Valence-bond isomer chemistry. Part 12 [1]. Pyrolysis of the hexafluorobicyclo[2.2.0]Hexa-2,5-diene-diazomethane adduct. Sigmatropic fluorine shifts and elimination of difluorocarbene from hexaflourocycloheptatrienes

Flow pyrolysis of the title adduct yields tetrafluoroethylene and all three tetrafluorobenzenes.     

Molecular structure of hexafluorobicyclo [2.2.0]hexa-2,5-diene (hexafluoro-dewar-benzene) in the vapour phase

Hexafluoro-Dewar-benzene has been studied by the electron-diffraction method. A model with C_2v symmetry gives excellent agreement between experimental and theoretical data. The structural parameters with error limits are (cf. Fig. 1): r(C₁-C₄)= 1.598 ±0.017 Å, r(C₁-C₂) = 1.505 ±0.005 Å, r(C₂-C₃) = 1.366 ± 0.015 Å, r(C₁-F₁) = 1.328±0.015 Å, r(C₂-F₂) = 1.319±0.007 Å, ∠F₁C₁C₄ = 118.7±0.7°, ∠F₂C₂C₃ = 133.6±0.7°, τ= 121.8±2.0°, and δ = -7.5±2.0°. Molecular orbital calculations by the CNDO/2 method gave τ = 119.8° and δ = ?4.2°.     

Diels-Alder approach to biaryls: elucidation of competing tandem [2+2] cycloaddition/[1,3] sigmatropic shift pathway

Reaction of 2-halo-6-nitrophenylacetylene with an electron deficient diene gives rise to a [4+2] cycloaddition/cycloreversion biaryl product and a bicyclo[4.2.0]octadiene resulting from a competing [2+2] cycloaddition pathway. The cyclobutene can be opened to give a mixture of cyclooctatriene and biaryl in varying amounts depending on heat and light exposure. The conversion of the cyclobutene into biaryl occurs through a [1,3] sigmatropic carbon shift followed by [4+2] cycloextrusion of ethylene gas.     

Acetylenic chemistry: Part 15 [1]. Reactions of 1,2,3,4-Tetrahydro-2,4-dioxopyrido[2,3-d]pyrimidine with 3-bromoprop-1-yne

Reaction of 1,2,3,4-tetrahydro-2,4-dioxopyrido[2,3-d]pyrimidine with 3-bromoprop-1-yne gave 1-prop-2′-ynylpyrido[2,3-d]pyrimidine-2,4-dione ( 4a ), 3-prop-2′-ynylpyrido[2,3-d]pyrimidine-2,4-dione ( 4b ), and 1,3-diprop-2′-ynylpyrido[2,3-d]pyrimidine-2,4-dione ( 4c ). Subsequent boiling of 1,3-diprop-2′-ynylpyrido-[2,3-d]pyrimidine-2,4-dione ( 4c ) in formic acid afforded 1-methylimidazo[1,2-a]pyridyl-N-prop-2′-ynylamide ( 5 ) and 1-acetonyl-3-prop-2′-ynylpyrido[2,3-d]pyrimidine-2,4-dione ( 6 ).     

Thermal rearrangement of dienolesters. Sequential [1.3] sigmatropic rearrangement and intramolecular diels-alder cycloaddition of 1-methylene-2-methyl-2-propenyl hex-5-enoate

The high temperature thermal rearrangement of the title dienol ester is reported. Its reactions entail a [1.3] acyl shift followed by intramolecular Diels-Alder cyclo-addition of the resulting β-diketone.     

Nitroxide chemistry. Part XIV [1]. Reactions of trifluoromethyl 2,3-dimethylbut-3-en-2-yl and trifluoromethyl 2,3-dimethylbut-2-yl nitroxides

The nitroxide CF₃N(?)CMe₂CMeCH₂ abstracts the aldehydic hydrogen from benzaldehyde, yielding the benzoyl compound PhCO₂N(CF₃)CMe₂CMeCH₂ (82%), the benzylic hydrogen from cumene, yielding PhCMe₂ON(CF₃)CMe₂CMeCH₂ (80%), and the methylene group hydrogen from fluorene, oxidises benzoin to benzil, and hydroquinol to quinone, attacks the Si'&'2.sbnd;H bond of trimethylsilane, and adds to tetrafluoroethylene to give the compound R¹N(CF₃)OCF₂CF₂ON(CF₃)R¹ (R¹ = CMe₂CMeCH₂, 97%), and to hexafluoropropene to give the 2:1-adduct.In a similar manner, the nitroxide CF₃N(?)CMe₂CHMe₂ adds to tetrafluoroethylene to give the compound R²N(CF₃)OCF₂CF₂ON(CF₃)R² (R² = CMe₂CHMe₂, 75%) and to hexafluoropropene to give a similar adduct (71%), and abstracts a benzylic hydrogen from toluene.     

Five-membered 2,3-dioxoheterocycles 15. Synthesis and [1,3]-sigmatropic rearrangement of 1,5-diaryl-3-diphenylmethoxy-4-ethoxycarbonyl-2,5-dihydropyrrol-2-ones

Ethyl oxaloacetate reacts with a mixture of an aromatic aldehyde and an arylamine to give 1,5-diaryl-4-ethoxycarbonyltetrahydropyrrole-2,3-diones, which react with diphenyldiazomethane to give the O-alkylation products. On heating, the latter undergo suprafacial [1,3]-sigmatropic rearrangement to 1,5-diaryl-4-diphenylmethyl-4-ethoxycarbonyltetrahydropyrrole-2,3-diones. The effects of the type of migrating group on the rearrangement are discussed.For Communication 14, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 753–757, June, 1990.     

