Carcerands and Hemicarcerands

Abstract

Carcerands and hemicarcerands are spherical, hollow host molecules with inner cavities that are large enough to accommodate smaller organic guest molecules. Since the first synthesis of a carcerand in the mid‐1980s, a large variety of different hemicarcerands and carcerands have been prepared and their properties investigated. In this review, mechanism and scope of (hemi)carcerand synthesis, which is a templated reaction, is discussed. This includes recent work on dynamic hemicarcerands, in which building blocks are connected via dynamic covalent bonds, and gated hemicarcerands, which open and/or close in response to an external trigger such as irradiation or the addition of acid, base, or thiols. In the second part of this chapter, the molecular recognition properties of hemicarcerands are reviewed. As part of this, binding properties of chiral hemicarcerands and water‐soluble hosts are discussed. In the last part, applications of hemicarcerands as molecular reaction flasks are reviewed. Through‐shell reactions between an encapsulated reactant and a bulk‐phase reactant include amine protonation, oxidation/reduction, H/D exchange, and nucleophilic substitutions and additions. In most of these reactions, bond formation/breaking takes place in a size‐restricted portal in the host shell. Thus, scope and reaction rate depend on guest orientation in the inner phase as well as size and shape of the bulk‐phase reactant relative to those of the hemicarcerand's portal. Hemicarcerands have also been used extensively for the inner‐phase stabilization of reactive intermediates. If a reactive intermediate is generated photochemically or thermally in the inner phase of the hemicarcerand, the surrounding host prevents dimerization of the intermediate or reactions with bulk‐phase reactants that cannot pass through an opening in the host shell. Many otherwise fleeting intermediates gain longevity through this extrinsic stabilization, making possible their observation, for example, by NMR spectroscopy. Examples that are discussed include cyclobutadiene, benzyne, anti ‐Bredt olefins, cyclic allenes and ketenimines, carbenes, and nitrenes. Furthermore, investigations of photoelectron transfer and energy transfer across the shell of hemicarcerands have improved our understanding of these important photophysical processes. Recent advances in this area are discussed.

Supramolecular Polymer Networks: Preparation, Properties, and Potential

“…Several classes of macrocycles have been developed, including crown ethers [199,200], porphyrins [201,202], cyclophanes [203], catenanes [204], cavitands [205,206], cryptophanes [207], calix[n]arenes [208], and carcerands [209]. Macrocyclic-based supramolecular gels can either be formed from low molecular weight precursors or from macromolecular building blocks.…”

Extraordinary Modes of Bonding Enabled by the Triquinane Framework

“…Yet another approach involves the spatial confinement and chemical isolation of reactive intermediates within container molecules. Perhaps the most famous platform of this description is that of the carcerands, originally developed by Cram and co-workers in the 1980s, which were used to trap such elusive species as cyclobutadiene and benzyne …”

Synthesis and Structure of the Inclusion Compound COF300‐ALA

“…[ 1‐2 ] Host molecules can be broadly divided into two main categories: [ 3‐5 ] i) The more widespread group is unimolecular inclusion compounds where one host molecule interacts with one guest species. Familiar examples of this category include calixarenes, [ 2 ] cyclodextrins, [ 6‐7 ] crown ethers, [ 3,8 ] cavitands, [ 9 ] cucurbiturils, [ 10‐11 ] cyclophanes, [ 12 ] and various carcerands; [ 5,13 ] ii) the other is multimolecular inclusion compounds where two or more host molecules are involved in forming cavities, channels, or layers in the crystal structure to accommodate guest molecules. Host molecules of this kind include urea, [ 14 ] choleic acids, [ 15 ] Dianin's compound, [ 16 ] etc ., and crystalline porous materials such as metal‐organic polyhedral [ 17 ] and porous organic cages [ 18 ] are well‐known examples.…”