Comparison of Ullmann/RCM and Ullmann/Bis-hydrazone Coupling Reactions; New Access to Benzodithiophenes for Dye-Sensitized Solar Cell and Thiahelicene Applications

Abstract

The use of CuTC (Liebeskind’s catalyst), followed by methylenation and ring-closing metathesis, or bis-hydrazone coupling reactions is described. This approach establishes an alternative non-photochemical synthesis of the strategically important 1,2-b:4,3-b′ BDT regioisomer, which has previously been underused in applications such as dye-sensitized solar cells and nonlinear optics because of the difficulty of synthesis on a large scale.

Key words

benzodithiophene - Liebeskind’s catalyst - bis-hydrazone coupling - alkene metathesis - non-photochemical synthesis

• References and Notes


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• 24 Preparation of N-[(3-Bromothiophen-2-yl)methylene]cyclohexylimine (7): In a 1-L three-necked round-bottom flask, equipped with a Dean–Stark trap, a solution of 3-bromo-2-formylthiophene 9 (84.15 g, 0.44 mol, 1 equiv) and cyclohexylamine (54.6 g, 0.55 mol, 1.25 equiv) in toluene (700 mL) was heated at reflux under nitrogen for 16 h. The solution was then evaporated to afford an orange oil (120 g, 100%) which was used directly in the next step. IR (ATR): 3075, 2925, 2851, 1623 cm^–1. ¹H NMR (CDCl₃, 400 MHz): δ = 8.44 (s, 1 H), 7.34 (dd, J = 5.3, 1.1 Hz, 1 H), 7.00 (d, J = 5.2 Hz, 1 H), 3.22 (m, 1 H), 1.51–1.85 (m, 7 H), 1.18–1.41 (m, 3 H). ¹³C NMR (CDCl₃, 100 MHz): δ = 151.0, 136.5, 130.5, 128.4, 113.5, 69.8, 34.1, 25.5, 24.7. HRMS (GC, CI+): m/z [M–H]^– calcd for C₁₁H₁₃BrNS: 269.9947; found: 269.9947.
• 25 General Procedure: N-[(3-Bromothiophen-2-yl)methylene]cyclohexylimine (7; 1 equiv) was dissolved in anhydrous NMP under argon, CuTc (2.2 equiv) was added in several portions (to achieve good mixing), and the reaction mixture was stirred at 90 °C under argon for 17 h. After cooling, the mixture was filtered through a pad of kieselguhr, which was then washed with EtOAc until no more brown colour was released from the filter cake. The filtrate was washed with 15% aqueous ammonia, producing a clear deep-blue aqueous layer. The organic layer was separated and retained, and the aqueous layer was extracted with EtOAc. The combined organic layers were washed with brine to remove as much NMP as possible, dried over MgSO₄, filtered, and evaporated under reduced pressure. The brown oily residue was dissolved in CH₂Cl₂ and 15% aqueous AcOH was added and mixture was at stirred r.t. overnight. The organic layer was separated and retained, and the aqueous layer was extracted with CH₂Cl₂. The combined organic layers were washed with brine, filtered through a MgSO₄/neutral alumina pad, and evaporated under reduced pressure to give a solution of crude product in NMP (despite the washing, the NMP was not removed completely). The resultant brown oil was taken up in H₂O and shaken until the product precipitated. The mixture was then filtered and the residue was washed with H₂O and dissolved in CH₂Cl₂, dried over MgSO₄, filtered and evaporated. The solid residue was washed with a mixture of hexanes and EtOAc (8:1 v/v) and dried under vacuum to give [3,3′-bithiophene]-2,2′-dicarboxaldehyde 12a; for yields, see Table 1 and Table 2.
