Thiophene-Based Double Helices: Syntheses, X-ray Structures, and Chiroptical Properties

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• 作者：Sheng Zhang ; Xinming Liu ; Chunli Li ; Lu Li ; Jinsheng Song ; Jianwu Shi ; Martha Morton ; Suchada Rajca ; Andrzej Rajca ; Hua Wang
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：August 10, 2016
• 年：2016
• 卷：138
• 期：31
• 页码：10002-10010
• 全文大小：767K
• 年卷期：0
• ISSN：1520-5126

文摘

We demonstrate facile and efficient construction of conjugated double helical ladder oligomers from the saddle-shaped cyclooctatetrathiophene (COTh) building blocks. The key step involves deprotonation of tetra[3,4]thienylene (β,β-COTh) with n-BuLi which displays remarkably high ipsilateral selectivity. Three racemic double helical ladder oligomers, rac-DH-1, rac-DH-2, and rac-DH-3, containing two, three, and five COTh annelated moieties are efficiently synthesized by diastereoselective coupling of the racemic precursors. The X-ray crystallographic studies of rac-DH-1, rac-DH-2 and rac-DH-3 unambiguously revealed that each double helical scaffold has two single helices intertwined with each other via the C–C single bonds. Following removal of TMS groups, double helical ladder oligomer rac-DH-1-D had sufficient solubility to be resolved via chiral HPLC, thus enabling determination of its chirooptical properties such as CD spectra and optical rotation. (+)-DH-1-D has a large barrier for racemization, with lower limit of ΔG^‡ > 48 kcal mol^–1, which may be compared to DFT-computed barrier of 51 kcal mol^–1. The enantiomers of DH-1-D show 1 order of magnitude stronger chirooptical properties than the carbon–sulfur [7]helicene, as determined by the anisotropy factor g = Δε/ε = −0.039, based on Δε_max = −11 and ε = 2.8 × 10² L mol^–1 cm^–1 in cyclohexane at 327 nm.