Pathways for Concerted [2 + 2] Cycloaddition to Cumulenes.

A computational search has revealed concerted pathways for [2 + 2] cycloaddition of ethylene to all 10 of the cumulenes with the formula X═C═Y, where X, Y = C, N, O, and S. Four different concerted pathways have been found, three of them pseudopericyclic plus another based on sp-hybridized carbon. In the case of 2 of the 16 possible cycloadditions, a pair of novel three-membered ring intermediates has been discovered.

Cycloaddition Chemistry of Tetrafluorothiophene S,S-Dioxide.

Tetrafluorothiophene S,S-dioxide has been found to be a powerful and versatile cycloaddend that undergoes a wide range of reactions as a Diels-Alder diene, dienophile, and [2 + 2] addend. Because it dimerizes only slowly at high temperatures, a broad range of conditions are available for these transformations. Reactions with terminal alkynes yield products of both Diels-Alder and [2 + 2] cycloaddition.

Synthesis and properties of the strained alkene perfluorobicyclo[2.2.0]hex-1(4)-ene.

The title fluoroalkene has been generated by dehalogenation of dibromide and diiodide precursors and trapped in situ. retro-Diels-Alder reaction of its adduct with N-benzylpyrrole has made the alkene available in high yield and purity. In sharp contrast to its extremely labile hydrocarbon counterpart, the fluoroalkene is very stable yet highly reactive.

Tetrafluorothiophene S,S-dioxide: a perfluorinated building block.

Tetrafluorothiophene S,S-dioxide, a highly reactive diene and dienophile, has been synthesized. A new route to 3,4-difluoro- and tetrafluorothiophene has been realized, and the previously unknown 2,3,4-trifluorothiophene has been obtained. The reactivity of tetrafluorothiophene S-oxide has been compared with that of the S,S-dioxide.

Photochemistry of o-fluoranil.

Initial exploration of the photochemical behavior of o-fluoranil has revealed dimer formation, cycloaddition to alkenes, and hydrogen abstraction from hydrocarbons, aldehydes, and ethers.

The multifaceted reactivity of o-fluoranil.

In addition to Diels-Alder and hetero-Diels-Alder reactions, tetrafluoro-o-benzoquinone (o-fluoranil) undergoes nucleophilic additions, addition-eliminations, dioxole formation, and charge-transfer complexation, reacting at every site on the molecular skeleton. It also effects dehydrogenations and other oxidations. The quinone can function as a (CF)(4) synthon.

Chemistry of hexafluorobicyclo[1.1.0]butane: a computational study.

Despite its very weak central C-C bond, the yet-unknown hexafluorobicyclobutane is predicted to be quite robust. Unlike the parent hydrocarbon, which undergoes thermal rearrangement to butadiene, the perfluoro compound will yield hexafluorocyclobutene upon heating. The fluorobicyclobutane will react under mild conditions with a variety of reagents, in particular with certain alkenes in a new type of concerted transformation.

o-Fluoranil: stereochemistry and mechanism of its Diels-Alder reactions.

In the absence of significant steric effects, Diels-Alder reactions of the title quinone generally take place with preservation of configuration, and are therefore probably concerted. However, hybrid density functional calculations indicate that often these reactions are highly asynchronous. Steric hindrance can result in reaction at quinone oxygen instead of carbon.

The effect of fluorine substitution on ring inversion in bicyclo[1.1.0]butanes.

Degenerate ring inversion in bicyclo[1.1.0]butane and eight of its fluorinated derivatives has been studied at the B3LYP/6-311G**+ level of theory.

o-Fluoranil chemistry: Diels-Alder versus hetero-Diels-Alder cycloaddition.

The title quinone undergoes [4 + 2] cycloadditions in two ways, Diels-Alder on the ring and hetero-Diels-Alder by attack at the oxygens. The latter mode of reaction is strongly favored thermodynamically, but there is a kinetic bias favoring the normal Diels-Alder addition that often prevails, especially with cycloaddends that are not electron-rich.