• 26 Preparation of N-{[3-Bromo-5-(trimethylsilyl)thiophen-2-yl]methylene}cyclohexylimine (14): A solution of n-BuLi (1.6 M in hexanes, 53 mL, 84.5 mmol, 1.15 equiv) was added dropwise to diisopropylamine (12 mL, 8.5 g, 84.5 mmol, 1.15 equiv) in anhydrous THF (600 mL) at 0 °C under nitrogen. After stirring for 45 min at 0 °C, N-[(3-bromothiophen-2-yl)methylene]cyclohexylimine (7; 20 g, 73.5 mmol, 1 equiv) in anhydrous THF (50 mL) was added dropwise over 10 min. After stirring for a further 45 min at 0 °C under nitrogen, the reaction mixture was cooled to –78 °C and trimethylsilyl chloride (10.7 mL, 9.2 g, 84.5 mmol, 1.15 equiv) was added dropwise. After stirring for 1 h at –78 °C, the reaction mixture was allowed to warm to r.t., sat. aq NH₄Cl (700 mL) was added, and the organic layer was separated and retained. The aqueous layer was extracted with EtOAc (2 × 400 mL) and the combined organic layers were washed with brine (500 mL), filtered through a MgSO₄/basic alumina pad, and evaporated to give 14 (25.2 g, 99%) as an orange oil. IR (ATR): 2927, 2853, 1624 cm^–1. ¹H NMR (CDCl₃, 400 MHz): δ = 8.43 (s, 1 H), 7.10 (s, 1 H), 3.22 (m, 1 H), 1.22–1.87 (m, 10 H), 0.31 (s, 9 H). ¹³C NMR (CDCl₃, 100 MHz): δ = 151.0, 144.4, 140.5, 136.5, 114.5, 70.0, 34.1, 25.5, 24.7, –0.6. HRMS (ESI): m/z [M+H]⁺ calcd for C₁₄H₂₃BrNSSi: 344.0498; found: 344.0503.
• 27 Rajca A, Wang H, Rajca S. Angew. Chem. Int. Ed. 2000; 39: 4481
• 28 Preparation of N-{[3-Iodo-5-(trimethylsilyl)thiophen-2-yl]methylene}cyclohexylimine (10): A solution of N-{[3-bromo-5-(trimethylsilyl)thiophen-2-yl]methylene}cyclohexylimine (14; 6.94 g, 20.2 mmol, 1 equiv) in anhydrous THF (350 mL) was cooled to –78 °C, under nitrogen. n-BuLi (1.6 M in hexanes, 13.9 mL, 22.2 mmol, 1.1 equiv) was added dropwise. The mixture was stirred for 30 min at –78 °C and a solution of iodine (7.7 g, 30.3 mmol, 1.5 equiv) in anhydrous THF (25 mL) was added dropwise until the red iodine colour persisted. After 15 min at –78 °C, the reaction mixture was allowed to warm to r.t., H₂O (350 mL) was added and the mixture was extracted with CH₂Cl₂ (3 × 250 mL). The combined organic layers were concentrated to 300 mL, washed with sat. aq sodium sulfite (2 × 300 mL), dried over MgSO₄, filtered, and evaporated to give 10 (7.12 g, 90%) as a brown oil, that crystallised upon standing. Mp 59 °C. IR (ATR): 2928, 2851, 1618 cm^–1. ¹H NMR (CDCl₃, 400 MHz): δ = 8.34 (s, 1 H), 7.20 (s, 1 H), 3.24 (br. m, 1 H), 1.85–1.57 (m, 7 H), 1.38–1.23 (m, 3 H), 0.31 (s, 9 H). ¹³C NMR (CDCl₃, 100 MHz): δ = 153.2, 145.4, 143.7, 141.4, 85.1, 69.9, 34.2, 25.5, 24.7, –0.5. HRMS (ESI): m/z [M–H]^– calcd for C₁₄H₂₁NISSi: 390.0203; found: 390.0203.
• 29 General Procedure: A solution of N-{[3-bromo-5-(trimethylsilyl)thiophen-2-yl]methylene}cyclohexylimine 14 (1 equiv) in anhydrous THF was cooled to –78 °C under nitrogen. n-BuLi (1.05 equiv) was added dropwise and the mixture was stirred at –78 °C for 30 min. Then CuI-P(OEt)₃ (1.5 equiv) was added in one portion and the mixture was stirred for a further 30 min at –78 °C before a solution of N-{[3-iodo-5-(trimethylsilyl)thiophen-2-yl]methylene}cyclohexylimine 10 in anhydrous THF was added dropwise. The reaction mixture was allowed to warm to r.t. and stirred at r.t. for 60 h. The reaction was quenched with H₂O and the reaction mixture was diluted with CH₂Cl₂, and 15% aqueous AcOH was added. The mixture was at stirred r.t. overnight, then the organic layer was separated and retained and the aqueous layer was extracted with CH₂Cl₂. The combined organic layers were washed with brine, filtered through a MgSO₄/neutral alumina pad and evaporated under reduced pressure. Crude material was purified by column chromatography (silica; hexanes–EtOAc, 100:0 to 2:1 v/v) to give 5,5′-bis(trimethylsilyl)-[3,3′-bithiophene]-2,2′-dicarbaldehyde (12b); for yields, see Table 1.
