- Email: [email protected]
Enantioselective sulfenylation of α-nitroesters catalyzed by diarylprolinols
Tetrahedron Letters 55 (2014) 387–389
Contents lists available at ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Enantioselective sulfenylation of a-nitroesters catalyzed by diarylprolinols Ling Fang a,b,⇑, Aijun Lin a, Yan Shi a, Yixiang Cheng a, Chengjian Zhu a,c,⇑ a
School of Chemistry and Chemical Engineering, State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, PR China College of Environmental and Biological Engineering, Chongqing Technology and Business University, Chongqing 400067, PR China c State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, PR China b
a r t i c l e
i n f o
Article history: Received 29 August 2013 Revised 27 October 2013 Accepted 11 November 2013 Available online 19 November 2013
a b s t r a c t The organocatalytic sulfenylation of a-nitroesters mediated by diaryl-L-prolinols was developed. A range of a-sulfenylated a-nitroesters were obtained in good yields with moderate to good enantioselectivities. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Organocatalysis Hydrogen bonding Sulfenylation a-Nitroesters Asymmetric
Many optically active sulfur-containing compounds exhibit pharmaceutical or biological activities, which were also employed as ligands, auxiliaries, and synthetic intermediates in organic chemistry.1 Asymmetric introduction of sulfur bonded directly to a stereogenic center is a challenge, which has attracted attention from organic chemists.2,3 The former strategy, involving nucleophilic process of thiols and thioacetic acids, has proven to be useful and is well documented in the literature.2 Recently, electrophilic sulfenylation as a complementary procedure was reported by several groups.3 Therefore, development of new methodologies for constructing this framework with various types of substrates has been an active area of research. On the other hand, although L-proline derivatives have been widely developed in asymmetric organocatalysis,4 reports related to L-proline derivatives exploited as hydrogen bond acceptor are limited so far.5,3e,f Nitro compounds are valuable precursor for dyes and insecticides,6 however, to our knowledge, no catalytic process are available for the preparation of chiral a-sulfenylated nitro compounds to date. As a continuation of our efforts to exhibit the diversity of the reaction substrates activated by secondary amines as the H-bond acceptor, herein, we wish to report the first asymmetric sulfenylation of a-nitroesters catalyzed by diaryl-L-prolinols. A model reaction of 2a and 3a was examined under a set of conditions, with catalyst variety, solvent, temperature, and catalyst ⇑ Corresponding authors. Tel./fax: +86 25 83594886. E-mail addresses: [email protected] (L. Fang), [email protected] (C. Zhu). 0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tetlet.2013.11.037
loading as parameters. The results are summarized in Table 1. Investigations into the catalysts (Fig. 1) revealed that diaryl-Lprolinols were superior to thioureas with regard to enantioselectivity and catalytic efficiency (entries 1–6). The sulfenylation product 4aa was isolated in 87% yield with 56% ee in the presence of catalyst 1b (20 mol % catalyst loading) in CH2Cl2 at room temperature (entry 2). However, the electronic properties of the aromatic ring of diarylprolinol had a substantial impact on the catalysts’ performance. A sharp decrease in catalyst activity was observed when 1d, bearing an electron-withdrawing group on the aromatic ring, was tested (entry 4). Solvent screening indicated that ether was the best choice among aprotic solvents (entries 2 and 7–10). Moreover, lower temperature (0 °C) led to some loss of catalyst efficiency (from 95% to 47% yield) albeit with a minor increase in enantioselectivity (entries 8 and 11). Decreasing the catalyst loading also resulted in diminished yields (entries 12 and 13). Under the optimal reaction conditions, we then focused on the variation in a-nitroesters and sulfur reagents. The reaction was conducted with 20 mol % catalyst 1b in ether at room temperature. As highlighted in Table 2, compared with more sterically hindered a-nitroesters, improved enantioselectivities (ca. 70% ee) could be achieved when those with simple structures 2a–c were investigated (entries 1–9). Furthermore, the aryl sulfur reagents afforded the corresponding products with higher yields and ee values than benzyl sulfur reagent (entries 4 and 9). When ethyl 2-nitro-2phenylacetate 2d was examined in the identical reaction
388
L. Fang et al. / Tetrahedron Letters 55 (2014) 387–389
Table 1 Optimization of the catalysts and reaction conditions for the enantioselective sulfenylation of a-nitroester 2aa O
O Me
Catalyst 1
+ PhS N
OEt NO2
O
2a
PhS
Solvent, r.t.
