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[email protected] Current Organic Synthesis, 2014, 11, 156-160
156
Selective Oxidation of Sulfides to Sulfoxides with Tert-Butylnitrite as an Alternative Oxidant Bing Yu, Zhen-Feng Diao, An-Hua Liu, Xu Han, Bin Li, Liang-Nian He* and Xiang-Ming Liu State Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, People's Republic of China Abstract: Tert-butylnitrite was proved to be an efficient oxidant for the selective oxidation of sulfides to sulfoxides. The reaction was promoted by Fe(NO3)3·9H2O under mild conditions and various substrates were effectively converted into the corresponding sulfoxides in good yields and excellent selectivity.
Keywords: Fe-promoted, oxidation, selectivity, sulfide, sulfoxide, Tert-Butylnitrite. INTRODUCTION
RESULTS AND DISCUSSION
Sulfoxides are important intermediates for the synthesis of several biologically and pharmaceutically significant compounds [1-3]. In particular, chiral sulfoxides have found application in asymmetric synthesis as ligand or chiral auxiliary [4, 5]. Consequently, much effort has been directed toward the preparation of sulfoxides. In this context, one of the most favored and straightforward routes to sulfoxides could be selective oxidation of the corresponding sulfides [6, 7]. As shown in Scheme 1, a myriad of oxidative systems have been developed for the oxidation of sulfides, such as Nsulfonyloxaziridines [8], Re(O)Cl3(PPh3)2 [9], Cu(acac)2/H2O2 [10], thiourea dioxide/TBHP [11], TetMe-IBX [12], Fe/SBA-15/H2O2 [13], [(n-C4H9)4N]4(-Mo8O26)/H2O2 [14], Br2/NaNO2/H2O/O2 [15], etc. Nevertheless, despite considerable progress in the field of sulfide oxidation, the selective sulfoxidation is still a major challenge as most protocols involve overoxidation or high energy consumption. As a consequence, oxidation methods with high selectivity are urgently desirable.
In the preliminary study, thioanisole 1a was chosen as the model substrate. A series of solvents were examined by performing the reaction of thioanisole and equimolar TBN at 100 oC for 12 h, and the results are shown in Table 1. It was found that aprotic solvents like toluene, 1,4-dioxane and ethyl acetate afforded phenyl methyl sulfoxide 2a in good selectivities but low yields (entries 13). Other aprotic solvents such as acetone, propylene carbonate were demonstrated to be inefficient (entries 4 and 12). Protic solvents like methanol, ethanol and water gave the desired sulfoxide in low yields (entries 6-8). Moreover, good conversions were achieved by using dimethyl carbonate, diethyl carbonate and ethylene carbonate as solvent, whereas the overoxidation product i.e. sulfone 3a was also detected (entries 9-11). Interestingly, the reaction in acetonitrile performed well with excellent selectivity toward sulfoxide 2a (entry 5). As a consequence, acetonitrile was employed for further investigation.
Scheme 1. Catalytic oxidation of sulfides to sulfoxides.
Tert-butylnitrite (t-BuONO, TBN) is a versatile reagent with essential properties, such as low boiling point and favorable solubility, which leads to the feasibility of mixing or separating with other reagents [16]. Therefore, TBN has been widely applied as diazotizating reagent [17-19], nitrosation reagent [20-23] and nitration reagent [24-26]. In particular, TBN played a key role in activation of molecular oxygen in oxidative reactions [27-29]. Recently, TBN has been employed as oxidant in C-C bond cleavage of 1,3diketones [30] and oxidation of benzylic alcohols [31]. Moreover, selective oxidation of sulfides using HBr and TBN as catalysts, O2 as an oxidant was also achieved by Zhang and coworkers in 2008 [32]. Encouraged by these developments, we wish to report herein an inexpensive Fe(III)-promoted oxidation of sulfides with TBN as oxidant in which hazardous halide can be avoided.
*Address correspondence to this author at the State Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, China; Tel: +86-22-23503878; E-mail:
[email protected] 1875-6271/14 $58.00+.00
Subsequently, the effects of TBN amount and reaction temperature on the reaction were evaluated (Table 2). By changing the temperature, the highest conversion and selectivity were attained at 60 oC (entries 1-6). When 2.0 equiv. of TBN was applied, the conversion can be improved slightly at either 80 oC or 60 oC (entries 7 and 8). However, the corresponding sulfone 3a was detected under 80 oC (entry 8). Furthermore, the sulfoxide 2a was obtained in 73% conversion and 70% yield by increasing the amount of TBN to 3.0 equiv. at 60 oC (entry 9). Further increasing the oxidant amount to 5.0 equiv., the similar result was observed (entry 9 vs. 10). Therefore, 3.0 equivalents of TBN were employed for further investigation. With 3.0 equiv. TBN as oxidant in acetonitrile at 60 oC for 12 h, various additives were examined for this reaction as listed in Table 3. The reactivity was slightly decreased when 10 mol% of the quaternary ammonium salts like Me4NBr, Bu4NBr and Bu4NI was added (entries 1-3). In the cases of NBS, I2 and DTBP, the conversions and yields still remained at the same level (entries 4-6). However, Cu(NO3)2·3H2O and TEMPO could partly suppress the sulfoxidation (entries 7 and 8). To our delight, the reaction was promoted by Fe(NO3)3·9H2O resulting in a 84% yield of the sulfoxide 2a with excellent selectivity (entry 9). Further prolonging the reaction time cannot further increase the yield (entry 10). On the other hand, 20 mol% Fe(NO3)3·9H2O gave an almost quantitative yield of 2a (entry 11). The control experiment without air was conducted (entry 12). However, the reaction could also proceed smoothly under inert atmosphere, which indicated the insignificant effect of air. In addition, ESI-MS analysis of the reaction mixture indicated the adduct of sulfide, TBN and iron salt (see supporting information). It © 2014 Bentham Science Publishers
Selective Oxidation of Sulfides
Current Organic Synthesis, 2014, Vol. 11, No. 1 157
Table 1. Oxidation of Sulfides by TBN in Different Solventsa
Entry
a b
Conv.(%)b
Solvent
Yield (%)b 2a
3a
1
Toluene
37
21
0
2
1,4-Dioxane
23
12
0
3
Ethyl acetate
44
43
0
4
Acetone
6
2
0
5
Acetonitrile
55
49
0
6
Methanol
19
14
0
7
Ethanol
12
10
0
8
Water
19
18
0
9
Dimethyl carbonate
45
39
3
10
Diethyl carbonate
57
50
2
11
Ethylene carbonate
66
60
4
12
Propylene carbonate
8
6
0
o
Reaction conditions: thioanisole (1.0 mmol, 0.1242 g), TBN (1.0 mmol, 0.1031 g), solvent (5 mL), 100 C, 12 h. Yields determined by GC with area normalization.
Table 2. The Influence of Temperature and TBN Amount on Sulfoxidationa S
TBN
O S
CH3CN, 12 h
T (oC)
+
2a
1a
Entry
O
TBN (equiv.)
S
O
3a Yield (%)b
Conv.(%)b 2a
3a
1
30
1.0
46
44
0
2
45
1.0
61
58
0
3
60
1.0
66
61
0
4
80
1.0
66
56
0
5
100
1.0
55
49
0
6
120
1.0
42
34
0
7
80
2.0
72
65
2
8
60
2.0
70
67
0
9
60
3.0
73
70
0
10
60
5.0
74
69
