Comparative X-ray structural study of laterally mono-ethyl substituted 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetra-methoxycalix[4]arene and its non-substituted parent compound including guest free and solvated forms. Chemical straightening of guest channels

Abstract

Comparative X-ray structural study of laterally mono-ethyl substituted 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetramethoxycalix[4]arene and its non-substituted parent compound including guest free and solvated forms.Abstract Three solvate crystal structures of the laterally ethyl substituted tetra-tert-butyltetramethoxycalix[4]arene 1 [(1ÁTHF (1a), 1ÁCHCl 3 (1b) and 1ÁCH 2 Cl 2 (1c)] are compared to the corresponding solvent-free structure (1) using single crystal X-ray structure determination, isostructurality and molecular isometricity calculations. To study the effect of the lateral substitution, the laterally nonsubstituted host with the guest THF (2a) is also included to the comparison. The calixarene molecules in the different structures all adopt the partial cone conformation with different affection of the respective guest molecules, always being positioned interstitially. Depending on the lateral substitution and the size of the included guests, the molecular conformation of the calix[4]arene shows small differences relating to the alignment of the arene units. The channels disposable of the solvent guest molecules in the crystal structures straighten as the effect of lateral substitution of the host calix[4]arene. The orthorhombic crystal structures of 1a-c are isostructural irrespective of the included solvent molecules, while 1 and 2a crystallise in the same monoclinic space group.

Calixarenes containing modified meso bridges

“…Calixarenes containing modified meso bridges [46][47][48] are synthesized via substitution of methylene bridges [49][50][51][52] or via their oxidation to keto groups; 53,54 they have not been as intensively studied as calixarenes functionalized at wide 55,56 or narrow 57.58 rims. Therefore it seems of interest to describe several selected examples of this class of compounds, showing their possible applications.…”

Structural conditions required for the bridge lithiation and substitution of a basic calix[4]arene

“…Besides the huge progress made in the modification of the upper- and lower-rim positions of basic calix[4]arenes such as 1 in Scheme 1, the substitution of at least one methylene bridge of the chalice opens up a perspective for the vertical expansion of the molecule [1–2]. Thus, during the past decade two main preparative routes for the methylene-bridge substitution of p - tert -butyltetramethoxycalix[4]arene have been established: A protocol described by Biali et al yields a stabilized methylene carbocation through bromination that is ready for electrophilic substitution under S N 1 conditions [3], whereas we follow a route involving the formation of a methylene carbanion through lithiation, which by nucleophilic substitution forms the desired bridge-substituted calixarenes [4–5].…”