Zigzag Hydrocarbon Belts

Abstract

Open AccessCCS ChemistryMINI REVIEW1 Feb 2021Zigzag Hydrocarbon Belts Tan-Hao Shi and Mei-Xiang Wang Tan-Hao Shi MOE Key Laboratory of Bioorganic Phosphorous and Chemical Biology, Department of Chemistry, Tsinghua University, Beijing 100084 and Mei-Xiang Wang *Corresponding author: E-mail Address: [email protected] MOE Key Laboratory of Bioorganic Phosphorous and Chemical Biology, Department of Chemistry, Tsinghua University, Beijing 100084 https://doi.org/10.31635/ccschem.020.202000287 SectionsAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Zigzag hydrocarbon belts have been fascinating chemists and materials scientists for decades because of their aesthetically appealing molecular structures, outstanding physical properties and intriguing chemical reactivities predicted by theoretical calculations, and potential applications as unique macrocyclic hosts in supramolecular chemistry. They may also serve as templates or seeds to grow structurally well-defined uniform zigzag carbon nanotubes. While there have been continuous computational studies on the structures and properties of belt[n]arenes or [n]cyclacenes since they were proposed as hypothetical molecules in 1954, the synthesis of (partially) conjugated zigzag hydrocarbon belts remains a great challenge, with no progress being reported in the past 20 years. Very recently, we have been witnessing the renaissance of synthetic interest in zigzag hydrocarbon belts, and the formation of the first fully conjugated one is on the horizon. This minireview focuses on the understanding of belt[n]arenes based on theoretical calculations and the synthesis and structure of zigzag hydrocarbon belts. Perspectives on the strategies to isolate and characterize the fully conjugated belt[n]arenes and on the applications of zigzag hydrocarbon belts are also discussed. Download figure Download PowerPoint Introduction Zigzag hydrocarbon belts are double-stranded macrocycles in which six-membered hydrocarbon rings are ortho-fused in a linear fashion. They include fully conjugated belt[n]arenes or [n]cyclacenes, partially reduced belt[n]arenes such as collar[n]arenes and belt[n]enes, and fully saturated belt[n]anes (Figure 1). Zigzag hydrocarbon belts have been fascinating chemists and materials scientists for decades because of their attractive molecular structures and intriguing electronic structures.1–3 In addition, the belt molecules, especially the fully conjugated ones, are predicted, based on theoretical calculations, to possess outstanding physical and chemical properties.4–12 Furthermore, the cylindrical cavity of varied sizes and electronic features would engender zigzag hydrocarbon belts unique and useful macrocyclic hosts in supramolecular chemistry.13 Moreover, the carbon skeletons of these belts can be viewed as the shortest segments of zigzag single-walled carbon nanotubes. They are potential templates to grow structurally well-defined and uniform zigzag carbon nanotubes, which are essential to—and the prerequisite for—practical applications of carbon nanotubes in advanced technology.14,15 For example, pure (8,0) and (12,0) zigzag single-walled carbon nanotubes are semiconducting and metallic, respectively.16,17 Finally, construction of zigzag hydrocarbon belts still remains one of the most formidable challenges in synthetic chemistry. Figure 1 | Structures of zigzag hydrocarbon belts (a) and zigzag single-walled carbon nanotubes (b). Download figure Download PowerPoint In this minireview, we first give a very brief historical overview of the field followed by a summary of theoretical studies of the structures and stabilities of belt[n]arenes. The status and recent advances in the synthesis of zigzag hydrocarbon belts are then discussed with the focus on synthetic strategies that aim to construct the core structures of partially saturated hydrocarbon belts and to convert them into fully conjugated ones. A Historical Overview Adapted from polyacenes, the terms “cyclic polyacenes”,4 “cyclacenes”,18 “super-acenes”,19 and “corannulenes”20 were used initially to denominate cyclic linear polyacene compounds. “Collar[n]arenes”,21“belt[n]enes”,22 and “belt[n]anes”22 were also coined to name partially