3,4-Bis(5-iodo-2-methylthien-3-yl)-2,5-dihydrothiophene: A Powerful Synthon­ for the Preparation of Photochromic Dithienylethene Derivatives

Abstract

A dithienylethene synthon, 3,4-bis-(5-iodo-2-methylthien-3-yl)-2,5-dihydrothiophene, was prepared and used as a starting material for the preparation of several photochromic dithien­ylethene derivatives via Suzuki cross-coupling reactions. The dithienylethene synthon was stable once stored in the dark at room temperature.

dithienylethene - photochromism - Suzuki coupling

References and Notes

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10

Preparation of Synthon 2a: To a cooled (-50 °C) solution of 3,4-bis-[2,2′-dimethyl-5,5′-(dichlorodithienyl)]-2,5-dihydrothiophene 1a [9] (2 mmol, 0.7 g) in THF (50 mL), n-BuLi was added dropwise (1.6 M in hexane; 3.6 mL) under nitrogen. The mixture was stirred for 0.5 h at -50 °C and then cooled to -78 °C. To the cooled mixture (-78 °C) I₂ (6 mmol, 1.5 g) was added in THF (5 mL), the reaction mixture was stirred for 1 h, then the temperature was allowed to rise to -20 °C. H₂O (50 mL) was then added to the residue and the product was extracted with Et₂O (3 × 20 mL). The combined organic phase was washed with a solution of NaHSO₃ (30%; 30 mL), H₂O (30 mL), a sat. solution of NaCl (30 mL), and dried over MgSO₄. After evaporation of the solvent, the crude product was purified by flash column chromatography (PE-EtOAc, 20:1). Yield: 63%; mp > 200 °C. MS (EI): m/z (%) = 530 (M⁺, 100), 276 (20), 71 (31), 43 (48). HRMS: m/z calcd for C₁₄H₁₂I₂S₃ [M⁺]: 530.2468; found: 530.2461.

11

Boronic Acid Reagents: The corresponding boronic acid reagents for the preparation of diarylethene compounds 3a-8a were purchased from ACROS. The corresponding boronic acid reagent for preparation of diarylethene compound 9a was synthesized as follows. A mixture of NBS (5.34 g, 30 mmol) and fluorene (4.98 g, 30 mmol) in distilled propylene carbonate (40 mL) was warmed in a 60 °C bath for 2 min. As the solution warmed the mixture turned orange and the solid dissolved. After stirring for 30 min at r.t. the color turned to yellow, and a negative starch-iodide test was obtained. The solution then was poured into H₂O (0.5 L), the precipitate was filtered, and crystallization from EtOH gave 6.7 g of 2-bromofluorene (91%). To a solution of DMSO (50 mL) and NaOH (4.0 g, 0.1 mol) was added 2-bromofluorene (2.45 g, 0.01 mol), the mixture was stirred for 1 h at r.t.. Iodomethane (7.1 g, 0.05 mol) in DMSO (10 mL) was then added to the above solution. The mixture was stirred for 4 h at r.t. When no 2-bromofluorene was detected by TLC anymore, the mixture was poured into ice water (100 mL) and the product was extracted with CHCl₃ (3 × 20 mL). The combined organic phase was washed with a sat. solution of NaCl and dried over MgSO₄. After evaporation of the solvent, the crude product was purified by flash column chromatography (PE) to afford 2-bromo-9,9-dimethyl-fluorene in 80% yield. To a solution of 2-bromo-9,9-dimethylfluorene (1.36 g, 5 mmol) in THF (100 mL), n-BuLi (1.6 M in hexane; 8 mL) was added slowly at -50 °C. The mixture was stirred for 30 min at -50 °C, followed by slow addition of B(OBu)₃ (1 mL) in THF (10 mL) at -78 °C. The mixture was stirred for 5 h, during this time the temperature was increased slowly to 0 °C. After no 2-bromo-9,9-dimethylfluorene was detected by TLC, the mixture was poured into HCl solution (1 M; 100 mL), the product was extracted with CHCl₃ (3 × 20 mL). The combined organic phase was washed with a sat. solution of NaCl and dried over MgSO₄. After evaporation of the solvent, the crude product was purified by flash column chromatography (PE-EtOAc, 2:1) to afford 9,9-dimethylfluorene-2-boronic acid in 60% yield. ¹H NMR (300 MHz, CDCl₃): δ = 8.34 (t, 1 H, J ₁ = 7.8 Hz, J ₂ = 7.9 Hz), 7.92 (d, 1 H, J = 7.7 Hz), 7.87-7.73 (m, 2 H), 7.53-7.40 (m, 3 H), 2.64 (s, 2 H), 1.64 (s, 3 H), 1.52 (s, 3 H).