Allylic alcohols formation by oxidation of allylic phenyl tellurides. A possible [2,3] sigmatropic rearrangement of allylic telluroxides

Treatment of cinnamyl, 3-methyl-2-butenyl, and 2-cyclohexenyl phenyl tellurides with an oxidizing agent such as H₂O₂, NaIO₄ or t-BuOOH at room temperature under nitrogen affords 1-phenyl-2-propenol, 2-methyl-3-butene-2-ol, and 2-cyclohexenol as a sole or main product respectively in a high yield. The formation of these allylic alcohols can be best explained by assuming a [2,3]-sigmatropic rearrangement of the intermediate allylic telluroxides. These tellurides also react with oxygen, the formation of α.β-unsaturated carbonyl compounds being much increased in this oxidation.     

[2,3]-Wittig sigmatropic rearrangement of crotyl propargyl ether system. An emerging tool for control of acyclic stereochemistry

The [2,3]-Wittig rearrangement of properly designated (E)- and (Z)-crotyl propargyl ether system has been shown to exhibit a remarkably high degree of threo- and erythro -selection, respectively, and the stereochemical outcomes are discussed on mechanistic grounds. Some useful transformations of the rearrangement product are also described within the context of the formal total synthesis of (±)-oudemansin. Further, the high level of diastereoselection is maintained in the reaction of the α-methylcrotyl counterparts, together with the exclusive formation of the (E)-olefinic bond.     

Asymmetric [2,3]Wittig sigmatropic rearrangement involving a chiral azaenolate as the migrating terminus. A simple synthesis of (+)-verrucarinolactone

The diastereo- and/or enantioselection are described in the title rearrangement of the chiral 2-(2-alkenyloxy)methyl 2-oxazolines which eventually provides optically active -hydroxy, γ,δ- unsaturated esters and (+)-verrucarinolactone.     

Cyclopropane chemistry. Part 5 [1,2]. Hexafluorocyclopropane as a source of difluorocarbene

Thermolysis of hexafluorocyclopropane in the presence of ethylene, propene, vinyl chloride, and vinyl bromide gives good yields of the corresponding 1,1-difluorocyclopropanes, formed by addition of difluorocarbene to the olefin. The tetrafluoroethylene formed dimerises to octafluorocyclobutane, co-dimerises with the olefin, or survives, depending on the reaction conditions. With allene, hexafluorocyclopropane gives 1-(difluoromethylene)cyclopropane, 2,2,3,3-tetrafluorospiropentane, and products derived from tetrafluoroethylene and allene.     

Studies in azide chemistry. Part X [1]. Synthesis of perfluoro-2-azido-1-azacyclohexene

Perfluoro-2-azido-1-azacyclohexene can be prepared by treating perfluoro-1-azacyclohexene with an equimolar proportion of sodium azide (in acetonitrile) or azidotrimethylsilane under mild conditions; ¹⁹F n.m.r. analysis reveals that this new imidoyl azide participates in ring ° chain valence tautomerism, the tetrazolo-isomer constituting 19% of the equilibrium mixture at 35 °C.     

Reactions of 1,3-thiazine-2,6-dithiones. Part 6 [1]. Synthesis of 2-alkylthio-2,3-dihydro-1,3-thiazine-6-thiones by reductive alkylation of 1,3-thiazine-2,6-dithiones

2-Alkylthio-2,3-dihydro-1,3-thiazine-6-thiones 4 are synthesized by the reductive alkylation of 1,3-thiazine-2,6-dithiones with thiolate anions.     

Tandem [2,3]sigmatropic rearrangement of a sulphonium ylide and cyclopropanation of the resulting electron-rich olefin on a 4-methoxy-2-pyrone derivative

Rhodium catalyzed reaction of 4-methoxy-6-phenylthiomethyl-2-pyrone with dimethyldiazomalonate affords 5-(bis(methoxycarbonyl))(phenylthio)methyl-4-methoxy-6-methylene-5,6- -dihydro-2-pyrone via [2,3]sigmatropic rearrangement of the corresponding sulphur ylide. Cyclopropanation of the 6-methylene double bond occurs by the less hindered side.     

Fluorine takes a hike: remarkable room-temperature rearrangement of the C1 isomer of C60F36 into the C3 isomer via a 1,3-fluorine shift

In a toluene/CDCl3 solution at room temperature, the C1 isomer of C60F36 rearranges into the C3 isomer over a period of four days, as a result of a unique 1,3-shift of fluorine; this rare example of addend migration across a fullerene cage surface is accelerated by air.     

Applications of the fluorinated 1,3-dipolar compounds as the building blocks of the heterocycles with fluorine groups. Part XII. Synthesis of trifluoromethylisoxazolines and their rearrangement into trifluoromethylaziridines [1]

$\text{N}$-Methyl-$\text{C}$-trifluoromethylnitrone cycloadded with various alkynes to give 3-trifluoromethyl-4-isoxazolines. The isolated isoxazolines further underwent the valence rearrangement into 2-trifluoromethylaziridines under more drastic conditions. Unusual stability of both trifluoromethylisoxazolines and -aziridines is discussed on the basis of the electronic effect of trifluoromethyl group.