• 30 Some S, Dutta B, Ray JK. Tetrahedron Lett. 2006; 47: 1221
• 31 Preparation of [3,3′-Bithiophene]-2,2′-dicarboxaldehyde (12a): Anhydrous DMSO (50 mL) was degassed under nitrogen for 30 min, then 3-bromo-2-formylthiophene (9; 1 equiv) was added and nitrogen gas was bubbled through the resulting solution for 10 min. Pd(PPh₃)₄ (0.1 equiv) and copper powder (3 equiv) were added and the solution was stirred and heated to 100 °C, under nitrogen for 15 h and then at 120 °C for 8 h. The progress of the reaction was monitored by TLC (hexanes–EtOAc, 3:1 v/v). The solution was cooled to r.t. before adding EtOAc (200 mL) and filtration through a pad of kieselghur. The filtrate was washed with H₂O (2 × 150 mL) and brine (150 mL), dried over MgSO₄, filtered and evaporated under reduced pressure to give a brown oil that was purified by chromatography (silica; hexanes–EtOAc, 95:5 to 3:1 v/v) to afford 12a (405 mg, 35%) as a yellow powder.
• 32 General Procedure: A dried 20-mL microwave vial was flushed with argon. To a solution N-[(3-bromothiophen-2-yl)methylene]cyclohexylimine (7; 1 equiv) in NMP (15 mL), CuTC (2.2 equiv) was added with stirring. The microwave vial was then sealed, vacuum was applied, and then the vial was filled with argon. The reaction mixture was irradiated (see Table 2), then diluted with EtOAc and 15% aqueous ammonia was added to produce a clear deep-blue aqueous layer. The organic layer was separated and retained and the aqueous layer was extracted with EtOAc. The organic layers were combined and evaporated and the resultant crude product (green oil) was dissolved in Et₂O. This solution was washed with brine, dried over MgSO₄, filtered, and evaporated to leave a brown oil, which was dissolved in CH₂Cl₂ (50 mL), 15% aqueous AcOH (50 mL) was added and mixture was stirred overnight at r.t. The organic layer was separated and retained and the aqueous layer was extracted with CH₂Cl₂. The combined organic layers were washed with brine, dried over MgSO₄, filtered and evaporated (first under reduced pressure on a rotary evaporator and then under high vacuum using a vacuum line) to give a brown oil. The oil was purified by column chromatography (silica; hexanes–EtOAc, 100:0 to 2:1 v/v) to afford [3,3′-bithiophene]-2,2′-dicarbaldehyde 12a as a yellow solid (for yields, see Table 2).
• 33 Preparation of 2,2′-Divinyl-3,3′-bithiophene (6): To a suspension of methyltriphenylphosphonium bromide (1.7 g, 4.75 mmol, 2.2 equiv) in distilled THF (50 mL), n-BuLi (1.6 M in hexanes, 2.96 mL, 4.75 mmol, 2.2 equiv) was added dropwise at –10 °C under nitrogen. The deep-orange solution was stirred at r.t. for 30 min, then a solution of [3,3′-bithiophene]-2,2′-dicarbaldehyde (12a; 460 mg, 2.16 mmol, 1 equiv) in distilled THF (10 mL) was added dropwise. The mixture was stirred at r.t. under nitrogen for 17 h, then the reaction was quenched with sat. aq NH₄Cl (20 mL). The aqueous layer was extracted with CHCl₃ (3 × 50 mL) and the combined organic layers were washed with brine (100 mL), dried over MgSO₄, and evaporated. The crude product was purified by column chromatography (silica; hexanes), to afford 6 (350 mg, 77%) as a viscous oil. The product was kept in the freezer in the dark, and used as soon as possible. IR (ATR): 3103, 3066, 3005, 2957, 2925, 2869, 1800, 1616 cm^–1. ¹H NMR (CDCl₃, 400 MHz): δ = 7.19 (dd, J = 5.3, 0.8 Hz, 1 H), 6.94 (d, J = 5.3 Hz, 1 H), 6.66 (ddd, J =17.3, 11.0, 0.8 Hz, 1 H), 5.58 (d, J = 17.3 Hz, 1 H), 5.13 (d, J = 11.0 Hz, 1 H). ¹³C NMR (CDCl₃, 100 MHz): δ = 139.1, 134.2, 130.1, 129.2, 123.1, 113.7. HRMS (ESI): m/z [M+H]⁺ calcd for C₁₂H₁₁S₂: 219.0299; found: 219.0297.