and fully saturated zigzag hydrcarbon belts. To unify the nomenclatures and also to follow the tradition in naming macrocyclic compounds in supramolecular chemistry, we recommend the use of “belt[n]arenes” to name double-stranded molecules in which aromatic rings are ortho-fused in a linear fashion. Collar[n]arenes, belt[n]enes, and belt[n]anes can be named therefore as partially or fully hydrogenated or saturated belt[n]arene derivatives. Moreover, this naming system can be easily extended to heteroaromatic ring-containing belt[n]arene analogs. For example, belt[4]arene[4]pyridine or tetraza-embedded belt[8]arene is a zigzag molecular belt, which contains four benzene and four pyridine units. As an illustrative chronology given in Table 1 shows, the chemistry of zigzag hydrocarbon belts is a relatively young or emerging research area. The first report of zigzag hydrocarbon belts as imaginary molecules may date back to 1954 when Heilbronner23 calculated the eigenvalues of molecular orbitals of the fully conjugated belt[12]arene. Noteworthily, it took more than three decades before the observation of carbon nanotubes by Iijima in 1991.31 In 1983, Vögtle proposed fully conjugated and partially aliphatic molecular belts for the first time as desirable synthetic targets, addressing “the challenges for generations of chemists.”19 The first attempted synthesis of belt[12]arene was pioneered by Stoddart18 in 1987. He established an elegant approach comprising repetitive Diels–Alder reactions between precisely predesigned oxygen-bridged bisdienes and bisdienophiles with molecular rigidity and curvature to construct Kohnkene, the core structure of a hydrocarbon belt. In the following years, he reported the synthesis of partially reduced belt[n]arenes.21,32,33 The strategy based on Diels–Alder reaction was later employed by Schlüter22 in 1989 and Cory25,26 in 1996 to synthesize partially hydrogenated belt[6]ene and belt[8]arene derivatives, respectively. All attempts to transform the partially saturated zigzag hydrocarbon belts into fully conjugated ones met with failure. Using ab initio methods, Kim5 calculated the structures, magnetism, and aromaticity of belt[n]arenes in 1999. The electronic structures and the strain energies of belt[n]arenes were investigated computationally by Chen and Jiang8 and Itami,27 respectively, in 2007 and 2016, while Su28 conducted a theoretical study of global aromaticity of belt[n]arenes in 2018. This year, Wang29,30 developed a new protocol to construct octahydrobelt[8]arenes and dodecahydrobelt[12]arenes by stitching all fjords of resorcin[n]arene (n = 4, 6) derivatives through multiple intramolecular alkylation reactions. The formation of a fully conjugated zigzag hydrocarbon belt was observed for the first time from retro-Diels–Alder reaction of belt[8]arene-DDQ4, an adduct between belt[8]arene and four equivalents of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), under matrix-assisted laser desorption/ionization (MALDI) conditions. Table 1 | Timeline of the Study of Zigzag Hydrocarbon Belts 1954 – Heilbronner: Calculation of the molecular orbital eigenvalues of belt[12]arene using LCAO method23 1983 – Vögtle: Proposal of hydrocarbon belts as desirable synthetic targets19 1985 – Alder: Calculation of molecular strain of belt[n]enes (n = 3–12)24 1987 – Stoddart: Synthesis of Kohnkene and attempted conversion to belt[12]arene18 1988 – Stoddart: Synthesis of partially reduced belt[12]arene21 1989 – Schlüter: Synthesis of a partially reduced belt[6]ene derivative22 1996 – Cory: Synthesis of partially reduced belt[8]arenes25,26 1999 – Kim: Calculation of the structure, magnetism, and aromaticity of belt[n]arenes (n = 5–14) using ab initio methods5 2007 – Chen and Jiang: Calculation of the electronic structures of belt[n]arenes (n = 4–14)8 2016 – Itami: Calculation of the strain energies of hydrocarbon belts27 2018 – Su: Calculation of the aromaticity of belt[n]arenes (n = 5–10)28 2020 – Wang: Synthesis of octahydrobelt[8]arenes and dodecahydrobelt[12]arenes29,30 2020 – Wang: Observation of the formation of belt[8]arene under MALDI condition29 Theoretical Study of Belt[n]arenes Early theoretical studies focused on the Hückel’s molecular orbital (HMO) and