Enantioselective Catalytic Synthesis of Inherently Chiral Calixarenes

Since the introduction of the concept of inherent chirality by Böhmer, an important part of research focused on the asymmetric synthesis of calixarene macrocycles. However, long synthetic procedures and tedious separation strategies hampered the application of this technology in many topics of organic chemistry, including enantioselective molecular recognition and catalysis. Very recently, a new generation of enantioselective catalytic methodologies has been reported, able to provide highly functionalized, inherently chiral calixarenes in a straightforward manner. In this review, we will discuss these new catalytic methods and the versatile properties of such macrocycles that call for potential applications in many areas of science.

Simultaneous construction of inherent and axial chirality by cobalt-catalyzed enantioselective C-H activation of calix[4]arenes

The simultaneous construction of multiple stereogenic elements in a single step is highly appealing and desirable in the field of asymmetric synthesis. Furthermore, the catalytic enantioselective synthesis of inherently chiral calix[n]arenes with high enantiopurity has long been a challenging endeavor. Herein, we report an enantioselective cobalt-catalyzed C-H activation/annulation for the efficient construction of inherently chiral calix[4]arenes bearing multiple C-N axially chiral element. By employing the benzamide tethered calix[4]arene as the substrate, the C-H annulation with alkynes can be successfully accomplished, leading to the generation of multiple stereogenic elements. A wide range of calix[4]arenes and alkynes are found to be well compatible, and exhibit good yields, high enantioselectivity and excellent diastereoselectivity. Notably, the gram-scale reaction, catalytic application, synthetic transformations, and chiral recognition further showcase the potential applications of this protocol.

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.

Inherently chiral calixarenes by a catalytic enantioselective desymmetrizing cross-dehydrogenative coupling

Under the catalysis of PdBr₂ and a chiral phosphoramidite ligand, the upper-rim mono (2-bromoaroyl)-substituted calix[4]arene derivatives underwent a facile enantioselective desymmetrization reaction to afford 9H-fluorene-embedded inherently chiral calixarenes in good yields with excellent enantioselectivities. The transannular dehydrogenative arene-arene coupling reaction proceeded most probably through an oxidative addition of the C_aryl-Br bond to a ligated palladium catalyst followed by a sequence of an enantioselective 1,5-palladium migration and an intramolecular C-H arylation sequence. This new family of inherently chiral calixarenes possesses unique chiroptical properties thanks to their highly rigid structure induced by the 9H-fluorene segment.

Direct meta substitution of calix[4]arenes

Calixarenes represent very popular building blocks in supramolecular chemistry. Compared to other macrocyclic families, they exhibit an almost infinite possibility of derivatization of the basic skeleton, which makes them ideal candidates for the design of new receptors or other functional systems. Although the chemistry of calixarenes is well established, there are still some substitution patterns that are unavailable or require a very lengthy synthetic approach. Among such synthetic challenges is the meta substitution of the aromatic skeleton (relative to phenolic oxygen), which, in conjunction with the 3D structure of calixarenes, leads to the inherent chirality and enables the synthesis of derivatives with a hitherto undescribed topology. This review deals with the current achievements in the meta substitution of calixarenes.

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.

Exploiting the p-Bromodienone Route for the Formation and Trapping of Calixarene Oxenium Cations with Enamine Nucleophiles

This study shows that calixarene p-bromodienone derivatives can act as precursors for the formation of oxenium cations, which can be trapped with enamine C-nucleophiles. When calixarene p-bromodienones were treated with enamines, in the presence of AgClO₄, the lower rim-substituted C-O-C products were obtained by an electrophilic attack of the intermediate calixarene-oxenium cation with a contemporary cone-to-partial-cone inversion of the involved aromatic ring.

Unexpected cleavage of upper rim-bridged calix[4]arenes leading to linear oligophenolic derivatives

Regioselective meta-mercuration followed by Pd-catalysed intramolecular bridging gave birth to a novel type of calixarene bearing a single bond bridge between the meta positions of the neighboring aromatic subunits. These bridged derivatives possess extremely distorted cavities that imply possible amended properties over common calix[4]arenes. This new type of calixarene reactivity can be documented by acid-/electrophile-mediated cleavage of the basic macrocyclic skeleton leading to open oligomeric structures-the behavior of which has never been observed before in classic calix[4]arenes bearing alkoxy groups on the lower rim.

meta-Bridged calix[4]arenes with the methylene moiety possessing in/out stereochemistry of substituents

Calix[4]arenes possessing two different substituents on an additional methylene bridge were prepared and the relationship between the in/out positions of these substituents and the complexation properties was studied.