Extraction and Transport of Particular Guests

Although a macrocyclic nature of the host is not a prerequisite for binding organic guests, the discovery of crown ethers by Pedersen [45], followed by extensive smdies of Cram [1] and Lehn [2], gave impetus to the birth of host-guest complex chemistry. Amines and crown ethers, particularly those of the 18-crown-6 family, constitute the first and most thoroughly studied to date subject in the field. 

In fact, even in his earliest works Pedersen found that primary ammonium compounds do form complexes with crowns whereas secondary, tertiary, and quaternary ammonium ions do not [45,46]. For the first time, it was ascribed to the mismatch of macrocyclic hole size and groups other than NH.f. 

Cram and co-workers studied thermodynamic and structural aspects of the interaction in depth. /-Butylamine was typically used as a standard guest, the association of which with a huge variety of crown ethers having different ring sizes, numbers and positions of oxygen atoms, and different substituents was measured in chloroform as a solvent. Picrate, thiocyanate, or hexafluorophosphate extraction was a common tool (for the later discussion of the role of the counterion in complexation see Ref. 47 as concerns pH dependence, it is determined typically by the ranges of existence of the ammonium ion in the protonated and the host in the neutral forms). Some illustrative examples of K ss for t-butylamine thiocyanate, extracted into chloroform at 24 C [9], are listed below  

Important conclusions may be drawn even from that minor part of Cram s results. First, the role of preorganization is evident, the macrocyclic 

The thorough analysis of these and other data on influence of the structural variations of the host, in conjunction with assembling Corey-Pauling-Koltun molecular models, have led to the following scheme of guest fixation by 18C6 systems  

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