Deformative Transition of the Menschutkin Reaction and Helical Atropisomers in a Congested Polyheterocyclic System

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• 作者：Yung-Yu Chang ; Tse-Lok Ho ; Wen-Sheng Chung
• 刊名：Journal of Organic Chemistry
• 出版年：2014
• 出版时间：November 7, 2014
• 年：2014
• 卷：79
• 期：21
• 页码：9970-9978
• 全文大小：624K
• ISSN：1520-6904

文摘

A 4,7-phenanthroline polycyclic 1A designed for probing the limits of the Menschutkin reaction was synthesized in a six-step sequence. The rotational barrier of the phenyl ring nearby the N-methyl group in rac-2A was estimated to be 鈮?8.1 kcal/mol from VT-NMR experiments, making them a new type of helical atropisomer. The methylation rate constants of 9 and 1A with MeI was found to be 2.22 脳 10^鈥? and 9.62 脳 10^鈥? s^鈥? mol^鈥? L, respectively; thus, the formation rate of (P/M)-2A is one of the slowest rates ever reported for a Menschutkin reaction. The N-methyl protons in (P/M)-2A exhibit a significant upfield shift (螖未 1.0 ppm) in its ¹H NMR, compared to those without a nearby phenyl, indicating a strong CH-蟺 interaction is involved. Conformational flexibility in dipyridylethene 9 is clearly shown by its complexation with BH₃ to form helical atropisomers (P,P/M,M)-10. The pK_a values of the conjugate acids of 1A and 9 in acetonitrile were determined to be 4.65 and 5.07, respectively, which are much smaller compared to that of pyridine 14a (pK_a = 12.33), implying that the basicity, nucleophilicity, and amine alkylation rates of 1A and 9 are markedly decreased by the severe steric hindrance of the flanking phenyl rings in the polyheterocycles.