利用三中心氢键限制芳酰胺构象的核磁共振氢谱分析

设计并合成了9个可形成三中心氢键和6个可形成二中心氢键的N-芳基芳酰胺模型化合物, 基于它们在氯仿和二甲基亚砜(DMSO)中的一维核磁共振波谱, 系统地分析了羰基对βH和γH的去屏蔽效应. 将Δ(δ_βH)和Δ(δ_γH)的值结合在一起, 分析了三中心氢键对芳酰胺分子的构象限制效果, 发现N-(2-氟苯基)-2-氟苯甲酰胺、 N-(2-甲氧基苯基)-2-氟苯甲酰胺和N-(2-氟苯基)-2-甲氧基苯甲酰胺这3个N-芳基芳酰胺在酰胺基团的左右两侧都能展现出很好的构象控制效果, 因此认为这3种结构单元在构建折叠体方面具有更大的潜力. 此外, 本文还发现, 当NH与第二个氢键受体形成氢键时, 其和第一个氢键受体之间的氢键就被削弱了, 即在芳酰胺形成三中心氢键时, 2个氢键受体争相与NH形成氢键并取得了某种平衡.

1H NMR Study on the Conformation of Aromatic Amides Limited by Three-center Hydrogen Bonds

We designed and synthesized nine N-aryl aromatic amides that could form three-center hydrogen bonds， and six N-aryl aromatic amides that could form two-center hydrogen bonds. Based on their one-dimensional ¹H nuclear magnetic resonance（NMR） spectra in chloroform and dimethye sufoxide（DMSO）， the de-shielding effects of carbonyl groups on βH and γH were analyzed separately. The conformational restriction of aromatic amides by three-center hydrogen bonds was evaluated by combining Δ（δ_βH） and Δ（δ_γH）. It was disclosed that three aromatic amides ［2-fluoro-N-（2-methoxyphenyl）benzamide， N-（2-fluorophenyl）-2-methoxybenzamide and 2-fluoro-N-（2-fluorophenyl）benzamide］ display excellent planarization of molecular comformations at both sides of their carbonyl groups， which would have greater potential in constructing new foldamers. Furthermore， it was inferred that， when an NH group forms hydrogen bond with the second hydrogen-bond acceptor， the hydrogen bond between the NH group and the first hydrogen-bond acceptor would be weakened. In other words， for an aromatic amide with a three-center hydrogen bond， two hydrogen-bond acceptors contend with each other for the NH group， resulting in a balanced situation.