12

Diarylethene Derivatives 3a-9a (Suzuki Coupling Reaction); General Procedure: To a solution of 2a (270 mg, 0.5 mmol) in anhyd THF (15 mL), Pd(PPh₃)₄ (40 mg, 3 mol%) and a solution of K₂CO₃ solution (2.5 M; 2 mL) were added The resulting solution was stirred for 10 min at ambient temperature. To the resulting solution was added the corresponding boronic acid reagent (1.5 mmol) under nitrogen at r.t. The resulting solution was refluxed under nitrogen and the reaction was monitored by TLC. After the starting material 2a disappeared, H₂O (20 mL) was added to the residue, and the product was extracted with Et₂O (3 × 20 mL). The combined organic phase was washed with H₂O (30 mL) and a sat. solution of NaCl (30 mL), respectively, and dried over MgSO₄. After evaporation of the solvent, the crude product was purified by flash column chromatography (PE-EtOAc).
3a: Yield: 81%; mp 165-167 °C. ¹H NMR (300 MHz, CDCl₃): δ = 7.51 (d, 4 H, J = 6.0 Hz), 7.35 (t, 4 H, J ₁ = 7.2 Hz, J ₂ = 7.6 Hz), 7.26 (d, 2 H, J = 7.1 Hz), 7.04 (s, 2 H), 4.21 (s, 4 H), 2.01 (s, 6 H). ¹³C NMR (400 MHz, CDCl₃): δ = 140.4, 135.7, 134.4, 134.2, 133.1, 128.9, 127.5, 125.4, 123.5, 43.0, 14.4. MS (EI): m/z (%) = 430 (M⁺, 100), 369 (40), 121 (36), 77 (8), 59 (10). HRMS: m/z calcd for C₂₆H₂₂S₃ [M⁺]: 430.6578; found: 430.6581. Anal. Calcd for C₂₆H₂₂S₃: C, 72.51; H, 5.15; S, 22.34. Found: C, 72.48; H, 5.17; S, 22.29.
4a: Yield: 84%; mp 174-175 °C. ¹H NMR (300 MHz, CDCl₃): δ = 7.43 (d, 4 H, J = 8.5 Hz), 6.91 (s, 2 H), 6.87 (d, 4 H, J = 8.5 Hz), 4.19 (s, 4 H), 3.83 (s, 6 H), 1.99 (s, 6 H). ¹³C NMR (400 MHz, CDCl₃): δ = 159.0, 140.2, 134.6, 134.2, 133.0, 127.1, 126.7, 122.5, 114.3, 55.4, 43.0, 14.3. MS (EI): m/z (%) = 490 (M⁺, 32), 429 (13), 215 (16), 199 (18), 71 (58), 43 (100). HRMS: m/z calcd for C₂₈H₂₆O₂S₃ [M⁺]: 490.7094; found: 490.7090. Anal. Calcd for C₂₈H₂₆O₂S₃: C, 68.54; H, 5.34; S, 19.60. Found: C, 68.61; H, 5.29; S, 19.68.
5a: Yield: 88%; mp > 200 °C. ¹H NMR (300 MHz, CDCl₃): δ = 8.31 (s, 2 H), 8.10 (d, 2 H, J = 8.0 Hz), 7.78 (d, 2 H, J = 7.7 Hz), 7.52 (t, 2 H, J ₁ = 7.0 Hz, J ₂ = 7.2 Hz), 7.15 (s, 2 H), 4.22 (s, 4 H), 2.01 (s, 6 H). ¹³C NMR (400 MHz, CDCl₃): δ = 148.7, 137.8, 137.6, 135.7, 134.8, 133.4, 131.0, 129.9, 125.2, 121.7, 119.9, 43.0, 14.5. MS (EI): m/z (%) = 520 (M⁺, 28), 518 (100), 488 (59), 197 (23), 166 (20), 59 (18). HRMS: m/z calcd for C₂₆H₂₀N₂O₄S₃ [M⁺]: 520.6520; found: 520.6526. Anal. Calcd for C₂₆H₂₀N₂O₄S₃: C, 59.98; H, 3.87; N, 5.38; S, 18.48. Found: C, 59.91; H, 3.82; N, 5.41; S, 18.54.
