Recyclable Nano Copper Oxide Catalyzed Stereoselective Synthesis of Vinyl Sulfides under Ligand-Free Conditions

Abstract

A simple and efficient protocol for the cross-coupling of vinyl halides with thiols catalyzed by recyclable CuO nanoparticles under ligand-free conditions is reported. This methodology results in the synthesis of a variety of vinyl sulfides in excellent yields with retention of stereochemistry.

Key words

vinyl halides - thiols - vinyl sulfides - ligand free - nano­crystalline copper oxide

References and Notes

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16

CuO nanoparticles (mean particle size: 33 nm; surface area: 29 m^²/g and purity: 99.99%) were purchased from Sigma-Aldrich. Analytical thin layer chromatography (TLC) was carried out using silica gel 60 F₂₅₄ pre-coated plates. Visualization was accomplished with a UV lamp or I₂ stain. ^¹H and ^¹³C NMR were recorded on 200 and 300 MHz instruments, in CDCl₃ using TMS as the internal standard, chemical shifts (δ) are reported in parts per million(ppm) downfield from tetramethylsilane. Melting points were determined on a Fischer-Johns melting point apparatus. Centrifugation was carried out using Kubota centrifuge (model 3500), for 1 h at 15000 rpm.
Synthesis of Vinyl Sulfides; Typical Procedure: To a stirred solution of trans-β-iodostyrene (1.0 mmol) and benzenethiol (1.0 mmol) in anhydrous DMSO (2.0 mL) at r.t., was added CuO nanoparticles (1.5 mol%) followed by KOH (1.5 equiv) and heated at 80 ˚C for 4 h. The progress of the reaction was monitored by TLC. After the reaction was complete, the reaction mixture was allowed to cool, and a 1:1 mixture of ethyl acetate-water (20 mL) was added and CuO was removed by centrifuging for 1 h at 15000 rpm. The combined organic extracts were dried with anhydrous Na₂SO₄. The solvent and volatiles were completely removed under vacuum to give the crude product, which was purified by column chromatography (petroleum ether-ethyl acetate, 99:1) to yield the expected product 1a (205.71 mg, 97% yield) as a colorless oil. The identity and purity of the product was confirmed by ^¹H and ^¹³C NMR spectroscopic analysis.
Synthesis of Vinyl Iodides; Typical Procedure:
To a solution of trans-2-phenyl vinylboronic acid (1.0 mmol) in MeCN (6 mL), which was protected from light, was added N-iodosuccinimide (1.2 mmol). After stirring for 2 h at r.t., the product was extracted with ethyl acetate (3 × 30 mL), washed with aqueous Na₂S₂O₅ (2 × 20 mL), water (2 × 20 mL), and dried (MgSO₄). Solvent evaporation in vacuo and purification by flash column chromatography afforded the vinyl iodides.
Synthesis of Vinyl Bromides; General Procedure: α,β-Unsaturated carboxylic acid (2 mmol) was added to a solution of LiOAc (0.2 mmol) in MeCN-H₂O (97:3 v/v, 4.5 mL). After the mixture was stirred for 5 min at room temperature, N-bromosuccinimide (2.1 mmol) was added as a solid. The progress of the reaction was monitored by TLC. After completion of the reaction, the product was extracted with ethyl acetate (3 × 30 mL), washed with aqueous Na₂S₂O₅ (2 × 20 mL), water (2 × 20 mL), and dried (MgSO₄). Solvent evaporation in vacuo and purification by flash column chromatography afforded the vinyl bromides.
Recycling of the Catalyst:
After the reaction was complete, the reaction mixture was allowed to cool, and a 1:1 mixture of ethyl acetate-water (2.0 mL) was added and CuO was removed by centrifu-gation. After each cycle, the catalyst was recovered by simple centrifugation, washing with deionized water and ethyl acetate and then drying in vacuo. The recovered nano-CuO was used directly in the next cycle.