Table 2 Organocatalytic asymmetric sulfenylation of a-nitroestersa
O
R2
Catalyst
Yieldb (%)
eec (%)
1 2 3 4 5 6 7 8 9 10 11 12 13
CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 Hexane Et2O MTBEf Toluene Et2Og Et2Oh Et2Oi
1a 1b 1c 1d 1e 1f 1b 1b 1b 1b 1b 1b 1b
94 87 75 —d 47 52 88 95 91 80 47 80 68
24 56 22 —e 14 5 69 72 70 69 75 72 68
a Reaction conditions: 3a (0.22 mmol) was added to the solution of a-nitroester 2a (0.2 mmol) and catalyst (0.04 mmol, 20 mol %) in solvent (2 mL) at room temperature and stirred for 24 h. b Isolated yields. c Determined by chiral HPLC analysis. d <5% Yield. e Not determined. f MTBE = methyl tert-butyl ether. g Reaction ran at 0 °C. h 10 mol % of catalyst 1b was used. i 5 mol % of catalyst 1b was used.
3a-3d 3a: R=Ph; 3b: R=pMe-Ph; 3c: R=pCl-Ph 3d: R=PhCH2
O O
R2
NO2 4
2a: R1=Me, R2=Et; 2b: R1=Et, R2=Et; 2c: R1=Me, R2=Me; 2d: R1=Ph, R2=Et; 2e: R1=Et, R2=2,6-(CH3)2C6H3; 2f: R1=Et, R2=t-Bu;
4aa
Solvent
Et2O, r.t.
R1 RS
O
2a-2f
Entry
1b (20 mol%)
+ R S N
NO2
OEt NO2
3a
O
O
R1
O Me
Entry
2
Product 4
1
2a
4aa
S
Yieldb (%)
eec (%)
95
72
93
67
80
63
67
40
90
67
96
60
97
67
98
64
60
32
—
—
O
78
10
OBut
84
28
81
30
O
Me
OEt NO2
2
2a
Me
S
4ab
O OEt
NO2
3
2a
S
4ac
2a
O OEt
NO2
Cl
4
Me
Me
S
4ad
O OEt
NO2
5
2b
S
4ba
O
Et
OEt NO2
Ar Ar N H HO
1a, Ar = Ph 1b, Ar = 3,5-(CH3)2C6H3 1c, Ar = 4-CH3OC6H4 1d, Ar = 3,5-(CF3)2C6H3
N H 1e
CF3
N
N H
4bc
7 N H
2c
S
4ca
OEt
O OMe
8
2c
S
4cc
Figure 1. Catalysts employed in this reaction.
