Toward a Four-Toothed Molecular Bevel Gear with C2-Symmetrical Rotors

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• 作者：Chen-Yi Kao ; Ya-Ting Hsu ; Hsiu-Feng Lu ; Ito Chao ; Shou-Ling Huang ; Ying-Chih Lin ; Wei-Ting Sun ; Jye-Shane Yang
• 刊名：Journal of Organic Chemistry
• 出版年：2011
• 出版时间：July 15, 2011
• 年：2011
• 卷：76
• 期：14
• 页码：5782-5792
• 全文大小：1220K
• 年卷期：v.76,no.14(July 15, 2011)
• ISSN：1520-6904

文摘

The design, synthesis, conformational analysis, and variable-temperature NMR studies of pentiptycene-based molecular gears Pp₂X, where Pp is the unlabeled (in 1H) or methoxy groups-labeled (in 1OM) pentiptycene rotor and X is the phenylene stator containing ortho-bridged ethynylene axles, are reported. The approach of using shape-persistent rotors of four teeth but C₂ symmetry for constructing four-toothed molecular gears is unprecedented. In addition, the first example of enantioresolution of chiral pentiptycene scaffolds is demonstrated. Density functional theory (DFT) and AM1 calculations on these Pp₂X systems suggest two possible correlated torsional motions, geared rocking and four-toothed geared rotations, which compete with the uncorrelated gear slippage. The DFT-derived torsional barriers in 1H for rocking, four-toothed rotation, and gear slippage are approximately 2.9, 5.5, and 4.7 kcal mol^鈥?, respectively. The low energy barriers for these torsional motions result from the low energy cost of bending the ethynylene axles. Comparison of the NMR spectra of 1OM in a mixture of stereoisomers (1OM-mix) and in an enantiopure form (1OM-op) confirms a fast gear slippage in these Pp₂X systems. The effect of the methoxy labels on rotational potential energy surface and inter-rotor dynamics is also discussed.