Triptycene-Derived Macrocyclic Arenes
With unique structural features, easy functionalization, and wide applications, macrocyclic arenes, including calixarenes, resorcinarene, cyclotriveratrylene, pillararenes, and their analogues, have attracted much attention and also become one of the most important and studied synthetic macrocyclic hosts during the last decades. Triptycene has been proved to be useful building block for the development of new synthetic hosts with specific structures and properties. Consequently, a series of triptycene-derived calixarenes, heteracalixarenes, tetralactam macrocycles, and their analogues have conveniently been synthesized with satisfactory yields by one-pot method or two-step fragment-coupling reactions. Moreover, the triptycene-derived macrocyclic hosts had large enough cavities and showed fixed conformation in solution. These structural features made them exhibit not only well molecular recognition properties toward small organic molecules, fullerenes, and organic dyes but also wide potential applications in self-assemblies. Especially, we have recently developed a new kind of chiral macrocyclic arenes named as helicarenes based on chiral 2,6-dihydroxyl-substituted triptycene subunits bridged by methylene groups. It was found that the helicarenes exhibited convenient synthesis, high stability, good solubility, fixed conformation, easy functionalization, and wide complexation with different kinds of chiral and achiral organic guests. In particular, the switchable complexation based on these macrocycles could be efficiently controlled by multi-stimuli including acid-base, redox, anion, or light stimulus in the presence of photoacid. Moreover, the helicarenes could also show potential applications in molecular assembly and molecular machines.
Since Pedersen first reported the synthesis of crown ethers and their cation-complexing properties in 1967, the development of new class of macrocyclic hosts has always been one of the most important topics in host-guest chemistry, and also supramolecular chemistry during the last half century. Consequently, various macrocyclic hosts [1, 2, 3, 4] including crown ethers, cryptands, cyclodextrins, cavitands, cyclophanes, cucurbiturils, calixpyrroles, cyclopeptides, and others have been reported, and these hosts have undoubtedly played a very important role in the emergence and development process of both host-guest chemistry and supramolecular chemistry.
In the known synthetic macrocycles, calixarenes have become one of most important macrocyclic hosts and thus found wide applications on supramolecular chemistry. Since calixarenes were first efficiently synthesized and named by Gutsche and coworkers in the late 1970s, they and their analogues including resorcinarenes, cyclotriveratrylenes, pillararenes, and others have attracted much attention during the last decades. So calixarenes were also called as “the third generation of host molecules” after crown ethers and cyclodextrins. Since calixarenes and their analogues are all composed of substituted aromatic rings bridged by methylene or methenyl groups, we can also call them as a type of macrocyclic arenes.
Previously, we [5, 7] have developed a new kind of synthetic hosts by the combination of triptycene building block with unique Y-shaped rigid structure and crown ether chains, including triptycene-derived cylindrical macrotricyclic polyethers [8, 9, 10, 11] and tweezer-like triptycene-derived crown ethers [12, 13, 14]. The rigid triptycene moiety favors these hosts to generate multi-cavity structures, while the flexible crown ether moiety facilitates the hosts to adjust their conformation for the encapsulated guests. These specific structural features make the hosts show the diversified complexations with different kinds of guests, especially, multiple stimuli responsive complexation, which will be useful for the design and construction of functionalized supramolecular assemblies. Recently, we also applied the tritopic triptycene-derived tri(crown ethers) into the design and construction of molecular switches and machines [15, 16].
Triptycene-Derived Calixarenes and Analogues
Triptycene-Derived Calixarenes and Analogues
Triptycene-Derived Heteracalixarene and Analogues
Triptycene-Derived Tetralactam Macrocycles
Structures in Solution
Due to the different linking modes of the triptycene moieties, 4a and 5a are a pair of diastereomers. 4a is a syn orientation of the two triptycene moieties, while 5a is an anti orientation isomer . Both of them showed one singlet for the tert-butyl protons, one singlet for the methoxy protons and two singlets for the bridgehead protons of the triptycene moiety in the 1H NMR spectra, while there was only one signal for the methylene carbons in their 13C NMR spectra. Moreover, there were no obvious changes in the methylene proton signals with increasing temperature in their variable-temperature 1H NMR spectra. These observations suggested that both 4a and 5a have highly symmetric structures and fixed conformations. However, their 1H NMR spectra showed a large difference from each other. For the methylene protons, a pair of doublet signals at 3.35 and 4.25 ppm (Δδ = 0.90) were observed in 4a, whereas 5a showed a pair of doublet signals at 3.67 and 3.86 ppm with a Δδ value of about 0.19 ppm. This result implied that macrocycles 4a and 5a are a pair of diastereomers, 4a has a syn orientation of the two triptycene moieties, while 5a is an anti orientation isomer. Similarly, 4b and 5b, 7a–c and 8a–c, 11a–d and 12a–d, 13, and 14 are also diastereomers with symmetric structures and fixed conformations, respectively, in which the formers were syn isomers, while the latter ones were trans isomers [19, 20].
