Synthesis of a Contrapositionally Substituted Cyclohexa-meta-phenylene: A Ready-to-Use Precursor for Cyclohexa-meta-phenylene-Based Materials

Abstract

A contrapositionally substituted derivative of cyclohexa-meta-phenylene ([6]CMP) was synthesized by an intramolecular Yamamoto coupling reaction of an appropriate terphenyl unit containing a trimethylsilyl substituent. Iododesilylation of the trimethylsilyl groups of the product with iodine monochloride was used to incorporate iodo groups, an important functionality for metal-catalyzed coupling reactions. The iodo groups were also converted into a (pinacolato)boryl groups, another important functionality for coupling reactions. The diborylated [6]CMP is expected to be a versatile potential comonomer and a precursor for the synthesis of CMP-based materials. The synthetic route to the disubstituted [6]CMP included lithiation, Pd-catalyzed borylation, Suzuki coupling, and Yamamoto coupling. The structure of the product was established by NMR spectroscopy and mass spectrometry.

Key words

cyclohexaphenylene - Yamamoto coupling - borylation

• References and Notes

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• 8 (3,3′′-Dibromo-[1,1′:3′,1′′-terphenyl]-5′-yl)(trimethyl)silane (3)Argon was bubbled through a solution of the diborylated derivative 2 (1 g, 2.48 mmol, 1 equiv), 1,3-dibromobenzene (2.92 g, 12.43 mmol, 5 equiv), and K₂CO₃ (1.46 g, 15 mmol, 5 equiv) in 3:2:1 toluene–EtOH–H₂O. Pd(PPh₃)₄ (71 mg, 2.5 equiv) was added, and the mixture was stirred at 90 °C for 24 h. The organic phase was separated, washed with H₂O, dried (MgSO₄), and concentrated. The residue was purified by column chromatography (silica gel, hexane; R_f  = 0.50) to give a colorless powder; yield: 0.92 g (81%); mp 230 °C.¹H NMR (400 MHz, CDCl₃) δ = 7.76 (t, J = 1.7 Hz, 2 H), 7.66 (s, 3 H), 7.56 (dd, J = 1.7, 1.1 Hz, 1 H), 7.54 (dd, J = 1.7, 1.1 Hz, 1 H), 7.52 (dd, J = 1.9, 1.0 Hz, 1 H), 7.50 (dd, J = 1.9, 1.0 Hz, 1 H), 7.33 (t, J = 7.8 Hz, 2 H), 0.36 (s, 9 H). ¹³C NMR (100 MHz, CDCl₃): δ = 143.9, 143.0, 138.1, 133.8, 132.9, 132.0, 128.0, 127.4, 125.9, 121.5, 1.2.
• 9 1⁵,4⁵-Bis(trimethylsilyl)cyclohexa-meta-phenylene (4)2,2′-bipyridine (312 mg, 2.0 mmol), cod (216 mg, 2.0 mmol), and [Ni(cod)₂] (550 mg, 2.0 mmol) were placed in a well-dried Schlenk flask and dissolved in anhyd DMF (80 mL). The mixture was then stirred for 30 min at 80 °C in darkness. A solution of the dibromo compound 3 (500 mg, 1.08 mmol) in degassed anhyd toluene (150 mL) was added quickly, and the resulting mixture was stirred at 80 °C for 3 d. The reaction was stopped by adding 10% aq HCl, and the organic phase was separated, washed with H₂O, dried (MgSO₄) and concentrated in vacuo. The residue was purified by column chromatography [silica gel, PE–CH₂Cl₂ (20:1); R_f  = 0.29] to give a colorless crystalline solid; yield: 397 mg (61%, 0.66 mmol).¹H NMR (400 MHz, CDCl₃) δ = 8.34 (s, 4 H), 8.31 (s, 2 H), 7.86 (d, J = 1.7 Hz, 4 H), 7.75 (t, J = 8.8 Hz, 8 H), 7.58 (t, J = 7.6 Hz, 4 H), 0.39 (s, 18 H).
• 10 1⁵,4⁵-Bis(pinacolatoboryl)cyclohexa-meta-phenylene (6)A 50 mL flask containing the diiodo compound 5 (1.0 g, 2.1 mmol), bis(pinacolato)diborane (1.42 g, 5.6 mmol), KOAc (2.06 g, 21 mmol), and Pd(dppf)Cl₂ (73.2 mg, 0.1 mmol) was evacuated and refilled with argon three times. DMF (30 mL) was then transferred into the flask from a syringe under argon at r.t., and the mixture was stirred at 110 °C for 48 h. The crude product was purified by column chromatography [silica gel, PE–CH₂Cl₂ (4:1)] to give a white solid; yield: 0.8 g (58%).¹H NMR (400 MHz, CDCl₃) δ = 8.19 (d, J = 1.7 Hz, 6 H), 7.84 (d, J = 8.7 Hz, 4 H), 7.73 (d, J = 7.1 Hz, 8 H), 7.56 (t, J = 7.7 Hz, 4 H), 1.41 (s, 24 H).