Structural properties of benzimidazole cavitand and its selective recognition toward 4-methylbenzamide over 4-methylanilide
Structural properties of benzimidazole cavitand and its selective recognition toward 4-methylbenzamide over 4-methylanilide
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DOI：
10.1021/jo0506478
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发表时间：
2005-07-22
影响因子：
3.6
通讯作者：
Paek, K
中科院分区：
文献类型：
--
作者：
Choi, HJ;Park, YS;Paek, K
用1H-1 NMR研究了含4个苯并咪唑瓣的空穴配体5的构象和性质。苯并咪唑空穴配体5可以通过与四种含羟基分子X-OH（如甲醇（X = Me）、乙酸（X = CH 3CO）和三氟乙酸（X = CF 3CO））的分子间氢键结合形成具有强制凹腔的非常稳定的花瓶结构。X-OH的氢键给体强度越强，相邻苯并咪唑片段的NH和N原子之间形成的氢键越强，5中心点4 HOX的花瓶结构越稳定。用400 MHz H-1 NMR观察到5在CDCl 3/CD 3 OD（9：1，v/v）中由于相邻苯并咪唑片段的NH和N原子之间的质子交换而产生的环状互变异构，并测量了在210 K的聚结温度下的活化自由能Δ G（210）（双匕首）= 10.2 kcal/mol。Cavitand 5在水饱和的CDCl 3中与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形成包合络合物。然而，同构的4-甲基苯胺客体如N-4-甲苯基乙酰胺8不能在5的凹腔中识别。4-甲基苯甲酰胺对4-甲基苯胺的这种高选择性似乎归因于4-甲基苯甲酰胺客体7的NH质子与最近的水分子的氧原子之间的氢键相互作用。
The conformations and properties of cavitand 5 with four benzimidazole flaps are studied by H-1 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 = CH3CO), and trifluoroacetic acid (X = CF3CO). 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 center dot 4HOX are stable. The annular tautomerism of 5 in CDCl3/CD3OD (9:1, v/v) due to the proton exchange between NH and N atoms of the neighboring benzimidazole fragments is observed by 400 MHz H-1 NMR, and the free energy of activation is measured as Delta G(210)(double dagger) = 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 CDCl3. 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.