Chemical Synthesis of Cycloparaphenylenes

1 Introduction

Since carbon nanotubes (CNTs) were first discovered by Iijima in 1991 [1], the synthesis of CNTs has attracted the interest of many scientists because of their outstanding physical properties as well as their potential applications in technology [2–7]. Single-walled CNTs can be considered as rolled-up structures of graphene sheets. The manner in which the graphene sheet is wrapped is given by the chiral index, which is represented by a pair of numbers (n, m) as shown in Figure 1. The structures corresponding to m = 0, n = m and n > m > 0 are called “zigzag,” “armchair” and “chiral” CNTs, respectively. The chiral index (n, m) can be used to deduce a variety of electrical, optical, magnetic and mechanical properties of the CNTs. For example, (n, m)CNTs in which n – m is zero or a multiple of three are metallic, whereas other CNTs are semiconducting [8, 9]. Therefore, the controlled, chirality-specific synthesis of CNTs is important for imparting defined macroscopic material properties.

Figure 1:

Chiral index of CNTs.

Ring-shaped structures can be obtained by “slicing” CNTs perpendicular to the main axis. These ring-shaped aromatic molecules, called “carbon nanorings,” represent the sidewall segment of CNTs with a specific chirality. Representative structures of carbon nanorings, cycloparaphenylene (CPP), cycloparaphenylene-2,6-naphthylene and cyclacene are shown in Figure 2. Thus, these carbon nanorings can be considered as ideal templates or building blocks for the construction of uniform CNTs. In this chapter, syntheses of carbon nanorings, especially CPP and its derivatives, are reviewed.

Figure 2:

Structures of CNTs and carbon nanorings.

2 Synthetic Efforts toward CPPs

Carbon nanorings consisting solely of n benzene rings connected via the para position are called [n]cycloparaphenylenes ([n]CPPs, Figure 3). Although synthetic studies on CPP had already begun in the 1930s, the synthesis of CPP was not accomplished for almost 80 years. Figure 3 shows representative early synthetic attempts toward CPPs.

Figure 3:

Synthetic attempts toward CPPs.

In 1934, Parekh and Guha reported an attempt toward the synthesis of CPPs [10]. Their strategy was to synthesize CPP by the copper-mediated desulfurization of dithia[1₂]paracyclophane (1a). Attempts involving similar large macrocycles 1b and 2 were reported by the group of Vögtle in 1984 [11]. However, CPP could not be synthesized by these methods. In 1993, Vögtle and coworkers proposed three synthetic approaches toward CPP (Figure 3). Their first approach involved the synthesis of cyclohexane-inserted CPP 4 by magnesiation followed by the copper-mediated homocoupling of cis-1,4-bis(4-halophenyl)cyclohexanes 3a and b. However, this approach only resulted in the formation of acyclic oligomers and polymers. The second approach involved the Diels–Alder reaction of macrocyclic enyne compounds 7a and b. Although the preparation of 7a,b by a Wittig reaction between 5 and 6a,b was successful, the [4+2] cycloaddition did not occur. They also attempted the synthesis of macrocycle 9b as a possible precursor to [5]CPP based on the synthesis of 9a from 8a reported by McMurry et al. [13]. The McMurry coupling of diketone 8b under the influence of TiCl₃/Zn produced 9b. However, 9b was only detected in mass spectrometry, and further transformation to [5]CPP was not investigated.

In 2008, Bertozzi and Jasti reported the first synthesis of CPP [14]. A few months later, the Itami group reported the first size-selective synthesis of CPP [15], and Yamago reported an alternative approach in 2010 [16]. Three groups of researchers under Jasti, Itami and Yamago have developed a synthetic methodology for the selective or random formation of CPPs, and [n]CPPs with n = 5–16, 18 have now been synthesized.

3 Synthetic Strategies toward CPPs

The biggest difficulty in the synthesis of [n]CPPs is the formation of a macrocycle, which usually has high levels of ring strain because of the connection of benzene rings in the para position. The Jasti/Bertozzi, Itami and Yamago groups addressed this problem with the initial formation of unstrained macrocyclic precursors. Three of their synthetic strategies and the mechanisms of the key steps are shown in Figure 4. The method employed by Jasti and Bertozzi uses L-shaped building blocks consisting of phenyl-substituted cyclohexadiene units. Because 1,4-dimethoxycyclohexadiene moieties can be converted into benzene rings via reductive aromatization, the L-shaped units are regarded as triphenylene precursors. Sequential coupling reactions of the L-shaped units afforded macrocycles with cyclohexadiene moieties occupying the corners, thus generating the CPP precursors. In the final step, these cyclohexadiene moieties were subjected to reductive aromatization to furnish the corresponding CPPs. Itami, on the other hand, used cis-1,4-diphenyl-1,4-alkoxycyclohexanes as triphenylene precursors. The macrocycles obtained by coupling the L-shaped units were subsequently transformed into CPPs by one-pot deprotection and oxidative aromatization. Yamago applied Bäuerle’s cis-platinum complex method [17] to unstrained CPP precursors. Initially, macrocyclic platinum-bridged biphenyls were synthesized, and a subsequent reductive elimination between the biphenyl ligands generated the CPPs. Reductive elimination of the two aryl units from platinum was promoted by oxidizing the platinum center with an oxidant. These methods have been developed by the three above-mentioned research groups and applied to the synthesis of a series of CPPs and related carbon nanorings.

