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01:13Electrophilic Aromatic Substitution: Fluorination and Iodination of Benzene
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Bromination and chlorination of aromatic rings by electrophilic aromatic substitution reactions are easily achieved, but fluorination and iodination are difficult to achieve. Fluorine is so reactive that its reaction with benzene is difficult to control, resulting in poor yields of monofluoroaromatic products. To address this, Selectfluor reagent is used as a fluorine source in which a fluorine atom is bonded to a positively charged nitrogen.
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01:26Thermal and Photochemical Electrocyclic Reactions: Overview
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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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01:16Cycloaddition Reactions: Overview
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Cycloadditions are one of the most valuable and effective synthesis routes to form cyclic compounds. These are concerted pericyclic reactions between two unsaturated compounds resulting in a cyclic product with two new σ bonds formed at the expense of π bonds. The [4 + 2] cycloaddition, known as the Diels–Alder reaction, is the most common. The other example is a [2 + 2] cycloaddition.
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01:17Thermal Electrocyclic Reactions: Stereochemistry
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The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
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03:07Electron Affinity
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The electron affinity (EA) is the energy change for adding an electron to a gaseous atom to form an anion (negative ion).
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01:20Diazonium Group Substitution with Halogens and Cyanide: Sandmeyer and Schiemann Reactions
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Arenediazonium substitution reactions occur when the diazonium group is substituted by various functional groups such as halides, hydroxyl, nitrile, etc. For instance, arenediazonium salts react with copper(I) salts of chloride, bromide, or cyanide to form corresponding aryl chlorides, bromides, and nitriles. These reactions are named Sandmeyer reactions. Although the mechanism of this reaction is complicated, as illustrated in Figure 1, they are believed to progress via an aryl copper...
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Fluorenyl Cations: Synthesis and Reactivity.
Kanika Vashisth1, Sameera Ranasinghe1, Ayesha Begum1
1Department of Chemistry and Biochemistry, Baylor University, One Bear Place #97348, Waco, TX, 76798, USA.
Chemistry (Weinheim an Der Bergstrasse, Germany)
|June 12, 2025
Summary
This review details the 71-year history of fluorenyl cations, from fleeting species to isolable compounds. Advancements in synthesis and characterization, including X-ray diffraction, mark significant progress in fluorenyl cation chemistry.
Area of Science:
- Organic Chemistry
- Spectroscopy
- Cation Chemistry
Background:
- Fluorenyl cations, first reported in 1954, were historically observed only as transient species.
- Characterization relied on techniques like mass spectrometry, UV/Vis, IR, and NMR spectroscopy.
- Limited understanding of their stability and reactivity due to their fleeting nature.
Purpose of the Study:
- To provide a comprehensive historical overview of fluorenyl cation research.
- To highlight synthetic methodologies and advancements in their characterization.
- To discuss related aromatic species with modified cyclopentadienyl cores.
Main Methods:
- Review of historical literature from 1954 to the present.
- Analysis of synthetic routes, including dihydroxylation and halide abstraction.
- Examination of spectroscopic and diffraction data for characterization.
Main Results:
- Detailed history of fluorenyl cation research spanning over seven decades.
- Identification of common synthetic pathways for fluorenyl cation precursors.
- Emergence of isolable fluorenyl cations, with two characterized by X-ray diffraction.
Conclusions:
- Significant progress has been made in stabilizing and isolating fluorenyl cations.
- Modern synthetic and characterization techniques have overcome previous limitations.
- The field continues to expand, including related aromatic systems.