Unusual reactivity of upper-rim bridged calix[4]arenes – Friedel–Crafts alkylation via cleavage of the macrocyclic skeleton

Upper rim-bridged calixarenes

Calix[n]arenes represent a very attractive family of macrocyclic compounds with many potential supramolecular applications. Due to their well-established chemistry and many different synthetic strategies, enabling practically any derivatization of the basic skeleton, calixarenes are among the very popular building blocks used for the design and construction of various receptors, sensors and other sophisticated supramolecular systems. Regio- and/or stereo-selective derivatization of calixarenes currently represents a very extensive set of reactions, the overview of which would fill many thick books. Therefore, this review deals with only a small part of the above-mentioned reactions, specifically describes possible ways of bridging the upper rim of calixarenes, often leading to interesting rigidified structures, and also briefly mentions the potential use of these compounds.

Nucleophile-induced transformation of phenoxathiin-based thiacalixarenes

Oxidized phenoxathiin-based macrocycles, easily accessible thiacalix[4]arene derivatives, consist of a unique set of structural elements representing a key prerequisite for the unexpected reactivity described in this paper. As proposed, the internal strain, imposed by the presence of a heterocyclic moiety, together with a number of electron-withdrawing groups (SO₂) opens the way to the cleavage of the macrocyclic skeleton through a cascade of three S_NAr reactions triggered by the nucleophilic attack of an SH^- anion. The whole transformation, which is unparalleled in classical calixarene chemistry, leads to unique linear sulfinic acid derivatives with a rearranged phenoxathiin moiety that can serve as building blocks for macrocyclic systems of a new type.

Vanadium complexes derived from oxacalix[6]arenes: structural studies and use in the ring opening homo-/co-polymerization of ε-caprolactone/δ-valerolactone and ethylene polymerization

Oxacalix[6]arene vanadium complexes have been employed for the ROP of cyclic esters and ethylene polymerization.

Rearrangement of meta-Bridged Calix[4]arenes Promoted by Internal Strain

The meta-bridged calixarenes possess a rigidified and highly distorted cavity, where the additional single-bond bridge imposes an extreme internal strain on the whole system. As a consequence, these compounds exhibit a reasonably amended reactivity, compared with common calix[4]arene derivatives, which is governed by the release of internal strain. This can be documented by the reaction of the bridged calix[4]arene with P₂O₅ or Nafion-H, leading (apart from polymers) to a macrocyclic product with a rearranged basic skeleton. The methylene bridge next to the fluorene moiety is intramolecularly shifted from position 2 to position 4 of the phenolic subunit to minimize the tension. As revealed by single-crystal X-ray analysis and by application of the residual dipolar coupling method, the rearrangement occurs without altering the original conformation.