Anesthetic properties of some fluorinated oxolanes and oxetanes.

Background: The search for new potent inhaled anesthetics has slowed, in large part because of the excellence of the two most recent additions, desflurane and sevoflurane. Nonetheless, neither desflurane nor sevoflurane are ideal anesthetics, desflurane causing cardiorespiratory stimulation, and sevoflurane having a slower (albeit rapid) recovery from anesthesia. Sevoflurane also can produce convulsions and postoperative agitation.

Synthesis, mechanism of formation, and dynamics of a highly fluorinated methylenecyclobutene.

Treatment of the hydrazone of 2H-perfluoro-2-methyl-3-pentanone with triethylamine at elevated temperatures yields a methylenecyclobutene via degradation to an acetylene followed by dimerization. The dimerization occurs even at -78 degrees C, and details of the reaction pathway have been elucidated. Both the acidity and the conformational dynamics of the methylenecyclobutene are influenced by buttressing effects in this crowded molecule.

Conformational and configurational dynamics of a highly fluorinated hydrazone.

The relative populations of two rotamers in the hydrazone of 2H-perfluoro-2-methyl-3-pentanone can be altered from one extreme to the other by increasing the Lewis basicity of the solvent, and the equilibrium E/Z ratio grows correspondingly. Both trends reflect an increase in the effective size of the amino group as a result of hydrogen bonding. The rate of E/Z interconversion is insensitive to the choice of solvent, consistent with the conclusion that the isomerization occurs via N-inversion and not C=N bond rotation.

Synthesis, basicity, and dynamics of a perfluorocyclohexenyl anion.

Electron transfer to perfluoro-1,3-dimethylcyclohexane in moist THF has yielded two quite different products. Tetrabutylammonium iodide irradiated with ultraviolet light gives a tetrabutylammonium enolate, but potassium fluorenone ketyl affords a cyclohexenyl anion. This allylic anion was isolated as its conjugate acid, a rather strong carbon acid.

Novel concerted fragmentation upon alcoholysis of a urazole.

[reaction: see text] Instead of yielding the expected hydrazine, alcoholysis of the above heterocycle results in fragmentation via a highly unusual pathway.

Perspective on fluorocarbon chemistry.

Fluorocarbons, organic molecules with carbon skeletons and fluorine "skins", differ fundamentally from their hydrocarbon counterparts in interesting and useful ways. A selection of the myriad applications fluorocarbons and their derivatives have found in modern life is described and related to molecular properties. Salient aspects of the nature and reactivity of fluorocarbon compounds are highlighted by comparison with their more familiar hydrocarbon analogues.

Diels-Alder reactions of perfluorobicyclo[2.2.0]hex-1(4)-ene with aromatics.

[reaction: see text] The title perfluoroalkene cycloadds to a variety of aromatic hydrocarbons, including benzene. It is the first alkene to yield a Diels-Alder adduct with benzene and is thus among the most potent dienophiles known.

New syntheses and chemistry of hexafluorotropone.

Two new routes to hexafluorotropone have been developed, one from hexachlorotropone and a superior synthesis from hexafluorobenzene. Hexafluorotropone was found to be a very weak base, with a conjugate acid pK(a) of -6.2 +/- 0.

A remarkable [2.2.2]propellane.

To explore the effects of fluorine substitution on the highly strained [2.2.2]propellane skeleton, a new representative of this ring system, perfluorotricyclo[2.

Octachloroazulene.

The title compound, the first perhaloazulene, has been synthesized from hexachlorobutadiene and cyclopentadiene. Further chlorination of 1,3,4,5,6,7-hexachloroazulene results in addition, not substitution, under electrophilic as well as free radical conditions. Radical chlorination of the hexachloroazulene affords in good yield a single decachlorotetrahydroazulene.