Ying-Xian Ma

Iptycenes and Their Derivatives in Molecular Machines

Triptycene with three arene units fused to the [2.2.2]bicyclooctatriene bridgehead system has a D 3h symmetric structure, and thus resembles a macroscale gearwheel. This structural feature makes the triptycene a potential component for the construction of molecular machines. Between the 1960s and 1970s, the highly hindered compounds with appreciable rotation barriers between sp 3 -hybridized carbon atoms won a considerable attention. Especially, triptycene and its derivatives, in which the rotation around the sp 3 –sp 3 single bond between the bridgehead carbon atom and the attached substituent was restricted to give long-lived conformational isomers, became the subject of several reports. In the early work, Iwamura (J Chem Soc Chem Commun 7:232–232, 1973) synthesized a triptycene derivative, 9,10-bis(1-cyano-1-methylethyl)triptycene (1, Fig. 7.1) and found that the triptycene 1 contained a rotational barrier of 37.7 kcal/mol which was a considerably high barrier for rotation around the sp 3 –sp 3 single bond.

Iptycenes and Their Derivatives in Molecular Self-Assembly

The unique three-dimensional rigid structural features of iptycenes and their derivatives make them promising candidates as the building blocks for the formation of a variety of supramolecular structures in the solid states via self-assembly. In 1986, Bashir-Hashemi et al. (J Am Chem Soc 108:6675–6679, 1986) found that the tritriptycene 1 could pack into a channel-shaped three-dimensional structure filled by the disordered acetone molecules, in which the p–p interactions between the phenyl rings of molecule 1 played the key roles. Similarly, Venugopalan et al. (Tetrahedron Lett 36:2419–2422, 1995) reported the crystallographic investigation of heptiptycene 2, which was crystallized from its chlorobenzene solution. The authors found that molecule 2 could pack into the zigzag ribbons in the solid state, which further self-assembled into the channels separated by the chlorobenzene molecules via the T-shaped fashion interactions between the hydrogen atoms of chlorobenzene and the aromatic rings of 2. In addition, Lu et al. (J Am Chem Soc 124:8035–8041, 2002) found that the dodecaphenyltriptycene 3 with the enhanced size of the clefts could also self-assemble into the well-ordered extended structure in the solid state. The channel-shaped void spaces were almost filled with the benzene molecules, and the p–p interactions between the solvent molecules and the phenyl rings of 3 stabilized this crystal packing.

Iptycenes and Their Derivatives in Coordination Chemistry

It is the rigid structures that make the triptycenes promising candidates as the building blocks for the construction of metal carbonyl complexes. Pohl and Willeford (J Organomet Chem 23:C45–C46, 1970) prepared the first triptycene p complex, tricarbonyl(h 6 -triptycene) chromium, via the reaction between triptycene and hexacarbonyl-chromium in the refluxing n-butyl ether (1) in 1970. This complex could be soluble in most organic solvents, but decomposed quickly under sunlight. In 1985, Gancarz et al. (Organometallics 4:2028–2032, 1985) firstly determined the crystal and molecular structures of tricarbonyl(h 6 -triptycene)chromium 1 and nonacarbonyl(h 6 -triptycene)tetracobalt 2. As shown in Fig. 11.1, the deformation of the triptycene skeletons was observed in both molecular structures due to the effect of nonbonded repulsions between one of the triptycene rings and the Cr(CO)3 or Co4(CO)9 tripods, and complex 2 displayed greater deformation since the much more bulky tripod. Moreover, they (Organometallics 5:2327–2332, 1986) further found that the relative rates of the bridging–equatorial, bridging–axial, and axial–equatorial transposition processes for complex 2 showed obvious temperature dependence, according to the variable-temperature 13C nuclear magnetic resonance (NMR) studies.

