Designed Heterocycles for Supramolecular Cation Recognition

Extensions from cation recognition to supramolecular recognition have recently been done in both natural and synthetic receptor chemistry. Four types of receptor-guest complexes are generally postulated based on the molecular sizes (1) smaU receptor-smaU guest complexes, (2) small receptor-big guest 

Several crown ether derivatives bound - NH3+, - C02 M, or other functional moieties exposed on the protein surface, and formed n 1 supramo-lecular complexes, van Unen et al. [26] and Itoh et al. [27] have demonstrated that the crown ether complexation remarkably enhanced reactivity 

Shinoda et al. reported a similar cychzation reaction with 3-bromomethyl-pyridine to give cyclic pyridinium tetramer 26 (Fig. 10) [55]. The protons of the cyclic tetramer had acidic nature, and were rapidly deuterated in D2O so- 

A series of cahxarene-type cychc oUgomers were systematically prepared. Cabildo et al. synthesized dicationic macrocycles 27 from a, a -dibromo-p-xylene with pyrazole and imidazole [56]. The energy barrier of the ring flipping inter conversion was estimated in solution (AG = 17kcalmol for 

Alcalde et al. determined the crystal structure of dicationic imidazoliiun macrocycle 28 in which two Cl anions formed two hydrogen bonds with the C-H protons of the receptor (Fig. 11) [57]. The quantitative investigations revealed that the ring closure reaction was promoted by Cl complexation with the monocationic hnear precursor [58]. Recently, Chellappan et al. reported the crystal structure of cahx[4]imidazolium[2]pyridine 29-F complex (Fig. 11), where the imidazohum C-H protons participated in hydrogen bonding with the F anion [59]. 

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