title: Structural properties of benzimidazole cavitand and its selective recognition toward 4-methylbenzamide over 4-methylanilide.
abstract: 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.
authors: Department of Applied Chemistry, Kyungpook National University, Daegu 702-701, Korea. choihj@knu.ac.kr
authors: Heung-Jin
authors: HJ
authors: Choi
authors: Yeon Sil
authors: YS
authors: Park
authors: Jie
authors: J
authors: Song
authors: Seok Ju
authors: SJ
authors: Youn
authors: Hong-Seok
authors: HS
authors: Kim
authors: Sung-Hoon
authors: SH
authors: Kim
authors: Kwangnak
authors: K
authors: Koh
authors: Kyungsoo
authors: K
authors: Paek