force field calculations of molecular structures and orbital features of belt[n]arenes.20,34,35 High-temperature superconductivity and ferromagnetism of belt[n]arenes were predicted in 1983.4 In the 1990s, semiempirical methods were used to shed light on the influence of ring size on the molecular and electronic structures.36–47 Since 1999, ab initio calculations and density functional theory (DFT) methods have been widely applied, providing interesting information and predictions of the molecular structures, strain energies, aromaticity, and electronic configurations of fully conjugated hydrocarbon belts. Molecular Geometry Kim5 proposed that belt[n]arenes can form three possible valence isomers of Dnh, Cn, and Dn symmetry, respectively (Figure 2). Optimizations of molecular geometries at different levels of theory conclude that the structure having Dnh symmetry is the most stable isomer.5–9,12,48 Notably, the zigzag peripheral C–C bonds (1.446–1.475 Å) are always longer than the fused C–C bonds (1.404–1.421 Å) irrespective of the electronic configuration of the ground state. Because of the bond length alternation, belt[n]arenes have therefore been viewed as a stack of two [2n]trannulenes linked by single C–C bonds. Based on optimized structures at the M06-L/6-31G(d) level of theory, the fused C–C bond lengths increase slightly with increasing molecular radius, with a total change of 0.012 Å from belt[6]arene to belt[12]arene. In addition, the bond lengths in triplet states are 0.004 Å greater than that in singlet states.9 Obviously, the diameter (d) of belt molecules increases with the value of n, and according to Sancho-García,12 it can be estimated by the equation d = 0.741n + 0.341 Å. Figure 2 | Structures of valance isomers of belt[n]arenes. Download figure Download PowerPoint Strain Energy Since the formula of belt[n]arenes is expressed as (C4H2)n, the strain energy of a macrocycle can be defined as energy difference per C4H2 unit between a belt[n]arene and hypothetical strain-free parent compound [Estrain = E(C4H2) − Estrain-free(C4H2)]. The energy of a C4H2 fragment [E(C4H2)] in belt[n]arenes is calculated by dividing the total energy (Etotal) of the molecules by n (Etotal/n). According to Cramer,9 who conducted calculations at the M06-L/6-31 g(d) level, the energy of “unstrained” belt[n]arene (Estrain-free) was obtained from extrapolation of a fitted curve for energy as a function of 1/n to n = ∞. Itami27 found a linear correlation between the normalized energies per C4H2 unit and n−2 as Etotal/n = 1365·n−2 − 96,409 (R2 = 0.99708). The strain energies of belt[n]arenes ( E strain ′ ) were simply obtained as 1365/n2 kcal/mol. Sancho-García12 applied hypothetical homodesmotic chemical reaction in Figure 3 to calculate strain energies as ΔstrainH(belt[n]arene) = ΔfH°(anthracene) − ΔfH°(belt[n]arene)/n − ΔfH°(naphthalene). It should be noted that the data compiled in Table 2 show clearly that strain energies varied considerably depending on the calculation methods. Nevertheless, small belt[n]arenes (n = 6–8) are highly strained, and the strain energies decrease with the increase of belt sizes. Figure 3 | Hypothetical homodesmotic reactions of belt[n]arenes. Download figure Download PowerPoint Table 2 | Strain Energies of Different Belt[n]arenes Using Different Calculated Method (kcal/mol) Estrain E strain ′ ΔstrainH(belt[n]arene) M06-L/6-31g(d) B3LYP/6-31g(d) M06-2X/6-31+g(d) M06-2X-D3(BJ)/6-31 + g(d) Belt[6]arene 33.0 37.9 37.7 37.6 Belt[7]arene 25.5 27.9 29.2 29.0 Belt[8]arene 18.2 21.3 22.4 22.2 Belt[9]arene 15.3 16.9 19.4 19.3 Belt[10]arene 11.8 13.7 15.6 15.5 Belt[11]arene 10.1 11.3 14.4 14.3 Belt[12]arene 8.2 9.5 – – Aromaticity Aromaticity of belt[n]arenes was computed by Kim5 at the UB3LYP/6-31G(d) level in terms of nucleus-independent chemical shift (NICS) at the molecular center. Large negative NICS values were obtained for belt[n]arenes with n = even, suggesting that these molecules are aromatic. The belt[n]arenes (n = odd) are almost nonaromatic, as the negative NICS values are very small and approach zero. Recently, aromaticity of belt[n]arenes was further studied by Su. Computed NICS values and anisotropy of the current-induced density (AICD) diagrams showed