Bowl-to-Bowl Inversion Accelerated by the Introduction of Bulky 1,1,4,4-Tetramethylbutane-1,4-diyl Groups on Sumanene

Abstract

A variable-temperature ¹H NMR study revealed that the bowl-to-bowl inversion of sumanene is accelerated by the introduction of bulky 1,1,4,4-tetramethylbutane-1,4-diyl groups on the peripheral aromatic ring. This is likely to be induced by a bowl flattening, as shown in the DFT-optimized structures.

Key words

bowl-to-bowl inversion - sumanene - bowl-shaped polyaromatic molecule - Friedel–Crafts alkylation - variable-temperature ¹H NMR

• References and Notes

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• 19 Preparation Procedure for 4 and 5 To a mixture of sumanene (1, 10 mg, 0.038 mmol) and AlCl₃ (45.3 mg, 0.34 mmol) in dry CH₂Cl₂ (1 mL) was added 2,5-dichloro-2,5-dimethylhexane (31.7 mg, 0.17 mmol) under a nitrogen atmosphere at room temperature. The reaction mixture was stirred for 1 h. The reaction mixture was filtered through silica gel (eluent: CH₂Cl₂), and the solvent was evaporated in vacuo to give the crude product (17 mg) including 3, 4, and 5. They were separated by recycling preparative HPLC with the GPC columns (eluent: CHCl₃, column: tandemly arranged JAIGEL-1H and JAIGEL-2H, Japan Analytical Industry Co., Ltd.). Compound 4: FT-IR (KBr): ν = 2925, 2867, 2956, 1457, 1361 cm^–1. ¹H NMR (400 MHz, toluene-d ₈, –40 °C, ref.: δ = 2.08 ppm): δ = 7.10 (s, 2 H), 4.85 (d, J= 19.0 Hz, 1 H), 4.50 (d, J= 19.2 Hz, 2 H), 4.14 (d, J= 19.2 Hz, 1 H), 3.57 (d, J= 19.2 Hz, 2 H), 1.80–1.60 (m, 8 H), 1.55 (s, 6 H), 1.50 (s, 6 H), 1.29 (s, 6 H), 1.18 (s, 6 H) ppm; ¹³C NMR (100 MHz, CDCl₃, room temperature, ref.: δ = 77.0 ppm): δ = 147.4, 146.8, 144.7, 143.9, 143.65, 143.56, 142.3, 142.1, 122.8, 47.4, 44.7, 2 × 36.7, 36.1, 36.0, 32.9, 32.8, 2 × 29.7 ppm. HRMS (MALDI TOF): m/z [M⁺] calcd for C₃₇H₄₀: 484.3125; found: 484.3106. Compound 5: FT-IR (KBr): ν = 2959, 2925, 2864, 1457, 1361 cm^–1. ¹H NMR (400 MHz, toluene-d ₈, –60 °C, ref.: δ = 2.08 ppm): δ = 4.90 (d, J= 19.0 Hz, 3 H), 4.21 (d, J= 19.2 Hz, 3 H), 1.82–1.65 (m, 12 H), 1.60 (s, 9 H), 1.34 (s, 9 H) ppm. ¹³C NMR (100 MHz, toluene-d ₈, –40 °C, ref.: δ = 20.4 ppm): δ = 143.9, 143.7, 141.6, 47.7, 36.8, 36.1, 32.9, 29.8 ppm. HRMS (MALDI TOF): m/z [M⁺] calcd for C₄₅H₅₄: 594.4220; found: 594.4237.
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