Zigzag Hydrocarbon Belts

Open AccessCCS ChemistryMINI REVIEW1 Feb 2021Zigzag Hydrocarbon Belts Tan-Hao Shi and Mei-Xiang Wang MOE Key Laboratory of Bioorganic Phosphorous Chemical Biology, Department Chemistry, Tsinghua University, Beijing 100084 Google Scholar More articles by this author *Corresponding author: E-mail Address: [email protected] https://doi.org/10.31635/ccschem.020.202000287 SectionsAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Zigzag hydrocarbon belts have been fascinating chemists materials scientists for decades because their aesthetically appealing molecular structures, outstanding physical properties intriguing chemical reactivities predicted theoretical calculations, potential applications as unique macrocyclic hosts in supramolecular chemistry. They may also serve templates or seeds grow structurally well-defined uniform zigzag carbon nanotubes. While there continuous computational studies on the structures belt[n]arenes [n]cyclacenes since they were proposed hypothetical molecules 1954, synthesis (partially) conjugated remains a great challenge, with no progress being reported past 20 years. Very recently, we witnessing renaissance synthetic interest belts, formation first fully one is horizon. This minireview focuses understanding based calculations structure belts. Perspectives strategies isolate characterize are discussed. Download figure PowerPoint Introduction double-stranded macrocycles which six-membered rings ortho-fused linear fashion. include [n]cyclacenes, partially reduced such collar[n]arenes belt[n]enes, saturated belt[n]anes (Figure 1). attractive electronic structures.1–3 In addition, belt molecules, especially ones, predicted, possess properties.4–12 Furthermore, cylindrical cavity varied sizes features would engender useful chemistry.13 Moreover, skeletons these can be viewed shortest segments single-walled nanotubes, essential to—and prerequisite for—practical nanotubes advanced technology.14,15 For example, pure (8,0) (12,0) semiconducting metallic, respectively.16,17 Finally, construction still most formidable challenges Figure 1 | Structures (a) (b). minireview, give very brief historical overview field followed summary stabilities belt[n]arenes. The status recent advances then discussed focus that aim construct core convert them into ones. A Historical Overview Adapted from polyacenes, terms “cyclic polyacenes”,4 “cyclacenes”,18 “super-acenes”,19 “corannulenes”20 used initially denominate cyclic polyacene compounds. “Collar[n]arenes”,21“belt[n]enes”,22 “belt[n]anes”22 coined name hydrcarbon To unify nomenclatures follow tradition naming compounds chemistry, recommend use “belt[n]arenes” aromatic Collar[n]arenes, named therefore hydrogenated belt[n]arene derivatives. system easily extended heteroaromatic ring-containing analogs. belt[4]arene[4]pyridine tetraza-embedded belt[8]arene belt, contains four benzene pyridine units. As an illustrative chronology given Table shows, chemistry relatively young emerging research area. report imaginary date back 1954 when Heilbronner23 calculated eigenvalues orbitals belt[12]arene. Noteworthily, it took more than three before observation Iijima 1991.31 1983, Vögtle aliphatic time desirable targets, addressing “the generations chemists.”19 attempted belt[12]arene was pioneered Stoddart18 1987. He established elegant approach comprising repetitive Diels–Alder reactions between precisely predesigned oxygen-bridged bisdienes bisdienophiles rigidity curvature Kohnkene, belt. following years, he belt[n]arenes.21,32,33 strategy reaction later employed Schlüter22 1989 Cory25,26 1996 synthesize belt[6]ene derivatives, respectively. All attempts transform ones met failure. Using ab initio methods, Kim5 magnetism, aromaticity 1999. strain energies investigated computationally Chen Jiang8 Itami,27 respectively, 2007 2016, while Su28 conducted study global 2018. year, Wang29,30 developed new protocol octahydrobelt[8]arenes dodecahydrobelt[12]arenes stitching all fjords resorcin[n]arene (n = 4, 6) derivatives through multiple intramolecular alkylation reactions. observed retro-Diels–Alder belt[8]arene-DDQ4, adduct equivalents 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), under matrix-assisted laser desorption/ionization (MALDI) conditions. Timeline Study – Heilbronner: Calculation orbital using LCAO method23 1983 Vögtle: Proposal targets19 1985 Alder: belt[n]enes 3–12)24 1987 Stoddart: Synthesis Kohnkene conversion belt[12]arene18 1988 belt[12]arene21 Schlüter: derivative22 Cory: belt[8]arenes25,26 1999 Kim: structure, 5–14) methods5 Jiang: 4–14)8 2016 Itami: belts27 2018 Su: 5–10)28 2020 Wang: dodecahydrobelt[12]arenes29,30 Observation MALDI condition29 Theoretical Belt[n]arenes Early focused Hückel’s (HMO) force belt[n]arenes.20,34,35 High-temperature superconductivity ferromagnetism 1983.4 1990s, semiempirical methods shed light influence ring size structures.36–47 Since 1999, density functional theory (DFT) widely applied, providing interesting information predictions energies, aromaticity, configurations Molecular Geometry form possible valence isomers Dnh, Cn, Dn symmetry, respectively 2). Optimizations geometries at different levels conclude having Dnh symmetry stable isomer.5–9,12,48 Notably, peripheral C–C bonds (1.446–1.475 Å) always longer fused (1.404–1.421 irrespective configuration ground state. Because bond length alternation, stack two [2n]trannulenes linked single bonds. Based optimized M06-L/6-31G(d) level theory, lengths increase slightly increasing radius, total change 0.012 Å belt[6]arene triplet states 0.004 greater singlet states.9 Obviously, diameter (d) increases value n, according Sancho-García,12 estimated equation d 0.741n + 0.341 Å. 2 valance Strain Energy formula expressed (C4H2)n, energy macrocycle defined difference per C4H2 unit strain-free parent compound [Estrain E(C4H2) ? Estrain-free(C4H2)]. fragment [E(C4H2)] dividing (Etotal) n (Etotal/n). According Cramer,9 who M06-L/6-31 g(d) level, “unstrained” (Estrain-free) obtained extrapolation fitted curve function 1/n ?. Itami27 found correlation normalized n?2 Etotal/n 1365·n?2 96,409 (R2 0.99708). ( E ? ) simply 1365/n2 kcal/mol. Sancho-García12 applied homodesmotic 3 calculate ?strainH(belt[n]arene) ?fH°(anthracene) ?fH°(belt[n]arene)/n ?fH°(naphthalene). It should noted data compiled show clearly considerably depending calculation methods. Nevertheless, small 6–8) highly strained, decrease sizes. Hypothetical Energies Different Calculated Method (kcal/mol) Estrain M06-L/6-31g(d) B3LYP/6-31g(d) M06-2X/6-31+g(d) M06-2X-D3(BJ)/6-31 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 computed UB3LYP/6-31G(d) nucleus-independent shift (NICS) center. Large negative NICS values even, suggesting aromatic. odd) almost nonaromatic, zero. Recently, further studied Su. Computed anisotropy current-induced (AICD) diagrams showed belt[9]arene although open-shell molecules. Interestingly, internal magnetic means scan, method enabling determination any point space main current position inside revealed maximum three-dimensional 0.8 away cross-section height 0.6 center.28 Electronic debated decades.5–12 initial DFT studies5,7 that, values, singlet-triplet gaps (?ES-T) decreased positive ?ES-T even) increased positive, favored structures. These results refined Jiang,8 performed unrestricted broken spin-symmetry (USB-B3LYP/6-31G(d)), indicating ? states. However, contrary expectation, resulted size. Leininger complete active self-consistent second-order n-electron perturbation theory. state singlet. exponentially size, reaching 0.14 0.16 eV approaches infinity.11 note polyradical character large contrast plethora only handful literature. studies, instability arising high lack Clar’s sextet, major hurdles Apart direct macrocyclization precursors overcome enormous leading thermodynamically unfeasible. circumvent spike stepwise aromatization appears practical access successful so far includes both iterative reactants straightforward cyclization monomacrocyclic calix[n]arene Stoddart Stoddart, Nobel laureate late 1980s18,21 early 1990s.32,33 first-attempted belt[12]arene, skeleton belt[12]arene.18 achieve ring, had used. After considering face selectivity reactions, designed bisdiene 7-oxabicyclio[2.2.1]heptane framework. depicted Scheme 1, consecutive furan bisbenzyne formed situ afforded syn-bisdienophile 22% yield, 4 prepared overall yield 24% steps. Heating toluene led fragments 5 6 61 dienophile fragments. pressure produced 20% 8, spectacular six oxygen atoms distributed around its outer surface (Scheme after Kohnke, synthesis. refluxing xylene, dimerization gave product 8 but low yield. When however, improved greatly 48%. Later on, his co-workers high-yielding