For calixtriptycenearenes 16–18, they adopted fixed cone conformation in solution as well although they have bigger cavities than the classic calixarene . For 16a, variable-temperature 1H NMR experiments showed its coalescence temperature is more than 100 °C, which is much higher than those of the classic p-tert-butylcalixarene and p-tert-butylcalixarene. This indicated that the significant contributor in determining the conformational mobility of 16a is not due to the intramolecular hydrogen bonds and the bulky tert-butyl groups but mainly attributed to the introduction of the triptycene moiety with rigid structure. For triptycene-derived calixarenes 16b–c, they also showed the similar spectral features to those of 16a. The results showed that all of the triptycene-derived calixarenes 16a–c containing two dimethoxy groups had Cs symmetric structures with a fixed cone conformation in solution. Their demethylated compounds also have the similar 1H NMR spectra features and the same fixed cone conformations as those of their precursors. Moreover, the variable-temperature 1H NMR experiments of 16a–c in DMSO-d6 showed no obvious changes of the methylene proton signals with the increase of the temperatures even up to 373 K. These observations not only confirmed their fixed conformations but also indicated that the conformational inversion barriers of these compounds are very high. Similarly, 21, 22, and 24 with different substituents at the upper rim also kept fixed cone conformation in solution due to the rigid structure of triptycene and the intramolecular hydrogen bonding of the adjacent phenol groups. But after 22 and 24 were all methyl etherified, products 23 and 25 showed 1,2-alternate conformations in the tested temperatures .
We also investigated the structures of triptycene-derived calixresorcinarene-like hosts 32–33  in solution by the 1H NMR, 13C NMR, and variable-temperature 1H NMR experiments. The spectra features showed that these calixresorcinarene-like hosts 32–33 are all the cis isomers with fixed cone conformation in solution. Similar to triptycene-derived calixarenes, triptycene-derived oxacalixarenes 35a–d and 36a–d are also a pair of diastereomers with high symmetric structures and fixed conformation in solution, in which 35a–d are cis isomers and 36a–d are trans isomers [24, 26]. For triptycene-derived homooxacalixarene analogues, the 1H NMR spectra of 39a–d  showed two singlets for the bridgehead protons with small Δδ value, which implied that they were cis isomer with a high symmetric structure. Meanwhile, the 1H NMR spectra of 40a–d showed the relatively significant different chemical shifts for bridgehead protons, which suggested that they were the trans isomers. It was noteworthy that the two sets of doublet signals of trans isomer 40c for the methylene group were gradually changed to one set of doublet signals above 370 K, which meant that at very high temperature, the rigid conformation of 40c was no longer existed. However, for cis isomer 39c, the methylene proton signals exhibited no obvious changes even up to 380 K. When the two p-phenyl-substituted benzene rings were linked together by crown ether chains, the conformations of macrocycles 41 and 42 could be fixation up to 380 K without free rotation.
Triptycene-derived N(H)-bridged azacalixarenes 44a–47a and 44b–47b  are also pairs of diastereomers. It was found that the 1H NMR spectra of cis isomers 44a–47a showed the close chemical shifts of the aromatic protons and small different shifts for the benzylic protons with the high symmetry boat conformation, while the trans isomers 44b–46b showed four singlets with significant different chemical shifts for bridgehead protons and two singlets for the protons of N−H bridged groups. These observations suggested that trans isomers 44b–46b do not adopt the chair conformation but fixed curved-boat conformation without the high symmetry at room temperature, which are different from those of the triptycene-derived oxacalixarenes with trans isomers . However, the 1H NMR and 13C NMR spectra of trans isomer 47b showed its high symmetrical structure with a chair conformation in solution. We deduced that the different properties of dynamic conformational interconversion probably resulted in the different conformations between 47b and 44b–46b [24, 29]. Similarly, triptycene-derived diazadioxacalixarenes 50 and 51a–c are also pairs of diastereomers due to the 3D structural characteristic of triptycene unit . By the 1H NMR spectroscopy, their spectra exhibit that the cis isomers 50a–c adopt twisted boat conformation, while the trans isomers 51a–c are in a symmetrical chair conformation.
For triptycene-derived tetralactam macrocycles 52a and 52b , they are a pair of diastereomers because their 1H NMR spectra are greatly different from each other. Both of cis isomer 52a and trans isomer 52b have highly symmetrical structures, and they exhibit only one signal for the N-H protons and two single signals for the bridgehead protons of the triptycene moieties in their 1H NNR spectra.