Figure 4:

Synthetic strategies toward CPPs of three research groups.

4 Synthesis of [5]–[12] and [18]CPP by Bertozzi and Jasti

The first synthesis of CPP was achieved by Bertozzi, Jasti and coworkers in 2008 (Figure 5) [14]. For the synthesis of the macrocyclic CPP precursor, L-shaped unit 10a was prepared from monolithiated 1,4-diiodobenzene and p-benzoquinone, and subsequent borylation of 10a led to 10b (B(pin) = 4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl). The palladium-catalyzed cross-coupling of 10a and 10b afforded three different macrocycles 11a, 12a and 13. Finally, reductive aromatization of the isolated macrocycles 11a, 12a and 13 with lithium naphthalenide (LiNaph) afforded [9]CPP [12]CPP and [18]CPP.

Figure 5:

Synthesis of [9]CPP, [12]CPP and [18]CPP.

Later, Jasti succeeded in the selective synthesis of [6]–[12]CPP. The strategy toward [7]–[12]CPP relied on sequential Suzuki–Miyaura cross-coupling involving three L-shaped units 10b–d (Figure 6) [18, 19]. For this purpose, silyl-protected 4-(4′-bromophenyl)phenol 14 underwent oxidative dearomatization with iodobenzene diacetate (PIDA), resulting in the formation of ketone 15a. Subsequently, 15a was treated with 4-chlorophenyllithium to yield the unsymmetrical L-shaped unit 10c, which was borylated to afford 10d. Using the two L-shaped units 10c and d, the corresponding macrocyclic precursors for CPPs were synthesized by orthogonal Suzuki–Miyaura cross-coupling reactions. The coupling of L-shaped units 10b–d with linear units (18 or 19) afforded macrocycles 11a, 12a and 20–23, which were converted into the corresponding CPPs by reductive aromatization using LiNaph or sodium naphthalenide (NaNaph). Synthesis of [6]CPP was first accomplished in 2012 (Figure 7) [20]. Ketone 15a was combined with silyl-protected biphenyl unit 24 to afford 25a. The tert-butyldimethylsilyl group on 25a was removed by tetrabutylammonium fluoride (TBAF) to give 25b, and the phenol moiety was the oxidized by PIDA. Nucleophilic addition of 4-bromophenyllithium yielded U-shaped unit 26a, and subsequent coupling with 1,4-diborylbenzene 18 afforded 27, which furnished [6]CPP after reductive aromatization.

Figure 6:

Selective synthesis of [7]–[12]CPP.

Figure 7:

Selective synthesis of [6]CPP.

For [8]CPP and [10]CPP, Jasti was able to establish a gram-scale synthesis (Figure 8) [21]. The synthesis of macrocycle 28a was achieved by cross-coupling of L-shaped unit 10b and U-shaped unit 26a, followed by reductive aromatization, which afforded 1.04 g of [8]CPP. Correspondingly, cross-coupling of 26a and 26b afforded 1.03 g of [10]CPP through the macrocycle 29a.

Figure 8:

Gram-scale synthesis of [8]CPP and [10]CPP.

Interestingly, Jasti and Yamago independently published the synthesis of [5]CPP, the smallest CPP isolated to date, almost simultaneously. In Jasti’s [5]CPP synthesis (Figure 9) [22], homocoupling reaction between the C–B bonds of 26b provided macrocycle 30a. Treatment of 30a with NaNaph promoted partial demethoxylation to afford 30b, and further reductive aromatization of 30b with lithium diisopropylamide (LDA) afforded [5]CPP.

Figure 9:

Synthesis of [5]CPP.

5 Synthesis of [7]–[16]CPP by Itami

In 2009, Itami reported the first size-selective synthesis of [12]CPP (Figure 10) [15]. For the synthesis of [12]CPP, L-shaped unit 31a was firstly synthesized from 1,4-diiodobenzene and cyclohexane-1,4-dione. Then, for the subsequent Suzuki–Miyaura coupling, 31a was converted into borylated L-shaped unit 31b and MOM-protected L-shaped unit (MOM = methoxymethyl) 31c. Palladium-catalyzed cross-coupling of 31b and an excess of 31c selectively afforded acyclic C-shaped unit 32a, which was subsequently coupled with 31b to furnish the macrocycle 33a. After 33a was synthesized, the cyclohexane units were deprotected, dehydrated and dehydrogenated in a p-toluenesulfonic acid (TsOH)-mediated one-pot reaction with microwave irradiation under air, thus producing [12]CPP.

Figure 10:

Selective synthesis of [12]CPP (R = MOM unless otherwise noted).

