Role of preorganization in host-guest-chemistry

Abstract

The concept of preorganization in the complexation of alkali cations is illustrated by a systematic variation of the structure of molecular cavities. The parent hemispherand 2 has been compared with novel hemispherands that contain a nitrogen (pyrido) or oxygen (4H-pyran) atom. These two types of hemispherands have been synthesized using pyrylium building blocks. A novel approach to the parent hemispherands that have a functional group at the outer sphere, involves the macrocyclization of a flexible precursor to give 5 and subsequent reaction of 5 with nitromalondialdehyde. Finally, modification at the inner sphere of hemispherands has been achieved via pyrylium hemispherands (13). These compounds have been attained by reaction of the corresponding 4H-pyran hemispherand (12) with the triphenylmethyl carbenium ion. Comparison of the X-ray structures of two hemispherands (2 and 9) with the corresponding sodium picrate complexes shows the degree of preorganization. This is also reflected in the free energies of complexation with different picrates. © 1988 IUPAC