6a: Yield: 80%; mp 123-125 °C. ¹H NMR (300 MHz, CDCl₃): δ = 7.19 (d, 2 H, J = 6.0 Hz), 7.07 (d, 2 H, J = 2.8 Hz), 6.99 (t, 2 H, J ₁ = 3.8 Hz, J ₂ = 3.7 Hz), 6.90 (s, 2 H), 4.17 (s, 4 H), 1.99 (s, 6 H). ¹³C NMR (400 MHz, CDCl₃): δ = 137.3, 135.2, 134.0, 133.7, 133.1, 127.8, 124.1, 124.0, 123.3, 42.8, 14.3. MS (EI): m/z (%) = 442 (M⁺, 100), 381 (50), 190 (22), 171 (23), 127 (30), 45 (18). HRMS: m/z calcd for C₂₂H₁₈S₅ [M⁺]: 442.7142; found: 442.7146. Anal. Calcd for C₂₂H₁₈S₅: C, 59.69; H, 4.10; S, 36.21. Found: C, 59.72; H, 4.15; S, 36.28.
7a: Yield: 82%; mp > 200 °C. ¹H NMR (300 MHz, CDCl₃): δ = 7.62-7.56 (m, 12 H), 7.45 (t, 4 H, J ₁ = 7.4 Hz, J ₂ = 7.5 Hz), 7.36 (d, 2 H, J = 7.2 Hz), 7.01 (s, 2 H), 4.23 (s, 4 H), 2.04 (s, 6 H). ¹³C NMR (400 MHz, CDCl₃): δ = 140.9, 140.5, 140.0, 135.8, 134.4, 133.1, 128.8, 127.8, 127.5, 127.4, 126.9, 125.7, 123.5, 43.0, 14.5. MS (EI): m/z (%) = 582 (M⁺, 85), 581 (M⁺ - 1, 100), 529 (21), 235 (26), 174 (81), 157 (56), 145 (50), 130 (84), 78 (38), 53 (71). HRMS: m/z calcd for C₃₈H₃₀S₃ [M⁺]: 582.8530; found: 582.8534. Anal. Calcd for C₃₈H₃₀S₃: C, 78.31; H, 5.19; S, 16.50. Found: C, 78.38; H, 5.16; S, 16.58.
8a: Yield: 78%; mp > 200 °C. ¹H NMR (300 MHz, CDCl₃): δ = 8.09 (d, 2 H, J = 8.2 Hz), 7.85 (dd, 4 H, J ₁ = 8.3 Hz, J ₂ = 8.2 Hz), 7.51-7.34 (m, 8 H), 6.96 (s, 2 H), 4.27 (s, 4 H), 2.27 (s, 6 H). ¹³C NMR (400 MHz, CDCl₃): δ = 138.0, 136.2, 134.0, 133.9, 133.6, 132.2, 131.6, 128.5, 128.4, 128.3, 127.9, 126.6, 126.0, 125.6, 125.2, 42.7, 14.3. MS (EI):
m/z (%) = 529 (M⁺ - 1, 100), 513 (15). HRMS: m/z calcd for C₃₄H₂₆S₃ [M⁺]: 530.7774; found: 530.7779. Anal. Calcd for C₃₄H₂₆S₃: C, 76.94; H, 4.94; S, 18.12. Found: C, 76.89; H, 4.98; S, 18.20.
9a: Yield: 87%; mp > 200 °C. ¹H NMR (300 MHz, CDCl₃): δ = 7.70 (t, 4 H, J ₁ = 7.9 Hz, J ₂ = 8.0 Hz), 7.52-7.44 (m, 4 H), 7.42 (t, 2 H, J ₁ = 7.7 Hz, J ₂ = 7.8 Hz), 7.35-7.31 (m, 4 H), 7.11 (s, 2 H), 4.24 (s, 4 H), 2.08 (s, 6 H), 1.48 (s, 12 H). ¹³C NMR (400 MHz, CDCl₃): δ = 154.4, 153.8, 140.9, 138.7, 138.6, 135.5, 134.5, 133.2, 130.5, 127.3, 127.1, 124.5, 123.5, 122.6, 120.4, 120.0, 119.6, 46.9, 42.9, 27.2, 14.5. MS (EI): m/z (%) = 664 (M⁺ + 2, 60), 661 (M⁺ - 1, 100), 471 (24), 46 (28). HRMS: m/z calcd for C₄₄H₃₈S₃ [M⁺]: 662.9822; found: 662.9827. Anal. Calcd for C₄₄H₃₈S₃: C, 79.71; H, 5.78; S, 14.51. Found: C, 79.65; H, 5.81; S, 14.58.