Preparation of Iptycene-Containing Polymers and Oligomers

In this chapter, the methods for the preparation of various iptycene-containing polymers and oligomers will be discussed in details. The structural features and the properties of these polymers and oligomers will also be described in the following corresponding sections. When a 3D structural unit, especially, a rigid 3D skeleton was introduced into a polymer, some special physical properties could be shown. As early as 1968, Klanderm and Faber (J Polym Sci Part A1 Polym Chem 6:2955–2965, 1968) attempted to prepare the polymers containing 9,10-bis(hydroxylmethyl)triptycene unit, which was expected to enhance the glass transition temperature (T g). Consequently, they obtained the modified polyesters with 9,10-bis(hydroxylmethyl)triptycene instead of the common glycol components, and found that the target poly(ethylene terephthalate)s expectedly showed the high heat-distortion temperatures and high glass transition temperatures, but the crystallizability was simultaneously declining. Obviously, the rigid bulky triptycene moiety seemed to interfere with the segmental rotation of the polymer chain, leading to an influence on the thermal properties of the polyesters.

Synthesis and Reactions of Triptycenes and Their Derivatives

In order to verify if the triptycyl (the analog of triphenylmethyl in which the three phenyl groups were united to a CH) free radical 1a should be more instable than the triphenylmethyl radical itself, Bartlett et al. (J Am Chem Soc 64:2649–2653, 1942) first synthesized triptycene 1 in a low total yield by a multistep route starting from the Diels–Alder addition reaction of anthracene and benzoquinone in 1942. Several years later, Bartlett et al. (J Am Chem Soc 72 (2):1003–1004, 1950) also reported the synthesis of 9-bromotriptycene (2) from the 9-bromoanthracene and benzoquinone according to the similar synthetic strategy. However, it was noted that the target compound 2 could be immediately afforded by the deamination of the diamine with NaNO2 in the presence of 50 % H3PO2 at − 3 °C.

Iptycenes and Their Derivatives in Material Science

In general, all compounds with liquid crystalline properties would be consisted of a rigid core and long flexible chains, which blocked the formation of completely ordered system. Thus, the systems containing the rigid iptycene core and the flexible long alkyl or alkoxy chains seemed to be potential candidates as liquid crystalline materials (Chem Soc Rev 38:3301–3316, 2009). In the early 1990s, Simon and Norvez (J Chem Soc Chem Commun 1990:3407–3412, 1990; J Org Chem 58:2414–2418, 1993) synthesized triptycene derivative 1 containing five long paraffinic chains (Fig. 8.1) and found that it showed the mesomorphic behavior at room temperature. The rigid triptycene core of this pentasubstituted derivative (1) regularly arrayed in a hexagonal lamellar structure, along with the long chains extending above and below the layer, which was indicated by its X-ray diffraction patterns. The triptycene derivative 1 exhibited the mesomorphic property at room temperature, probably because the cell areas were big enough to hold the chains at disordered state. On the other hand, it was found that the triptycene derivative 2 (Fig. 8.1) with six alkoxy chains and an aromatic core would form the symmetric compatible lamellar lattices. However, the cell areas in these lamellar lattices were too small to be available for all chains in a disordered state, which led to the crystalline state instead of the mesomorphic state.

Synthesis and Reactions of Other Iptycenes and Their Derivatives

In contrast to triptycene or pentiptycene, the researches on heptiptycene and noniptycene are very few, probably due to the difficulties of synthesis. In the early 1970s, Huebner et al. (Tetrahedron Lett 359–362, 1970) synthesized the first heptiptycene 1 with very low and instable yields by the reaction of 11-chloro-9,10-di-hydro-9,10-etheno anthrace 2) with n-butyl lithium at 25 °C. With the help of high-resolution mass spectrum, ultraviolet spectrum, and the space group of crystalline 1:1 chlorobenzene complex, the structure of compound 1 could be determined. Moreover, molecule 1 showed a remarkable thermal stability with high melting point. Even when melted to 580 °C in a sealed tube, it would not decompose with some minor sublimation.

