., 2008, 130, 12886.
5. (a)J. Xuan and W. J. Xiao, Angew. Chem. Int. Ed., 2012, 51, 6828;
(b)J. W. Tucker and C. R. J. Stephenson, J. Org. Chem., 2012, 77,
1617; (c)M. A. Ischay and T. P. Yoon, Eur. J. Org. Chem., 2012,
3359; (d)F. Teply, Collect. Czech. Chem. Commun., 2011, 76, 859;
(e)J. M. R. Narayanam and C. R. J. Stephenson, Chem. Soc. Rev.,
2011, 40, 102; (f)T. P. Yoon, M. A. Ischay and J. N. Du, Nature
Chemistry, 2010, 2, 527; (g)K. Zeitler, Angew. Chem. Int. Ed., 2009,
48, 9785.
18. (a)A. W. Jensen and C. Daniels, J. Org. Chem., 2003, 68, 207; (b)D.
Madhavan and K. Pitchumani, Tetrahedron, 2001, 57, 8391; (c)S. D.
Green, C. Monti, R. F. W. Jackson, M. S. Anson and S. J. F.
Macdonald, Chem. Commun., 2001, 2594.
55
19. C. L. Gu, C. S. Foote and M. L. Kacher, J. Am. Chem. Soc., 1981,
103, 5949.
4
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