conditions, it nearly completely converted into ethyl benzoate7 and no sulfenylation product was isolated (entries 10).8 Possibly due to the conjugative effects of the adjacent aryl group, the carbon-centered anion intermediate formed under basic medium underwent rearrangement prior to electrophilic substitution. Interestingly, increasing the bulky hindrance of the substrate ester moiety by introducing tert-butyl or 2,6-dimethylphenyl, reduced the reaction stereoselectivity (entries 11 and 12). In contrast to the general literatures related to asymmetric catalysis, the results indicated that steric effect of the substrate was not the significant factor for reaction enantioselectivity in this catalytic system. Nevertheless, ethyl 2-cyanopropanoate 5 only afforded the sulfenylation product 6 with 30% ee (entry 13). These results revealed that the structure of nitroesters had a substantial impact on reaction enantioselectivity. Supported by the experiment results and the analogy to sulfenylation of b-ketoesters,3e it is suggested that the reaction intermediate was formed by the strong interaction of hydrogenbonding between substrate with nitro group and the catalyst although the reaction mechanism is not clear. In summary, we have developed a facile protocol for preparation of a-sulfenylated a-nitroesters9 in moderate-to-good ee’s, and this is another example that L-proline derivatives could serve
Me
NO2
CF3
1f
O
NO2
Cl
S
S N H
2b
CF3
CF3
NH
6
Et
S
2c
O OMe
NO2
Cl
9
Me
Me
S
4cd
O OMe
NO2
10
11
2d
2e
4da — S
4ec
NO2
Cl
12
2f
S
4fc
13
5
Et
O
NO2
Cl
6
Et
O
S
Me
O OEt
CN a Reaction conditions: 3 (0.22 mmol) was added to the solution of a-nitroester (0.2 mmol) and catalyst 1b in Et2O (2 mL) at room temperature, and stirred for 24– 36 h. b Isolated yields. c Determined by chiral HPLC analysis.
as hydrogen bond acceptor for more substrates with a a-acidic hydrogen atom.
L. Fang et al. / Tetrahedron Letters 55 (2014) 387–389
Acknowledgments We gratefully acknowledge the National Natural Science Foundation of China (21172106 and 21074054) and the National Basic Research Program of China (2010CB923303) for their financial support. Research Fund for the Doctoral Program of Higher Education of China (20120091110010) is also acknowledged.
4.
5.
Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2013. 11.037. References and notes 1. (a) Evans, D. A.; Campos, K. R.; Tedrow, J. S.; Michael, F. E.; Gagné, M. R. J. Am. Chem. Soc. 2000, 122, 7905–7920; (b) Masdeu-Bultó, A. M.; Diéguez, M.; Martin, E.; Gómez, M. Coord. Chem. Rev. 2003, 242, 159–201. 2. For selected examples, see: (a) McDaid, P.; Chen, Y.; Deng, L. Angew. Chem., Int. Ed. 2002, 41, 338–340; (b) Leow, D.; Lin, S.; Chittimlla, S. K.; Fu, X.; Tan, C.-H. Angew. Chem., Int. Ed. 2008, 47, 5641–5645; (c) Wang, Z.; Sun, X.; Ye, S.; Wang, W.; Wang, B.; Wu, J. Tetrahedron: Asymmetry 2008, 19, 964–969; (d) Kimmel, K. L.; Robak, M. T.; Ellman, J. A. J. Am. Chem. Soc. 2009, 131, 8754–8755; (e) Sun, J.; Fu, G. C. J. Am. Chem. Soc. 2010, 132, 4568–4569; (f) Tian, X.; Cassani, C.; Liu, Y.; Moran, A.; Urakawa, A.; Galzerano, P.; Arceo, E.; Melchiorre, P. J. Am. Chem. Soc. 2011, 133, 17934–17941; (g) Shibatomi, K.; Soga, Y.; Narayama, A.; Fujisawa, I.; Iwasa, S. J. Am. Chem. Soc. 2012, 134, 9836–9839. 3. For selected examples, see: (a) Zhao, G.-L.; Rios, R.; Vesely, J.; Eriksson, L.; Córdova, A. Angew. Chem., Int. Ed. 2008, 47, 8468–8472; (b) Jereb, M.; Togni, A. Org. Lett. 2005, 7, 4041–4043; (c) Sobhani, S.; Fielenbach, D.; Marigo, M.; Wabnitz, T. C.; Jøgensen, K. A. Chem. Eur. J. 2005, 11, 5689–5694; (d) Cai, Y.; Li, J.;
6. 7. 8.
9.