that belt[8]arene and belt[9]arene are aromatic although they are open-shell molecules. Interestingly, the internal magnetic field of belt[8]arene was studied by means of NICS values scan, a method enabling determination of aromaticity of any point in space and the main current position inside the belt. It was revealed that the maximum aromaticity in the three-dimensional space of belt[8]arene is 0.8 Å away from the fused C–C bond of the cross-section at the height of 0.6 Å from the molecular center.28 Electronic configuration The electronic configurations of the ground states of belt[n]arenes have been debated for almost two decades.5–12 For example, initial DFT studies5,7 showed that, with increasing n values, the singlet-triplet energy gaps (ΔES-T) of belt[n]arenes (n = odd) decreased from positive to negative while the ΔES-T of belt[n]arenes (n = even) increased from negative to positive, suggesting the different electronic configurations are favored depending on the molecular structures. These results were later refined by Chen and Jiang,8 who performed calculations using unrestricted broken spin-symmetry DFT (USB-B3LYP/6-31G(d)), indicating that all belt[n]arenes (n ≥ 6) have open-shell singlet ground states. However, contrary to expectation, the calculations resulted in ΔES-T values, which increased with increasing belt size. Very recently, Leininger conducted calculations using the complete active space self-consistent field method and the second-order n-electron valence perturbation theory. They found that the ground state is an open-shell singlet. The ΔES-T values decrease exponentially with increasing belt[n]arene size, reaching between 0.14 and 0.16 eV when n approaches infinity.11 It is also interesting to note that a polyradical character was predicted for large belt[n]arenes. Synthesis of zigzag hydrocarbon belts In contrast to the plethora of theoretical calculations, there are only a handful of synthetic studies in literature. As predicted by the computational studies, instability arising from high macrocyclic strain energies, the lack of a Clar’s aromatic sextet, and the open-shell singlet ground state are major hurdles in the synthesis of fully conjugated zigzag hydrocarbon belts. Apart from electronic configurations and aromaticity, direct macrocyclization of linear precursors into belt[n]arenes would have to overcome enormous strain energies, leading this approach to be thermodynamically unfeasible. To circumvent the spike of strain energies, a stepwise construction of partially saturated belt[n]arene derivatives followed by aromatization appears as the practical strategy to access belt[n]arenes. The successful construction of partially saturated belt[n]arene structures or derivatives reported so far includes both iterative Diels–Alder reactions between reactants and the straightforward cyclization reactions of monomacrocyclic calix[n]arene derivatives. The Stoddart strategy Stoddart, a Nobel laureate in chemistry, pioneered the synthesis of hydrocarbon belts in the late 1980s18,21 and the early 1990s.32,33 In the first-attempted synthesis of belt[12]arene, he developed an elegant method using repetitive Diels–Alder reactions to construct the core skeleton of belt[12]arene.18 To achieve the formation of a double-stranded macrocyclic ring, bisdienes and bisdienophiles of rigidity and curvature had to be used. After considering the face selectivity of Diels–Alder reactions, he designed bisdiene and bisdienophiles of the 7-oxabicyclio[2.2.1]heptane framework. As depicted in Scheme 1, consecutive Diels–Alder reactions of furan with bisbenzyne formed in situ from 1 afforded syn-bisdienophile 2 in 22% yield, while bisdiene 4 was prepared in an overall yield of 24% from 3 in four steps. Heating 2 with two equivalents 4 in toluene led to the formation of fragments 5 and 6 in 61 and 24% yield, respectively. Scheme 1 | Synthesis of dienophile and bisdiene fragments. Download figure Download PowerPoint The reaction between 2 and 5 under high pressure produced 20% of Kohnkene 8, a spectacular double-stranded macrocycle with six oxygen atoms distributed around its outer surface (Scheme 2). The compound was named after Kohnke, who conducted the synthesis. In refluxing