Kite conformation

Cavitands 89 flutter between a C2v-symmetric kite conformation (favoured at low temperature) and a C4v-symmetric vase conformation (favoured at higher temperatures) in which the walls are upright oriented see Reference 397a... 

The versatility of resorcinarene-detived cavitands has generated a wealth of host molecules (Rebek 1999 Jasat and Sherman 1999 Hof et al. 2002 Rudkevich 2002 Laughrey et al. 2003 Shivanyuk et al. 2002 Barrett et al. 2002 Paek et al. 2003 Pirondini et al. 2003). Tetraquinoxaline-spanned resorcinarene cavitands 182 were introduced by Cram in the early 1980s (1983) and have been used extensively to study host-guest interactions (Moran et al. 1982 Dalcanale et al. 1989 Moran et al. 1991 Soncini et al. 1992 Careri et al. 1997 Bianchi et al. 2003). Of particular interest is therin to undergo a reversible thermal switch liom the vase to the kite conformation (Fig. 5.13). The vase exists at temperatures of 45 C and above, while the kite exists exclusively below —60 °C. 

Bridged resorcinarenes are known as cavitands (such as 31 or 32). However, cavitand 32 is not yet fully preorganized as it can give two main conformers, namely a cylindrical vessel or the open kite (Figure 3.16). 

The first deep-cavity cavitand to be synthesized was the 2.3-quinoxaline derivative 4 (Fig. 4). This second row of aromatic rings in this cavitand. and related cavitands formed by the condensation of resorcinarenes with 2,3-dichloropyrazines. is conformationally flexible. Thus, they exist in a range of conformers between the vaselike C4v form (shown) and a flat C21 form. These families of compounds were termed velcrands, because in the flat, "kite-like" form, they have a propensity to dimerize.When not binding themselves, these types of cal-itands also bind mono-" or di-substituted"benzene derivatives. 

The resulting ring structure is therefore somewhat kite-shaped, with the metal residing in relatively close proximity to the P-carbon of the metallacyclobutane ring. It is important to note that evidence indicates that the non-agostic complexes 8 and 9 are conformationally fluxional on an NMR time scale at 25 °C, and... 

One of the early examples of resorcin[4]arenes with add-on features due to non-covalent chirality was deep cavitand amide 76 (Fig. 2.20) [98]. Cavitand 76 has doors at the upper rim that are craitroUed by a unidirectional cooperative belt of hydrogen bmids. The doors can close clockwise or counterclockwise, and, in the presence of additional chirality centers, one of the directions is preferred (de 50 %) [98]. The chiral vessels exhibit preferential binding of enantiomers of various small molecules, e. g. /rans-cyclohexanediol (60% de). Other resorcin[4]arenes with inherently chiral conformations were reported by Schmidt et al. [99] and the group of Szumna [100,101]. Amide substituted resorcin[4]arenes like 77,78 and 79 exist in inherently chiral kite conformatiOTis that are stabilized by an unidirectional... 

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