389
Chen, W.; Xie, M.; Liu, X.; Lin, L.; Feng, X. Org. Lett. 2012, 14, 2726–2729; (e) Fang, L.; Lin, A.; Hu, H.; Zhu, C. Chem. Eur. J. 2009, 15, 7039–7043; (f) Lin, A.; Fang, L.; Zhu, X.; Zhu, C.; Cheng, Y. Adv. Synth. Catal. 2011, 353, 545–549; (g) Li, X.; Liu, C.; Xue, X.-S.; Cheng, J.-P. Org. Lett. 2012, 14, 4374–4377. For reviews, see: (a) Mukherjee, S.; Yang, J. W.; Hoffmann, S.; List, B. Chem. Rev. 2007, 107, 5471–5569; (b) Jensen, K. L.; Dickmeiss, G.; Jiang, H.; Albrecht, Ł.; Jørgensen, K. A. Acc. Chem. Res. 2012, 45, 248–264; (c) Mielgo, A.; Palomo, C. Chem. Asian J. 2008, 3, 922–948. For examples, see: (a) Lattanzi, A. Tetrahedron: Asymmetry 2006, 17, 837–841; (b) Lattanzi, A. Adv. Synth. Catal. 2006, 348, 339–346; (c) Russo, A.; Lattanzi, A. Adv. Synth. Catal. 2008, 350, 1991–1995; (d) Li, Y.; Liu, X.; Yang, Y.; Zhao, G. J. Org. Chem. 2007, 72, 288–291; (e) Zheng, C.; Li, Y.; Yang, Y.; Wang, H.; Cui, H.; Zhang, J.; Zhao, G. Adv. Synth. Catal. 2009, 351, 1685–1691; (f) Russo, A.; Lattanzi, A. Org. Biomol. Chem. 2010, 8, 2633–2638. (a) Metcalf, R. L. Organic Insecticides: Their Chemistry and Mode of Action; Interscience: New York, 1955; (b) Martin, H. EP 0114705, 1984. This byproduct was confirmed according to the literature and 1H NMR and 13C NMR analysis: Wang, A.; Jiang, H. J. Am. Chem. Soc. 2008, 130, 5030–5031. Similar phenomena were also observed in these literatures: (a) Ramírez, J.; Huber, D. P.; Togni, A. Synlett 2007, 1143–1147; (b) Han, B.; Liu, Q.-P.; Li, R.; Tian, X.; Xiong, X.-F.; Deng, J.-G.; Chen, Y.-C. Chem. Eur. J. 2008, 14, 8094–8097. General procedure for the organocatalytic sulfenylation of a-nitroesters: A solution of catalyst 1b (12.4 mg, 0.04 mmol) and a-nitroesters 2 (0.2 mmol) in dry ether (2 mL) was stirred for 5 min at room temperature before the sulfur reagent 3 (0.22 mmol) was added, and the resulting mixture was stirred for 24-36 h. The crude reaction mixture was diluted with ethyl acetate and then directly purified by flash chromatography on silica gel (petroleum ether/EtOAc) to afford the corresponding product 4. Ethyl 2-nitro-2-(phenylthio)propanoate (4aa): Colorless oil; yield: 95% (48.4 mg); 72% ee [HPLC (Chiralcel OJ–H, hexane/i-PrOH = 90:10, flow rate 0.5 ml/min), retention time, 19.8 min (major), 23.2 min (minor)]. ½a25 D ¼ 12:9 (c 0.45, EtOAc). 1H NMR (300 MHz, CDCl3): d 7.64–7.61 (m, 2H), 7.50–7.45 (m, 1H), 7.42–7.37 (m, 2H), 4.20 (dq, J = 10.8, 7.2 Hz, 1H), 4.14 (dq, J = 10.8, 7.2 Hz, 1H), 1.95 (s, 3H), 1.18 (t, J = 7.2 Hz, 3H) ppm. 13C NMR (75 MHz, CDCl3): d 164.4, 137.0, 130.8, 129.2, 127.8, 99.1, 63.4, 23.7, 13.5 ppm. EI-MS: m/z 208 (35) [MHNO2]+, 135 (100).
