Synthesis Of Pillar[5] Arene Derivatives And Their Selective Binding Behaviors

Supramolecular chemistry is one of the most attractive fields in chemistry, lifescience and materials science. The arrival of any new class of macrocycles canaccelerate the development of supramolecular chemistry. Macrocycles, such as crownethers, cyclodextrins, cucurbiturils and calixarenes (CAs), play a prominent role inhost-guest chemistry. Pillar[n]arene is a new type of calixarene analogue made up ofhydroquinone units that are linked by methylene (-CH2-) bridges. It possesses thesymmetrical pillar architecture with two identical cavity portals. On the other hand,Pillararenes（PAs） are more rigid than the traditional CAs. As a new class ofmacrocyclic hosts, PAs are attracting considerable attention because of theirapplications in molecular recognition and supramolecular self-assembly. A series ofpillar[5]arene derivatives were synthesized to investigate the binding modes andrecognition ability with some typical guests by multiple means. The mechanisms ofmolecular recognition based on PAs and their derivatives were further revealed. Themain contents are as follows:1. An anionic water-soluble pillar[5]arene—carboxylatopillar[5]arene （CP5A）and some organic positivly-charged compounds （i.e.1,4-bis(pyridinium)butanederivatives and isoquinoline derivatives）were synthesized. The binding behaviors ofCP5A with these guests mentioned above have been comprehensively investigated byNMR spectroscopy, UV absorption and fluorescence spectroscopy in aqueousphosphate buffer solution (pH7.2).(1) The results obtained indicate undoubtedly theformation of [2]pseudorotaxanes and/or external geometries between CP5A and1,4-bis(pyridinium)butane derivatives, where the electrostatic interaction betweennegative carboxylate groups and positive pyridinium groups significantly reinforcesthe complex stability. The position of the substituents attached to the pyridinium ringsin1,4-bis(pyridinium)butane dramatically affects the binding affinities and thebinding modes.(2) CP5A presents very strong binding ability to isoquinolinederivatives. Furthermore, the solvent effects and salt effects during the course of complexation have also been investigated, showing they significantly influence thebinding ability and selectivity of CP5A with isoquinoline derivative guests.Electrostatic and hydrophobic interactions and C-H···π interactions play a crucial rolein the binding between CP5A and isoquinoline derivative cations in aqueous media.2. Four kinds of alkyl-substituted pillar[5]arenes (AlkP5As) were synthesized.The complexation of AlkP5As with some neutral guests has been investigated byNMR titrations.(1) It reveals that AlkP5A can efficiently form interpenetratedgeometries with neutral dinitrile. Slow exchange on the NMR timescale was observedfor this complex in CDCl3. AlkP5As exhibit very strong binding affinities towards theshort and size-fit butanedinitrile and hexanedinitrile. While for the long dinitriles, thebinding constants sharply decreased. Dipole-dipole forces and C H···π andC H···N/C H···O hydrogen bond interactions are important driving forces.(2)Theexperimental results indicate the formation of interpenetrated complexes betweenAlkP5As and1,4-dihalobutanes, where the dispersive interactions dominate thecomplex stability. Fast exchange on the NMR timescale was observed for thiscomplex in CDCl3. For1,4-dihalidebutane guests, the association constant increasesin the order of F <Cl <Br <I.3. A fully functionalized P5A has been synthesized by CuAAC “click” reactionand the possible synthetic strategies have been discussed. The host showed effectivebinding affinities towards neutral alkanediamines, which are much larger than thoseobserved for simple methyl-substituted pillar[5]arene.4. A highly effective self-assembly in chloroform of [2]pseudorotaxane,composed of EtP5A/EtP6A and two secondary ammonium salts, withtetrakis[3,5-bis(trifluoromethyl)phenyl]borate counteranion (1·BArF and2·BArF)induced by very loose ammonium ion pairs, is prepared. In addition, a firstpillararene-based self-sorting system consisting of four components is constructed,demonstrating the spontaneous high affinity of molecules for themselves.