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ortho–para-Directing Activators: –CH3, –OH, –⁠NH2, –OCH301:11

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All ortho–para directors, excluding halogens, are activating groups. These groups donate electrons to the ring, making the ring carbons electron-rich. Consequently, the reactivity of the aromatic ring towards electrophilic substitution increases. For instance, the nitration of anisole is about 10,000 times faster than the nitration of benzene. The electron-donating effect of the methoxy group in anisole activates the ortho and para positions on the ring and stabilizes the corresponding...
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Radical reactions can occur either intermolecularly or intramolecularly. In an intermolecular radical reaction, a nucleophilic radical adds to an electrophilic alkene or vice versa. In such reactions, the radical and generally the alkene, which is also called the radical trap, are two different molecules. Additionally, for such intermolecular reactions to occur, the radical trap must be active, present in an excess concentration, and the radical starting material must have a weak...
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Due to their highly strained structures, epoxides can readily undergo ring-opening reactions through nucleophilic substitution, either in the presence of an acid or a base. The nucleophilic substitution reactions in the presence of acid are called acid-catalyzed ring-opening reactions, and nucleophilic substitution reactions in the presence of a base are called base-catalyzed ring-opening reactions. Epoxides undergo base-catalyzed ring-opening reactions in the presence of a strong nucleophile...
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Radicals, the highly reactive species, gain stability by undergoing three different reactions. The first reaction involves a radical-radical coupling, in which a radical combines with another radical, forming a spin‐paired molecule. The second reaction is between a radical and a spin‐paired molecule, generating a new radical and a new spin‐paired molecule. The third reaction is radical decomposition in a unimolecular reaction, forming a new radical and a spin‐paired...
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Epoxides that are three-membered ring systems are more reactive than other cyclic and acyclic ethers. The high reactivity of epoxides originates from the strain present in the ring. This ring strain acts as a driving force for epoxides to undergo ring-opening reactions either with halogen acids or weak nucleophiles in the presence of mild acid. The acid catalyst converts the epoxide oxygen, a poor leaving group, into an oxonium ion, a better leaving group, making the reaction feasible. The...
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The presence of electron-donating, electron-withdrawing, or conjugating groups adjacent to a radical center, imparts electronic stabilization to the radicals. Examples of such electronically-stabilized radicals are triphenylmethyl, tetramethylpiperidine‐N‐oxide, and 2,2‐diphenyl‐1‐picrylhydrazyl. These radicals are remarkably stable and are known as persistent radicals. Some of the persistent radicals can even be isolated and purified.
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Diradical Interactions in Ring-Open Isoxazole.

Adam A Wallace1, Yerbolat Dauletyarov1, Andrei Sanov1

  • 1Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721, United States.

The Journal of Physical Chemistry. A
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Electron capture in isoxazole causes ring opening and forms diradical states. Photoelectron spectra reveal distinct isoxazole diradical properties compared to oxazole, explained by molecular orbital interactions.

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Area of Science:

  • Physical Chemistry
  • Quantum Chemistry
  • Molecular Spectroscopy

Background:

  • Isoxazole and oxazole are heterocyclic compounds with distinct electronic structures.
  • Understanding diradical states is crucial for reaction mechanisms and molecular properties.
  • Previous studies have investigated related systems, providing a basis for comparison.

Purpose of the Study:

  • To investigate the electronic structure and diradical manifold of isoxazole.
  • To compare the diradical properties of isoxazole with those of oxazole.
  • To elucidate the factors governing the differences between isoxazole and oxazole diradical states.

Main Methods:

  • Photoelectron spectroscopy was used to probe the electronic states of isoxazole diradicals.
  • Computational modeling, specifically a coupled-fragments molecular-orbital model, was employed.
  • Analysis of through-bond and through-space interactions within the diradical species.

Main Results:

  • Electron capture by isoxazole leads to O-N bond dissociation and ring opening, forming a diradical.
  • A dense manifold of diradical states (triplet, singlet) was observed via photoelectron spectra.
  • Isoxazole diradicals exhibit a significantly different electronic structure compared to oxazole diradicals.

Conclusions:

  • The unique diradical manifold of isoxazole arises from the terminal placement of the nitrogen atom.
  • Through-space interactions in oxazole stabilize a closed-shell singlet state, unlike in isoxazole.
  • The electronic configuration of isoxazole diradicals is influenced by the near degeneracy of π and σ* orbitals, favoring triplet states.