Contents lists available at ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Enantioselective sulfenylation of a-nitroesters catalyzed by diarylprolinols Ling Fang a,b,⇑, Aijun Lin a, Yan Shi a, Yixiang Cheng a, Chengjian Zhu a,c,⇑ a
School of Chemistry and Chemical Engineering, State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, PR China College of Environmental and Biological Engineering, Chongqing Technology and Business University, Chongqing 400067, PR China c State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, PR China b
a r t i c l e
i n f o
Article history: Received 29 August 2013 Revised 27 October 2013 Accepted 11 November 2013 Available online 19 November 2013
a b s t r a c t The organocatalytic sulfenylation of a-nitroesters mediated by diaryl-L-prolinols was developed. A range of a-sulfenylated a-nitroesters were obtained in good yields with moderate to good enantioselectivities. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Organocatalysis Hydrogen bonding Sulfenylation a-Nitroesters Asymmetric
Many optically active sulfur-containing compounds exhibit pharmaceutical or biological activities, which were also employed as ligands, auxiliaries, and synthetic intermediates in organic chemistry.1 Asymmetric introduction of sulfur bonded directly to a stereogenic center is a challenge, which has attracted attention from organic chemists.2,3 The former strategy, involving nucleophilic process of thiols and thioacetic acids, has proven to be useful and is well documented in the literature.2 Recently, electrophilic sulfenylation as a complementary procedure was reported by several groups.3 Therefore, development of new methodologies for constructing this framework with various types of substrates has been an active area of research. On the other hand, although L-proline derivatives have been widely developed in asymmetric organocatalysis,4 reports related to L-proline derivatives exploited as hydrogen bond acceptor are limited so far.5,3e,f Nitro compounds are valuable precursor for dyes and insecticides,6 however, to our knowledge, no catalytic process are available for the preparation of chiral a-sulfenylated nitro compounds to date. As a continuation of our efforts to exhibit the diversity of the reaction substrates activated by secondary amines as the H-bond acceptor, herein, we wish to report the first asymmetric sulfenylation of a-nitroesters catalyzed by diaryl-L-prolinols. A model reaction of 2a and 3a was examined under a set of conditions, with catalyst variety, solvent, temperature, and catalyst ⇑ Corresponding authors. Tel./fax: +86 25 83594886. E-mail addresses: [email protected] (L. Fang), [email protected] (C. Zhu). 0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tetlet.2013.11.037
loading as parameters. The results are summarized in Table 1. Investigations into the catalysts (Fig. 1) revealed that diaryl-Lprolinols were superior to thioureas with regard to enantioselectivity and catalytic efficiency (entries 1–6). The sulfenylation product 4aa was isolated in 87% yield with 56% ee in the presence of catalyst 1b (20 mol % catalyst loading) in CH2Cl2 at room temperature (entry 2). However, the electronic properties of the aromatic ring of diarylprolinol had a substantial impact on the catalysts’ performance. A sharp decrease in catalyst activity was observed when 1d, bearing an electron-withdrawing group on the aromatic ring, was tested (entry 4). Solvent screening indicated that ether was the best choice among aprotic solvents (entries 2 and 7–10). Moreover, lower temperature (0 °C) led to some loss of catalyst efficiency (from 95% to 47% yield) albeit with a minor increase in enantioselectivity (entries 8 and 11). Decreasing the catalyst loading also resulted in diminished yields (entries 12 and 13). Under the optimal reaction conditions, we then focused on the variation in a-nitroesters and sulfur reagents. The reaction was conducted with 20 mol % catalyst 1b in ether at room temperature. As highlighted in Table 2, compared with more sterically hindered a-nitroesters, improved enantioselectivities (ca. 70% ee) could be achieved when those with simple structures 2a–c were investigated (entries 1–9). Furthermore, the aryl sulfur reagents afforded the corresponding products with higher yields and ee values than benzyl sulfur reagent (entries 4 and 9). When ethyl 2-nitro-2phenylacetate 2d was examined in the identical reaction
388
L. Fang et al. / Tetrahedron Letters 55 (2014) 387–389
Table 1 Optimization of the catalysts and reaction conditions for the enantioselective sulfenylation of a-nitroester 2aa O
O Me
Catalyst 1
+ PhS N
OEt NO2
O
2a
PhS
Solvent, r.t.