While the synthetic route to [12]CPP has the advantage of being selective, several reaction steps are required. Therefore, Itami developed a concise method for the synthesis of [9]CPP and [12]CPP in 2011 (Figure 11) [23, 24]. The homocoupling reaction of L-shaped units 31c or brominated derivative 31d, promoted by Ni(cod)₂/bpy (cod = 1,5-cyclooctadiene, bpy = 2,2′-bipyridyl), afforded macrocycles 34a and 33b, which are easily separated by silica gel column chromatography. The final aromatization step was also improved using sodium hydrogen sulfate (NaHSO₄) as an acid and m-xylene/DMSO (DMSO = dimethyl sulfoxide) as solvent, thus forgoing the necessity of microwave heating. As a result [9]CPP and [12]CPP were synthesized in four steps from commercially available materials. This concise method can be scaled up easily, and consequently large amounts of [9]CPP and [12]CPP are now commercially available.

Figure 11:

Synthesis of [9]CPP and [12]CPP (R = MOM).

The Itami group also established a size-selective synthesis for [7]–[16]CPP by a combination of L-shaped building blocks with linear units. First, the size-selective synthesis of large CPPs, [14]– [16]CPP, was accomplished by assembling L-shaped unit 31d with 1,4-diborylbenzene 35 and 4,4′-diborylbiphenyl 36 to yield U-shaped units 37a and 38a, respectively (Figure 12) [25]. The bromo substituents on the U-shaped units 37a and 38a were converted into boryl groups by palladium-catalyzed Miyaura borylation, resulting in the formation of 37b and 38b. Cross-coupling reactions between suitable pairs among 37a,b and 38a,b afforded the corresponding rectangular macrocycles 39–41 (39: 37a + 37b, 40: 37b + 38a, 41: 38a + 38b). Alternatively, macrocycles 39 and 41 can also be obtained by the nickel(0)-mediated homocoupling of 37a and 38a, respectively [26]. Final oxidative aromatization of 39–41 with NaHSO₄ yielded [14]–[16]CPP. In addition, the size-selective synthesis of [9]–[11] and [13]CPP was also accomplished in 2012 (Figure 13) [26]. C-shaped units 32b and 42, obtained from 31b,d and 1,4-dibromobenzene, were used as precursors for [9]–[11]CPP. Connecting the C–Br terminals of 32b furnished 34a and subsequent aromatization led to [9]CPP, whereas cross-coupling of 32b with 1,4-diborylbenzene yielded 43, which was converted to [10]CPP. Assembling 31b,d and 1,4-dibromobenzene afforded 44 through the macrocyclization of 42, which was then converted into [11]CPP. The coupling reaction of U-shaped unit 37b with L-shaped unit 31d yielded 45; the terminal C–Br moieties in 45 were connected using Ni(cod)₂/bpy to give 46 as a precursor for [13]CPP. The aromatization of 46 was accelerated by the addition of o-chloranil as an oxidant.

Figure 12:

Synthesis of [14]–[16]CPP (R = MOM).

Figure 13:

Selective synthesis of [9]–[11] and [13]CPP (R = MOM).

In 2014, the size-selective synthesis of smaller CPPs, [7]CPP and [8]CPP, was achieved (Figure 14) [27]. Because [n]CPP with n < 9 cannot be synthesized by assembling the previously described L-shaped units, Itami synthesized a new smaller L-shaped unit 47a, which can be considered a biphenyl-convertible unit. The two-fold addition of dilithiated L-shaped unit 31d to 47a afforded C-shaped unit 48, and subsequent homocoupling of the terminal groups in 48 afforded macrocycle 49, which was used as the precursor for [7]CPP. Cross-coupling of 48 and 1,4-diborylbenzene yielded 50, which was subsequently converted to [8]CPP. Selective synthesis of [10]CPP was also improved using the small L-shaped unit 47a (Figure 14) [28]. Nucleophilic addition of dilithiated 1,4-dibromobenzene to 47a and subsequent MOM-protection generated U-shaped unit 51. The Ni(cod)₂/bpy-mediated homocoupling of 51 afforded rectangular macrocycle 52, which was converted into [10]CPP by acid-mediated aromatization. With this study, Itami completed the comprehensive size-selective synthesis of [7]–[16]CPP.

Figure 14:

Selective synthesis of [7], [8] and [10]CPP (R = MOM).

5.1 Synthesis of [5]–[13] and [16]CPP by Yamago

The Yamago group reported an alternative approach toward [8]CPP by applying Bäuerle’s cycloarene synthesis using platinum-containing macrocycles (Figure 15) [16]. Complexation of 4,4′-bis(trimethylstannyl)biphenyl 53 and Pt(cod)Cl₂ afforded macrocycle 54a. Because the ligand exchange on platinum is reversible, tetraplatinum complex 54a was selectively formed as it represents the least strained isomer. Reductive elimination of the aryl groups from the platinum center promoted by ligand exchange from cod to dppf (dppf = 1,1′-bis(diphenylphosphino)ferrocene), followed by oxidation with bromine resulted in the formation of [8]CPP.

