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. As simple model reactions, cycloaddition of ethylene to formaldehyde, thioformaldehyde, and formaldimine is also described.

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. Remarkably, the orbital topology-forbidden [2 + 2] process sometimes dominates over the allowed Diels-Alder reaction.

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. Its characterization includes its electron affinity, photoelectron spectrum, and the previously reported structure determination by electron diffraction.

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. This "fluorohomoene" reaction is shown to be a pseudopericyclic process, as is condensation of the bicyclobutane with water. Whereas the carbene 3-butenylidene rearranges primarily to butadiene, its perfluoro counterpart is predicted to be an efficient precursor for hexafluorobicyclobutane.

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. Fluorine substitution lowers the inversion barrier impressively; reasons for this phenomenon are discussed.

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. Preference for endo over exo cycloaddition is observed, and is reinforced by a repulsive secondary orbital interaction in exo transition states.

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.

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.

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.

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, 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.

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.

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. Ring inversion in the anion, measured by (19)F NMR line shape analysis, is characterized by these activation parameter values: DeltaH(++) = 8.84 +/-0.14 kcal/mol and DeltaS(++) = 0.81 +/- 0.6 cal mol(-1) K(-1).

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. Treatment of this Cl(10) compound with one equiv of a phosphazene base gives a nonachlorodihydroazulene, but the addition of a second equiv results in dechlorination to 1,2,3,4,5,6,7-heptachloroazulene as well as dehydrochlorination to octachloroazulene. The former azulene is obtained cleanly from the Cl(9) compound with acid catalysis or by reduction with mercury. In the presence of calcium carbonate, however, the Cl(9) intermediate yields the dark green octachloroazulene. Although octachloronaphthalene is readily converted into its octafluoro counterpart, the isomeric octachloroazulene is far too sensitive to undergo the analogous transformation.

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[2.2.2.01,4]octan-2-one ethylene ketal, was prepared by a rapid and quantitative [2+2] cycloaddition to the strained alkene perfluorobicyclo[2.2.0]hex-1(4)-ene. The propellane displays impressive thermal stability, and the vulnerable C-C bond joining the bridgeheads is very resistant to attack by electrophilic reagents. On the other hand, that electron-deficient bond is cleaved quickly at room temperature by a variety of nucleophiles and mild reducing agents. The behavior of this compound contrasts dramatically with that of the only known [2.2.2]propellane lacking fluorine substituents.

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 +/- 0.5. The tropone adds in [6 + 4] fashion to cyclopentadiene and photocyclizes to hexafluorobicyclo[3.2.0]hepta-3,6-dien-2-one. Lithium hydroxide in benzene transforms the tropone into pentafluorotropolone, which functions as a bidentate ligand.