Palladium-Catalyzed Direct Thienylation of Monobromide-Substituted Difluorinated Benzothiadiazoles for Organic Photovoltaics

Abstract

A palladium-catalyzed cross-coupling between monobromide-substituted difluorinated benzothiadiazoles (DFBT) and simple thiophenes has been developed. The significant feature of this protocol is synthetic simplicity without using toxic stannanes, thus providing a facile access to unsymmetrical DFBT–thiophene structures that are of great interest in organic photovoltaics.

Key words

difluorinated benzothiadizaoles - C–H functionalization - palladium - photovoltaics

• References and Notes

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• 12 Typical Procedure for Pd-Catalyzed Cross-Coupling of 1a with 2a In a flame-dried Schlenk tube under nitrogen atmosphere, Pd(OAc)₂ (2.2 mg, 0.01 mol), dppe (8.0 mg, 0.02 mol), AdOH (54.1 mg, 0.3 mmol), K₂CO₃ (55.3 mg, 0.4 mmol), and thienylated bromo-DFBT (1a) (86.4 mg, 0.2 mmol) were added, followed by toluene (2 mL) with stirring. Thiophene 2a (67.3 mg, 0.4 mmol) was then added. The reaction mixture was stirred in a preheated oil bath at 120 °C for 12 h. The reaction mixture was cooled to r.t., diluted with CH₂Cl₂, washed with brine, dried over Na₂SO₄, filtered, and concentrated. The residue was purified with silica gel chromatography (PE– CH₂Cl₂, 8:1) to give pure compound 3a.
• 13 tert-Butyl 5-{5,6-Difluoro-7-(5-hexylthiophen-2-yl)benzo[c]-[1,2,5]thiadiazol-4-yl}thiophene-2-carboxylate (3a) Yield 78 mg (75%); orange solid; mp 112.1 °C. ¹H NMR (400 MHz, CDCl₃): δ = 8.16 (d, J = 3.6 Hz, 1 H), 8.10 (d, J = 3.6 Hz, 1 H), 7.80 (d, J = 3.2 Hz, 1 H), 6.92 (d, J = 2.8 Hz, 1 H), 2.90 (t, J = 7.6 Hz, 2 H), 1.80–1.72 (m, 2 H), 1.63 (s, 9 H), 1.45–1.39 (m, 2 H), 1.37–1.29 (m, 4 H), 0.91 (t, J = 7.0 Hz, 3 H). ¹⁹F NMR (376 MHz, CDCl₃): δ = –126.06 (d, J = 14.7 Hz), –129.14 (d, J = 14.7 Hz). ¹³C NMR (100 MHz, CDCl₃): δ = 161.5, 150.9 (dd, J = 262.3 Hz, 19.9 Hz), 150.8 (d, J = 6.7 Hz), 149.0 (dd, J = 260.0 Hz, 20.0 Hz), 148.7 (d, J = 9.3 Hz), 148.6 (d, J = 9.0 Hz), 137.4 (d, J = 6.3 Hz), 137.3 (dd, J = 5.9 Hz, 3.7 Hz), 132.6, 131.6 (d, J = 9.4 Hz), 130.6 (d, J = 9.5 Hz), 128.7 (dd, J = 6.1 Hz, 3.6 Hz), 124.9, 113.2 (d, J = 12.3 Hz), 110.1 (d, J = 12.1 Hz), 82.1, 31.6, 31.5, 30.1, 28.9, 28.3, 22.6, 14.1. MS (MALDI): m/z (%) = 520.1 [M⁺], 521.1 [M⁺], 522.1 [M⁺]. HRMS: m/z calcd for C₂₅H₂₆O₂N₂F₂S₃: 520.1119; found: 520.1122.