Figure 15:

Selective synthesis of [8]CPP.

In 2011, Yamago reported both the random and selective synthesis of CPPs via the platinum complexation method (Figure 16) [29]. For example [12]CPP was synthesized selectively using bis(trimethylstannyl)terphenyl 55 via macrocyclic platinum complex 56. In addition, the attempted reaction between 55 and diplatinum unit 57a was expected to selectively furnish [10]CPP. However, [8]–[13]CPP were obtained randomly, depending on the reaction time at elevated temperatures. Yamago achieved an unexpected size-selective synthesis of [10]CPP (Figure 17) [30]. The homocoupling reaction of L-shaped platinum complex 59 was attempted to obtain cyclic dimers, trimers and tetramers as precursors for [8]CPP, [12]CPP and [16]CPP, respectively. When Ni(cod)₂/bpy and silver tetrafluoroborate were used for the homocoupling and reductive elimination, respectively, [8]CPP, [10]CPP, [12]CPP and [16]CPP were generated randomly. When Pd(dba)₂ (dba = dibenzylideneacetone) was used instead of Ni(cod)₂ [10]CPP was obtained selectively. In 2013, the selective synthesis of [6]CPP and [8]CPP was achieved and the method for the preparation of [10]CPP was improved [31]. The reaction of diplatinum complexes 60, 57b and 61 with 4-bromophenyllithium afforded U-shaped diplatinum complexes 62–64, and the subsequent homocoupling of 62–64 afforded macrocycles 65, 54b and 66. [6]CPP, [8]CPP and [10]CPP were obtained in the reductive elimination promoted by Br₂ (for [8]CPP and [10]CPP) or by XeF₂ (for [6]CPP).

Figure 16:

Selective and random synthesis of CPPs.

Figure 17:

Synthesis of [6], [8], [10], [12] and [16]CPP.

As mentioned in the synthesis of [5]CPP by Jasati, Yamago also achieved the synthesis of [5]CPP (Figure 18) [32]. They used the triethylsilyl group instead of the methyl group for hydroxy group protection. The nickel(0)-mediated homocoupling of 26c afforded macrocycle 30c. Removal of the triethylsilyl group and reduction of 30c with tin(II) chloride furnished [5]CPP.

Figure 18:

Selective synthesis of [5]CPP (R = SiEt₃).

After the synthesis of [5]CPP, Yamago applied the tin(II)-promoted reductive aromatization method to the selective synthesis of [7]–[12]CPP (Figure 19) [33]. First the building blocks were prepared: arylcyclohexadienone (15a,15b), symmetric and asymmetric L-shaped units with triethylsilyl protection (10e–h) and U-shaped units (26c,d). C-shaped unit 17b was synthesized from 10e via dilithiation and nucleophilic addition to 15a, followed by protection. U-shaped unit 16b was synthesized by the Suzuki–Miyaura cross-coupling reaction of 10f and 10h. The macrocycles used as the precursors of [7]–[12]CPP were synthesized as follows: cross-coupling of 26c and 10g (67 as [7]CPP precursor), homocoupling of 17b (28b as [8]CPP precursor), cross-coupling of 16b and 10g (11b as [9]CPP precursor), homocoupling of 16b (29b as [10]CPP precursor), cross-coupling of 16b and 26d (68 as [11]CPP precursor) and cross-coupling of 26c and 26d (12b as [12]CPP precursor). Each precursor was subjected to TBAF-mediated deprotection and subsequent reductive aromatization to selectively afford [7]–[12]CPP.

Figure 19:

Selective synthesis of [7]–[12]CPP (R = SiEt₃).

Yamago’s [6]CPP synthesis was a hybrid of the platinum method and the reductive aromatization method (Figure 20) [34]. The transmetallation reaction of dimetallated L-shaped units 10g, 10i and 10j to the platinum(II) complex produced macrocyclic diplatinum complex 69. Reductive elimination promoted by triphenylphosphine proceeded to yield strained macrocycle 70 as a precursor to [6]CPP. Removal of the silyl groups of 70 followed by reductive aromatization afforded [6]CPP.

Figure 20:

Selective synthesis of [6]CPP (R = SiEt₃).

6 Synthesis of Armchair Carbon Nanorings

The previously reported synthetic strategies for [n]CPPs can also be applied to the synthesis of a variety of CPP-related, strained ring-shaped compounds. When substituted benzene rings, polycyclic aromatic hydrocarbons (PAHs) or heteroarenes are used in addition to benzene rings, a range of CPP derivatives can be obtained. Soon after Jasti, Itami and Yamago developed CPP synthesis methods, a variety of carbon nanorings were synthesized by them and other groups. In this section, carbon nanorings, which represent segments of armchair CNTs, are described. Armchair carbon nanorings can be subdivided into multiarylCPPs, CPP dimers and π-extended carbon nanorings.

