Choi Heung-Jin, Park Yeon Sil, Song Jie, Youn Seok Ju, Kim Hong-Seok, Kim Sung-Hoon, Koh Kwangnak, Paek Kyungsoo
Department of Applied Chemistry, Kyungpook National University, Daegu 702-701, Korea.
J Org Chem. 2005 Jul 22;70(15):5974-81. doi: 10.1021/jo0506478.
The conformations and properties of cavitand 5 with four benzimidazole flaps are studied by (1)H NMR. The benzimidazole cavitand 5 can form very stable vase structures with an enforced concave cavity by intermolecular hydrogen bonding with four hydroxyl-containing molecules, X-OH, such as methanol (X = Me), acetic acid (X = CH(3)CO), and trifluoroacetic acid (X = CF(3)CO). The stronger hydrogen bond donor strengths of X-OH are, the stronger hydrogen bonds are formed between the NH and N atoms of the neighboring benzimidazole fragments and the more vase structures of 5.4HOX are stable. The annular tautomerism of 5 in CDCl(3)/CD(3)OD (9:1, v/v) due to the proton exchange between NH and N atoms of the neighboring benzimidazole fragments is observed by 400 MHz (1)H NMR, and the free energy of activation is measured as DeltaG++(210) = 10.2 kcal/mol at a coalescence temperature of 210 K. Cavitand 5 forms inclusion complexes with 4-methylbezamide guests such as 4-methyl-N-p-tolylbenzamide 6 and N,4-dimethylbenzamide 7 in water-saturated CDCl(3). However, an isomorphic 4-methylanilide guest such as N-4-tolylacetamide 8 cannot be recognized in the concave cavity of 5. This high selectivity toward 4-methylbenzamide over 4-methylanilide seems attributable to the hydrogen-binding interaction between the NH proton of 4-methylbezamide guest 7 and the oxygen atom of the closest water molecule.
通过¹H NMR研究了具有四个苯并咪唑侧翼的穴状配体5的构象和性质。苯并咪唑穴状配体5可以通过与四个含羟基分子X-OH（如甲醇（X = Me）、乙酸（X = CH₃CO）和三氟乙酸（X = CF₃CO））形成分子间氢键，形成具有强制凹腔的非常稳定的花瓶状结构。X-OH的氢键供体强度越强，相邻苯并咪唑片段的NH和N原子之间形成的氢键越强，5·4HOX的花瓶状结构越稳定。在400 MHz¹H NMR下观察到，由于相邻苯并咪唑片段的NH和N原子之间的质子交换，5在CDCl₃/CD₃OD（9:1，v/v）中的环状互变异构现象，在210 K的聚结温度下，活化自由能测定为ΔG++(210) = 10.2 kcal/mol。在水饱和的CDCl₃中，穴状配体5与4-甲基苯甲酰胺客体（如4-甲基-N-对甲苯基苯甲酰胺6和N,4-二甲基苯甲酰胺7）形成包合物。然而，同构的4-甲基苯胺客体（如N-4-甲苯基乙酰胺8）在5的凹腔中无法被识别。这种对4-甲基苯甲酰胺相对于4-甲基苯胺的高选择性似乎归因于4-甲基苯甲酰胺客体7的NH质子与最接近的水分子的氧原子之间的氢键相互作用。