xylene, macrocyclic dimerization of 6 also gave product 8 but in a low yield. When high pressure was applied, however, the yield was improved greatly to 48%. Later on, Stoddart and his co-workers reported the high-yielding synthesis of 8 by means of refluxing bisdiene 4 in toluene with bisdienophile 7, an intermediate from the reaction between fragments 2 and 632 (Scheme 2). Applying the identical strategy by the same authors, similar belt derivatives 9– 11 (Figure 4) were synthesized successfully.32 Scheme 2 | Synthesis of hexaepoxy-octacosahydrobelt[12]arene 8. Download figure Download PowerPoint To synthesize hydrocarbon belt molecules, Stoddart21 carried out the deoxygenation reaction of 8 (Scheme 3). Treatment of 8 with a low-valent titanium reagent resulted in the removal of two oxygen atoms to form tetraepoxy-tetracosahydrobelt[12]arene 12 in good yield. The belt gives a very rigid cavity to include a water molecule in the crystalline state. Further dehydration of 12 led interestingly to the formation of hydrocarbon belt 13 as the major product. The structure that contains one anthracene, one benzene, and two naphthalene units was elucidated based on the observation of four anisochromous AB systems corresponding to four 1,4-cyclohexadiene units from its 1H NMR spectrum. Attempted synthesis of belt[12]ene through the Birch reduction of 13 gave a mixture of compounds. The most intense molecular peak from the mass spectrum, and a prominent AB system in the 1H NMR spectrum, supported the formation of dodecahydrobelt[12]arene (Scheme 3). Scheme 3 | Synthesis of hydrocarbon belts 13 and 14 from 8. Download figure Download PowerPoint The Stoddart method, which uses the curved 7-oxabicyclio[2.2.1]heptane framework to direct macrocyclization, has provided a springboard for others to synthesize different derivatives of hydrocarbon belts. For example, in the synthesis of a partially saturated belt[18]arene derivative, Schlüter49 also used Diels–Alder reaction of the benzyne intermediate to introduce rigid 7-oxabicyclio[2.2.1]heptane segments into a linear precursor 17. Thermal decomposition of 17 in refluxing toluene generated isobenzofuran-terminated benzoquinone intermediate 18, which underwent spontaneous head-to-tail [4+2] cycloaddition reaction to give a mixture of macrocycle 19 and ladder polymers 20. Taking advantage of the equilibrium between 19 and 20 owing to the reversibility of Diels–Alder reaction, belt product was synthesized in an improved yield (45%) by heating 20 in decalin after the ring–chain equilibrium was reached. In theory, belt molecule 19 can be obtained almost quantitatively from 20 if iterative procedures to equilibrate 20 into 19 are implemented (Scheme 4).49 Scheme 4 | Synthesis of a partially saturated belt[18]arene derivative. Download figure Download PowerPoint Recently, application of a rigid 7-oxabicyclio[2.2.1]heptane scaffold to facilitate the formation of belt structures has been extended successfully by Gross, Peña, and their co-workers50 to the synthesis of tetraepoxyoctahydrobelt[8]arene 25 and tetraepoxyoctahydrobelt[10]arene 29. Shown in Scheme 5 is macrocyclic tetramerization of the highly reactive 5,6-didehydroisobenzofuran 24, which is derived from 21. Compound 25 was obtained in 5% yield from the reaction in a dilute solution. When bisenophile 2 reacted with 1,2,4,5-tetrazine 22 followed by treatment with bisbenzyne formed in situ from 27, a pair of syn-isomers 28 were isolated in 24% yield from the reaction mixture. The curvature of the molecule caused by two 7-oxabicyclio[2.2.1]heptane subunits favored the macrocyclization reaction with bisdiene 26 under high-dilution conditions via twofold Diels–Alder reactions, furnishing product 29 in 15% yield (Scheme 6). Scheme 5 | Synthesis of tetraepoxyoctahydrobelt[8]arene 25. Download figure Download PowerPoint It is interesting to note that the authors50 took a surface chemistry approach to dissociate oxygen atoms of highly oxygenated belts using an STM-based atom manipulation technique. Judging from the zoomed-in AFM images of the area which were imaged after STM scanning at a sample voltage of 3 V, molecules of 29 deposited on a Cu-(111) surface underwent deoxygenation reactions to form both isomers