Jasti synthesized tetraphenylated [12]CPP in 2012 (Figure 21) [35], whereby an L-shaped unit with four 4-n-butylphenyl groups (71) was prepared from tetrabromoquinone, 4-n-butylphenylboronic acid and 1,4-diiodobenzene. Macrocycle 72 was obtained by stepwise cross-coupling of 71 with the L-shaped unit bearing boryl and chloro groups (10d) and diboryl L-shaped unit 10b. Finally, the reductive aromatization of 72 mediated by NaNaph and followed by quenching with iodine furnished tetraaryl [12]CPP 73.

Figure 21:

Synthesis of tetraaryl [12]CPP 73 (Ar = 4-n-butylphenyl).

Several multiarylCPPs were synthesized by Nishiuchi and Müllen and were used as potential precursors for the synthesis of belt-shaped molecules. In 2012, dodecaaryl [9]CPP was synthesized using Jasti’s method (Figure 22) [36]. Initially, L-shaped units with four phenyl or 4-tert-butylphenyl groups 74a,b were prepared. Nickel(0)- or copper(I)-mediated homocoupling reactions of 74a,b furnished macrocycles, which were converted into dodecaaryl [9]CPP 75a,b via reductive aromatization using TiCl₄/LiAlH₄. In 2014, a series of multiarylCPPs 75c–f were prepared (Figure 22) [37]. Nishiuchi and Müllen synthesized multiphenyl-substituted L-shaped units 74c–f, which were subjected to a coupling reaction and aromatization to form carbon nanorings 75c–f. Nishiuchi and Müllen also conducted the synthesis of methylated or ethylene-bridged nanorings 75g,h in 2015 (Figure 22) [38]. Although they attempted oxidative intramolecular cyclization, or the Scholl reaction, on nanorings 75a–f to obtain belt-shaped molecule, the desired product was not obtained because of undesired 1,2-phenyl shift or other side reactions.

Figure 22:

Synthesis of multiarylCPPs 75a–h.

In 2014, Wegner reported the synthesis of [8]CPP derivatives 79a–d containing tetraaryl [8]CPP derivative 79d (Figure 23) [39]. Itami’s L-shaped unit 31c and monoprotected diynes 76a–c were connected by sequential Sonogashira coupling and deprotection to form unstrained macrocycles 77a–c. The diyne moieties of 77a–c were converted into benzene rings by rhodium-catalyzed [2+2+2] cycloaddition reaction with 3-hexyne or di(4-n-butylphenyl)acetylene. The resulting macrocycles 78a–d were finally subjected to acid-mediated aromatization to obtain [8]CPP derivatives 79a–d.

Figure 23:

Synthesis of substituted [8]CPP derivatives 79a–d (R = MOM).

When two CPPs are connected, an ideal belt-shaped molecule or the so-called carbon nanobelt can be obtained. As a first step toward the synthesis of such carbon nanobelts, the synthesis of CPP dimers was examined. In 2012, Jasti succeeded in the synthesis of p-phenylene- and 1,5-naphthylene-bridged [8]CPP dimers 80 and 81, respectively (Figure 24) [40]. By using a bromine-containing L-shaped unit (10k) along with the previously reported U-shaped unit 26b, bromine-containing macrocycle 28c was prepared. Subsequently, two 28c macrocycles were connected with 1,4-diborylbenzene (18) or 1,5-diborylnaphthalene (80) by the cross-coupling reaction, followed by reductive aromatization to form [8]CPP dimers 81 and 82. Jasti reported that directly connected [8]CPP dimer could not be obtained from 28c.

Figure 24:

Synthesis of p-phenylene- or 1,5-naphthylene-bridged [8]CPP dimers 81, 82.

In 2014, Itami accomplished the synthesis of a directly connected CPP dimer through the bottom-up synthesis of chloroCPP. Considering the tolerance required for the reaction conditions of CPP synthesis such as palladium-catalyzed cross-coupling and acid-mediated aromatization, the chloro group is expected to be an efficient handle for further synthetic manipulation. The preparation of chloro [10]CPP was achieved by a simple modification of a previously reported route for [10]CPP (Figure 25) [41]. Instead of 1,4-diborylbenzene 18, which was used for the synthesis of [10]CPP, 1,4-diboryl-2-chlorobenzene 83 was used as the chloro-containing linear unit. A cross-coupling reaction of 1,4-diboryl-2-chlorobenzene and C-shaped unit 32b formed chloro-containing macrocycle 34b. The subsequent aromatization of 34b furnished chloro [10]CPP 84, which was the first example of a monosubstituted CPP. Finally, a homocoupling reaction of 84 with Ni(cod)₂/bpy afforded the directly connected [10]CPP dimer 85.

Figure 25:

Synthesis of chloro [10]CPP 84 and [10]CPP dimer 85 (R = MOM).

Next, π-extended carbon nanorings including p-phenylene-PAH hybrid rings and all-PAH rings are presented. In 2013, PAH-containing ring was reported by Swager’s group, who applied Itami’s “cyclohexane” method to the synthesis of carbon nanoring 88, bearing naphthalene and benzene rings (Figure 26) [42]. Naphthalene-containing L-shaped unit 86 was prepared from 1,4-dibromonaphthalene and 1,4-cyclohexanedione. Subsequently, macrocycle 85 was obtained by the nickel(0)-mediated homocoupling of 86. Oxidative aromatization of cyclic tetramer 87 with TsOH yielded cyclo [4]paraphenylene [8]1,4-naphthylene 88.