Miscellaneous Applications of Iptycenes and Their Derivatives

In 1999, Perchellet et al. (Anti-Cancer Drugs 10(8):749–766, 1999) reported a class of triptycene bisquinones containing the para-quinone moieties, and found that they could be used as the bi-functional anti-cancer drugs. These drugs could not only block the synthesis of nucleic acid and protein but also inhibit the proliferation of cancer cells in vitro. Especially, the bisquinone 1 and its C-2 brominated derivative 2 exhibited strong anti-proliferative activities, which were at the same level as the anti-proliferative ability of daunomycin (DAU). In 2002, Wang et al. (Cancer Lett 188(1–2):73–83, 2002) further designed and synthesized the amino-functionalized triptycene bisquinones 3a–b which had the potent anticancer activities. It was noteworthy that these triptycene-based anticancer drugs could maintain their activity in multidrug-resistant tumour cells (daunorubicin-resistant HL-60 cell lines) with the ability to induce poly(ADP-ribose) polymerase-1 (PARP-1) cleavage. On the basis of the further investigations, Wang et al. (Anticancer Drugs 14(7):503–514, 2003) found that these synthetic triptycene analogs could inhibit both DNA topoisomerase (topo) I and II activities, which could be used as dual inhibitors. In particular, the amino-functionalized triptycene showed the same level of power to the existing topo I inhibitor, like camptothecin. Simultaneously, the triptycene derivative also exhibited the stronger inhibited effect on topo II than amsacrine. It was noteworthy that these synthetic triptycene analogs were cytostatic and cytotoxic, which could inhibit the L1210 leukemic cell growth and mitochondrial metabolism.

Iptycenes and Their Derivatives in Sensors

Generally speaking, the higher performance fluorescent chemosensors based on the conjugated polymers could be achieved through the amplification of the inherent sensitivity of the fluorescent material (Zhou and Swager, J Am Chem Soc 117(50):12593–12602, 1995; Zhou and Swager, J Am Chem Soc 117(26):7017–7018, 1995). In the iptycene-incorporated conjugated polymers, the rigid structure of iptycene can block the p–p stacking and the excimer formation in the solid state and these features make them potential candidates for fluorescent chemosensors. In 1998, Yang and Swager (J Am Chem Soc 120(21):5321–5322, 1998) took advantage of the films of pentiptycene-based conjugated polymers 1–3 with high fluorescence quantum yields and stabilities to act as the sensing materials for fluorescent chemosensors. These fluorescent chemosensors displayed wonderful abilities for the trace detection of the electron-deficient unsaturated species such as 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), and benzoquinone (BQ). Especially, the polymer 1 exhibited high sensitivity to high-explosive vapors (TNT, DNT), and repetitively great spectroscopic stability along with a fast response time. All of the excellent performances made it a commercially available device called “Fido”, which had been applied for the detection technology of robotic and hand-held explosive sniffers. The electron transfer from the excited polymer to the electron acceptors leading to the fluorescence quenching seemed to be regarded as the detection mechanism. Further studies (Yang and Swager, J Am Chem Soc 120(46):11864–11873, 1998) suggested that the electron-rich environment, good balance of the electrostatic interactions, and the porous structures in the films played the key roles in the high sensitivity. Moreover, there were also varied factors that could determine the degree of fluorescence quenching, including the porosity and thickness of the film itself, the vapor pressure and the diffusion ability of analytes, and the exergonicity of electron transfer, the strength of binding between the polymer and the analytes.

Synthesis and Reactions of Pentiptycenes and Their Derivatives

Compared with the first synthesis of triptycene in 1942 [Bartlett et al. in J Am Chem Soc 64(11):2649–2653, 1942], it was not until 32 years later that Skvarche and Shalaev [Dokl Akad Nauk SSSR 216(1):110–112, 1974] first accomplished the synthesis of pentiptycene (2). As shown in Scheme 3.1, pentiptycene 2 was synthesized in about 10 % total yield by a three-step route starting from 2-aminotriptycene 1. Similar to the way for the synthesis of triptycene, the Diels–Alder reaction between anthracene and benzyne was also the key process to pentiptycene.