Neutral guest-cyclophane complex

Since the seminal work of Cram on cyclophanes and resorci-narenes (1) the host-guest chemistry of container molecules has been extensively investigated and more sophisticated examples with other forms and larger cavities have been reported. Thus a large number of host-guest complexes containing neutral,... 

The first practical application for cyclophanes was found in host-guest chemistry [4]. Molecular recognition of ions and neutral molecules in functionalized cavities of cyclophanes has been demonstrated for numerous examples. Here, special interest has been attracted by complexes in water since it can directly model molecular recognition events in biological systems. The development of suitable water soluble hosts with high binding selectivity is an active field of research [5] that has recently been reviewed [6]. 

Cyclophano-crown ether host (Scheme 8), possessing cyclic polyether and cyclophane moieties in a single molecule and ability to simultaneously bind a potassium cation and 6-methoxy-2-naphthonitrile in a water-methanol mixture, is of particular interest, since the contrasting solvent effects can be examined in the same host. Indeed, neutral 6-methoxy-2-naphthonitrile guest forms a 160-fold more stable complex with the cyclophane moiety in more hydrophilic 60 40 water-methanol solvent than in pnre methanol. In contrast, the affinity toward potassium ion is enhanced by a factor of 120 in pure methanol rather than in 60 40 water-methanol. Scheme 8... 

With the demonstration of the existence of a successful host-guest system between BPP34C10 and [PQT][PF6]2, the next question to be addressed was the reversal of the roles of the host and the guest, i.e., the formation of a receptor incorporating bipyridium units, which would complex with a neutral hyrdroquinol derivative as the substrate. To this end, the tetracationic cyclophane [BBIPYBIXYCY][PF6]4 was synthesised Scheme 2) from the known [BBIPYXY][PF6]2 - obtained from 4,4 -bipyridine and... 

Synthetic molecular receptors command widespread interest as mimics of membrane transport agents and enzyme active sites.[1,2] The development of new host systems for the selective complexation of organic and inorganic cations and anions has mushroomed in recent years however, examples of solution phase coordination of neutral organic molecules are few. Most of these examples involve inclusion of an aromatic hydrocarbon into the hydrophobic pocket of a water soluble cyclodextrin or cyclophane receptor.[3-6] A smaller subset of synthetic receptors possess lipophilic exteriors and hydrophilic cavities for the association of polar substrates in nonpolar media. [7-9] We report here the synthesis and characterization of a new system of the latter type designed to bind polar guests in chloroform solution. 

---