Figure 26:

Synthesis of cyclo [4]paraphenylene [8]1,4-naphthylene 88 (R = MOM).

In 2014, Wang synthesized a carbon nanoring consisting of naphthalene and benzene via an alternative route (Figure 27) [43]. cis-Configurated L-shaped unit 89a was selectively obtained by the Diels–Alder reaction of 1,4-bis(4-bromophenyl)butadiene and p-benzoquinone. Homocoupling reaction of 89a afforded macrocycle 90, and cyclo [6]paraphenylene [3]1,4-naphthylene 91 was obtained by the oxidative aromatization of the cyclohexadiene moiety using 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ).

Figure 27:

Synthesis of cyclo [6]paraphenylene [3]1,4-naphthylene 91.

Synthesis of the pyrene-containing carbon nanoring, cyclo [12]paraphenylene [2]2,7-pyrenylene (94) was reported by Itami in 2014 (Figure 28) [44]. The synthetic route was based on the synthesis of [16]CPP. Macrocycle 93 was obtained by the homocoupling reaction of pyrene-containing U-shaped units, which was generated by the cross-coupling reaction of 2,7-diborylpyrene 92 and L-shaped unit 31d. Oxidative aromatization of 93 under high temperatures produced cyclo [12]paraphenylene [2]2,7-pyrenylene 94.

Figure 28:

Synthesis of cyclo [12]paraphenylene [2]2,7-pyrenylene 94.

In 2012, Itami reported the synthesis of [9]cyclo-1,4-naphthylene 97, which consists exclusively of naphthalene rings (Figure 29) [45]. For the generation of 97, a synthesis based on a reductive aromatization pathway was selected. Firstly, L-shaped unit 95 was prepared from 1,4-dibromonaphthalene and 1,4-naphthoquinone. Nickel(0)-mediated macrocyclization of 95 yielded cyclic trimer 96, which was subsequently converted into 97 by reductive aromatization with lithium metal. Interestingly, previously reported aromatization conditions, e.g., the use of LiNaph, were not suitable for this reaction.

Figure 29:

Synthesis of [9]cyclo-1,4-naphthylene 97.

In 2014, Yamago reported the synthesis of [4]cyclo-2,7-pyrenylene 100 (Figure 30) [46]. In order to increase the solubility of the synthetic intermediates, 4,5,9,10-tetrahydropyrene was used instead of pyrene. Macrocyclic tetraplatinum complex 99, consisting of four tetrahydropyrene rings, was synthesized from 2,7-distannyl-4,5,9,10-tetrahydropyrene 98 by using Yamago’s macrocyclization method. The reductive elimination of 99 promoted by triphenylphosphine provided a ring-shaped structure, which was subsequently dehydrogenated using palladium on carbon to afford [4]cyclo-2,7-pyrenylene 100.

Figure 30:

Synthesis of [4]cyclo-2,7-pyrenylene 100.

6.1 Synthesis of Chiral and Zigzag Carbon Nanorings

In this section, the syntheses of segments of chiral and zigzag CNTs (chiral/zigzag carbon nanorings) are summarized. A possible explanation for the lower number of reported examples of chiral/zigzag carbon nanorings relative to that of armchair carbon nanorings could be the inherently lower symmetry of the former with respect to the latter.

In 2011, Itami reported the first synthesis of a chiral carbon nanoring, representing a (15,14)CNT segment (Figure 31) [47]. A modular method for the synthesis of [14]–[16]CPP was used to synthesize naphthalene-containing macrocycle 102, employing L-shaped unit 31d, U-shaped unit 37b and 2,6-diborylnaphthalene 101 as the chirality-inducing units. Cycloparaphenylene-2,6-naphthylene 103 was obtained by the subsequent oxidative aromatization of 102. Itami mentioned that many possible segments of chiral CNTs can be obtained by varying the acene unit and/or the number of paraphenylene moieties. For example, segments of (n+2,n+1), (n+3,n+1) and (n+4,n+1)CNTs can be obtained by inserting a 2,6-naphtylene, 2,6-anthrylene or 2,8-tetracenylene unit, respectively, into the [n]CPP ring. As the 2,6-naphthylene moiety in 103 can easily rotate at ambient temperature, a separation of the enantiomers of 103 was not possible.

Figure 31:

Synthesis of [13]cycloparaphenylene-2,6-naphthylene 103.

