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Tri‐ and Tetraurea Piperazine Cyclophanes: Synthesis and Complexation Studies of Preorganized and Folded Receptor Molecules
Author(s) -
Raatikainen Kari,
Beyeh N. Kodiah,
Rissanen Kari
Publication year - 2010
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201001695
Subject(s) - cyclophane , chemistry , intramolecular force , hydrogen bond , crystallography , stereochemistry , molecule , ether , monomer , piperazine , crystal structure , calixarene , organic chemistry , polymer
Abstract A series of symmetrical tri‐ and tetrameric N ‐ethyl‐ and N ‐phenylurea‐functionalized cyclophanes have been prepared in nearly quantitative yields (86–99 %) from the corresponding tri‐ and tetraamino‐functionalized piperazine cyclophanes and ethyl or phenyl isocyanates. Their conformational and complexation properties have been studied by single‐crystal X‐ray diffraction, variable‐temperature NMR spectroscopy, and ESI‐MS analysis. The rigid 27‐membered trimeric cyclophane skeleton assisted by a seam of intramolecular hydrogen bonds results in a preorganized ditopic recognition site with an all‐ syn conformation of the urea moieties that, complemented by a lipophilic cavity of the cyclophane, binds molecular and ionic guests as well as ion pairs. The all‐ syn conformation persists in acidic conditions and the triprotonated triurea cyclophane binds an unprecedented anion pair, H 2 PO 4 − ⋅⋅⋅HPO 4 2− , in the solid state. The tetra‐ N ‐ethylurea cyclophane is less rigid and demonstrates an induced‐fit recognition of diisopropyl ether in the solid state. The guest was encapsulated within the lipophilic interior of a quasicapsule, formed by intramolecular hydrogen‐bond‐driven folding of the 36‐membered cyclophane skeleton. In the gas phase, the essential role of the urea moieties in the binding was demonstrated by the formation of monomeric 1:1 complexes with K + , TMA + , and TMP + as well as the ion‐pair complexes [KI+K] + , [TMABr+TMA] + and [TMPBr+TMP] + . In the positive‐mode ESI‐MS analysis, ion‐pair binding was found to be more pronounced with the larger tetraurea cyclophanes. In the negative mode, owing to the large size of the binding site, a general binding preference towards larger anions, such as the iodide, over smaller anions, such as the fluoride, was observed.