Structures in Solid State
For triptycene-derived calixarene 16a and its demethylated macrocycle 17a, the crystal structures showed they adopted cone conformations (Fig. 5e, f), in which intramolecular hydrogen bonding between the adjacent phenol hydroxyl groups or between the ether oxygen atoms and their adjacent phenol hydroxyl protons might play an important role in formation of the fixed conformations . We also inferred that these intramolecular hydrogen bonding interactions might play an important role in the formation of their fixed cone conformations. Pentabromo-substituted calixarene 24 also kept cone conformations, but 25 with the phenol hydroxyl groups all substituted by methoxy groups showed 1,2-alternate conformation due to the lack of intramolecular hydrogen bonding (Fig. 5h) .
Fullerenes and their derivatives have drawn much attention for their wide potential applications. Design and synthesis of new classes of supramolecular containers for fullerenes are of great interest in relation to the development of fullerene-based functional materials. Calixtriptycenearenes have enough large and well-defined electron-rich cavity for fullerenes . Consequently, 4b and 5b could form 1:1 stable complexes with C60 and C70 with the association constants (Ka) more than 1 × 104 M−1 by the fluorescence titrations, which were significantly higher than those ones (9–1300 M−1) of 1:1 complexes between C60 and the classical calixarene derivatives . This probably revealed the introduction of the triptycene moiety not only fixed conformations of the macrocycles but also enhanced the interaction of concave cavities of the macrocycles with the electron-deficient convex surface of the fullerenes. Oxacalixarene 35d with extended cavity could also form 1:1 complexes with C60 and C70, and Ka values for 35d•C60 and 35d•C70 were (7.54 ± 0.29) × 104 and (8.96 ± 0.31) × 104 M−1, respectively . Similarly, homooxacalixarene analogues 39a–d and 40a–d with fixed conformations and large electron-rich cavities showed efficient complexation abilities toward fullerenes C60 and C70 as well , and Ka values for the 1:1 complexes were over 104 M−1.
Helicarenes: New Chiral Macrocyclic Arenes
During the past decades, chiral synthetic hosts based on the macrocyclic arenes have attracted much attention for their wide applications in chiral recognition and self-assembly. Generally, chiral macrocyclic arenes could be obtained by introducing chiral auxiliary into the macrocyclic skeleton . Introducing inherent chirality is another strategy to build chiral macrocyclic arenes [39, 40], but their fussy synthesis and the difficulty in utilizing the macrocyclic cavities limit their practical applications to some extent. Recently, Ogoshi and coworkers  reported a new type of planar chiral macrocyclic arenes based on pillararenes. Undoubtedly, chiral building blocks can provide an efficient and direct way to construct chiral macrocyclic arenes, but no such examples have been reported before.
It was known 2,6-dihydroxy substituted triptycene is an easily available chiral compound. Based on this chiral triptycene building block, a new class of chiral macrocyclic arene composed of 2,6-dihydroxyltriptycene subunits bridged by methylene groups could be obtained. Since the macrocycle adopts a hex nut-like structure with a helical chiral cavity, we can name them as helicarenes .
Applications in Molecular Recognitions and Self-Assemblies
Conclusion and Perspectives
In summary, it has been proved that triptycene derivatives with unique three-dimensional structure could be utilized as useful building blocks for the design and construction of new kinds of macrocyclic arenes and their analogues. Consequently, a series of triptycene-derived calixarenes, heteracalixarenes, tetralactam macrocycles, and their analogues were conveniently synthesized with satisfactory yields by one-pot method or two-step fragment-coupling reactions. Moreover, these triptycene-derived macrocyclic hosts had large enough cavities and showed fixed conformation in solution. These structural features made them exhibit not only well molecular recognition toward small organic molecules, fullerenes, and organic dyes but also wide potential applications in molecular recognition and self-assembly. Especially, we developed a new kind of chiral macrocyclic arenes named as helicarenes based on chiral 2,6-dihydroxyl-substituted triptycene subunits bridged by methylene groups. It was found that the helicarenes exhibited convenient synthesis, high stability, good solubility, fixed conformation, easy functionalization, and wide complexation with different kinds of chiral and achiral organic guests. Especially, the switchable complexation based on these macrocycles could be efficiently controlled by multi-stimuli including acid-base, redox, anion, or light stimulus in the presence of photoacid. However, the researches on these triptycene-derived macrocyclic hosts, especially helicarenes, are still in the infancy. Expanding new members of helicarene family and development of their potential applications are an urgent work in supramolecular chemistry. But we believe that the triptycene-derived macrocyclic arenes, especially helicarenes with special structural features and varied complexation behaviors, could become a new kind of synthetic hosts and thus attract more and more attention in macrocyclic and also supramolecular chemistry in the near future.
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