In 2011–2013, Isobe synthesized carbon nanorings solely consisting of chrysenes or anthanthrylenes as shown in Figure 32 [48–50]. Using Yamago’s method for the synthesis of CPPs, macrocyclic tetraplatinum complexes 105a–c were prepared by the transmetalation of Pt(cod)Cl₂ with diborylated PAHs 104a–c, which can be synthesized from chrysene and pigment red, respectively. In contrast to Yamago’s original macrocyclization that includes the use of Pt(cod)Cl₂ and distannylarenes, Isobe’s modification employs CsF or K₃PO₄ as promoters for the transmetalation of the diborylarenes. Treating macrocycles 105a–c with triphenylphosphine at elevated temperatures provided a mixture of rotational isomers of carbon nanorings 106a–c. One of the rotational isomers of [4]cyclo-2,8-chrysenylene (106a), as well as that of [4]cyclo-2,8-anthanthrylene (106b), represents armchair (10,10)CNT segments, and the other types of rotational isomers represent (11,9)CNT and (12,8)CNT segments. In addition, each isomer consists of two enantiomers, whose separation was also successfully accomplished. One of the rotational isomers of [4]cyclo-3,9-chrysenylene (106c) can be regarded as a sidewall segment of zigzag (16,0)CNTs (Figure 32) [50].

Figure 32:

Synthesis of [4]cyclo-2,8-chrysenylene 106a [4],cyclo-2,8-anthanthrylene 106b and [4]cyclo-3,9-chrysenylene 106c (R = n-hexyl (a–c) or triisopropylsilylethynyl (b)).

7 Synthesis of Heteroatom-Containing Carbon Nanorings

By including heteroarenes in the components of carbon nanorings, heteroatom-containing carbon nanorings can be obtained. The first synthesis of a CPP derivative containing heteroatoms was reported by Itami and coworkers in 2012. They synthesized a pyridine-containing carbon nanoring by using 5,5′-dibromo-2,2′-bipyridyl (107) as a building block (Figure 33) [51]. Stepwise Suzuki–Miyaura cross-coupling reaction of 107 and diborylated U-shaped unit 37b generated macrocycle 108, which bears four pyridine rings. Aromatization reaction mediated by NaHSO₄ yielded cyclo [14]paraphenylene [4]2,5-pyridylene (109a). The pyridine-containing ring 109a showed halochromic property because of reversible protonation (109b) and deprotonation. Complexation of 109a to palladium(II) also occurred, generating dipalladium complex 109c.

Figure 33:

Synthesis of cyclo [14]paraphenylene [4]2,5-pyridylene 109a and its reactions with acid and palladium(II) (R = MOM).

Itami also synthesized anthraquinone-containing carbon nanoring 112a as the first donor–acceptor carbon nanoring (Figure 34) [52]. Diborylated 9,10-anthraquinone 110 was connected to U-shaped unit 51, which was used for high-yielding selective synthesis of [10]CPP, to obtain an acyclic intermediate. Then, nickel-mediated homocoupling reaction produced anthraquinone-containing macrocycle 111, which was successfully converted into carbon nanoring 112a by an oxidative aromatization reaction. By using malononitrile, 112a can be easily derivatized to quinodimethane-containing ring 112b. Because the fluorescent color of 112a,b changed, depending on the polarity of solvent (solvatofluorochromism), it can be concluded that anthraquinone and tetracyanoanthraquinodimethane behaved as π-acceptors and parahenylene as a π-donor in 112a,b.

Figure 34:

Synthesis of cyclo [10]paraphenylene-9,10-anthraquinon-2,6-ylene 112a and its tetracyanoquinodimethane derivative 112b (R = MOM).

The oxidative aromatization route was applied to the synthesis of thiophene-containing rings (Figure 35) [53]. Itami and coworkers prepared a new L-shaped unit containing two ethynyl groups 113 as the starting material. Macrocycles were generated by the copper-catalyzed homocoupling reaction of terminal alkynes. Cyclic tetramer (114), pentamer (115) and hexamer (116) were obtained together with a cyclic dimer, trimer and acyclic oligomers. The butadiyne moieties of 114–116 were then converted into thiophene rings by treating them with sodium sulfide to yield thiophene-containing precursors 117–119. Finally, acid-mediated aromatization reaction of 117–119 produced cycloparahenylene-2,5-thienylenes 120–122.

Figure 35:

Synthesis of [4]–[6]cycloparaphenylene-2,5-thienylene 120–122 (R = MOM).

Wang and coworkers synthesized ring-shaped molecules consisting of benzene, naphthalene and thiophene rings by using their aromatization method (Figure 36) [54]. Their L-shaped unit 89a was borylated by Miyaura borylation to form 89b, which was then coupled with 2-iodothiophene. The thiophene moieties were then subjected to iodination, which generated thiophene-attached L-shaped unit 123. The homocoupling reaction of 123 and oxidative aromatization of the resulting cyclic dimer produced cycloparaphenylene-1,4-naphthylene-2,5-thienylene 124. Larger ring 125 was synthesized by the sequential Suzuki–Miyaura cross-coupling reaction of 123 with 89b and DDQ-mediated oxidative aromatization reactions.

Figure 36:

Synthesis of cycloparaphenylene-1,4-naphthylene-2,5-thienylene 124 and 125.