Table 2 Organocatalytic asymmetric sulfenylation of a-nitroestersa
O
R2
Catalyst
Yieldb (%)
eec (%)
1 2 3 4 5 6 7 8 9 10 11 12 13
CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 Hexane Et2O MTBEf Toluene Et2Og Et2Oh Et2Oi
1a 1b 1c 1d 1e 1f 1b 1b 1b 1b 1b 1b 1b
94 87 75 —d 47 52 88 95 91 80 47 80 68
24 56 22 —e 14 5 69 72 70 69 75 72 68
a Reaction conditions: 3a (0.22 mmol) was added to the solution of a-nitroester 2a (0.2 mmol) and catalyst (0.04 mmol, 20 mol %) in solvent (2 mL) at room temperature and stirred for 24 h. b Isolated yields. c Determined by chiral HPLC analysis. d <5% Yield. e Not determined. f MTBE = methyl tert-butyl ether. g Reaction ran at 0 °C. h 10 mol % of catalyst 1b was used. i 5 mol % of catalyst 1b was used.
3a-3d 3a: R=Ph; 3b: R=pMe-Ph; 3c: R=pCl-Ph 3d: R=PhCH2
O O
R2
NO2 4
2a: R1=Me, R2=Et; 2b: R1=Et, R2=Et; 2c: R1=Me, R2=Me; 2d: R1=Ph, R2=Et; 2e: R1=Et, R2=2,6-(CH3)2C6H3; 2f: R1=Et, R2=t-Bu;
4aa
Solvent
Et2O, r.t.
R1 RS
O
2a-2f
Entry
1b (20 mol%)
+ R S N
NO2
OEt NO2
3a
O
O
R1
O Me
Entry
2
Product 4
1
2a
4aa
S
Yieldb (%)
eec (%)
95
72
93
67
80
63
67
40
90
67
96
60
97
67
98
64
60
32
—
—
O
78
10
OBut
84
28
81
30
O
Me
OEt NO2
2
2a
Me
S
4ab
O OEt
NO2
3
2a
S
4ac
2a
O OEt
NO2
Cl
4
Me
Me
S
4ad
O OEt
NO2
5
2b
S
4ba
O
Et
OEt NO2
Ar Ar N H HO
1a, Ar = Ph 1b, Ar = 3,5-(CH3)2C6H3 1c, Ar = 4-CH3OC6H4 1d, Ar = 3,5-(CF3)2C6H3
N H 1e
CF3
N
N H
4bc
7 N H
2c
S
4ca
OEt
O OMe
8
2c
S
4cc
Figure 1. Catalysts employed in this reaction.
