Synthesis and Conformational Analysis of 10-Mesitylanthracene-1,8-diyl Oligomers

Published as part of the Cluster Atropisomerism

Abstract

Oligomers consisting of 10-mesitylanthracene-1,8-diyl units were synthesized by Ni-mediated coupling of the corresponding dibromide as new oligoarene compounds. The structures and properties of these oligomers were investigated by NMR and electronic spectroscopy. Conformational analyses with the aid of DFT calculations revealed that each biaryl axis adopted a nearly perpendicular conformation, and the trimers and tetramers existed as a mixture of diastereomeric conformers.

Key words

biaryl - conformation - NMR - DFT - coupling - anthracene

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• 19 Synthesis of Oligomers An: A solution of 2,2′-bipyridine (86.3 mg, 0.552 mmol), 1,5-cyclooctadiene (68 μL, 0.553 mmol), and Ni(cod)₂ (155 mg, 0.564 mmol) in a degassed mixture of toluene (2 mL) and DMF (2 mL) was stirred for 30 min at 80 °C under Ar. To the solution was slowly added a solution of 3 (101 mg, 0.223 mmol) in degassed toluene (5 mL). The mixture was then heated at 80 °C for 19 h under Ar. After cooling to r.t., the mixture was treated with aq HCl (5 mL, 1.0 M). After addition of sat. aq NaCl (100 mL), the mixture was extracted with toluene (3 × 100 mL). The combined organic extracts were dried over Na₂SO₄, filtered and evaporated to give a mixture of oligomers (87.5 mg). Components of the crude product were separated by chromatography (silica N60, hexane/CH₂Cl₂, 5:1) and then by recycling GPC (CHCl₃) to give A2 (6.7 mg, 10.2%), A3 (5.6 mg, 8.5%), A4 (5.1 mg, 7.7%), and other large oligomers (ca. 22 mg) as yellow solids. A2: mp 245–246 °C. ¹H NMR (400 MHz, CDCl₃): δ = 1.80 (s, 6 H), 1.89 (s, 6 H), 2.50 (s, 6 H), 7.14 (s, 2 H), 7.17 (s, 2 H), 7.28 (m, 4 H), 7.50 (m, 4 H), 7.60 (d, J = 6.6 Hz, 2 H), 7.68 (m, 4 H), 8.13 (s, 2 H), ¹³C NMR (100 MHz, CDCl₃): δ = 20.29, 20.36, 21.45, 125.10, 125.21, 125.49, 125.85, 125.93, 126.14, 127.48, 128.47, 129.20, 129.74, 130.15, 131.15, 131.65, 131.74, 134.95, 136.05, 137.30, 137.83, 139.21. HRMS–FAB: m/z calcd for C₄₆H₃₈ [M]⁺: 590.2974; found: 590.2939. All other experimental procedures and characterization data of A2 and other oligomers can be found in the Supporting Information.
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• 21 We obtained another unstable energy-minimum structure, the dihedral angle in which was –108.8° (or +108.8°).
• 22 For the calculation of anti-3′ and syn-3′, we obtained only one energy-minimum structure, even though we started the calculations from various conformations.