Li, Xiao, Ng, Steigerwald, Nuckolls and coworkers reported the synthesis of a coral-shaped molecule containing perylene diimide (PDI) by using Yamago’s method (Figure 37) [55]. Distannylated 1,7-diarylPDI 126a, prepared from dibromoPDI by the Stille coupling reaction, was converted into diplatinum complex 126b. Complexation of 126b with 5,5′-bis(trimethylstannyl)-2,2′-bithiophene and subsequent reductive elimination accelerated by triphenylphosphine yielded PDI-containing ring 127. Rotational isomers of 127 were also isolated.

Figure 37:

Synthesis of PDI-containing nanoring 127 (R = CH(C₅H₁₁)₂).

Stępień synthesized π-extended [6]CPP by using carbazole or benzocarbazole as building blocks (Figure 38) [56]. Reaction of dibromocarbazole 128 and 1,3,5-tris(bromomethyl)benzene produced intermediate 130. Nickel(0)-mediated homocoupling reaction of 130 proceeded intramolecularly to yield [3]cyclo-2,7-carbazolylene 132. When benzocarbazole derivative 129 was used instead [3],cyclo-2,6-benzo[def]carbazolylene 133 can be obtained through intermediate 131.

Figure 38:

Synthesis of [3]cyclo-2,7-carbazolylene 132 and [3]cyclo-2,6-benzo[def]carbazolylene 133.

Osuka reported the synthesis of macrocyclic molecule consisting solely of porphyrin rings via platinum complexes (Figure 39) [57]. Diarylporphyrin 134 was subjected to iridium-catalyzed C–H borylation to yield 2,12-diborylporphyrin 135a. The incorporated metal (zinc) was then replaced by nickel (135b). Complexation of 135b with platinum was carried out to obtain a mixture of cyclic platinum complex 136, which was converted into cyclo-2,12-porphyrinylenes by treatment with triphenylphosphine to yield [3]–[5]cyclo-2,12-porphyrinylene 137–139.

Figure 39:

Synthesis of [3]–[5]cyclo-2,12-porphyrinylene 137–139 (Ar = 3,5-di-tert-butylphenyl).

8 Synthesis of Carbon Nanocages

The [n.n.n]carbon nanocages with C₃ symmetry consist of a pair of trisubstituted benzene rings and three [n]paraphenylene moieties, each linking two benzene rings. In 2013, Itami, Segawa and Kamada reported the first synthesis of [6.6.6] carbon nanocage 143 (Figure 40) [58]. Similar to the synthesis of [14]–[16]CPP by Itami, trifurcated unit 141 was obtained by a three-fold Suzuki–Miyaura coupling of 1,3,5-triborylbenzene 140 with L-shaped unit 31b. Nickel(0)-mediated dimerization of 141 provided a nonstrained bicyclic precursor 142, and its six cyclohexane moieties were aromatized to afford [6.6.6]carbon nanocage 143. Itami and Segawa then expanded this synthetic methodology to obtain carbon nanocages of varying sizes; use of appropriately modified starting materials resulted in the synthesis of [4.4.4]carbon nanocage 147 and [5.5.5]carbon nanocage 148 shown in Figure 41 [59]. Based on Itami’s synthesis of [7]CPP and [8]CPP, the smaller trifurcated unit 144a was prepared in several steps from 1,3,5-tribromobenzene and small L-shaped unit 47b. After transforming the chloro groups on 144a into boryl groups (144b), two bicyclic macrocycles 145 and 146 were synthesized under the influence of Ni(cod)₂/phen (phen = 1,10-phenanthroline) or palladium catalyst, depending on the reaction conditions and starting materials used. Precursors 145 and 146 were subsequently subjected to acid-mediated aromatization reactions, affording the corresponding carbon nanocages (147 and 148).

Figure 40:

Synthesis of [6.6.6]carbon nanocage 143 (R = MOM).

Figure 41:

Synthesis of [4.4.4] and [5.5.5]carbon nanocage 147 and 148 (R = MOM).

Yamago and Kim reported cage-shaped molecule (151) consisting of 16 benzene rings (Figure 42) [60]. First, 1,3,5-tris(4-bromophenyl)benzene 149a was converted into stannyl derivative 149b and then to platinum complex 149c. Mixing of 149b and 149c generated hexaplatinum complex 150a, which was an analogue of self-assembled molecular capsules developed by Fujita [61]. After exchanging the ligand cod with dppf, reductive elimination was performed to obtain cage-shaped molecule 151.

Figure 42:

Synthesis of cage-shaped molecule 151.

9 Summary

In this chapter, syntheses of CPPs and derivatives are reviewed. Development of the synthetic methods of CPPs allowed to uncover interesting size-dependent photophysical and redox properties of CPPs [62–79]. Characteristic CPP properties, such as host–guest behavior [80–91] and the complexation with transition metal [92], will provide a better understanding of the fundamental structure–property relationships and allow the application of CPPs in materials science. Growth of CNT by using CPP as a template will open the new field of selective synthesis of CNT [93]. For further details about the properties and applications of CPPs and their derivatives, see also accounts and review articles [94–100].