of diepoxytetrahydrobelt[10]arenes (Figure 5 left). In the case of 25, removal of both one and two oxygen atoms occurred similarly (Figure 5 right). Complete removal of oxygen atoms was not observed. Figure 5 | AFM images of the self-assembled islands of 29 (left) and 25 (right) acquired after STM scanning with voltage of 3 V and 2.8 V (a). Zoomed-in AFM images of molecules as indicated by the Download figure Download PowerPoint The same repetitive Diels–Alder reactions between and bisbenzyne have been used very by to construct a tetraepoxyoctahydrobelt[10]arene As in Scheme 7, the reaction between bisdiene and bisbenzyne generated in situ from the of with afforded a pair of syn-isomers in yield. Further Diels–Alder reaction of with in the of an of led to the formation of in yield. Notably, after of a conditions to by the found the of from the treatment of with an of and the remains results indicated that and give a and an respectively (Scheme Scheme | Synthesis of tetraepoxyoctahydrobelt[10]arene and its conversion to Download figure Download PowerPoint The and method To synthesize structurally well-defined polymers using iterative Diels–Alder cycloaddition reactions, designed and prepared a from the [4+2] cycloaddition reaction between and benzoquinone followed by When a highly of was in ring took to form a benzoquinone intermediate which underwent dimerization of in dilute enabling the synthesis of a hydrogenated belt[6]ene in yield (Scheme of belt structures was not observed. The formation of is most to the of the first Diels–Alder which and dienophile into Compound the hydrocarbon belt in the literature. Scheme 8 | Synthesis and molecular structure of hydrogenated belt[6]ene in molecular structure are for Download figure Download PowerPoint Taking advantage of [4+2] cycloaddition reaction between benzoquinone and reported in 1996 a straightforward synthesis of a partially saturated belt[8]arene under When and bisdiene in were and into refluxing macrocyclic Diels–Alder reactions to product in yield (Scheme Compound was prepared in two from while reactions of with followed by cyclization bisdiene Scheme | Synthesis and molecular structure of a hydrogenated belt[8]arene in molecular structure are for Download figure Download PowerPoint of the partially saturated belt[8]arene were investigated by with the aim to synthesize fully conjugated zigzag belt As in Scheme reactions of using different generated interesting derivatives, the to the For example, of with at in benzene produced a belt while the treatment of with afforded a compound Further dehydration of using product in yield. When or was with in benzene, an belt was isolated in a yield. Attempted aromatization of the two rings of with oxygen in resulted in the formation of a mixture of belt molecules. Scheme | Chemical of Download figure Download PowerPoint It should be noted that in a as a in in in the reduction of in and and also the reaction of with The synthesis of a belt of was also to the of no of these studies have been Scheme 6 | Synthesis of tetraepoxyoctahydrobelt[10]arene 29. Download figure Download PowerPoint strategy In to the methods based on iterative Diels–Alder reactions between predesigned and a different synthetic strategy to construct zigzag hydrocarbon belts has been reported very The of the method is to the of through multiple intramolecular alkylation reactions. Figure 4 | Structures of belt derivatives synthesized by Download figure Download PowerPoint are monomacrocyclic compounds easily from the reaction of or its derivatives with They which two in into Taking these of we the synthesis of belt molecules simply by into and then stitching the fjords via intramolecular alkylation As depicted in Scheme which is a compound derived from underwent reaction with a reagent to a macrocycle by intramolecular alkylation reaction took at to give product in a good yield. compound underwent intramolecular alkylation reactions to the two fjords molecular belt To hydrocarbon belts, was into an intermediate following a reaction comprising the reaction with and the reaction with In the of reagent in a intramolecular cyclization reaction of under conditions to

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