bisdienophile 7, intermediate 632 Applying identical same authors, similar 9– 11 4) synthesized successfully.32 hexaepoxy-octacosahydrobelt[12]arene 8. Stoddart21 carried out deoxygenation 3). Treatment low-valent titanium reagent removal tetraepoxy-tetracosahydrobelt[12]arene 12 good gives rigid water molecule crystalline Further dehydration interestingly 13 product. anthracene, benzene, naphthalene units elucidated anisochromous AB systems corresponding 1,4-cyclohexadiene 1H NMR spectrum. Attempted belt[12]ene Birch reduction mixture intense peak mass spectrum, prominent supported dodecahydrobelt[12]arene 14 method, uses curved framework macrocyclization, has provided springboard others belt[18]arene derivative, Schlüter49 benzyne introduce precursor 17. Thermal decomposition 17 generated isobenzofuran-terminated benzoquinone 18, underwent spontaneous head-to-tail [4+2] cycloaddition 19 ladder polymers 20. Taking advantage equilibrium owing reversibility reaction, (45%) heating decalin ring–chain reached. quantitatively if procedures equilibrate implemented 4).49 derivative. application scaffold facilitate successfully Gross, Peña, co-workers50 tetraepoxyoctahydrobelt[8]arene 25 tetraepoxyoctahydrobelt[10]arene 29. Shown tetramerization reactive 5,6-didehydroisobenzofuran 24, derived 21. Compound 5% dilute solution. bisenophile reacted 1,2,4,5-tetrazine 22 treatment 27, pair syn-isomers 28 isolated mixture. caused subunits 26 high-dilution conditions via twofold furnishing 29 15% 6). 25. authors50 dissociate oxygenated STM-based atom manipulation technique. Judging zoomed-in AFM images area imaged STM scanning sample voltage V, deposited Cu-(111) diepoxytetrahydrobelt[10]arenes left). case 25, occurred similarly right). Complete not observed. self-assembled islands (left) (right) acquired V 2.8 (a). Zoomed-in indicated colored boxes. bis(isobenzofuran) recently Miao51 pyrene-bearing 30. illustrated 30 interaction 31 CsF 32 36% presence excess amount 33 17% examination few aromatize deoxygenation, authors production octahydrotetrabenzobelt[10]arene 34 NaI TMSI, though mechanism unclear. Single-crystal X-ray diffraction box-like oval-shaped cavity, 7). 7 34. Schlüter Cory ribbon-shaped masked AB-type (diene-dienophile type) monomer 37 diene 35 36 oxidation reaction.22 concentrated solution heated toluene, electrocyclic opening place diene-containing 38, polymerization reaction. Amazingly, 38 became dominant solution, gram-scale derivative 39 70% 8). Formation other exclusive likely due folded conformation adduct, brings moieties close proximity. represents smallest documented 39. Alkyl chains hidden clarity. facile diene, 42 pseudo-high-dilution conditions.25 bisenophile1,4,5,8-anthradiquinoe 40 41 solutions 1,4-dioxane added slowly simultaneously 1,4-dioxane, annulation involving double proceeded smoothly furnish 69% 9). steps 2-bromo-1,4-dimethoxybenzene fourfold nucleophilic substitution tetrabromide 1-octynyllithium Cp2Zr-promoted reductive yielded 41. 9 42. Transformations compounds.26 summarized 10, oxidants some failed obtain desired products. dehydrogenation DDQ ambient temperature naphthalene-containing 43 m-CPBA bis-expoxide 44. bis-epoxide 44 p-toluenesulfonic acid furnished 45 60% oxidized PCC hot anthraquinone-embedded 46 moderate oxidative cyclohexene alcoholic potassium hydroxide diethoxy-substituted 10 transformations review article book chapter Advances Theoretically Interesting Molecules published 1998, Cory52 mentioned carbonyl groups MeMgI. larger analog mentioned. Unfortunately, best our knowledge, details disclosed. Fjord-stitching addition dienes dienophiles, completely recently. key feature fjord regions resorcin[n]arenes 9–11 Stoddart. Resorcin[4]arenes available condensation resorcinol various aldehydes. usually adopt cone conformation, phenolic hydroxy each region proximity.53 advantages resorcin[4]arenes, explored converting alkylating reactions.29,30 11, cyclophane 48, known resorcin[4]arene, Suzuki coupling boron produce tetraisopropenyl-bearing 49. Mediated Tf2O, Friedel–Crafts efficiently room 50 Under forcing conditions, TMSOTf-promoted up last affording 51. prepare aromatizable reactant 48 transformed alcohol-containing 54 sequence Stille vinylstannane, ozonolysis, Grignard ArMgBr. Eaton’s (7.5 wt % phosphorous pentoxide methanesulfonic acid), one-pot alkylative cycliza