Demonstration of Heterogeneous Catalysis:
To a stirred solution of trans-β-iodostyrene (1.0 mmol) and benzenethiol (1.0 mmol) in anhydrous DMSO (2.0 mL) at r.t., was added CuO nanoparticles (1.5 mol%) followed by KOH (1.5 equiv) and the mixture was heated at 80 ˚C for 1.5 h. The reaction mixture was allowed to cool and the catalyst was separated via centrifugation and 0.5 mL of the reaction mixture was worked-up. The ^¹H NMR spectrum of the reaction mass indicated 50% product formation. This filtrate obtained after the catalyst separation was further stirred for 2.5 h at 80 ˚C and the reaction mixture was worked-up. No further progress of the reaction was observed as seen by ^¹H NMR spectroscopy and the product remained at 50% only. This experiment clearly demonstrated that no leaching of
the catalyst was taking place and that the reaction was heterogeneous. TEM images of the catalyst indicated no change before and after the reaction, which further confirms the heterogeneous nature of the catalyst (Figure  [¹] a and b).
( E )-(4-Methoxyphenyl)(styryl)sulfane (Table 4, entry 3): White solid; mp 58-60 ˚C; IR (KBr): 3055, 2953, 1598, 1415, 1232, 957, 742 cm^-¹; ^¹H NMR (300 MHz, CDCl₃, TMS): δ = 7.35 (d, J = 8.87 Hz, 2 H), 7.28-7.09 (m, 5 H), 6.84 (d, J = 8.87 Hz, 2 H), 6.75 (d, J = 15.48 Hz, 1 H), 6.44 (d, J = 15.48 Hz, 1 H), 3.80 (s, 3 H); ^¹³C NMR (100 MHz, CDCl₃, TMS): δ = 159.4, 136.6, 133.3, 130.0, 128.8, 128.5, 127.1, 124.4, 114.8, 114.5, 55.2; MS (ESI): m/z = 265 [M + Na]; Anal. Calcd for C₁₅H₁₄OS: C, 74.34; H, 5.82; S, 13.23. Found: C, 74.28; H, 5.76; S, 13.17. ( E )-(4-Fluorostyryl)(naphthalen-2-yl)sulfane (Table 4, entry 6): White solid; mp 101-103 ˚C; IR (KBr): 3049, 2992, 1602, 1453, 1274, 954, 739 cm^-¹; ^¹H NMR (200 MHz, CDCl₃, TMS): δ = 7.81-7.72 (m, 4 H), 7.52-7.40 (m, 3 H), 7.32-7.24 (m, 2 H), 6.97 (t, J = 8.30 Hz, 2 H), 6.86 (d, J = 15.86 Hz, 1 H), 6.68 (d, J = 15.86 Hz, 1 H); ^¹³C NMR (100 MHz, CDCl₃, TMS): δ = 163, 133.8, 132.7, 132.4, 132.2, 130.9, 128.8, 128.2, 127.8, 127.6, 127.5, 127.3, 126.7, 126.2, 122.9, 115.8, 115.5; MS (ESI): m/z = 303 [M + Na]; Anal. Calcd for C₁₈H₁₃FS: C, 77.11; H, 4.67; S, 11.44. Found: C, 77.01; H, 4.58; S, 11.36. ( E )-[2-(Biphenyl-4-yl)vinyl](naphthalen-2-yl)sulfane (Table 4, entry 7): Yellow solid; mp 138-140 ˚C; IR (KBr): 3059, 2987, 1653, 1471, 1201, 944, 759 cm^-¹; ^¹H NMR (300 MHz, CDCl₃, TMS): δ = 7.83 (s, 1 H), 7.75 (t, J = 8.68 Hz, 3 H), 7.60-7.36 (m, 11 H), 7.31-7.23 (m, 1 H), 6.97 (d, J = 15.48 Hz, 1 H), 6.77 (d, J = 15.48 Hz, 1 H); ^¹³C NMR (100 MHz, CDCl₃, TMS): δ = 140.5, 140.3, 135.5, 133.7, 132.5, 132.2, 131.6, 128.7, 128.2, 127.7, 127.6, 127.3, 126.8, 126.7, 126.4, 126.1, 123.2; MS (ESI): m/z = 361 [M + Na]; Anal. Calcd for C₂₄H₁₈S: C, 85.17; H, 5.36; S, 9.47. Found: C, 85.11; H, 5.28; S, 9.39. ( Z )-Ethyl 3-(4-Chlorophenylthio)acrylate (Table 6, entry 2): Yellow solid; mp 64-65 ˚C; IR (KBr): 3056, 2925, 1702, 1577, 1214, 959, 744 cm^-¹; ^¹H NMR (200 MHz, CDCl₃, TMS): δ = 7.39 (d, J = 8.49 Hz, 2 H), 7.31 (d, J = 8.49 Hz, 2 H), 7.11 (d, J = 10.00 Hz, 1 H), 5.89 (d, J = 10.00 Hz, 1 H), 4.21 (q, J = 6.98 Hz, 2 H), 1.33 (t, J = 7.17 Hz, 3 H); ^¹³C NMR (100 MHz, CDCl₃, TMS): δ = 166.3, 148.8, 134.5, 134.4, 132.2, 129.4, 113.7, 60.3, 14.2; MS (ESI): m/z = 265 [M + Na]; Anal. Calcd for C₁₁H₁₁ClO₂S: C, 54.43; H, 4.57; S, 13.21. Found: C, 54.37; H, 4.49; S, 13.15.

Figure 1 TEM images of (a) fresh nano-CuO particles and (b) nano-CuO particles after the fourth reaction cycle