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Aromatic Hydrocarbon Cations: Structural Overview

Aromatic Hydrocarbon Cations: Structural Overview

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...

Structure of Benzene: Molecular Orbital Model

Structure of Benzene: Molecular Orbital Model

According to the molecular orbital (MO) model, benzene has a planar structure with a regular hexagon of six sp2 hybridized carbons. As shown in Figure 1, each carbon is bonded to three other atoms with C–C–C and H–C–C bond angles of 120°. The C–H bond length is 109 pm, and the C–C bond length is 139 pm which is midway between the single bond length of sp3 hybridized carbons (154 pm) and sp2 hybridized carbons (133 pm).

Photochemical Electrocyclic Reactions: Stereochemistry

Photochemical Electrocyclic Reactions: Stereochemistry

The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation

[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction

[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction

The Diels–Alder reaction is an example of a thermal pericyclic reaction between a conjugated diene and an alkene or alkyne, commonly referred to as a dienophile. The reaction involves a concerted movement of six π electrons, four from the diene and two from the dienophile, forming an unsaturated six-membered ring. As a result, these reactions are classified as [4+2] cycloadditions.

Prochirality

Prochirality

The concept of prochirality leads to the nomenclature of the individual faces of a molecule and plays a crucial role in the enantioselective reaction. It is a concept where two or more achiral molecules react to produce chiral products. A typical process is the reaction of an achiral ketone to generate a chiral alcohol. Here, the achiral reactant reacts with an achiral reducing agent, sodium borohydride, to generate an equimolar mixture of the chiral enantiomers of the product. For example, an...

Nomenclature of Aromatic Compounds with Multiple Substituents

Nomenclature of Aromatic Compounds with Multiple Substituents

When more than one substituent is present on the benzene ring, the IUPAC nomenclature depends on the number of substituents present.
For disubstituted benzene derivatives, with two groups attached to the benzene ring, three constitutional isomers are possible. For example, consider dimethyl benzene, often called xylene, where the second methyl group can be substituted at the second, third, or fourth carbon. The relative position of the substituents is represented by prefixes ortho, meta, or...

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Benzidine/Quinoidal-Benzidine-Linked, Superbenzene-Based π-Conjugated Chiral Macrocycles and Cyclophanes.

Guangwu Li¹, Taisuke Matsuno², Yi Han¹

¹Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore, Singapore.

Angewandte Chemie (International Ed. in English)

|March 19, 2020

Summary

This summary is machine-generated.

Researchers synthesized novel cyclophanes using large polycyclic aromatics, achieving unique quinoidal structures with paramagnetic properties. These advanced materials show potential for selective fullerene encapsulation.

Keywords:

chiral macrocycles cyclophane diradicaloids host-guest chemistry polycyclic aromatic hydrocarbons

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Published on: August 22, 2018

Area of Science:

Organic Chemistry
Supramolecular Chemistry
Materials Science

Background:

Synthesizing fully conjugated cyclophanes with large polycyclic aromatic hydrocarbons (PAHs) is a significant synthetic challenge.
Hexa-peri-hexabenzocoronene (superbenzene) derivatives offer unique structural and electronic properties for advanced materials.

Purpose of the Study:

To develop a synthetic route for novel benzidine-linked, superbenzene-based cyclophanes.
To investigate the structural, electronic, and magnetic properties of the synthesized cyclophanes and their quinoidal derivatives.
To explore the potential of these macrocycles for guest encapsulation, specifically fullerene C70.

Main Methods:

Intermolecular Yamamoto coupling of pre-organized precursors to synthesize cyclophane frameworks.
Oxidative dehydrogenation to form quinoidal benzidine-linked cyclophanes.
X-ray crystallographic analysis for structural determination.
Spectroscopic (UV-Vis absorption) and electrochemical (cyclic voltammetry) methods for electronic property investigation.
Density Functional Theory (DFT) calculations.

Main Results:

Successful synthesis of three benzidine-linked, superbenzene-based cyclophanes (ortho-, para-, meta-isomers).
Characterization of quinoidal benzidine-linked cyclophanes exhibiting thermally populated paramagnetic activity and a significant singlet-triplet energy gap.
Isolation and structural identification of two enantiomers of the ortho-cyclophane, revealing a chiral figure-of-eight macrocyclic structure.
Demonstration of selective C70 fullerene encapsulation by the cage-like cyclophanes 2-NH and 3-NH.

Conclusions:

The study presents a viable synthetic strategy for complex cyclophane architectures incorporating large PAHs.
The synthesized quinoidal cyclophanes possess unique electronic and magnetic properties, including paramagnetism.
The cage-like structures show promise for applications in molecular recognition and host-guest chemistry, particularly for fullerene derivatives.