conditions, it nearly completely converted into ethyl benzoate7 and no sulfenylation product was isolated (entries 10).8 Possibly due to the conjugative effects of the adjacent aryl group, the carbon-centered anion intermediate formed under basic medium underwent rearrangement prior to electrophilic substitution. Interestingly, increasing the bulky hindrance of the substrate ester moiety by introducing tert-butyl or 2,6-dimethylphenyl, reduced the reaction stereoselectivity (entries 11 and 12). In contrast to the general literatures related to asymmetric catalysis, the results indicated that steric effect of the substrate was not the significant factor for reaction enantioselectivity in this catalytic system. Nevertheless, ethyl 2-cyanopropanoate 5 only afforded the sulfenylation product 6 with 30% ee (entry 13). These results revealed that the structure of nitroesters had a substantial impact on reaction enantioselectivity. Supported by the experiment results and the analogy to sulfenylation of b-ketoesters,3e it is suggested that the reaction intermediate was formed by the strong interaction of hydrogenbonding between substrate with nitro group and the catalyst although the reaction mechanism is not clear. In summary, we have developed a facile protocol for preparation of a-sulfenylated a-nitroesters9 in moderate-to-good ee’s, and this is another example that L-proline derivatives could serve
Me
NO2
CF3
1f
O
NO2
Cl
S
S N H
2b
CF3
CF3
NH
6
Et
S
2c
O OMe
NO2
Cl
9
Me
Me
S
4cd
O OMe
NO2
10
11
2d
2e
4da — S
4ec
NO2
Cl
12
2f
S
4fc
13
5
Et
O
NO2
Cl
6
Et
O
S
Me
O OEt
CN a Reaction conditions: 3 (0.22 mmol) was added to the solution of a-nitroester (0.2 mmol) and catalyst 1b in Et2O (2 mL) at room temperature, and stirred for 24– 36 h. b Isolated yields. c Determined by chiral HPLC analysis.
as hydrogen bond acceptor for more substrates with a a-acidic hydrogen atom.
L. Fang et al. / Tetrahedron Letters 55 (2014) 387–389
Acknowledgments We gratefully acknowledge the National Natural Science Foundation of China (21172106 and 21074054) and the National Basic Research Program of China (2010CB923303) for their financial support. Research Fund for the Doctoral Program of Higher Education of China (20120091110010) is also acknowledged.
4.
5.
Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2013. 11.037. References and notes 1. (a) Evans, D. A.; Campos, K. R.; Tedrow, J. S.; Michael, F. E.; Gagné, M. R. J. Am. Chem. Soc. 2000, 122, 7905–7920; (b) Masdeu-Bultó, A. M.; Diéguez, M.; Martin, E.; Gómez, M. Coord. Chem. Rev. 2003, 242, 159–201. 2. For selected examples, see: (a) McDaid, P.; Chen, Y.; Deng, L. Angew. Chem., Int. Ed. 2002, 41, 338–340; (b) Leow, D.; Lin, S.; Chittimlla, S. K.; Fu, X.; Tan, C.-H. Angew. Chem., Int. Ed. 2008, 47, 5641–5645; (c) Wang, Z.; Sun, X.; Ye, S.; Wang, W.; Wang, B.; Wu, J. Tetrahedron: Asymmetry 2008, 19, 964–969; (d) Kimmel, K. L.; Robak, M. T.; Ellman, J. A. J. Am. Chem. Soc. 2009, 131, 8754–8755; (e) Sun, J.; Fu, G. C. J. Am. Chem. Soc. 2010, 132, 4568–4569; (f) Tian, X.; Cassani, C.; Liu, Y.; Moran, A.; Urakawa, A.; Galzerano, P.; Arceo, E.; Melchiorre, P. J. Am. Chem. Soc. 2011, 133, 17934–17941; (g) Shibatomi, K.; Soga, Y.; Narayama, A.; Fujisawa, I.; Iwasa, S. J. Am. Chem. Soc. 2012, 134, 9836–9839. 3. For selected examples, see: (a) Zhao, G.-L.; Rios, R.; Vesely, J.; Eriksson, L.; Córdova, A. Angew. Chem., Int. Ed. 2008, 47, 8468–8472; (b) Jereb, M.; Togni, A. Org. Lett. 2005, 7, 4041–4043; (c) Sobhani, S.; Fielenbach, D.; Marigo, M.; Wabnitz, T. C.; Jøgensen, K. A. Chem. Eur. J. 2005, 11, 5689–5694; (d) Cai, Y.; Li, J.;
6. 7. 8.
9.
