Resolving the magnetic asymmetry of the inner space in self-assembled dimeric capsules based on tetraurea-calix[4]pyrrole components
<p> The encapsulation of N,N, N',N'-tetramethyl-1,5-pentanediamine-N,N'-dioxide 2 in a non-chiral capsular assembly formed by dimerization of tetraurea-calix[4]pyrrole 1a produced the observation of the N-methyl groups of the encapsulated guest as two separated singlets...
| Authors: | , , |
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| Format: | article |
| Publication Date: | 2015 |
| Country: | España |
| Institution: | Varias* (Consorci de Biblioteques Universitáries de Catalunya, Centre de Serveis Científics i Acadèmics de Catalunya) |
| Repository: | Recercat. Dipósit de la Recerca de Catalunya |
| OAI Identifier: | oai:recercat.cat:2072/305848 |
| Online Access: | http://hdl.handle.net/2072/305848 https://doi.org/10.2533/chimia.2015.652 |
| Access Level: | Open access |
| Keyword: | CALIX[4]PYRROLE MAGNETIC ASYMMETRY MOLECULAR ENCAPSULATION SUPRAMOLECULAR CHIRALITY |
| Summary: | <p> The encapsulation of N,N, N',N'-tetramethyl-1,5-pentanediamine-N,N'-dioxide 2 in a non-chiral capsular assembly formed by dimerization of tetraurea-calix[4]pyrrole 1a produced the observation of the N-methyl groups of the encapsulated guest as two separated singlets resonating highly upfield in the 1H NMR spectrum. In order to clarify the origin of the observed signal splitting we assembled and studied a series of structurally related dimeric capsules. We used the tetraurea-calix[4]pyrrole 1a , the enantiomerically pure tetraurea-calix[4] pyrrole R-1b and the tetraurea-bisloop calix[4]pyrrole 1c as components of the produced assemblies. The 1H NMR spectra of the assembled encapsulation complexes with bis-N-oxide 2 evidenced diverse splitting patterns of the N-methyl groups. In addition, 2D EXSY/ROESY NMR experiments revealed the existence of chemical exchange processes involving the separated methyl signals of the encapsulated guest. The capsular assemblies were mainly stabilized by a belt of eight head-to-tail hydrogen-bonded urea groups. The interconversion between the two senses of rotation of the unidirectionally oriented urea groups was slow on the 1H NMR timescale. These characteristics determined the appearance of a new asymmetry element (supramolecular conformational chirality) in the assemblies that accounted for some of the magnetic asymmetries featured by the capsule's inner space. The racemization of the supramolecular chirality element was fast on the EXSY timescale and produced the chemical exchange processes detected for the encapsulation complexes.</p> |
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