Enantioselective synthesis of inherently chiral sulfur-containing calix[4]arenes via chiral sulfide catalyzed desymmetrizing aromatic sulfenylation

Inherently chiral calixarenes hold great potential for applications in chiral recognition, sensing, and asymmetric catalysis due to their unique structures. However, due to their special structures and relatively large sizes, the catalytic asymmetric synthesis of inherently chiral calixarenes is challenging with very limited examples available. Here, we present an efficient method for the enantioselective synthesis of inherently chiral sulfur-containing calix[4]arenes through the desymmetrizing electrophilic sulfenylation of calix[4]arenes. This catalytic asymmetric reaction is enabled by a chiral 1,1'-binaphthyl-2,2'-diamine-derived sulfide catalyst and hexafluoroisopropanol. Various inherently chiral sulfur-containing calix[4]arenes are obtained in moderate to excellent yields with high enantioselectivities. Control experiments indicate that the thermodynamically favored C-SAr product is formed from the kinetically favored N-SAr product and the combination of the chiral sulfide catalyst and hexafluoroisopropanol is crucially important for both enantioselectivity and reactivity.

Organocatalytic Enantioselective Synthesis of Inherently Chiral Calix[4]arenes

Inherently chiral calix[4]arenes are an excellent structural scaffold for enantioselective synthesis, chiral recognition, sensing, and circularly polarized luminescence. However, their catalytic enantioselective synthesis remains challenging. Herein, we report an efficient synthesis of inherently chiral calix[4]arene derivatives via cascade enantioselective cyclization and oxidation reactions. The three-component reaction features a broad substrate scope (33 examples), high efficiency (up to 90 % yield), and excellent enantioselectivity (>95 % ee on average). The potential applications of calix[4]arene derivatives are highlighted by their synthetic transformation and a detailed investigation of their photophysical and chiroptical properties.

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.

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.

Calix[4]arene Bridge Mononitration with tert-Butyl Nitrite: Synthesis of Bridging Chiral p-tert-Butylcalix[4]arene with a Mononitro Bridge Substituent

To explore the reaction universality of bridge nitration, the mononitration of different p-tert-butylcalix[4]arene derivatives was executed with tert-butyl nitrite as a nitration reagent. The effects of calix[4]arene conformations, substituents on the lower rim, and reaction conditions on bridge mononitration are systematically studied. The bridge nitration of p-tert-butylcalix[4]arene derivatives in 1,3-alternate, 1,2-alternate, and partial cone conformations can be smoothly executed while that of p-tert-butylcalix[4]arene derivatives strictly regulated in a cone conformation cannot. The nitration product complexity shows a positive correlation with the bridge-hydrogen types, and the optimal bridge-mononitrated substrate is calix[4]arene with only one bridge-hydrogen type. The electron-withdrawing substituent on the lower rim is apparently beneficial for the bridge mononitration. As a result, a variety of bridging chiral p-tert-butylcalix[4]arenes with a mononitro bridge substituent have been successfully synthesized. The highest bridge-mononitrated yield can reach 27% from 1,3-alternate p-tert-butylcalix[4]arene biscrown-5 under optimal reaction conditions.

Bridging chiral de-tert-butylcalix[4]arenes: Optical resolution based on column chromatography and structural characterization

As the third-generation supramolecular main structure, calixarenes, especially chiral calixarenes, have been applied to various fields. In this study, the bridging chiral de-tert-butylcalix[4]arene derivatives with an amide group attached to a chiral point was synthesized for the first time, which provided a new group for its structural derivation at the bridging chiral position. The racemic compound 2 was optically resolved by column chromatography on silica gel with the aid of the chiral auxiliary (1S)-(+)-10-camphorsulfonyl chloride, and finally a pair of optically pure bridging chiral de-tert-butylcalix[4]arene derivatives 4a and 4b were obtained. The results of experimental and calculated ECD showed that compounds 4a and 4b were a pair of enantiomers, and their absolute configurations were designated S and R, respectively. This study provides new idea for the derivatization of specific chiral groups based on bridging chiral calix[4]arenes and their chiral resolution.

Mononitration of a Calix[4]arene Methylene Bridge: Synthesis and Preliminary Catalysis Performances of Bridging Chiral p-tert-Butylcalix[4]arenes with a Monoamino Bridge Substituent in a 1,3-Alternate Conformation

In order to prepare bridging chiral p-tert-butylcalix[4]crown-5 with a mononitro bridge substituent in a 1,3-alternate conformation, a mononitration method of calix[4]arene bridging methylene has been first developed with tert-butyl nitrite as a nitration reagent. The effects of solvent, reaction temperature, reaction time, and nitration reagent dosage on bridge mononitration have been deeply explored to obtain an optimal nitration condition. The facile nitration presents a new key for calix[4]arene bridge derivatization. After further modification and diastereoisomeric resolution, a pair of bridging chiral p-tert-butylcalix[4]arenes with a monoamino bridge substituent were produced from the bridge-mono-nitrated calix[4]arene. Their preliminary catalysis results in the Henry reaction show good catalytic activities (up to 95% yield) and still low but obviously enhanced enantioselectivities (up to 22.3% ee from 7a, 6% ee from 1), which confirms that the structural transformation indeed improves asymmetric catalysis performances of bridging chiral calix[4]crown-5 amines in a 1,3-alternate conformation.

Thermally Stable Nitrothiacalixarene Chromophores: Conformational Study and Aggregation Behavior

Achieving high thermal stability and control of supramolecular organization of functional dyes in sensors and nonlinear optics remains a demanding task. This study was aimed at the evaluation of thermal behavior and Langmuir monolayer characteristics of topologically varied nitrothiacalixarene multichromophores and phenol monomers. A nitration/azo coupling alkylation synthetic route towards partially O-substituted nitrothiacalixarenes and 4-nitrophenylazo-thiacalixarenes was proposed and realized. Nuclear magnetic resonance (NMR) spectroscopy and X-ray diffractometry of disubstituted nitrothiacalix[4]arene revealed a rare 1,2-alternate conformation. A synchronous thermal analysis indicated higher decomposition temperatures of nitrothiacalixarene macrocycles as compared with monomers. Through surface pressure/potential-molecular area measurements, nitrothiacalixarenes were shown to form Langmuir monolayers at the air–water interface and, through atomic force microscopy (AFM) technique, Langmuir–Blodgett (LB) films on solid substrates. Reflection-absorption spectroscopy of monolayers and electronic absorption spectroscopy of LB films of nitrothiacalixarenes recorded a red-shifted band (290 nm) with a transition from chloroform, indicative of solvatochromism. Additionally, shoulder band at 360 nm was attributed to aggregation and supported by gas-phase density functional theory (DFT) calculations and dynamic light scattering (DLS) analysis in chloroform–methanol solvent in the case of monoalkylated calixarene 3. Excellent thermal stability and monolayer formation of nitrothiacalixarenes suggest their potential as functional dyes.