891045-39-9Relevant articles and documents
N-(Acetamido)thiourea based simple neutral hydrogen-bonding receptors for anions
Liu, Wen-Xia,Yang, Rui,Li, Ai-Fang,Li, Zhao,Gao, Yu-Feng,Luo, Xing-Xing,Ruan, Yi-Bin,Jiang, Yun-Bao
experimental part, p. 4021 - 4028 (2009/12/06)
N-(Acetamido)-N′-phenylthioureas (4-6) were found to be efficient anion receptors with higher anion affinity than their N-benzamido-N′- phenylthiourea counterparts (1 and 2). The N′-phenylthiourea moiety in 4-6 was shown to be the chromophore with an absorption maximum at ca. 270 nm. It was found that, in the presence of anions, the absorption at ca. 270 nm of 4-6 (except 5f) in acetonitrile (MeCN) was blue shifted and enhanced while a red-shifted shoulder appeared at ca. 295 nm, together with an isosbestic point at ca. 240 nm. The 1:1 anion binding constants of 4-6, for example at 10 6-107 M-1 order of magnitude for AcO - in MeCN, were found to be higher than those of 1 and 2, although the acidity of the thioureido -NH protons in 4-6 is lower than that in 1 and 2. 1H NMR data indicates that the N-N single bond in 4-6 is twisted but less than that in 1 and 2. A conformation change at the N-N single bond of 4-6 was suggested to occur upon anion binding which leads to a planar hydrogen-bonding network in the anion binding complex in which a charge transfer takes place with the N-acyl moiety being the electron acceptor. Variations in the CD signals of a proline derivative 6 bearing a chiral center in the N-amido moiety provide direct evidence for this conformation change upon its binding with anions in MeCN. The amplified effect of substituent X at the N′-phenyl ring of 5 on the anion binding constant supports the conclusion of anion-binding switched charge transfer in the anion binding complex. 1H NMR and absorption titrations for 5 indicated that the anion-receptor interaction was of a hydrogen-bonding nature until the N′-phenyl substituent X is as electron-withdrawing as m-CF3 (5e). With X being the more electron-withdrawing p-NO2 (5f), deprotonation of the thioureido -NH occurs in the presence of anion. Results reported here confirm that N-amidothioureas derived from both N-aliphatic and N-aromatic amides can in general be a family of efficient hydrogen-bonding receptors, with the aliphatic N-amido derivatives being more efficient. This provides a wider structural diversity for designing thiourea-based functional molecules such as anion receptors and organocatalysts. Preliminary experiments confirm that 6 could catalyse efficiently the reduction of nitrostyrene in CH2Cl2 and MeCN.