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Chen, W.; Xie, M.; Liu, X.; Lin, L.; Feng, X. Org. Lett. 2012, 14, 2726–2729; (e) Fang, L.; Lin, A.; Hu, H.; Zhu, C. Chem. Eur. J. 2009, 15, 7039–7043; (f) Lin, A.; Fang, L.; Zhu, X.; Zhu, C.; Cheng, Y. Adv. Synth. Catal. 2011, 353, 545–549; (g) Li, X.; Liu, C.; Xue, X.-S.; Cheng, J.-P. Org. Lett. 2012, 14, 4374–4377. For reviews, see: (a) Mukherjee, S.; Yang, J. W.; Hoffmann, S.; List, B. Chem. Rev. 2007, 107, 5471–5569; (b) Jensen, K. L.; Dickmeiss, G.; Jiang, H.; Albrecht, Ł.; Jørgensen, K. A. Acc. Chem. Res. 2012, 45, 248–264; (c) Mielgo, A.; Palomo, C. Chem. Asian J. 2008, 3, 922–948. For examples, see: (a) Lattanzi, A. Tetrahedron: Asymmetry 2006, 17, 837–841; (b) Lattanzi, A. Adv. Synth. Catal. 2006, 348, 339–346; (c) Russo, A.; Lattanzi, A. Adv. Synth. Catal. 2008, 350, 1991–1995; (d) Li, Y.; Liu, X.; Yang, Y.; Zhao, G. J. Org. Chem. 2007, 72, 288–291; (e) Zheng, C.; Li, Y.; Yang, Y.; Wang, H.; Cui, H.; Zhang, J.; Zhao, G. Adv. Synth. Catal. 2009, 351, 1685–1691; (f) Russo, A.; Lattanzi, A. Org. Biomol. Chem. 2010, 8, 2633–2638. (a) Metcalf, R. L. Organic Insecticides: Their Chemistry and Mode of Action; Interscience: New York, 1955; (b) Martin, H. EP 0114705, 1984. This byproduct was confirmed according to the literature and 1H NMR and 13C NMR analysis: Wang, A.; Jiang, H. J. Am. Chem. Soc. 2008, 130, 5030–5031. Similar phenomena were also observed in these literatures: (a) Ramírez, J.; Huber, D. P.; Togni, A. Synlett 2007, 1143–1147; (b) Han, B.; Liu, Q.-P.; Li, R.; Tian, X.; Xiong, X.-F.; Deng, J.-G.; Chen, Y.-C. Chem. Eur. J. 2008, 14, 8094–8097. General procedure for the organocatalytic sulfenylation of a-nitroesters: A solution of catalyst 1b (12.4 mg, 0.04 mmol) and a-nitroesters 2 (0.2 mmol) in dry ether (2 mL) was stirred for 5 min at room temperature before the sulfur reagent 3 (0.22 mmol) was added, and the resulting mixture was stirred for 24-36 h. The crude reaction mixture was diluted with ethyl acetate and then directly purified by flash chromatography on silica gel (petroleum ether/EtOAc) to afford the corresponding product 4. Ethyl 2-nitro-2-(phenylthio)propanoate (4aa): Colorless oil; yield: 95% (48.4 mg); 72% ee [HPLC (Chiralcel OJ–H, hexane/i-PrOH = 90:10, flow rate 0.5 ml/min), retention time, 19.8 min (major), 23.2 min (minor)]. ½a25 D ¼ 12:9 (c 0.45, EtOAc). 1H NMR (300 MHz, CDCl3): d 7.64–7.61 (m, 2H), 7.50–7.45 (m, 1H), 7.42–7.37 (m, 2H), 4.20 (dq, J = 10.8, 7.2 Hz, 1H), 4.14 (dq, J = 10.8, 7.2 Hz, 1H), 1.95 (s, 3H), 1.18 (t, J = 7.2 Hz, 3H) ppm. 13C NMR (75 MHz, CDCl3): d 164.4, 137.0, 130.8, 129.2, 127.8, 99.1, 63.4, 23.7, 13.5 ppm. EI-MS: m/z 208 (35) [MHNO2]+, 135 (100).