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Synthesis and Anion‐Selective Complexation of Homobenzylic Tripodal Thiourea Derivatives
Author(s) -
Hisaki Ichiro,
Sasaki Shinichi,
Hirose Keiji,
Tobe Yoshito
Publication year - 2007
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.200600647
Subject(s) - chemistry , steric effects , thiourea , cryptand , titration , binding constant , bromide , hydrogen bond , intramolecular force , stability constants of complexes , medicinal chemistry , selectivity , inorganic chemistry , stereochemistry , binding site , molecule , organic chemistry , ion , biochemistry , catalysis
Cryptand‐ and tripod‐type thiourea derivatives 4b and 5a – d , which have binding functionalities at the homobenzylic positions, were synthesized as possible neutral receptors toward anions with an expectation that the three binding sites work cooperatively to bind an anion selectively. 1 H NMR spectroscopic monitoring of the titration of cryptand 4b with CH 3 CO 2 – , Cl – , and F – in CDCl 2 CDCl 2 at 100 °C showed that the binding constants were considerably smaller than those of tripodal thiourea 5a , presumably owing to the presence of strong intramolecular hydrogen bonding in 4b . Complexation constants of tripodal receptors 5a – d with H 2 PO 4 – , CH 3 CO 2 – , Cl – , and Br – anions were evaluated by 1 H NMR and/or UV/Vis spectroscopic analysis of the titration in DMSO. Though tripodal receptors 5a , b undergo complexation with phosphate anion in a 1:1 stoichiometry, their association constants were not as large as simple reference compound 14 probably because of the steric hindrance around the binding sites and the large entropy cost for cooperative binding. Receptor 5c exhibits complexation in a 1:2 stoichiometry with H 2 PO 4 – and CH 3 CO 2 – , whereas it forms 1:1 complexes with chloride and bromide anions because of the subtle balance between the steric hindrance and the binding ability. However, by increasing the acidity of the thiourea functionality, receptor 5d exhibited remarkably enhanced binding ability and selectivity toward H 2 PO 4 – compared to those of reference compound 15 presumably through cooperative complexation of the three binding sites to the guest anion. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)