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Woodward–Hoffmann Selection Rules and Microscopic Reversibility01:34

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Electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are concerted pericyclic reactions that proceed via a cyclic transition state. These reactions are stereospecific and regioselective. The stereochemistry of the products depends on the symmetry characteristics of the interacting orbitals and the reaction conditions. Accordingly, pericyclic reactions are classified as either symmetry-allowed or symmetry-forbidden. Woodward and Hoffmann presented the selection criteria for...
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Chair Conformation of Cyclohexane02:02

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The chair conformation is the most stable form of cyclohexane due to the absence of angle and torsional strain. The absence of angle strain is a result of cyclohexane’s bond angle being very close to the ideal tetrahedral bond angle of 109.5° in its chair conformer. Similarly, the torsional strain is also absent owing to the perfectly staggered arrangement of bonds.
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Conformations of Cyclohexane02:11

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Cyclohexane does not exist in a planar form due to the high angle and torsional strain it would experience in the planar structure. Instead, it adopts non-planar chair and boat conformations.
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Aromatic Hydrocarbon Cations: Structural Overview01:18

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Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon that consists of an odd number of carbon atoms and an intervening sp3 carbon in the ring. The three double bonds in the ring correspond to 6 π electrons, which is a Huckel number, and therefore satisfies the criteria of 4n + 2 π electrons. However, the intervening sp3 carbon disrupts the continuous overlap of p orbitals. As a result, cycloheptatriene is not aromatic.
Removing one hydrogen from the intervening CH2 group...
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Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.
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The inscribed polygon method is consistent with Hückel’s 4n + 2 rule and helps to learn whether the given cyclic compound is aromatic or not. The compound is stable and aromatic if every bonding molecular orbital (MO) is completely filled with a pair of electrons. However, if the non-bonding or antibonding orbitals are filled with electrons, the compound is unstable and not aromatic. Consider the Frost circle diagrams for cycloalkenes containing 4 to 8 carbons.
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Twisted Macrocycles with Folded ortho-Phenylene Subunits.

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Area of Science:

  • Supramolecular Chemistry
  • Organic Chemistry
  • Polymer Science

Background:

  • Foldamers are oligomers known for adopting defined secondary structures and exhibiting functional properties.
  • Incorporating foldamer subunits into larger architectures is crucial for biomacromolecular higher-order structures but remains underexplored.
  • Aromatic foldamers, such as o-phenylenes, offer a simple platform for investigating self-assembly into complex architectures.

Purpose of the Study:

  • To investigate the dynamic covalent assembly of o-phenylene foldamers into twisted macrocyclic structures.
  • To characterize the resulting macrocycles' shape persistence, symmetry, and conformational dynamics.
  • To explore the influence of macrocyclic confinement on foldamer subunit behavior.

Main Methods:

  • Utilized dynamic covalent imine formation to assemble o-phenylene tetramers with various rod-shaped linkers (p-phenylene, tolane, diphenylbutadiyene).
  • Characterized the synthesized [3 + 3] macrocycles using Nuclear Magnetic Resonance (NMR) spectroscopy, including variable-temperature NMR.
  • Analyzed conformational distributions and distinguished between homochiral (D3-symmetric) and heterochiral (C2-symmetric) conformers.

Main Results:

  • Efficient macrocyclization was achieved, leading to the formation of shape-persistent, triangular macrocycles with twisted cores and internal diameters up to ~2 nm.
  • NMR spectroscopy successfully differentiated between D3- and C2-symmetric conformers.
  • Confinement within the macrocyclic architecture was observed to slow down the inversion dynamics of the o-phenylene moieties.

Conclusions:

  • Dynamic covalent chemistry provides an effective route for constructing complex foldamer-based macrocycles.
  • The resulting macrocycles exhibit significant shape persistence and tunable symmetry.
  • Macrocyclic confinement influences the conformational dynamics of foldamer subunits, offering insights into higher-order structural control.