• 34 Preparation of Benzo[1,2-b:4,3-b′]dithiophene (1) by RCM: Under argon, Grubbs’ 1st generation catalyst [Ru(Pcy₃)₂(CHPh)Cl₂] (30 mg, 0.1 equiv) was added to a solution of 2,2′-divinyl-3,3′-bithiophene (6; 80 mg, 0.36 mmol, 1 equiv) in anhydrous CH₂Cl₂ (25 mL). The reaction mixture was stirred at r.t. for 8 h, then the solvent was removed under reduced pressure and the crude product was purified by column chromatography (silica; hexanes), to afford 1 (67 mg, 96%).
• 35 Preparation of N′,N′′-{[3,3′-Bithiophene]-2,2′-diylbis(methanylylidene)}bis(4-methylbenzenesulfonylhydrazone) (8): [3,3′-Bithiophene]-2,2′-dicarbaldehyde 12a (1.05 g, 4.7 mmol, 1 equiv) and tosylhydrazide (1.75 g, 9.4 mmol, 2 equiv) were dissolved in distilled THF (300 mL) and stirred at r.t. overnight. The reaction mixture was dried over MgSO₄, filtered and evaporated to give 8 (2.62 g, 100%) as a bright-yellow-orange solid foam. Mp 129 °C. IR (ATR): 3176, 2958, 2923, 2867, 1645, 1594 cm^–1. ¹H NMR (400 MHz, DMSO-d ₆): δ = 11.33 (s, 2 H), 7.77 (d, J = 0.9 Hz, 2 H), 7.69 (dd, J = 5.1, 0.7 Hz, 2 H), 7.67 (d, J = 8.4 Hz, 4 H), 7.40 (dd, J = 8.1, 0.6 Hz, 4 H), 7.03 (d, J = 5.1 Hz, 2 H), 2.36 (s, 6 H). ¹³C NMR (100 MHz, DMSO-d ₆): δ = 143.7, 140.7, 136.4, 136.0, 134.7, 130.3, 129.8, 128.7, 127.2, 21.1. HRMS (ESI): m/z [M+H]⁺ calcd for C₂₄H₂₃N₄O₄S₄: 559.0597; found: 559.0587.

The intramolecular McMurry cyclisation of 12a was unsuccessful under a variety of conditions [e.g., TiCl₄/Zn and TiCl₃(DME)_1.5/Zn(Cu)], despite its use as an intermolecular coupling reaction to obtain the 1,2-dithiophenylethene starting material for the photochemical route. For typical reaction conditions, see:
• 36a Yoshida S, Fujii M, Aso Y, Otsubo T, Ogura F. J. Org. Chem. 1994; 59: 3077
• 36b See also ref. 15c.
• 37 Preparation of Benzo[1,2-b:4,3-b′]dithiophene (1) by Bis-hydrazone Coupling: [3,3′-Bithiophene]-2,2′-dicarbaldehyde (12a; 4 g, 18 mmol, 1 equiv) and tosylhydrazide (6.59 g, 36 mmol, 2 equiv) were dissolved in distilled THF (850 mL), and the mixture was stirred at r.t. overnight, dried over Na₂SO₄, and transferred to a 1 L three-necked round-bottom flask that has been flame-dried under nitrogen. The reaction mixture was cooled to 0 °C and NaH (95%; 1.08 g, 45 mmol, 2.5 equiv) was added in portions. The reaction mixture was allowed to warm to r.t. and then heated at reflux for 3 h under nitrogen. After cooling, the solution was concentrated under reduced pressure to 100 mL, and sat. aq NH₄Cl (300 mL) was added. The mixture was extracted with EtOAc (2 × 400 mL) and the combined organic layers were dried over MgSO₄, filtered, and evaporated to leave a brown solid (4 g). The crude product was purified by column chromatography (silica; hexanes) to afford 1 (1.21 g, 37%) as colourless crystals.
