Structure−Reactivity Relationship in Interlocked Molecular Compounds and in Their Supramolecular Model Complexes
作者:Masumi Asakawa、Christopher L. Brown、Stephan Menzer、Françisco M. Raymo、J. Fraser Stoddart、David J. Williams
DOI:10.1021/ja962937z
日期:1997.3.1
Examination of the pseudorotaxane-like geometries adopted in the solid state by a series of 1:1 complexes revealed significant differences in the hydrogen bonding interactions between oxygen atoms in some hydroquinone-based guests carrying polyether/polyester functions and the acidic hydrogen atoms on the bipyridinium units of the host-cyclobis(paraquat-p-phenylene). These differences are reflected directly in the stabilities of the complexes in solution and dramatic changes in the magnitudes of their association constants (K-a values ranging from 130 to 4300 M(-1) in MeCN at 25 degrees C) are observed upon varying the location of the carbonyl ester function(s) along the polyether/ester chains. A similar effect (K-a values ranging from 5 to 730 M(-1) in Me(2)CO at 25 degrees C) was observed in the binding of paraquat as its bipyridinium bis(hexafluorophosphate) salt by analogous macrocyclic hydroquinone-based mono- and bis-lactones. Investigations of the kinetics of hydrolyses of the ester functions revealed that - while inert in their free forms - the macrocyclic mono- and bis-lactones undergo hydrolysis when incorporated within [2]catenanes composed of one of these macrocyclic lactones and cyclobis(paraquat-p-phenylene). Presumably, the enhanced reactivity of the ester functions is a result of [C-H ... O] hydrogen bonding interactions involving the ester carbonyl oxygen atoms and the acidic hydrogen atoms on the bipyridinium units, as suggested by single-crystal X-ray crystallographic analyses. Thus, cyclobis(paraquat-p-phenylene) can act as a mechanically-interlocked ''catalyst''.
Enantioselective Discrimination in the Self-Assembly of [2]Pseudorotaxanes
作者:Masumi Asakawa、Henk M. Janssen、E. W. Meijer、Dario Pasini、J. Fraser Stoddart
closer the chiral centers are to the hydroquinone templating unit. CD spectroscopy reveals that the different modes of binding of the enantiomeric substrates by the axiallychiral tetracationic cyclophane are not accompanied by drastically different core geometries for the [2]pseudorotaxanes. Thus, the chirality of the complex is governed primarily by the properties of the rigid receptor. The combination