Understanding Reactivity Patterns of Radical Cations

Schmittel, Michael; Burghart, Armin

doi:10.1002/anie.199725501

Cited by 365 publications

(221 citation statements)

References 736 publications

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“…Compared with the structure of radical cation 1 þ , the Se1-C bond distances (Se1-C1, Se1-C11) basically keep unchanged, while the Se2-C bonds (Se2-C17, Se2-C8) are lengthened and the intramolecular Se-Se distance (Se1-Se2) of 2.8815 (9) 55 . It is also worth noting that dialkyl dichalcogen radical cation (MeSe) 2 þ , containing a two-center three-electron Se-Se p-bond, dimerizes as a rectangular species (MeSe) 4 2 þ by a p*-p* interaction with long Se-Se bonds (2.974(1) Å) 56 that are comparable to those of [1-1] 2 þ . However, in these cases reversibility was not observed [53][54][55][56] .…”

Section: Resultsmentioning

confidence: 99%

“…O dd-electron species exist as intermediates in many chemical reactions and play an important role in bond formation and cleavage processes [1][2][3][4][5][6][7] . Odd-electron bonding is of both fundamental and practical interest [8][9][10][11][12][13] .…”

mentioning

confidence: 99%

“…Gerson et al 32 and Alder et al 33 reported cyclic framework-constrained N'N three-electron s-bonds almost 30 years ago. In 1997, Drews and Seppelt 34 isolated and structurally characterized the Xe 2 þ ion containing a Xe'Xe three-electron s-bond.…”

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confidence: 99%

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Isolation and reversible dimerization of a selenium–selenium three-electron σ-bond

Zhang

Wang

et al. 2014

Nat Commun

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Three-electron s-bonding that was proposed by Linus Pauling in 1931 has been recognized as important in intermediates encountered in many areas. A number of three-electron bonding systems have been spectroscopically investigated in the gas phase, solution and solid matrix. However, X-ray diffraction studies have only been possible on simple noble gas dimer Xe'Xe and cyclic framework-constrained N'N radical cations. Here, we show that a diselena species modified with a naphthalene scaffold can undergo one-electron oxidation using a large and weakly coordinating anion, to afford a room-temperature-stable radical cation containing a Se'Se three-electron s-bond. When a small anion is used, a reversible dimerization with phase and marked colour changes is observed: radical cation in solution (blue) but diamagnetic dimer in the solid state (brown). These findings suggest that more examples of three-electron s-bonds may be stabilized and isolated by using naphthalene scaffolds together with large and weakly coordinating anions.

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Section: Resultsmentioning

confidence: 99%

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confidence: 99%

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Isolation and reversible dimerization of a selenium–selenium three-electron σ-bond

Zhang

Wang

et al. 2014

Nat Commun

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“…The lack of reaction with this substrate shows that the first step in the reaction is likely to be oxidation of the enol to a radical cation. 34 Use of methanol (MeOH) as a solvent medium provided the corresponding methyl ester in good yields. Use of ceric tetra-nbutylammonium nitrate (CTAN) in CH 2 Cl 2 provided comparable yields to reaction of 6 with CAN in CH 3 CN (Table 2, entries 2 and 3).…”

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confidence: 99%

Mild Conversion of β-Diketones and β-Ketoesters to Carboxylic Acids

2006

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A mild protocol for the conversion of β-ketoesters and β-diketones to carboxylic acids using CAN in CH 3 CN is described. The method is compatible with a number of functional groups, and can generate carboxylic acids under neutral conditions at room temperature. The reaction is fast and general, providing an alternative method to the commonly used malonic ester acid preparation. Initial mechanistic studies show that initial oxidation of the enol form of the β-dicarbonyl initiates the reaction. The presence of nitrate as an oxidant ligand or as an additive is critical for success of the reaction.

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“…Radical ions are intermediates in some important reactions [5] and are typically generated by electrochemistry or by the action of a strong reducing or oxidizing inorganic reagent, (e.g., eqs 3 and 4).…”

Section: Synthetic Paths Via Photoinduced Electron Transfermentioning

confidence: 99%

Environment-friendly organic synthesis. The photochemical approach

Albini

Fagnoni

Mella

2000

Pure and Applied Chemistry

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Photochemical reactions are expected to have an increasing role in organic synthesis with the drive towards environment-friendly reactions. Some examples illustrating the scope and versatility of radical alkylation of electrophilic alkenes via photoinduced electron transfer are presented. A few applications in special fields are mentioned. PHOTOCHEMISTRY FOR ORGANIC SYNTHESISPhotoinitiated reactions are promising candidates for environment-friendly organic synthesis. The central issue of organic chemistry is to make quite stable organic molecules susceptible to reactions. This is obtained either by weakening a covalent bond (e.g., by complexation or adsorption on a catalyst surface) or by polarizing it (e.g., again by complexation or by forming a hydrogen bond) or by cleaving a covalent bond. The last choice is the one requiring more energy and more control of the medium, as typified by the deprotonation of a carbonyl or carboxyl derivative to form an enolate, probably the most largely used strategy for the formation of the carbon-carbon bond.Photochemistry is different, since electronically excited states (see eq. 1) differ from ground states not only for the high energy level, but also, and perhaps more importantly, for the dramatic change in the electronic distribution by which they are characterized as well as by the fact that such a change does not require the addition of a chemical reagent or-at least in principle-a special care of the medium. To take a simple example, a ketone is a weak electrophile, due to the polarization of the p C-O bond. As such, it reacts only with quite active (usually charged) nucleophiles. The electrophilicity can be increased by increasing the polarization (e.g., by complexation by means of a Lewis acid), and under these conditions weaker electrophiles add. However, electronic excitation brings about a more deepseated change. The np* excited state is no more a C-electrophile (3 electrons now crowd in the p space) but the single electron remaining in the n O orbital gives to this species a strong radical reactivity centerd at the oxygen atom (see Scheme 1). Indeed, the typical reactions are fast (typical rates ≥ 1 ¥ 10 6 M -1 s -1 ) H-abstractions from a C-H bond or addition to an alkene, i.e., the same reactions that an alkoxy radical would give.

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Understanding Reactivity Patterns of Radical Cations

Cited by 365 publications

References 736 publications

Isolation and reversible dimerization of a selenium–selenium three-electron σ-bond

Isolation and reversible dimerization of a selenium–selenium three-electron σ-bond

Mild Conversion of β-Diketones and β-Ketoesters to Carboxylic Acids

Environment-friendly organic synthesis. The photochemical approach

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