• 38 Preparation of 2,7-Bis(trimethylsilyl)benzo[1,2-b:4,3-b′]dithiophene (15) by Bis-hydrazone Coupling: 5,5′-Bis(trimethylsilyl)-[3,3′-bithiophene]-2,2′-dicarbaldehyde (12b; 2.62 g, 7.15 mmol, 1 equiv) and tosylhydrazide (2.66 g, 15.30 mmol, 2 equiv) were dissolved in distilled THF (450 mL) and stirred at r.t. overnight. The THF solution was dried over Na₂SO₄, and transferred to a 500-mL three-necked round-bottom flask that had been flame-dried under nitrogen. The reaction mixture was cooled to –78 °C, n-BuLi (1.6 M in hexanes, 4.7 mL, 7.5 mmol, 1.05 equiv) was added dropwise and the mixture was stirred for 5 min at –78 °C. The reaction mixture was allowed to warm to r.t., then heated at reflux for 5 h. After cooling, sat. aq NH₄Cl (200 mL) was added and the mixture was extracted with EtOAc (200 mL). The organic layer was concentrated under reduced pressure to 100 mL, diluted with Et₂O (200 mL), washed with brine (2 × 250 mL), dried over MgSO₄, filtered, and evaporated to leave a brown solid (5.1 g). Crude material was purified by column chromatography (silica; hexanes) to afford 15 (770 mg, 32%) as colourless crystals. Mp 128 °C. IR (ATR): 3053, 2985, 2959, 2897 cm^–1. ¹H NMR (CDCl₃, 400 MHz): δ = 7.88 (s, 2 H), 7.80 (s, 2 H), 0.44 (s, 18 H). ¹³C NMR (CDCl₃, 100 MHz): δ = 142.3, 140.4, 135.9, 128.7, 118.4, –0.2. HRMS (ESI): m/z [M]⁺ calcd. for C₁₆H₂₂S₂Si₂: 334.0696; found: 334.0696.
• 39 Preparation of N′,N′′-{[5,5′-Bis(trimethylsilyl)-(3,3′-bithiophene)-2,2′-diyl]bis(methanylylidene)}bis(4-methylbenzenesulfonylhydrazone) (16): Using the method employed for the synthesis of 8 (see ref. 35) 5,5′-bis(trimethylsilyl)-(3,3′-bithiophene)-2,2′-dicarbaldehyde (12b; 2 g, 5.45 mmol, 1 equiv) and tosylhydrazide (2.03 g, 10.9 mmol, 2 equiv) were dissolved in distilled THF (250 mL) and the mixture was stirred at r.t. overnight, dried over MgSO₄, and evaporated to afford 16 (3.83 g, 100%) as a bright-yellow-orange solid foam. Mp 156 °C. IR (ATR): 3190, 3065, 2955, 2926, 2898, 2856, 1597 cm^–1. ¹H NMR (DMSO-d ₆, 400 MHz): δ = 11.38 (s, 2 H), 7.74 (s, 2 H), 7.67 (d, J = 8.2 Hz, 4 H), 7.40 (dd, J = 8.2, 0.7 Hz, 4 H), 7.16 (s, 2 H), 2.36 (s, 6 H), 0.30 (s, 18 H). ¹³C NMR (DMSO-d ₆, 100 MHz): δ = 143.5, 142.5, 140.2, 139.2, 137.3, 137.0, 136.1, 129.7, 127.0, 21.0, –0.5. HRMS (ESI): m/z [M+H]⁺ calcd for C₃₀H₃₉N₄O₄S₄Si₂: 703.1387; found: 703.1387.
• 40a Bamford WR, Stevens TS. M. J. Chem. Soc. 1952; 4735
• 40b Chamberlain AR, Bloom SH. Org. React. 1990; 39: 1
• 41a Shapiro RH, Duncan JH, Clopton JC. J. Am. Chem. Soc. 1967; 89: 471
• 41b Shapiro RH. Org. React. 1976; 23: 405
• 41c Aldington RM, Barrett AG. M. Acc. Chem. Res. 1983; 16: 55
• 42a Chamberlin AR, Stemke JE, Bond FT. J. Org. Chem. 1978; 43: 147
• 42b Miranda R, Hernandez A, Angeles E, Cabrera A, Salmon M, Joseph-Nathan P. Analyst 1990; 115: 1483
• 43 Chamberlin AR, Bond FT. J. Org. Chem. 1978; 43: 154
• 44 Carbene intermediates have also been proposed for the Shapiro reaction, see ref. 41c.
• 45 A referee has suggested that the dianion (see Scheme 5, box) is the intermediate in the cyclisation reaction, which is entirely reasonable, especially in the sodium hydride procedure (see ref. 37) in which the base was used in excess, but when 1.05 equiv butyllithium is employed (see ref. 38), the second deprotonation is probably effected by the toluenesulfinate anion in a reversible step that is driven, ultimately, by the irreversible loss of nitrogen, and the reaction then probably follows the mechanism drawn in Scheme 5.
• 46 Jung tentatively proposes (see ref. 18a) that when the base is sodium hydride, both tosylhydrazones deprotonate and eliminate the tosylsulphinate, before ring closure occurs.
• 47 Kerr WJ, Morrison AJ, Pazicky M, Weber T. Org. Lett. 2012; 14: 2250